Microstructure Modification

Wrought steels are frequently heat treated by producers to obtain optimal machinability. Annealing or normalizing treatments are used to produce a relatively coarse pearlitic or spheroidal microstructure that possesses good machinability. Higher carbon steels require longer heat treatment cycles intended to produce coarse carbides dispersed in ferrite. In contrast, low-carbon steels can be partially hardened to produce a microstructure that is less ductile and adhesive than fully annealed steels. Research has shown that reducing cooling rates from sintering can improve the machinability of FC-0208; however, the coarser microstructure reduces mechanical properties (Ref 35). A significant improvement in machinability indicated by drill motor load is only obtained at lower strength levels (Table 22).

Table 22 Comparison of mechanical properties and motor load

Cooling rate

Yield

Ultimate tensile

Hardness,

Motor load,

strength, ksi

strength, ksi

HRB

hp

8.75

36.7

36.7

48.9

4.39

16.7

50.5

50.5

57.2

4.55

39.8

58.3

58.3

73.0

4.76

Sintered density, 6.8 g/cm3

Sintered density, 6.8 g/cm3

A similar result is obtained in improving the machinability of a 0.85%Mo-2%Ni-0.5% graphite by tempering and annealing. The annealing treatments do not produce the anticipated improved machinability. The 0.85% molybdenum steel also proves to be very temper resistant (Table 23). It is noteworthy that the annealing treatment at 871 °C (1600 °F) changes the failure mode of the high-speed steel drill. The drill appears to adhere to the workpiece and snap rather than overheat.

Table 23 Ancorsteel 85 HP: 2 wt% Ni, 0.5 wt% graphite

Treatment

Temperature, °C (°F)

Time, h Cool

Holes to failure

Hardness, HRB

Temper

149 (300)

1 Natural

4

78

302 (575)

1 Natural

8

71

454(850)

1 Natural

14

82

621 (1125)

1 Natural

6

74

Anneal

871 (1600)

1 Furnace

7

57

As-sintered

3

73

The experiment indicates that the heat treatments employed did not change the microstructure and machinability of P/M steels sufficiently. The results do not justify the extra process step. Further study is necessary to define the optimal microstructure and heat treatment for machinability. It is possible that increasing tempering temperatures to 648 to 732 °C (1200 to 1350 °F) for subcritical annealing or controlled transformation annealing after sintering can improve machinability further. Because such treatments add an extra process step, they are justified only where parts require extensive machining prior to heat treatment.

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