フェライト系高温用鋼溶接熱影響部のクリープ強度低下現象と 組織変化に及ぼすCrの影響

－フェライト系高温用鋼のHAZ軟化機構(第2報)－

抄録

The effect of chromium content on creep rupture strength and microstructural change at high temperature service was investigated to clarify the mechanism of deterioration of creep rupture strength in Heat Affected Zone (HAZ) of heat resistant ferritic steel. Simulated weld thermal cycles, a temperature range from 1103 to 1473 K, were applied to test specimens machined from 2% Cr-1.5% W-V, Nb ferritic steel plates. After heat treatment at 1013 K for 1.8 ks, simulating Post Weld Heat Treatment (PWHT), creep rupture strength were evaluated at 923 K with the applied stress of 98 MPa and various microstructural examinations were carried out. In 2% Cr steel as well as 10% Cr steel shown in previous report, creep rupture strength was deteriorated in HAZ heated in the peak temperature range of 1173-1273 K, and that HAZ after creep test had the lower density of fine particle mainly consisting of MX type carbo-nitride less than 0.1 μm in diameter effective for dispersion strengthening as compared in base metal. However, a degree of deterioration of creep rupture strength in HAZ was smaller in 2% Cr steel than in 10% Cr steel. The growth rate of fine particles such as M₂₃C₆ type carbide and MX type carbo-nitride was lower in HAZ of 2% Cr steel compared with in that of 10% Cr steel. The growth of fine MX in 2% Cr HAZ was confirmed in accordance with the Ostwald ripening dominated by chromium diffusion as well as 10% Cr HAZ. The growth rate in the Ostwald ripening was described as a function of chromium content which increased as the increase of chromium content. As results, the mechanism of deterioration of creep rupture strength suggested in HAZ of 10% Cr steel, which the partial dissolution of carbide during weld thermal cycle could accelerate Ostwald ripening, was confirmed to have the universal validity to that in HAZ of 2% Cr steel.