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Machining is a process for finishing workpiece to specified dimensions, tolerances and surface finish. The type of surface that a machining operation generates and its characteristics are of great importance in manufacturing (Isik, 2007). It is a function of the machining conditions which depend on the cutting tool, workpiece, machine tool, cutting fluids and cutting parameters (Ezugwu et al., 2004). Thus, new materials development and manufacturing technologies requires the adoption of new conditions in production and production strategies.
Milling is a process of rapid metal removal in which the work piece is fed past a multipoint cutter which rotates at a high speed. It is a commonly used machining process in the metal cutting industry. Jobs are machined at a faster rate compared to single point tools and the surface finish is also better. In industrial machining, the face milling process is regularly used to machine large, flat surfaces in a very fast and precise way.
Ductile irons, also known as nodular cast irons or spheroidal graphite irons, have been progressively used by manufacturing industry ever since their first development in the late 1940s because they offer wide range of properties that are not seen in other types of cast irons such as high tensile strength, excellent wear resistance, fatigue resistance, toughness and ductility. While graphite flakes in grey cast irons act as stress raisers resulting in poor toughness, graphite nodules in ductile irons act as crack arresters resulting in superior toughness and ductility (Ductile Iron Society, 2014).
Generally, machining processes and the final finishing operations alter the integrity of the surfaces of the workpiece. Surface integrity indicates the surface characteristics (surface roughness, surface residual stress, microstructure and hardness) that influence the functionality of a part. Amidst these characteristics, surface residual stress plays a key role as the fatigue life, corrosion and wear resistance of a component can be severely compromised (or improved) if tensile (or compressive) surface residual stress is present. Thus, within the framework of surface integrity investigations, special emphasis is given to the measurement of surface residual stresses since they contribute directly to premature failure of components.
Several authors have studied the influence of feed rate and cutting speed on the surface residual stress of the machined surfaces of various materials but there has been no agreement among them. Navas et al. (2012) studied the effect of cutting speed and feed rate in turning AISI 4340 steel. They observed that as feed rate increased, residual stresses became more tensile due to increase in temperature; surface roughness also increased. Capello (2005) observed in turning three different steels (Fe37O, C45 and 39NiCrMo3) that an increase in feed rate and tool nose radius increased residual stresses on the surfaces generated. Gunnberg et al. (2006) studied the influence of rake angle, tool nose radius, cutting speed, cutting depth and feed rate on surface residual stress during hard turning of 18MnCr5 case carburized steel using polycrystalline cubic-boron-nitride cutting tool inserts. They observed that while high speed generated high tensile residual stress, increased feed rates generated high compressive surface residual stresses. Dahlman et al. (2004) also studied the hard turning of AISI 52100 hardened steel and reported that increased feed rate generated higher compressive residual stress. Rech and Moisan (2003) however reported from their own study on the hard turning of case-hardened 27MnCr5 that feed rate has non-significant effect on the surface residual stress generated.
Indeed, considerable research has been carried out on the machining of different cast irons and steels using different cutting tools. However, there is dearth of information on the effect of cutting parameters on the surface residual stress of machined pearlitic ductile iron. This study examined the effect of some processing parameters on the surface residual stress of pearlitic ductile iron face-milled with cemented carbide cutting tool.