Weld HAZ mis-matching design is highly important for assuring joint performances is important. High chromium steel is expected as a material of the primary coolant pipes for the next generation of sodium cooled fast breeder reactor in Japan, whereas austenitic stainless steel is used for that of Monju. A lot of elbows in the piping system should be necessary for Monju because of relaxation of thermal stress. Thermal expansion of high chromium steel is lower than that of austenitic stainless steel, thus numbers of elbows shall be reduced, by using this material.

Piping system is fabricated to connect by welding process. Mechanical properties, for example, hardness and stress-strain characteristic curve, of the weld metal and heat-affected zone, HAZ, differ from base metal in the welded joint. The welded joint of high chromium steel has naturally softened HAZ due to multiple welding heat cycles. Tensile strength limit in a welded joint is affected by HAZ softening, and it is desirable to consider the softening effect and to increase tensile strength not in HAZ but in a whole welded joint. The effect of HAZ softening effect on tensile strength limit of the welded pipe joint is investigated in this paper.

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