Machinability Studies on 17-4 PH Stainless Steel Under Cryogenic Cooling Environment

Abstract

17-4 precipitated hardened stainless steel (PH SS) is widely used in various areas including nuclear reactor components, marine constructions, jet engine parts, aircraft fittings, missile fittings, oil field valve parts and rotors of the centrifugal compressors owing to excellent properties like high corrosion resistance, high strength and good ductility. Productivity improvement while machining of 17-4 PH SS is a difficult work due to the limitation of higher cutting conditions. 17-4 PH SS material is treated as difficult to cut material due to formation of built up edges (BUE) on the cutting tool during machining and difficulty in chip control, which causes for poor surface quality as well as increases the number of tools required for machining. One of the methods to overcome above mentioned problems is to use of conventional coolants. But, in the recent years, environmental conscious regulations have became stringent in terms of disposal of chemically contaminated conventional coolants from the health and environmental safe prospective. Because of these reasons, nowadays, metal cutting industries are looking towards new sustainable machining method to reach the target set by the environmentally conscious regulations in terms of usage and disposal of chemical contaminant conventional coolants without sacrificing the productivity. Hence, the present work, focused on cryogenic machining which is recently developed eco-friendly as well as efficient cooling technology. The present work is divided into three phases while machining of 17-4 PH SS. In the first phase, experiments were conducted based on the one factor at a time approach to study the individual effect of process parameters like cutting velocity, feed rate and depth of cut on performance characteristics like cutting temperature, tool flank wear, material removal rate (MRR), chip morphology and surface integrity (surface topography, surface finish, microhardness, white layer thickness) under various cooling environments like cryogenic (liquid nitrogen), minimum quantity lubrication (MQL), wet machining and dry conditions. It was found that as the cutting velocity, feed rate and depth of cutincreases, response like cutting temperature, flank wear and MRR were increased respectively under all the cooling environments. Whereas, in the case of surface roughness, decreasing trend was observed at the cutting velocity variation and increasing trend was found for feed rate and depth of cut variations conditions respectively. In overall, it was also evident from the experimental results that cryogenic machining significantly improved the machining performance and product performance all the cutting conditions. From result, it was found that cryogenic machining is selected as a best feasible machining method for 17-4 PH SS and it was selected for next phases of the work. On the other way machining efficiency, quality of the product and machining cost highly depending on the selection of optimum machining conditions. In the second phase, Taguchi L9 orthogonal array experimental design has been used for optimization of cutting conditions for single and multiple objective responses under the cryogenic cooling environment. Taguchi method was used for single response optimization and ANOVA was used to find the most influenced process parameters on each response. Gray relational analysis (GRA) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) optimization techniques have been applied for multi response optimization, best multi optimization tool which suits for the current study have been selected through conformation tests. From the conformation test results, it was observed that Taguchi determined optimum cutting conditions significantly improved the turning performance characteristic during machining of 17-4 PH SS. Whereas, in the case of multi response optimization condition, GRA technique substantially improved the turning performance characteristic when compared to the TOPSIS technique. In the third phase, correlation models were developed for modeling of cryogenic turning process by finding out the relation between the input process parameters and output responses using Response Surface Methodology (RSM) for cost effective research methodology. In additions to this, interaction effects of process parameters on turning performance characteristics were studied using 3D surface plots. From the modelingconformation test results, it was observed that close agreement was found between the actual and predicted values. From interaction plots of surface roughness, it was observed that the high level of cutting velocity and low levels of feed rate and depth of cut could be contributed to generate lower surface roughness respectively. Whereas, from interaction plots of flank wear and MRR, it was found that the highest levels of process parameters could produce high flank wear and maximum MRR respectively.