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Abstract

A fracture mechanics approach, involving high temperature fracture toughness tests, is used to investigate embrittlement during post-weld heat treatment in a low-alloy Mn-Cr-Mo-V steel. A large decrease in the fracture toughness of the coarse-grained heat-affected zone occurs upon heating to 600°C, and coincides with the temperature range for reheat cracking in this steel. The very low fracture toughness (K_Ic=32 MN/m^3/2) is due to severe embrittlement of the prior austenite grain boundaries, which is shown to be similar to temper embrittlement. Exposed crack faces consist of grain boundary facets, devoid of evidence of creep rupture. This kind of fracture is found in cases of severe reheat cracking, and differs fronz the grain boundary creep rupture mode otherwise observed. It is proposed that fracture occurs by unstable extension of pre-existing defects under the action of residual welding stresses, and follows the embrittled grain boundary network.

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Embrittlement of the weld heat-affected zone in a Mn-Cr-Mo-V steel

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