Performance of coolant strategies when turning hardened martensitic stainless steel using tialn coated carbide tool

Abstract

Coolant strategies in turning hardened stainless steel are important, due to the fact that heat cannot be removed efficiently from the cutting area. This heat issue shortens the tool life and reduces machined surface integrity, resulting in higher machining cost and lower productivity. Conventional cutting fluids cause health problems, workshop pollution and higher recycling cost. Dry, minimum quantity lubricant (MQL) and cryogenic machining are alternatives of green coolant to eliminate conventional cutting fluids. Thus, the objective of this research is to study the feasibility and performance of using new green coolant strategies that contribute to the sustainable process. Experiments were carried out in two different stages when turning 48 ±1 HRC martensitic stainless steel (AlSI420) uses a wiper PVD-TiAIN coated carbide cutting tool. Cutting speeds (l00, 135, and 170 m/min) and feed rates (0.16, 0.2, and 0.24 mm/rev) were investigated. The depth of cut was kept constant at 0.2 mm. Nitrogen gas pressure was 0.5 MPa and the oil mist (castor oil) flow rate was 40 ml/h. In the first stage, comparison between three cutting conditions were evaluated, namely cold nitrogen gas (cold N2), nitrogen gas with oil mist (N2+MQL) and cold nitrogen gas with oil mist conditions (cold N2+MQL). Dry cutting was used as the benchmark. In the second stage, the best cutting condition from first stage was used for further experiments to investigate the effect of cutting speed and feed on machining responses such as tool life (TL), volume of material removed (VMR), surface roughness (Ra) and cutting forces (Fx, Fy and Fz), chip morphology and microstructures of machined surface. Full factorial design was used to model the relationship between cutting responses (tool life, surface roughness, and cutting forces) and different cutting speeds and feed rates. These models were verified by performing confirmation experiments. The results obtained showed that cold N2+ MQL improved performance in terms of tool life, surface roughness and cutting forces in comparison to dry, cold N2, and N2+MQL conditions. At cutting speed of 100 m/min and feed rate of 0.16 mm/rev, cold N2+MQL condition prolongs the tool life by 135%, decreases the cutting forces by 18%, and improves surface roughness by 19% as compared to dry cutting. Flank and crater were observed at the tool nose. Abrasion and adhesion were the dominant wear mechanisms when turning hardened martensitic stainless steel. The machined surface had less alteration of grain microstructure and higher hardness in cold N2+MQL condition compared to the dry cutting condition. The longest tool life was obtained at low cutting speed and low feed rate, whereas lower cutting forces and better surface roughness were observed at high speed and low feed rate. Analysis based on the mathematical models of machining responses (tool life, surface roughness and cutting forces) would be helpful in selecting cutting variables for optimization of turning hardened stainless steel, which is in line with sustainable and green machining by using cold N2+MQL condition.