Neutron irradiation effect on the mechanical properties of type 316L SS welded joints
Introduction
In the design activity for International Thermonuclear Experimental Reactor (ITER), the vacuum vessel (VV) is one of the most important components from the view point of safety design [1], [2]. The VV is designed as a double walled structure so that some parts are not qualified by the conventional design standards. JAERI has executed the preparation activity for the new design standards [3] and obtained the technical data to support them. The irradiation data on the mechanical properties of 316L or 316LN base metal at low temperature (<523 K) and low dose (<1 dpa) are reported in the literature [4], [5], [6], [7], [8], [9], [10], [11], [12]. These data show that low temperature irradiation causes considerable irradiation hardening at low doses. For welded joints, a few data have been reported [6], [13], [14], [15]. The irradiation effects on weld metals (WMs) or heat-affected zone (HAZ) of welded joints are not fully understood. Therefore it is required to obtain irradiation data on welded joints at low temperature.
In this study, neutron irradiation tests were performed at the Japan Material Testing Reactor (JMTR) to investigate neutron irradiation effects on the mechanical properties of 316L SS welded joints. The irradiated specimens were cut from the welded joints made by the candidate welding processes for the fabrication of the ITER VV.
Section snippets
Materials and welded joints
Type 316L SS plates 40 mm thick were used in this study. The plates were solution-annealed at 1400 K, followed by water quenching. Butt-joint welds of the plates were made by tungsten inert gas (TIG) welding and T-joints were made by electron beam (EB) and metal active gas (MAG) welding. These welded joints were selected as representative examples of the candidate welding processes for the fabrication of the ITER VV. The chemical composition of the 316L SS base metal and its welding rods are
Tensile properties
The results of tensile tests on the base metal are summarized in Table 2 and the stress–displacement curves are shown in Fig. 1. The results indicate that the 0.2–0.5 dpa irradiation caused considerable irradiation hardening and degradation of ductility. It seems there were no differences in the results compared to the RD. The appearance of a yield point is a typical phenomenon for low temperature (−573 K) irradiation of austenitic steel [5]. The yield strength (YS) increased with dose level.
Conclusions
The results obtained from this study are as follows:
- 1.
Neutron irradiation up to 0.5 dpa at 473 K caused considerable irradiation hardening and degradation of ductility for base metal and welded joints. However, all specimens except MAG–WM showed sufficiently large RA and impact values. The specimens fractured in a ductile manner.
- 2.
The dpa dependences of the YS increase and the YS values of the base metal have almost the same trend as that in the literature.
- 3.
The RA values of MAG–WM specimens were
Acknowledgements
We greatly appreciate the helpful comments and supports given by the members of reactor structure laboratory and staffs of JMTR during this study.
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