Analysis on Type Ⅳ Cracking of Main Steam Sampling Tube in USC Unit

The leakage reason of main steam sampling tube of one 1 000 MW USC unit was analyzed by means of macroscopic and microscopic analysis, hardness testing and piping stress calculation. The results showed that the properties of leaking was the type Ⅳ cracking in heat affected zone. The main reason of early failure was the high secondary stress for the poor flexibility of sampling tube layout. It would be paid attention to the layout of the leading-out tubes. It was suggested that the creep life evaluation of weld should take not only creep strain but also the stress triaxiality factor into account to improve the accuracy of creep life evaluation.