Characterisation and modelling of tempering during multi-pass welding

Weld-induced in-process tempering of martensite/bainite was studied through characterisation and modelling. Three-pass gas tungsten arc (GTA) and submerged arc (SA) welds were produced in grooved plates made from a low-alloy ferritic (SA508) steel. A thermal-metallurgical-mechanical model was developed to simulate multi-pass welding while accounting for tempering kinetics. Significant tempering of martensite, in the heat affected zone that was produced by the first pass, occurred during the second and third passes, resulting in a coarsened lath structure, increased carbide precipitation and reduced hardness. The tempering effect was more extensive in the SA weldments than in the GTA weldments, since the tempering mainly occurred in a martensite-dominant region without re-austenitisation for the former while in a partially re-austenitised region for the latter. The predictions for tempered microstructures were consistent with microscopic observations, and the predicted micro-hardness agreed well with measurements when tempering was considered in modelling. The peaks in predicted tensile and compressive residual stresses were reduced by considering tempering effects, since the local yield strength reduced as a consequence of the tempering, thereby limiting the stresses that could be sustained.