Inclusion engineering of steel to prevent chemical tool wear

Trent introduced the concept of tribological conditions of seizure at the tool–chip interface, when the normal pressure exceeds the flow stress of asperities so that the asperities are squeezed to make atomic contact. In consequence, chemical dissolution of the tool into the chip occurs by a diffusion mechanism, causing chemical wear. Oxley incorporated the concept of seizure in his quantitative model for flow chip morphology. Oxley introduced the concept of equilibrium shear angle in his quantitative model for flow chip morphology, which incorporates the work of shear in the secondary shear zone instead of friction, once atomic contact is established, but Oxley's model ignored potential interaction from metallurgical softening events in the secondary shear zone, which led to prediction of unrealistically high temperature at high cutting speeds. In fact, metallurgical softening events do occur particularly at high cutting speeds, causing shear localisation, which leads to significant deviation from Oxley's model predictions. In this paper, Oxley's model will be extended to capture the interaction of shear localisation in the secondary shear zone on the mechanics of metal cutting. Dynamic recrystallisation, phase transformation of the matrix and geometric softening owing to second phase particles are identified as important microstructural softening events causing shear localisation, which could intervene before the equilibrium shear angle is reached. The occurrence of shear localisation is shown to alter the mechanics of metal cutting, chip morphology and the tool wear mechanism. A phenomenological database in model alloys will be presented to validate the model assumptions. The application of the model in the design of self-lubricating free cutting steel for moderate and high cutting speeds will be examined.