Stress-relief heat treatment of submerged-arcvvelded microalloyed steels

Abstract

A wide range of micro alloyed steels containing various combinations of Al, Ti, V, Nb, and submerged-arc welded using wires of either high or low Mo additions, have been studied in both the as-welded and stress-relieved conditions, with particular reference to impact toughness and microstructure. It has been found that, irrespective of thermal treatment, the heat-affected base metal always has a much lower impact transition temperature (based on 27 J) than the weld metal. In the as-welded condition, the most important constituent with respect to toughness is the amount of pro-eutectoid ferrite present which, in combination with brittle grain boundary carbide films, has a strong negative effect on impact toughness. During reheating these carbide films break up, which tends to improve toughness, but the precipitation of microalloying elements as carbides or nitrides tends to decrease the toughness. However, these precipitation effects after reheating are largest in alloys containing high Mo and less pro-eutectoid ferrite. The complex interplay between these various factors is discussed and recommendations are made concerning the stress-relieving heat treatments of the alloys studied.