Improved mechanical properties by nanosize tungsten-molybdenum carbides in tungsten containing hot work die steels

AISI H13 die steel with substitution of tungsten for vanadium (H13–W) was produced through vacuum induction melting and forging, followed by 1050°C-quenching and 600°C-tempering. Through 10–40 h of 600°C-tempering, H13–W exhibits superior red hardness as compared to H13 steel. The increments of hardness (4-6HRC) as well as yield strength were deducted to the precipitated M₆C-type carbides with sizes ranging from 10 nm to 400 nm. Two classifications of carbides are identified. Type I of carbides has the nano size of 8–50 nm, and type II of undissolved primary carbides ((Fe_1.34Mo_1.62 Cr_2.95W_0.09)C) has the bigger size of 100–400 nm. The yield strength increments ($\Delta {\sigma }_y$) were modeled by a bivariate distribution of nano and undissolved precipitates. The calculated major strengthening effects from type I nano carbides in H13–W/H13 steels were close to the deduced values by Hall-Petch theory, they were 1053 MPa and 965.8 MPa, respectively. According to the calculation on equilibrium solubility of precipitations of MoC/Mo₂C and WC/W₂C, it was testified that MoC, Mo₂C, WC and W₂C were wholly dissolved into austenite at 1050 °C of quenching, followed by precipitating in large quantities in the form of nanosize carbides at 600^oC-tempering.