MACHINABILITY OF AN FE-CU-C BASED POWDER METALLURGY STEEL IN GREEN, PRE-SINTERED AND SINTER-HARDENED STATES

Abstract

Machining is involved in the fabrication of most powder metallurgy (PM) components to upgrade near-net-shape powder compacts to end-usable net-shapes, produce geometrical features in the compacts which cannot be produced during powder compaction, and attain the desired tolerance level on the compacts. Nevertheless, sintered powder compacts are usually challenging for machining due to their porous nature, as well as due to the presence of hard-to-cut metallurgical phases in some high-performance PM materials like sinter-hardened steel. Machining powder compacts prior to the commencement of sintering activities, i.e., green compacts, and machining pre-sintered compacts, i.e., compacts sintered at a temperature below the final sintering temperature, are promising avenues that circumvent the difficulties in machining sinter-hardened steel compacts. These avenues define the scope of the present study, besides the investigation on machining sinter-hardened compacts for machinability improvements. Machining green compacts is commonly known as green machining in the PM fraternity, which suggests sinter-hardening of green machined compacts instead of machining sinter-hardened compacts. Low cutting forces, significantly reduced tool wear rate, and higher material removal rate are the prominent advantages of green machining over the machining of sinter-hardened compacts. Nevertheless, green machining has the limitation of resulting in a poor-quality machined surface and exit-edge (the edge from where the cutting tool leaves the green compact workpiece when machining ends) in comparison. The feasibility of overcoming the limitation with the help of machining parameters is a scarcely explored area in the literature. Exploring this area through a thorough understanding of the material removal mechanism constitutes the motivation of the present investigation in the green machining domain. Green compacts of 7.00 g/cm3 density, i.e., nearly 90% relative density, prepared from a Rio-Tinto-produced FLC-4608 (Metal Powder Industries Federation denomination, standard 35) sinter-hardenable steel powder-premix containing proprietary binder-lubrication system FLOMET HGS were analysed in machining. The sinter-hardened compacts, sintered at 1150 oC and cooled with a rate of 1.5 oC/s, which attained nearly 90% relative density, were also machined in this study for comparative analysis. Turning operation was selected as a representative machining process. Results indicated that, in green machining, a lower feed rate regulates the attribute of powder particles’ plucking in the material removal mechanism and improves the machined surface quality. Increased cutting velocity and feed rate were found to reduce the size of exit-edge damage, i.e., exit-edge-breakout, and improve the exit-edge quality. However, these quality improvements could not feasibly achieve the machined surface and exit-edge quality comparable to a compact machined post-sinter-hardening, as identified in the present study. It was determined that green machining has significance in its advantages over machining sinter-hardened compacts, i.e., lower Fc, and favourable chip morphology. Machining pre-sintered compacts has potential to deal with the issues in green machining, besides the issues in the machining of sinter-hardened compacts. With the motivation of verifying this potential, the present investigation considers performing a turning operation on the FLC-4608 compacts sintered at 600 oC, 800 oC, 1000 oC, and 1150 oC, having nearly 90% relative density. The cooling rate in these sintering cycles was maintained equal to 1.5 oC/s, which causes the cycle of sintering at 1150 oC to convert to a sinter-hardening cycle. Results indicated that compacts pre-sintered at 600 oC are equivalent to green compacts free from binder-lubrication system, and sintering activities commence above this temperature. At a fixed cutting velocity and feed rate, the influence of an increase in the sintering temperature on the machinability was found to be as follows: on the one hand, it increases Fc and enhances difficulties in chip formation and handling, but on the other, it improves the surface finish of the machined surface and reduces the size of the exit-edge-breakout. Given these opposite trends, this study proposes a novel methodology to identify the range of appropriate sintering temperatures that enables the optimum machinability of pre-sintered compacts over the machinability of both green compacts free from binder-lubrication system and sinter-hardened compacts. In this methodology, a newly defined machinability criterion, the ratio of machining outcome to machining effort, was graphically represented with machining effort. The comprehensive and generic nature of this methodology and its significance in the context of design for manufacturing (DFM) could be referred to in the current report. Nevertheless, based on the methodology, from 750 oC to 850 oC was identified as the range of appropriate sintering temperature for optimum machinability of pre-sintered compacts. This temperature range was found to produce the inter-particle necks and a mixture of metallurgical phases (ferrite, pearlite, and some extent of martensite) to such an extent that it produces a considerably smooth machined surface (low Ra) and good-quality exit-edge compared to the machining of green compact free from binder lubrication system while exhibiting a substantially low Fc, which in turn, will result in low tool wear, compared to the machining of sinter-hardened compacts. The compacts pre-sintered within this determined temperature range also resulted in an improved quality machined surface compared to the green compacts containing binder-lubrication system; however, the comparative results of exit-edge-breakout were found to be the opposite. Chip characteristics have received scarce attention in earlier studies on machining sintered metals. Therefore, analysing the effect of cutting velocity and feed rate on Fc with the help of chip characteristics is a novel approach. With this motivation, a turning operation was performed on the sinter-hardened FLC-4608 compacts with nearly 90% relative density. Results indicated that the combination of high cutting velocity and low feed rate provides the appropriate condition to attain low Fc. Results also indicated that machining configurations under consideration produce shear localised segmented chips, also known as saw-tooth chips, and the chip formation process involves near-full densification of uncut chip material. Other than chip length, all the studied characteristics of chips, minimum and maximum chip thickness, the microstructure of the shear band, and the structure underside the tip of the chip segment were consistent with the Fc results. Increasing feed rate exhibited increasing minimum and maximum chip thickness, consistent with increasing Fc. Similarly, through the microstructure of the adiabatic shear band and structure underside the tip of the chip segment, increasing cutting velocity exhibited the predominance of the thermal softening effect over strain hardening and strain rate hardening, consistent with reducing Fc. The mathematical formulation of a few chip formation characteristics and machinability assessment based on these characteristics constitute the motivation for further study on orthogonal machining (groove turning) of FLC-4608 sinter-hardened steel compacts having 90% relative density. It was found that machining typically results in shear localised chip segments exhibiting plastic deformation and near-full densification of uncut chip elements. Both these phenomena were found to have a combined influence on the mathematical formulae of chip formation characteristics: efficiency (given by cutting ratio and shear angle) and process stability (given by chip segmentation frequency). Increased cutting velocity can improve the chip formation efficiency, whereas decreased cutting velocity and increased feed rate can improve the process stability, as determined in the study. A modification in the conventional nomograph was identified to use it for machining porous metals that exhibit densification of uncut chip elements during chip segmentation. It was also rationalized that densification likely progresses from the tool tip towards the free surface of the workpiece during chip segmentation. Overall, the present study is potentially helpful to PM practitioners for improving the process flow of fabricating sinter-hardened steel components with consideration of green machining and machining of pre-sintered compacts. This study also has the potential to help attain better control over the process of machining hard-to-machine porous metals, like sinter-hardened steels, through a thorough comprehension of chip characteristics. In summary, this report can be contemplated as machining-related guidelines for facilitating the fabrication of sinter-hardened steel components.