Effect of morphology of martensite-austenite phase on fracture of weld heat affected zone in vanadium and niobium microalloyed steels

In multipass welding, the intercritically reheated coarse grained heat affected zone (HAZ) demonstrates the worst toughness in the welded joint, since it contains a high carbon martensite with some retained austenite, known as M-A phase, which is brittle and associated with the high cooling rates following welding. The purpose of the present work was to explore those aspects of the morphology of the M-A phase which determined the ease or otherwise of crack development in welded vanadium and niobium high strength low alloy steels. Four steels were subjected to heat treatment to simulate the microstructure of an intercritically reheated coarse grained HAZ. The toughness of the simulated intercritically reheated coarse grained HAZ was assessed using both Charpy and CTOD tests. Microstructural features were characterised by scanning and transmission electron microscopy and optical microscopy. Fractographic examination of the Charpy and CTOD specimens were carried out to understand the micromechanism of fracture under different microstructural and test conditions. Evidence of both cracking and debonding of M-A phase and carbides was found, and many of the cracks appeared to develop by linking up of voids resulting from debonding. The importance of the dihedral angle 2 theta in determining the interfacial energy of the two main morphologies of the M-A phase, blocky and elongated stringer particles, was considered. While both carbides and inclusions were observed, these features appear to have a minor role in determining the degree of toughness of the steels.