FAILURE ANALYSIS OF A WELD REPAIRED STEAM-TURBINE CASING

The present study was aimed at analysing the failure of a weld repaire d turbine casing after 30 years of total service including 5 years aft er weld repair. The casing was weld repaired by a high Cr-Ni weld meta l (24Cr-32Ni-4Mn-Fe). The base metal, a low alloy ferritic steel (1Cr- 0.5 Mo steel) with ferrite-pearlite structure did not show any abnorma lity to indicate significant degradation. Fracture surface showed void s and microcracks. The cracks might have initiated from the voids pres ent in the weld region possibly by thermal fatigue mechanism. The high level of thermal stress appears to be operative due to thicker sectio n of the casing and also due to high mismatch of thermal expansion coe fficients of the base metal and the weld metal. Dilution of alloying e lements took place and delta-ferrite was observed at the grain boundar ies as predicted by the Schaeffer diagram. The delta-ferrite was trans formed to sigma-phase and alloy carbides during high temperature servi ce (500 degrees C). The propagation of cracks generated by thermal fat igue was facilitated by the formation of embrittled sigma-phase at the austenite grain boundary and this ultimately led to an early failure of the casing. (C) 1998 Elsevier Science Ltd. All rights reserved.