Overheating of low-alloy steels

Abstract

When low-alloy steels are reheated to high temperatures for forging, a deterioration in the ambient-temperature mechanical properties after a full heat treatment may be observed. This is linked with the appearance of intergranular facets on the fracture surface. The phenomenon has been described as overheating, and an extensive literature has developed describing experimental studies of the mechanical-property and microstructural changes induced in low-alloy Ni-Cr-Mo steels. This literature is reviewed, emphasis being placed upon the property changes, methods of detecting overheating, and the theories that have been advanced to account for the observed effects. The important role played by non-metallic inclusions, particularly sulphides, is described. The methods of preventing overheating through control of steel composition, particularly in relation to sulphur and manganese, are described. Finally, an assessment is made of the methods that have been recommended for the reclamation of steels which may have been inadvertently overheated during forging.