Abstract
The results of this study are combined with earlier findings to present a wider account of the causes of machinability variation in low-carbon steel. It was found that a given steel can exhibit differences in machinability that cannot be directly correlated with any of the obvious microstructural features that may be determined by optical, or even electron, microscopy, nor predicted on the basis of mechanical properties. During air cooling, ferrite may become supersaturated with interstitials, but even these low concentrations of interstitial elements are sufficient to cause considerable changes in machining properties. Similar effects are obtained when small cementite particles dissolve during chip formation, thereby providing the interstitial solutes. The effect of cementite morphology, size, and distribution is appraised in terms of its stability under the extreme conditions of strain and temperature typical in metal-cutting operations. Although this study mainly involved the machining of low-carbon steel with high-speed-steel tools, evidence is presented to show that the effect may also be relevant to other steels and tool materials.
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