Fracture mechanical characterisation of ferrite-austenite dissimilar metal welds (DMWs) for elevated temperature service in view of metallurgical mis-match

A characteristic feature of dissimilar metal weld (DMW) is the metallurgical and material property mis-match resulting from a steep gradient of microstructures with significantly different strength and toughness properties. This mis-match inevitably affects the entire failure behaviour of DMWs under external operational loads, not only from crack initiation standpoint, but especially regarding the development of crack driving force accentuating crack path deviation and subsequent crack growth. The determination of relevant fracture toughness properties for all the different microstructural regions of the DMW is of utmost importance for successful structural integrity and lifetime analyses. This paper deals with experimental material characterisation of two configurations of ferrite (SA508)-austenite (Type 304/316) DMWs made using (i) a beveled V-groove and Alloy 82/182 filler metal with a buttering layer and (ii) a narrow-gap weld and Alloy 52 filler metal without any buttering. Results of hardness surveys and fracture mechanical (J-R curve) tests are presented and analysed. The role of mis-match (i.e. metallurgical constraint) in e.g. promoting sudden crack path deviations is discussed in the light of detailed post-test sectioning metallography and specimen fractography identifying the actual crack initiation and propagation path.