B添加高Cr鋼と低合金鋼異材溶接金属における溶接後熱処理過程のBの存在状態

Abstract

Recently, higher steam inlet temperature and pressure have been required for fossil power plants in order to increase the power generation efficiency. Therefore, boron bearing high chromium steels with higher long-term creep strength are applied to structure materials in these plants. Dissimilar (metal) weld joints between a boron bearing high chromium steel and a low alloy steel are applied for various parts of boiler and other equipment in the plants. Caution has to be paid to maintain the quality of these weld joints because boron is diffused to the low alloy steel weld deposits by dilution from the boron bearing high chromium base metal. However, the existence form of boron and the effect of boron on reheat cracking susceptibility during post weld heat treatment (PWHT) have not been surveyed in the previous literature. Therefore, using Cr-Mo low alloy steels with varied contents of B and Cr, reheat cracking susceptibility of the weld metal was evaluated, and the precipitation behavior of the carbides during PWHT was observed. >From these results, the existence form of boron in boron bearing Cr-Mo steel weld metals was discussed in this study.Reheat cracking susceptibility clearly increased with boron addition, whereas it decreased with increasing chromium content. It was shown by thermo-dynamic calculation that possible existence forms of boron were four types, namely BN, M2B, M23(C, B)6 and dissolved boron. The amount of M23C6 type carbides was increased with increasing chromium content. On the basis of these results, it was presumed that the amount of dissolved boron in boron bearing low chromium steel weld metals was larger than in those with high chromium. It was suggested that the larger amount of dissolved boron enhanced strength difference between the matrix and the grain boundary in low chromium steels, hence it led to the higher susceptibility to reheat cracking.