Tempering of high-purity and commercially based steels containing 10 wt-% tungsten or 5 wt-% molybdenum

The effect of similar atomic percent additions of tungsten and molybdenum on the tempering behaviour of high-purity ternary and commercially based medium-carbon steels has been examined. A high-purity steel containing 5.6 wh%Mo (3.3 at.-%) showed substantially higher secondary hardening than an alloy with 9.9 wt-%W (3.2 at.-%). Marked differences in hardness were not observed when additions of 3 wt-%Cr and 0.3 wt-%V, frequently found in die steels, were present. Reasons for this behaviour are discussed.

On tempering the ternary steels the alloy carbide precipitation sequence was M₂C → M₆C and the close association of M₂C and M₆C in some regions of the steel containing molybdenum suggested that an in situ transformation might be occurring. In the commercially based steels the presence of chromium led additionally to the formation of relatively small amounts of M₂₃C₆ during the early stages of tempering. Analytical electron microscopy has been used to examine the composition of the M₂C and M₆C carbides in the ternary alloys during tempering at 700°C. The solubility of iron in M0₂C was appreciably lower than for W₂C, and this may have contributed to the much lower rate at which the M₂C → M₆C transformation occurred in the molybdenum alloy compared with the tungsten alloy.