Effect of minimum temperatures on the mechanical properties and reversed austenite content of 9%Ni steel subjected to cryogenic treatment

Abstract:

The effect of minimum temperature on the mechanical properties and reversed austenite of 9%Ni steel subjected to cryogenic treatment was experimentally investigated in the present work. Cryogenic treatments with different temperatures and soaking time were conducted by combining with the newly developed quenching, lamellarizing and tempering (QLT) heat treatment of 9% Ni steel. The results showed that the execution of cryogenic treatment at -80 and -110 °C had no obvious influence in the reversed austenite and mechanical properties of 9%Ni steel. However, the room temperature impact toughness was improved by cryogenic treatment at -140 °C soaking for 24 hours, which was attributed to the modification of reversed austenite morphology from bulks into strips. The volume fraction of reversed austenite was reduced slightly due to the isothermal martensitic transformation at -140 °C. The conduction of cryogenic treatment at -196 °C soaking for 24 hours increased the volume fraction of reversed austenite as well as refined the secondary martensite laths, as a result of improving both the room temperature impact toughness and cryogenic ductility. This observed results were explained through the precipitation of ultra-fine carbides and increase of internal stress during cryogenic treatment which provided more nucleation points for the reversed austenite in the process of tempering.

Key words:9%Ni steel; cryogenic treatment; reversed austenite; mechanical properties; microstructure