Microstructural degradation of the HAZ in 11Cr-0.4Mo-2W-V-Nb-Cu steel (P122) during creep

Abstract

The microstructural degradation of the heat affected zone (HAZ) in 11Cr-0.4Mo-2W-V-Nb-Cu steel (P122) during creep was investigated. Creep testing was conducted using two types of specimens at 650°C and 675°C, and ruptured in the fine grained HAZ, known as type IV failure. Weldments were known to be weaker in creep strength than base metal in this test condition. The coarse and fine grained microstructures were observed, and the average grain sizes were measured. The HAZ adjacent to the base metal was characterized by a fine grained microstructure consisting of subgrains with low dislocation density. Hardness of the intercritical area between HAZ and the base metal was the lowest after PWHT and during creep. Creep cavities tended to form at the grain boundaries in the fine grained HAZ due to creep. Small cracks gathered with cavities were observed in the fine grained HAZ after creep, and these corresponded to the fracture portion. M23C6, M7C3 and MX type carbides had already precipitated in the HAZ before the creep test. A Laves phase arose at the grain boundary of the coarse and fine grained zones of the HAZ during the test. It is presumed that Laves phase precipitation in the coarse grained HAZ is slower than in the fine grained HAZ and base metal during creep.