Wear mechanisms of HSS cutting tools

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 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, Sweden1. IntroductionMetal cutting puts extreme demands on the tool and tool material through conditions of highforces, high contact pressures, high temperatures, and intense chemical attack by difficult tocut work materials. In addition, the tool geometry and cutting conditions in terms of sharpedges, cyclic engagement and presence of cutting fluid will add to the severity. Most oftencutting tools are used close to their ultimate resistance against these loads, especially to thelimiting thermal and mechanical stresses.In spite of the increasing use of high performance tool materials, such as CVD and PVDcoated cemented carbides, cermets, ceramics, cubic boron nitride and diamond, high speedsteels (HSS) are still frequently used in tools for metal cutting applications. The relatively hightoughness and the possibility of economic manufacturing of tools with complicated geometriesstill justify the use of HSS in many cutting operations. The introduction of powdermetallurgical grades in combination with Electro Slag Heating (ESH) and Physical VapourDeposition (PVD) coating technologies has further improved the performance of HSS cuttingtools.Since the successful introduction of the PVD-TiN-coating in the late 70:ies, the academicresearch on HSS metal cutting tools has been concentrated to developing even better coatingmaterials and techniques for their deposition.This paper is a brief overview of the mechanisms of wear of HSS cutting tools and includesillustrations 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 arefound in Refs [1-10].2. The cutting process in briefTo 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, seeFig. 1. The common model illustrates orthogonal cutting, but it applies to any cuttingoperation including turning, milling, sawing, drilling, tapping, broaching, etc. Through plasticshear of the work material and sliding of work material against the tool flank and rake face acharacteristic temperature profile is established. The principle heat sources are located at theprimary 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 faceat some distance from the edge.To illustrate the forces and mechanical stresses acting on the tool edge in one picture is lessstrait forward since they change considerably with cutting operation and cutting parameters. Inintermittent cutting they also may change completely from entrance to exit during the Wear mechanisms of HSS cutting tools 2individual edge engagements. Generally, the over all cutting force F is related to cutting speedand feed as indicated in Fig. 2. It is indicated that a low friction coating can lower the cuttingforce and thereby giving a lower edge temperature, which can be utilised to increase theproductivity.We know from the type of failure mechanisms that HSS cutting tools are used close to theirlimits of yield and fracture stresses, see § 6 and on. Since the cutting edge is forcing its waythrough 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 externaloxygen or cutting fluids to this region means that there is no formation of oxide films or anyother protecting interlayer. Consequently, the tool edge is also exposed to extremely severeconditions. Primary shear zoneFig. 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 properties3.1. High temperature strengthA metal cutting tool must be able to combine high hardness (or high yield strength) with highfracture strength at elevated temperature, see Fig. 3a. The latter is especially important in Wear mechanisms of HSS cutting tools 3interrupted cutting. A high thermal conductivity is also a desired tool property since it willreduce 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 formof 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. hardnessHigh hardness is associated with brittleness, and strengthening metallic materials such as HSS bymartensitic hardening, dispersion of har ...