Thermal embrittlement in 18Ni cobalt free and 18Ni-8Co-5Mo maraging steels

Abstract

The thermal embrittlement behaviour of 18Ni cobalt free (250 grade) maraging steel (T250) compared with that of 18Ni-8Co-5Mo (250 grade) maraging steel (M250) was studied for different heat treatment conditions. The effect of heat treatment was evaluated in terms of Charpy V notch impact energy. Extensive optical microscopy, TEM, SEM, and Auger electron spectroscopy established that T250 and M250 steels showed identical embrittling behaviour for all the heat treatment conditions studied. It was observed that on direct cooling from high temperatures to intermediate temperatures in the range 1073-1273 K and isothermal annealing at these temperatures, both the steels became embrittled due to the precipitation at grain boundaries of Ti(C,N) from the supersaturated solid solution of Fe-Ti-C. However, no embrittlement was observed when the steels were quenched to room temperature from high temperature treatment and subsequently reheated rapidly to the intermediate temperatures. These findings are thoroughly discussed in light of the distribution of Ti(C,N) in the prior austenite grains in the steels for the two heat treatment conditions. © 1996 The Institute of Materials.