Precipitation Process of M₂₃C₆ and Sigma Phase in Type 316KL Weld Metal

An author's previous paper on heat-treating of Type 316L weld metal revealed that the decomposition of δ ferrite in the weld metal (EWM) containing mainly an eutectic δ-ferrite occured rapidly as compared to that in the weld metal (PWM) containing mainly a primary δ ferrite and this reason was attributed to the difference in the lattice coherency at the δ/γ interface. In this paper, to clarify metallographically the results described above, the precipitation process of M₂₃C₆ and sigma phases was investigated by transmission electron microscopy using the weld metals heattreated at 923 or 1123K. In the EWM specimen, at both the heat-treating temperatures, the M₂₃C₆ phase precipitated initially at the δ/γ interface and then within δ ferrite grains and the precipitation of σ phase occured finally. However, in the PWM specimen, the precipitation of M₂₃C₆ could not be observed at all within δ ferrite grains. This suggests that M₂₃C₆ formed readily in the EWM specimen, of which the δ ferrite phase contained larger amounts of molybdenum than that in the PWM specimen. On the other hand, the M₂₃C₆ at δ/γ interface in the EWM specimen precipitated rapidly and in large amount, comparing with that in the PWM specimen. Particularly, as this tendency was remarkable at 1123K, the coherency of the δ/γ interface was considered to influence considerably the precipitation of M₂₃C₆.