Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Part 7 potx

1 Properties of cast carbon steels as a function of carbon content and heat treatment.. Further information on the elements used in alloy steel castings is provided in the section "Low-A

Fig 1 Properties of cast carbon steels as a function of carbon content and heat treatment (a) Tensile strength

and reduction of area (b) Yield strength and elongation (c) Brinell hardness (d) Charpy V-notch impact energy

Low-alloy steels contain, in addition to carbon, alloying elements up to a total alloy content of 8% Cast steels

containing more than the following amounts of a single alloying element are considered low-alloy cast steel:

Element Amount, %

9500 9500 Manganese, nickel, chromium, molybdenum

The 8000, 8400, 2300, and 9500 alloy types are used extensively as cast steels There are additional alloy types that are infrequently specified as cast steels, that is, 3100 (nickel-chromium), 3300 (nickel-chromium), 4000 (molybdenum), 5100 (chromium), 6100 (chromium-vanadium), 4600 (nickel-molybdenum), and 9200 (silicon) Further information on the elements used in alloy steel castings is provided in the section "Low-Alloy Cast Steels" of this article

Specifications Steel castings are usually purchased to meet specified mechanical properties, with some restrictions on

chemical composition Tables 1 and 2 list the requirements given in various specifications of the American Society of Testing and Materials (ASTM) and in SAE J435c Table 1 primarily lists carbon steel castings (with some comparable low-alloy types), while Table 2 lists several low-alloy cast steels and some cast steels with chromium contents up to 10.0%

Table 1 Summary of specification requirements for various carbon steel castings

Unless otherwise noted, all the grades listed in this table are restricted to a phosphorus content of 0.40% max and a sulfur content of 0.045% max

Tensile

strength (a)

Yield strength (a)

Chemical composition (b) , %

in 50 mm (2 in.), %

Minimum reduction

in area, %

Other requirements

ASTM A 148: carbon steel castings for structural applications

80-40 550 80 275 40 18 30 (e) (e) (e) 0.06% S,

0.05% P

Composition and heat treatment necessary to achieve specified mechanical properties

80-50 550 80 345 50 22 35 (e) (e) (e) 0.06% S,

0.05% P

Composition and heat treatment necessary to achieve specified mechanical properties

90-60 620 90 415 60 20 40 (e) (e) (e) 0.06% S,

0.05% P

Composition and heat treatment necessary to achieve specified mechanical properties

105-85 725 105 585 85 17 35 (e) (e) (e) 0.06% S,

0.05% P

Composition and heat treatment necessary to achieve specified mechanical properties

SAE J435c: see Table 2 for alloy steel castings specified in SAE J435c

0.50-0.60 187 HB max Low carbon steel

suitable for carburizing

0025 415 60 207 30 22 30 0.25(c) 0.75(c) 0.80 187 HB max Carbon steel welding

0.50-0.80 170-229 HB Carbon steel

medium-strength grade

0050B 690 100 485 70 10 15

0.40-0.50

0.90

0.50-0.80 207-225 HB Carbon steel

ASTM A 216: carbon steel castings suitable for fusion welding and high-temperature service

(a) Where a single value is shown, it is a minimum

(b) Where a single value is shown, it is a maximum

(c) For each reduction of 0.01% C below the maximum specified, an increase of 0.04% Mn above the maximum specified is permitted up to the maximums given in the applicable ASTM specifications

(d) Grades may also include low-alloy steels; see Table 2 for the stronger grades of ASTM A 148

(e) Unless specified by purchaser, the compositions of cast steels in ASTM A 148 are selected by the producer in order to achieve the specified mechanical properties

(f) Purchased on the basis of hardenability, with manganese and other elements added as required

(g) Specified residual elements include 0.30% Cu max, 0.50% Ni max, 0.50% Cr max, 0.20% Mo max, and 0.03% V max, with the total residual elements not exceeding 1.00%

(h) These ASTM specifications also include alloy steel castings for the general type of applications listed in the Table

(i) Testing temperature of -32 °C (-25 °F)

(j) Charpy V-notch impact testing at the specified test temperature with an energy value of 18 J (13 ft · lbf) min for two specimens and an average

in 50 mm (2 in.), %

Minimum reduction

SAE J435c: see Table 1 for the carbon steel castings specified in SAE J435c

0.50- 1.10

0.70- 0.65

0.45-(j) , (k)

0.50- 1.00

0.60- 1.20

0.90-(j) , (k)

1.00-0.50(i)

0.45-0.65

(i) , (j)

2.00-0.50(i)

0.9-1.20

(i) , (j)

0.50- 0.60

0.30- 1.75

1.00-0.50(i)

0.45-0.65

(i) , (l)

4.00-0.50(i)

0.45-0.65

(i) , (j)

8.00-0.50(i)

0.90-1.20

(i) , (j)

ASTM A 389: alloy steel castings (NT) suitable for fusion welding and pressure-containing parts at high temperatures

C23 485 70 275 40 18 35 0.20

0.30-0.80

0.60 1.50

1.00- 1.00- 1.00-

0.45-0.65

(h) , (m)

C24 550 80 345 50 15 35 0.20

0.30-0.80

0.60 1.25

1.00- 1.00- 1.00-

0.90-1.20

(h) , (m)

ASTM A 487: alloy steel castings (NT or QT) for pressure-containing parts at high temperatures

0.40- 0.30

0.15-(k) , (p)

0.40- 0.30

0.15-(k) , (p)

0.40- 0.30

0.15-(k) , (p)

4D (QT) 690 100 515 75 17 35 0.30 1.00 0.80

0.40-0.80

0.80

0.40- 0.30

0.15-(k) , (p)

4E (QT) 795 115 655 95 15 35 0.30 1.00 0.80

0.40-0.80

0.80

0.40- 0.30

0.15-(k) , (p)

6A (NT) 795 115 550 80 18 30 0.38

1.30-1.70

0.80 0.80

0.80

0.40- 0.40

0.30-(k) , (p)

6B (QT) 825 120 655 95 15 35 0.38

1.30-1.70

0.80 0.80

0.80

0.40- 0.40

0.30-(k) , (p)

0.40- 1.00

0.70- 0.60

0.40-(k) , (p) , (r)

2.00- 2.00- 2.00-

0.90-1.10

(k) , (p)

8B (QT) 725 105 585 85 17 30 0.20

0.50-0.90

0.80 2.75

2.00- 2.00- 2.00-

0.90-1.10

(k) , (p)

8C (QT) 690 100 515 75 17 35 0.20

0.50-0.90

0.80 2.75

0.75-0.50(i)

0.15-0.30

(i) , (p)

9B (QT) 725 105 585 85 16 35 0.33

0.60-1.00

0.80 1.10

0.75-0.50(i)

0.15-0.30

(i) , (p)

0.75-0.50(i)

0.15-0.30

(i) , (p)

10A (NT) 690 100 485 70 18 35 0.30

0.60-1.00

0.80 0.90

0.55- 2.00

1.40- 0.40

0.20-(k) , (p)

10B (QT) 860 125 690 100 15 35 0.30

0.60-1.00

0.80 0.90

0.55- 2.00

1.40- 0.40

0.20-(k) , (p)

0.50- 1.10

0.70- 0.65

0.45-(p) , (s)

0.50- 1.10

0.70- 0.65

0.45-(p) , (s)

0.50- 1.00

0.60- 1.20

0.90-(p) , (s)

0.50- 1.00

0.60- 1.20

0.90-(p) , (s)

0.20-(p) , (t)

0.20-(p) , (t)

0.20-(p) , (t)

(b) When a single value is shown, it is a minimum

(c) When a single value is shown, it is a maximum

(d) Unless specified by the purchaser, the compositions of cast steels in ASTM A 148 are selected by the producer and therefore may include either carbon or alloy steels; see Table 1 for the lower-grade steels specified in ASTM A 148

(e) 0.06% S (max), 0.05% P (max)

(j) 0.050% Cu (max), 0.10% W (max), 0.045% S(max), 0.04% P (max)

(k) When residual maximums are specified for copper, nickel, chromium, tungsten, and vanadium, their total residual content shall not exceed 0.60%

(l) 0.35% Cu (max), 0.03% V (max), 0.015% S (max), 0.020% P (max)

(m) 0.15-0.25% V

(n) The specified residuals of copper, nickel, chromium, and molybdenum (plus tungsten), shall not exceed a total content of 1.00%

(o) Includes the residual content of tungsten

(p) 0.50% Cu (max), 0.10% W (max), 0.03% V (max), 0.045% S (max), 0.04% P (max)

(q)

Material class 7A is a proprietary steel and has a maximum thickness of 63.5 mm (21

2 in.)

(r) Specified elements include 0.15-0.50% Cu, 0.03-0.10% V, and 0.002-0.006% B

(s) When residual maximums are specified for copper, nickel, chromium, tungsten, molybdenum, and vanadium, their total content shall not exceed 0.50%

(t) When residual maximums are specified for copper, nickel, chromium, tungsten, and vanadium, their total content shall not exceed 0.75%

(u) Low-carbon grade with double austenitization

(v) For each reduction of 0.01% C below the maximum, an increase of 0.04% Mn is permitted up to a maximum of 2.30%

(w) 0.20% Cu (max), 0.10% W (max), 0.02% V (max), 0.02% S (max), 0.02% P (max)

In the low-strength ranges, some specifications limit carbon and manganese content, usually to ensure satisfactory weldability In SAE J435c, carbon and manganese are specified to ensure that the minimum desired hardness and strength are obtained after heat treatment For special applications, other elements may be specified either as maximum or minimum, depending on the characteristics desired

The ASTM specifications that include carbon and low-alloy grades of steel castings are A 216, A 217, A 352, A 356, A

389, A 487, and A 757 The ASTM specifications with grades of carbon steel castings are listed in Table 1 Table 2 lists the requirements for the low-alloy classes of steel castings given in some of the ASTM specifications mentioned above In addition, ASTM specifications may address common requirements of all steel castings for a particular type of application For example, ASTM A 703 specifies the general requirements of steel castings for pressure-containing parts

If only mechanical properties are specified, the chemical composition of castings for general engineering applications is usually left to the discretion of the casting supplier For specific applications, however, certain chemical composition limits have been established to ensure the development of specified mechanical properties after proper heat treatment, as well as to facilitate welding, uniform response to heat treatment, or other requirements Hardness is specified for most grades of SAE J435c to ensure machinability, ease of inspection for high production rate items, or certain characteristics pertaining to wear

SAE J435c includes three grades, HA, HB, and HC, with specified hardenability requirements Figure 2 plots hardenability requirements, both minimum and maximum, for these steels Hardenability is determined by the end-quench hardenability test described in the article "Hardenability of Carbon and Low-Alloy Steels" in this Volume Other specifications require minimum hardness at one or two locations on the end-quench specimen In general, hardenability is specified to ensure a predetermined degree of transformation from austenite to martensite during quenching, in the thickness required This is important in critical parts requiring toughness and optimum resistance to fatigue

Fig 2 End-quench hardenability limits for the hardenability grades of cast steel specified in SAE J435c The

nominal carbon content of these steels is 0.30% C (see Table 1) Manganese and other alloying elements are added as required to produce castings that meet these limits

Among the most commonly selected grades of steel castings are, first, a medium-carbon steel corresponding to ASTM A

27 65-35 or SAE 0030 and second, a higher-strength steel, often alloyed and fully heat treated, similar to ASTM A 148 105-85 or SAE 0105

Particularly when the purchaser heat treats a part after other processing, a casting will be ordered to compositional limits closely equivalent to the AISI-SAE wrought steel compositions, with somewhat higher silicon permitted As in other steel

castings, it is best not to specify a range of silicon, but to permit the foundry to utilize the silicon and manganese combination needed to achieve required soundness in the shape being cast The silicon content is frequently higher in cast steels than for the same nominal composition in wrought steel Silicon above 0.80% is considered an alloy addition because it contributes significantly to resistance to tempering

Railroad equipment manufacturers and other major users of steel castings may prefer their own or industry association specifications Users of steel castings for extremely critical applications, such as aircraft, may use their own, industry association, or special-purpose military specifications Foundries frequently make nonstandard grades for special applications or have their own specification system to meet the needs of the purchaser Savings may be realized by using

a grade that is standard with a foundry, especially for small quantities

Low-Carbon Cast Steels

Low-carbon cast steels are those with a carbon content of less than 0.20% Most of the tonnage produced in the carbon classification contains between 0.16 and 0.19% C, with 0.50 to 0.80% Mn, 0.05% P (max), 0.06% S (max), and 0.35 to 0.70% Si In order to obtain high magnetic properties in electrical equipment, the manganese content is usually held between 0.10 and 0.20% The properties of these dynamo steels may be slightly below those of typical low-carbon cast steels because of their manganese content

low-Figure 1 includes the mechanical properties of carbon cast steels with low-carbon contents within the range of about 0.10

to 0.20% There is very little difference between the properties of the low-carbon steels resulting from the use of normalizing heat treatments, and the properties of those that are fully annealed In cast steels, as in rolled steels of this composition, increasing the carbon content increases strength and decreases ductility Although the mechanical properties

of low-carbon cast steels are nearly the same in the as-cast condition as they are after annealing, low-carbon steel castings are often annealed or normalized to relieve stresses and refine the structure

Low-carbon steel castings are made in two important classes One may be termed railroad castings, and the other miscellaneous jobbing castings The railroad castings consist mainly of comparatively symmetrical and well-designed castings for which adverse stress conditions have been carefully studied and avoided

Miscellaneous jobbing castings present a wide variation in design and frequently involve the joining of light and heavy sections Varying sections make it more difficult to avoid high residual stress in the as-cast shape Because residual stresses of large magnitude cannot be tolerated in many service applications, stress relieving becomes necessary Therefore, the annealing of those castings is decidedly beneficial even though it may cause little improvement of mechanical properties Castings for electrical or magnetic equipment are usually fully annealed because this improves the electrical and magnetic properties

An increase in mechanical properties can be obtained by quenching and tempering, provided the design of the casting is such that it can be liquid quenched without cracking Impact resistance is improved by quenching and tempering, especially if a high tempering temperature is employed

Uses As has been mentioned, important castings for the railroads are produced from low-carbon cast steels Some

castings for the automotive industry are produced from this class of steel, as are annealing boxes, annealing bottoms, and hot-metal ladles Low-carbon steel castings are also produced for case carburizing, by which process the castings are given a hard, wear-resistant exterior while retaining a tough, ductile core The magnetic properties of this class of steel make it useful in the manufacture of electrical equipment Free-machining cast steels containing 0.08 to 0.30% sulfur are also produced in low-carbon grades

Medium-Carbon Cast Steels

The medium-carbon grades of cast steel contain 0.20 to 0.50% C and represent the bulk of steel casting production In addition to carbon, they contain 0.50 to 1.50% Mn, 0.05% P (max), 0.06% S (max), and 0.35 to 0.80% Si The mechanical properties at room temperature of cast steels containing from 0.20 to 0.50% C are included in Fig 1 Steels in this carbon range are always heat treated, which relieves casting strains, refines the as-cast structure, and improves the ductility of the steel

Unlike low-carbon castings, when medium-carbon steel castings are fully annealed, it is possible to increase the yield strength, the reduction of area, and the elongation over the entire range, compared to as-cast properties (Fig 14) This

increase is pronounced for steel with a carbon content between 0.25 and 0.50% The hardness and tensile strength can be expected to fall off slightly following full annealing

Fig 14 Effect of annealing on the mechanical properties of medium-carbon steel castings

A very large proportion of steel castings of this grade are given a normalizing treatment, following by a tempering treatment The improvement in mechanical properties of medium-carbon cast steel that may be expected after normalizing

or normalizing and tempering is shown in Fig 1

If the design of a casting is suitable for liquid quenching, further improvements are possible in the mechanical properties

In fact, to develop mechanical properties to the fullest degree, steel castings should be heat treated by liquid quenching and tempering Commercial procedure calls for tempering to obtain the desired strength level Tempering temperatures of

650 to 705 °C (1200 to 1300 °F) are usually used to obtain higher ductility and impact properties

High-Carbon Cast Steels

Cast steels containing more than 0.50% C are classified as high-carbon steels This grade also contains 0.50 to 1.50% Mn, 0.05% P (max), 0.05% S (max), and 0.35 to 0.70% Si The mechanical properties of high-carbon steels at room temperature are shown in Fig 1 High-carbon cast steels are often fully annealed Occasionally, a normalizing and tempering treatment is given, and for certain applications an oil quenching and tempering treatment may be used

The microstructure of high-carbon steel is controlled by the heat treatment Carbon also has a marked influence, for example, giving 100% pearlitic structure at eutectoid composition (~0.83% carbon) Higher proportions of carbon than eutectoid composition will increase the proeutectoid cementite, which is detrimental to the casting if it forms a network at the grain boundaries because of improper heat treatment (for example, slow cooling from above the Acm temperature) Faster cooling will prevent the formation of this network and, hence, improve the properties

Low-Alloy Cast Steels

Low-alloy cast steels contain a total alloy content of less than 8% These steels have been developed and used extensively for meeting special requirements that cannot be met by ordinary plain carbon steels with low hardenability The addition

of alloys to plain carbon steel castings may be made for any of several reasons, such as to provide higher hardenability, increased wear resistance, higher impact resistance at increased strength, good machinability even at higher hardness, higher strength at elevated and low temperatures, and better resistance to corrosion and oxidation than the plain carbon steel castings These materials are produced to meet tensile strength requirements of 485 to 1380 MPa (70 to 200 ksi), together with some of the above special requirements

Alloy cast steels are used in machine tools; high-speed transportation units; steam turbines; valves and fittings; railway, automotive, excavating, and chemical processing equipment; pulp and paper machinery; refinery equipment; rayon machinery; and various types of marine equipment They are also used in the aeronautics field

Low-alloy cast steels may be divided into two classes according to use: those used for structural parts of increased strength, hardenability, and toughness, and those resistant to wear, abrasion, or corrosive attack under low- or high-temperature service conditions There can be no sharp distinction between the two classes because many steels serve in both fields

The present trend toward decreasing weight through the use of high-strength materials in lighter sections has had a

marked effect on the development of low-alloy cast steels Low-alloys grades, such as those in the 86xx, 41xx, and 43xx

families, are capable of producing mechanical properties with a yield strength 50% higher and a tensile strength 40% higher than carbon steels, with a ductility and impact resistance at least equal to unalloyed steels Some 75 to 100 combinations of the available alloying materials have been regularly or occasionally used It is doubtful that this many variations in composition are necessary or economical

Alloying Elements The compositions of low-alloy cast steels are primarily characterized by carbon contents under

0.45% and by small amounts of alloying elements, which are added to produce certain specific properties Low-alloy steels are applied when strength requirements are higher than those obtainable with carbon steels Low-alloy steels also have better toughness and hardenability than do carbon steels

Carbon-Manganese Cast Steels. Manganese is the cheapest of the alloying elements and has an important effect in increasing the hardenability of steel For this reason, many of the low-alloy cast steels now contain between 1 and 2% manganese In the normalized steels in which grain refinement is also needed, vanadium, titanium, or aluminum is often added

Carbon-manganese steels containing 1.00 to 1.75% Mn and 0.20 to 0.50% C have received considerable attention from engineers in the past because of the excellent properties that can be developed with a single, relatively inexpensive alloying element and by a single normalizing or normalizing and tempering heat treatment Carbon-manganese steels are also referred to as medium-manganese steels and are represented by the cast 1300 series of steels (1.60 to 1.90% Mn)

characteristics of high yield strength at elevated temperatures, higher ratio of yield strength to tensile strength at room temperature, greater freedom from temper embrittlement, and greater hardenability Therefore, these steels have replaced medium-manganese steel for certain applications

There are two general grades of manganese-molybdenum cast steels:

• 8000 series (1.0 to 1.35% Mn, 0.10 to 0.30% Mo)

• 8400 series (1.35 to 1.75% Mn, 0.25 to 0.55% Mo)

For both of these alloy types, the selected carbon content is frequently between 0.20 and 0.35%, depending on the heat treatment employed and the strength characteristics desired

produced for their high hardenability Sections exceeding 125 mm (5 in.) in thickness can be quenched and tempered to obtain a fully tempered martensitic structure The composition range employed for the 9500 series is:

Nickel or molybdenum with manganese refines the grain structure to a lesser extent than does vanadium, titanium, or aluminum, but each is important for increasing the ability of the steel to air harden Chromium and vanadium impart considerable hardenability Vanadium-containing steels are sometimes precipitation hardening and, therefore, may have higher tensile and yield strengths

Nickel and nickel-vanadium steels are used for parts exposed to subzero conditions (such as return headers, valves, and pump castings in oil-refinery dewaxing processes) because of good notch toughness at lower temperatures These steels are characterized

by high tensile strength and elastic limit, good ductility, and excellent resistance to impact The cast steels of the 2300 series contain 2.0 to 4.0% Ni, depending on the grade required

purposes requiring wear resistance and high strength The manganese-molybdenum cast steels are also used in these applications

improves hardenability and makes the steel relatively immune to temper embrittlement Nickel-chromium-molybdenum cast steel is particularly well suited to the production of large castings because of its deep-hardening properties In addition, the ability of these steels to retain strength at elevated temperatures extends their usefulness in many industrial applications

in elevated-temperature properties Cast steels containing chromium, molybdenum, vanadium and tungsten have given good service in valves, fittings, turbines, and oil refinery parts, all of which are subjected to steam temperatures up to 650

The chromium cast steels (5100 series, 0.70 to 1.10% Cr) are not in common use in the steel casting industry Although chromium leads the field as an alloying element for wear-resistant steels, it is seldom used alone For example, the chromium-molybdenum steels are widely used

types is primarily based on either their atmospheric-corrosion resistance (weathering steels) or the age-hardening characteristics that copper adds to steel

these strength levels and with considerable toughness and weldability were originally developed for ordnance applications These cast steels can be produced from any of the above medium-alloy compositions by heat treating with liquid-quenching techniques and low tempering temperatures Cast 4300 series steels or modifications thereof are usually employed

Mechanical Properties Figure 4 shows typical room-temperature mechanical properties of low-alloy steels plotted

against yield strength These properties are, of course, a function of alloy content, heat treatment, and section size

Figure 15 shows the wide range of properties obtainable through changes in carbon and alloy content and heat treatment (note the properties for 0.30% C, 1.50% Mn, and 0.35% Mo steel) Figure 16 shows the variations in mechanical properties of a water-quenched cast 8630 steel as a function of tempering temperature Section size effects were discussed

in the section "Mechanical Properties" of this article

Fig 15 Distribution of mechanical properties and carbon and alloy contents for alloy steel castings (a) Cr-Mo-V

steel, 1.00Cr-1.00Mo-0.25V, normalized and tempered; 25 heats (b) Cr-Mo steel, 1.00Cr-1.00Mo, normalized and tempered; 25 heats (c) Nickel steel, 0.20C-2.25Ni, normalized and tempered; 200 heats (d) Mn-Mo steel, 0.30C-1.50Mn-0.35Mo, normalized and tempered; 40 heats (e) Mn-Mo steel, 0.30C-1.50Mn-0.35Mo, quenched and tempered; 268 to 302 HB; 50 heats (f) Mn-Mo steel, 0.30C-1.50Mn-0.35Mo, quenched and tempered; 300

to 321 HB; 50 heats

Physical Properties

The physical properties of cast steel are generally similar

to those of wrought steel

Elastic constants of carbon and low-alloy cast steels

as determined at room temperature are only slightly affected by changes in composition and structure The

modulus of elasticity, E, is about 200 GPa (30 × 106 psi), Poisson's ratio is 0.3, and the modulus of rigidity is 77.2 GPa (11.2 × 106 psi) Increasing temperature has a marked effect on the modulus of elasticity and the modulus of rigidity A typical value of the modulus of elasticity at 200 °C (400 °F) is about 193 GPa (28 × 106psi); at 360 °C (680 °F), 179 GPa (26 × 106 psi); at 445

°C (830 °F), 165 GPa (24 × 106 psi); and at 490 °C (910

°F), 152 GPa (22 × 106 psi) Above 480 °C (900 °F), the value of the modulus of elasticity decreases rapidly

Density of cast steel is sensitive to changes in

composition, structure, and temperature The density of medium-carbon cast steel is about 7.8 Mg/m3 (490 lb/ft3) The density of cast steel is also affected somewhat by mass or size of section (Fig 13d)

Electrical properties of carbon and low-alloy steel castings do not significantly affect usage The only electrical

property that may be regarded as having any importance is resistivity, which, for various annealed carbon steel castings with 0.07 to 0.20% C, is 0.13 to 0.14 μΩ· m Resistivity increases with carbon content and is about 0.20 μΩ· m at 1.0% C

Magnetic Properties Steel castings from the housings for electrical machinery and magnetic equipment and carry

only stray fluxes around the machines; hence, the magnetic properties of steel castings are less important than they were formerly when core material was manufactured from commercial cast iron and steel Low-carbon cast dynamo steel has supplanted other cast metals for housings and frames for magnetic circuits

The carbon content of the steel is very important in determining the magnetic properties The maximum permeability and the saturation magnetization decrease, and the coercive force increases, as the carbon content increases Manganese, phosphorus, sulfur, and silicon also increase the magnetic hysteresis loss in cast steels This loss is equal to about 10 J/m3

per cycle for B = 1 T for each 0.10% Mn, 0.01% S, and 0.01% P Other factors being equal, the magnetic hysteresis loss

is unaffected by more than 0.02% P Magnetic properties change considerably, depending on the mechanical treatment and heat treatment of the steel

Cast dynamo steels contain about 0.10% C, with other alloying elements held to a minimum; the castings are furnished in the annealed condition Specifications require 0.05 to 0.15% C, 0.20% Mn, and 0.35 to 0.60 or 1.50 to 2.00% Si

The magnetic properties of annealed cast dynamo steel that may normally be expected are:

Maximum permeability, mH/m 18.6

Hysteresis loss (induction for H 1.91

Fig 16 Mechanical properties of water-quenched cast

8630 steel

= 11.9 kA/m), T

Saturation magnetization, T 2.14

Residual induction, T 1.10

Coercive force, A/m 29

As the carbon content is increased, maximum permeability and saturation magnetization decrease, and coercive force increases Also, an increase in manganese and sulfur content increase the magnetic hysteresis loss

Silicon and aluminum eliminate the allotropic transformation in iron and permit annealing at high temperature without recrystallization during cooling; thus, large grains can be obtained These elements can be added in large quantities without affecting magnetic properties, but they do reduce the saturation value and increase the brittleness of the metal Hysteresis loss varies directly with grain size number; therefore, the larger the grain size, the better the properties Residual alloy content should be low because it lowers saturation value

The factors that improve the machinability of dynamo steel decrease the magnetic properties A disadvantage in the use of pure iron for dynamo steel is low resistivity The iron must be rolled thin to keep eddy currents down; otherwise, the magnetic properties will be poor

Volumetric Changes In the foundry, all volume changes of a metal are pertinent, whether they occur in the liquid

state, during solidification, or in the solid state Of particular interest is the contraction that results when molten steel solidifies

Volume changes that occur in the liquid state as the cast metal cools affect the planning for adequate metal to fill the mold Contraction is of the order of 0.9% per 100 °C (180 °F) for a 0.30% C steel The exact amount of contraction will vary with the chemical composition, but it is usually within the range of 0.8 to 1.0% per 100 °C (180 °F) for carbon and low-alloy steels A larger contraction occurs upon solidification (2.2% for nearly pure iron to 4% for a 1.00% C steel) For cast carbon and low-alloy steels, a solidification contraction of 3.0% is generally assumed

The greatest amount of contraction occurs as the solidified metal cools to room temperature Solid-state contraction from the solidus to room temperature varies between 6.9 and 7.4% as a function of carbon content Alloying elements have no significant effect on the amount of this contraction The rigid form of the mold hinders contraction and results in the formation of stresses within the cooling casting that may be great enough to cause fracture or hot tears in the casting The hot metal has low strength just after solidification The rigidity of the mold makes the proper relation of casting configuration to accommodate this contraction one of the most important factors in producing a successful casting

In commercial production, a combination of all three contraction components may operate simultaneously Molten metal

in contact with the mold wall solidifies quickly and proceeds to solidify toward the center of the casting The solid envelope undergoes contraction in the solid state, while a portion of the still-molten metal is solidifying The remaining molten metal contracts as its temperature decreases toward the freezing point Because of contraction factors, many casting designs require considerable development to produce a sound casting

Engineering Properties

Wear Resistance Cast steels have wear resistance comparable to that of wrought steel of similar composition and

condition Chromium leads the field as an alloying element for wear-resistant steels but is seldom used alone vanadium, manganese-molybdenum, and nickel-manganese cast steels are used for numerous structural purposes requiring wear resistance and high strength

Nickel-Corrosion resistance of cast steel is similar to that of wrought steel of equivalent composition Data published on the

corrosion resistance of wrought carbon and low-alloy steels under various conditions may be applied to cast steels

Low-alloy steels are generally not considered corrosion resistant, and casting compositions are not normally selected on the basis of corrosion resistance In some environments, however, significant differences are observed in corrosion behavior such that the corrosion rate of one steel may be half that of another grade In general, steels alloyed with small amounts of copper tend to have somewhat lower corrosion rates than copper-free alloys As little as 0.05% Cu has been shown to exert a significant effect In some environments, nominal levels of nickel, chromium, phosphorus, and silicon may also bring about modest improvements, but when these four elements are present, the addition of copper holds little

if any advantage Detailed information on the corrosion resistance of steels is available in the articles "Corrosion of

Carbon Steels," "Corrosion of Alloy Steels," and "Corrosion of Cast Steels" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Soil Corrosion Cast steel pipe has been tested for various periods up to 14 years in different types of soil The results

of these tests were compared directly with results from tests on wrought steel pipe of similar composition, and no significant difference in the corrosion of the two materials could be detected However, the actual corrosion rate and rate

of pitting of the cast pipe varied widely, depending on the soil and aeration conditions

Elevated-Temperature Properties Steels operating at temperatures above ambient are subject to failure by a

number of mechanisms other than mechanical stress or impact These include oxidation, hydrogen damage, sulfide scaling, and carbide instability, which manifests itself as graphitization

The environmental factors involved in elevated-temperature service (370 to 650 °C, or 700 to 1200 °F) require that steels used in this temperature range be carefully characterized As a consequence, four ASTM specifications have been developed for cast carbon and low-alloy steels for elevated-temperature service One of these specifications, ASTM A

216, describes carbon steels; the other three, A 217, A 356, and A 389, cover low-alloy steels

The two alloying elements common to nearly all the steel compositions used at elevated temperatures are molybdenum and chromium Molybdenum contributes greatly to creep resistance Depending on microstructure, it has been shown that 0.5% Mo reduces the creep rate of steels by a factor of at least 103 at 600 °C (1110 °F)

Chromium also reduces the creep rate, although modestly, at levels to approximately 2.25% At higher chromium levels, creep resistance is somewhat reduced Vanadium improves creep strength and is indicated in some specifications Other elements that improve creep resistance include tungsten, titanium, and niobium The effect of tungsten is similar to that of molybdenum, but on a weight percent basis more tungsten is required in order to be equally beneficial Titanium and niobium have been shown to improve the creep properties of carbon-free alloys, but because they remove carbon from solid solution, their effect tends to be variable None of the latter three elements appears in U.S specifications for cast steels for elevated-temperature service

Low-Temperature Toughness. In addition to the soundness, strength, and microstructure of a metal, toughness, too,

is strongly affected by temperature Steel castings suitable for low-temperature service are specified in ASTM A 352 and

A 757 Figure 17 shows the effect of temperature on the impact resistance of three grades of cast steels conforming to ASTM A 352 Figure 17(b) also shows the effect of heat treatment on the impact resistance of grade LC2-1

Fig 17 Effect of temperature on the Charpy V-notch energy of a carbon steel and two low-alloy cast steels

specified in ASTM A 352 for low-temperature service (a) Charpy V-notch energies for a carbon cast steel, 0.30% C (max) with 1.00% Mn (max), quenched, tempered, and stress relieved, taken from a 50 × 230 × 210

mm (2 × 9 × 81

4in.) test block and from a 75 × 230 × 283 mm (3 × 9 × 111

8 in.) test block (b) Charpy notch energies for nickel-chromium-molybdenum cast steel specimens (taken from 50 × 230 × 210 mm, or 2 ×

V-9 × 81

4 in., test block) from steel with two different tempering and aging treatments after being air cooled from 955 °C (1750 °F), reheated to 900 °C (1650 °F), and then water quenched (c) Charpy V-notch energies for 21

2% Ni cast steel specimens (taken from 75 × 230 × 283 mm, or 3 × 9 × 111

8in., test block) after being air cooled (normalized, N) from 900 °C (1650 °F) and either tempered (T) at 620 °C (1150 °F) or reheated to

900 °C (1650 °F), water quenched (Q) and then tempered at 620 °C (1150 °F) All specimens were taken at locations greater than one-fourth the thickness in from the surface of test blocks having an ASTM grain size of 6

to 8 The curves represent average values for several tests at each test temperature

cast steel specified in ASTM A 216 Table 7 shows the effect of lower temperatures on the fracture toughness of five cast

steels As described below, the fracture toughness values in Table 7 are reported in terms of either JIc or Jc when

circumstances prevented the direct determination of KIc

Fig 18 Temperature dependence of plane-strain fracture toughness of a carbon steel casting (grade WCC of

ASTM A 216) Test blocks were 508 × 508 × 1219 mm (20 × 20 × 48 in.), annealed 8 h at 900 °C (1650 °F), furnace cooled to 315 °C (600 °F), tempered 8 h at 605 °C (1125 °F), furnace cooled to 315 °C (600 °F), reheated to 955 °C (1750 °F) and held 8 h, furnace cooled to 900 °C (1650 °F), equalized, accelerated cooled

to 95 °C (200 °F), final tempered 8 h at 650 °C (1200 °F), and air cooled Compact tension specimens of three thicknesses as indicated were used The open data points are the only symbols that indicate valid test results Source: Ref 4

Table 7 Fracture toughness values for five cast steels at room temperature and -45 °C (-50 °F)

Steel Room temperature

(a) Average value

50 to 500 mm (2 to 20 in.) for valid KIc determinations The larger dimensions are unreasonable for cost-effective testing

Therefore, the J-integral used to estimate fracture toughness characteristics without the use of large specimens For linear elastic plane strain, JIc is related to KIc for linear elastic plane strain:

2(1 ²)

where G is the strain energy release rate per unit crack extension, E is Young's modulus, and ν is Poisson's ratio

Valid values of JIc, and hence conservative estimates of KIc, were obtained for four of the cast steels at room temperature

and two cast steels at -45 °C (-50 °F) (Table 7) Only Jc, and hence estimates of Kc, were obtained for the other material

and temperature conditions The tests that did not produce valid JIc value were in the Charpy V-notch transition

temperature energy region Landes et al., using A 471 wrought steel, showed that Jc fracture toughness values obtained in

this region from small J-integral specimens had substantial scatter and that the lower limit of this Jc scatter band was

similar to that of equivalent toughness measured on larger specimens (Ref 6) Landes et al suggested that the lower boundary value of the Jc scatter may be reasonable to use for conservative-design criteria in the Charpy V-notch transition temperature region

As shown in Table 7, Mn-Mo cast steel exhibited the highest fracture toughness (JIc or KIc) at both room temperature and

-45 °C (-50 °F), while the 0050A cast steel showed the lowest fracture toughness (Jc or Kc) at both temperatures The three martensitic cast steels (C-Mn, Mn-Mo, and 8630) had better fracture toughness at room temperature than the two ferritic-pearlitic cast steels (0030 and 0050A) The 8630 cast steel had the largest decrease in fracture toughness at -45 °C

(-50 °F) compared to room temperature The C-Mn and Mn-Mo steels had ductile stable crack growth and the highest J values at both room temperature and -45 °C (-50 °F) Based on J-integral tests, they were the best steels at both

temperatures

Machinability Extensive lathe and drilling tests on steel castings have not revealed significant differences in the

machinability of steels made by different melting processes, nor of wrought and cast steel, provided strength, hardness, and microstructure are equivalent The skin or surface on a sand mold casting often wears down cutting tools rapidly, possibly because of adherence of abrasive mold materials to the casting Therefore, the initial cut should be deep enough

to penetrate below the skin, or the cutting speed may be reduced to 50% of that recommended for the base metal

Microstructure has considerable effect on the machinability of cast steels It is sometimes possible to improve the machining characteristics of a steel castings by 100% through normalizing, normalizing and tempering, or annealing

Weldability Steel castings have welding characteristics comparable to those of wrought steel of the same composition,

and welding these castings involves the same considerations

The severe quenching effect produced when using a small welding rod to weld a large section results in the formation of martensite in the base metal area immediately adjacent to the weld (in the heat-affected zone) This can happen even in low-carbon steel, causing loss of ductility in the heat-affected zone Usually cast steels with a maximum of 0.20% C and 0.50% Mn present fewer problems from this effect However, it is essential that all of the carbon steels (with more than 0.20% C) and the air-hardening alloyed steels be preheated before welding at the standard recommended temperatures, maintaining a proper interpass temperature, and then postweld heat treated to produce sufficient ductility

To prevent cracking in carbon and low-alloy cast steels, the hardness of the weld bead should not exceed 350 HV, except where conditions are such that only compressive forces result from the welding This value may not be low enough in configurations in which extreme restraint is involved

Virtually all castings receive a stress-relief heat treatment after welding, even composite fabrications in which steel castings are welded to wrought steel shapes

The maximum compositional limits that have been placed by the industry on readily weldable grades of castings are 0.35% C, 0.70% Mn, 0.30% Cr, and 0.25% Mo (max) plus W, with a total of 1.00% undesirable elements, predicated on the widespread use of stress-relief heat treatment in the steel casting industry For each 0.01% decrease in the specified maximum carbon content, most specifications permit an increase of 0.04% Mn above the maximum specified, up to a maximum of 1.00% (ASTM A 27, grade 70-40, and A 216, grade WCC, allow up to a maximum of 1.40% Mn) Specifications for weldable grades of cast steel are ASTM A 27, A 216, A 217, A 352, A 487, and A 757 Specifications covering control of weld quality are ASTM E 164 and E 390

Many welds that fail do not fail in the weld but in the zone immediately adjacent to the weld While the weld is being made, this zone is heated momentarily to a melting temperature The temperature decreases as the distance from the weld increases Such heating induces structural changes, specifically the development of hard, brittle areas adjacent to the weld deposits, which reduce the toughness of the area and frequently cause cracking during and after cooling Likewise, certain alloying elements other than carbon, such as nickel, molybdenum, and chromium, bring about air hardening of the base metal For these reasons, the quantity of alloying elements to be used must be limited unless special precautions are taken, such as the preheating of the base metal to 150 to 315 °C (300 to 600 °F) Increased hardness in the heat-affected zone of the base metal can be removed by postweld heat treating the welded casting or by heating it for definite periods at 650 to

675 °C (1200 to 1250 °F) This also relieves stresses from welding

For the arc welding of steel castings, a high-grade heavily coated electrode (AWS E7018 type), granular flux, or CO2

atmosphere is generally desirable These coatings contain little or no combustible material Mineral coatings are often used to keep hydrogen absorption at a minimum and thereby limit underbead cracking Selection of the number of passes and of welding conditions is similar to welding practice for wrought steels

Welds in castings may be radiographed by gamma- or x-ray methods to ascertain the degree of homogeneity of the welded section The most common imperfections are incomplete fusion, slag inclusions, and gas bubbles Magnetic particle inspection is also useful in the detection of surface and near-surface cracks

The mechanical properties of welds joining cast steel to cast steel and of welds joining cast steel to wrought steel are of the same order as similar welds joining wrought steel to wrought steel Most tensile specimens machined across the weld break outside the weld, in the heat-affected zone This does not mean that the weld is stronger than the casting base metal Closely controlled welding techniques and stress relieving are necessary to prevent brittleness in the heat-affected zone

References cited in this section

3 "Fatigue and Fracture Toughness of Five Carbon or Low-Alloy Steels at Room or Low Climatic Temperatures," Research Report 9A, Steel Founders' Society of America, Oct 1982

4 H.D Greenberg and W.G Clark, Jr., A Fracture Mechanics Approach to the Development of Realistic

Acceptance Standards for Heavy Walled Steel Castings, Met Eng Q., Vol 9 (No 3), Aug 1969, p 30-39

5 J.R Rice, A Path Independent Integral and the Approximate Analysis of Strain Concentration by Notches

and Cracks, J Appl Mech., Vol 35, June 1968, p 379

6 J.D Landes and D.H Shaffer, Statistical Characteristics of Fracture in the Transition Region, in Fracture Mechanics, STP 700, American Society for Testing and Materials, 1980, p 368

Nondestructive Inspection

Highly stresses steel castings for aircraft and for high-pressure or high-temperature service must pass rigid nondestructive inspection ASTM specifications E 186, E 280, and E446 cover radiographic standards for steel castings; E 94 covers ASTM recommended practice for radiographic testing; and E 142 covers the quality control of radiographic testing

Radiographic acceptance standards must be agreed upon by the user and producer before production begins Critical areas

to be radiographed may be identified on the casting drawing

Magnetic particle inspection is used on highly stressed steel castings to detect surface discontinuities or imperfections at

or just below the surface ASTM E 709 should be consulted for establishing cause for rejection before the castings are produced The imperfections are evaluated by using ASTM E 125, "Reference Photographs for Magnetic Particle Indications on Ferrous Castings." The extent of inspection and the acceptance standards must be agreed upon by the user and producer at the time of the order

Liquid penetrant inspection can be used on steel castings, but it is primarily used to inspect nonmagnetic materials such as nonferrous metals and austenitic steels for possible surface discontinuities ASTM E 165 is the specification used, and E

433 gives the standard reference photographs for liquid penetrant inspection

Ultrasonic testing is sometimes used on steel castings to detect imperfections below the surface in heavy sections that are 0.3 to 8.5 m (1 to 28 ft) thick Test surfaces of castings should be free of material that would interfere with the ultrasonic test These castings may be as cast, blasted, ground, or machined The standard specification for ultrasonic testing is ASTM A 609, "Longitudinal-Beam Ultrasonic Inspection of Carbon and Low-Alloy Steel Castings." This technique is intended to complement ASTM recommended practice E 94 for the use of radiographic testing in the detection of discontinuities

Hydrostatic testing, or pressure testing, is used on valves and castings intended to contain steam or fluids, such as those made to ASTM specifications A 216, A 217, A 352, A 356, A 389, and A 487 If a casting must pass a pressure test, essential factors must be noted on the blueprint, and the details of the test should be understood by the buyer and producer

References

1 J.M Barson and S.T Rolfe, Correlation Between Charpy V-Notch Test Results in the

Transition-Temperature Range, in Impact Testing of Metals, Special Technical Publication 466, American Society for

Testing and Materials, 1970, p 280-302

2 M.T Groves and J.F Wallace, Cast Steel Plane Fracture Toughness, Charpy V-Notch and Dynamic Tear

Test Correlations, Steel Cast Res., No 70, March 1975, p 1-9

3 "Fatigue and Fracture Toughness of Five Carbon or Low-Alloy Steels at Room or Low Climatic Temperatures," Research Report 9A, Steel Founders' Society of America, Oct 1982

4 H.D Greenberg and W.G Clark, Jr., A Fracture Mechanics Approach to the Development of Realistic

Acceptance Standards for Heavy Walled Steel Castings, Met Eng Q., Vol 9 (No 3), Aug 1969, p 30-39

5 J.R Rice, A Path Independent Integral and the Approximate Analysis of Strain Concentration by Notches

and Cracks, J Appl Mech., Vol 35, June 1968, p 379

6 J.D Landes and D.H Shaffer, Statistical Characteristics of Fracture in the Transition Region, in Fracture Mechanics, STP 700, American Society for Testing and Materials, 1980, p 368

Bearings for normal service conditions, a category that includes over 95% of all rolling-element bearings, are applicable when:

• Maximum temperatures are of the order of 120 to 150 °C (250 to 300 °F), although brief excursions to

175 °C (350 °F) may be tolerated

• Minimum ambient temperatures are about -50 °C (-60 °F)

• The contact surfaces are lubricated with such materials as oil, grease, or mist

• The maximum Hertzian contact stresses are of the order of 2.1 to 3.1 GPa (300 to 450 ksi)

Bearings used under normal service conditions also experience the effects of vibration, shock, misalignment, debris, and handling Therefore, the fabrication material must provide toughness, a degree of temper resistance, and microstructural stability under temperature extremes The material must also exhibit the obvious requirement of surface hardness for wear and fatigue resistance

Bearing Steels

Harold Burrier, Jr., The Timken Company

High-Carbon or Low-Carbon Steels

Traditionally, bearings have been manufactured from both carbon (1.00%) and low-carbon (0.20%) steels The carbon steels are used in either a through-hardened or a surface induction-hardened condition in special integral bearing configurations, such as the automotive wheel spindle shown in Fig 1 Low-carbon bearing steels are carburized to provide the necessary surface hardness while maintaining other desirable properties in the core

high-Fig 1 Cross-section of a surface-hardened high-carbon steel automotive spindle

The parallel development of both high- and low-carbon steels for bearing applications is rooted in history The early European manufacturers chose to use familiar chromium-type tool steels The American bearing manufacturers, on the other hand, added a carburized case to their soft, plain steel bearings to meet the higher hardness requirements of more highly loaded rolling-element bearings

Both high-carbon and low-carbon materials have survived because each offers a unique combination of properties that best suits the intended service conditions For example, high-carbon steels:

• Can carry somewhat higher contact stresses, such as those encountered in point contact loading in ball bearings

• Can be quenched and tempered, which is a simpler heat treatment than carburizing

• May offer greater dimensional stability under temperature extremes because of their characteristically lower content of retained austenite

Carburizing steels, on the other hand, offer:

• Greater surface ductility (because of their retained austenite content) to better resist the stress-raising effects of asperities, misalignment, and debris particles

• A higher level of core toughness to resist through-section fracture under severe service conditions

• A compressive residual surface stress condition to resist bending loads imposed on the ribs of roller bearings and reduce the rate of fatigue crack propagation through the cross section

• Easier machining of the base material in manufacturing

Figure 2 illustrates this last point in a comparison of tool life in machining tubing of a high-carbon steel (AISI 52100) and

a carburizing steel (4320 mod) at similar hardness levels

Fig 2 Comparison of tool life at various cutting speeds in the machining of 52100 and 4320 steel tubing

Table 1 compares selected mechanical properties of the core material in a quenched and tempered carburizing steel with those of the unhardened portion of an induction-hardened high-carbon steel component The values for both strength and

toughness are greater in the quenched and tempered low-carbon steel The surface properties are largely a function of the hardness

Table 1 Core properties of carburized versus induction-hardened components

Yield strength Ultimate tensile strength Impact energy Material Hardness, HB

Machining, %(a)

8620(b) 30-40 HRC 825-965 120-140 1105-1240 160-180 55-110 40-80 65

5160(c) 197 275 40 725 105 10 7 55

(a) Where 1212 represents 100%

(b) Quenched and tempered

(c) Annealed

In rolling-contact bearings, it is essential to maintain an adequate strength throughout the region of maximum subsurface shear stresses Figure 3 shows an estimated relationship between hardness and shear yield strength that is applicable to either steel type The success of a given steel in a bearing application is not as much a function of the steel type as how it

is treated Fatigue resistance generally increases with hardness; the maximum depends on the steel type Figure 4 compares the bending fatigue lives of through-carbon and carburized steels as a function of surface hardness In bending fatigue, the combination of compressive residual surface stresses with a higher composite section toughness gives the advantage to the carburized steel

Fig 3 Estimated relationship between hardness and shear yield strength

Fig 4 Rotating-beam fatigue strength of case-hardening, through-hardening, and tool steels as a function of

surface hardness (a) Testpiece diameter of 6 mm (0.25 in.), triangular torque (b) Testpiece diameter of 12

mm (0.5 in.), constant torque Source: Ref 1

Reference cited in this section

1 The Influence of Microstructure on the Properties of Case-Hardened Components, American Society for

Metals, 1980

Bearing Steels

Harold Burrier, Jr., The Timken Company

Microstructure Characteristics

High-Carbon Bearing Steels Figure 5 shows a typical microstructure of a hardened and tempered high-carbon

bearing steel such as AISI 52100 The matrix is high-carbon martensite, containing primary carbides and 5 to 10% retained austenite The hardness throughout the section is typically 60 to 64 HRC Table 2 lists the compositions of selected high-carbon bearing steels currently in use The first three grades are listed in order of increasing hardenability; they are applied to bearing sections of increasing thickness to ensure freedom from nonmartensitic transformations in hardening Grade TBS-9 is a lower-chromium bearing steel, which, because of its residual alloy content, has a hardenability similar to that of AISI 52100 The remaining steels are representative of overseas steels applied to bearing components

Table 2 Nominal compositions of high-carbon bearing steels

(c) Russian grade

Fig 5 Typical microstructure of a high-carbon through-hardened bearing component

Carburizing Bearing Steels Figure 6 shows typical case and core microstructures of carburized bearing components

The case microstructure consists of high-carbon martensite with retained austenite in the range of 15 to 40% Case hardness is typically 58 to 64 HRC In the core of carburized bearings, the microstructure consists of low-carbon martensite; it also often contains variable amounts of bainite and ferrite The core hardness may vary from 25 to 48 HRC

Fig 6 Typical microstructures of carburized bearing components (a) Case (b) Core

Table 3 lists the compositions of typical carburizing bearing steels The AISI grades are listed in approximate order of increasing hardenability or section size applicability SCM420 and 20MnCr5 are Japanese and German grades, respectively, found in carburized bearing components In addition to standard AISI grades, bearing steels can also be designed so that their hardenability matches the requirements of specific section thicknesses Alloy conservation and a more consistent heat-treating response are benefits of using specially designed bearing steels

Table 3 Carburizing bearing steels

in Fig 7 In particular, the presence of pearlite, resulting from a mismatch of quenching conditions and case hardenability,

is shown to have a detrimental effect

Fig 7 Effect of surface microstructure on the shape of S-N curve for surface fatigue (pitting) Source: Ref 1

Reference cited in this section

1 The Influence of Microstructure on the Properties of Case-Hardened Components, American Society for

Metals, 1980

Bearing Steels

Harold Burrier, Jr., The Timken Company

Bearing Steel Quality

Apart from a satisfactory microstructure, which is obtained through the proper combination of steel grade and heat treatment, the single most important factor in achieving high levels of rolling-contact fatigue life in bearings is the cleanliness, or freedom from harmful nonmetallic inclusions, of the steel Bearing steels can be produced by one of these techniques:

• Clean-steel air-melt practices

• Electroslag remelting

• Air melting followed by vacuum are remelting

• Vacuum induction melting followed by vacuum arc remelting

Cleanliness, cost and reliability can increase depending on which practice is chosen

Bearing steel cleanliness is most commonly rated by using microscopic techniques, such as those defined in ASTM A 295 for high-carbon steels and A 534 for carburizing steels The worst fields found in metallographically prepared sections of the steel can be compared with rating charts (J-K charts) according to the type of inclusion: sulfides, stringer-type oxides, silicates, and globular-type oxides

Table 4 lists the current levels of each of the inclusion types allowed for air-melted bearing quality steels Several manufacturers produce bearings with significantly lower levels of nonmetallic inclusion content than allowed by rating charts

Table 4 Nonmetallic inclusion ratings

Specification

Thin Heavy Thin Heavy Thin Heavy Thin Heavy

ASTM A 534-76 (carburizing steel) 3.0 2.0 3.0 2.5 2.5 1.5 2.0 1.5

Bearing steel cleanliness can also be rated by oxygen analysis, the magnetic particle method (AMS 2301, AMS 2300), and ultrasonic methods Of these, the ultrasonic method appears to show a superior correlation with bearing fatigue life when the oxygen content of the steel is less than 20 ppm This is due to the larger volume of material sampled by the technique

Bearing Steels

Harold Burrier, Jr., The Timken Company

Effect of Heat Treatment

The importance of matching the hardenability and quenching of a bearing steel has been pointed out above However, within this restriction, other heat treatment variables have been found to affect the performance of bearings, particularly under the less-than-ideal conditions of debris contamination

Retained austenite in the microstructure is known to reduce the surface hardness of high-carbon steels, as shown in Fig 8 The ductility of the surface, as expressed by the ratio of yield strength to fracture strength in Fig 9, is improved by increasing amounts of retained austenite This improved ductility often results in improved rolling-contact fatigue performance Figure 10 illustrates this improvement by showing the results of a fatigue performance study of 14 carburizing steel compositions The retained austenite level can be raised by adding nitrogen to the case (Fig 11) or by increasing carburizing cycles (Fig 12) Improved performance has been demonstrated in high-carbon steels when carburizing increased the carbon in solution and the austenite content Figure 13 shows the improved performance of carburized AISI 52100 steel bars in comparison with uncarburized bars of the same material Other means of increasing surface ductility, such as optimizing the austenitizing temperature (Fig 14), controlling the quenching rate (Fig 15), and austempering to produce a completely bainitic microstructure (Fig 16), have been shown to improve the fatigue performance of high-carbon bearing steels for certain applications

Fig 8 Influence of retained austenite on the surface hardness of carburized alloy steels (reheat quenched and

tempered at 150 to 185 °C, or 300 to 365 °F) Source: Ref 1

Fig 9 Influence of retained austenite on surface ductility (a) Yield strength data (b) Yield-to-fracture-strength

ratio

Fig 10 Variation of 90% life of rolling fatigue with the amount of retained austenite Source: Ref 2

Fig 11 Effect of carbonitriding to increase retained austenite on rolling-contact fatigue Source: Ref 3

Fig 12 Performance of through-hardened and carburized bearings in a debris environment with a load of 17.6

kN (1800 kgf) and a rotation speed of 2000 rev/min Source: Ref 4

Fig 13 Effect of surface carbon enrichment of 52100 steel on rolling-contact performance A, carburized test

bars; B, uncarburized test bars Source: Ref 5

Fig 14 Effect of heat treatment on hardness and life of 52100 steel Source: Ref 6

Fig 15 Effect of controlling quench severity on the performance of 52100 steel bearings Source: Ref 7