Strength of Initially Virgin Martensites at — 196 °C after Aging and Tempering

Abstract

The compressive strength at —196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero Ms temperatures, has been determined in the virgin condition and after one hour at temperatures from —80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to account for this effect of molybdenum.