When the hardness of die steel is below 58 HRC, wear resistance increases with increasing hardness; in other words, as we often say, the higher the hardness, the better the wear resistance.
However, when the hardness of die steel exceeds 58 HRC, wear resistance is determined by the type and morphology of the carbides.
The microstructure of die steel after heat treatment consists of martensite, retained austenite, and carbides. Among these, carbides provide the best wear resistance, followed by martensite.
Cold work die steel exhibits better wear resistance than low-alloy tool steel, primarily because it has a higher total alloy content, superior carbide composition, and a greater quantity of carbides. Although both Cr12MoV and GCr15 have a carbon (C) content of 1.5% and can be heat-treated to a hardness of 58–62 HRC, the wear resistance of Cr12MoV is three times that of GCr15. This is primarily because Cr12MoV contains 12% chromium (Cr), whereas GCr15 contains only 1.5% chromium (Cr). The higher the total alloy content, the better the quality and greater the quantity of carbides formed, resulting in naturally superior wear resistance.
Although the hardness of high-speed steel appears to be only 2–3 HRC higher than that of cold-work tool steel, its wear resistance increases exponentially. This is because high-speed steel is an alloy with high tungsten (W) and vanadium (V) content; after heat treatment, it develops a large amount of hard carbide structure, significantly enhancing its wear resistance. Furthermore, the finer and more uniformly distributed these carbides are, the better the wear resistance. Powdered high-speed steel exhibits better wear resistance than conventional cast high-speed steel precisely because its carbides are fine and uniformly dispersed—a benefit of the advanced powder metallurgy process.
Therefore, the statement that “the higher the hardness of mold steel, the better its wear resistance” is subject to certain conditions.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.