Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

Abstract

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.