Atom probe investigations on temper embrittlement and reversible temper embrittlement in S 690 steel weld metal

Severe embrittlement was observed in weld material of a brand new penstock of a huge hydro power plant. Temper embrittlement (TE) was found as root case of embrittlement. Reversible temper embrittlement (RTE) treatment characterised by a short-time heating at about 600 degrees C, by which the toughness of embrittled weld material can significantly be recovered, was qualified and successfully applied in the plant. Basic investigations were performed to explain the embrittlement as well as the de-embrittlement effect. By the application of high resolution analytics as Atom Probe Tomography (APT) applied on TE as well as on the RTE-treated material, revealed phosphorus segregation in the grain boundaries as root cause of embrittlement. By application of RTE treatment the APT results revealed, that the phosphorus segregation in the grain boundaries disappeared. The mechanism of this behaviour can be explained by referring the McLean [Grain boundaries in metals. Oxford: Clarendon Press; 1957] based grain boundary equilibrium segregation of phosphorous. During RTE treatment, which occur at higher temperatures (600 degrees C) that segregation (which starts during cooling at about 550 degrees C), desegregation occurs. During this higher temperature, the diffusion is much faster than segregation producing the fast recovery of toughness.