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ASM Metals Handbook Volume 01 - Properties and Selection Irons, Steels, and High-Performance Allo

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3.36 0.61 2.75 0.74 1.96 0.35 0.52 0.47 0.158 0.070
(a) CC, combined carbon. (b) GC, graphite carbon. Source: Ref 18
Mechanical Properties. As-quenched gray iron is brittle. Tempering after quenching improves strength and toughness but
decreases hardness. A temperature of about 370 °C (700 °F) is required before the toughness (impact strength) approaches the
as-cast level. The tensile strength after tempering may be from 35 to 45% greater than the strength of the as-cast iron. Changes in
properties brought about by quenching and tempering are shown in Fig. 20 .
Fig. 20 Changes in mechanical properties of gray iron as a function of tempering temperature
mm
1=16 in.
increments Plain Iron Mo(A) Mo(B) Ni-Mo Cr-Mo Cr-Ni-Mo
3.2 2 54 56 53 54 56 55
6.4 4 53 56 52 54 55 55
9.5 6 50 56 52 53 56 54
12.7 8 43 54 51 53 55 54
15.9 10 37 52 50 52 55 53
19.0 12 31 51 49 52 54 53
22.2 14 26 51 46 52 54 52
25.4 16 26 49 45 52 54 53
28.6 18 25 46 45 52 53 52
31.8 20 23 46 44 51 50 51
34.9 22 22 45 43 47 50 50
38.1 24 22 43 44 47 49 50
41.3 26 21 43 44 47 47 49
44.4 28 20 40 41 45 47 48
47.6 30 19 39 40 45 44 50
50.8 32 17 39 40 45 41 47
54.0 34 18 36 41 44 38 46
57.2 36 18 40 40 45 36 45
60.3 38 19 38 37 45 34 46
63.5 40 22 38 36 42 35 46
66.7 42 20 35 35 42 32 45
Source: Ref 18
Table 27 Compositions of irons for which hardenability data are given in Table 26
Iron
Composition
TC CC(a) GC(b) Mn Si Cr Ni Mo P S
Plain 3.19 0.69 2.50 0.76 1.70 0.03 ... 0.013 0.216 0.097
Mo(A) 3.22 0.65 2.57 0.75 1.73 0.03 ... 0.47 0.212 0.089
Mo(B) 3.20 0.58 2.62 0.64 1.76 0.005 Trace 0.48 0.187 0.054
Ni-Mo 3.22 0.53 2.69 0.66 2.02 0.02 1.21 0.52 0.114 0.067
Cr-Mo 3.21 0.60 2.61 0.67 2.24 0.50 0.06 0.52 0.114 0.071
Cr-Ni-Mo 3.36 0.61 2.75 0.74 1.96 0.35 0.52 0.47 0.158 0.070
(a) CC, combined carbon. (b) GC, graphite carbon. Source: Ref 18
Mechanical Properties. As-quenched gray iron is brittle. Tempering after quenching improves strength and toughness but
decreases hardness. A temperature of about 370 °C (700 °F) is required before the toughness (impact strength) approaches the
as-cast level. The tensile strength after tempering may be from 35 to 45% greater than the strength of the as-cast iron. Changes in
properties brought about by quenching and tempering are shown in Fig. 20 .
Fig. 20 Changes in mechanical properties of gray iron as a function of tempering temperature
ASM Handbook,Volume 1 Gray Iron 01 Sep 2005
Copyright ASM International. All Rights Reserved. Page 60
Heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings, because the strength
of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements.
When gray iron is quenched and tempered, it usually is done to increase resistance to wear and abrasion by increasing hardness.
A structure consisting of graphite embedded in a hard martensitic matrix is produced by heat treatment. Localized heat treatment
such as flame or induction hardening can be used in some applications in which alloy iron or chilled iron has traditionally been
used, often with a savings in cost. Heat treatment can be used when chilling is not feasible, as with complicated shapes or large
castings, or when close tolerances that can be attained only by machining are required. Heat treatment extends the field of
application of gray iron as an engineering material.
The hardness of quenched and tempered gray iron measured with either a Brinell or Rockwell C tester is a composite hardness
that superimposes the effects of graphite type, distribution, and size on the hardness of the metal matrix. The true hardness of the
matrix, measured with a microhardness test, is generally from 8 to 10 HRC points higher than the hardness indicated by the
conventional Rockwell C test. This is shown in Fig. 21 , which compares composite hardness with matrix hardness. The
composite hardness was obtained by conventional Rockwell testing; the matrix hardness was measured with a Vickers indentor
and a 200 g load, and the values were converted to equivalent Rockwell hardness. The hardness of the matrix and the change in
matrix hardness with tempering temperature are about the same as in an alloy steel containing approximately 0.7% C.
Fig. 21 Effect of tempering temperature on the hardness of quenched gray iron (3.30% C, 2.35% Si)
Heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings, because the strength
of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements.
When gray iron is quenched and tempered, it usually is done to increase resistance to wear and abrasion by increasing hardness.
A structure consisting of graphite embedded in a hard martensitic matrix is produced by heat treatment. Localized heat treatment
such as flame or induction hardening can be used in some applications in which alloy iron or chilled iron has traditionally been
used, often with a savings in cost. Heat treatment can be used when chilling is not feasible, as with complicated shapes or large
castings, or when close tolerances that can be attained only by machining are required. Heat treatment extends the field of
application of gray iron as an engineering material.
The hardness of quenched and tempered gray iron measured with either a Brinell or Rockwell C tester is a composite hardness
that superimposes the effects of graphite type, distribution, and size on the hardness of the metal matrix. The true hardness of the
matrix, measured with a microhardness test, is generally from 8 to 10 HRC points higher than the hardness indicated by the
conventional Rockwell C test. This is shown in Fig. 21 , which compares composite hardness with matrix hardness. The
composite hardness was obtained by conventional Rockwell testing; the matrix hardness was measured with a Vickers indentor
and a 200 g load, and the values were converted to equivalent Rockwell hardness. The hardness of the matrix and the change in
matrix hardness with tempering temperature are about the same as in an alloy steel containing approximately 0.7% C.
Fig. 21 Effect of tempering temperature on the hardness of quenched gray iron (3.30% C, 2.35% Si)
ASM Handbook,Volume 1 Gray Iron 01 Sep 2005
Copyright ASM International. All Rights Reserved. Page 61
After tempering at 370 °C (700 °F) for maximum toughness, the hardness of the metal matrix is still about 50 HRC. Where
toughness is not required and a tempering temperature of 150 to 260 °C (300 to 500 °F) is acceptable, the matrix hardness is
equivalent to 55 to 60 HRC. High matrix hardness and the presence of graphite result in a surface with good wear resistance for
some applications, for instance, farm implement gears, sprockets, diesel cylinder liners, and automotive camshafts.
Dimensional changes resulting from the hardening of gray iron are quite uniform and predictable if the prior structure and
composition are uniform. Such dimensional changes can be allowed for in machining before heat treatment.
Complex shapes or nonuniform sections may be distorted as a result of the relief of residual casting stresses or a differential
rate of cooling and hardening during quenching, or both. The former condition can be minimized by stress relieving at about 565
to 590 °C (1050 to 1100 °F) before machining. The latter can be minimized by either marquenching or austempering; both of
these processes are used for cylinder liners where out-of-roundness must be held to a minimum. Through hardening is employed
for gears, sprockets, hub bearings, and clutches.
Localized Hardening. In parts requiring only localized areas of hardness, conventional induction hardening or flame
hardening may be used.
For flame hardening, it is generally desirable to alloy the iron with