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In this era of design driven material science where new metals and alloys having advantageous material properties are generated to meet the design requirements. Material modification at microstructure level using heat treatment in order to induce the required material properties satisfying the functional requirement of a design application. It becomes necessary to understand how effectively and efficiently these newly emerging materials can be machined. Almost 70% of the assembled or individual product has got machining involved in it at some of stage of its production process (Polishetty, 2012). The materials in discussion in this paper are Austempered Ductile Iron (ADI), Duplex Stainless Steel and Nano-Structured Bainitic Steel (NSBS). Another important factor leading to steady emergence of new materials and alloys is to meet the design requirement of the product to function within the envelope of strict environmental laws. Machining is defined as a production process in which the metal is removed in the form of chips (swarf) by a plastic deformation process. The deformation temperature and the force significantly contribute to the quality of the process. Temperature affects the cutting tool material and the forces effect the power and strength needed to perform the process (G.T Smith, 1989). There are two general ways to machine described so far by researchers-orthogonal and oblique cutting. Orthogonal cutting has cutting edge perpendicular to the direction of cut and oblique cutting involves cutting edge at an acute angle to the tool/work feed direction (Sandvik Coromant, 1994).
Machinability is defined as the ability of a material to produce acceptable outcomes on machining. Some of the outcomes under consideration are surface texture, power consumed, metal removal rate and tool wear. Generally, machinability is qualitative than a quantitative evaluation of the process. The term machinability assumes significance especially for materials which are problematic to machine (G T Smith, 1995). The common problem experienced in machining are rapid tool wear/tool failure, surface finish off-limits, out of tolerance parts, dimensional inaccuracy, strain hardening due to plastic deformation and lower productivity. Machinability research is carried on to look at ways to reduce the weight of the automotive, aerospace engine and ancillaries by replacing heavy and traditional materials such as steel and grey cast iron with materials having high strength to weight ratio such as ADI and NSBS. Cast iron machining has been noteworthy in establishing metal cutting theories by eminent researchers such as G. Boothroyd, M.C. Shaw, E. J. A. Armarego and R. H. Brown. (Armarego & Brown, 1969; Boothroyd, 1965; Shaw, 1986). With the introduction of new cutting tool materials such as silicon carbide, Polycrystalline Cubic Boron Nitride (PCBN) and ceramics, the cutting tools are able to survive in adverse cutting conditions. The machinery has advanced significantly offering wide range of speeds, array of spindle options and multiple axis machining. The demand for higher productivity, lower manufacturing costs and better quality of products has led to development of high speed machining (Childs, Maekawa, Obikawa, & Yamane, 2000).
Machinability research is a way to find solutions to problems experienced during machining and ensure that the economy and efficiency of the process stays optimum. One of the problems under consideration is strain hardening due to plastic strain. Strain Induced Transformation (SIT) is a common problem experienced during machining of ductile materials or materials having unstable microstructural phases. For a ductile material, micro cracks are developed around the tool/chip interface and these micro cracks initiate the process of strain hardening that leads to adiabatic shearing process. As a result of strain hardening the gross crack extends from the free surface to a point in the shear plane where the rate of strain hardening is greater than crack propagation and leads to arrest of the crack formation process (Shaw, 1986).
Figure 1. SEM image of SAF 2205 duplex microstructure consisting of α-ferrite, γ-austenite phase