The effect of ferrite phases on the micromechanical response and crack initiation in the inter-critical heat-affected zone of a welded 9Cr martensitic steel

Abstract

This paper presents a crystal plasticity model to predict the tensile response and crack initiation in a mixed ferrite-martensite material with a low volume fraction of pro-eutectoid ferrite, representative of a welding-induced inter-critical heat-affected zone (IC-HAZ). It is shown that small volume fractions of ferrite can have a significant effect on material strength and ductility depending on the ferrite grain orientation. For relatively ‘soft’ ferrite grains, micro-cracks can grow across inter-ferrite ligaments with damage accumulating in the ferrite, leading to a reduction in strength and strain hardening, but with little influence on ductility; in contrast, relatively ‘hard’ ferrite grains act to accelerate micro-crack initiation, leading to reduced ductility, with negligible influence on strain hardening up to the maximum load.