Effect of High Temperature Prestraining on Notch Toughness of Rimmed Steel

Abstract

A series of tests was performed to investigate the effect of high temperature prestraining on the notch toughness of a rimmed steel plate, 25 mm thick, by using standard V-Charpy specimens, and also, on the behaviours of the fracture stress curves by using the notched bar tension specimens.
As the result, a harmful effect was marked conspicuously in 15 tf-lbs and 50% shear transition temperatures after prestraining at 200°C or 300°C. The value of Tr₁₅ became about 75°C higher than that of the base metal after prestraining by 10% at 300°C. Less adverse effect was recognized by prestraining at higher temperatures, and at 500°C embrittlement was hardly observed after prestraining.
Moreover, the role of aging was investigated by comparing the effect of high temperature straining followed by heating at the prestraining temperature with that of aging after room temperature prestraining under various heating times, and it was found out that the part of the embrittlement, caused by excess in high temperature prestraining as compared with room temperature treatment, was of the non aging-type one and the remaining part was due to strain-aging.
As a whole, it was suggested after examining the behaviors of both cleavage and fibrous fracture stress curves that the phenomena of embrittlement of steel caused by high temperature prestraining ought not be discussed from the results of the retained ductility tests but from those of the notch toughness tests.