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
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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.
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
Primary
shear zone
Wear mechanisms of HSS cutting tools
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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
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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.
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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.