EMBRITTLEMENT OF 5. 5%Ni STEELS ON SLOW COOLING FROM THE FINAL TEMPERING.

Embrittlement on slow cooling was studied in 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 degree C for 20 min and cooled at a rate of about 0. 2 K/min to temperatures at which cooling was interrupted by water quenching. Three types of embrittlement took place depending on chemical compositions and interrupted temperatures. Intergranular failure was a characteristic feature in Mo free and 0. 4 Mo steels. In the case 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 cooling proceeded the yield strength decreased or remained at a low level at first, increased gradually and reached a constant level. For 0. 6Cr-0. 4 and 0. 8Mo steels, martensitic transformation of the unstable austenite appeared at a low tensile stress level and decreased both yield strength and toughness.