Wire Electro-discharge Machining of Sub-cooled AISI P20 Tool Steel

Abstract

AISI P20, also known as “General Mold Steel Grade” steel is a chrome-moly (added nickel) tool steel created explicitly for zinc die casting and plastic molding applications like hydroforming tools, frames for plastic pressure dies and plastic molds to fill the requirements for its strength and machined cavities. Sub-cooling of metals is an effective method that is acknowledged for many years for the enhancement of wear life and reduction of residual stress for tool steel. The major problem in the whole process of heat-treatment of tool steel is the retained austenite. It should be noted that retained austenite is unstable and soft and freshly formed martensite is brittle. Hence, the tool steel can only be suitable for use after tempering, which also relieves its residual stresses. The extent of sub-cooling decides the capability of austenite to transform into martensite. Thus, to increase the wear properties of the tool steel, it is heat-treated with sub cooling and tempering. In this research work, the P20 samples are heat-treated with six distinct steps, stated as groups (FS50, FS100, FS150, Q, WSN and F600WSN). The physical and mechanical properties of all the groups are studied. The physical characterization is done revealing the microstructure and phases present in the metal. The mechanical characterization includes hardness test, impact resistance test, compressive strength test, wear test. It is then concluded that the sub cooled groups (FS50, FS100 and FS150) gives better properties than others, and are considered for further studies. Extensive wear test and sub-surface micro-hardness measurement of three sub-cooled groups are studied to find out the influence of sub-zero temperature on the wear characteristics of the metal. The tool steel sub-cooled at -150 ◦C (FS150 group) has maximum wear resistance and micro-hardness as compared to other sub-cooling temperatures. This group has the most suitable properties for die material and is taken for further studies involving non-conventional machining. Wire electro-discharge machine (WEDM) is considered to study the non-conventional machining of metal. The machining characteristics of the sub-cooled group (FS150 group) are compared with that of the parent metal and the results are discussed. The machining of sub-cooled tool steel is faster than untreated tool steel (parent metal) showing more suitable surface characteristics. The machining and surface characteristics of the sub-cooled group (FS150 group) is elaborately studied further. Grey Relational Analysis (GRA) is used to optimize the machining of sub-cooled metal (FS150). This optimum parametric setting to obtain better cutting speed, kerf width, surface roughness, not only increases productivity, but also improves the quality of the finished surface. To know more about the machining characteristics, the WEDMed surfaces machined with optimum and initial parametric settings are compared with the un-machined surface. Sub-surface micro-hardness and wear tests are also done to study the effect of the optimum and initial parametric WEDM setting on the sub-cooled metal for achieving better surface characteristics. Wear test done also reveals that due to the formation of recast layer while machining, the wear resistance of the machined surface increases. The micro-hardness of the surface machined with the optimized setting gives a higher hardness value than the surfaces of that machined with initial parametric machining conditions and un-machined metal. Thus, the tool steel sub-cooled at -150 ◦C (FS150 group) has much higher wear resistance and micro-hardness with a significant enhancement in its compressive strength and energy absorption capacity. By decreasing the sub-cooling temperature involved in different heat treatment processes, the wear resistance of tool steel increases. The WEDMing of sub-cooled tool steel is more efficient than un treated tool steel showing more suitable surface characteristics. The GRA optimized parametric setting to obtain better MRR, KW and SR improves the productivity and quality of the component and also produces a thicker recast layer. Wear test revealed that the surface machined with an optimized parametric setting gives higher wear resistance. The recast layer produced by WEDMing helps to improve the wear resistance of tool steel. The micro-hardness of the surface machined with the optimized setting gives a higher hardness value than both the surfaces of that machined with initial parametric machining conditions and un-machined metal.