Recently, boron bearing high chromium steel is applied to structural material of a fossil power generation plant. Dissimilar weld joints between this steel and low alloy steel are applied for various parts such as boiler or other equipments in this plant. In this weld joint, boron added to low alloy steel weld metals by the dilution from boron bearing 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 previous literature. So, reheat cracking test, precipitated carbide observation of boron bearing Cr-Mo low alloy steel weld metal was carried out in this study. From these test results, the reheat cracking susceptibility clearly increased by boron addition. Moreover, it decreased according to increasing chromium content. It was estimated that the structural form of boron was recognized in four kinds, such as BN, M2B, M23(C,B)6 and dissolved boron by thermo-dynamic calculation. The amount of M23C6 type carbide was increased with increasing chromium content. On the basis of these results, it was presumed that large amounts of dissolved boron existed in boron bearing low chromium steel weld metal compared with high chromium one during PWHT.

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