On the effect of equal-channel angular pressing on creep of tempered martensite ferritic steels

The effect of long-term aging and equal-channel angular pressing (ECAP) on creep of the tempered martensite ferritic steel P91 at 650 °C and 120 MPa was studied. We compare the creep behaviour of a material which was subjected to a standard heat treatment (austenitizing and air cooling followed by tempering and air cooling) to the creep properties of an overaged (standard heat treatment followed by 1 week overaging at 700 °C) and an overaged and severely plastically deformed (ECAP, one 90° pass at 250 °C) material. During overaging, carbides coarsen and as a consequence, the creep-rate of the overaged material is higher than that of the material after a standard heat treatment. Severe plastic deformation of the overaged material results in an increase of room-temperature hardness. But as a striking new result, the creep-rate of this material increases after ECAP. This result is discussed on the basis of a scenario where a high density of mobile dislocations results in higher creep rates.