Micromechanisms of tool wear in machining free cutting steels

A quantitative investigation of tool crater wear was carried out in free cutting steels with and without lead addition (commercial grade AISI 12L14 and AISI 1215 respectively) at moderately high cutting speeds (140–220 m min−1) using cemented carbide cutting tools. Crater wear was quantitatively measured by determining the amount of tungsten carried into the chips using instrumental neutron activation analysis. The bulk of tungsten in the chips occurs as soluble tungsten dissolved in the steel matrix rather than as tungsten carbide confirming that dissolution of the tool into the workpiece is the dominant mechanism of tool crater wear. Experimental results have confirmed that lead decreases the cutting force and the contact length but is ineffective in suppressing tool dissolution wear.Since dissolution of the tool occurs by a diffusion mechanism, it should be possible to design a diffusion barrier at the tool-chip interface to suppress dissolution wear. It is demonstrated that deformable oxide inclusions (CaOAl2O32SiO2) engineered into the workpiece (AISI 1215 IE) form a glassy layer at the tool-chip interface that suppresses dissolution wear. Alternatively a HfN coating put on the tool acts as an effective diffusion barrier, as the solubility of HfN is seven orders of magnitude (10 million times) less than that of tungsten carbide in the austenite phase of the steel at the tool-chip interface temperature. Thus, inclusion engineering of the workpiece and coating of the tool are identified as two viable and attractive options to replace lead in free cutting steels. Theoretical analysis of the above experimental observations constitutes the subject of Section 4.The effect of tribology of seizure occurring at higher cutting speeds on the tool-chip interface temperature is analyzed using finite element modelling. The shear flow of the chip material under the compressive stress of the seized region is described using Bowden and Tabor's equation. The effect of temperature distribution of the seized region on the diffusional transport is analyzed. A comparison of the experimentally measured tungsten transported to the chip with the theoretical prediction suggests that an enhanced diffusion operates at the tool-chip interface. High diffusivity paths contribute to an enhancement in the diffusion coefficient that is two orders of magnitude greater than the lattice diffusion coefficient.