Friction, Lubrication, and Wear Technology (1997) Part 12 ppt

4 Effect of hardness and carbon content on the abrasive wear of selected steel rods used in an ore crusher Improving Wear Properties of Mild Steels.. Therefore, the ferritic and martens

Cr-Mo-B 0.30 0.82 0.22 0.35 0.04 0.27 0.010 0.017 0.03Cu,

0.026Ti, 0.039Al, 0.001B, 0.006N

0.30- 0.30- 0.30- Little used; mainly for small thin

parts such as needle rollers

1019

0.15-0.20

1.00

0.70- 0.70- 0.70-

1020

0.18-0.23

0.60

0.30- 0.30- 0.30-

1118

0.14-0.20

1.60

1.30- 1.30- 1.30- Free-machining carburizing grade

resulfurized

4023

0.20-0.25

0.90

0.70- 0.30

0.15- 0.15- 0.15-

0.20-0.30

4027

0.25-0.30

0.90

0.70- 0.35

0.20- 0.20- 0.20-

0.20-0.30 Moderate strength and toughness

4022

0.20-0.25

0.90

0.70- 0.35

0.20- 0.20- 0.20-

0.35-0.45

5120

0.17-0.22

0.90

0.70- 0.30

0.15- 0.90

0.70- 0.70- 0.70-

4118

0.18-0.23

0.90

0.70- 0.30

0.15- 0.60

0.40- 0.40- 0.40-

0.08-0.15

4720

0.17-0.22

0.70

0.50- 0.55

0.55

0.35- 1.20

0.90- 0.25

0.15- 0.15- 0.15- Cr and Ni produce increased

hardenability for heavy sections (large roller bearings)

4820

0.18-0.23

0.70

0.50- 0.30

0.15- 0.15- 0.15-

3.25-3.75

0.30

0.20-4320

0.17-0.22

0.65

0.45- 0.30

0.15- 0.60

0.40- 2.00

1.65- 0.30

0.20-8620

0.18-0.23

0.90

0.70- 0.30

0.15- 0.60

0.70

0.40- 0.25

0.15-9310

0.08-0.13

0.65

0.45- 0.30

0.15- 1.40

1.00- 3.50

3.00- 0.15

0.08- 0.08- 0.08- For very high shock resistance;

high fatigue resistance; surface carbon content must not exceed 0.9% to avoid retained austenite

3310

0.08-0.13

0.60

0.45- 0.45- 0.45- 1.75

1.40- 3.75

3.25- 3.25- 3.25-

Table 2 Abrasive wear data for selected steels

HB

Gouging wear

ratio

Rubber wheel weight loss,

g AISI 4140 (0.40 C) 845 °C (1550 °F), oil quenched 582 0.219

AISI 4140 (0.40 C) 845 °C (1550 °F), oil quenched, 540 °C (1000 °F) 550 0.235

AISI 4140 (0.40 C) 845 °C (1550 °F), oil quenched, 540 °C (1000 °F) 499 0.410

AISI 4140 (0.40 C) 845 °C (1550 °F), oil quenched, 540 °C (1000 °F) 363 0.531

AISI 4340 (0.40 C) Quenched 650 °C (1200 °F) 340 0.716

AISI 4340 (0.40 C) Quenched 205 °C (400 °F) 520 0.232

6Cr-1Mo (0.88 C) 1065 °C (1950 °F), air cooled, 230 °C (450 °F) 601 0.112

6Cr-1Mo (0.88 C) 1065 °C (1950 °F), air cooled, 540 °C (1000 °F) 601 0.148

5Mn-1Mo-2Cr (1.0 C) 1040 °C (1900 °F), furnace cooled 288 0.245

6Mn-1Mo (1.27 C) 1040 °C (1900 °F), water quenched, 60 °C (140 °F) 200 0.192

6Mn-1Mo, Ti (1.23 C) 1040 °C (1900 °F), water quenched, 60 °C (140 °F) 200 0.170

6.5Mn-1Mo (1.01 C) 1040 °C (1900 °F), furnace cooled 240 0.292

6.5Mn-1Mo-2Cr (0.99 C) 1040 °C (1900 °F), furnace cooled 241 0.316

6.5Mn-1Mo-5Cr (1.0 C) 1040 °C (1900 °F), furnace cooled 246 0.324

6.5Mn-3Mo-2Cr (1.02 C) 1040 °C (1900 °F), furnace cooled 241 0.294

6.5Mn-2Cr (1.00 C) 1040 °C (1900 °F), furnace cooled 474 0.329

8Mn-1Mo-1Cr (1.00 C) 1040 °C (1900 °F), furnace cooled 229 0.337

9Mn-1Mo (1.27 C) 1065 °C (1950 °F), water quenched, 60 °C (140 °F) 206 0.219

9Mn-1Mo, Ti (1.24 C) 1065 °C (1950 °F), water quenched, 60 °C (140 °F) 199 0.213

Table 3 Hardness, toughness, and abrasive wear data for abrasion-resistant steel alloys

(b) A, austenite; M, martensite; F, ferrite; B, bainite; C and c, carbide; P and p, pearlite;

lower case letters indicate small amounts

(c)

size impact specimen

Steel Transformation Diagram

When carbon steel is heat treated by quenching from a high temperature, the resulting room-temperature microstructures are not those shown in the equilibrium diagram Instead, a temperature-time-transformation diagram (TTT) diagram is used to describe these changes A typical diagram for AISI 1095 steel is shown in Fig 3 The left side of the diagram shows the temperature in both centigrade and fahrenheit The right side of the diagram shows the hardness of the steel at room temperature Note that the maximum hardness attainable for this steel is 66 HRC The first heavy curved line in the chart represents the time at which transformation begins In hypoeutectoid steels, this initial transformation consists of the separation of proeutectoid ferrite This is followed by the separation of ferrite and carbide in the form of pearlite The beginning of the pearlite formation is represented by the second heavy curved line Below the knee of the curve ( 540

°C, or 1000 °F), the transformation product is bainite, a phase similar in morphology to martensite but not quite as hard If the solid curved line is not crossed before the martensite start temperature (Ms) horizontal dashed line, martensite is the product of austenite decomposition This diagram shows that a very rapid quench is required to produce martensite (that

is, one must go from 885 °C, or 1625 °F, past the knee in <1 s a quench rate of approximately 360 °C/s, or 650 °F/s) Bainite can be obtained by rapid quenching to a temperature just below the knee and holding at that temperature until the transformation is complete, followed by a quench to room temperature

Fig 3 Time-temperature-transformation diagram for SAE 1095 eutectoid carbon steel (0.9C-0.3 Mn)

Austenitized at 885 °C (1625 °F); grain size, 4 to 5 Ae1, equilibrium transformation temperature

A variety of microstructures can be obtained by controlling the quench processes Martensite is the hardest phase and provides the highest wear resistance in the absence of any fracture-initiating conditions If a tougher material is desired, bainite is the best wear-resistant phase Of course, tempering (that is, heating the martensite to an elevated temperature below the austenitizing temperature and holding to reduce the lattice strain) is a process that can be used to increase the ductility and toughness of martensite The maximum martensitic hardness attainable in a steel is a function of the carbon content (Fig 2)

Wear Properties of Carbon Steel

Hardness as a Function of Carbon Content. Under the abrasive conditions found in mining and construction operations, wear rates of steel can be related to hardness and carbon content For example, wear tests were conducted on steel rods used on a vibrating screen at an ore crushing plant These screens were equipped with rods 6.5 mm ( in.) or 4.8 mm ( in.) in diameter, 585 mm (23 in.) long, and equally spaced in several rows across the rod deck of vibrating screens used to size -25 mm (-1 in.) siliceous ore into ±9.5 mm (± in.) sizes The wear rate of all test rods was compared with that of 1070 high-carbon steel that was oil quenched and tempered to 44 HRC To ensure that each type of

steel would be exposed to the same abrasion, rods were alternated in groups of five across the screen Wear rates based on the loss of weight for the different steels and hardnesses are shown in Fig 4 Although the 0.30C-13Cr stainless steel showed the best resistance to wear, 1080 steel austempered to 57 HRC was found to be more cost effective

Fig 4 Effect of hardness and carbon content on the abrasive wear of selected steel rods used in an ore crusher

Improving Wear Properties of Mild Steels. Mild steel demonstrates poor wear resistance and resistance to surface damage during dry sliding The use of mild steel in sliding surface contact requires surface treatment, such as hardening

or coating, and the selection of a "compatible" mating material such as bronze or babbitt Where hard minerals come in contact with steel, wear is very rapid unless the steel surface is hardened or coated with a very hard material

Corrosion Resistance Improved by Altering Microstructure. Steel is subject to accelerated wear in a corrosive environment Unprotected steel is also susceptible to fretting damage or the formation of oxidized wear debris between two contacting surfaces in low-amplitude oscillating motion A wide variety of microstructures is possible in the heat treatment of steel or cast iron Wear properties can be related to specific microstructures

Steel Selection Based on Relative Costs

When selecting a steel based on its wear-resistance properties, the total cost of the steel and its heat treatment must be considered The following steels, which may have suitable wear-resistance properties in specific applications, are listed in order of increasing total costs:

• Low-carbon steels, such as 1020, not heat treated

• Simple high-carbon steels, such as 1095, not heat treated

• Directly hardened carbon or low-alloy steels, either through-hardened or surface-hardened by induction

or flame process

• Low-carbon or low-alloy steels that are surface-hardened by carburizing, cyaniding, or carbonitriding

• Medium-carbon chromium or chromium-aluminum steels that are hardened by nitriding

• Directly hardened high-alloy steels, such as D2 high-carbon high-chromium tool steel (1.50C-12Cr), that contain particles of free carbide

• Precipitation-hardening stainless steels (mainly for applications involving elevated temperatures and corrosive environments, as well as excessive wear)

• Specialty steels produced by powder metallurgy (P/M) or mechanical alloying techniques

• Alloy carbides bonded by steel matrices

Other ferrous materials, such as high-manganese austenitic steels and various classes of cast irons, are also widely used for wear-resistance applications

Depth of Hardened Regions

Skids, grinding rods, chute liners, and similar parts may be considerably reduced in section before replacement is necessary In such parts, a more expensive deep-hardening steel may be more economical than a shallow-hardening steel For example, a 64 mm (2 in.) diameter bar with a surface hardened of 50 HRC may be made of either a water-quenched

1040 or an oil-quenched 5160 steel However, by the time the bar has been worn to three-fourths of its original diameter ( 48 mm, or 1 in.), the 1040 steel will have a surface hardness of 25 HRC and thus would wear at a much faster rate than the 5160 steel, which has a hardness of 37 HRC at the same location (Ref 2)

Toughness

Wear resistance tends to increase with hardness, but it decreases as toughness increases This is an important relationship

in applications that require both wear resistance and impact resistance

The correlation between wear resistance and toughness for a variety of ferrous alloys is shown in Fig 5 The scatter arises, at least in part, from microstructural effects For example, point 22B refers to AISI 4340 steel, quenched and tempered at a high temperature of 650 °C ( 1200 °F) to produce fine carbides in a ferrite matrix Point 22A represents the same steel, except normalized to produce fine pearlite; point 22C represents a quenched sample tempered at 205 °C (400 °F), a relatively low tempering temperature Steels in the lower band of Fig 5 combine toughness with wear resistance; these are mainly the austenitic manganese steels The data in Fig 5 indicate that, for most ferrous alloys, there

is a trade-off between wear resistance and toughness In some alloys, altering the carbon content is a simple method for adjusting these properties

Fig 5 Relationship between resistance to gouging abrasion and toughness of selected materials Area A,

wrought and cast low-alloy steels; area B, austenitic manganese steels; area C, variety of heat-treated steels; area D, high-chromium white cast irons Source: Ref 3

Carbon Content

The wear resistance of ferritic steel is improved by hardening, either throughout the section or superficially The maximum hardness depends on the carbon content of the steel and the amount of martensite (that is, efficiency of quenching) (Fig 2)

Standard hardness measurements may indicate that a martensitic steel is largely transformed, although it may retain some austenite Exposure to ultralow temperatures, followed by tempering, can help complete the transformation to martensite and improve wear resistance Because martensite is a metastable structure, it begins to transform to more stable structures

as the temperature is increased Consequently, martensitic steels are not suitable for wear resistance at elevated temperatures (>200 °C, or 390 °F) or for applications in which the heat of friction can raise the temperature significantly Special alloy steels, such as tool steels or martensitic stainless steels, are appropriate for service at higher temperatures The thermal instability of martensite should also be considered during finishing operations (such as grinding) when a heat-affected zone (HAZ) could be produced at the surface The resultant tempering effects could be localized or general;

in either case, wear resistance is likely to be reduced

Carbon content also affects hardness and wear resistance through the formation of various simple and complex carbides Wear properties depend on the type, amount, shape, size, and distribution of carbides present, in addition to the properties

of the matrix (for example, hardness, toughness, and stability) Despite this complexity, a correlation for relative wear content is possible

Relation of Hardness to Microstructure

The frequent use of bulk hardness as a guide to abrasive wear resistance is supported by the data shown in Fig 6 for annealed unalloyed metals These data were obtained using an abrasive cloth (two-body abrasion) with an abrasive hardness much greater than that of the metal samples The data points are approximate; the experimental scatter of the measurements is not shown

Fig 6 Abrasive wear resistance versus hardness for annealed unalloyed metals and steel Source: Ref 4

Corresponding correlations with other properties related to hardness (such as elastic modulus) have also been presented

In all cases, if the metals are unalloyed, a simple correlation is obtained for controlled tests of two-body abrasion Different crystal structures would be expected to yield different correlations, but the data in Fig 6 do not show such an effect

Care must be exercised in extending the simple hardness correlation to metals containing impurities or solutes, or to more complicated alloys Figure 7 shows how wear resistance relates to hardness for various types of materials The linear plot shown for pure metals in Fig 6 is repeated as the steep line in Fig 7 Another straight line describes brittle ceramics reasonably well The differences in bonding type may account for these two distinct lines

Fig 7 Plot of wear resistance versus hardness for selected materials (metals, alloys, and ceramics) having

various microstructures Source: Ref 5

Between the curves for pure metals and brittle ceramics are curves for heat-treated steels, cold-worked metals, and precipitation-hardening alloys of varying hardnesses For a typical precipitation-hardening alloy, large increases in hardness provide very little improvement in wear resistance The same is true of simple cold working alloys (assuming no complicating phase changes), except that wear resistance actually decreases at high hardness levels Heat-treated steels show initial improvement in wear properties as hardness increases, but eventually the correlation is reversed It is possible that the reduced wear resistance at high hardness for cold-worked metals and heat-treated steels is related to the reduction

in fracture toughness Zum Gahr (Ref 6) has evaluated the effect of low fracture toughness on reduced abrasion resistance

of metal alloys He suggests that during abrasion a critical load can be reached by a given abrasive particle, above which fracture toughness becomes a significant factor in the abrasive wear process Although carbon steels are not likely to fall into the category of materials for which fracture toughness related abrasive wear is a problem, the effect of heat treatment and carbon content on fracture toughness should be kept in mind As the carbon content of carbon steel increases, the maximum martensitic hardness attainable by quenching increases and fracture toughness decreases If one tries to use very hard untempered martensite in a part, the chances of the microfracture dominating wear in abrasive wear increases

Pearlitic steels are inexpensive materials and above a certain hardness level exhibit reasonable wear resistance Clayton (Ref 7) demonstrated in pin-on-ring experiments that a hardness of >250 HV 30 (Vickers hardness of >250 measured under a load of 30 kgf applied for 10 to 15 s, the normal loading time) wear resistance increased by almost an order of magnitude (Fig 8) Note that specific wear, which is defined as the volume removed per total sliding distance, has units

of mm3/mm Wear resistance, which is the reciprocal of specific wear, has units of mm/mm3 or mm-2

Fig 8 Dry pin-on-ring test used to evaluate effect of microstructure on the wear resistance and the hardness of

pearlitic steel pins Test parameters: pressure, P, 34.71 MPa (5.033 ksi); velocity, v, 0.4 m/s (1.3 ft/s);

atmosphere, dry air P, pearlite; P/F, pearlite and ferrite Source: Ref 6

In lubricated block-on-ring wear experiments with pearlitic carbon steel where deformation wear was the principal wear mode, pearlite was observed to minimize the depth of extreme plastic deformation (Ref 8) During heavy working, the interlath spacing in pearlite will decrease and the ferrite will work harden Thus, the hardness of pearlite increases with decreased lamellae spacing (Fig 9) and the structure tends to resist recrystallization (Ref 9) The result is a thinner zone

of heavy working and a smaller volume of metal extruding out of the contact zone

Fig 9 Abrasive wear resistance and bulk hardness of 0.7% C steel as a function of pearlite interlamellar

spacing Abrasion data obtained using pin abrasion test apparatus with pressure, P, of 710 kPa (105 psi); two

different 220-mesh abrasives (flint [96 to 99% Si] and silicon carbide) used in separate tests Source: Ref 6

Under boundary lubrication conditions, where heavier loads are possible than under dry conditions, a pearlitic structure with tight interlamellar spacing in the pearlite phase is appropriate to minimize the depth of shear instability Shear instability (Ref 10) below the contact surface is the source for deformation wear

Austenitic steels with varying manganese content show wear resistance to be a function of manganese content The resistance depends on the transformation of the unstable austenite to martensite (that is, the hard wear-resistant phase) Manganese tends to promote the retention of room-temperature austenite in carbon steel The retained austenite is unstable and will transform to martensite under heavy deformation The room-temperature austenite becomes more stable

as the manganese content of the alloy is increased Pin-on-disk experiments performed by Jost and Schmidt (Ref 11) showed that the amount of austenite transformation decreased with increasing manganese content Figure 10 shows an almost linear increase in wear resistance with decreasing concentration of manganese in the alloy

Fig 10 Plot of wear resistance versus unstable austenite content as a function of manganese content

Pin-on-disk test specimens had 4 to 8% Mn content Fe-Mn-C steel pins rubbed against a steel Pin-on-disk Test parameters, pressure, 4.2 MPa (610 psi); velocity, 0.18 m/s (0.59 ft/s) Source: Ref 6

References

1 G Krauss, Physical Metallurgy and Heat Treatment of Steel, Metals Handbook Desk Edition, H.E Boyer

and T.L Gall, Ed., American Society for Metals, 1985, p 28-2 to 28-10

2 Properties and Selection of Irons and Steels, Vol 1, 9th ed., Metals Handbook, 1978, p 606

3 D.E Diesburg and F Borik, Optimizing Abrasion Resistance and Toughness in Steels for the Mining

Industry, Symposium on Material for the Mining Industry, Climax Molybdenum Co., 1974

4 M.M Khruschov, The Principles of Abrasive Wear, Wear, Vol 28, 1974, p 97-99

5 E Hornbogen, The Role of Fracture Toughness in the Wear of Metals, Wear, Vol 33, 1975, p 251-259

6 K.H Zum Gahr, Abrasive Wear of Metallic Materials, Metallurgical Aspects of Wear, DGM, 1981, p

73-104

7 P Clayton, The Relationship between Wear Behavior and Basic Material Properties for Pearlitic Steels,

Proceedings of the International Conference on Wear of Materials, American Society of Mechanical

Engineers, 1979, p 395-396

8 W.A Glaeser, High Strain Wear Mechanisms in Ferrous Alloys, Wear, Vol 123, 1988, p 155-169

9 N Jost and I Schmidt, Friction Induced Martensite in Austenitic Fe-Mn-C Steels, Proceedings of the

International Conference on Wear of Materials, American Society of Mechanical Engineers, 1985, p

205-214

10 G Langford, Deformation of Pearlite, Metall Trans A, 1977, p 861-875

11 A.A Rosenfield, A Shear Instability Model of Sliding Wear, Wear, Vol 116, 1987, p 319-328

Wear of Stainless Steels

John H Magee, Carpenter Technology Corporation

The selection of a particular type of stainless steel for an application involves the consideration of such factors as the corrosion resistance of the alloy, mechanical properties, fabricability, and cost However, for applications such as pumps, valves, bearings, fasteners, and conveyor belts, where one contacting metal surface moves relative to the other, the wear and galling resistance of the metals in contact should also be considered in the selection process

Stainless steels are characterized as having relatively poor wear and galling resistance, but are often required for a particular application, because of their corrosion resistance Therefore, finding the most effective alloy to withstand wear and galling can be a difficult problem for design engineers Lubricants and coatings are often used to reduce wear, although lubricant use is precluded in may applications, such as high-temperature environments, in which they can break down, or food and pharmaceutical processing equipment, which require sanitation

Additionally, a critical part, such as a valve in a power plant, must resist galling or seizing, because it can shut down or endanger the entire plant (Ref 1, 2)

This article discusses each stainless steel family, specifically in terms of wear resistance Information on wear and galling, laboratory wear and galling tests, and the associated data from these tests is presented Applications and design considerations are also discussed

Classification of Stainless Steels. In the United States, grades of stainless steels are generally designated by one or more of these methods: the American Iron and Steel Institute (AISI) numbering system, the Unified Numbering System (UNS), and proprietary name of designation In addition, designations have been established by most of the major industrial nations Of the two institutional numbering systems, AISI is the oldest and most widely used Most of the grades have a three-digit designation; the 200 and 300 series are generally austenitic stainless steels, whereas the 400

series are either ferritic or martensitic Some of the grades have a one- or two-letter suffix that indicates a particular modification of the composition

The UNS system is a broader-based system that comprises a list of metal alloys, including stainless steel This system includes a considerably greater number of stainless steels than AISI, because it incorporates all of the more recently developed stainless steels The UNS designation for a stainless consists of the letter S, followed by a five-digit number For those alloys that have an AISI designation, the first three digits of the UNS designation usually correspond to an AISI number When the last two digits are 00, the number designates a basic AISI grade Modifications of the basic grades use two digits other than zeroes

Table 1 provides the compositional limits for select stainless steels, listed by UNS and AISI type designations and separated into the basic families Where AISI type designations are not available, common trade names are listed in parentheses These names, the third commonly used identification of stainless steels, have often become the popular means of identifying a particular alloy A complete listing of all stainless alloys available is provided in Ref 3

Table 1 Composition of selected standard and special stainless steels

0.25- 1.00

0.40- 0.40- 0.40-

S42020 420F 0.15(a) 1.25 1.00 0.60 0.15(a)

12.00-14.00

0.60

3.00-0.040 0.040

15.00-18.00

6.00

16.00- 6.50

1 3.00

1.50- 0.40

0.20-0.10-0.30 Nb; 0.10-0.30 V

16.00- 6.00

3.50-0.040 0.030

16.00-18.00

9.00

6.00- 6.00- 6.00-

S30200 302 0.15 2.00 1.00 0.045 0.030

17.00-19.00

10.00

8.00- 8.00- 8.00-

S30300 303 0.15 2.00 1.00 0.20 0.15(a)

17.00-19.00

10.00

7.00-0.75

S30323 303Se 0.15 2.00 1.00 0.20 0.060

17.00-19.00

10.00

8.00- 8.00- 8.00- 0.15 min Se

S30330 303 Cu(b) 0.15 2.00 1.00 0.15 0.10(a)

17.00-19.00

10.00

8.00- 8.00- 8.00-

S30403 304L 0.03 2.00 1.00 0.045 0.030

18.00-20.00

12.00

8.00- 8.00- 8.00-

S30430 302 HQ(b) 0.10 2.00 1.00 0.045 0.030

17.00-19.00

10.00

9.00- 9.00- 9.00- 1.30-2.40 Cu

S30452 304 HN(b) 0.08 2.00 1.00 0.045 0.030

18.00-20.00

10.50

10.00- 10.00- 10.00-

S30900 309 0.20 2.00 1.00 0.045 0.030

22.00-24.00

15.00

12.00- 12.00- 12.00-

S30908 309S 0.08 2.00 1.00 0.045 0.030

22.00-24.00

15.00

12.00- 12.00- 12.00-

S31000 310 0.25 2.00 1.50 0.045 0.030 24.00- 19.00-

26.00 22.00

S31008 310S 0.08 2.00 1.50 0.045 0.030

24.00-26.00

22.00

19.00- 19.00- 19.00-

S31600 316 0.08 2.00 1.00 0.045 0.030

16.00-18.00

4.00

10:00- 3.00

2.00- 2.00- 2.00-

S31603 316L 0.030 2.00 1.00 0.045 0.030

16.00-18.00

14.00

10.00- 3.00

2.00- 2.00- 2.00-

S31620 316F 0.08 2.00 1.00 0.20 0.10(a)

17.00-19.00

14.00

12.00- 2.50

1.75- 1.75- 1.75-

S31700 317 0.08 2.00 1.00 0.045 0.30

18.00-20.00

15.00

11.00- 4.00

3.00- 3.00- 3.00-

S31703 317L 0.030 2.00 1.00 0.045 0.030

18.00-20.00

15.00

11.00- 4.00

3.00- 3.00- 3.00-

S32100 321 0.08 2.00 1.00 0.045 0.030

17.00-19.00

12.00

9.00- 9.00- 9.00- 5 × C min Ti

S34700 347 0.08 2.00 1.00 0.045 0.030

17.00-19.00

13.00

9.00- 9.00- 9.00- 10 × C min Nb

S34720 347F(b) 0.08 2.00 1.00 0.045

0.18-0.35

19.00

17.00- 12.00

17.00- 17.00- 17.00-

N08020 20Cb-3(e) 0.07 2.00 1.00 0.045 0.035

19.00-21.00

38.00

3 3.00

4.50- 6.50

2.50- 0.20

0.08- 0.08- 0.08-

S32550 Alloy 255(c) 0.04 1.50 1.00 0.04 0.03

240-27.0

6.50

4.50- 4.00

2.00- 0.25

0.10-1.50-2.50 Cu

S32900 329 0.20 1.00 0.75 0.040 0.030

23.00-28.00

5.00

2.50- 2.00

3.50- 2.50

1.00- 0.35

7.50- 2.50

2.00-0.01 0.90-1.35 Al

S15500 15-5PH(g) 0.07 1.00 1.00 0.040 0.030

14.00-15.50

5.50

6.50- 3.00

2.00- 2.00- 2.00- 0.75-1.50 Al

S17400 17-4PH(g) 0.07 1.00 1.00 0.040 0.030

15.50-17.50

5.00

0.50-0.50 0.040 0.030

16.00-17.00

5.00

4.00- 3.25

2.50- 0.13

0.07- 0.07- 0.07-

S35500 634(b)

0.10-0.15

1.25

0.50-0.50 0.040 0.030

15.00-16.00

5.00

4.00- 3.25

2.50- 0.13

5.00- 1.00

7.50-0.50 0.10-0.50 Nb;

1.50-2.50 Cu 0.80-1.40 Ti

S66286 A286(c) 0.08 2.00 1.00 0.040 0.030

13.50-16.00

27.0

24.0- 1.50

1.00- 1.00- 1.00- 0.35 Al;

0.0010-0.010 B 1.90-2.35 Ti; 0.10-0.50 V

Note: All compositions include Fe as balance

(b) Designation resembles AISI type, but is not used in that system

(d) Trade name of Crucible Inc

Families of Stainless Steels

On the basis of microstructure, there are five major families of stainless steels: ferritic, austenitic, martensitic, precipitation-hardenable (PH), and duplex (austenite and ferrite)

Ferritic stainless steels are so named because their body-centered-cubic (bcc) crystal structure is the same as iron at room temperature These alloys are magnetic and cannot be hardened by heat treatment In general, ferritic stainless steels

do not have particularly high strength Their annealed yield strengths range from 275 to 350 MPa (40 to 50 ksi), and their poor toughness and susceptibility to sensitization limit their fabricability and the usable section size Their chief advantages are their resistance to chloride stress-corrosion cracking, atmospheric corrosion, and oxidation, at a relatively low cost

Ferritic stainless steels contain between 11 and 30% Cr, with only small amounts of austenite-forming elements, such as carbon, nitrogen, and nickel Their general use depends on their chromium content

The low-chromium (11%) alloys (S40500 and S40900) have fair corrosion and oxidation resistance and good fabricability

at low cost They have gained wide acceptance for use in automotive exhaust systems

The intermediate-chromium (16 to 18%) alloys (S43000 and S43400) are used for automotive trim and cooking utensils These alloys are not as readily fabricated as the lower Cr alloys, because of their poor toughness and weldability

The high-chromium (19 to 30%) alloys (S44200 and S44600) are used for applications that require a high level of corrosion and oxidation resistance These alloys often contain either aluminum or molybdenum and have a very low carbon content Their fabrication is possible because of special melting techniques that can achieve very low carbon, as well as very low nitrogen contents Stabilizing elements, like titanium and niobium, can be added to prevent sensitization and to improve as-welded properties

Austenitic stainless steels constitute the largest stainless family, in terms of number of alloys and usage Like the ferritic alloys, they cannot be hardened by heat treatment However, their similarity ends there The austenitic alloys are nonmagnetic, and their structure is face-centered-cubic (fcc), like high-temperature (900 to 1400 °C, or 1650 to 2550 °F) iron They possess excellent ductility, formability, and toughness, even at cryogenic temperatures In addition, they can be substantially hardened by cold work

Although nickel is the chief element used to stabilize austenite, carbon and nitrogen are also used, because they are readily soluble in the fcc structure A wide range of corrosion resistance can be achieved by balancing the ferrite-forming elements, such as chromium and molybdenum, with austenite-forming elements

Austenitic stainless steel can be subdivided into two categories: chromium-nickel alloys, such as S30400 and S31600, and chromium-manganese-nitrogen alloys, such as S20100 and S24100 The latter group generally contains less nickel and maintains the austenitic structure with high levels of nitrogen Manganese (5 to 20%) is necessary in these low-nickel alloys to increase nitrogen solubility in austenite and to prevent martensite transformation The addition of nitrogen also increases the strength in austenitic alloys Typical chromium-nickel alloys have tensile yield strengths from 200 to 275 MPa (30 to 40 ksi) in the annealed condition, whereas the high-nitrogen alloys have yield strengths up to 500 MPa (70 ksi)

As previously mentioned, austenitic alloys can be substantially hardened by cold working The degree of work hardening depends on alloy content, with increasing alloy content decreasing the work-hardening rate Figure 1 depicts the higher work-hardening rate of type 301 (7% Ni) versus type 305 (11.5% Ni), which is primarily due to its lower nickel content Austenitic stainless steels that have a low alloy content, such as S20100, S20161, S30100, and S30400, often become magnetic because of the transformation to martensite when sufficiently cold worked or heavily deformed in machining or

forming operations The rapid work hardening of S20161 is a major advantage in sliding wear In S30430, copper is intentionally added to lower the work-hardening rate for enhanced headability in the production of fasteners

Fig 1 Effect of cold working on mechanical properties of stainless steels (a) Type 301 (b) Type 305 Source:

Ref 4

Another property that depends on alloy content is corrosion resistance Molybdenum is added to S31700 and S31600 to enhance corrosion resistance in chloride environments High-chromium grades (S30900 and S31000) are used in oxidizing environments and high-temperature applications, whereas a high-nickel grade (N08020) is used in severe reducing acid environments To prevent intergranular corrosion after elevated-temperature exposure, titanium or niobium

is added to stabilized carbon in S32100 or S34700 Also, lower-carbon grades (AISI L or S designations), such as S30403 (type 304L), have been established to prevent intergranular corrosion

Martensitic stainless steels are similar to iron-carbon alloys that are austenitized, hardened by quenching, and then tempered for increased ductility and toughness These alloys are magnetic, and their heat-treated structure is body-centered tetragonal In the annealed condition, they have a tensile yield strength of about 275 MPa (40 ksi) and are generally machined, cold formed, and cold worked in this condition

The strength obtained by heat treatment depends on the carbon content of the alloy Increasing carbon content increases strength, but decreases ductility and toughness The most commonly used alloy in this family is S41000, which contains about 12% Cr and 0.1% C This alloy is tempered to a variety of hardness levels, from 20 to 40 HRC Both chromium and carbon contents are increased in alloys S42000, S44002, S44003, and S44004 The first of these contains 14% Cr and 0.3% C and has a hardness capability of 50 HRC The other three alloys contain 16% Cr and from 0.6 to 1.1% C These alloys are capable of 60 HRC, and a tensile yield strength of 1900 MPa (280 ksi) The amount of primary carbides increases with increased carbon content in these three alloys

Wear resistance for martensitic stainless steels is very dependent on carbon content S44004 (1.1% C) has excellent adhesive and abrasive wear, similar to tool steels, whereas S41000 (0.1% C) has relatively poor wear resistance The key

to adhesive wear resistance is a high hardness Abrasive wear resistance requires both high hardness and primary carbides

Molybdenum and nickel can be added to martensitic stainless steel to improve corrosion and toughness properties However, the addition of these elements is somewhat restricted, because higher amounts results in a microstructure that is not fully martensitic

PH stainless steels are chromium-nickel grades that can be hardened by an aging treatment These grades are classified as austenitic (such as S66286), semi-austenitic (such as S17700), or martensitic (such as S17400) The classification is determined by their solution-annealed microstructure The semi-austenitic alloys are subsequently heat treated, so that the austenite transforms to martensite Cold work is sometimes used to facilitate the aging reaction Various alloying elements, such as aluminum, titanium, niobium, or copper, are used to achieve aging They generally form intermetallic compounds, but in S17400, fine copper precipitates are formed

Like the martensitic stainless steels, PH alloys can attain high tensile yield strengths, up to 1700 MPa (250 ksi) However, these alloys have superior ductility, toughness, and corrosion resistance, compared with the martensitic alloys These properties are related to their higher chromium, nickel, and molybdenum contents, as well as their restricted carbon (0.040 max.) levels The low carbon content of the martensitic PH stainless steels is especially critical for toughness and good ductility However, this low carbon content reduces the wear resistance of these alloys

Duplex stainless steels are chromium-nickel-molybdenum alloys that are balanced to contain a mixture of austenite and ferrite, and are magnetic, as well Their duplex structure results in improved stress-corrosion cracking resistance, compared with the austenitic stainless steels, and improved toughness and ductility, compared with the ferritic stainless steels They are capable of tensile yield strengths ranging from 550 to 690 MPa (80 to 100 ksi) in the annealed condition, which is approximately twice the strength level of either phase alone

The original alloy in this family was the predominantly ferritic S32900 The addition of nitrogen to duplex alloys, such as S32950 and S31803, increases the amount of austenite to nearly 50% In addition, nitrogen improves aswelded corrosion properties, chloride corrosion resistance, and toughness The improvement in toughness is probably related to the higher amount of austenite present, which makes it possible to produce heavier product forms, such as plates and bars

Physical and Mechanical Properties of Stainless Steels

The physical and mechanical properties of stainless steels are quite different from those of aluminum and copper alloys However, when comparing the various stainless families with carbon steels, many similarities in properties exist, although there are some key differences Like carbon steels, the density of stainless steels is 8.0 g/cm3, which is approximately three times greater than that of aluminum alloys (2.7 g/cm3) Like carbon steels, stainless steels also have a high modulus

of elasticity (200 MPa, or 30 ksi), which is nearly twice that of copper alloys (115 MPa, or 17 ksi) and nearly three times that of aluminum alloys (70 MPa, or 10 ksi)

Differences between these materials are evident in thermal conductivity, thermal expansion, and electrical resistivity, as well Figure 2 shows the large variation in thermal conductivity between various types of materials: type 6061 aluminum has a very high thermal conductivity, followed by aluminum bronze, 1080 carbon steel, and then stainless steels For stainless steels, alloying additions, especially nickel, copper, and chromium, greatly decrease thermal conductivity

Fig 2 Comparison of thermal conductivity for carbon steel, copper alloy, aluminum, and stainless steels

Source: Ref 5, 6

Thermal expansion (Fig 3) is greatest for type 6061 aluminum alloy, followed by aluminum bronze and austenitic stainless alloys, and then ferritic and martensitic alloys For austenitic stainless alloys, additions of Ni and Cu can decrease thermal expansion Stainless steels have high electrical resistivity (Fig 4) Alloying additions tend to increase electrical resistivity Therefore, the ferritic and martensitic stainless steels have lower electrical resistivity than the austenitic, duplex, and PH alloys, but higher electrical resistivity than 1080 carbon steel Electrical resistivity of stainless steels is 7.5 times greater than aluminum bronze and nearly 20 times greater than type 6061 aluminum alloy (Ref 5, 6)

Fig 3 Comparison of thermal expansion for carbon steel, copper alloy, aluminum, and stainless steels Source:

Ref 5, 6

Fig 4 Comparison of electrical resistivity for carbon steel, copper alloy, aluminum, and stainless steels Source:

Ref 5, 6

Table 2 lists tensile properties and toughness for selected stainless alloys representing the five families The four grades listed under austenitic alloys have relatively low yield strength, compared with the heat-treatable alloys, but have the highest tensile ductility and toughness The latter two alloys, S20161 and S21800, were specifically developed to have superior resistance to galling and metal-to-metal wear for stainless steels Alloy N08020 is a high-nickel (33%) stainless alloy for use in harsh corrosive environments

Table 2 Properties of selected stainless steels relative to various ferrous and nonferrous alloys

Average tensile properties Yield strength,

0.2% offset

Ultimate tensile strength

Reduction

of area, %

J ft · lbf Austenitic stainless

Source: Ref 5, 6

The ferritic stainless steels (type 405 and 409) listed have tensile yield strengths similar to the austenitic grades, but lower values for ultimate tensile strength, ductility, and toughness However, strength, ductility, and toughness are still excellent, compared with other materials, such as type 6061 aluminum and aluminum bronze The duplex stainless alloy (S32950) listed has twice the tensile yield strength of the austenitic and ferritic grades, and approximately half the toughness Again, its toughness is far superior to heat-treat-hardened alloys

The martensitic alloys listed in Table 2 have a large variation in strength, ductility, and toughness In the annealed condition, their properties are similar to the ferritic alloys, with strength increasing and ductility decreasing with increasing carbon content The higher carbon-containing alloys, type 420 and type 440C, are generally tempered at a low temperature (330 °C, or 625 °F max.) to maximize their strength On the other hand, type 410 is tempered over a wide temperature range, from 260 to 650 °C (500 to 1200 °F) The tensile properties of type 410 are similar to carbon steel (AISI 1080)

The martensitic PH alloys, such as S45500 and S17400, have higher annealed strength and lower ductility than the martensitic alloys, and are aged at temperatures ranging from 480 to 620 °C (895 to 1150 °F) Their strength is dependent

on the hardener (Ti, Nb, Cu), the amount of hardener, and the aging temperatures used Toughness is either similar to superior to the martensitic alloys at a given strength level

Wear and Galling

Types of Wear

The types of wear described below include abrasive, fretting, corrosive, fatigue, and adhesive wear

Abrasive wear involves the plowing of localized surface contacts through a softer mated material The wear is most frequently caused by nonmetallic materials, but metallic particles can also cause abrasion (Ref 7) Generally, a material is seriously abraded or scratched only by a particle harder than itself Abrasive wear is commonly divided into three types: low stress, high stress, and gouging

Low-stress abrasion (scratching) is defined as wear that occurs due to relatively light rubbing contact of abrasive particles with the metal Wear scars usually show scratches, and the amount of subsurface deformation is minimal Consequently, the surface does not work harden appreciably Parts such as screens, chute liners, blades, and belts that are exposed to sand slurries or abrasive atmospheres could experience low-stress abrasion Many machine components such

as bushings, seals, and chains that operate in dust will wear by low-stress abrasion (Ref 8)

High-stress abrasion is wear under a level of stress that is high enough to crush the abrasive Considerably more strain hardening of the metal surface occurs The abrasion or ore grinding balls is an example of high-stress abrasion in the mining industry Other examples include abrasion experienced by rolling-contact bearings, gears, cams, and pivots

Gouging Abrasion. The term gouging abrasion is used to describe high-stress abrasion that results in sizable grooves

or gouges on the worn surface (Ref 9) It occurs on parts such as crusher liners, impact hammers in pulverizers, and dipper teeth handling large rocks Strain hardening and deformation are the dominant factors

For ferrous materials, abrasion resistance is highly dependent on three metallurgical variables: microstructure, hardness, and carbon content The inherently hard martensitic structure is preferable to the softer ferritic and austenitic structures This is especially significant in low-stress abrasion, where little subsurface deformation occurs When high-stress abrasion is encountered, alloys with high work-hardened hardness values have improved wear resistance, when compared with alloys with low work-hardened hardness values Although austenitic stainless less steels will work harden more readily than the other stainless families of alloys, martensitic stainless alloys are preferred in applications where high-stress abrasion is encountered, because of their higher hardness by heat treatment Increased carbon content, regardless of structure, favors better abrasive wear resistance, and so does an increased volume of carbides, as long as their hardness is not exceeded by that of the abrasive medium

For stainless steels, knowledge of low-stress abrasion resistance is important, because abrasive particles can be found in applications where stainless steels are used Austenitic stainless steels with a high work-hardening rate could be used where gouging abrasion is a problem and toughness is required However, austenitic manganese steels, such as Hadfield

Mn steel, are more resistant Generally, stainless steels are considered for abrasive wear conditions when the environment

is corrosive or when elevated temperature are encountered

Fretting wear is material loss that is due to very small amplitude vibrations at mechanical connections, such as riveted joints This type of wear is a combination of oxidation and abrasive wear Oscillation of two metallic surfaces produces tiny metallic fragments that oxidize and become abrasive Subsequent wear proceeds by mild adhesive wear in combination with abrasive wear

Fretting wear is influenced by contact conditions, environmental conditions, and material properties and behavior These factors may interact to influence both the nature and the extent of fretting damage For example, the influence of an environmental factor depends on its accessibility to the metal contact area Only if the environmental conditions have ready access to fretting damage sites, then environmental factors will strongly influence fretting

Key parameters in fretting include load, frequency, amplitude of fretting motion, number of fretting cycles, relative humidity, and temperature Fretting wear rate is virtually independent of amplitude up to a critical value Beyond that, the wear rate increases almost linearly with the amplitude The effect of frequency has been studied on mild steel Up to 30

Hz, the fretting wear decreases with increasing frequency, while wear is not affected above 30 Hz A threshold number of cycles appears to be required for the onset of steady-state fretting wear rate This is marked by the appearance of microspall pits, which indicate that a surface fatigue mechanism is operative The environmental factors, that is, relative humidity and temperature, generally favor the use of stainless steels, because of their superior corrosion and thermal properties

Corrosive wear involves an interaction between the wear surface and the corrosive reagent Corrosion in aqueous media is an electrochemical action that results in material removal by dissolution, whereas wear involves material removal that is due to physical interaction between surfaces under relative motion When these two processes are combined, the material loss may be significantly accelerated, because of synergistic behavior The wear-corrosion process involves the disruption and removal of the oxide film, leading to exposure of active metal surface to the environment, dissolution or repassivation of the exposed metal surface, interaction between elastic fields at asperities in contact with the environment, and interaction between plastically deformed areas and the environment The exact nature of the wear-corrosion process is very dependent on the specific metal/corrosive reagent (Ref 10)

Corrosive wear can occur in the mining industry, where abrasive wear combines with a wet corrosive environment Abrasion-resistant alloy steels can be ineffective in this application, whereas stainless steels often perform well In these conditions, carbon steel readily forms iron oxide, which is removed by the sliding and bumping of moving coal/ore (Ref 11) If the rust is repeatedly removed, then there will be continuing loss of metal thickness

In South Africa, a ferritic grade (3Cr12) provides a cost-effective solution to corrosive wear problems in chutes, liners, and conveyor belt equipment used in ore handling This alloy is a modified type 409 stainless steel In the United States, type 304 has been widely used in coal-handling equipment such as chutes, bins, hoppers, and screens, because of its good corrosion resistance Another reason for the popularity of type 304 is the great improvement over alloy steels with regard

to "slideability." Type 304 retains a smooth surface finish, whereas abrasion-resistant alloy steels rust, which causes material buildup and lowers flow rate Coal hopper cars lined with type 304 can be discharged three times faster than unlined cars, because of improved slideability (Ref 12)

Corrosive wear is clearly a situation in which the use of stainless steels is attractive, because of their ability to resist the removal of their oxide film

Fatigue wear, or contact fatigue, occurs when a surface is stressed in a cyclic manner This type of wear can be found

in parts subjected to rolling contact, such as ball bearings and gears The fatigue wear rate of metals is affected by surface conditions, such as finish, residual stress, hardness, and microstructure Surface treatments such as nitriding, carburizing, and shot peening, which increase surface hardness and improve residual stress distribution, are performed to prevent fatigue wear

Cavitation, a combination of corrosive wear and fatigue wear, occurs when a liquid is subjected to rapidly alternating changes in pressure, during which bubbles form Subsequent equalization of pressure causes the bubbles to collapse at the liquid-metal interface, resulting in cavitation wear High-velocity pumps, hydraulic turbines, and fluid flow valves are applications where cavitation is a concern (Ref 13)

Because of the corrosive aspect of cavitation, austenitic stainless steels that work harden rapidly can be used satisfactorily A cobalt-base hardfacing alloy, such as Stellite 6B, is used to achieve the best resistance

Adhesive wear occurs when two metallic components slide against each other under an applied load where no abrasives are present This type of wear is called "adhesive" because of the strong metallic bonds formed between surface asperities (that is, surface high spots) of the materials Wear results from the shear failure of the weaker of the two metallic mating surfaces One theory postulates that subsurface crack nucleation and growth follow asperity shearing and flattening (Ref 14)

When the applied load is low enough, the surface oxide film characteristic of stainless steels can prevent the formation of metallic bonds between the asperities on the sliding surfaces, resulting in low wear rates This form of wear is called mild wear, or oxidative wear, and can be tolerated by most moving components When the applied load is high, metallic bonds will form between the surface asperities, and the resulting wear rates are high The load at which there is a transition from mild to severe wear is called the transition load

Adhesive wear is more prevalent in parts where a lubricant cannot be used Examples include chain-link conveyor belts, fasteners, and sliding components in a valve For stainless steels, hardness affects adhesive wear resistance For martensitic alloys, a minimum hardness of 53 HRC is required for excellent wear resistance For austenitic stainless alloys, the work-hardened hardness is critical, as are alloying additions that increase the stability of the oxide film These factors tend to increase the transition load required for severe wear to occur

Galling

Galling can be considered a severe form of adhesive wear With high loads and poor lubrication, surface damage can occur on sliding metal components The damage is characterized by localized macroscopic material transfer, that is, large fragments or surface protrusions that are easily visible on either or both surfaces This gross damage is usually referred to

as galling, and it can occur after just a few cycles of movement between the mating surfaces Severe galling can result in seizure of the metal surfaces

The terms scuffing and scoring are also used to describe similar surface damage under lubricated conditions Scuffing is the preferred term when the damage occurs at lubricated surfaces, such as the piston ring-cylinder wall contact Scoring typically describes damage that takes the form of relatively long grooves (Ref 15)

Materials that have limited ductility are less prone to galling, because under high loads surface asperities will tend to fracture when interlocked Small fragments of material may be lost, but the resultant damage will be more similar to scoring than to galling For highly ductile materials, asperities tend to plastically deform, thereby increasing the contact area of mated surfaces; eventually, galling occurs

Another key material behavior during plastic deformation is the ease with which dislocations cross slip over more than one plane In fcc materials, such as austenitic stainless steels, dislocations easily cross slip The rate of cross slip for a given alloy or element is usually indicated by its stacking-fault energy Dislocation cross slip is hindered by the presence

of stacking faults, and a high stacking-fault energy indicates a low number of impeding stacking faults and an increased tendency to cross slip and, hence, gall Table 3 lists the stacking-fault energies of four fcc elements Nickel and aluminum have poor galling resistance, whereas gold and copper have good galling resistance Austenitic stainless steels with high work-hardening rates will have relatively low stacking-fault energies, and have been shown to have less tendency to gall

Table 3 Stacking-fault energies of some common metals

Metals Stacking-fault energy, eV gs/cm2

Materials that have a hexagonal close-packed (hcp) structure with a high c/a ratio have a low dislocation cross slip rate

and are less prone to galling This explains why cobalt-base alloys and cadmium-plated alloys resist galling while titanium alloys tend to gall

Factors Affecting Wear and Galling

The factors that affect wear and galling can be design, lubrication, environmental, and material related Component design is probably the most critical factor When stainless steels are required, proper design can minimize galling and wear Similar applications, like valve parts, can often result in wear-related problems for one company or be of very little concern for another, despite their use of the same alloy

When stainless steels are used in sliding surface applications, a key consideration is reduction of contact stress The load

on the parts should be minimized, and contact area should be maximized Design tolerance of the parts should be tight with sufficient clearance because tightly fit parts will be more prone to wear and galling Lubricants should be used where possible, because they are very effective at reducing contact stress However, the design should ensure that lubrication can be effectively applied Often, lubricants are ineffective because of poor design, which renders the parts inacessible to lubrication

Another important factor is the surface roughness of the parts Highly polished surfaces (<0.25 m, or 10 in.) or very rough finishes (>1.5 m, or 60 in.) increase the tendency for wear and galling It is theorized that very smooth surfaces lack the ability to store wear debris, because of the absence of valleys between asperities, which means the asperities will have greater interaction Also, lubricants will tend to wipe off the smoother surface Too rough a finish results in interlocking asperities, which promote severe tearing and galling

A final design option is surface treatments, such as nitriding, carburizing, hard-face coatings, ion implantation, and shot peening These treatments are effective at reducing wear and galling, provided that the part configuration or the added cost is acceptable

Wear and galling can also be affected by the environment Stainless steels are selected instead of carbon steels when the environment requires either oxidation or corrosion resistance Alloy selection is dependent on the specific environment that the parts will encounter in service Thus, laboratory and field tests of candidate alloys are highly recommended Once again, component design is a critical factor in reducing corrosive wear

In the mining industry, proper design of equipment can reduce corrosive wear, thereby increasing service life Sharp bends in flames and pipelines should be avoided, and the angle of discharge from belts or chutes should be minimized Another consideration is the possibility of galvanic corrosion, which can occur where dissimilar metals are connected in the presence of an electrolyte An example would be the use of carbon steel fasteners to join stainless components The dissimilar metal combination, plus the adverse surface area ratio between the fastener and the surrounding surface, will accelerate the corrosion of the carbon steel

The proper design of equipment should allow for the drainage of pipes, flumes, and tanks during shutdown, as well as spillage or hosedown water within the plant Examples of proper versus improper design of coal-handling equipment are shown in Fig 5

Fig 5 Effect of design on susceptibility to corrosive wear Source: Ref 11

Material selection is dependent on the type of wear encountered To resist adhesive wear, a high work-hardened hardness

in an austenitic alloy and a stable oxide film can have excellent results For hardenable stainless steels, high carbon content (0.3% minimum) to increase hardness (53 HRC minimum) is critical Thus, martensitic PH alloys with their low carbon content, but high hardness, tend to have poor wear resistance Because ferritic stainless steels cannot be hardened

by heat treatment nor readily work hardened, they may also have poor metal-to-metal wear resistance

The comments on adhesive wear apply to galling resistance, except that the martensitic PH alloys with high hardness, which is achieved by using a low aging temperature (480 °C, or 895 °F), can have improved galling resistance Elements that form inclusions in steel, such as sulfur, tin, bismuth, and lead, can affect the sliding behavior of the mated materials These inclusions act as solid lubricants Thus, the sulfur-bearing stainless steels, such as types 303 and 416, have better galling resistance, but poorer adhesive wear resistance than their non-sulfur-bearing parent alloys, types 304 and 410 Another alloy example is Waukesha 88, which contains a tin- and bismuth-bearing second phase that results in excellent galling resistance, despite the high nickel content of the alloy (Ref 16)

Hardness and microstructure are critical to abrasive wear resistance Unlike adhesive wear resistance, which favors austenitic alloys with a high work-hardening rate, abrasive wear resistance favors the hard martensitic matrix structure versus the softer austenitic or ferritic structure This is especially significant in low-stress abrasion, where little subsurface deformation occurs Also, high-carbon alloys with primary carbides have better wear resistance Thus, martensitic alloy S44004 has good resistance to abrasion, whereas low-carbon martensitic PH alloy S17400 has poor abrasion resistance For gouging abrasion, Hadfield Mn steel is used, because strain hardening and impact resistance are critical

To resist fretting, thermal wear, and corrosive wear, corrosion and oxidation resistance become critical The type of stainless steel to be used depends on the environment, with austenitic stainless steels favored in severe environments A key factor for resisting these types of wear is the alloying addition, because silicon, aluminum, and chromium improve the corrosion and oxidation resistance of a particular alloy regardless of stainless family

Wear and Galling Tests Commonly Used for Stainless Steels

Dry sand/rubber wheel test is an ASTM standard test (Ref 17) used to determine the resistance of metallic materials to low-stress (scratching) abrasion The test involves the abrading of a test specimen with a grit of controlled size and composition (that is, rounded grain quartz sand) The abrasive is introduced between the test specimen and a rotating wheel with a chlorobutized rubber tire or rim of a specified hardness The test specimen is pressed against the rotating wheel at a specified force by means of a lever arm while a controlled flow of grit abrades the test surface The rotation of the wheel is such that its contact face moves in the direction of the sand flow A schematic diagram of the test apparatus is shown in Fig 6 The test duration and force applied by the lever arm vary depending on the relative wear resistance of the materials evaluated For stainless steels, the test duration is one-third that of the more abrasion-resistant tool steel alloys such as D2 Specimens are weighed before and after the test, and the loss in mass is recorded Abrasive wear is generally reported in terms of volume loss (in cubic millimeters) by dividing mass loss by the density of the alloy

Fig 6 Dry sand abrasion test apparatus Source: Ref 18

Corrosive wear testing commonly involves hub and ball mill tests The apparatus used for the hub test, which is stress abrasive test, is shown in Fig 7 The slurry container has three hubs, each of which holds eight specimens, which are driven through slurry, as shown in Fig 8

low-Fig 7 Hub wear test machine Source: Ref 12, p 3

Fig 8 Specimen arrangement on hub Source: Ref 12, p 3

Slurries can differ, depending on the application that needs to be simulated Often, alloys are evaluated both wet (corrosive slurry) and dry to demonstrate the effect of corrosion on corrosive wear Specimens are weighed before and after the test, and volume loss is determined by dividing the weight loss by the alloy density

The ball mill test is used extensively by the U.S Bureau of Mines and in Canada to determine corrosive effects found in nickel mines For this test, a 5.3 L (1.4 gal) porcelain jar serves as the ball mill, and the specimens are free to tumble in the corrosive slurry Tests are performed for a particular time period, such as 8 h, after which the weight loss is determined and volume loss is calculated The samples are then tested for additional time periods, using fresh slurry for each period At the conclusion of the test, corrosive wear versus time is plotted for the alloys being evaluated

Block-on-ring is an ASTM standard test (Ref 19) for determining the resistance of materials to sliding wear The test utilizes a block-on-ring friction and wear testing machine to rank pairs of materials according to their sliding wear characteristics under various conditions Rotational speed and load can be varied to correspond to service requirements In addition, tests can be run with various lubricants, liquids, or gaseous atmospheres

The test consists of a block specimen loaded against a ring specimen at a given speed for a given number of revolutions (Fig 9) Block scar volume is calculated from the block scar width and depth, and ring scar volume is calculated from ring weight loss The friction force required to keep the block in place is continuously measured during the test with a

load cell and is then recorded The choice of test parameters is left to the user (with the exception of sliding distance, which is specified because wear does not usually vary linearly with distance in this test)

Fig 9 Key components of a block-on-ring test apparatus The coefficient of sliding friction, sf , is calculated using the equation sf = (friction force)/(applied load) Source: Ref 2

Crossed-cylinder is an ASTM standard test (Ref 20) for determining the resistance of metallic materials to metal wear This test ranks the adhesive wear resistance of materials and evaluates the compatibility of different metal couples It is the most commonly used test for evaluating metal-to-metal wear resistance of stainless steels

metal-to-The test configuration consists of two cylindrical specimens that are positioned perpendicular to each other One sample

is rotated at a specified test speed while the other sample is kept stationary The stationary specimen is pressed against the rotating specimen at a specified load by means of a lever arm and attached weights The setup results in deadweight loading A photograph of the test apparatus is shown in Fig 10 Elevated-temperature tests have also been performed using the crossed-cylinder apparatus The test duration, number of cycles, and rotational speed are varied depending on the relative wear resistance of the mated materials The amount of wear is determined by weighing the specimen before and after the test Weight loss is converted to volume loss by dividing the density of the material Volume loss is determined for both the stationary and rotating specimens, and the total volume loss is recorded When dissimilar materials are being tested, it is recommended that each alloy be tested in both the stationary and rotating positions

Fig 10 Typical crossed-cylinder test apparatus Source: Ref 21

Pin-on-disk is an ASTM standard test for determining the wear of material during sliding (Fig 11) The coefficient of friction can also be determined using this test For the pin-on-disk wear test, two specimens are required: one specimen is

a pin with a radiused tip that is positioned perpendicular to the other specimen, which is usually a flat circular disk A ball, rigidly held, often is used as the pin specimen The test machine causes either the disk specimen or the pin specimen

to revolve about the disk center In either case, the sliding path is a circle on the disk surface The plane of the disk may

be oriented either horizontally or vertically The pin specimen is pressed against the disk at a specified load, usually by means of an arm or lever and attached weights Wear results are reported as volume loss (in cubic millimeters) for the pin and disk separately When two different materials are tested, it is recommended that each material be tested in both the pin and disk positions

Fig 11 Pin-on-disk wear test apparatus (a) Key components of the device (b) Close-up showing pin motion

relative to flat when subjected to load Source: Ref 2

The amount of wear is determined by measuring appropriate linear dimensions of both specimens before and after the test, or by weighing both specimens before and after the test Linear measurements of wear are converted to wear volume

by using appropriate geometric relationships, while mass loss is converted to volume loss by dividing mass loss by the appropriate density values for the specimens Wear results are usually obtained by conducting a test for a selected sliding distance, load, and speed Graphs of wear volume versus sliding distance using different specimens for different distances can be plotted

The galling test is an ASTM standard test (Ref 22), which ranks the galling resistance of material couples This test, commonly referred to as the button-on-block test, was developed during the 1950s and is the most commonly used procedure to test the galling resistance of stainless steels Although it is generally performed on bare metals, nonmetallics, coatings, solid lubricants, and surface-modified alloys can be tested as well

The test method uses available laboratory equipment that is capable of maintaining a constant compressive load between two flat specimens, such as hydraulic or screw-fed compression testing machines A specimen with a 13 mm (0.5 in.) diameter section, referred to as the button, is slowly rotated one revolution (360°) relative to the other specimen under a specified compressive load The test surfaces are ground so that the surface roughness range is from 0.4 to 1 m (15 to 40 in.) and the specimens are flat to ensure 100% contact between the mated surfaces

The surfaces are examined for galling after sliding The criterion for whether galling occurs is the appearance of the specimens, based on unassisted visual examination Galling is characterized by at least one of the contacting surfaces exhibiting torn or raised metal If the specimens have not galled, then a new button is tested on a new block location at an increased load This procedure is continued until galling occurs Similarly, if galling does occur on the first test, then lower loads are evaluated until no galling occurs The galling test set-up is shown in Fig 12

Fig 12 Button-on-block galling test Source: Ref 23

The loads applied correspond to a contact stress for the 13 mm (0.5 in.) diameter button The stress midway between the highest nongalled test and the lowest galled test is referred to as the threshold galling stress The higher the threshold galling stress, the more resistant the mated materials are to galling Galling resistance can be determined for a particular selfmated material or dissimilar-mated materials For the latter, it is recommended that each alloy be evaluated as button and block specimens

To simulate repeated part performance under service conditions, such as those existing with sliding valve parts, the button specimen can be rotated multiple revolutions, in which the direction is reversed after each 360° revolution, instead of just rotating it once Because of the increased severity of the multiple-rotation method, threshold galling stresses will be significantly lower than those of the single-rotation test

To test highly resistant alloys, such as cobalt base Stellite 6B, a similar button-on-block test is used The button is rotated through a 120° arc ten times at three different loads Surface profilometry is then used to assess the degree of damage (Ref 24)

Another test that is similar to the button-on-block test is the ring-on-ring test, which uses ring specimens, rather than solid button/block samples The test is used to assess galling resistance for tubular products related to oil production (Ref 25)

Galling of threaded parts, such as 9 m (30 ft) drill collars, is evaluated by a make/break test The test procedure involves making a box/pin connection at a specified torque, breaking the connection, and recording the breakout torque The procedure is repeated a given number of times, and the threads are periodically examined for galling Unlike the button-on-block test, which simply tries to rank materials, this test simulates the actual service connections that are made for drill collars in the oil-drilling industry Tests that simulate the specific application are very beneficial, but are usually quite expensive to perform Thus, the button-on-block test can be used as a screening test to choose the best candidate alloy for

a more specific test tailored to the service conditions of a particular application

Wear Data for Stainless Steels

Abrasive/Corrosive Wear. The low-stress abrasion resistance of stainless steels can be determined by using either the dry sand abrasion or the hub test Dry sand abrasion data (Table 4) clearly show that austenitic stainless steels have poorer wear resistance than the harder martensitic alloys, whose increased carbon content increases wear resistance Type

410, which has 0.1% C, has wear resistance similar to the best austenitic alloys, whereas the wear resistance of type 440C

is three to four times greater than the austenitic alloys The abrasive wear resistance of type 440C is inferior to D2, an abrasive wear resistant tool steel Test results shows that D2 has a volume loss of 32 mm3, which is similar to type 440C, when the test time for D2 is tripled

Table 4 Abrasive wear resistance of stainless steels

Alloy Rockwell

hardness

Volume loss, mm3 Austenitic

Mn steel and AISI 4340 However, when evaluated in a wet slurry of 5% NaCl plus 0.5% acetic acid, the stainless alloys were more resistant Similar trends are shown in various wet environments, such as distilled water, synthetic nickel mine water, and synthetic seawater (Fig 14, 15, 16) using the ball mill test

Fig 13 Abrasive wear of alloy and stainless steels under dry and wet corrosive conditions Source: Ref 12, p 4

Fig 14 Corrosive wear of alloy and stainless steels relative to an aluminum alloy, based on ball mill test using

1500 mL distilled water Source: Ref 12, p 5

Fig 15 Corrosive wear of alloy and stainless steels, based on ball mill test using synthetic nickel mine water

Source: Ref 12, p 8

Fig 16 Corrosive wear of alloy and stainless steels, based on ball mill test using synthetic seawater Source:

Ref 12, p 9

Stainless steels have superior corrosive wear resistance in coal-handling equipment Service life is greatly improved, compared with carbon steels (Fig 17) In the mining industry, corrosion is the primary factor in the deterioration of equipment Figure 18 shows the effect of chromium content on atmospheric corrosion resistance Without chromium, corrosion rates are at least ten times greater than stainless steels

Fig 17 Service life of coal-handling equipment (a) Coal unloading time, 50 ton hopper car (b) Coal conveyor

bottom × 0 coal at 49 to 82 °C (120 to 180 °F) 6000 h/yr (c) Coal conveyor bottom 5 × 0 clean coal 6000 h/yr Source: Ref 13

Fig 18 Effect of chromium content on atmospheric corrosion resistance Source:Ref 11

Tests under service conditions have been performed on stainless steels, abrasion-resistant steels, and carbon steels Tests were performed for up to 5.3 years, and the reduction of thickness determined Results of two studies show the superiority

of stainless steels, compared with nonstainless alloys (Table 5) In general, type 304 is superior to type 410 However, alloy selection is dependent on the particular coal-handling equipment

Table 5 Wear rates for test plates in drag conveyor bottoms

g (mils/million tons)

of coal conveyed

Test time, months

125 × 9.5 mm (5 × in.)

Type 304 stainless steel 0.67 (24) 27

Type 410 stainless steel 0.73 (26) 27

Type 316 stainless steel 0.90 (32) 27

Abrasion-resistant steel 1.01 (36) 15

9.5 mm ( in.) by 0 thermal dryer product

Type 304 stainless steel 0.045 (1.6) 25

Type 316 stainless steel 0.053 (1.9) 25

Type 410 stainless steel 0.100 (3.6) 25

Abrasion-resistant steel 2.3 (83) 6

63.5 × 6.35 mm (2.5 × 0.25 in.)

Type 304 stainless steel 0.07 (2.6) 47

Type 410 stainless steel 0.11 (3.9) 47

Crushed middlings conveyor

Type 304 stainless steel 0.17 (6) 48

Abrasion-resistant steel 7 (250) (failed in 18 months)

Source: Ref 11

Adhesive Wear. The metal-to-metal wear resistance of stainless steels can be determined by using the crossed-cylinder wear test Unlike low-stress abrasion resistance, austenitic stainless steels generally have superior resistance, compared with martensitic stainless alloy (Table 6) The excellent wear resistance of type 201, type 301, S20161, and S21800 can

be attributed to a high work-hardening rate Additionally, the latter two alloys have a high silicon content, which not only increases the work-hardening rate, but results in a more adherent oxide film, thereby preventing the transition from mild oxidation wear to severe wear Like abrasive wear resistance, a high hardness value is critical for martensitic stainless steels in order to achieve good adhesive wear resistance For type 440C, high hardness (56 HRC) results in exceptional metal-to-metal wear resistance, whereas for type 440C (26 HRC), volume loss is high Generally, high volume loss is characteristic of these alloys, even at 50 HRC

Table 6 Adhesive wear resistance of stainless alloys

Table 7 Wear compatibility of dissimilar-mated stainless steels

S17400 (43 HRC)

S24100 (95 HRB)

S20910 (99 HRB)

S21800 (95 HRB)

Type 440C (57 HRC) Type 304 16.4