Strengthening and embrittlement mechanisms in stainless steels

The creep strengthening mechanisms which are believed to operate in austenitic stainless steels are discussed. These mechanisms have been proposed on the basis of simple metallography and mechanical tests. The same simple assessment of strengthening mechanisms has been applied to experimental casts of type 316 steel, where it was previously shown that additions of phosphorus reduce the minimum creep rate up to 1,000 times. This assessment has been unable to identify the reason why phosphorus confers such potent strengthening. The addition of tin to type 316 steel has previously been shown to reduce the ductility. Two possible causes of the embrittlement by tin are changes in either grain boundary diffusivity or grain boundary sliding behaviour. No evidence has been found that tin alters the grain boundary diffusivity and no useful measurements of grain boundary sliding have been possible. The reason why tin promotes intergranular creep damage is still unknown. (author)