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The variation in composition of carbides formed during the tempering at 700°C of complex medium-carbon steels containing tungsten or molybdenum has been followed by analytical electron microscopy. Quantitative analysis of carbides containing tungsten has been achieved using LαX-ray intensity measurements. Direct measurement of carbide compositions has meant that chemical as well as structural observations can be made during the tempering of alloy steels.

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 situtransformation might be occurring. In the commercially based steels the presence of chromium led additionally to the formation of relatively small amounts of M23C6during 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.