Experimental Investigations on Diamond Burnishing of 17-4 Ph Stainless Steel under Sustainable Cooling Environments

Abstract

The materials which initiate more tool wear, heat, cutting force, and poor surface finish during machining are termed to be difficult-to-cut materials. The precipitation hardenable (PH) stainless steel is one of the interesting family of steels which can attain hardness up to 49 HRC. In these family of stainless steels, 17-4 PH stainless steel has attracted engineers across the world because of its superior corrosion resistance and high strength, which is not possible to find in any of the steel grades. Owing to low thermal conductivity, high strength and admirable wear resistance properties, it has been classified under difficult to cut materials. It is a special type of PH martensitic stainless steel which consists of martensite along with a small quantity of austenite. Compressor blades of steam turbines are subjected to high temperature, vibration, and stress inducement. These issues can cause damage to the engine. Hence, the first set of compressor blades can be manufactured with PH stainless steel to avoid the problems arising due to foreign object damage. Machining of such kind of steels results in poor surface quality and also the production cost is more. Burnishing is one of the preferred secondary finishing operations which is usually performed after machining to achieve the mirror finish of the surface. To achieve the superior surface characteristics of the difficult to cut material, it is preferred to cool the burnishing zone with an appropriate lubricant. Millions of workers throughout the world get affected by working under different kinds of cutting fluids or coolants. Aerosol particles or mist are some of the hazardous elements which will be generated during the application of different types of cutting fluids during machining and which affects the operator’s health. Cryogenic machining has emerged as an alternative cutting fluid in the last two decades. Liquid nitrogen (LN2) will be sprayed at the interface of the tool and workpiece. It is environmental friendly coolant when compared to other conventional coolants. During the burnishing process, because of the pressure created in the burnishing zone, the temperature at that region increases. By the application of LN2, the temperature can be reduced, which results in improved surface integrity of the material.A high-quality finishing of the mechanical parts is necessary to attain the improved fatigue resistance and a low friction ratio. Hence the finishing processes are turned out to be a major drive for industrial innovation all over the globe. Some of the secondary finishing processes such as grinding, lapping, honing, and polishing have been widely used to achieve the super finish of the surface. However, to improve the surface quality and geometrical accuracy of the component, burnishing has been introduced. Burnishing is also one of the well-known secondary finishing process used to improve the functional performance of the component. Diamond burnishing is one of the chipless finishing processes where the spherical tip of the tool made up of natural diamond, slides on the surface of the workpiece which causes plastic deformation. Directly after machining, the workpiece can be diamond burnished to acquire improved surface integrity. It is an economical and compatible process which can be applied on ferrous and nonferrous materials to achieve the mirror-like surface finish. It has a higher level of efficiency when compared to grinding, lapping, and polishing processes. The main objective of this research work is to investigate the influence of process parameters on the surface integrity characteristics while diamond burnishing of 17-4 PH stainless steel under varying working environments. To achieve the best feasible surface integrity properties of the material, the present research work has been classified into four phases. In the first phase, one factor at the time approach (OFATA) was used to find out the influence of control factors such as burnishing speed, burnishing feed and burnishing force on performance characteristics such as surface roughness, surface hardness, surface morphology, surface topography, subsurface microhardness and residual stress using a commercially available diamond burnishing tool. The cryogenic cooling, minimum quantity lubrication (MQL), and dry environments were considered for the study. In the second phase, a novel diamond burnishing tool was designed and fabricated to improve the performance characteristics of the material. To analyze its performance under all the three environments, OFATA was used. Further, the study was extended to investigate the influence of two more process parameters such as the number of tool passes and diamond sphere diameter on the performance characteristics in the cryogenic cooling condition. Inthe third phase, the optimization of process parameters was performed by Taguchi’s Grey Relational Analysis (TGRA). In the fourth phase, a mathematical model was developed for surface roughness and surface hardness by Response Surface Methodology (RSM). The developed regression equation was used to perform multi-objective optimization using genetic algorithm (GA). The optimal process parameters were achieved, which will be beneficial in improving the performance of the component.