Wear mechanisms of HSS cutting tools pps

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 b

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

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

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

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

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

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

Al-alloys 100 - 150 Oxides, AlFeSi 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

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

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

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)

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)

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

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

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

Fig 9 Small (a) and somewhat larger (b) edge chippings due to local overloading and fatigue of hob teeth

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

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

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

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

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

25 µm

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

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