THE HEAT -TREATMENT AND MICROSTRUCTURE OF A SINTERED NICKEL-MOLYBDENUM STEEL

Abstract

The effects of post-sintering heat-treatment on the properties and microstructure of a 5Ni–0·5Mo–0·5C sintered steel have been studied in detail. It has been demonstrated that by using a relatively high tempering temperature after oil-quenching, UTS and elongation can be simultaneously raised above their as-sintered values, while impact-resistance is at least maintained. By tempering at 650–675°C a UTS of 900 N/mm² can thus be combined with 4% tensile elongation and with a Charpy unnotched impact value of 26 J. Properties close to these can be achieved even if oil-quenching is replaced by cooling in a furnace cold zone. Alternatively, by quenching and then tempering at 200°C the UTS can be raised to ∼1400 N/mm² with 1 ½ elongation. The constituents of the microstructures, which are generally not homogeneous, have been identified for each heat treatment condition. It is concluded that for this steel, which is predominantly martensitic on quenching, the properties are controlled by the tempering of the martensite, and the heterogeneity generally does not have any pronounced effect on properties. Studies of the effect of raw material variables have indicated: (a) that iron powder compressibility and even the final density of the steel are not a safe guide to properties after high-temperature sintering; (b) that costs may be reduced without detriment to properties by using ferromolybdenum powder instead of elemental molybdenum; (c) that the use of ferronickel powder or codecomposed iron/nickel gives results inferior to those achieved with elemental carbonyl nickel powder.

Notes

* Manuscript received 11 December 1972.