Reasons for cracking of 304 stainless steel bent pipes used in the drainage system of nuclear power plants

Cracking occurred on the outer bending side of the stainless steel elbow at the outlet of the steam turbine bypass drainage system in a power plant, resulting in steam leakage. The leakage causes of the elbow were analyzed by means of macroscopic observation, scanning electron microscope analysis, chemical composition analysis, metallographic examination and hardness test. The results show that the elbow had high hardness, high brittleness and high content of matrix inclusions due to the absence of heat treatment after cold working. In addition, there were pit defects on the outer surface of the bend. Under the vibration condition caused by the operation of the trap, the cracks at the surface defects propagated along the distribution direction of the inclusions, so that the elbow finally cracked.