The role of the constituent phases in determining the low temperature toughness of 5.5Ni cryogenic steel

Ferritic Fe-Ni steels that are intended for service at low temperature are usually given an intercritical temper as the final step in their heat treatment. The temper dramatically decreases the ductile-brittle transition temperature, T_B. Its metallurgical effect is to temper the lath martensite matrix and precipitate a distribution of fine austenite particles along the lath boundaries. Prior research suggests that the low value of T_B is a consequence of the small effective grain size of the ferrite-austenite composite. The present research was done to test this suggestion against the counter-hypothesis that the low T_B is due to the inherent toughness of the constituent phases. The approximate compositions of the tempered martensite and precipitated austenite phases in the composite microstructure of tempered 5.5Ni steel are known from STEM analysis. Bulk alloys were cast with these two compositions. Their mechanical properties were measured after heat treatment and compared to those of the parent alloy in the toughened ‘QLT’ condition. Both of the constituent phases are brittle at low temperature. It follows that the outstanding low-temperature toughness of the tempered alloy cannot be attributed to the inherent properties of the constituent phases, but must reflect their cooperative behavior in the composite microstructure. The austenitic bulk alloy was also used to investigate the stability of the precipitated austenite phase. The thermomechanical stability of the bulk alloy approximates that of the precipitated austenite within tempered 5.5Ni steel. This result is consistent with previous data, and supports the conclusion that the stability of the precipitated austenite is determined mainly by its chemical composition.