Prediction of sigma-phase embrittlement and its influence on repair weldability for type 316FR stainless steel weld metals with different solidification modes

To clarify the influence of long-term σ phase embrittlement on repair weldability for type 316FR stainless steel weld metals (316FR-AF and 316FR-FA), the prediction of σ-phase embrittlement and a spot-Varestraint test were performed. The precipitation of σ and χ phases were characterized during accelerated aging treatments for both weld metals, and the precipitation behavior (σ+χ phases) could be expressed by a Johnson−Mehl-type kinetic equation. By using the variation of mechanical properties upon precipitation (σ+χ phases) and the determined kinetic equation, σ-phase embrittlement for 316FR-AF and 316FR-FA weld metals during service exposure in FBR plants could be successfully predicted. Specifically, a 30.7% decrement of the absorbed impact energy as compared with the as-welded specimens could be anticipated just after one year of service for 316FR−FA weld metal at 823 K. By using the simulated weld metal with σ-phase embrittlement, the possibility of weld cracking caused by σ-phase embrittlement during repair welding was confirmed, showing micro-crack formation within the σ phase after the spot-Varestraint test.