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Laser welding of irradiated 316L(N) ERHII 1MM plate: NET task deliverable PSM1 GB6 ECN-1
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ECN-C--98-043 ECN rapport
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Welding of parts that have received a significant neutron dose will benecessary in thermonuclear reactors like ITER and DEMO in view of the foreseen remote repair, maintenance and replacement of specific components. Welding of irradiated or helium containing austenitic stainless steel has been shown to be problematic because of weld cracking. Results of laser weldments of 0.5 and 5 dpa irradiated 316L(N)-ER11H plates, containing 7 respectively 30 appm helium, to unirradiated 316L(N)-ERHU plate are discussed with respect to welded joint quality, including mechanical properties, fractography and microstructure. All the welds are free of macroscopic HAZ- and fusion zone cracking. This is remarkable when compared to the 1 appm helium cracking threshold that is usually assumed for TIG welding. The yield stress and tensile strength of laser weldments of irradiated to unirradiated plates are comparable to laser weldments of unirradiated plates. The ductility tends to be significantly lower. The reduction of area tends to decrease with higher welding speeds (i.e. lower heat input) and higher dose. This is reflected in the fractures, which, for the irradiated-unirradiated combinations, occurred in the fusion zone. This while the welded unirradiated-unirradiated combinations fractured in the base material. The fractographic study showed a shift from transdendritic to interdendritic dimple fracture with higher welding speeds and higher dose. This means a more brittle fracture, which is reflected in a lower reduction of area. In the hardness measurements a dependence of the HAZ-width is found on the heat input in the case of 0.5 dpa irradiated material, higher heat inputs giving a wider HAZ. Lower heat inputs obtained by higher welding speeds can cause an increase of the hardness in the fusion zone. In the microstructure no pores were found in the fusion zone. The welds were overall free of hot cracks, except for some minor ones at the fusion line on the side of the irradiated part. This leads to the conclusion that with laser welding, sound welds can be made in 316L(N)-ERHU material with helium concentrations of up to 30 appm. The irradiation is responsible for a reduction of the ductility of the welded joints. This seems to be caused by a weakening of the dendrite grain boundaries in the fusion zone. The current work does not show whether the dendrite grain boundary weakening is caused by the accumulation or presence of helium alone. 9 refs.

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