As shown schematically in the graph, high-temperature hardness is made possible only by a very important straightening mechanism: secondary hardening, promoted by the precipitation of fine alloy carbides. The stronger the secondary hardening (meaning more intense carbide precipitation), the higher the tempering resistance of hot-work tool steels. Such precipitation intensity depends on the amount of alloy elements in solid solution, which is related to the alloy composition and heat treating practice obtained by molybdenum, vanadium, or tungsten alloying.
The graph shows a schematic of hardness after tempering and the effect of secondary hardening in high-alloy steels. Observe that high-temperature hardness is only possible through precipitation hardening caused by alloy carbides (secondary hardening). At low temperatures, hardness is less than martensite due to the presence of retained austenite, which is eliminated after high-temperature tempering.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Hot-Work Tool Steels ⇒ Introduction
