Investigation into Cracking of a Weld Repaired Turbine Casing

The present study was aimed at analysing the failure of a weld repaired turbine casing after 30 years of total service including 5 years after weld repair. The casing vvas weld repaired by a high alloyed weld metal (24Cr-32Ni-4Mn-Fe). The base metal consisting of ferrite-pearlite microstructure did not show any appreciable degradation during service. δ ferrite was observed at the interface of weldment and HAZ as predicted by the Schaeffler diagram. The δ- ferrite phase appeared to transform to alloy carbides and a-phase during high temperature service.

The difference between thermal expansion coefficients of ferritic and austenitic stainless steel led to the generation of stress in addition to the usual thermal stress. The resultant stress was estimated to be near to the yield stress indicating that the weld zone experienced a typical condition of low cycle fatigue. The presence of striations on the fracture surface confirmed thermal fatigue as the failure mode. Crack growth took place along the grain boundaries embrittled by a-phase and led to failure. The correct choice of the filler metal should have been a high Nibase alloy having similar coefficient of thermal expansion as the ferritic steel base metal.