SUPERPLASTIC BEHAVIOR OF POWDER-METALLURGY 1.3-PERCENT-C-1.6-PERCENT-CR-0.8-PERCENT-B STEEL

The high temperature deformation behaviour of an ultrahigh carbon boro n tool steel containing 1.3wt-%C-1.6wt-%Cr-0.8wt-%B was investigated. The steel was processed by powder metallurgy methods, involving argon atomisation and consolidation by hot isostatic pressing at 180 MPa for 2 it at 700 degrees C. The microstructure of the consolidated materia l retains the dendritic solidification structure of the rapidly solidi fied powders. A ferritic grain size of 1 mu m was measured. This steel presents a superplastic behaviour in the temperature range 660 to 105 0 degrees C with stress exponents of similar to 2 at low strain rates and tensile elongations as high as 520% at 1000 degrees C. A change in the activation energy at the alpha-gamma transformation temperature ( 738 degrees C) was clearly observed. The creep data are in agreement w ith grain boundary sliding and slip creep models. (C) 1997 The Institu te of Materials.