Cause of Tube Burst on High Temperature Superheater of 630 MW Supercritical Boiler

The tube burst accident occurred in the TP347HFG austenitic stainless steel high temperature superheater of a supercritical boiler. The causes of the tube burst were analyzed by means of macro observation, chemical composition analysis, metallographic examination and mechanical property test. The results show that brittle fracture occurred in the high temperature superheater tube. Due to the small size of orifice in header at the upper of tube panel, which was inconsistent with design drawing, causing the water medium flow in the tube was too small, the cooling effect was poor, and the tube wall serviced in over temperature environment for a long time. However, the steel precipitated a lot of σ phase at the grain boundary due to long-term high temperature operation, and the fine grains also promoted the formation of σ phase, which led rapid aging of material and reduced the high temperature strength, finally led to tube burst.