Effect of Precipitation Behavior of Microalloying Elements during PWHT on Notch Toughness of Weld Metal

Abstract

This paper deals with the precipitation behavior of some microalloying elements in the weld metal during post weld heat treatment (PWHT).
An investigation was carried out to define the effects of Ti, B, Nb, V etc. on notch toughness of the weld metal which was produced by the large current MIG arc process. Attention was focused into three main areas as fo flows:
1. Correlation between toughness degradation attributed to PWHT and oxygen content of weld metal which governs the morphologies of Ti and B in the weld metal before PWHT.
2. A quantitative estimation of increase in hardness accompanied with toughness degradation which is attributed to the precipitation of Nb as NbC.
3. Determination of precipitation behavior of V existing solely or coexisting with Nb.
As a result, it becomes clear that the precipitation of Ti and B as carbonitrides during PWHT is not decisive of the embrittlement of the weld metal in the oxygen range above 200 ppm, where the larger proportion of both elements exists as oxides. The precipitation hardening of Nb as carbonitrides accompanied with significant toughness degradation reaches the maximum at 600°C in the PWHT temperature range from 500°to 650°C. On the other hand, the precipitation of V is suppressed when V coexists with Nb.