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°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°C)
that segregation (which starts during cooling at about 550°C), desegregation occurs. During this
higher temperature, the diffusion is much faster than segregation producing the fast recovery of
toughness.