THERMAL EMBRITTLEMENT IN 18NI COBALT FREE AND 18NI-8CO-5MO MARAGING STEELS

The thermal embrittlement behaviour of 18Ni cobalt free (250 grade) ma raging steel (T250) compared with that of I8Ni-8Co-5Mo (250 grade) mar aging steel (M250) was studied for different heat treatment conditions . The effect of heat treatment was evaluated in terms of Charpy V notc h impact energy. Extensive optical microscopy, TEM SEM, and Auger elec tron spectroscopy established that T250 and M250 steels showed identic al embrittling behaviour for all the heat treatment conditions studied . It was observed that on direct cooling from high temperatures to int ermediate temperatures in the range 1073-1273 K and isothermal anneali ng 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 tr eatment and subsequently reheated rapidly to the intermediate temperat ures. These findings are thoroughly discussed in light of the distribu tion of Ti(C,N) in the prior austenite grains in the steels for the tw o heat treatment conditions. (C) 1996 The Institute of Materials.