Cause analysis of circumferential cracking of reheat steam hot section pipeline of 300MW unit

Abstract

During the operation of reheat steam hot leg pipeline, circumferential cracking occurs, leading to the failure of high temperature steam leakage. In order to find out the causes of the cracking of the reheat steam pipeline and understand its health status, the comprehensive inspection and analysis of the cracked reheat steam pipeline are carried out by using the test methods of macro morphology analysis, non-destructive testing analysis, chemical composition analysis, microstructure analysis, mechanical property testing and scanning electron micro area analysis. The results show that the chemical composition of the cracked reheat steam pipeline meets the requirements of the design standard, the metallographic microstructure is ferrite + bainite + pearlite, the tensile strength and elongation after fracture are qualified, and the yield strength is slightly lower than the requirements of the standard. The main causes of the pipeline circumferential cracking: the low-temperature medium in the cold section of reheat steam is continuously discharged into the high-temperature pipeline and directly contacts with the inner wall of the pipeline repeatedly, forming alternating thermal stress, which causes the pipeline thermal fatigue damage cracking to leakage; and the insufficient yield strength will accelerate the speed of fatigue cracking to a certain extent.