Creep properties and microstructure change of FBR grade type 316 stainless steel weld zone

Hasebe, Shinichi; Onizawa, Takashi  ; Kato, Shoichi 

We conducted a long-term creep test of weld metal and welded joint made from what we consider optimal filler metals for more than 10000 hours at 550C, in order to evaluate their long-term properties at high temperatures, and select the appropriate filler metals for FBR grade type 316 stainless steel. We also conducted an evaluation of the long-term high temperature strength of the weld metals by observing changes in the microstructures that was subjected to material deterioration. The results obtained are as fo1lows. (1)Weld metal and welded joint made from 316 type and a specific optimal material for 16-8-2 type showed better creep properties than current materials. Especially for 16-8-2 type, the quality improved so much that predominance microstructure stability in the region of long-term. (2)We clearly showed that when -ferrite phase decomposed by long time heating. the Laves phase, phase and austenite phase were precipitated and the remaining -ferrite phase was changed to an -ferrite phase (Cr≒12%, Ni≒2%) as it became a low-alloy and reached equilibrium. (3)The long-term creep strength of the 316 type weld metal tends to decrease as phase separation increases due to a high Cr concentration in -ferrite phase. On the other hand, we confirmed that 16-8-2 type weld metal could maintain long-term creep strength almost as high as the base metal, because there was little separation of inter-metallic compounds due to its low concentration of Cr. (4)We found that change in the microstructure can be easily captured by analyzing the composition of the remaining -ferrite phase. This is an effective method to evaluate long-term high temperature strength.