Causes Analysis on Fracture of a Q235B Steel Circulating Water Main Pipe

A circulating water main pipe of Q235B steel in a 1 000 MW coal-fired power generation of a power plant fractured during operation. The fracture causes of the circulation water main pipe were analyzed by means of macroscopic inspection, chemical compositions analysis, mechanical properties testing, metallographic inspection, scanning electron microscope analysis, energy spectrum analysis and so on. The results show that: incomplete penetration phenomenon and inclusions, such as welding slag, welding beading and so on, existed in the welding seam of the circulating water pipe; low welding quality and abnormal microstructure caused poor bearing capacity of the welding seam. In the corrosion environment, cracks generated in the area of welding slag and expanded rapidly, resulting in the fracture failure of the circulating water main pipe.