Mechanical properties of two ultrahigh carbon-boron tool steels

The microstructure and the mechanical behavior of two modified boron-containing tool steels were investigated at low and high temperatures. Both steels were processed by powder metallurgy methods, involving gas atomization and hot isostatic pressing at 180 MPa for 2 h at 900, 1000 and 1100 °C. The microstructure of the consolidated tool steels consist of a ferrite matrix and carboboride M23(C,B)6 particles. The 1wt.%C-1wt.%B tool steel contains, in addition, small particles of vanadium carbide and vanadium boride. An ultimate tensile strength of 300 MPa at 700 °C was obtained in the 1wt.%C-1wt.%B tool steel. At testing temperatures in the austenitic phase, a stress exponent of about 5 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that slip creep is the controlling deformation mechanism in the 1wt.%C-1wt.%B tool steel tested in this temperature range. At testing temperatures in the ferritic phase the creep rate was slower than that predicted by the slip creep equation. This was attributed to the presence of fine second phase particles. On the other hand, the 1.5wt.%C-0.5wt.%B tool steel showed an ultimate tensile strength somewhat lower than 300 MPa at 700 °C. At testing temperatures in the austenitic phase, a stress exponent of about 2 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that grain boundary sliding is the controlling deformation mechanism in the 1.5wt.%C-0.5wt.%B tool steel tested in this temperature range. A contribution from slip creep exists at testing temperatures in the ferritic phase.