The Effect of Microstructure Constituents on the Static and Dynamic Fracture Behavior of High Strength Quenched and Tempered Martensitic Steels

Abstract

The aim of the present work is to contribute to a microstructural-based predictive tool of Charpy impact toughness for the design of new grades of quenched and tempered (QT) martensitic steels. The effect of carbides on the true strain to fracture under uniaxial tension has been extensively studied; but very little attention has been paid yet to the effect of carbides on the upper shelf energy (USE) of QT martensite. The present work focuses on the contribution of microstructural constituents, carbide in particular, to the fracture behavior in the USE domain of the ductile-to-brittle transition curve. A hot-rolled, martensitic 40CrMo4 steel bar, quenched from 875°C and tempered at 600°C was used. In order to keep similar matrix while varying the carbide precipitation state, an additional tempering at either 690°C or 720°C was applied to some specimen blanks. The three matrix microstructures and carbide populations were characterized in detail. The impact toughness, M3C carbide size and intercarbide spacing were shown to increase with tempering temperature. Instrumented Charpy impact curves were used to derive the respective contributions of initiation energy and propagation energy to the overall fracture energy of each microstructure. The propagation energy gives a major contribution and a correlation has been proposed with the intercarbide spacing distribution. The reported results shed new light on the effect of carbide size and spatial distribution on the impact toughness behavior in the USE domain of QT martensitic steels.