In the previous report, it was pointed out that martensite-like structure formed at weld bond of welded α-steels and γ-steels reduced the ductility of weld bond remarkably.
The substance of martensite-like structure has not been cleared, because the structure is very fine.
In this report, the substance of the structure was cleared up by transmission electron microscopic observation. The effect of C content in α-steels on the amount of above mentioned martensite-like structure was also investigated. Moreover, investigations were made on the hardness change of the structure with post-weld heat treatment and so on.
Martensite-like structure is not formed uniformly along the whole weld bond, but formed mainly at a stagnation area where flow of the molten metal contacting with the fusion line does not occur.
Martensite-like structure was identified as a mixed structure of low carbon martensite or bainite (in case of using low carbon α-steels as base metal), or high carbon martnesite or bainite (in case of using high carbon α-steels as base metal) and fine chromium carbide (Cr₂₃C₆).
The amount of martensite-like structure formed at weld bond increases with increasing C content in a-steels, but it decreases with too much C.
In the post-weld heat treatment at 650°C, the weld bond is softened with increasing heating time, but bigin to be hardened when heating time is over app. 2 h and hardened remarkably over app. 7 h. This hardening is caused by large amounts of Cr₂₃C₆ precipitates which are induced by the carburization from haz of α-steel.
On the microscopic observation of weld bond, the hardly etched region was observed between the martensite-like structure and weld metal. This region is very hard and hardened further with post-weld heat treatment. However, the substance of the structure of this region could not be cleared up.