18・8 ステンレス鋼溶接部の高温強度について

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An investigation was made to examine mechanical properties of welded type 304 stainless steel at elevated temperatures. The stainless steel plate 14mm in thickness was welded with inert-gas arc-welding, without pre-heating. Three different kinds of filler metals, 3.2mm in diameter, were used and their chemical composition and the mechanical properties are shown in Table 1. Fatigue and tensile test specimens were machined from the welded joint so as to get the weld metal at mid portion of the specimens, as shown in Fig. 2. V-notch Charpy impact specimen was also made from the welded joint. A rotating beam fatigue test, running 3,400rpm, was carried out at 650℃, while the tensile test, hardness test and impact test were carried out at temperature ranging between 350℃ and 750℃. The highest fatigue strength was observed for the specimen welded with Fox 16/13 Co metal. The specimen failed consistently at base metal and the endurance limit is about the same value as that of the base metal. The specimens welded with either NC-36L or NC-38 metal showed lower endurance limit than that of the base metal and failed always at weld metal. (See Fig. 3) The highest fatigue resistance observed in the specimen welded with Fox 16/13 Co may be attributed to the strain induced precipitation of niobium carbide at plastically strained region. This was also verified by the fact that the most remarkable coaxing effect in fatigue was observed in this specimen, as shown in Fig. 6. While the fatigue strength of the welded specimens was markedly decreased by solution heat treatment at 1100℃ for 1hr after welding, little change was observed in the fatigue strength after stress relief heat treatment at 870℃ for 2hr. (See Fig. 5) Tensile test was carried out at temperatures from 600℃ to 700℃ and failure of the specimens occured at or near the weld metal. The proof stress was increased by welding. The endurance limit observed at 650℃ was rather higher than the proof stress measured at the same temperature. A fairly low impact value was obtained on the weld metal and also at the heat affected zone, comparing with higher toughness of the base metal. (See Fig. 8) The impact value of weld metals increases with increasing test temperature up to about 600℃, while a sharp drop is observed at around 650℃ in each specimen. In the specimens welded with NC-36L or NC-38,the impact value increased rapidly with increasing temperature after reached their minimum at around 650℃. This minimum is attributed mainly to the precipitation of chromium carbide. (See Fig. 9) On the other hand, NbC can precipitate fairly rapidly only at high temperature of the order of 950℃ so that there is no recovery appeared in the impact value but further decrease with increasing temperature was observed in the specimen welded with Fox 16/13 Co metal. An about 5mm wide hardened zone was observed both sides of the bead. From the result of the present investigation, it may be possible to conclude that the mechanical properties of welded portion of austenitic steel at elevated temperature is much affected by their micro-and macro-structure, especially by the carbides precipitation.
社団法人溶接学会の論文
1965-11-25

18・8 ステンレス鋼溶接部の高温強度について

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