Induction Hardening of Gray Iron

Gray iron castings can be surface hardened by the induction method when the number of castings to be processed is large enough to warrant the relatively high equipment cost and the need for special induction coils. Considerable variation in the hardness of the cast irons may be expected because of a variation in the combined carbon content. A minimum combined carbon content of 0.40 to 0.50% C (as pearlite) is recommended for cast iron to be hardened by induction, with the short heating cycles that are characteristic of this process. Heating castings with lower combined carbon content to high hardening temperatures for relatively long periods of time may dissolve some free graphite, but such a procedure is likely to coarsen the grain structure at the surface and will result in undesirably large amounts of retained austenite in the surface layers. The recommended minimum induction hardening temperature for gray iron is 870 to 925 °C (1600 to 1700 °F).

The surface hardness attained from the induction hardening of gray iron is influenced by the carbon equivalent (%C + 1/3% Si) when this hardness is measured by conventional Rockwell tests. The more graphite that is present in the microstructure, the lower the surface hardness will appear to be after hardening. Table 10 shows the surface hardness of induction-hardened gray iron castings of various carbon equivalents from 3.63 to 4.23. The microstructure of these castings, which were cast in the same manner and cooled at similar rates, contained more and larger graphite flakes as the carbon equivalent increased. This resulted in lower apparent surface hardness after hardening, yet the hardened matrix was consistently 57 to 61 HRC (converted from microhardness).

Table 10 Effect of carbon equivalent on surface hardness of induction-hardened gray irons

Composition, %(a)

Carbon equivalent(b)

Hardness HRC, converted from

C

Si

As read

Rockwell 30-N

Microhardness

3.13

1.50

3.63

50

50

61

3.14

1.68

3.70

49

50

57

3.19

1.64

3.74

48

50

61

3.34

1.59

3.87

47

49

58

3.42

1.80

4.02

46

47

61

3.46

2.00

4.13

43

45

59

3.52

2.14

4.23

36

38

61

(a) Each iron also contained 0.50 to 0.90 Mn, 0.35 to 0.55 Ni, 0.08 to 0.15 Cr, and 0.15 to 0.30 Mo.

(a) Each iron also contained 0.50 to 0.90 Mn, 0.35 to 0.55 Ni, 0.08 to 0.15 Cr, and 0.15 to 0.30 Mo.

Distortion. Induction hardening causes less distortion than would a similar quenching treatment from a furnace. The maximum warpage in a 560 mm (22 in.) length of bar was found to be 0.03 mm (0.0015 in.) after induction hardening, compared with 0.17 to 0.25 mm (0.007 to 0.010 in.) for the same bars quenched from a furnace. For thin-walled cylinders, distortion is not a problem unless the thickness of the induction-hardened layer exceeds 20% of the wall thickness.

For selective hardening, in place or by scanning, to depths up to 3.8 mm (0.150 in.), water quenching can be used safely. However, the hardening of keyways, cross-drilled holes, or extremely thin walls may require the use of oil to prevent excessive distortion or cracking.

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