Effect of chromium content on loss of creep rupture strength in the heat affected zone of heat-resistant ferritic steel

摘要

The degree of creep rupture strength loss in the HAZ of the 2 percent Cr steel tends to be less than in the HAZ of the 10 percent Cr steel. The 2 percent Cr steel also sustains heavy loss of creep rupture strength in the HAZ heated at a peak temperature near 1273 K. This temperature region also corresponds well with that where carbides, specifically M9,Cg, enter partial solid solution. The 2 percent Cr steel shows slower coarsening of precipitates and a gentler loss of density of precipitates than the 10 percent Cr steel. This suggests the smaller degree of creep rupture strength loss in the HAZ of the 2 percent Cr steel than in the HAZ of the 10 percent Cr steel as being at tributable to the particle dispersion strengthening effect being maintained in the longer term. The results obtained in the theoretical investigation of the growth behaviour of carbides suggest that the precipitates formed in the HAZ heated at a peak temperature of 1273 K in the 2 percent Cr steel broadly correspond with the predicted growth behaviour of M^C6 and MX under Cr diffusion control. The foregoing results suggest that loss of creep rupture strength in the HAZ of the 2 percent Cr steel can be explained by the previously proposed model involving M_(23)C_6 carbides partially entering solid solution in the welding thermal cycle, fine M_(23)C_6 carbides being formed, and the long-term stability of the fine MX carbonitrides being lost, resulting in loss of creep rupture strength.