An evaluation of heat partition in the high-speed turning of AISI/SAE 4140 steel with uncoated and TiN-coated tools

In manufacturing by machining, thermal loads on cutting tools can have a major influence on tool wear and hence process cost, especially at higher cutting speeds. An investigation has been undertaken to determine heat partition into the cutting tool for high-speed machining of AISI/SAE 4140 high-strength alloy steel with uncoated and TiN-coated tools. The cutting tests have been performed at cutting speeds ranging between 100 and 880 m/min with a feed rate of 0.1 mm/rev and a constant depth of cut of 2.5 mm. Cutting temperatures are measured experimentally using an infrared thermal imaging camera. The sticking and sliding regions are investigated from an examination of the tool-chip contact region using a scanning electron microscope (SEM). In addition, non-uniform heat intensity is modelled according to the contact phenomena. In this work, evaluation of the fraction of heat flowing into the cutting tool is carried out by iteratively reducing the available heat flux until the finite element method (FEM) temperatures are simultaneously matched at multiple points with the experimentally measured temperatures. This paper elucidates on the differences in thermal shielding for uncoated and TiN-coated tools. It is found that heat partition into the cutting tool decreases from a fraction of 0.41 to 0.17 for conventional cutting speeds and increases from 0.19 to 0.24 for high-speed machining when using uncoated carbide cutting tools. On the other hand, with TiN-coated tools, heat partition varies from 0.35 down to 0.095 for the whole range of cutting speeds. These results clearly show that the use of TiN-coated tools generally reduces heat partition into the cutting tool, but does so more significantly in high-speed machining (HSM) as compared with conventional machining speeds. The driver behind this study on heat partition in machining with TiN coatings is the design of coatings with enhanced thermal shielding properties.