Volume 18 - Friction, Lubrication, and Wear Technology Part 20 ppsx

Table 3 Bimetal bearing material systemsBearing performance characteristicsa Load capacity ratingc Class Backing layer Surface layer Compatibility Conformability Embeddability Fatigu

Fig 8 Compressive yield strength levels typically obtained in the individual layers of a trilayer construction

bearing material

Corrosion Resistance Bearing failure due to corrosion alone is rare Corrosion usually interacts with mechanical and

thermal factors to produce failure by fatigue or seizure under conditions the bearing normally would be able to tolerate

To a considerable extent, bearing corrosion can be avoided by use of oxidation inhibitors in commercial lubrication oils, and by periodic oil changes There are, however, many situations in which neither of these practices is dependable and where bearing materials with inherently high corrosion resistance should be used

Commercially pure lead is susceptible to corrosion by fatty acids Lead-base and copper-lead bearing alloys can suffer severe corrosion damage in acidic lubricating oils Tin additions in excess of 5% provide effective protection against this kind of corrosion, and for this reason tin is used extensively in lead-base bearing alloys Both copper and lead are attacked by acidic oils that contain sulfur This is of particular concern with copper-lead and leaded bronze bearing alloys Effective protection can be obtained by employing layered construction, with a surface layer of either a lead alloy containing tin or a tin alloy As long as the corrosion-resistant surface layer is intact, the underlying copper-lead alloy will not suffer damage by corrosion

Tin and aluminum bearing alloys are substantially impervious to corrosion by the products of oil oxidation, and they are used extensively in applications where the potential for lubricating oil corrosion is known to be high Although lubricating oil oxidation and contamination are the most common causes of bearing damage by corrosion, other sources of bearing corrosion, such as seawater, animal and vegetable oils, and corrosive gas, should be recognized Selection and specification of a bearing material for a specific application should take into account the anticipated service conditions under which the bearings will have to operate, and the potential for corrosion that these conditions may stimulate

Heat and Temperature Effects The reduced mechanical strength of bearing liner materials at elevated temperatures

is an important consideration in the selection of a bearing material for a given application Fatigue strength, compressive yield strength, and hardness decrease significantly with increased bearing operating temperature As shown by the softening curves in Fig 9, lead-base and tin-base bearing alloys are most severely limited in this respect, and copper alloys the least

Fig 9 Strength retention at elevated temperatures for selected bearing alloys (a) Copper-base alloys (b)

Aluminum-base alloys (c) Zinc-base alloys (d) Lead-base alloys and tin-base alloys

Load Capacity The load capacity of a bearing material is defined as the maximum unit pressure under which the

material can operate without excessive friction or wear damage Load capacity ratings published as guides for machinery designers generally represent upper limits, which may be safely employed only under very good conditions of lubricant film integrity, counterface finish, mechanical alignment, and temperature control

In cyclic-load service (for example, in crankshaft bearings), load capacity is primarily limited by fatigue strength In steady-load service, it depends more strongly on compressive yield strength, reflected in indentation hardness In all cases, the strength of the material at operating temperature will be the determining factor that governs the choice of bearing material Temperature and its control are therefore critically important to the successful operation of sliding bearings

Although useful to the designer as reference values, load capacity ratings must be recognized as imprecise and somewhat judgmental approximations They are not guaranteed or directly measurable material properties

Bearing Material Systems

Because of the widely varying conditions under which bearings must operate, commercial bearing materials have evolved

as specialized engineering materials systems rather than as commodity products They are used in relatively small tonnages and are produced by a relatively small number of manufacturers Much proprietary technology is involved in

alloy formulation and processing methods Successful selection of a bearing material for a specific application often requires close technical cooperation between the user and the bearing producer

Single-Metal Systems

Most single-metal sliding bearing are made from either copper alloys or aluminum alloys Some use is also made of cast zinc-base alloys which serve as lower-cost substitutes for solid bronze Commercially significant alloys that are used as single-metal bearings are listed in Table 2

Table 2 Single-metal bearing material systems

Bearing performance characteristics(a) Load

capacity rating(c)

Class Material

Compatibility Conformability Embeddability Fatigue

strength

Corrosion resistance (b)

E E E C D 28 4 Wrist pin bushings, pump bushings, electric-motor bushings,

track-roller bushings, farm-equipment gear bushings, mill-machinery bearings, machine-tool bearings

Low lead (1-4% Pb) F F F B B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool

bearings, earth-moving machinery bearings, farm-equipment gear bushings

2

Unleaded F F F A B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool

bearings, railroad-car wheel bearings

3 Aluminum alloy,

low tin

D D D D A 28 4 Connecting-rod main bearings, bushings, mill-machinery bearings

Zinc alloy 12% Al E E F B E 28 4 Compressor bearings, pump bushings, mill-machinery bearings,

earth-moving machinery bearings

4

27% Al E F F A E 34 5 Compressor bearings, pump bushings, mill-machinery bearings,

earth-moving machinery bearings Porous metal

Bronze

C C C D B 14 2 Electric motor bushings, home appliance bearings, agricultural

equipment bushings Iron D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural

equipment bushings

5

Iron-bronze D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural

equipment bushings (a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest)

(b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils

(c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication

Wide ranges of compositions and properties are available in the older copper group Brasses and bronzes have been widely used in bearing applications since the mid-1800s Interest in the use of aluminum alloys was stimulated by World War II metal shortages and greatly accelerated by the commercial introduction of aluminum-tin bearing alloys in 1946 Since then, metal economics have dictated the use of aluminum alloy bearings, but brasses and bronzes continue to be preferred by many designers of heavy and special-purpose machinery

Single-metal systems do not exhibit outstandingly good surface properties, and their tolerance of boundary and thin-film lubrication conditions is limited As a result, the load capacity rating for a single-metal bearing usually is low relative to the fatigue strength of the material from which it was made Because of their metallurgical simplicity, these materials are well suited for small-lot manufacturing from cast tubes or bars, using conventional machine shop processes

Copper Alloys Except for commercial bronze and low-lead tin bronze, copper alloys in single-metal systems are almost

always used in cast form This provides thick bearing walls ( 3.20 mm, or 0.125 in.) that are strong enough so that the bearing is retained in place when press fitted into the housing

Commercial bronze and medium-lead tin bronze alloys C83420 and C83520 are used extensively in the form of wrought strip for thin-wall bushings, which are made in large volumes by high-speed press forming The relatively poor compatibility of these alloys can be improved by embedding a graphite-resin paste in rolled or pressed-in indentations, so that the running surface of the bushing consists of interspersed areas of graphite and bronze Such bushings are widely used in automotive engine starting motors

The lead in leaded tin bronzes is present in the form of free lead that is dispersed throughout a copper-tin matrix so that the bearing surface consists of interspersed areas of lead and bronze In general, the best selection of materials from this group for a given application will be the highest-lead composition that can be used without risking excessive wear, plastic deformation, or fatigue damage

Aluminum Alloys Virtually all solid aluminum bearings used in the United States are made from alloys containing

from 5.5 to 7% Sn, plus smaller amounts of copper, nickel, silicon, and magnesium Starting forms for bearing fabrication include cast tubes as well as rolled plate and strip, which can be press formed into half-round shapes As is the case with solid bronze bearings, relatively thick bearing walls are employed in solid aluminum alloy bearings

The tin in these alloys is present in the form of free tin that is dispersed throughout an aluminum matrix so that the bearing surfaces consist of interspersed areas of aluminum and tin Surface properties are enhanced by the free tin in much the same way that those of bronze are improved by the presence of free lead

The high thermal expansion of aluminum poses special problems in maintaining press fit and running clearances Various methods are employed for increasing yield strength (for example, heat treatment and cold work) to overcome plastic flow and permanent deformation under service temperatures and loads

Zinc Alloys. During the past 20 years, zinc-aluminum-copper casting alloys have been used to replace cast bronze alloys

in certain low-speed machinery bearing applications This practice has advanced farthest in Europe, as an outgrowth of World War II material substitution efforts

These alloys do not contain any soft microconstituents that correspond to the lead used in bearing bronzes and to the tin in cast aluminum bearing alloys To a considerable degree, compatibility of the zinc-base alloys seems to derive from their chemical behavior with hydrocarbon lubricants Formation of a stable low shear strength film of zinc-base soap appears to

be an important factor

Porous Metal Bushings Oil-impregnated porous metal bushings can also be included in the single-metal systems

category The materials used for these bushings include unleaded and leaded tin-bronze, bronze-graphite, iron-carbon, iron-copper, and iron-bronze-graphite compositions Oil content of these materials constitutes 8 to 30% of total volume

Bimetal Systems

All bimetal systems employ a strong bearing back to which a softer, weaker, relatively thin layer of a bearing alloy is metallurgically bonded Low-carbon steel is by far the most widely used bearing-back material, although alloy steels, bronzes, brasses, and (to a limited extent) aluminum alloys are also used The bimetal bearing material systems currently

in significant commercial use are classified in Table 3

Table 3 Bimetal bearing material systems

Bearing performance characteristics(a) Load

capacity rating(c)

Class Backing layer Surface layer

Compatibility Conformability Embeddability Fatigue

strength

Corrosion resistance (b)

MPa ksi

Typical applications

Tin babbitt:

0.25-0.50 mm (0.010-0.020 in.)

1 Steel

0.102 mm (0.004 in.)

Connecting-rod and main bearings, camshaft bearings, electric-motor bushings, pump bushings, thrust washers

Lead babbitt:

0.25-0.50 mm (0.010-0.020 in.)

2 Steel

0.102 mm (0.004 in.)

Connecting-rod and main bearings, camshaft bearings, transmission bushings, pump bushings, thrust washers

Aluminum alloy:

High tin B C C D A 41 6 Connecting-rod and main bearings, camshaft

bearings, transmission bushings, pump bushings, thrust washers

Medium-tin B C C C A 55 8 Connecting-rod and main bearings, camshaft

bearings, transmission bushings, pump bushings, thrust washers

High-lead B C C C A 55 8 Connecting-rod and main bearings, camshaft

bearings, transmission bushings, pump bushings, trust washers

thrust washers

3 Steel

thrust washers Copper alloy:

Copper-lead C C C C F 38 5.5 Connecting-rod and main bearings, camshaft

bearings High-lead bronze D D D C E 45 6.5 Camshaft bearings, turbine bearings, pump

bushings, thrust washers

4 Steel

Medium-lead bronze

E E E B D 55 8 Piston pin bushings, rocker-arm bushings, wear

plates, steering-knuckle bushings, guide bushings, thrust washers

5 Medium-lead

bronze

Tin babbitt, 0.25-0.50

mm (0.010-0.020 in.)

washers, railroad-car journal bearings, machinery bearings

mill-6 Medium-lead

bronze

Lead babbitt: 0.50 mm (0.010- 0.020 in.)

(a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest)

(b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils

(c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication

The strengthening effect of a steel bearing back is illustrated clearly for classes 3 and 4 in Table 3; these ratings can be compared with those for the aluminum and copper alloy single-metal systems in Table 2 When steel bearing backs are employed, load-capacity ratings for both copper and aluminum alloys are sharply increased above those of the corresponding single metals without degrading any other properties Similarly, in classes 1, 2, 5, 6, and 7, the strong bearing-back materials permit use of lead and tin alloys that have extremely good surface properties but that are so low in strength that they can be used as single-metal bodies only under very light loads

The strengthening effect of thin-layer construction on lead and tin alloys is illustrated in Table 3 (classes 1 and 2), where

a 50% increase in load capacity is achieved by reducing babbitt layer thickness Although similar behavior has been observed with aluminum and copper alloys, the thin-liner effects are less dramatic Liner thicknesses employed with these stronger alloys are established by metal economics and manufacturing process considerations, rather than by strength/thickness relationships

Deterioration in surface properties with increasing liner alloy fatigue strength is clearly seen by the comparison of classes

1 and 2 with classes 3 and 4, and by comparisons within classes 3 and 4 (Table 3) In practice, only those systems with surface properties rated "D" or better are successful under boundary and thin-film lubrication conditions This restricts the use of bimetal materials in connecting-rod and main bearings to loads of 55 MPa ( 8 ksi)

Bronze-back bearings (see Table 3, classes 5, 6, and 7) do not exhibit combinations of performance characteristics substantially different from those of steel-back bearings The practical advantages of bronze as a bearing-back material lie partly in the economics of small-lot manufacturing and partly in the relative ease with which worn bronze-back bearings can be salvaged by rebabbitting and remachining From the standpoint of performance, the advantage of bronze over steel

as a bearing-back material is the protection bronze affords against catastrophic bearing seizure in case of severe liner wear

or fatigue Similar protection is provided by the aluminum alloy bearing back in class 8

Although the surface properties of bronze bearing-back materials are not impressive, they are superior to those of steel, and these "reserve" bearing properties can be of considerable practical importance in large expensive machinery used in certain critical applications

Trimetal Systems

Virtually all trimetal systems employ a steel bearing back, an intermediate layer of relatively high strength, and a tin alloy

or lead alloy surface layer The systems in current commercial use are listed by classes in Table 4 Most of these systems are derived from the bimetal systems of Table 3 (classes 3 and 4) by the addition of a lead-base or tin-base surface layer

Table 4 Trimetal bearing material systems

capacity rating(c)

MPa ksi

Typical applications

1 Steel Medium-lead bronze Tin babbitt,

0.25-0.50 mm 0.020 in.)

main bearings, bushings

2 Steel High-lead bronze Tin babbitt,

0.25-0.50 mm 0.020 in.)

main bearings, bushings

3 Steel Copper-lead Lead babbitt,

0.075 mm (0.003 in.)

bearings, camshaft bearings

4 Steel Copper-lead Lead babbitt,

0.025 mm (0.001 in.)

bearings, bushings

5 Steel High-lead bronze Lead babbitt,

0.025 mm (0.001 in.)

bearings, thrust washers

6 Steel Medium-lead bronze Lead babbitt,

0.025 mm (0.001 in.)

bearings

7 Steel Aluminum, low tin Lead babbitt,

0.025 mm (0.001 in.)

bearings

8 Steel Aluminum, tin free, low

alloy

Lead babbitt, 0.025 mm (0.001 in.)

bearings

9 Steel Aluminum, tin free, low

alloy, precipitation hardened

Lead babbitt, 0.025 mm (0.001 in.)

bearings

10 Steel Aluminum, tin free,

high alloy

Lead babbitt, 0.025 mm (0.001 in.)

bearings

11 Steel Silver Lead babbitt,

0.025 mm (0.001 in.)

bearings for aircraft reciprocating engines (a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest)

(b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils

(c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication

The strengthening effects of thin-layer construction are notable in those systems that incorporate electroplated lead alloy surface layers 0.025 mm ( 0.001 in.) thick (Table 4, classes 4 through 11) Comparison of fatigue strength and load capacity ratings of these systems with those of the corresponding bimetal systems in Table 3 shows that the thin lead alloy surface layer upgrades not only surface properties but also fatigue strength The increase in fatigue strength can be attributed at least in part to the elimination of stress raisers, from which fatigue cracks can propagate

Class 1 and class 2 trimetal systems comprise leaded bronze intermediate layers with relatively thick tin alloy surface layers These represent an evolution from bronze-back babbitt construction wherein steel has replaced most of the bronze This produces the expected economy and bearing-back yield strength, but retains the desirable "reserve" bearing properties exhibited by bronze-back construction

Class 11 trimetal systems, which have silver intermediate layers, are too costly for most commercial applications However, they provide an unequaled combination of high load capacity and corrosion resistance They continue to have limited use in radial piston engines for aircraft

Trimetal systems with electroplated lead-base surface layers and copper or aluminum alloy intermediate layers provide the best available combinations of cost, fatigue strength, and surface properties Such bearings have high tolerances for boundary and thin-film lubrication conditions, and thus can be used under higher loads than can any of the bimetal systems Although more costly than the corresponding steel-back bimetal systems, they are used in some high-volume automotive applications as well as in larger mobile and stationary engines A highly developed body of mechanical, metallurgical, and chemical manufacturing technology has been established in the plain bearing industry, and this technology permits mass production of precision trimetal bearings without a severe cost penalty

• Additional metallic materials (gray cast irons and cemented carbides)

• Nonmetallic materials (nylon, PTFE, carbon-graphite, wood, rubber, and laminated phenolics)

Tin-Base Alloys

Tin-base bearing materials (babbitts) are alloys of tin, antimony, and copper that contain limited amounts of zinc, aluminum, arsenic, bismuth, and iron The compositions of tin-base bearing alloys, according to ASTM B 23, SAE, and ISO specifications, are shown in Table 5

Table 5 Designations and nominal composition of tin-base bearing alloys

Designation(a) Composition, %

(B 23)

Sn Sb Cu Other

Product form Applications

L13910 Alloy 1 91 4.5 4.5 Cast on steel or bronze back Bimetal surface layer

L13840 Alloy 3 84 8 8 Cast on steel or bronze back Bimetal surface layer

SnSb12Cu6Pb 80 12 6 2 Pb Cast on steel back Bimetal surface layer (a) UNS, unified numbering system; SAE, Society of Automotive Engineers; ISO, International Organization for

Standardization; ASTM, American Society for Testing and Materials

The presence of zinc in these bearing metals generally is not favored Arsenic increases resistance to deformation at all temperatures; zinc has a similar effect at 38 °C (100 °F) but causes little or no change at room temperature Zinc has a marked effect on the microstructures of some of these alloys Small quantities of aluminum (even <1%) will modify their microstructures Bismuth is objectionable because, in combination with tin, it forms a eutectic that melts at 137 °C (279

°F) At temperatures above this eutectic, alloy strength is decreased appreciably

Bulk mechanical properties of ASTM grades 1 to 3 are shown in Table 6 These properties have some value for initial materials screening comparisons of alloys; but they are not reliable predictors of the performance of thin layers bonded to

a strong backing, which is the manner in which tin-base babbitts are usually used in modern bearing practice Layer thickness effects (Fig 5) and temperature effects (Fig 7) are more important practical considerations than the mechanical property differences among the various alloy compositions

Table 6 Typical mechanical properties of chill cast tin-base bearing alloys

Compressive yield strength(a)

100 °C (212 °F) L13910 Alloy 1 30.3 4.40 18.3 2.65 64 9.3 2 17.0 8.0

L13890 Alloy 12 SnSb8Cu4 Alloy 2 42.1 6.10 20.7 3.00 77 11.2 24.5 12.0

L13840 Alloy 3 45.5 6.60 21.7 3.15 69(b) 10.0(b) 1(b) 27.0 14.5

(b) Values are for die-cast alloy specimens

Compared with most other bearing materials, tin alloys have low resistance to fatigue, but their strength is sufficient to warrant their use under low-load conditions These alloys are commercially easy to bond and handle and they have excellent antiseizure qualities Their excellent corrosion-resistant properties make these alloys especially well-suited for bearing applications in compressors, electric motors, and food-processing equipment

Tin-base alloys vary in microstructure in accordance with their composition Alloys that contain about 0.5 to 8% Cu and

<8% Sb are characterized by a solid-solution matrix in which needles of a copper-rich constituent and fine rounded particles of precipitated SbSn are distributed The proportion of the copper-rich constituent increases with copper content SAE 12 (ASTM grade 2) has a structure of this type in which the needles often assume a characteristic hexagonal starlike pattern Alloys that contain 0.5 to 8% Cu and >8% Sb exhibit primary cuboids of SbSn, in addition to needles of the copper-rich constituent in the solid-solution matrix In alloys with 8% Sb and 5 to 8% Cu, rapid cooling suppresses formation of the SbSn cuboids This is particularly true of alloys containing lower percentages of copper

Lead-Base Alloys

Lead-base bearing materials (lead babbitts) are alloys of lead, tin, antimony, and in many cases, arsenic Many such alloys have been used for centuries as type metals, and were probably first employed as bearing materials because of the properties they were known to possess The advantage of arsenic additions have been generally recognized since 1938 Nominal compositions of the most widely used lead babbitts according to ASTM, SAE, and ISO specifications are listed

in Table 7 Additional information on mechanical properties of some of these alloys is given in Table 8

Table 7 Designations and nominal composition of lead-base bearing alloys

84 15 1 1 Cast on steel back Bimetal surface layer

PbSb14Sn9CuAs 77 14 9 0.5 1 Cu Cast on steel back Bimetal surface layer

L53565 Alloy 8 80 15 5 0.5 Cast on steel back Bimetal surface layer

(a) 87.5 9 3.5 Cast on bronze back Bimetal surface layer

Table 8 Typical mechanical properties of chill cast lead-base bearing alloys

Compressive yield strength(a)

100 °C (212 °F) L53346 Alloy 13 PbSb10Sn6 Alloy 13 22.8 3.30 10.7 1.55 69 10.0 5 19 8.5

L53585 Alloy 14 PbSb15Sn10 Alloy 7 24.5 3.55 11.0 1.60 72 10.5 4 22.5 10.5

L53620 Alloy 15 PbSb15SnAs Alloy 15 24.8 3.60 14.5 2.10 71 10.4 2 21 13

Comments made in the section "Tin-Base Alloys" in this article concerning the significance of bulk mechanical properties

of tin-base babbitt alloys apply equally to those of lead-base alloys

For many years, lead-base bearing alloys were considered to be only low-cost substitutes for tin alloys However, the two groups of alloys do not differ greatly in antiseizure characteristics, and when lead-base alloys are used with steel backs and in thicknesses <0.75 mm (<0.03 in.), they have fatigue resistance that is equal to, if not better than, that of tin alloys Bearings of any of these alloys remain serviceable longest when they are 0.13 mm ( 0.005 in.) thick (See Fig 5)

In the absence of arsenic, the microstructures of these alloys comprise cuboid primary crystals of SbSn or of antimony embedded in a ternary mixture of Pb-Sb-SbSn in which lead forms the matrix The number of these cuboids per unit volume of alloy increases as antimony content increases If antimony content is >15%, the total amount of the hard constituents increases to such an extent that the alloys become too brittle to be useful as bearing materials

Arsenic is added to lead babbitts to improve their mechanical properties, particularly at elevated temperatures All lead babbitts are subject to softening or loss of strength during prolonged exposure to the temperatures (95 to 150 °C, or 200 to

300 °F) at which they serve as bearings in internal combustion engines Addition of arsenic minimizes such softening Under suitable casting conditions (see the section "Casting Processes" in this article), the arsenical lead babbitts (for example, SAE 15 [ASTM grade 15]) develop remarkably fine and uniform structures They also have better fatigue strength than arsenic-free alloys

Pouring temperature and rate of cooling markedly influence the microstructures and properties of lead alloys, particularly when they are used in the form of heavy liners for railway journals High pouring temperatures and low cooling rates, which typically result from the use of overly hot mandrels, promote segregation and formation of a coarse structure A coarse structure may cause brittleness, low compressive strength, and low hardness Therefore, low pouring temperatures (325 to 345 °C, or 620 to 650 °F) usually are recommended Because these alloys remain relatively fluid almost to the point of complete solidification ( 240 °C, or 465 °F, for most compositions), they are easy to manipulate and can be handled with no great loss of metal from drossing

Lead-Base Electroplated Overlays The improvement in fatigue life that can be achieved by decreasing babbitt

layer thickness has already been noted Economically as well as mechanically, it is difficult to consistently achieve very thin uniform babbitt layers bonded to bimetal shells by casting techniques Therefore, the process of electroplating (see the section "Electroplating Processes" in this article) a thin precision babbitt layer on a very accurately machined bimetal shell was perfected Specially designed plating racks allow the thickness of the plated babbitt layer to be regulated so accurately that further machining is usually not required

Electroplated tin alloys were found to be generally inferior to lead alloys, and only lead alloys are in commercial use as electroplated bearing overlays Table 9 lists the four most commonly used compositions SAE alloy 192 is the most frequently used Tin in alloys 191, 192, and 193 and indium in alloy 194 impart corrosion resistance Tin also increases wear resistance Both copper and indium enhance fatigue resistance

Table 9 Designations and nominal composition of lead-base electroplated overlay alloys

Copper-Base Alloys

Copper-base bearing alloys comprise a large family of materials with a wide range of properties They include commercial bronze, copper-lead alloys, and leaded and unleaded tin bronzes They are used alone in single-metal bearings, as bearing backs with babbitt surface layers, as bimetal layers bonded to steel backs, and as intermediate layers

in steel-backed trimetal bearings (see Tables 2, 3, and 4)

The moderate strength and hardness of pure copper are readily increased by alloying, most commonly with tin (with which copper forms a solid solution) Lead is present in cast copper-base bearing alloys as a nearly pure, discrete phase, because its solid solubility in the matrix is practically nil The lead phase, which is exposed on the running surface of a bearing, constitutes a site vulnerable to corrosive attack under certain operating conditions

The antifriction behavior of copper-base bearing alloys improves as lead content increases, although at the same time strength is degraded because of increased interruption of the continuity of the copper alloy matrix by the soft weak lead Thus, through judicious control of tin content, lead content, and microstructure, a large family of bearing alloys has evolved to suit a wide variety of bearing applications

Table 10 gives specification numbers and nominal compositions of copper-base bearing alloys, as well as the forms in which the alloys are used and general notations on typical product applications The information in Table 10 should be used in conjunction with the appropriate portions of Tables 2, 3, and 4 and with the brief descriptions that follow

Table 10 Designations and nominal composition of copper-base bearing alloys

No

Product form Applications

Unleaded tin bronzes

2 C52100 CnSn8P 92(a) 8 Wrought strip Solid bronze bushings and

washers

3 C90300 88 8 4 Cast tubes Solid bronze bearings

4 C90500 88 10 2 Cast tubes Solid bronze bearings

5 C91100 84 16 Cast tubes Solid bronze bearings

6 C91300 81 19 Cast tubes Solid bronze bearings

Low-lead tin bronzes

7 C92200 88.5 6 1.5 4 Cast tubes Solid bronze bearings

8 C92300 87 8.5 0.5 4 Cast tubes Solid bronze bearings

9 C92700 87.5 10 2 0.5 Cast tubes Solid bronze bearings

Medium-lead tin bronzes

12 C83600 CuPb5Sn5Zn5 85 5 5 5 Cast tubes Solid bronze bearings,

bronze bearing backs

13 C93200 CuSn7Pb7Zn3 83 7 7 3 Cast tubes Solid bronze bearings

88 4 8 Sintered on steel back Bimetal surface layer

16 C93700 CuPb10Sn10 80 10 10 Cast tubes Solid bronze bearings,

bronze bearing backs

17 Alloy

792

CuPb10Sn10(G) 80 10 10 Cast on steel back Bimetal surface layer,

trimetal intermediate layer

18 Alloy

792

CuPb10Sn10(P) 80 10 10 Sintered on steel back Bimetal surface layer

High-lead tin bronzes

19 C93800 78 7 15 Cast tubes Solid bronze bearings,

bronze bearing backs

CuPb24Sn4(G) 73.5 3.5 23 Cast on steel back Bimetal surface layer,

trimetal intermediate layer

22 Alloy

794

CuPb24Sn4(P) 73.5 3.5 23 Sintered on steel back Bimetal surface layer,

trimetal intermediate layer

23 F112 72.5 2.5 25 Cast on steel back Trimetal intermediate layer

24 C94300 70 5 25 Cast tubes Solid bronze bearings

CuPb30(P) 70 30 Sintered on steel back Bimetal surface layer,

trimetal intermediate layer

Commercial Bronze Lead-free copper alloys are characterized by poor antifriction properties but fairly good

load-carrying ability Wrought commercial bronze strip (SAE 795) with 10% Zn can be readily press formed into cylindrical bushings and thrust washers Strength can be increased by cold working this inexpensive material

Unleaded Tin Bronze The unleaded copper-tin alloys are known as phosphor bronzes because they are deoxidized

with phosphorus They are used principally in cast form as shapes for specific applications, or as rods or tubes from which solid bearings are machined They have excellent strength and wear resistance, both of which improve with increasing tin content, but poor surface properties They are used for bridge turntables and trunnions in contact with high-strength steel, and in other slow-moving applications

Low-Lead Tin Bronzes The inherently poor machinability of tin bronzes can be improved by adding small amounts

of lead Such additions do not significantly improve surface properties, however, and applications for these alloys are essentially the same as those for unleaded tin bronzes

Medium-Lead Tin Bronzes The only wrought strip material in this group of alloys is SAE 791, which is press formed

into solid bushings and thrust washers C83600 is used in cast form as bearing backs in bimetal bearings SAE 793 is a low-tin, medium-lead alloy that is cast or sintered on a steel back and used as a surface layer for medium-load bimetal bushings SAE 792 is higher in tin and slightly higher in lead; it is cast or sintered on a steel back and used for heavy-duty applications such as wrist pin bushings and heavy-duty thrust surfaces

High-Lead Tin Bronzes These contain medium-to-high amounts of tin, and relatively high lead contents to markedly

improve antifriction characteristics SAE 794, widely used in bushings for rotating loads, has the same bronze matrix composition as SAE 793 (4.5% Sn) but three times as much free lead It is cast or sintered on a steel back and used for somewhat higher speeds and lower loads than alloy 793 The bronze matrix of SAE 794 is much stronger than that of a plain 75-25 copper-lead alloy Alloy 794 can be used as the intermediate layer with a plated overlay in heavy-duty trimetal bearing applications such as main and connecting-rod bearings in diesel truck engines This construction provides the highest load-carrying ability available in copper alloy trimetal bearings

Copper-Lead Alloys These are used extensively in automotive, aircraft, and general engineering applications These

alloys are cast or sintered to a steel backing strip from which parts are blanked and formed into full-round or half-round shapes depending on final application Copper-lead alloys continuously cast on steel strip typically consist of copper dendrites perpendicular and securely anchored to the steel back, with an interdendritic lead phase In contrast, sintered copper-lead alloys of similar composition are composed of more equiaxed copper grains with an intergranular lead phase

High-lead alloy SAE 48 can be used bare on steel or cast iron journals Tin content in this alloy is restricted to a minimum value to maintain a soft copper matrix, which together with the high lead content improves the antifriction/antiseizure properties of the alloy Bare bimetal copper-lead bearings are used infrequently today because the lead phase, present as nearly pure lead, is susceptible to attack by corrosive products that can form in the crankcase lubricant during extended oil-change periods Therefore, most copper-base alloys with lead contents >20%, including both SAE alloy 48 and alloy

49, are now used with plated overlays in trimetal bearings for automotive and diesel engines

SAE 485 is a special sintered and infiltrated composite material, produced by methods described in the section "Powder Metallurgy Processes" in this article By these methods, it is possible to combine a very strong continuous copper alloy matrix structure with a very high lead content, and to alloy the lead-rich constituent with sufficient tin to make it resistant

to corrosion SAE 485 is used principally for bushing and bearing applications that involve alignment, shaft surface finish, or unusual dirt contamination problems

Mechanical Properties of Copper-Base Bearing Alloys Table 11 shows the ranges of mechanical strength

properties that are exhibited by copper-base bearing alloys, according to alloy families and forms as listed in Table 10 Indentation hardness tests provide the most generally useful indications of behavior under compressive loads, and are the only standard strength tests that are applicable to all of the alloy forms Conventional tensile and compression tests can be performed only on solid alloy bodies, which represent a relatively small fraction of total copper-base bearing alloy applications

Table 11 Typical room-temperature mechanical properties of copper-base bearing alloys

Compressive yield strength(a)

Ultimate tensile

strength Alloy family Product form

Medium-lead tin bronzes

Steel backed 50-130 Cast tubes 75-85 11-12 185-210 27-30 48-55

High-lead tin bronzes

The ready availability of aluminum and its relatively stable cost have provided an incentive for continuing development

of its use in plain bearings Aluminum single-metal, bimetal, and trimetal systems can now be used in the same load ranges as babbitts, copper-lead alloys, and high-lead tin bronzes Moreover, the outstanding corrosion resistance of aluminum has become an increasingly important consideration in recent years, and has led to widespread use of aluminum alloy materials (in the place of copper-lead alloys and leaded bronzes) in automotive engine bearings

Designations and Compositions Alloy designations and nominal compositions of the aluminum-base bearing

alloys in most extensive commercial use are listed in Table 12 In these alloys, additions of silicon, copper, nickel, magnesium, and zinc function to strengthen the aluminum through solid-solution and precipitation mechanisms Fatigue resistance and the opposing properties of conformability and embeddability are largely controlled by these elements and

by the use of appropriate heat treatments Tin and lead are instrumental in upgrading the inherently poor compatibility of aluminum Cadmium is also used as an alloy addition for this reason Silicon has a beneficial effect on compatibility in addition to its moderate strengthening effect Although not well understood theoretically, this compatibility-enhancing mechanism is of considerable practical value Silicon is used effectively in many alloys for this reason (usually in conjunction with tin, lead, or cadmium)

Table 12 Designations and nominal composition of aluminum-base bearing alloys

No UNS SAE ISO Other Al Si Cu Ni Mg Sn Other

Product form Applications

Bimetal surface layer

2 Alloy

786

59.5 0.5 40 Wrought strip,

bonded to steel back

Bimetal surface layer

Bimetal surface layer

4 Alloy

787

Al-6 88.5 4 0.5 1 6 Pb Wrought strip,

bonded to steel back

Bimetal surface layer

Bimetal surface layer

Bimetal surface layer

Bimetal surface layer

Wrought strip, bonded to steel back

Bimetal surface layer

14 Alloy

770

AlSn6CuNi 91.5 1 1 6.5 Wrought strip,

bonded to steel back

Bimetal surface layer, trimetal intermediate layer

15 A08280 Alloy

780

90.5 1.5 1 0.5 6.5 Wrought strip,

bonded to steel back

Bimetal surface layer, trimetal intermediate layer

Bimetal surface layer, trimetal intermediate layer

17 A04002 Alloy

781

AlSi4Cd F-154 95 4 0.1 0.1 1 Cd Wrought strip,

bonded to steel back,

precipitation hardened

Trimetal intermediate layer

18 Alloy

782

AlCd3CuNi 95 1 1 3 Cd Wrought strip,

bonded to steel back

Bimetal surface layer, trimetal intermediate layer

19 Alloy AlSi11Cu 88 11 1 Wrought strip, Trimetal

784 bonded to steel

back

intermediate layer

Trimetal intermediate layer

Microstructural Features The cast low-tin alloys (numbers 9 through 11 in Table 12) all display similar

microstructures consisting of equiaxed aluminum grains with NiAl3, free silicon (if present), and free tin precipitated in the grain boundaries Tin forms a nearly complete envelope around each aluminum grain The copper and magnesium are mostly or completely in solid solution in the aluminum and are not visible under the microscope Microstructures of the wrought low-tin, intermediate-tin, and high-tin alloys exhibit the expected effects of rolling and annealing, with the as-cast aluminum grains replaced by new recrystallized grains and the insoluble phases (NiAl3, and silicon) uniformly distributed throughout The original continuous grain boundary envelope of free tin assumes a completely new configuration, the tin now appearing as somewhat elongated, discontinuous "lakes." This characteristic structure, often termed "reticular," results in much greater ductility than that of the cast alloys

Microstructures of the lead-aluminum alloys (numbers 3 and 4 in Table 12) exhibit a recrystallized aluminum matrix with

a fine uniform dispersion of free silicon The lead is present as thin stringers or ribbons of the lead-tin constituent, elongated in the rolling direction During recrystallization, this constituent does not coalesce into lakes as does free tin, and the ribbon-like configuration persists in finished bearings The effectiveness of the modest lead concentrations in these alloys in imparting surface compatibility probably is related to the favorable orientation of the lead-tin ribbons relative to the bearing surface

The wrought tin-free alloys (numbers 16 through 20 in Table 12) display very simple microstructures, consisting of a recrystallized aluminum matrix with the soluble strengthening additions (copper, zinc, and magnesium) in solid solution Insoluble phases (NiAl3, silicon, cadmium, and lead) are present in fine and uniform dispersion

Mechanical Properties of Aluminum-Base Bearing Alloys Conventional mechanical properties are somewhat

like microstructural features in that they are of more value in predicting the fabrication behavior of aluminum-base bearing alloys than in predicting their bearing performance With the exception of solid aluminum alloy bearings, in which there is no steel back and where press-fit retention depends entirely on the strength of the aluminum alloy, mechanical properties of finished bearings are rarely specified, usually for control purposes only Consideration of some

of these properties (Table 13) does, however, contribute to an understanding of these alloys as a family of related engineering materials, and of their relationship to the better-known structural aluminum alloys in addition to the copper-base, tin-base, and lead-base bearing alloys discussed previously

Table 13 Typical room-temperature mechanical properties of aluminum-base bearing alloys

Compressive yield strength(a)

Ultimate tensile

strength Alloy family Product form

Hardness, HB

High-tin alloys Steel backed 100-130 15-19 25-40

High-lead alloys Steel backed 40-50

Intermediate-tin alloys Steel backed 50-60

Cast tubes 70-140 10-20 125-220 18-32 45-65 Wrought plate 80-140 12-20 140-170 20-25 40-55

Low-tin alloys

Steel backed 35-45

Tin-free alloys Steel backed 35-65

Product Applications The majority of the current commercial applications of aluminum-base bearing alloys involve

steel-backed bimetal or steel-backed trimetal bearings To determine the most cost-effective aluminum material for any specific application, consideration should be given to the economic advantages of bimetal versus trimetal systems The higher cost of the high-tin and high-lead alloys usually is offset by eliminating the cost of the lead alloy overlay plate The cost effectiveness of the aluminum bimetal materials is clearly demonstrated by the fact that approximately 75% of the passenger car engines built in the United States use high-lead aluminum alloy bimetals for main and connecting-rod

bearings In Europe and Japan, intermediate and high-tin aluminum bimetals similarly dominate passenger car engine bearing markets

If the higher load capacity of a trimetal material is required, it then becomes important to select an aluminum liner alloy that provides adequate but not excessive strength, so that conformability and embeddability are not sacrificed unnecessarily The tin-free alloy group (alloys 16 to 20 in Table 12) offers a wide range of strength properties, and the most economical choice usually is found in this group

Silver-Base Alloys

Use of silver in bearings is largely confined to unalloyed silver (AMS 4815) electroplated on steel shells, which then are machined to very close dimensional tolerances and finally precision plated to size with a tin overlay of soft metal The overlay may be lead-tin, lead-tin-copper, lead-indium, or in some cases, pure lead As a bearing material, plated silver is invariably used with an overlay Silver on steel with an overlay is regarded as the ultimate fatigue-resistant bearing material

Silver was widely used during and after World War II in aircraft applications, where its high cost could be justified With the phasing out of piston engines, however, the use of silver in bearings has greatly declined Current applications are specialized, chiefly in the aircraft and locomotive industries In view of the rapidly rising cost of silver, any increase in demand for this material would stimulate the search for a comparable less-expensive substitute

Zinc-Base Alloys

The base alloys that have been used successfully for machinery bearings are standard zinc foundry alloys of the aluminum-copper-magnesium high-performance type Tubular shapes made by conventional sand, permanent mold, and pressure die-casting methods are machined into bearings in the same way that solid bronze bearings are made Most applications have been direct substitutions for solid bronze bearings; the substitutions are made primarily to reduce costs Alloy designations, nominal compositions, and typical mechanical properties are shown in Tables 14 and 15 for the two predominant alloys in the United States

zinc-Table 14 Designations and nominal composition of zinc-base bearing alloys

Microstructurally, these alloys display a eutectic or peritectic zinc matrix, surrounding zinc-rich or rich primary dendrites Copper is in solid solution Grain size varies greatly with casting method from coarse in sand castings to finest in pressure die castings Some experimental evidence associates the coarse sand cast structures with superior bearing wear resistance

aluminum-Because of their low cost, the zinc-base alloys will probably continue to replace copper-base alloys in certain bearing applications in construction, earth-moving, mining, and mill machinery markets However, technical limitations with respect to high-temperature strength and corrosion resistance will prevent any massive movement away from bronzes and into zinc alloys

Additional Metallic Bearing Materials

Additional commercially available metallic bearing materials include gray cast irons and cemented carbides

Gray cast irons are standard materials for certain applications involving friction and wear (for example, brake drums,

piston rings, cylinder liners, and gears) Cast irons perform well in such applications, and thus should be given consideration as bearing materials Gray iron bearings have proved successful in refrigeration compressors where bearing pressures are <4500 kPa (<650 psi) for main bearings and <5500 kPa (<800 psi) for connecting rod bearings Normally, the journals in refrigeration compressors are made either of steel (carburized and hardened to 55 to 60 HRC) or of pearlitic malleable or ductile iron (hardened to 44 to 48 HRC and having a surface finish of 0.3 m, or 12 in., root-

mean-square, Rq Because of occasional dilution of the oil with liquid refrigerant and heavy foaming of the oil, lubrication may become marginal for short periods of time Fine-grain iron with uniformly distributed No 6 (or finer) graphite flakes usually performs well during these periods The bearings are often phosphate coated to improve their seizure resistance This type of coating also creates a spongelike surface that promotes retention of oil

For good wear resistance, gray cast iron should be pearlitic with randomly distributed graphite flakes Cast irons have been heat treated to obtain martensitic structures for use as cylinder liners, but the benefits of such heat treatment have not been economically justifiable Hardened cast iron has been used successfully as a material for the ways on machine tools

Cemented Carbides. Extremely hard materials, including cemented tungsten carbides, titanium carbides, and other

combinations have been used successfully for various specialized bearing and seal applications In terms of the bearing performance characteristics listed in Table 1 these materials exhibit essentially zero conformability and embeddability, but rank high in strength, hardness, corrosion resistance, and compatibility Cemented carbides have been of greatest interest in high-temperature aerospace applications, but have also been used to advantage in certain machinery and machine tool applications

Nonmetallic Bearing Materials

Nonmetallic bearing materials are widely used for a variety of applications They have many inherent advantages over metals, including better corrosion resistance, lighter weight, better resistance to mechanical shock, and the ability to function with very marginal lubrication or with no lubricant present at all The major disadvantages of most nonmetallics are their high coefficients of thermal expansion and their low thermal conductivity characteristics For many years, carbon-graphites, wood, rubber, and laminated phenolics dominated the field of nonmetallic bearing materials In the early 1940s, development of nylon and polytetrafluoroethylene (PTFE, or Teflon) gave engineering designers two new nonmetallics with very unique characteristics, particularly the ability to operate dry

A wide variety of polymer composites is now being used very successfully in bearing applications The addition of fiber reinforcements and fillers such as solid lubricants and metal powders to the resin matrix can significantly improve the physical, thermal, and tribological properties of these plastics

Casting Processes

Single-Metal Systems Except for porous metal oil-impregnated bushings, all the single-metal systems listed in Table

2 are commercially produced by casting, either with or without subsequent mechanical working Plate, strip, and sheet forms of commercial bronze, of low-lead and lead-free tin bronzes, and of aluminum-tin alloys are initially cast as ingots, slabs, or bars by static and continuous casting methods similar to those used for other brass and aluminum mill products Subsequent rolling and annealing operations are also similar to those used for conventional mill products Because of the

extreme hot shortness of leaded tin bronzes and aluminum-tin alloys, these alloys must be rolled either cold or at only slightly elevated temperatures, with frequent intermediate annealing

The recrystallized wrought structures of bronze and aluminum-tin bearing alloys are substantially different from the initial cast structures, with respect to the configurations of the copper and aluminum phases and of the free-lead and free-tin phases The improvements in ductility and forming characteristics that result from these structural changes are of great importance in subsequent bearing manufacturing operations Bearing performance properties are not strongly affected by these changes Both the as-cast and wrought forms of these alloys are in commercial use and are equally acceptable in bearing applications

Tubular and cylindrical bronze, zinc, and aluminum-tin alloy shapes are produced by static, centrifugal, and continuous casting methods, and subsequently are machined into bearings The high-lead bronzes are used only in the as-cast condition because of their low ductility and extreme hot shortness, which preclude any substantial amount of plastic deformation of cast shapes Cast aluminum-tin alloy tubes can withstand a limited amount of cold work, however, and in some instances cold compression of 4 to 5% is employed to increase yield strength and improve press-fit retention in the finished bearings

Bimetal Systems Specialized casting methods are widely employed for producing bimetal bearing materials in both

tubular and flat strip forms Except for aluminum alloy systems (Table 3, classes 3 and 8), all bimetal systems in commercial use can, at least in principle, be produced by casting methods, and systems that incorporate tin and lead babbitt liners >0.1 mm (>0.004 in.) thick are universally produced by casting

Babbitt Centrifugal Casting. Short tubular steel and bronze shapes (bearing shells) are commonly lined with tin or lead alloys by various forms of centrifugal casting In these processes, a machined steel or bronze shell is first pre-heated and coated by immersion in molten tin or tin alloy The prepared shell is then placed in a lathelike "spinner" and rotated at

a controlled speed about its axis Molten babbitt is admitted through one end and is uniformly distributed around the inside wall of the shell by centrifugal action The molten layer then is cooled and solidified by spraying water against the outside of the rotating bearing shell When properly controlled, these processes produce fine-grain liner layers of reasonably uniform thickness, completely bonded to the steel or bronze bearing-back material Centrifugal casting methods are especially well suited to large-diameter thickwall bearings, which are made in relatively small quantities, and

to full-round seamless bearings, which cannot be fabricated from flat strip

Bronze Centrifugal Casting. Leaded tin bronzes also can be applied to the inner walls of steel shells by centrifugal casting Various methods of shell preparation are employed, including both molten-salt and controlled-atmosphere pre-heating Oxidation must be entirely absent from the inner wall of the steel shell for complete metallurgical bonding Centrifugal casting of bronzes is most successful with alloys containing >3% Sn and 20% Pb Outside this composition range, leaded tin bronze and copper-lead alloys are sensitive to lead segregation and consequent nonuniform

"centrifuged" microstructures Within these composition limits and under well-controlled process conditions, mechanically sound well-bonded bronze layers with reasonably uniform microstructures can be produced

Bronze Gravity Casting. All copper-lead alloys and leaded bronzes containing 35% Pb can be successfully cast in and bonded to steel shells by gravity casting methods, in which centrifugal forces are not a factor In these processes, a core usually is used to form an annular space inside the shell, into which molten bronze or copper-lead alloy is poured Several different processes of this kind are in commercial use, utilizing a variety of preheating methods, core materials, pouring methods, and quenching procedures

As in centrifugal casting, absence of oxides on the inner shell wall is necessary for complete bonding of the alloy layer to the steel back Liner microstructures produced by gravity shell casting methods generally are more uniform than those obtained by centrifugal casting For low-tin and high-lead compositions, gravity casting is preferred because of the absence of centrifuging effects on the solidifying alloy

Babbitt Strip Casting. Steel-backed tin alloy and lead alloy bearing strip materials are commonly produced by continuous casting in specially designed process lines in which separate cleaning, etching, hot tinning, liner-alloy casting, and quenching operations are carried out continuously on a moving steel bearing-back strip One or more in-line machining operations may also be incorporated so that the strip emerges with a closely controlled thickness, suitable for bearing fabrication

Bronze Strip and Slab Casting. The oldest commercial processes for producing steel-back copper-lead and leaded bronze bearing strip also utilize continuous casting on a moving steel strip Steel preheating, alloy casting, and quenching operations are performed under a strongly reducing atmosphere to ensure freedom from oxidation Some in-line machining also can be done, but the cast strip usually is machined in a separate line for close control of thickness Additional cold rolling and annealing operations are also employed particularly with the low and medium lead-tin bronze alloys, in which recrystallized structures are frequently preferred for their superior fabrication properties

Strip casting of copper alloys is a difficult technology that requires close process control, a high level of operator skill, and relatively expensive special-purpose equipment It is used by only a few bearing manufacturers, but with considerable commercial success It is employed not only for thin-gage coiled materials but also for heavy-gage slabs with steel thicknesses as great as 15 mm (0.60 in.)

Trimetal Systems Trimetal materials with relatively thick surface layers (Table 4, classes 1, 2, and 3) are used mostly

in large bearings These bearings are produced in relatively low volumes from steel shells initially lined by casting with copper-lead alloys or bronze After intermediate machining to remove excess liner alloy, such shells are commonly relined with tin or lead babbitt by centrifugal casting The methods used are essentially the same as those for casting in bare steel or solid bronze shells

Powder Metallurgy Processes

Single-Metal Systems The only commercial use of powder metallurgy (P/M) methods for making single-metal

bearing materials is in the fabrication of copper-base and iron-base porous metal bushings, which are subsequently impregnated with oil The methods used are similar to those for making structural P/M shapes (that is, pressing in a closed die and sintering under a reducing atmosphere) Bars, tubes, and finished parts are made in this way Post-sinter coining and repressing operations are frequently used to control final dimensions of finished parts

Bimetal and Trimetal Systems No powder metallurgy processes that use lead-base or tin-base bearing alloys are in

commercial use nor are there at present any commercially developed processes for lining bearing shells by means of P/M methods In the manufacture of steel-back copper-lead alloy and leaded bronze strip, however, P/M methods are employed more extensively than any other method

Continuous Sintering Process. A wide variety of steel-back copper alloy materials, including counterparts of all of the cast copper-lead and leaded bronze bearing alloys (Table 3, class 4), can be produced by continuous sintering on a steel backing strip In these processes, prealloyed (PA) powder particles are spread uniformly onto moving steel strip As the strip passes through a furnace under a reducing atmosphere, the particles become sintered together, forming an open grid bonded to the steel strip After cooling, this bimetal is rolled to densify the liner alloy and then resintered to develop complete interparticle and alloy/steel bonds After resintering, the strip material may receive further rolling to attain finish stock size, and sometimes to strain harden the alloy liner for increased strength

Strip sintering technology makes possible the production of steel-core "sandwich" material, which is especially suitable for applications requiring two bearing surfaces (such as in some thrust washers) In this instance, powder spreading, sintering, cooling, and rolling are repeated on the opposite side, after which the strip is finally resintered Sintered strip for most automotive and truck bearing applications is processed in coils 5mm ( 0.2 in.) in overall thickness Thick-wall materials with steel layers up to about 16 mm ( in.) thick also can be processed into flat slab lengths

Impregnation and Infiltration. Both bimetal and trimetal bearing materials also can be made by impregnation or

infiltration of a lower-melting lead alloy into a layer of sintered copper alloy powder In impregnation, a bilayer strip made from PA copper-lead alloy or leaded bronze powder is immersed in a bath of molten lead-tin alloy heated above the melting point of lead During immersion, some of the lead at the surface of the strip is replaced by the lead-tin alloy In infiltration, the copper alloy powder layer is free-sintered and not compacted after sintering The open-grid sintered layer

is then infiltrated with material having a lower melting temperature than that of the grid alloy

The infiltrant is usually molten lead or a lead alloy but it can be a nonmetallic material such as PTFE, which can be introduced in paste or slurry form A very useful class of self-lubricating trilayer structures is produced commercially in this way; the PTFE-base infiltrant also forms a thin low shear strength surface film in these structures

Powder Rolling. One very useful application of direct powder rolling that has been developed commercially in the plain bearing industry is production of an aluminum-lead alloy strip for subsequent bonding to a steel back (see third item under class 3 in Table 3) In this method, PA lead-aluminum powder and unalloyed aluminum powder are fed simultaneously in separate streams to a powder rolling mill and continuously compacted into a bilayer aluminum strip After sintering, this strip is roll bonded to low-carbon steel, with the unalloyed aluminum bonding layer next to the steel This steel-back strip is used as a bimetal material for bearing applications where the unit loading is beyond the capability

of tin or lead babbitt bimetal material

Roll Bonding Processes

Virtually all commercial manufacturer of bimetal aluminum alloy bearing strip materials (see Table 3, class 3) is currently done by roll bonding the liner alloy to a steel backing strip Both batch and continuous processes are employed, the latter being favored for economical high-volume processing of lighter-gage material

In all roll bonding processes, whether batch or continuous, very clean aluminum and steel surfaces are forced together under intense pressure in a rolling mill, so that solid-phase bonding (cold welding) can occur between the two metals at many sites in the interface Heat, which may be applied simultaneously with pressure in hot rolling and subsequently in postroll annealing, serves to develop complete diffusion bonding from the initial weld sites and to recrystallize the aluminum alloys so that the final bimetallic strip product exhibits useful liner-alloy ductility and complete bonding

Tin-aluminum alloys usually are not bonded directly to steel because of undesirable interactions between the free tin constituent and the steel backing A layer of electrolytic nickel plating on the steel surface is commonly used to alleviate these effects with both low-tin and high-tin alloy compositions

Another method commonly used with tin-aluminum alloys employs a tin-free aluminum interlayer This is accomplished

by the use of Alclad tin-aluminum alloy strip The tin-free cladding layer serves as the bonding surface and is present as a distinct bond interlayer in the finished bimetal strip

Direct roll bonding to steel is most commonly employed with tin-free aluminum alloys (fifth item under class 3, Table 3) and with lead-aluminum strip materials

Electroplating Processes

Plated Overlays. Lead alloy surface layers (overlays) whose thickness must be limited to <0.05 mm (<0.002 in.)

(Table 4, classes 4 to 12) are most commonly produced by electroplating the lead alloy on bimetallic bearings that have previously been finish machined Specially designed plating racks are used to ensure uniform distribution of plating current over the bearing surface With close control of current, critical dimensional tolerances often can be maintained so precisely that no machining of the electrodeposited alloy surface is required One manufacturer has commercialized a process in which the lead alloy electroplating is applied continuously to precision-rolled bimetal strip In this process, all forming and machining operations are done after electroplating

Electroplated lead babbitts comprise booth binary lead-tin and ternary lead-tin-copper compositions, all of which are commercially codeposited from fluoborate electrolytes To ensure against bond and plate defects, extreme care is exercised in preparing the basis metal

In addition to cleaning and etching treatments, preplating basis-metal preparation usually includes deposition of one more very thin metallic interlayers A thin layer of nickel is most frequently used over copper-lead alloys and bronzes to prevent diffusion of tin from the plated surface layer into the copper basis metal Copper is most often used over aluminum alloys to ensure complete adhesion of the plated lead alloy layer, and nickel sometimes is plated over the copper to prevent diffusion of tin from the lead alloy layer into the copper layer

Binary lead-indium alloy overlays are also used with copper-lead and leaded bronze intermediate layers These alloys are produced by electroplating separate layers of pure lead and pure indium and subsequently diffusing the indium into the lead in a low-temperature heat treatment operation In this case, no diffusion barrier is required between the overlay and the intermediate alloy layer

Plated Silver Intermediate Layers Pure silver and silver-lead alloy bearing liners are applied to steel shells by

electrodeposition from cyanide plating baths Final machining usually is done after plating, leaving a substantially thick

layer (typically 0.25 to 0.38 mm, or 0.010 to 0.015 in.) of bonded silver liner material Although as-plated thickness tolerances are not critical, special racking and masking techniques are employed to restrict plating to the surfaces where it

is required and to eliminate local concentrations of high current density If the structure of the plated layer is to be uniform, and the bond strength of the liner uniformly high, the steel basis metal must be prepared very carefully and plating-bath compositions and cleanness must be properly controlled Although the basic principles involved in silver plating of bearing liners are the same as for decorative silver plating, the unusually thick deposits involved (normally

>0.50 mm, or 0.020 in.) and the extremely high quality requirements for bond and plated-metal soundness have led to development of several unique operating and control practices

Bearing Material Selection

It must be emphasized that selection of a bearing material system for a specific application and of a mechanical design for the bearing itself are closely interrelated processes Neither process is entirely straightforward, neither can be approached independently, and both require a good understanding of other interacting components of the machine system

Although this article considers the principles involved in bearing operation, it has not attempted to present a detailed discussion of mechanical design factors The reader should therefore not expect to make final decisions on materials for specific applications on the basis of this text alone

Most manufacturers of plain bearings have experienced engineering staff personnel available to aid potential users with both mechanical design and material selection Because of the wide material selection offered by most of these experienced specialized producers and their background of experience in practical applications, full advantage should be taken of the engineering services such sources can provide

Selected References

• Bearing and Bushing Alloys, SAE J459c, SAE Information Report, SAE Handbook 1990, Part 1,

Society of Automotive Engineers, 1990

• Bearing and Bushing Alloys, SAE J460e, SAE Information Report, SAE Handbook 1990, Part 1,

Society of Automotive Engineers, 1990

• E.R Booser, Bearing Materials and Properties, Mach Des., 10 Mar 1966, p 22-28

• K.G Budinski, Surface Engineering for Wear Resistance, Prentice-Hall, 1988, p 15-42

• T Calayag and D Ferres, "High Performance, High-Aluminum Zinc Alloys for Low-Speed Bearings

and Bushings," Technical Paper Series Paper No 820643, Society of Automotive Engineers, 1982

• "Custom Plain Bearing Products," Engine Parts Div., Gould Inc., 1986

• G.J Davies, G.S Senior, and O Beaurepaire, "The Development and Application of Polymer

Bearings," Technical Information Paper No 3, Glacier-Vandervell, Inc (Great Britain), 1990

• A.O DeHart, Basic Bearing Types, Mach Des., 10 Mar 1966, p 15-21

• "Fluid Film Bearing Products," Bushings and Bearings Div., JPI Transportation Products, Inc., 1990

• M.L MacKay, L.J Cawley, and G.R Kingsbury, "A New Aluminum-Lead Bearing Material for

Automotive Engine Service," Technical Paper Series Paper No 760113, Society of Automotive Engineers, 1976

• J Masounave and G Huard, Comparison between Continuously Cast and Sand Cast Zinc-Aluminum

Alloys Used in Bearing Applications, Wear Resistance of Metals and Alloys, conference proceedings,

ASM International, 1988, p 65-71

• I.D Massey, N.A MacQuarrie, D.R Eastham, "Development of Crankshaft Bearing Materials for

Highly Loaded Applications," Technical Information Paper No 2, Glacier-Vandervell, Inc (Great Britain), 1990

• S Mohan, V Agarwala, and S Ray, Wear Characteristics of Rheocast and Stircast Al-Pb Metal-Metal

Composites, Tribology of Composite Materials, conference proceedings, ASM International, 1990, p

189-193

• L.J Pesek and W.A Weinkamer, Strip-Type Bearings, Mach Des., 10 Mar 1966, p 35-39

• "Plain Bearings Copper Alloys Part 1: Cast Copper Alloys for Solid and Multilayer Plain Bearings,"

International Standard 4382/1, International Organization for Standardization, 1982

• "Plain Bearings Copper Alloys Part 2: Wrought Copper Alloys for Solid Plain Bearings,"

International Standard 4382/2, International Organization for Standardization, 1982

• "Plain Bearings Lead and Tin Casting Alloys for Multilayer Plain Bearings," International Standard

4381, International Organization for Standardization, 1981

• "Plain Bearings Metallic Multilayer Materials for Thin-Walled Plain Bearings," International

Standard 4383, International Organization for Standardization, 1981

• G.C Pratt and C.A Perkins, "Aluminum Based Crankshaft Bearings for the High Speed Diesel

Engine," Technical Paper Series Paper No 810199, Society of Automotive Engineers, 1981

• G.C Pratt and W.J Whitney, "Progress with Aluminum-Lead Crankshaft Bearing Alloys," Technical

Paper Series Paper No 890552, Society of Automotive Engineers, 1989

• A.E Roach and C.L Goodzeit, Why Bearings Seize, General Motors Engineering Journal, Sept-Oct

1955

• K Sakamoto, Y Ogita, Y Sato, and T Tanaka, "Development of New Aluminum-Zinc-Silicon

Bearings for Heavy Load Applications in Uprated Engines," Technical Paper Series No 900124, Society of Automotive Engineers, 1990

• "Standard Designations for Copper and Copper Alloys," Application Data Sheet, Copper

Development Association Inc., 1990

• "Standards Handbook, Part 2 Alloy Data, Wrought Copper and Copper Alloy Mill Products," Copper

• D.F Wilcox and E.R Booser, Bearing Design and Application, McGraw-Hill, 1957, p 367-391

Friction and Wear of Hardfacing Alloys

Paul Crook, Haynes International, Inc.; Howard N Farmer, Consultant

Introduction

HARDFACING may be broadly defined as the application of a wear-resistant material, in depth, to the vulnerable (or worn) surfaces of a component by a weld overlay or thermal spray process This discussion deals with the weld overlay materials used to resist wear; the thermal spray materials are covered in the article "Thermal Spray Coatings" in this Volume

The weld overlay materials fall into five categories:

• Build-up alloys

• Metal-to-metal wear alloys

• Metal-to-earth abrasion alloys

• Tungsten carbides (for extreme earth sliding and cutting wear)

• Tractor rails

• Railroad rail ends

• Steel mill table rolls

• Large slow-speed gear teeth

The metal-to-metal wear alloys are martensitic air-hardening steels that, with care, can be applied (without cracking) to wearing areas of machinery parts Typical applications of this alloy family include:

• Undercarriage components of tractors and power shovels

• Steel mill work rolls

• Crane wheels

Most of the materials in the final three categories consist of hard particles within a metallic matrix, and, for many, it is the hard constituent that provides resistance to wear The cobalt-base alloys are an exception in this regard because they exhibit resistance to a wider variety of wear forms, largely by virtue of the deformation and fracture characteristics of the cobalt-rich matrix

The primary function of the metal-to-earth abrasion alloys and tungsten carbides is abrasion resistance The metal-to-earth alloys are high-chromium white irons in which chromium carbides are formed during alloy solidification The tungsten carbides are actually composite materials, and their use involves the transfer of discrete tungsten carbide particles (which

in the welding consumable forms are encased in a steel tube) across the welding are and into the molten weld pool, where they are subsequently "frozen" into the overlay structure by the matrix formed from the melting of the steel tube

The metal-to-earth alloys possess resistance to sliding and crushing (that is, low- and high-stress) abrasion on a moderate scale, whereas the tungsten carbides are intended for use under extreme sliding and cutting conditions Typical applications for these material types include:

• Cobalt-base/carbide type

• Cobalt-and nickel-base/Laves type

• Nickel-base/boride type

The cobalt-base alloys are especially to deformation and chemical attack at high temperatures (500 °C < T 900 °C, or

930 °F < T 1650 °F) and are used to protect dies and guide rolls in the steel industry Other common applications of the nonferrous hardfacing alloys include:

• Valve seating surfaces (both control valves and diesel exhaust valves

• Pump parts

• Extrusion screw flights

• Rock bit bearings

• Marine bearings

• Glass molding hardware

The discussion up to this point has been devoted to briefly introducing the five families of weld overlay material It is now appropriate to distinguish, in a general way, between the individual family members This is best done by considering that within each family (or category), several levels of abrasion resistance are available Increasing abrasion resistance, however, is accompanied by increasing brittleness, which can cause cracking during cooling, after welding, or under impact loading in service In certain situations (for example, seating surfaces in chemical control valves), a material with less than optimum abrasion resistance must be chosen in order to obtain a crack-free overlay or to ensure that the overlay does not crack in service

This "abrasion level" concept is particularly applicable to the metal-to-earth alloys, the tungsten carbide composites, the cobalt-base/carbide-type alloys, and the nickel-base/boride type alloys The level of abrasion resistance within these material groups is generally proportional to the carbon, hence carbide, or the boron content

Although the cracking of weld overlays is of great concern in many applications, it may be desirable (for example, for the relief of residual stresses) for others In the mining and construction industries, where brittle metal-to-earth abrasion alloys are used, relief of stresses prior to the use of equipment is important, the objective being to reduce the risks of underbead (substrate/overlay interface) cracking during service and the subsequent spalling of the overlay

Several welding processes are used to apply the hardfacing materials They range from the traditional (for example, oxyacetylene torch) to the new and sophisticated (for example, plasma-transferred arc, PTA, and synergic metal inert gas, MIG) To accommodate these different overlay processes, the hardfacing materials are available in a variety of forms The most popular processes, and the forms most commonly associated with each process, are:

Weld overlay process Consumable form

Oxyacetylene Bare cast or tubular rod

Shielded metal arc (SMA) Coated solid or tubular rod (stick electrode)

Gas tungsten arc (TIG) Bare cast or tubular rod

Gas metal arc (MIG) Tubular or solid wire

Submerged arc Tubular or solid wire

Plasma-transferred arc (PTA) Powder

In choosing the process to be used, the following factors are important:

The high strength of the austenitic manganese steels is the result of a synergism between manganese and carbon (Ref 1)

In suppressing the formation of phases other than austenitic, manganese not only increases carbon solubility at lower temperatures but also encourages carbon supersaturation of the structure Both high inherent strength and a high work hardening rate result from this

Because the austenitic manganese steels are metastable, problems of carbide embrittlement tend to arise when the alloys are cooled slowly or reheated Manganese steel components are therefore kept as cool as possible during the build-up (repair) process Often, the bulk of the part is submerged in water during the welding process

An overview of the abrasion and impact properties of the build-up alloys relative to the metal-to-metal wear alloys is presented in Table 1 Specific build-up alloy compositions are given in Table 2 In the austenitic manganese steels, chromium is added to increase strength but promotes the formation of embrittling carbides Molybdenum and nickel, on the other hand, suppress the precipitation of embrittling carbides (molybdenum by modifying the type and geometry of carbides formed, and nickel by increasing the stability of the austenitic structure)

Table 1 Impact resistance and abrasion resistance properties of build-up alloys and metal-to-metal wear alloys

Resistance(a) Impact Abrasion

Substrate

Min Max Min Max

Build-up weld overlay

Table 2 Composition, hardness, and abrasion data for build-up alloys and metal-to-metal wear alloys

Abrasion, volume loss Composition, wt%

Build-up weld overlay

EFe 1 (a) bal 2 0.1 1.0 1 1.5 37 88 5.4 49 3.0

EFeMn-C (a) bal 4 0.8 1.3 14 4 18 65 4.0 57 3.5

EFeMn-Cr (a) bal 15 0.5 1.3 15 2.0 1 24 93 5.7 46 2.8

Metal-to-metal weld overlay

EFe2 (a) bal 3 0.2 1.0 1 1.0 1 48 54 3.3 66 4.0

EFe3 (a) bal 6 0.7 1.0 1 1.0 59 60 3.7 68 4.1

ER420 (b) bal 12 0.3 1.0 2 45 84 5.1 62 3.8

(a) Two-layer SMA deposit process

(b) Two-layer SAW deposit process

(c) Dry sand/rubber wheel test (ASTM G 65, Procedure B):load 13.6 kg (30 lb); 2000 rev

(d) Slurry/steel wheel test(ASTM B 611, modified):load 22.7 kg (50 lb); 250 rev

Room-temperature hardness and low- and high-stress abrasion data for the compositions are also listed in Table 2 The low-stress data were generated using the dry sand/rubber wheel (ASTM G 65) test, and the high-stress results were produced using the slurry/steel wheel (ASTM B 611, modified) test The high-stress test is the more severe, and the volume losses reported are for only 250 rev of the steel wheel (as compared with 2000 rev of the rubber wheel in the low-stress test)

As may be deduced from these data, as-deposited hardness is a poor indicator of low- or high-stress abrasion resistance for these alloys

Metal-to-Metal Wear Alloys

Alloys in the metal-to-metal category (commonly referred to as machinery hardfacing alloys) are martensitic, hardening steels that can be applied several layers thick and can be finish machined (although with some difficulty) if adequate equipment is used They are recommended for wearing, industrial, heavy-duty, nonlubricated parts These materials are also sometimes referred to as "super build-up" alloys If a machined finish is required, three layers are generally applied, with the assumption that the top layer will be removed during machining A single layer is usually inadequate because of the effects of substrate/overlay intermixing (dilution)

air-Typical compositions and properties of the metal-to-metal wear alloys are presented in Tables 1 and 2 As with the

build-up alloys, there appears to be no correlation between room-temperature hardness and abrasion properties

In hostile environments, a high chromium content is beneficial ER420 (American Welding Society, AWS, classification), and modified versions containing nickel, molybdenum, and niobium (or vanadium), are therefore the natural choice when high temperatures and mildly corrosive environments are encountered For applications using steel mill hot-work rolls (which demand considerable hot hardness, resistance to oxidation, and resistance to thermal fatigue) both ER420 and EFe3 have been found suitable Other applications for the metal-to-metal wear alloys in Table 2 include tractor rollers and crane wheels (EFe2), pincer guide shoes (EFe3), and blast furnace bells (ER420)

Metal-to-Earth Abrasion Alloys

The high-chromium irons encompass a wide range of compositions in which chromium may vary between approximately

6 and 35 wt%, and carbon may vary from about 2 to 6 wt% Other possible alloying additions include molybdenum, manganese, and silicon

The most important microstructural feature in the high-chromium irons, at least from a wear standpoint, is an M7C3

carbide, which forms in abundance during solidification and contains chromium, iron, and (if present) molybdenum The matrix around these carbide particles can be austenitic, pearlitic, or martensitic (Ref 2) In general, the austenitic alloys rely on manganese for austenite stability

As mentioned in the introduction of this article, carbon content is a good indicator of abrasion resistance for this class of materials To illustrate this fact, low- and high-stress data are plotted for selected alloys in Fig 1 This information was generated by the use of six different alloys (open arc/flux-cored wires) and nine different sets of welding parameters (Ref 3) The carbon contents referred to in the figures are those of the second layer of the two-layer overlays tested The use of

a shaded zone, rather than a line, indicates that there was considerable scatter in the data (particularly at the lower carbon contents, in the case of high-stress abrasion) The test methods and parameters were identical to those described in the section "Build-Up Alloys" in this article

Fig 1 Plot of volume loss versus carbon content for high-chromium iron metal-to-earth abrasion alloys (a)

Low-stress condition (b) High-stress condition Source: Ref 3

At high carbon and chromium levels, the formation of a hypereutectic microstructure, containing large, spinelike carbide particles (with a hexagonal cross section), is favored At lower carbon and chromium contents, the microstructure is hypoeutectic The microstructures of three-high-chromium irons (as deposited by the open arc welding process) are shown in Fig 2 The nominal compositions of the same three alloys are presented in Table 3 As can be seen, ERFeCr-A3 (at a chromium content of 11 wt% and a carbon content of 2.6 wt%) exhibits a hypoeutectic (primary austenite) microstructure The other two alloys, ERFeCr-A4(Mod) (29Cr/3.5C) and ERFeCr-A2 (28Cr/4.3C), possess a hypereutectic (primary carbide) structure In Fig 2, the large, spinelike carbides are shown in cross section

Table 3 Composition of metal-to-earth abrasion alloys

Composition, wt%

Alloy

Cr C Si Mn Mo Ni B Fe ERFeCr-A3 11 2.6 1.3 1.8 1.5 bal

ERFeCr-A4(Mod) 29 3.5 1.1 0.9 2.6 0.7 bal

Fig 2 Microstructure of high-chromium iron metal-to-earth abrasion alloys hardfaced with two-layer open arc

deposit welding process (a) ERFeCr-A3 (b) ERFeCr-A4(Mod) (c) ERFeCr-A2 300×

In addition to M7C3, deposits of ERFeCr-A2 contain small quantities of M6C and deposits of ERFeCr-A4(Mod) contain small quantities of both M6C and M3C With regard to the matrix, both ERFeCr-A3 and ERFeCr-A4(Mod) exhibit a face-centered cubic (fcc) austenitic structure as deposited ERFeCr-A2 is largely austenitic but may also contain small quantities of ferrite or martensite The presence of martensite in the matrix structure of a high-chromium iron is believed

to be beneficial to its high-stress abrasion resistance because of the additional support it provides to surface carbides

For a given hardfacing consumable, deposit (overlay) microstructure is strongly influenced by the welding process and parameters used Factors of concern, in this regard, include:

• Overlay/substrate intermixing

• Changes in composition due to losses in the arc

• Deposit cooling rate

With regard to industrial applications of the metal-to-metal abrasion alloys, the low-carbon (2 to 3% C) hypoeutectic materials are usually selected for situations involving moderate abrasion and impact, whereas the higher carbon (4 to 6% C) hypereutectic alloys are used in applications involving severe abrasion and little or no impact Specific uses include:

• Crushers cones and pump casings (ERFeCr-A3)

• Bulldozer blades and crusher hammers [ERFeCr-A4(Mod)]

• Coal pulverizer liners and gravel pumps (ERFeCr-A2)

Tungsten Carbides

In contrast to the other weld overlay materials, the tungsten carbide composites do not rely upon the formation of suitable hard phases during weld pool solidification Instead, these overlay materials rely on the transfer of tungsten carbide particles from the welding consumable to the overlay It is important, therefore, to limit the heat input of the welding process in order to prevent melting of the tungsten carbide particles If the tungsten carbide particles melt, they mix with

iron to form much softer iron-tungsten carbides, thus reducing abrasion resistance For this reason, oxyacetylene deposits usually exhibit higher abrasion resistance than arc-welded tungsten carbide overlays

An advantage of the tungsten carbide composites is that the size of the hard particles in the overlay can be controlled This is important because abrasion resistance is dependent upon the size relationship between microstructural features (such as carbides) and the abrading particles It is believed that, if the abrading particles are large in comparison to the microstructural particles, then, after a running-in period (during which the softer matrix material at the surface is worn down), the abrading particles ride over the hard microstructural outcrops On the other hand, if the abrading particles are small in comparison to the microstructural particles, it is believed that the opportunity exists for wear of the matrix around the microstructural particles Eventually, these may drop out, having played only a small in resisting abrasion

The tungsten carbide particles themselves are typically manufactured by the melting of tungsten and carbon in a graphite crucible During subsequent cooling, a two-phase mixture of WC and W2C is formed After crushing and screening (size selection), this material is incorporated into a tabular welding product with a carbon steel sheath

Several tungsten carbide composites are available in a variety of tabular product forms Popular compositions are 38, 50,

55, and 60 wt% tungsten carbide, with the carbon steel tube making up the balance For each composition, several carbide size ranges are available As an example, for the 60% WC oxyacetylene welding consumable, four mesh size ranges are available:

AWS designation Mesh size range

• Carbide volume fraction

• Size relationship between the carbides and the abrasive medium

• Welding technique applied

Important factors are the distribution of carbides in the overlay (because the particles tend to sink, turbulence in the molten weld pool is an advantage), and the amount of carbide dissolution and reprecipitation in the steel matrix during welding Impact strength generally decreases with increasing carbide volume fraction

Table 4 relates tungsten carbide composites and the previously mentioned alloys on the basis of low-stress and high-stress abrasion data (generated under identical test conditions) These values should be compared with the values in Table 2 and Fig 1 Photomicrographs of the test deposits are shown in Fig 3 Noteworthy features include:

• The difference in size of the tungsten carbide particles in the two materials

• The extent to which secondary carbides have precipitated within the matrix

Table 4 Abrasion data for tungsten carbide composites

Abrasion, volume loss Material

Low-stress (a) High-stress (b)

Carbide, wt% Mesh size mm 3 in. 3 × 10 -3 mm 3 in. 3 × 10 -3

60 20-30 7.3 0.45 28.7 1.75

61 100-250 10.6 0.65 24.4 1.49

(a) Dry sand/rubber wheel test (ASTM G 65,

Procedure B): load 13.6 kg (30 lb); 2000 rev

(b) Slurry/steel wheel test (ASTM B 611, modified):

load 22.7 kg (50 lb); 250 rev

Fig 3 Microstructures of tungsten carbide composites with carbides of different size in the weld overlay

material (a) 60% WC, 20 to 30 mesh particles (b) 61% WC, 100 to 250 mesh particles Overlay process applied is one-layer shielded metal arc (SMA) deposit 120×

The tungsten carbide composites have been used to solve a wide variety of industrial sliding and drilling abrasion problems Some of the common applications are plowshares, ditchdigger teeth, ripper teeth, and oil well drilling tools For extremely hostile environments, some nonferrous tungsten carbide products (cobalt- and nickel-base products in the form

of bare cast rods) are available Also, several alternative composite materials, utilizing other carbides (for example, vanadium, titanium, or niobium), are available that have the advantage of creating a more homogeneous deposit because

of their lower densities

Nonferrous Alloys

Nonferrous hardfacing alloys are used either for high resistance to specific types of wear (other than abrasion), or for wear resistance (including abrasion) in environments that are too corrosive or beyond the service temperatures of ferrous alloys The cobalt-base alloys and bronzes are particularly resistant to galling and to those wear processes involving microfatigue as the degradation mechanism (such as cavitation erosion) The cobalt-base alloys possess high resistance to deformation at temperatures in excess of 750 °C (1380 °F)

Cobalt-Base/Carbide-Type Alloys Table 5 lists typical compositions of the nonferrous hardfacing alloys The chief

difference between the various cobalt-base/carbide-type alloys is in carbon content (hence, carbide volume fraction, room-temperature hardness, and level of abrasion resistance) Chromium-rich M7C3 is the predominant carbide in these alloys, although tungsten-rich M6C is evident in those alloys having a high tungsten content, and chromium-rich M23C6 is common in the low-carbon alloys

Table 5 Composition of selected nonferrous alloys

Fig 4 Comparison of hardfaced nonferrous alloys to tool steel and carbon steel reference materials using ASTM

G 65 low-stress abrasion test (a) Schematic of G 65 dry sand/rubber wheel test apparatus (b) Low-stress abrasion test data G 65 test parameters: procedure B; room temperature; 13.6 kg (30 lb) load; quartz grain sand diameter of 212 to 300 m (8.48 to 12 mil); 2000 rev at 200 rev/min; 390 g/min (0.86 lb/min) feed rate

Fig 5 Galling property evaluation of nonferrous alloys versus selected alloys (itself, stainless steels, and a

nickel-molybdenum-chromium alloy) Data obtained using pin-on-block test with following parameters: 2722 (6000 lb) load; 10 strokes through 120° arc GTA, gas tungsten arc

Fig 6 Comparison of hardfaced nonferrous alloys to a stainless steel and a nickel-molybdenum chromium alloy

using ASTM G 32 vibratory cavitation test (a) Schematic showing G 32 test apparatus, (b) Cavitation erosion test data G 32 test parameters: medium, distilled water; test temperature, 16 °C (61 °F); vibration, 20 kHz frequency at 0.05 mm (0.002 in.) amplitude A, type 410 stainless steel (hardness, 23 HRC); B, Hastelloy alloy C-276; C, ERCoCr-E (two-layer GTA deposit); D, ERCoCr-A (two-layer GTA deposit)

The low-stress abrasion data in Fig 4 were generated using the ASTM G 65 dry sand/rubber wheel test The vibratory cavitation test described in ASTM G 32 was used to generate the results given in Fig 6 The results in Fig 5 relate to a pin-on-block galling test (Ref 4)

Photomicrographs of plasma-transferred arc (two-layer) deposits of three cobalt-base/carbide-type alloys are shown in Fig 7 The differences in carbide volume fraction and geometry are evident from these photomicrographs

Fig 7 Microstructures of two-layer PTA deposited hardfacing on cobalt-base/carbide type alloys (a) ERCoCr-C

(b) ERCoCr-A (c) ERCoCr-E 425×

Iron and Nickel Substitutes for Cobalt Compositions Because of the relatively high cost of cobalt, attempts

have been made to design alternate materials, with iron and nickel as the predominant cobalt substitutes The compositions of three such materials are given in Table 6 A comparison of the sliding wear properties of the cobalt-base/carbide-type alloys and these three alternate alloys, as a function of temperature up to 1000 °C (1830 °F), is presented in Fig 8 From the data in Fig 8, it is evident that cobalt content is a very important factor up to 750 °C (

1380 °F), above which oxide glaze formation is observed

Table 6 Composition of alternate alloys to replace cobalt-base/carbide-type alloys

Composition, wt%

Alloy

Co Ni Fe Cr Mo W Si C B

Cobalt substitute

Alloy N-6 3 max bal 3 max 29 5.5 2 1.5 max 1.1 0.6

Alloy 716 11 23 bal 26 3 3.5 1.5 max 1.1 0.5

Alloy 2006 bal 8 18 31 8 1 1.3

High-silicon stainless steel

Fig 8 Plot of wear rate versus temperature to compare sliding properties of cobalt-base/carbide-type alloys

and selected alternate alloys, all of which have been GTA deposit hardfaced Test conditions include pressure of

21 MPa (3 ksi) and velocity of 8 mm/s (0.3 in./s)

The information in Fig 8 is also useful because it indicates a maximum sliding wear rate for the cobalt-base hardfacing alloys (self-mated) at 250 °C ( 480 °F) During these (reciprocating) sliding wear tests (Ref 5), the coefficient of friction was monitored continuously Not only was this parameter found to vary through each sliding cycle, but the values also drifted with test time (hence sliding distance) For reference, friction coefficient traces for ERCoCr-A, at 25, 500, and

1000 °C (75, 930, and 1830 °F), are shown in Fig 9 Note that relatively high values were obtained in the early stages of testing at 1000 °C (1830 °F) (presumably during the time interval in which the oxide glaze was forming)

Fig 9 Plot of coefficient of friction versus sliding distance for self-coupled ERCoCr-A (GTA deposit) hardfaced

cobalt-base/carbide-type alloy as a function of test temperature (a) 25 °C (75 °F) (b) 500 °C (930 °F) (c)

1000 °C (1830 °F) Test conditions include pressure of 21 MPa (3 ksi) and velocity of 8 mm/s (0.3 in./s)

High-Silicon Stainless Steel Alternate Material Recently, several new high-silicon stainless steels have been

introduced that may be considered as alternate materials These exhibit identical anti-galling characteristics and possess equal resistance to cavitation erosion Some of these have been developed for general use, and others for specific applications (for example, hydroelectric turbines and nuclear valves) Some are based on type 200 (manganese- and nitrogen-containing) austenitic stainless steel; others are based on type 300 stainless steel One of these latter compositions is presented in Table 6 These high-silicon hardfacing alloys differ from the traditional cobalt-base/carbide-type alloys in terms of their corrosion resistance (they are better in some aqueous media and worse in others), their mechanical properties (particularly at high temperatures), and their thermal stability

Laves-Type Alloy Compositions Three cobalt- and nickel-base/Laves-type alloy compositions are presented in

Table 5 In these materials, molybdenum and silicon are added at levels in excess of their solubility limit with the objective of inducing the precipitation of the hard (and corrosion-resistant) Laves phase (an intermetallic compound) Carbon is held as low as possible in these alloys to discourage carbide formation The microstructure of alloy T-800 (two-layer PTA deposit) is shown in Fig 10

Fig 10 Microstructure of Alloy T-800 (two-layer PTA deposit) cobalt-base/Laves-type nonferrous alloy 200×

Because the Laves intermetallic phase is so abundant in these alloys, its presence governs all the material properties Accordingly, the effects of the matrix composition in these alloys are less pronounced than is the case for the cobalt-base/carbide-type alloys, for example The Laves phase is especially responsible outstanding abrasion resistance, but it severely limits the material ductility and the impact strength In fact, it is difficult to attain crack-free overlays on all but the smallest components given adequate preheat For this reason, these alloys have been more successful as thermal spray materials

Nickel-Base/Boride-Type Alloys Of all the hardfacing alloys, the nickel-base/boride-type alloys are

microstructurally the most complex The alloy compositions represent a progression in terms of iron, chromium, boron and carbon contents (Table 5) Iron content is largely incidental, allowing the use of ferrocompounds during manufacture Together with nickel, the other three elements determine the level and type of hard face within the structure upon solidification, boron being the primary hard-face forming element (for which nickel and chromium compete) and carbon being the secondary hardphase former)

The chief purpose of silicon in the material is to provide, in conjunction with boron, self-fluxing characteristics However,

as an important matrix element and as a potential promoter of intermetallic precipitates, it also has a powerful influence

on the wear properties of the alloys

The actual phases that form in the nickel-base/boride-type alloys are listed in Table 7 on the basis of chromium content