Microstructure and Mechanical Properties of Different EUROFER Welds

M. Rieth, Forschungszentrum Karlsruhe, Eggenstein-Leopoldshafen (Germany) 
In contrast to austenitic steels untreated welded joints of ferritic-martensitic steels like EUROFER suffer from hardening and embrittlement due to uncontrolled martensite formation in the weld and from softening in the vicinity of the heat affecting zones. With respect to specific Test Blanket Module (TBM) design and assembly requirements for DEMO there is a significant discrepancy between the necessary post welding heat treatment and its applicability. Therefore, Tungsten-Inert-Gas (TIG) with EUROFER filler wire, Electron Beam (EB), and Laser welding have been applied to EUROFER plates in the condition as received. Prior to specimen fabrication the microstructure of the different welds have been investigated. Both TIG welds show coarse grain formation which is typical for solidification micro structures that form during the welding cycles. Both beam welds don’t show this severe grain coarsening. Also typical for TIG welds are the softened regions in the HAZ as can be seen from the hardness profiles. While the lateral extensions of the beam welds are significantly smaller, softening in the HAZ cannot be observed here. From these microstructural examinations it is already clear that TIG welds need a full two-step heat treatment (austenitization plus annealing) in order to recover a uniform distributed fine grain. This was also confirmed by poor charpy test results. But charpy tests on beam welded specimens have shown surprisingly good results, even without post-weld heat treatment. Compared to the EUROFER base material, the Ductile-to-Brittle-Transition-Temperature (DBTT) of Laser and EB welds is almost comparable. Further assets and drawbacks of these different welding technologies are discussed in detail based on tensile and charpy tests performed on welds with and without applied post-welding heat treatments. The examinations were completed by according microstructural examinations.

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