Quantitative model of the effect of quenched and tempered structure, 0.2‐% proof stress and grain size on fracture appearance transition temperature Fatt

Abstract

The transition temperature for the notched‐bar impact strength considerably varies as a function of the steel composition, the quenched and tempered (QT) structure and associated grain size as well as the 0.2‐% proof stress. To get a better insight into how these parameters affect the transition temperature, the Fatt of many steels resulting from identical quenching, but different tempering treatments was evaluated and a quantitative model developed. The correlation between similarly QT steels and the 0.2‐% proof stress can be described by similar parables, the summits of which lie on a straight line. The position of these peaks on the straight line is determined by the QT structure and the grain size. As the influence of the grain size is assumed to be the same for all QT structures, a measured Fatt ought to be sufficient to know the correlation between grain size and yield strength for a given QT structure. For evaluation purposes, it has been shown practical to characterize the QT structure on the straight line by the Fatt for the grain size dm = 1 mm. The influence of the QT structure proper is determined by the strongly varying correlation between carbide lamellae and grain size, which is independent of the grain size. This carbide lamellae size is thus ascribed the role of a critical crack size.