Effect of PWHT on Notch Toughness of Weld Metal by Large Current MIG Arc Process

Abstract

This paper deals with the notch toughness of the weld metal produced by the large current MIG arc process in as welded and post weld heat treated conditions.
When one-run welding is made by tandem electrode process, cooling rate after welding for the trailing weld bead is retarded with preheating effect by the leading weld bead. This brings forth unevenly distributed notch toughness of weld metal, especially toughness degradation at and near the plate surface.
In previous paper, the authors have indicated the relation of notch toughness versus Ti and B and/or microstructure, which provides a guiding principle to solve those problems above mentioned. Based on the principle, an investigation was carried out to find out the optimum Ti and B contents of wire-consumable intended to use the longitudinal seam welding of heavy thickness steel pipe of arctic grade. As a result, the use of high Ti-B type wire-consumable as a trailing electrode was determined to be effective to improve the notch toughness of the weld metal at and near the plate-surface.
Nextly, in this paper, toughness degradation behavior of the weld metal due to post weld heat treatment was investigated. From various tests, it was observed the embrittlement depended upon cooling rate after heat treatment, the Mn content of the weld metal. The fracture mode of embrittled specimen was entirely intergranular one. And also, it was clarified that those behavior was reversible. These phenomena feature typical temper embrittlement. Furthermore these embrittlement was evidenced to be suppressed considerably by reducing the Mn content of wire-consumable.