20th International Forgemasters Meeting - IFM 2017

During pressure test of a huge hydro power plant; embritteled weld material caused a catastrophic fracture of the closure head of a new penstock. Temper embrittlement (TE) in weldings of high strength steel type S 690.was identified as basic cause of theobserved failure. TE was caused by application of an unusual slow cooling rate during post weld heat treatment. A procedure, characterized by a short-time heating at about 600°C,recovers this embrittlement(Reversible Temper Embrittlement treatment, RTE) significantly, was developed, qualified and successfully applied to the welds in place. Basic investigations have been performed to explain the embrittlement (TE) as well as the de–embrittlement (RTE). As basic cause for the embrittlement, phosphorus segregation in the grain boundaries was identified by high resolution Atom Probe Tomography (APT). The APT results alsorevealed that by application of RTE treatment, the phosphorus segregation in the grain boundaries disappeared. According to Mc Lean, the reversal mechanism of the temper embrittlement is explained by grain boundary equilibrium segregation of phosphorous.