Metallurgical Factors Affecting the Creep Ductility of the Weld HAZ of Cr-Mo Pressure Vessel Steels

Abstract

Recently, the intergranular creep cracking was found at the weld joints of high temperature reactors made by the 1 1/4Cr-0.5Mo pressure vessel steel. The cracking is caused by the fact that the coarse grain region of weld heat affected zone of low creep rupture ductility is located at the stress concentration sites.
For the purpose of suitable material selections, creep rupture behavior of the synthetic HAZ was studied for various Cr-Mo steels containing 1 to 5% chromium. Both the creep rupture ductility and notched sensitivity of the weld HAZ were found to be remarkablely improved by increasing Cr contents.
Effects of the Cr content on the rupture ductility were discussed on various metallurgical view points. Some results imply that improved rupture ductility caused by higher Cr contents is attributed to reducing γ/α transformation temperatures, avoiding the formation of high density Mo₂C carbides and decreasing the size of grain boundary carbides.
In order to prevent the creep cracking of the weld Joint, therefore, the application of Cr-Mo steels of higher Cr content is recommended for the pressure vessel used in higher temperatures over 500°C.