Failure analysis of aweld repaired steam turbine casing

Abstract

The present studywas aimed at analysing the failure of a weld repaired turbine casing after 30 years of total serviceincluding 5 years after weld repair. The casing was weld repaired by a high Cr–Ni weldmetal (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 abnormalityto indicate significant degradation. Fracture surface showed voids and microcracks. The cracksmight have initiated from the voids present in the weld region possibly by thermal fatiguemechanism. The high level of thermal stress appears to be operative due to thicker section of thecasing and also due to high mismatch of thermal expansion coefficients of the base metal and theweld metal. Dilution of alloying elements took place and δ-ferrite was observedat the grain boundaries as predicted by the Schaeffler diagram. The δ-ferrite wastransformed to σ-phase and alloy carbides during high temperature service(500°C) . The propagation of cracks generated by thermal fatigue was facilitated by theformation of embrittled σ-phase at the austenite grain boundary and thisultimately led to an early failure of the casing.