Alloy Steels

Alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of such alloying elements as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of approximately 11% Cr. For many alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions. Alloy steels can be classified according to:

• Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels, and so on, as described in SAE-AISI designations and shown in Table 4

• Heat treatment, such as quenched and tempered, normalized and tempered, annealed, and so on Table 4 SAE-AISI system of designations for carbon and alloy steels

Numerals and digits

Type of steel and nominal alloy content, %

Carbon steels

10xx

Plain carbon (Mn 1.00 max)

11xx

Resulfurized

12xx

Resulfurized and rephosphorized

15xx

Plain carbon (max Mn range: 1.00-1.65)

Manganese steels

13xx

Mn 1.75

Nickel steels

23xx

Ni 3.50

25xx

Ni 5.00

Nickel-chromium steels

31xx

Ni 1.25; Cr 0.65 and 0.80

32xx

Ni 1.75; Cr 1.07

33xx

Ni 3.50; Cr 1.50 and 1.57

34xx

Ni 3.00; Cr 0.77

Molybdenum steels

40xx

Mo 0.20 and 0.25

44xx

Mo 0.40 and 0.52

Chromium-molybdenum steels

41xx

Cr 0.50, 0.80, and 0.95; Mo 0.12, 0.20, 0.25, and 0.30

Nickel-chromium-molybdenum steels

43xx

Ni 1.82; Cr 0.50 and 0.80; Mo 0.25

43BVxx

Ni 1.82; Cr 0.50; Mo 0.12 and 0.25; V 0.03 min

47xx

Ni 1.05; Cr 0.45; Mo 0.20 and 0.35

81xx

Ni 0.30; Cr 0.40; Mo 0.12

86xx

Ni 0.55; Cr 0.50; Mo 0.20

87xx

Ni 0.55; Cr 0.50; Mo 0.25

88xx

Ni 0.55; Cr 0.50; Mo 0.35

93xx

Ni 3.25; Cr 1.20; Mo 0.12

94xx

Ni 0.45; Cr 0.40; Mo 0.12

97xx

Ni 0.55; Cr 0.20; Mo 0.20

98xx

Ni 1.00; Cr 0.80; Mo 0.25

Nickel-molybdenum steels

46xx

Ni 0.85 and 1.82; Mo 0.20 and 0.25

48xx

Ni 3.50; Mo 0.25

Chromium steels

50xx

Cr 0.27, 0.40, 0.50, and 0.65

51xx

Cr 0.80, 0.87, 0.92, 0.95, 1.00, and 1.05

Chromium (bearing) steels

50xxx

Cr 0.50

C 1.00 min

51xxx

Cr 1.02

Cr 1.45

Chromium-vanadium steels

61xx

Cr 0.60, 0.80, and 0.95; V 0.10 and 0.15 min

Tungsten-chromium steel

72xx

W 1.75; Cr 0.75

Silicon-manganese steels

92xx

Si 1.40 and 2.00; Mn 0.65, 0.82, and 0.85; Cr 0 and 0.65

High-strength low-alloy steels

9xx

Various SAE grades

Boron steels

xxBxx

B denotes boron steel

Leaded steels

L denotes leaded steel

Note: The xx in the last two digits of these designations indicates that the carbon content (in hundredths of a percent) is to be inserted.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated condition, some overlap exists among the alloy steel classifications, which include (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa, or 50 to 150 ksi) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. The chemical compositions of typical QT low-carbon steels are given in Table 5. Many of the steels are covered by ASTM specifications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other similar steels, are produced as forgings or castings.

Table 5 Chemical compositions for typical alloy steels

Steel

Composition, wt%(a)

C

Si

Mn

P

S

Ni

Cr

Mo

Other

Low-carbon quenched and tempered steels

A 514/A 517 grade A

0.150.21

0.400.80

0.801.10

0.035

0.04

0.500.80

0.180.28

0.05-0.15 Zr(b), 0.0025 B

A 514/A 517 grade F

0.100.20

0.150.35

0.601.00

0.035

0.04

0.701.00

0.400.65

0.400.60

0.03-0.08 V, 0.15-0.50 Cu 0.0005-0.005 B

A 514/A 517 grade R

0.150.20

0.200.35

0.851.15

0.035

0.04

0.901.10

0.350.65

0.150.25

0.03-0.08 V

A 533 type A

0.25

0.150.40

1.151.50

0.035

0.04

0.450.60

A 533 type C

0.25

0.150.40

1.151.50

0.035

0.04

0.701.00

0.450.60

HY-80

0.120.18

0.150.35

0.100.40

0.025

0.025

2.003.25

1.001.80

0.200.60

0.25 Cu, 0.03 V, 0.02 Ti

HY-100

0.120.20

0.150.35

0.100.40

0.025

0.025

2.253.50

1.001.80

0.200.60

0.25 Cu, 0.03 V, 0.02 Ti

Medium-carbon ultrahigh-strength steels

4130

0.280.33

0.200.35

0.400.60

0.801.10

0.150.25

4340

0.380.43

0.200.35

0.600.80

1.652.00

0.700.90

0.200.30

300M

0.400.46

1.451.80

0.650.90

1.652.00

0.700.95

0.300.45

0.05 V min

D-6a

0.420.48

0.150.30

0.600.90

0.400.70

0.901.20

0.901.10

0.05-0.10 V

Carburizing bearing steels

4118

0.180.23

0.150.30

0.700.90

0.035

0.040

0.400.60

0.080.18

5120

0.170.22

0.150.30

0.700.90

0.035

0.040

0.700.90

3310

0.080.13

0.200.35

0.450.60

0.025

0.025

3.253.75

1.401.75

Through-hardened bearing steels

52100

0.981.10

0.150.30

0.250.45

0.025

0.025

1.301.60

A 485 grade 1

0.901.05

0.450.75

0.951.25

0.025

0.025

0.25

0.901.20

0.10

0.35 Cu

A 485 grade 3

0.951.10

0.150.35

0.650.90

0.025

0.025

0.25

1.101.50

0.200.30

0.35 Cu

Chromium-molybdenum heat-resistant steels

21Cr-1Mo 4

0.15

0.50

0.300.60

0.040

0.040

2.002.50

0.871.13

5Cr-4 Mo 2

0.15

0.50

0.300.60

0.030

0.030

4.006.00

0.450.65

9Cr-1Mo

0.15

0.501.00

0.300.60

0.030

0.030

8.0010.00

0.901.10

(a) Single values represent the maximum allowable.

(a) Single values represent the maximum allowable.

(b) Zirconium may be replaced by cerium. When cerium is added, the cerium/sulfur ratio should be approximately 1.5/1, based on heat analysis.

Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa (200 ksi). Table 5 lists typical compositions. Many of these steels are covered by SAE-AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.

Bearing steels used for ball and roller bearing applications are comprised of low-carbon (0.10 to 0.20% C) case-hardened steels and high-carbon (~1.0% C) through-hardened steels (Table 5). Many of these steels are covered by SAE-AISI designations.

Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.20%. Table 5 lists typical compositions. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered, or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants. Product forms include forgings, tubing, pipe, castings, and plate.

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