MICROMECHANISMS OF TOOL WEAR IN MACHINING FREE CUTTING STEELS

A quantitative investigation of tool crater wear was carried out in fr ee cutting steels with and without lead addition (commercial grade AIS I 12L14 and AISI 1215 respectively) at moderately high cutting speeds (140-200 m min(-1)) using cemented carbide cutting tools. Crater wear was quantitatively measured by determining the amount of tungsten carr ied 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 diss olution of the tool into the workpiece is the dominant mechanism of to ol crater wear. Experimental results have confirmed that lead decrease s the cutting force and the contact length but is ineffective in suppr essing tool dissolution wear. Since dissolution of the tool occurs by a diffusion mechanism, it should be possible to design a diffusion bar rier at the tool-chip interface to suppress dissolution wear. It is de monstrated that deformable oxide inclusions (CaO-Al2O3-2SiO(2)) engine ered 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) les s than that of tungsten carbide in the austenite phase of the steel at the tool-chip interface temperature, Thus, inclusion engineering of t he 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 subje ct of Section 4. The effect of tribology of seizure occurring at highe r cutting speeds on the toot-chip interface temperature is analysed us ing finite element modelling. The shear Bow 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 s eized region on the diffusional transport is analysed. A comparison of the experimentally measured tungsten transported to the chip with the theoretical prediction suggests that an enhanced diffusion operates a t the tool-chip interface. High diffusivity paths contribute to an enh ancement in the diffusion coefficient that is two orders of magnitude greater than the lattice diffusion coefficient.