Machinability Improvement of 17-4PH Stainless Steel by Cryogenic Cooling

Abstract

17-4PH stainless steel is a martensitic precipitation hardening stainless steel that provides an outstanding combination of high strength, good corrosion resistance, good mechanical properties, good toughness in both base metal and welds, and short time, low-temperature heat treatments that minimize warpage and scaling. This valuable alloy is widely used in the aerospace, nuclear, chemical, petrochemical, food processing, power generation, and naval industries; however, 17-4PH stainless steel is categorized as hard to machine materials due to low thermal conductivity and high toughness. Tool wear in traditional machining of 17-4PH stainless steel is high; hence, low tool life causes high tooling cost. In this paper, indirect cryogenic machining was used, in order to improve machinability of 17-4PH stainless steel in turning operation with TiN coated carbide insert tool. Pressurized-liquid-nitrogen (LN) was used as a cryogenic coolant. Nitrogen gas applied on the liquid nitrogen to pressurize it. A specific tool holder was designed and manufactured for cryogenic turning. Cryogenic machining decreases temperature-dependent tool wear and increases tool life by keeping tool temperature low. Cutting force, tool flank wear and maximum tool temperature have been studied as machinability parameters. Cutting force was measured by the Kistler 9121 piezoelectric dynamometer. The Dino-Lite digital microscope with 20-200X magnification was used to measure tool flank wear. The experimental results showed that cryogenically enhanced machining decreases cutting force and tool flank wear by 22 and 23 percent, respectively, compared with dry turning. Predicting of tool life using linear extrapolation showed that tool life in cryogenic turning improved by 39% over dry turning. In addition, cutting force in cryogenic machining became more stable than the force in dry condition. Thermal analysis of the carbide tool performed in the ANSYS Software using experimental data. Thermal analysis showed that the maximum temperature of cutting tool in cryogenic machining is 75 percent lower than dry condition.