Machinability effects of stainless steels with a HIPed NiTi coating in high-efficiency machining operations:Dissertation

The machinability effects of new high-strength stainless steels are researched due to specific properties arising from their structure. In grinding operations, HIPed (Hot Isostatically Pressed) austenitic 316L, duplex 2205 and super duplex 2507, and as-cast 304 stainless steel, in turning HIPed 316L, duplex stainless steel 2205 and X5 CrMnN 18 18 stainless steel, and in drilling HIPed PM (Powder Metallurgic) Duplok 27 and duplex stainless steel ASTM8190 1A and X2CrNi 1911 with HIPed NiTi coating were researched in revised machining testing environments using tool life testing, chip and workpiece surface morphology analysis. Chips, workpiece surfaces and cutting tools were analysed by SEM and EDS. High toughness, workhardening and low heat conductivity have a synergistic effect in inducing machinability difficulties e.g. decreased product quality, shorter tool life, increased power consumption and decreased chip evacuation. An increased amount of alloying elements is found to decrease machinability in the form of increased cutting force and workhardening rate of the machined surface, and decreased tool life and surface roughness. Also, the machinability of PM-produced stainless steels is decreased because of the increased amount of hard oxide particles included in the microstructure of PM-produced stainless steel. The formation of BUE (Built-up Edge) is found, affecting the machinability and tool life of tested high-strength stainless steels. In grinding operations HIPed austenitic 316L and duplex 2205 stainless steel are rated according to cutting force, workhardening rate and the amount of microvoids and microcracks in ground surfaces. In turning operations HIPed 316L, duplex stainless steel 2205 and X5 CrMnN 18 18 stainless steels are assessed in machinability order. The machinability of conventional cast duplex stainless steel ASTM8910 and HIPed duplex stainless steel Duplok27 were sorted according to the PRE-value (Pitting Resistance Equivalent). Finally in this study, the suitability of coated cemented carbide tools in the drilling of conventionally produced cast stainless steels with HIPed NiTi-coating was investigated. In drilling of difficult-to-cut X2CrNi 19 11 stainless steel with a pseudo-elastic coating, effective cutting parameters that maintain an adhesion layer between the NiTi coating and the stainless steel intact with an advantageous surface finish were generated