Cr-Mo鋼の溶接金属の再熱割れ感受性に及ぼすCおよびBの影響

Abstract

Reheat cracking during post weld heat treatment (PWHT) generally occurs in the coarse grain HAZ (heat affected zone) of Cr-Mo steel and 780MPa class high strength steel. Although reheat cracking in the weld metal of Cr-Mo steel has also become well-known recently, the effect of alloy elements in the weld metal on reheat cracking had not been investigated in detail. In particular, the small amounts of alloy elements such as carbon and boron in the weld metal are affected partly by dilution components from the base metal. The purpose of this study was to clarify the effect of the carbon and boron contents on reheat cracking susceptibility in the weld metal of Cr-Mo steel. Reheat cracking tests were carried out by the C-ring and constant stress methods. Reheat cracking susceptibility and the hardness of weld metal containing up to 0.08mass% carbon clearly increased with increasing boron contents over 3ppm. However, reheat cracking of weld metal containing approximately 7ppm boron was suppressed by increasing the carbon content in spite of an increase in hardness. The constant stress reheat cracking tests showed experimentally that the high temperature strength of the grain boundary in weld metal containing boron decreased with increasing temperature during PWHT. Secondary Ion Mass Spectrometry (SIMS) and precipitate analysis revealed that the existence form of boron on the grain boundary changed from dissolved to precipitated during PWHT, and thermo-dynamic calculation showed an increase of M23C6 carbide with increasing carbon contents. Based on these results, it was estimated that the reheat cracking susceptibility of weld metal increased by intergranular embrittlement due to precipitation of boron nitride and decreased by grain boundary strengthening due to the increase of M23C6 carbide.