Delta ferrite in modified 9Cr–1Mo steel weld metal

Abstract The role of chemical composition on the presence of δ-ferrite in P91 weld metal in the as-welded condition was studied by systematically varying five alloying elements (Cr, Ni, Mn, Si, and Mo) in 28 gas–metal arc welds. The effect of thermal treatment on the δ-ferrite content was also investigated. Metallography analysis of the as-welded beads showed that 18 of the 28 welds contained 0.3 to 6.2% δ-ferrite in martensite. Sixteen of the 28 welds fully complied with chemical composition specifications, including the limit of the sum of (Mn + Ni). Of the sixteen welds that complied with the AWS A5.28/A5.28 M:2022 (ER90S-B91) chemical composition specification, thirteen contained δ-ferrite. Based on these results, lower specification limits for Mo and Si are proposed to ensure a fully martensitic microstructure. Empirical formulae were applied to evaluate their accuracy in predicting the P91 as-welded microstructure: ten of the 28 welds (36%) were incorrectly predicted. Based on these results, the recommended Schaeffler Cr eq and Kaltenhauser ferrite factor maximum limits should be lowered to 11.5 and 6, respectively, to limit the presence of δ-ferrite in the P91 as-welded microstructure. Phase-transformation temperatures were evaluated against the amount of δ-ferrite in beads in the as-welded condition. It was observed that a wider (Ae 4 − Ae 3 ) temperature range was beneficial in suppressing δ-ferrite in the final as-welded microstructure. Higher peak temperatures and slower cooling rates during welding also reduced the amount of δ-ferrite in P91 weld metal.