Studies on End Milling of Maraging Steel Using Cryogenic Treated and PVD Coated Cemented Carbide Inserts Under Dry

Abstract

Maraging steel MDN 250 is an ultra-high strength steel which is developed to meet the large demand for high strength materials. The high strength of maraging steel is due to the precipitation of intermetallics during aging. Maraging steel finds wide applications in tool dies, a piston rod in heavy vehicles, rocket parts etc. The high strength combined with good hardness makes maraging steel difficult to machine material. Excessive tool wear, high heat generation, high power consumption, larger cutting forces, poor surface quality and/or difficulties in chip formation are some of the difficulties faced while machining difficult to cut materials. It is difficult to overcome these difficulties by the use of conventional cutting methods and tool materials. As the conventional cutting tools cannot withstand the high cutting temperature and cutting forces and results in tool wear while machining these difficult to cut materials. This early failure of cutting tools reduce the surface finish, increase the idle time and production cost. Some of the techniques used to overcome these difficulties are cryogenic treatment of cutting tools, coating of cutting tools, using sustainable cutting fluids like cryogenic liquid nitrogen etc. The cryogenic treatment is a new technique which improves the physical and mechanical properties of existing cutting tool in the most economic and sustainable way. It is reported that cryogenic treatment can improve some of properties of cutting tool like tool life, micro hardness, wear resistance, fatigue life, rupture strength and compressive residual stress. The cryogenic treatment use liquid nitrogen at -196°C for cooling the samples to cryogenic temperature and generally held for 24 hours to improve the properties of cutting tool. The present study investigates the effect of cryogenic treatment of cemented carbide (WC-Co) inserts at the different soaking period of 18 h (CT-18), 24 h (CT-24) and 32 h (CT-32) at a sub-zero temperature of−196 °C. The cryogenically treated inserts exhibited higher tool life, better surface finish and lower cutting forces during machining at different spindle speeds. The optimum soaking time for cryogenic treatment of WC-Co inserts is found to be 24 h (CT-24) beyond which there is no improvement in microhardness and wear resistance. However, as the spindle speed increased the effect of cryogenic treatment diminished. Hence the machining performance of cryogenic treated WC-Co inserts at a soakingvi period of 24 hours under three different environment of dry, wet and cryogenic has been investigated. The machining performance and tool life extended under cooling environments and highest tool life and machining performance is found to be during cryogenic machining. Coatings on the cutting tools are one of the outstanding strategies developed to avoid the difficulties in machining like rapid tool wear and lower tool life. A large number of PVD coatings are developed for the milling operations to have better performance. Aluminium and silicon based coatings find most promising applications in the end milling. Thus aluminium based coatings like AlTiN and AlCrN coatings synthesized by cathodic arc deposition (CAD) and silicon nitride based coatings like TiSiN and TiAlSiN synthesized by magnetron sputtering were studied for end milling performance. The coated tool along with the use of cutting fluid can minimize the tool wear and extend the life of the cutting tool. Also considering the environmental hazards, operator safety, recycling, and the disposal issues, use of conventional cutting fluids should be minimized. The liquid nitrogen is used in the experiment as nitrogen is abundant in the atmosphere and causes a rapid reduction in cutting temperature and quickly evaporates into the atmosphere. The tool life is maximum using AlCrN coated tool (125 min) compared to cryogenic treated and other coated tools at a spindle speed of 270 rpm under cryogenic environment. AlCrN > TiAlSiN > AlTiN > TiSiN > CT-24 is the order of tool life of cutting tools.