1973 Volume 59 Issue 6 Pages 752-763
In order to improve the low temperature toughness of a newly developed 6% Ni steel, a new process of the heat treatment has been studied. By this new heat treatment, the steel is partially austenitized at the temperature range of the ferrite-austenite region and is quenched. When this special heat treatment is added between the conventional process of quenching and tempering, the low temperature toughness of nickel bearing steels is remarkably improved.
The reasons why this special heat treatment remarkably improves the low temperature toughness are as follows. Firstly, a large amount of finely dispersed austenite islands, which are stable at a low temperature as -196°C, are formed by adding this heat treatment before tempering. These stable austenite islands act as a shock absorber and provides a resisting force against the propagation of brittle fracture. Secondly, temper brittleness is depressed since the embrittled prior austenite boundaries are covered by tempered martensite which is formed by this new process. Thirdly, the ductilicy of ferrite matrix is improved through the scavenging effect due to the formation of austenite during this heat treatment.
