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Investigation of ẟ-ferrite content in weld metal of modified 9Cr–1Mo electrodes using thermodynamic modelling and quenching experiments


During fabrication of modified 9Cr–1Mo steels, δ-ferrite formed in the weld metal and heat-affected zone may not transform completely to austenite during subsequent cooling. The influence of changes in weld metal composition on δ-ferrite content was investigated using weld pads produced using basic coated P91 electrodes from four different manufacturers. The weld pads were designated Electrodes 1, 2, 3, and 4 according to the different manufacturers. Electrodes 1 and 3 fully complied with the EN ISO 3580-A CrMo91 specification. Electrode 2 had very low nickel and high niobium contents, and Electrode 4 had carbon content above the maximum allowable limit. Thermo-Calc results showed that the temperature range over which a mixture of δ-ferrite and austenite is stable (the Ae4 − Ae3 temperature range) was smaller by more than 100 °C for Electrode 2 when compared with those of the other three electrodes. The limited (Ae4 − Ae3) temperature range, high ferrite factor, and chromium − nickel balance value of Electrode 2 were associated with an increase in the δ-ferrite content of the weld metal. Metallography results confirmed a significant amount of δ-ferrite in the as-welded microstructure of Electrode 2. Thermo-Calc estimates for the amount of δ-ferrite at high temperatures were supplemented by experimental anneal heat treatment on the weld metal. High-temperature anneal heat treatments were carried out at 1320 °C and 1420 °C. The amount of δ-ferrite in the high-temperature annealed and quenched samples was significantly lesser than predicted by Thermo-Calc property diagrams.

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I received financial support from the Southern African Institute of Welding.

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Correspondence to S. S. Mahlalela.

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Mahlalela, S.S., Pistorius, P.G.H. Investigation of ẟ-ferrite content in weld metal of modified 9Cr–1Mo electrodes using thermodynamic modelling and quenching experiments. Weld World 66, 1191–1198 (2022).

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