Influence of Co, Cu and W on microstructure of 9%Cr steel weld metals

Abstract

There is an increasing interest in the use of the 9-12%Cr grades of ferritic steel containing Mo, V, Nb, N and W with further possible Co additions, for high temperature applications. There is a parallel need for weld metal development to match these materials. A series of weld metals, based on 9CrMoV but with Cu and Co additions and W replacement of the Mo, have been examined and the effects on delta ferrite distributions and contents identified. The results show that the compositional differences have a direct effect on the delta ferrite content. The delta ferrite form and content are different for the last weld bead, or the as deposited structure, and the centre of the weld pad, where the structure has been reheated by subsequent weld beads. In general, a blocky form of ferrite is associated with the as deposited weld bead and an intergranular form for the reheated weld metal positions but these are not exclusive. A simplified mechanism is proposed for explaining the ferrite distribution. Finally, the measured ferrite contents are considered with respect to the existing empirical relationships from the literature, based on equivalent Cr and Ni levels, for estimation of the ferrite levels.