Abstract
The effect of weld thermal cycle 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 10% Cr-3% Co-3% W-V, Nb ferritic steel plate. 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. The experimental results showed that 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. In the HAZ heated at that peak temperature range M23C6 type carbide was partially dissolved and in creep tested specimen of that HAZ, MX type carbo-nitride contained chromium especially. The growth of MX in that HAZ was confirmed in accordance with the Ostwald ripening dominated by chromium diffusion. In conclusion, the deterioration of creep rupture strength in HAZ was caused by the decrease of dispersion strengthening effect by higher growth rate of fine particle. The mechanism to explain the acceleration of growth of fine particle in the HAZ was introduced as the following. M23C6 type carbide partially dissolved during weld thermal cycle precipitated again as fine particles and these fine particles caused the Ostwald ripening of MX dominated by chromium diffusion resulting in higher growth rate compared with that dominated by vanadium diffusion in case of base metal.
