Specific Welds for Test Blanket Modules

Michael Rieth¹

¹Forschungszentrum Karlsruhe, IMF I, 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 fusion zone and from softening in the vicinity of the heat affected zones. To improve the joint performance usually two-step post welding heat treatments are applied where the temperature of the first step (austenitization) typically varies around 1000 °C and the second step (tempering) below 800 °C.

An evaluation of different welding techniques as well as the necessity and type of post weld heat treatments play an important role for the fabrication of test blanket modules (TBMs) which will be inserted into ITER. Therefore, the weld fabrication and characterization was performed with respect to scalability and transferability to the real component.

The results of a comprising investigation program show that stiffening grids in the test blanket modules, which are typically fabricated with 8 mm Eurofer plates, can be welded by tungsten-inert-gas (TIG) and laser beam technique. But these welds need tempering treatments of at least 750 °C.

Another critical weld is the joint between the First Wall and the caps of the TBMs, since here the welding depth is about 40 mm. It is shown that such welds can be performed best by electron beam welding but certainly need a complete two-step post-welding heat treatment.

The weld characterisation was done by charpy and creep tests which were accompanied by microstructural examinations. The focus was laid on the analysis of fracture initiation in the different specimens.