Experimental Investigation on Different Machinability Aspects of AISI 4340 Hardened Alloy Steel Using Cermet Inserts Under Various Cutting Environments

Abstract

In engineering industries nowadays, the hardened steel materials of hardness above 45 HRC have great demand for the manufacturing of automotive, aircraft and machine tool components due to their better strength, wear resistance and high thermal stability. To meet this demand, there is a need to consider the technological developments that are associated with cutting tools, workpiece materials, machine tools, process conditions and the manufacturing environments. Furthermore, machinability of hardened steels is of utmost importance while establishing the technological feasibility and economic viability of the manufacturing process. In recent past, hard turning has been established as an effective and emerging metal cutting process of hard steels and is explored vigorously as a profitable alternative to traditional cylindrical grinding which involves a series of costly and time-consuming operations.

The current research concerns with some machining attributes such as machining forces, tool wear, surface roughness, chip morphology, economical feasibility and other surface integrity aspects of hardened alloy steel using cermet inserts under various cutting environments. The present experimental investigation has been divided into six stages. AISI 4340 steel hardened to 48 HRC has been considered as the workpiece material and cermet insert as a cutting tool in all the six stages of work.

In the first stage, the performances of uncoated carbide and coated cermet inserts for various machinability aspects during the machining of hardened steel in the dry cutting environment were compared. Cutting speed, feed, and depth of cut were considered as the controllable parameters. Workpiece surface temperature, machining forces, and tool flank wear were taken as performance measures of various cutting inserts. All the three input variables were found to possess significant influence over workpiece surface temperature, feed and radial forces in case of both uncoated carbide and cermet. Cermet performed better than carbide in terms of flank wear, cutting force and workpiece surface temperature, whereas carbide outperformed cermet in terms of feed and radial forces. The depth of cut was found to be the most vital for feed and cutting forces in case of cermet whereas it is vital for radial force in case of carbide. Cutting speed significantly affected workpiece surface temperature and flank wear for carbide whereas it affected the radial force for cermet. The feed was the vital parameter when the flank wear of cermet was taken into account. In the second stage, the experimental and statistical analyses of flank wear, material removal rate, tool tip temperature, surface roughness parameters (i.e., Ra, Rz and Rt), chip morphology, chip thickness and dimensional deviations (i.e., circularity and cylindricity) have been performed using coated cermet. The experimental study exhibited that the depth of cut was the predominant machining parameter influencing surface roughness followed by feed rate and cutting speed. A similar result was found for flank wear and dimensional deviations. However, cutting speed was found to be the most crucial input parameter for tool tip temperature whereas the feed rate was the most notable input variable for material removal rate (MRR). Abrasion and chipping were two major wear mechanisms found for flank wear in this study. During chip morphology study using scanning electron microscope material side flow, serrations, shear band, shear cracks, smooth and rough surfaces were observed. Further, the effect of flank wear on surface roughness parameters, dimensional deviations and effect of MRR on different patterns of crater wear were studied. For each response, a mathematical model was developed with regression analysis and all the models had high R-Sq values confirming favourable relationships between predicted and experimented values.

Effects of mist cooling and dry cutting on cutting force, flank wear, chip morphology, crater wear, surface roughness and microhardness of chip were evaluated during hard turning operation of the same test specimen in the third stage. Water-soluble oil was applied for cooling and lubricating purposes in mist cooling, and a comprehensive comparative analysis was performed with the dry cutting environment. Dry machining was found more effective as compared to mist cooling technique against the chip microhardness and cutting force. For other responses like crater wear, flank wear, surface roughness and chip morphology, mist cooling performed better than dry machining owing to the reduction of cutting zone temperature and complimentary modification in tool-chip interaction during hard turning. In the fourth stage, various machinability aspects were investigated of the same workpiece material in the context of improvement in cutting force, tool flank wear, crater wear, surface roughness, microhardness, machined surface morphology, chip morphology, chip reduction coefficient and apparent coefficient of friction in minimum quantity lubrication (MQL) with three different cutting fluids i.e., compressed air, water-soluble coolant and nanofluid using uncoated cermet insert. It was observed that Al2O3 enriched eco-friendly nano coolant outperformed both compressed air and water-soluble coolant in terms of all machinability aspects. Influences of various nanofluids taking two base fluids (i.e., deionized water and rice bran vegetable oil) were investigated under MQL on the machinability aspects during hard turning of the same material in the fifth stage. The effects of various fluid properties were analyzed for all nanofluids. Various types of nanofluids were prepared by dispersing different nanoparticles of ZnO, CuO, Fe2O3 and Al2O3 at 0.1% concentration in a base fluid. However, ZnO was not considered in case of vegetable oil because of its poisonous nature. The impacts of nanofluids on various machinability responses were measured and analyzed. Among all the nanofluids, CuO based nanofluid showed the best performance while Al2O3 based nanofluid performed the worst.

In the final or sixth stage, the comparative assessment of various responses such as cutting force, flank wear, crater wear, chip morphology and surface roughness were carried out during machining of the same hardened steel with both untreated and cryo-treated cermet inserts under dry cutting condition. Lastly, the input variables were optimized using Response Surface Methodology (RSM) to evaluate the tool life for the economic analysis. The experimental result demonstrated that the uncoated deep cryo-treated with tempered cermet insert delivered better results in comparison to other cermet inserts. According to cost analysis, uncoated and deep cryo-treated with tempered cermet insert was found to be the most economical among other cermet inserts at the optimum cutting condition.