Influence of Stress on Intergranufar Brittleness of a Martensitic Stainless Steel

摘要

Phosphorus intergranular segregation can strongly decrease grain boundary cohesion in steels. Martensitic stainless steel 17-4 PH was tempered 4 hours at 600℃ and aged for long times at 320℃ (up to 15700 h). Tempering treatment at 600℃ leads to phosphorus intergranular segregation, although there is no sign of intergranular brittleness because of the low hardness of the steel. Ageing treatment at 320℃ makes the steel much harder (α' precipitation) and it is then possible to obtain intergranular ruptures. It was observed that the shift of the ductile-to-brittle transition temperature due to ageing at 320℃ was more important if the ageing treatment was carried out under a tensile stress of 500 MPa, even though the hardness level is unchanged by the stress. This shift was attributed to an increase of phosphorus concentration in the grain boundaries that lie perpendicularly to the tensile stress axis, which corresponds to the longest side of the Charpy specimen. The interpretation of this effect is based on the following assumption: the segregation free energy of phosphorus is increased for grain boundaries normal to the tensile axis. Tempering treatment at 600℃ leads to phosphorus segregation in all the grain boundaries. But, during ageing at 320℃, the application of a tensile stress make the phosphorus diffuse from grain boundaries that lie parallel to the tensile axis toward grain boundaries that lie normal to the tensile axis. This assumption was confirmed by intergranular segregation measurements made on specimens that were heat treated under a flexion load.