308系ステンレス鋼溶接金属のクリープ中における組織変化とこれに及ぼすV添加および溶接法の影響

抄録

In order to improve the high temperature properties of austenitic stainless steel weld metals, it is important to clarify the mechanism of formation and growth of brittle structures as well as the relation between brittle structure and fractograph of the creep ruptured specimens.
The microstructure and the fractograph were determined by transmission and scanning electron microscopies on Tp 308 and Tp 308-V stainless steel weld metals made by GTAW. and SAW. methods. The results obtained are summarized as follows.
(1) In the early stage of the creep process, dislocations are multiplied in austenite and locked by fine precipitates formed in austenite. Besides, the precipitates formed in δ ferrite grow up to 0.05μ diameter.
(2) The formation of cell structure in austenite and the growth of precipitates in δ ferrite proceed during the creep process. These microscopic changes are affected by dissolution of the fine precipitates formed in austenite and their reprecipitation in δ ferrite. The addition of V restrains such dissolution and reprecipitation. These effects of V improve the creep rupture properties of Tp 308 stainless steel weld metals.
(3) Fractograph of GTAW. weld metal of Tp 308V consists of dimple patterns which have similar forms and sizes with δ ferrite network. In the case of submerged arc weld metal of Tp 308V, the dimple patterns are found to be formed by inclusion and δ ferrite network on the fractograph. Both of the elongated dimple patterns formed by precipitated δ ferrite and the small dimple patterns (clearly distinguishable from the elongated one) which have similar forms and sizes with small cell structure in austenite are observed on GTAW weld metal of Tp 308.