The influence of the microstructure of hardened tool steel workpiece on the wear of PCBN cutting tools

Due to the recent developments of advanced cutting tool materials in the superbarasive family, such as cubic boron nitride (CBN) tools, the interest in cutting hardened steels has increased significantly. High flexibility and ability to manufacture complex workpiece geometry in one set up is the main advantage of hard turning compared to grinding. The focus of this study is to investigate the performance and wear behavior of CBN tools in finish, dry turning of four different hardened steels, treated to the same hardness R_c = 54. The following four materials were machined: X155CrMoV 12 cold work steel (AISI D2), X38CrMoV5 (AISI H11) hot work steel, 35NiCrMo16 hot work steel and 100Cr6 bearing steel (AISI 52100). A large variation in tool wear rate was observed in the machining of these steels. The tool flank grooves have been correlated to the microstructure of these steels, namely the presence of various carbides. The chip study reveals that there is presence of different amounts of white layers in machining these steels.     

Machinability of BN free-machining steel in turning

In the past few years, extensive researches have been done to improve the machinability of work materials in order to increase productivity and reduce the effect on the environment. To satisfy these demands, various free-machining steels have been researched and developed. One of them is BN free-machining steel that contains hexagonal boron nitride (h-BN). However, the machinability was not stable. In this study, machining tests were carried out to clarify the machinability of steels and appropriate chemical composition of work material and tool material to achieve high efficient machining. Tested work materials were plane carbon steel JIS S45C and BN free-machining steels. The JIS S45C was used as the standard. The tool wear in turning BN free-machining steel was smaller than that in turning standard steel. In case of turning BN1 with P30 at 200, 300 m/min, the wear progress rate of flank wear and crater depth were about half as much as that in turning standard steel. BN free-machining steel showed slightly lower cutting temperature and smaller cutting force in comparison with standard steel at the tested cutting speeds. Al and N were detected as a layer at the tool wear region of P grade carbide tools after turning BN free-machining steel at high cutting speed. It is thought that one of the main reasons of outstanding machinability of BN free-machining steel is that the deposited layer containing Al and N acts as diffusion barrier at the tool–chip interface. In turning larger Al content BN-added steel with higher Ti content cutting tools, a larger wear reduction was observed. Therefore, it is said that not only added BN but also appropriate Al is necessary in work material.     

Machining of hardened steel—Experimental investigations,performance modeling and cooling techniques: A review

The researchers have worked on many facets of machining of hardened steel using different tool materials and came up with their own recommendations. Researchers have tried to investigate the effects of cutting parameters, tool materials, different coatings and tool geometry on different machinability aspects like, the tool life, surface roughness, cutting forces, chip morphology, residual stresses and the tool–chip interface temperature under dry and/or semi-dry and/or flood cooling environment during machining of hardened steels while many of them have ventured to characterize the wear phenomenon. Good amount of research has been performed on an analytical and/or numerical and/or empirical modeling of the cutting forces, tool–chip interface temperature, and tool wear under orthogonal/oblique cutting conditions during machining of hardened steels. This paper presents a comprehensive literature review on machining of hardened steels using coated tools, studies related to hard turning, different cooling methods and attempts made so far to model machining performance(s) so as to give proper attention to the various researcher works.     

Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool

High strength steel 30Cr3SiNiMoVA (30Cr3) is usually used to manufacture the key parts in aviation industry owing to its outstanding mechanical properties. However, 30Cr3 has poor machinability due to its high strength and high hardness. Hard milling is an efficient way in machining high strength steels. This paper investigated hard milling of 30Cr3 using a PVD-AlTiN coated cemented carbide tool with regard to cutting forces, surface roughness, chip formation and tool wear, respectively. The experimental results indicated that the increase of cutting speed from 70 to 110 m/min leads to direct reduction of cutting forces and improvement of surface finish, while both feed rate and depth of cut have negative effect on surface finish. The occurrence of oxidation on chip surfaces under high cutting temperature makes the chips show different colors which are strongly influenced by cutting speed. Saw-toothed chips were observed with the occurrence of the thermo-plastic instability within the primary shear zone. Micro-chipping and coating peeling were confirmed to be the primary tool failure modes. Serious abrasion wear and adhesive wear with some oxidative wear were confimed to be the main wear mode in hard milling of 30Cr3.     

Refrigerated cooling air cutting of difficult-to-cut materials

One approach to enhance machining performance is to apply cutting fluids during cutting process. However, the use of cutting fluids in machining process has caused some problems such as high cost, pollution, and hazards to operator's health. All the problems related to the use of cutting fluids have urged researchers to search for some alternatives to minimize or even avoid the use of cutting fluids in machining operations. Cooling gas cutting is one of these alternatives. This paper investigates the effect of cooling air cutting on tool wear, surface finish and chip shape in finish turning of Inconel 718 nickel-base super alloy and high-speed milling of AISI D2 cold work tool steel. Comparative experiments were conducted under different cooling/lubrication conditions, i.e. dry cutting, minimal quantity lubrication (MQL), cooling air, and cooling air and minimal quantity lubrication (CAMQL). For this research, composite refrigeration method was adopted to develop a new cooling gas equipment which was used to lower the temperature of compressed gas. The significant experimental results were: (i) application of cooing air and CAMQL resulted in drastic reduction in tool wear and surface roughness, and significant improvement in chip shape in finish turning of Inconel 718, (ii) in the high-speed milling of AISI D2, cooling air cutting presented longer tool life and slightly higher surface roughness than dry cutting and MQL. Therefore, it appears that cooling air cutting can provide not only environment friendliness but also great improvement in machinability of difficult-to-cut materials.     

Analysis of the influence of tool type, coatings, and machinability on the thermal fields in orthogonal machining of AISI 4140 steels

Micro-thermal imaging was used to determine the amount of heat flowing into the tool, chip and workpiece during orthogonal cutting at speeds up to 400 m min⁻¹. Two AISI 4140 steels with different machinability ratings and three types of tools were compared: (i) uncoated with 0° rake angle, (ii) coated with −6° rake angle and (iii) coated with chip breaker. A control volume approach was used to estimate the energy partition from thermal images and energy outflow was compared to direct measurement of the cutting power. This provides a new physical tool for examining machinability, tool wear and subsurface damage.     

An experimental and analytical analysis on chip morphology,phase transformation,oxidation, and their relationships in finish hard milling

Milling hardened steels has emerged as a key technology in mold and dye manufacturing industry. The effects of cutting parameters on chip morphology, phase transformation and oxidation reaction of the chips during finish milling AISI H13 tool steel (50±1 HRc) with coated inserts were investigated in this paper. The chip morphology and phase transformation were examined using an optical microscope and a scanning electron microscope (SEM). The X-ray photoelectron spectroscopy was used to measure the chemical compositions of oxidation layer on chip surfaces. Shear-induced lamella structures characterized by jagged and rough appearance are the basic features of free surfaces. The microstructural analysis indicated that saw-tooth chips and white layers are produced only under certain combinations of cutting parameters of high cutting speed and feed rate. Based on chip color, chip morphology and X-ray photoelectron spectroscopy (XPS) analysis of the chip oxidation layer, the maximal instantaneous temperature at the tool–chip interface is semi-qualitatively estimated using the analytical method developed. In addition, chip color can be predicted based on the oxidation compositions.     

Effects of minimum quantity lubrication on turning AISI 9310 alloy steel using vegetable oil-based cutting fluid

This paper presents the effects of minimum quantity lubrication (MQL) by vegetable oil-based cutting fluid on the turning performance of low alloy steel AISI 9310 as compared to completely dry and wet machining in terms of chip–tool interface temperature, chip formation mode, tool wear and surface roughness. The minimum quantity lubrication was provided with a spray of air and vegetable oil. MQL machining was performed much superior compared to the dry and wet machining due to substantial reduction in cutting zone temperature enabling favorable chip formation and chip–tool interaction. It was also seen from the results that the substantial reduction in tool wears resulted in enhanced the tool life and surface finish. Furthermore, MQL provides environment friendliness (maintaining neat, clean and dry working area, avoiding inconvenience and health hazards due to heat, smoke, fumes, gases, etc. and preventing pollution of the surroundings) and improves the machinability characteristics.     

Evaluation of layer adhered on PCBN tools during turning of AISI D2 steel

Polycrystalline cubic boron nitride (PCBN) tools have high abrasion resistance and are thus suitable for application in the machining of steels with a high volume fraction of primary carbides in their microstructure. These tools are usually applied in the machining of steels with hardness above 45–50 HRC and in the case of application to steels with hardness below 45 HRC, the formation of an adhered layer on the rake face of the tools often occurs. This paper reports a study on the impact of the layer adhered on PCBN tools during the turning of AISI D2 steel, with 35 and 50 HRC. The microhardness and microstructure of the adhered material were determined, as well as the tool wear based on volumetric wear parameters. The layer adhered on the PCBN tool rake face has the same chemical elements as the machined steel alloy. Its microstructure is oriented in the direction of the chip flow and the primary carbides were fragmented. For the sample with 35 HRC the amount of material adhered (W_AM) on the rake face of the PCBN tool was approximately 360% higher than the steel with 50 HRC. The material layer adhered on the PCBN tool rake surface in the case of the 35 HRC steel acts as an edge (assuming the cutting function), while for the 50 HRC steel, the adhered layer intensifies the adhesion wear mechanism through spalling on the tool rake face. The results obtained provide important information for the selection of materials and grades for the development of new cutting tools.     

Drilling of X2CrNi 19 11 stainless steel with hiped NiTi coating

This study investigated suitability of TiN and TiCN-coated cemented carbide tools in the machining of conventionally produced stainless steel with HIPed (Hot Isostatic Pressed) NiTi coating. Near-equiatomic nickel–titanium alloy (NiTi) has many attractive material properties, such as pseudo-elasticity and shape memory effects, which result into beneficial engineering properties, e.g. as cavitation resistant coatings in addition to its well-known shape memory properties. Stainless steels are often considered to be poorly machinable materials; materials with high elasticity are also difficult to machine. In drilling stainless steel with a pseudo-elastic coating material, machinability difficulties are caused by the high strength and work hardening rate of steel and the pseudo-elastic properties of the coating material. The machinability was studied by analyzing cemented carbide drills and chips. The interface between stainless steel and NiTi coating was examined with SEM (scanning electron microscopy) and EDS (energy dispersive spectroscopy) analysis. The effect of feed rate on chip formation and tool wear was analyzed. The cutting tests indicated that cutting speeds of 50 m/min, a feed rate of 0.1–0.2 mm/rev, and solid carbide drills can be applied, from a machinability standpoint. A HIPed pseudo-elastic coating decreases machinability. When effective cutting speeds and feed rates were utilized, optimal tool life was achieved without a decrease in coating properties.     

Tool life and tool wear in the semi-finish milling of inclined surfaces

Functional die and mold components have complex geometries and are made of high hardness materials, which make them difficult to machine. This work contributes to a better understanding of this type of process and of the wear mechanisms of tools used in semi-finishing operations of hardened steels for dies and molds. Several milling experiments were carried out to cut AISI H13 steel with 50 HR_C of hardness using the high-speed milling technique. The main goal was to verify the influence of workpiece surface inclination and cutting conditions on tool life and tool wear mechanisms. The main conclusions were the inclination of the machined surface strongly influences tool life and tool wear involves different mechanisms. At the beginning of tool life, the wear was caused mainly by abrasion on the flank face plus diffusion and attrition on the rake face. At the end of tool life, the mechanisms were adhesions and microchipping at the cutting edge.     

Chip formation, cutting forces, and tool wear in turning of Zr-based bulk metallic glass

The chip light emission and morphology, cutting forces, surface roughness, and tool wear in turning of Zr-based bulk metallic glass (BMG) material are investigated. Machining results are compared with those of aluminum 6061-T6 and AISI 304 stainless steel under the same cutting conditions. This study demonstrates that the high cutting speeds and tools with low thermal conductivity and rake angle activate the light emission and chip oxidation in BMG machining. For the BMG chip without light emission, serrated chip formation with adiabatic shear band and void formation is observed. The cutting force analysis further correlates the chip oxidation and specific cutting energy and shows the significant reduction of cutting forces for machining BMG at high cutting speeds. The machined surface of BMG has better surface roughness than that of the other two work materials. Some tool wear features, including the welding of chip to the tool tip and chipping of the polycrystalline cubic boron nitride (PCBN) tool edge, are reported for turning of BMG. This study concludes that BMG can be machined with good surface roughness using conventional cutting tools.     

PcBN刀具连续车削淬硬钢性能的研究

为了研究PcBN刀具在干式连续车削条件下刀具的性能,选用两种不同的PcBN刀具,在不同切削速度及不同工件硬度条件下,对淬硬钢进行车削试验。在此基础上,对刀具前、后刀面的显微形貌特征进行了观察,分析了刀具的失效机理。结果表明:切削速度对刀具的切削寿命影响很大,随着切削速度的增加刀具寿命几乎线性下降。硬度也是影响刀具切削寿命的重要因素之一,当v190 m/m in时,刀具切削硬度为(64±1)HRC工件的寿命比相同条件下切削硬度为(61±1)HRC工件的寿命下降约40%~60%。刀尖温度随着切削速度的增加不多,切屑温度要比刀尖温度高出许多。在本试验中,刀具的失效判据为崩刃或后刀面平均磨损超过0.3 mm。     

Measurement of specific cutting energy for evaluating the efficiency of bandsawing different workpiece materials

In cutting operations by multipoint cutting tools such as bandsawing, the layer of material removed per tooth (5–30 μm) is usually less than or equal to the cutting edge radius (5–15 μm). Furthermore, the bandsaw tooth is also restricted since it has to accommodate the chip in a gullet of limited size. This situation can lead to inefficient metal removal by a combination of piling up, discontinuous chip formation and ploughing action in contrast to the cutting operations by most of the single point cutting tools (e.g., turning). Specific Cutting Energy (E_SP) is a better way of measuring the efficiency of the metal cutting process compared to the other processes such as determining tool wear, cutting forces, chip ratio, etc. This paper reports on the full bandsawing tests of three different workpiece materials (Ball bearing steel, Stainless steel and Ni–Cr–Mo steel). The increase of E_SP throughout the life of the bandsaw reflected the degradation of the cutting performance due to the wear of the cutting edge geometry for Ball bearing and Stainless steels. However, there was no increase in E_SP when cutting Ni–Cr–Mo steel, which could be explained by the existence of a large protective built-up edge and/or minimal blade wear. The variation of the E_SP in different workpiece materials will also provide valuable information for bandsaw manufacturers and end users to estimate machinability characteristics for selected workpieces.     

Computational modelling of 3D turning: Influence of edge micro-geometry on forces,stresses, friction and tool wear in PcBN tooling

In this paper, computational modelling of 3D turning process in the presence of variable design cutting tool inserts is studied. Turning of alloy steel, AISI 4340, which utilized in high strength mechanical components for automotive and aerospace industry steel with uniform and variable edge design Polycrystalline cubic Boron Nitride (PcBN) inserts is performed. In the experiments chip geometry, forces and tool wear are measured. 3D computational modelling is utilized to predict chip formation, forces, stresses, temperatures and tool wear on uniform and variable edge design tools. Influence of variable edge tooling on resultant pressure-dependent friction has been investigated by utilizing 3D process simulations. Predicted forces are compared with experiments until pressure-dependent shear friction factor is solved. In general, a lower friction concentration is found for variable edge tooling. The temperature and stress distributions and tool wear contours reveal the advantages of variable edge micro-geometry design.     

The effect of machinability on thermal fields in orthogonal cutting of AISI 4140 steel

The micro-scale temperature fields in the cutting of two AISI 4140 steels with different machinability ratings were measured. A custom infrared microscope was constructed; each pixel was calibrated separately to reduce measurement uncertainty. Orthogonal cutting experiments were performed on a high speed machining center with surface speeds up to 500 m min⁻¹ and uncut chip thicknesses ranging from 0.1 mm to 0.3 mm. The results indicate that in certain critical regions of the thermal field, improved machinability correlates with significant reductions in temperature that exceed measurement uncertainties. Such micro-scale temperature measurements will help to design materials with further improved machinability.     

An experimental investigation on effect of minimum quantity lubrication in machining AISI 1040 steel

The growing demands for high productivity of machining need use of high cutting velocity and feed rate. Such machining inherently produces high cutting temperature, which not only reduces tool life but also impairs the product quality. Application of cutting fluids changes the performance of machining operations because of their lubrication, cooling, and chip flushing functions. But the conventional cutting fluids are not that effective in such high production machining, particularly in continuous cutting of materials likes steels. Minimum quantity lubrication (MQL) presents itself as a viable alternative for turning with respect to tool wear, heat dissipation, and machined surface quality. This study compares the mechanical performance of MQL to completely dry lubrication for the turning of AISI-1040 steel based on experimental measurement of cutting temperature, chip reduction coefficient, cutting forces, tool wears, surface finish, and dimensional deviation. Results indicated that the use of near dry lubrication leads to lower cutting temperature and cutting force, favorable chip–tool interaction, reduced tool wears, surface roughness, and dimensional deviation.     

Influence of the direction and flow rate of the cutting fluid on tool life in turning process of AISI 1045 steel

High-pressure coolant (HPC) delivery is an emerging technology that delivers a high-pressure fluid to the tool and machined material. The high fluid pressure allows a better penetration of the fluid into the tool–workpiece and tool–chip contact regions, thus providing a better cooling effect and decreasing tool wear through lubrication of the contact areas.The main objective of this work is to understand how the tool wear mechanisms are influenced by fluid pressure, flow rate and direction of application in finish turning of AISI 1045 steel using coated carbide tools.The main finding was that when cutting fluid was applied to the tool rake face, the adhesion between chip and tool was very strong, causing the removal of tool particles and large crater wear when the adhered chip material was removed from the tool by the chip flow. When cutting fluid was not applied to the rake face, adhesion of chip material to the face did occur, but was not strong enough to remove tool particles as it moved across the face, and therefore crater wear did not increase.     

An experimental study of the effect of high-pressure water jet assisted turning (HPWJAT) on the surface integrity

This study deals with the effect of High-Pressure Water Jet Assisted Turning (HPWJAT) of austenitic stainless steels on chip shape and residual stresses. The machining of the austenitic stainless steels represents several difficulties. Recently, research has shown that the introduction of a high-pressure water jet into the gap between the tool and the chip interface is a very satisfactory method for machining applications. In this article, the effect of a high-pressure water jet, directed into the tool-chip interface, on chip shapes breakage and surface integrity in face turning operations of AISI 316L steel has been investigated. Tests have been carried out with a standard cutting tool. The cutting speeds used were 80 and 150 m/min, with a constant feed rate of 0.1 mm/rev and a constant cutting depth of 1 mm. Three jet pressures were used: 20, 50 and 80 MPa. Residual stress profiles have been analysed using the X-ray diffraction method in both longitudinal and transversal directions. The results show that jet pressure and cutting parameters influence the residual stresses and the chip shapes. Using a high-pressure jet, it is possible to create a well fragmented chip in contrast to the continuous chip formed using dry turning. It is also possible to control the chip shape and increase tool life. When the jet pressure is increased the residual stress at the surface decreases; however it is increased by an increase in cutting speed. It can be concluded that surface residual stresses can be reduced by the introduction of a high-pressure water jet.     

Investigation of wear behavior and chip formation for cutting tools with nano-multilayered TiAlCrN/NbN PVD coating

A comprehensive investigation of the wear progress and chip formation was performed on an ultra-fine-grained cemented carbide ball nose end mill coated with a novel nano-multilayered TiAlCrN/NbN coating, by dry machining-hardened steel AISI H13 (HRC 55–57) at a cutting speed of 300 m/min. Flank wear and cutting forces were measured as the wear progressed; chip temperatures were estimated. The surface morphology of the tools were studied by using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis techniques. Results showed that protective oxide films (Al–O, Cr–O and Nb–O) were formed during cutting. With the combination of the protective oxide films and the fine-grain tough substrate, the tool wear rate was greatly reduced compared to the other coatings tested. Continuous and saw-tooth chips were identified, corresponding to a new sharp tool and a worn tool, respectively. The mechanisms of saw-tooth chip formation were found to be a combination of “crack theory” and “adiabatic shear theory”. The characteristics of the chips were studied in detail with the results showing that during formation the chips underwent a combined effect of strain hardening and thermal softening, followed by a quenching phenomenon.