Resistometric Investigation of the Precipitation in Iron-Nickel-Titanium Ternary Maraging Steels

Precipitation processes in ternary iron-nickel-titanium maraging steels containing 20 to 5% nickel and 1.8% titanium have been investigated by measuring electrical resistivity at liquid nitrogen temperature and hardness. Resistivity decrease is observed around 400°C in isochronal aging of iron-nickel binary alloys quenched from 950°C, the nickel contents of which are varied from 20 to 5%. Similar resistivity decrease is also observed in isochronal aging of the 15 to 5% nickel-1.8% titanium ternary maraging steels. However, resistivity increase is observed around 400°C in isochronal aging of the 17.5%Ni-0.4%Ti, 17.5%Ni-1.8%Ti and 20%Ni-1.8%Ti ternary maraging steels. The resistivity changes in isochronal aging are discussed. The greater part of precipitation hardening is produced by aging in temperatures above 400°C. It is considered from resistivity decrease above 400°C in isochronal and isothermal agings that the precipitation in the 5%Ni-1.8%Ti maraging steel proceedes in a single stage but that in the 10%Ni-1.8%Ti, 15%Ni-1.8%Ti and 20%Ni-1.8%Ti maraging steels proceed in two stages. Great resolution of a precipitate formed in the first stage of the 10%Ni-1.8%Ti maraging steel is observed in reversion experiment. However, the reversion of precipitates formed in the first stages of the 15%Ni-1.8%Ti and 20%Ni-1.8%Ti maraging steels is relatively small. Precipitates in the ternary maraging steels are also considered by reference to investigations published.