Improved Red Hardness and Toughness of Hot Work Die Steel through Tungsten Alloying

Abstract

In order to elucidate the relationship between red hardness/toughness and tungsten content, strong carbide-forming element tungsten was added to a 5 wt.% Cr medium carbon steel (Cr5) in melting. After normalization and forging, the vacuum gas-quenched experimental steels were tempered at 560 and 600 °C for 10-40 h, respectively. Rockwell hardness and un-notched impact energy were evaluated. It was revealed that the precipitation phases play a critical role in the mechanical properties of the steels. The tungsten element effectively improved the red hardness of the steel by 6.3-15.8% when tempering at 600 °C for 10-40 h, while the corresponding impact toughness holds at the same level, about 250J, as Cr5 steel. Through observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), mesoscales carbides were found with a structure of (Fe, Cr, W, Mo, V)6C. It was deduced that tungsten element acts a positive role in inhibiting the coarseness of secondary M6C-type carbides and impeding the formation of M23C6 and M7C3, and thus enhances the thermal stability of Cr5W steel at elevated temperature. Furthermore, the tempering stability prediction model was constructed to predict the red hardness of tungsten-containing Cr5W steel.