Effects of neutron irradiation on the microstructure and mechanical properties of the heat affected zone of stainless steel welds

The effects of neutron irradiation on austenitic stainless steels, usually used for the manufacturing of internal elements of nuclear reactors (e.g. the core shrouds), are the alteration of the microchemistry and the microstructure, and, as a consequence, of the mechanical properties. The present study is aimed at extending knowledge upon the impact of neutron-irradiation on the heat affected zone (HAZ) of welded materials, which was influenced by the thermal cycles upon fusion welding. Two types of austenitic stainless steels welds, AISI 304 and AISI 347, referred to as test materials, have been produced by FRAMATOME ANP (Germany) using a welding procedure that was a compromise between the conditions applied to real reactor components and the restrictions concerning dimensions and the allowable deformation imposed by this research project. The welded test materials have been irradiated with neutrons in a High Flux Reactor in Petten (The Netherlands) at a temperature of around 573 K (approximate operating temperature of light water reactors) to 0.3 dpa and 1 dpa. A welded AISI 304 type austenitic stainless steel, so-called in-service material, originating from a decommissioned light water reactor in Mol (Belgium) which had operated for 25 years and having accumulated different dose levels, to a maximum of 0.3 dpa, was also studied. The effect of neutron irradiation on the HAZ was evaluated by studying the microstructure and mechanical properties before and after irradiation. The characterisation of the microstructure was made by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The mechanical properties were determined by performing microhardness measurements and tensile testing. Tensile tests were conducted on small flat specimens at two deformation temperatures: room temperature and about 573 K. For the unirradiated and very low dose irradiated materials, optical microscopy observations showed that the grain size is larger in the HAZ as compared to the base material (BM) due to the high temperatures reached during welding. The HAZ extends over around 600 µm on both sides of the weld. TEM observations showed that the HAZ contains a higher dislocation density than the BM due to the thermal cycles upon welding. The HAZ also contains small ferrite islands dispersed in the austenite matrix. Concerning the irradiated materials, TEM observations have shown that the austenitic matrix contains a large number of irradiation-induced defects. These defects are black dots, too small to be identified in TEM, and Frank loops, which can be either of vacancy or of interstitial type. In the in-service material the irradiation-induced defect density was found to be higher in the HAZ as compared to the BM. The higher defect density in the HAZ may be due to the higher grain size in the HAZ as compared to the BM, leaving less sinks (e.g. grain boundaries) for irradiation-induced defects annihilation. No irradiation-induced defects have been observed by TEM in the bcc ferritic interphase, which confirms that the irradiation-induced defects accumulate at a smaller rate in bcc materials than in fcc ones. Following tensile testing at room temperature the microstructure of unirradiated materials contains mainly twins. Following tensile testing at high temperature, the microstructure appears composed of dislocation cells. These results are independent on the specimen position from the fusion line. In the case of irradiated materials the deformation microstructure contains mainly stacking faults and twins. It seems to present no significant dependence on the material type, the irradiation dose and the test temperature. Tensile tests performed on all irradiated materials revealed an increase in the yield strength (radiation hardening) and a decrease of the uniform elongation (loss of ductility), at both deformation temperatures. Radiation hardening presents lower values in the HAZ as compared to the BM. The loss of ductility is higher in the HAZ as compared to the BM. Radiation hardening was analysed using the dispersed obstacle hardening model. It was found that the measured radiation hardening cannot be explained solely by the presence of the irradiation-induced defects observed in TEM. Smaller irradiation-induced features (not resolvable in TEM) apparently also contribute to radiation hardening. In conclusion, the HAZ presents a resistance to neutron-irradiation that is similar to the one of the BM, in terms of accumulation of irradiation-induced defects (black dots and Frank loops) and changes in mechanical properties (hardening and loss of ductility). The degradation of the mechanical properties of the HAZ clearly results from irradiation and not from welding. It seems that the threshold dose for peculiar deterioration of the HAZ, in terms of apparition of cracks or microcracks, was not reached in the present study.

Baluc, Nadine Laurence
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3326-6

 Record created 2005-08-31, last modified 2018-01-27

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