Influence of Heat Treatments on Microstructure and Toughness of 9%Ni Steel

The 9%Ni low-carbon steel is applied to utilities and processes at temperatures as low as ??196 °C. However, the microstructural features play an important role on the mechanical properties. Notably, the cryogenic toughness and mechanical strength are strongly dependent on the final heat treatment. In this paper, the microstructure of a 9%Ni low-carbon steel was modified by different heat treatments. The hardness and cryogenic toughness were measured and correlated to microstructural features. The material shows a temper embrittlement with intergranular cracking and minimum cryogenic toughness after tempering around 400 °C. Austempering at 480 °C also produced very low toughness results. On the other hand, excellent cryogenic toughness was obtained with single tempering at 600 °C after quenching or normalizing. Even higher toughness was obtained with the double tempering at 670 °C/2 h plus 600 °C/2 h. The amount of reversed austenite and its morphology in the specimen quenched and tempered at 600 °C were shown in the paper.