Wear mechanisms of HSS cutting tools

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Metal cutting puts extreme demands on the tool and tool material through conditions of high forces, high contact pressures, high temperatures, and intense chemical attack by difficult to cut work materials. In addition, the tool geometry and cutting conditions in terms of sharp edges, cyclic engagement and presence of cutting fluid will add to the severity. Most often cutting tools are used close to their ultimate resistance against these loads, especially to the limiting thermal and mechanical stresses. ...

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Wear mechanisms of HSS cutting tools 1 Wear mechanisms of HSS cutting tools by Sture Hogmark, Uppsala University, The Ångström Laboratory, Sweden Mikael Olsson, Dalarna University, Sweden 1. Introduction Metal cutting puts extreme demands on the tool and tool material through conditions of high forces, high contact pressures, high temperatures, and intense chemical attack by difficult to cut work materials. In addition, the tool geometry and cutting conditions in terms of sharp edges, cyclic engagement and presence of cutting fluid will add to the severity. Most often cutting tools are used close to their ultimate resistance against these loads, especially to the limiting thermal and mechanical stresses. In spite of the increasing use of high performance tool materials, such as CVD and PVD coated cemented carbides, cermets, ceramics, cubic boron nitride and diamond, high speed steels (HSS) are still frequently used in tools for metal cutting applications. The relatively high toughness and the possibility of economic manufacturing of tools with complicated geometries still justify the use of HSS in many cutting operations. The introduction of powder metallurgical grades in combination with Electro Slag Heating (ESH) and Physical Vapour Deposition (PVD) coating technologies has further improved the performance of HSS cutting tools. Since the successful introduction of the PVD-TiN-coating in the late 70:ies, the academic research on HSS metal cutting tools has been concentrated to developing even better coating materials and techniques for their deposition. This paper is a brief overview of the mechanisms of wear of HSS cutting tools and includes illustrations from both uncoated and coated tools. More details on the metal cutting process, the mechanisms of tool degradation, and the properties of HSS materials and their coatings are found in Refs [1-10]. 2. The cutting process in brief To understand the wear mechanisms in metal cutting it is necessary to have a brief understan- ding of the severe contact conditions prevailing at the cutting tool/work material interface, see Fig. 1. The common model illustrates orthogonal cutting, but it applies to any cutting operation including turning, milling, sawing, drilling, tapping, broaching, etc. Through plastic shear of the work material and sliding of work material against the tool flank and rake face a characteristic temperature profile is established. The principle heat sources are located at the primary shear zone in the forming chip and in the frictional contact between chip and tool (secondary shear zone), and the highest temperature is consequently reached on the rake face at some distance from the edge. To illustrate the forces and mechanical stresses acting on the tool edge in one picture is less strait forward since they change considerably with cutting operation and cutting parameters. In intermittent cutting they also may change completely from entrance to exit during the
Wear mechanisms of HSS cutting tools 2 individual edge engagements. Generally, the over all cutting force F is related to cutting speed and feed as indicated in Fig. 2. It is indicated that a low friction coating can lower the cutting force and thereby giving a lower edge temperature, which can be utilised to increase the productivity. We know from the type of failure mechanisms that HSS cutting tools are used close to their limits of yield and fracture stresses, see § 6 and on. Since the cutting edge is forcing its way through the interior of the work piece like a propagating wedge, both surfaces of the opened “crack” represent highly chemically reactive metal. The fact that there is no access to external oxygen or cutting fluids to this region means that there is no formation of oxide films or any other protecting interlayer. Consequently, the tool edge is also exposed to extremely severe conditions. Primary shear zone Fig. 1. Principle action and temperature distribution of a HSS metal cutting edge exposed to its practical limit of thermal loading. a) b) Fig. 2. Schematics of cutting force F vs. cutting speed (a) and feed (b). (Linear scales). 3. Tool material properties 3.1. High temperature strength A metal cutting tool must be able to combine high hardness (or high yield strength) with high fracture strength at elevated temperature, see Fig. 3a. The latter is especially important in
Wear mechanisms of HSS cutting tools 3 interrupted cutting. A high thermal conductivity is also a desired tool property since it will reduce the tendency to local thermal softening. The high thermal resistance of carbides, nitrides and oxides indicates their potential as protec- tive thin PVD or CVD coatings, but also their strengthening ability when present in the form of small particles in the tool material. However, they are also common as strengthening ele- ments in most work materials where they contribute to abrasive wear, see § 6.1. 3.2. Fracture strength vs. hardness High hardness is associated with brittleness, and strengthening metallic materials such as HSS by martensitic hardening, dispersion of hard particles, etc. of a metallic materials most often results in a material with a lower fracture strength as indicated by Fig. 3b. a) b) Fig. 3. a) Hot hardness (HV) of HSS compared to that of carbon steel and austenitic stainless steel. The superior hot hardness of carbides, nitrides and oxides in the whole temperature interval is also indicated. b) Room temperature fracture strength (Rmb) vs. hardness (HV) of some common tool materials. 4. Common work materials for HSS cutting tools Generally, the work materials in metal cutting with HSS tools are macroscopically much softer than the tools, see Table 1. However, many work materials contain constituents (carbides, nitrides or oxides) that are harder (HV 1500 – 3000) and more temperature resistant than the HSS matrix, as indicated in Fig. 3a, and contribute to the tool degradation by abrasion. High toughness, large fracture elongation (ductility) and ability to work harden all add to generate a high temperature during chip formation. High temperatures reduce the strength of the HSS tool, but will also facilitate chemical reactions and possibility to form intermetallic phases between tool and work material. This will increase the friction between these materials and thus further aggravate the situation. Another fact that has to be considered when comparing the mechanical properties of tool materials with those of work materials is that chip formation generally occurs by extremely high shear rates. Taking high strain rate into account, the work material curves of Fig. 3a are
Wear mechanisms of HSS cutting tools 4 lifted up such that the corresponding RT hardness of a carbon steel may well match the hardness of the cutting edge at its working temperature, as indicated by the two ovals in this figure [11]. The illustrated situation is accentuated in intermittent cutting when a hot tool edge suddenly meets cold work material. Table 1. Work materials and their nominal properties related to tool wear in metal cutting Work material Hardness [HV] Hard particles Ductility Work harden C-steels 200 - 250 Cementite Yes Yes Cast irons 200 - 250 Cementite - - γ−steels 180 - 250 - Yes Yes Al-alloys 100 - 150 Oxides, AlFeSi Yes - Ti-alloys 200 - 350 - Yes Yes Ni-based alloys 200 - 350 Yes Yes Yes 5. Tool wear Taking orthogonal cutting as a model the general characteristics of a worn HSS cutting tool are schematically illustrated in Fig. 4. Primarily, depending on cutting operation, cutting parameters, cutting parameters, work material and tool material the performance of the tool is limited by nose wear, flank wear, crater wear, edge chippings, or combinations of these. Depending on the same parameters, the wear either occurs gradually by abrasive or adhesive wear, through plastic deformation, by more discrete losses of material through discrete fracture mechanisms, or by combinations of these. Below, illustrative micrographs from scanning and optical microscopy (SEM and OM, respectively) of used HSS tools will be used to demonstrate the wear mechanisms. Fig. 4. Schematic of tool wear distribution.
Wear mechanisms of HSS cutting tools 5 6. Wear mechanisms of uncoated tools 6.1. Abrasive wear Abrasive wear dominates the flank and crater wear of the HSS tool edge seen in Fig. 5. The grooved pattern is a combination of the scratching action of hard particles in the work material, and the protection against scratching offered by the hard phases in the tool material. Behind large tool carbides, seen in the chip flow direction, there are typical ridges of protected tool material. The individual abrasive scratches are too small to be resolved in the picture. Abrasive wear is counteracted by a high yield strength (high hardness) and large carbide volume of the HSS. a) b) Fig. 5. Typical appearance of abrasive wear. a) Wear dominates the crater and flank wear of a milling tool. The arrows point at ridges of HSS material relatively resistant to abrasion. There is also evidence of edge fracture. Work material: C-steel. b) Paper knife. An extremely fine-scaled abrasion, only resisted by the hard carbides, dominates the tool wear. 6.2. Adhesive wear When viewed in low magnification the dominating wear mechanism of the milling tooth of Fig. 6 appears to be abrasive, i.e. a ploughing action of hard constituents in the work material (carbon steel). However, higher magnification (Fig. 6b) reveals that it is rather a combination of abrasive and adhesive wear. This adhesive component, often referred to as mild adhesive wear, is a tearing of superficial HSS material by high shear forces resulting in a slow drag of the surface layer and removal of small fragments in the direction of chip flow. If the tool is used to its upper limit of heat resistance, severe adhesive wear may result as a large scale plastic flow of surface material in the direction of the chip flow, see Fig. 7.
Wear mechanisms of HSS cutting tools 6 Adhesive wear dominates the flank and crater wear of HSS tools if the edges reach high temperatures, i.e. at high cutting speed. Adhesive wear is further promoted when cutting chemically aggressive materials. Both mild and severe adhesive wear are primarily resisted by the HSS material through its high yield strength at elevated temperature (high hot hardness). a) b) Fig. 6. Crater in a milling tool that has been cutting in low carbon steel. In low magnification (a) the dominating wear mechanism appears to be abrasive. However, a close up (b) reveals that it is dominated by a mild adhesive component with shear fragmentation of the HSS material in the direction of chip flow (arrow). a) b) Fig. 7. a) Optical micrograph of cross-sectioned hob tooth after cutting austenitic stainless steel. b) Detail of a). The arrows indicate the chip flow direction and flow pattern of superficial HSS material, respectively. The latter is indicative of severe adhesive wear.
Wear mechanisms of HSS cutting tools 7 6.3. Large scale plastic deformation Sometimes, the HSS tool edge is loaded beyond its yield strength and deforms by large-scale plastic deformation, see Fig. 8, resulting in edge blunting. a) b) Fig. 8. Plastic deformation of HSS tool edge. a) The edge line in the central part of the picture is being plastically moved downwards and will soon leave the edge as the adjacent parts of the edge already have. b) Cross section of the plastically deformed edge showing signs of adiabatic shear. 6.4 Fatigue and fracture Macroscopic fracture of the whole tool can occur but is a rather scarce event. More common is localised chippings of the tool edge, see Fig. 9. Note that the chippings in (a) seem to be initiated by grinding marks running parallel to the edge. a) b) Fig 9. Small (a) and somewhat larger (b) edge chippings due to local overloading and fatigue of hob teeth.
Wear mechanisms of HSS cutting tools 8 7. Wear mechanisms of coated tools Since the late 70:ies when the TiN-coating was introduced on HSS metal cutting tools, PVD coating has become standard in tool wear protection, and today coating centres offer a considerable number of thin ceramic coatings for HSS tools [9]. A thin (1 – 10 µm) PVD coating will primarily protect the cutting edge in two ways: • Acting as a shield against abrasive and mild adhesive wear. • Reducing the tool temperature by reducing the friction between tool and work material, especially between chip and rake face. The coatings combine a superior hardness (abrasive wear resistance) with relatively low chemical reactivity with metallic materials (low solubility), the latter giving protection against the welding mechanism that is the prerequisite for adhesive wear. Consequently, most of the common PVD coatings of today rather fail by fatigue and discrete delamination/detachment than removal by slow gradual wear [9]. Once the coating is removed, the wear mechanisms of coated tools are the same as those of uncoated, although more severe because more severe cutting parameters are normally used for coated tools. 7.1. Coating removal due to poor substrate preparation There are primarily two ways by which failure in HSS substrate preparation can occur. • The surface temperature during grinding/polishing reaches above the austenitisation temperature resulting in a brittle interlayer of untempered martensite, see Fig. 10. • The resulting substrate surface is too rough, see Figs. 11 and 12. 10 µm 10 µm a) b) Fig. 10. Metallographic cross-sections through surface finished HSS materials. a) Superficial layer of untempered martensite due to excessive heat generation during finishing. b) Properly surface finished HSS. Used as substrate for PVD coating, the untempered martensite in Fig. 11a would constitute a brittle interlayer inferior to coating adhesion.
Wear mechanisms of HSS cutting tools 9 PVD coatings on HSS tools possess internal compressive stresses of the order of 1-5 GPa. Typically, TiN deposited on HSS has a lateral compressive residual stress of around 4 GPa. This stress acts positively for the coating cohesion, but negatively on its adhesion to the substrate. In combination with a rough substrate, excessively high compressive stresses may cause spontaneous detachment without any external loads [10, 12]. The reason is that lateral compressive stresses in the coating combined with a rough substrate will generate tensile stresses across the coating/substrate interface as illustrated in Fig. 11a [12]. If such a system is externally loaded, coating detachment is facilitated along regions of maximum tensile stress, i.e. along the coarse ridges on the tool of Fig. 11b. These ridges are the result of a too rough grinding process / incorrect grinding parameters. Another example of topographically induced coating failure is shown in Fig. 12 where it also is indicated that cracks nucleated in the coating may spread to the underlying HSS material. Through fatigue, they may later cause edge chippings and large-scale edge fracture. a) b) Fig 11. a) The lateral compressive stresses state σ present in most PVD coatings will generate interfacial stresses S. At the top of e.g. grinding ridges this stress is a tensile “lift off” stress that may reach the same order of magnitude as the residual stress σ [12]. Such ridges can result from rough grinding. b) TiN coating detachment along grinding ridges of a HSS cutting tool. 25 µm a) b) Fig. 12. Microscopic fatigue cracks observed on the rake face close to the edge of a hob tooth that has been cutting in carbon steel. b) Close up of a). Note that the direction of the cracks coincide with the direction of surface finishing.
Wear mechanisms of HSS cutting tools 10 7.2. Coating removal due to thermal softening of the substrate Once the HSS substrate material reaches a temperature level of excessive softening, it fails to resist the contact pressure, and the brittle coating fractures, see Fig. 13a. Note the dark etching contrast underneath the coating, which reveals thermal softening due to over tempering. The coating fractures and individual fragments are then detached in the form of small fragments, see Fig. 13b. a) b) Fig. 13. Coating detachment of hob tooth used for making gears of carbon steel. a) Coating fracture due to thermal softening of the substrate. b) Removal of small coating fragments and initial wear of the underlying HSS material. The thickness of the fragments is the same as the original coating thickness. 8. Distribution and evolution of edge wear The macroscopic wear pattern of a cutting tool edge was illustrated in Fig. 4. The mechanisms described above will eventually cause wear that exceed the worn out criteria, either as a certain width of the flank, the rake face or as a certain edge blunting. Figure 14 shows the development of a large crater in a TiN-coated hob tooth. The work material was carbon steel and the wear that eventually controlled tool life occurred on the rake face. Initially, the hob teeth suffered from limited edge chipping (Fig. 14b and c). At the same time, thermal softening of tool material in the rake face (over tempering revealed by the dark contrast adjacent to the coating) reduced the load bearing capacity of the coating, which failed by cracking and brittle fracture (Fig. 14b), cp. Fig. 13. Once the coating was removed, a large crater was rapidly developed in the unprotected HSS by severe adhesive wear, Fig 14d. Irrespective of the location of the critical wear, its evolution can be illustrated as in Fig. 15. An initial wear, often involving tip blunting through minor fractures (chipping) is followed by a linear, steady-state wear regime dominated by abrasive and adhesive wear. A gradual tip blunting is one of the reasons behind a successively increasing edge temperature, and eventually, a situation of accelerated wear through edge fracture or severe plastic deformation is reached.
Wear mechanisms of HSS cutting tools 11 It is also indicated in Fig. 15 that a smoothening of the tool surface and sharpening of the edge will prolong tool life. This is further accentuated by coating. However, the improved wear resistance obtained by coating is often used to increase the productivity rather than to obtain a longer tool life. a) b) c) d) Fig. 14. Successive wear of TiN-coated hob teeth revealed by SEM and etched metallographic cross-sections. a) Overview of one tooth from a used tool. b) Initial stage of edge chipping and thermal softening of the coating substrate on the rake face. c) Close up of edge chipping in cross-section. Note cracks running from both rake and clearance face d) Final stage of severe crater wear. The etching in b) and c) reveals superficial thermal effects.
Wear mechanisms of HSS cutting tools 12 Fig. 15. Tool wear evolution. The three ovals represent initial, steady state and catastrophic tool wear, respectively. Influence of tool surface preparation and coating is also indicated. Table 2 summarises the wear mechanisms of HSS cutting tools. Normally, the tool suffers from a combination of two or more of these mechanisms, and it can be difficult to judge which is dominating. It is also indicated in the table how the different wear mechanisms result from a combined effect of properties of work and tool materials as well as cutting parameters. Table 2. Common wear mechanisms of HSS cutting tools, their cause and how to fight them. Wear mechanism Cause of wear Counteractive tool properties Abrasive wear Hard particles or other hard phases in the High matrix hardness, large volume of work material remove material by a hard phases, hard coating ploughing action. Mild and severe High cutting speed generates high tool Smooth surface, sharp edge, high hot adhesive wear surface temperatures that facilitate strong hardness, high thermal conductivity, adhesion between work and tool materials. chemically inert (anti sticking) coating The worst situation prevails for tough, ductile and chemically reactive work materials with low thermal conductivity. Plastic deformation High cutting speed generates excessive High hot hardness, high thermal edge temperatures in combination with conductivity. high loads. Fracture and Interrupted cutting, especially in Smooth tool surface, high fracture fatigue combination with high cutting speed and toughness promoted by a defect free HSS use of cutting fluid, a tough and ductile with a fine grained structure of both matrix work material. Use of insufficiently sharp and hard phases tool edges.
Wear mechanisms of HSS cutting tools 13 7. Towards better performance of HSS cutting tools 7.1 Improving the HSS material From the demonstrated mechanisms of wear of HSS cutting tools we can draw the conclusion that hardness, heat resistance (hot hardness) and fracture toughness both macroscopically and microscopically are the prerequisites of high tool performance. Recent HSS development has focused on the homogeneity and cleanliness of the HSS steel. Through the PM technology it is possible to obtain a fine grained, homogeneous structure. However, the authors think that it is possible to further improve the hardness/toughness ration demonstrated in Fig. 2b. By further reducing the size of the matrix grains and hard phase particles down to the nanometer range (applying nano-technology) a further step is possible as illustrated by the general hardness/- toughness relation given in Fig. 16. Fig. 16. Reducing the size of the microstructure constituents and improving the cleanliness improves the hardness/toughness ratio. 7.2. Improving the surface integrity Another means to further improve HSS tool performance is to improve the surface finish, i.e. avoiding deterioration of the superficial HSS material by excessive heat generation. The macroscopic strength and the resistance to edge chipping of HSS materials (Fig. 14.c) is very sensitive to surface defects generated by the surface preparation, see Fig. 17, and consequently the surface of the cutting tool edge must be accurately prepared in order to avoid premature failure. A smooth tool surface contributes to the resistance against micro cracking and to avoid premature detachment of coatings as demonstrated in Fig. 11.
Wear mechanisms of HSS cutting tools 14 Fig. 17. Four point bend strength of AP 2023 illustrating the influence of surface condition on strength. References 1. Söderberg, S., Wear mechanisms and tool life of hhigh speed steels related to microstructure , PhD Thesis of Uppsala University, Sweden, Acta Universitatis Upsaliensis 1982 2. Söderberg, S., Jacobson S., Olsson, M., Wear Atlas of HSS Cutting Tools, Proceedings of the 5th International Congress on Tribology (Eurotrib 89), Helsinki, Finland, Finnish Society for Tribology, 1989 3. Trent, E.M., Metal Cutting. 3rd ed. Oxford: Butterworth-Heineman, 1991. 4. Alden Kendall, A., Friction and wear of cutting tools and cutting tool materials, ASM Handbook, Vol. 18, 1992, pp 609-620 5. Holmberg, K., Matthews, A., Coatings tribology, Properties, techniques and applications in Surface Engineering, Ed: D. Dowson, Elsevier 1994. 6. Alvelid, B., Wisell, H., Wear of HSS in orthogonal milling, Scand. J. of Metallurgy 9 (1980) 59-67 7. Lim, S.C., Lee, S.H., Liu, Y.B., Seah, K.H.W., Wear maps for uncoated high-speed steel cutting tools, Wear 170 (1993) 137-144. 8. Lim, C.Y.H., Lim, S.C., Lee, K.S., The performance of TiN-coated high speed steel tool inserts in turning, Tribology International 32 (1999) 393-398 9. Hogmark, S. Jacobson, S., Larsson, M., Wiklund, U., Mechanical and tribological requirements and evaluation of coating composites, In Modern Tribology 2000. Ed. B. Bushan, Vol II, 931-959 CRC Press 2001 10. Le May, I., Principles of mechanical metallurgy, Elsevier 1981 11. Larsson, M., Olsson M., Hedenqvist, P., Hogmark, S., Mechanisms of coating failure as demonstrated by scratch and indentation testing of TiN coated HSS - On the influence of coating thickness, substrate hardness and surface topography, Surface Engineering 16, 5 (2000) 436-444 12. Wiklund, U., Gunnars, J., Hogmark, S., Influence of residual stresses on fracture and delamination of thin hard coatings, Wear 232 (1999) 262-269