Microhardness and Surface Integrity in Turning Process of Duplex Stainless Steel (DSS) for Different Cutting Conditions

Duplex stainless steels (DSSs) are considered to be difficult to machine. Built-up-edge (BUE) and irregular wear are common challenges in machining operations (Ref 1 ). This degrades the surface finish of a machined product and results in reduced tool life. To utilize the useful properties of two-phase microstructure is necessary to study surface quality of machined stainless steels. According to Jang et al. (Ref 2 ), surface integrity (SI) is a measure of the quality of a machined surface and is interpreted as elements which describe the actual structure of both surface and subsurface. SI is generally defined by its mechanical, metallurgical, chemical, and topological states of surface properties. To ensure better SI, special attention must be paid when choosing cutting parameters (Ref 3 , 4 ), tool material and geometry (Ref 5 , 6 ), and tool coatings (Ref 7 , 8 ). SI is important for the components’ ability to adapt to high thermal and mechanical loads during their applications (Ref 9 , 10 ). SI and surface microhardness are important in determining corrosion resistance, and also in fatigue crack initiation. The DSS is widely used for many industrial applications due to its unique properties. These are more important in the machining of expensive machine parts. Cabrera et al. (Ref 11 ) and Park et al. (Ref 12 ) considered that the good combination of their mechanical properties (high strength and toughness) and corrosion resistance makes them of great interest for a wide range of applications, especially in the oil, chemical, and power industries. Therefore it is very important to control their microstructure evolution, physical, and mechanical properties. According to Solomon and Solomon (Ref 13 ) in the case of materials with austenitic structure, plastic deformation can induce transformation of austenite structure into martensite structure. According to Kundrak et al. (Ref 14 ), turning performed at high speed is an intensive technology in terms of the heat generated in the process. The temperature of the workpiece material in the cutting edge area may reach the transformation temperature. Sasahara (Ref 15 ) demonstrated, using 0.45% C steel test material, that for tool corner radius of 0.2 and 0.8 mm, the hardness on the machined surface becomes higher with a smaller radius. In addition, the feed rate did not affect the surface hardness for values between 0.05 and 0.4 mm/rev. Ezugwu et al. (Ref 16 ) demonstrated that the cutting tool geometry in machining nickel-based alloys represents an important parameter in tool life and in the quality of the machined surface. Ezugwu and Tang (Ref 17 ) have found that a machined surface obtained with the rhomboid shaped insert has a higher microhardness as compared to the round inserts. Javidi et al. (Ref 18 ) showed that plastic deformation of the grain boundaries was found in the first 3-4 μm of the subsurface layer. They also confirmed that no significant variation in hardness was observed beneath the machined surface obtained by different cutting conditions. Ozturk and Altan (Ref 19 ) presented the effects of cutting edge radius on the cutting and thrust forces. They showed that cutting forces increase with increasing edge radius for CuZn30 workpiece material. Storch and Zawada-Tomkiewicz (Ref 20 ) showed distribution of unit forces on the tool nose rounding in turning process for C55 steel. But these publications did not mention problems related to the microhardness as physical parameters of SI.