Role of delta ferrite in thermal aging of type 316 weld metals

The effect of ferrite on the precipitation processes in type 316 austenitic stainless weld metals has been studied. Two weld metals, one fully austenitic (17Cr–10Ni–2Mo) and one containing 5% δ-ferrite (19Cr–12Ni–3Mo) have been thermally aged at temperatures between 538 and 800°C for times of up to 1000 h. Thin foil specimens have been examined in the electron microscope to reveal the precipitate and dislocation structures. It was found that the fully austenitic deposit was particularly stable and over the time–temperature conditions investigated only discrete M₂₃C₆ carbides were observed with no evidence of any intermetallic phases. For comparison material from a steam pipe weld which had been exposed for more than 10⁵ h at 570°C was also examined. This particular weld metal had been given a prior solution heat treatment at 1050°C and so gave some indication of the long-term behaviour of a fully austenitic deposit. Once again, only M₂₃C₆ carbides were observed. In the duplex deposit the ferrite phase operated as a focus for virtually all precipitation processes and a variety of intermetallic phases was observed. No high-angle grain boundaries were seen but the δ-ferrite regions were linked by a dislocation subgrain boundary network which persisted during aging. The structural features that were observed have been considered in terms of their potential effects on creep strength and ductility.