Embrittlement of 5.5% Ni Steels on Slow Cooling from the Final Tempering

Abstract

The embrittlement on slow cooling was studied in the intercritically heat treated cryogenic nickel steels which contain reverted austenites. The 5.5% Ni steels with different Mo and Cr contents were QLT treated and reheated at 600°C for 20 min and then slow cooled at a rate of about 0.2 K/min to the temperatures at which the slow cooling was interrupted by water quenching.
Three types of embrittlement took place on slow cooling, depending on chemical compositions and interrupted temperatures. Intergranular failure was a characteristic feature of the embrittlement in Mo free and 0.4 Mo steels and this was attributable to temper embrittlement. In the cases of 0.2 and 0.8 Mo steels, no intergranular fracture was observed and quasi-cleavage fracture became more dominant with a decrease in the intermediate-quenching temperature. As the cooling proceeded the yield strength decreased or remained in a low level at first and increased gradually and reached a constant level. This may be concerned with the change in the stability of reverted austenite on slow cooling. For 0.6Cr-0.4 and 0.8Mo steels, martensitic transformation of the unstable austenite appeared to occur at low tensile stress level and decrease both yield strength and toughness.