1988 Volume 6 Issue 1 Pages 128-132
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.