Mikihiro Sakata

Existence Form of Boron in Dissimilar Weld Metals of Low Alloy Steel with Boron Bearing High Chromium Steel during Post Weld Heat Treatment

Recently, higher steam inlet temperature and pressure has been required for fossil power plants in order to increase the power generation efficiency. Therefore, boron-bearing high-chromium (high-Cr) steels with higher long-term creep strength are applied to structure materials in these plants. Dissimilar (metal) weld joints between a boron-bearing high-Cr steel and a low-alloy steel are applied for various parts of boilers 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-Cr base metal. However, the existing 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 behaviour of the carbides during PWHT was observed. From these results, the existing form of boron in boron-bearing Cr–Mo steel weld metals is 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 existing forms of boron were of 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-Cr. It was suggested that the larger amount of dissolved boron enhanced the strength difference between the matrix and the grain boundary in low-chromium steels, hence it led to higher susceptibility to reheat cracking.

Existence Form of Boron in Dissimilar Weld Metals of Low Alloy Steel With Boron Bearing High Chromium Steel

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.Copyright