The liquid metal embrittlement of 9Cr steel in sodium environments and the role of non-metallic impurities

Abstract

Liquid metal embrittlement (LME) of 9Cr steel has been studied in sodium environments, to which 0–50 w/o NaOH was added, at temperatures between 150 and 525°C. LME occurred under dynamic straining with increasing severity from 150–450°C by a quasi-cleavage mode of fracture. At a slightly higher temperature, which depended on strain rate, there was a transition in behaviour and LME did not occur. The steel was resistant to LME under constant load except at temperatures below the transition temperature when stresses were high enough to cause significant creep. LME could be greatly reduced, or eliminated, either by cold-trapping or hot-trapping the sodium; thus it is inferred that a non-metallic impurity species is involved in the cracking mechanism. For environmental conditions typical of sodium-cooled nuclear plants, the occurrence of LME only in the post-UTS region of the stress-strain curve, i.e. in the region of non-uniform elongation and approach to failure, indicates that it is not directly relevant to plant design.