Failure analysis of elbow pipe cracking in a residual heat-removal system of a nuclear power plant

Failure happened to a 316LN stainless steel elbow pipe of the nuclear power plant (NPP) located at south China. Detailed analysis has been made to figure out the mode and root cause. Non-destructive tests, chemical composition analysis, mechanical property tests, scanning electronic microscopy, energy spectrum and micro-structure analyses, and cold-work elongation analysis have been conducted. Results reveal lots of cold-work slip bands exist in the microstructure. Deformation coefficient is about 32% in the crack regime. The hardness is 390 HV0.1, and cracks present brittle characteristics and propagate along the grain boundary. Furthermore, the fracture possesses crystal characteristics. Thus, indicate failure of elbow pipe occurs due to inter-granular stress corrosion cracking (IGSCC) that penetrates the whole pipe wall, and the root cause of elbow pipe failure is the high-cold-work condition.