σ-Phase induced embrittlement in titanium containing austenitic stainless steel tie-bars in a condenser

In this investigation the failure of tie-bars in a condenser water box of a nuclear power plant was analysed to determine the cause of failure. The tie-bars were made of titanium containing austenitic SS and were used for providing stiffness to the condenser water box. Failures had occurred within 2 weeks of operation and the location of cracking was near the welds, between the tie-bars and the water box base plates. Microhardness measurements indicated high hardness of the bar material. The analysis for chemical composition indicated that the weld had a high amount of ferrite stabilizers which led to formation of a high amount of ferrite phase in the weld. Chromium equivalent parameter indicated that the bar material and the weld fusion zone were prone to precipitation of brittle phase, e.g. σ-phase. Metallographic examination of the weld fusion zone confirmed partial transformation of the ferrite phase into a brittle σ-phase. The brittle σ-phase was formed during the multi-pass welding of the bar to the base plate causing subsequent brittle failure of the tie-bars.