Klocke F. Manufacturing Processes 1: Cutting

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3.7 Wear 81

WC:

Dissolution into:

C; (FeW)

(FeW)

(Ti,

Ta, Nb, W) C

Co-W-C-mixedcrystal

ChipTool

X6CrNiMoTi17−12−2

Mass percent %

55µm

Fe, Cr, Ni

Material

Cutting material

Cemented carbide WC-Co

Fig. 3.40 Presentation of the diffusion phenomena in cemented carbide tools

• dissolution of tungsten carbide to form mixed and double carbides in the form of

C, (FeW)

C and (FeW)

The carbon released during the dissolution of tungsten carbide migrates in the

direction of lesser concentration, i.e. into the steel. The diffusion of carbon passes

into the cobalt phase. The maximum solubility of carbon in cobalt is about 0.7% at

1200

◦

C. In the presence of Fe the solubility is increased to 1.5–2%. The diffusing

iron thus introduces two reactions, both of which accelerate dissolution. It lends

itself to the formation of iron mixed carbides and increases the reception of carbon

in cobalt, which is in turn the prerequisite for the dissolution of tungsten carbide.

The diffusion processes that are possible between cemented carbide and steel

materials can be shown very effectively with the help of annealing tests. To this pur-

pose, cemented carbide-material pairs were annealed at a temperature of 1100

◦

a pressure of 12.6 MPa and a holding time of 2 h in a protective gas atmosphere

of argon. Then the samples that were fused together on the contact surface were

separated on an abrasive cut-off machine perpendicularly to the contact level, and

82 3 Fundamentals of Cutting

after these sample halves were grinded and buffed, the distribution of elements

was analysed with a microprobe perpendicularly to the contact surface cutting tool

material/material.

As the quantitative line scans in Fig. 3.40 show, diffusion phenomena take place

under the selected annealing parameters between the uncoated cemented carbide

and the austenitic steel material here selected. On the side of the steel material,

there is a diffusion of iron, chrome and nickel into the cutting tool material. In t he

opposite direction (i.e. from the cutting material into the material) tungsten and

cobalt diffuse.

The dissolution of tungsten carbide, the bearer of hardness and wear resistance,

caused by the diffusion of iron into the cemented carbide leads to a weakening

of the structure and lowers the resistance of the cemented carbide substrate against

abrasion. The consequence of this is the formation of distinct crater wear (Fig. 3.41).

Since diffusion phenomena depend on temperature, the cutting speed inﬂuences on

an elementary level the speed with which crater wear develops. Uncoated cemented

carbides can thus only be used at relatively low cutting speeds (v

< 100 m/ min).

The cemented carbide substrate also has a serious effect on crater formation. Mixed

carbides on the basis of titanium and tantalum/niobium have, in comparison to WC,

a much higher chemical stability towards iron. Mixed carbides reduce the diffusion

of iron into the cobalt binder phase, thus increasing the resistance of the cutting tool

material to crater wear and increasing its heat wear resistance quite substantially.

The most eff ective way to reduce crater wear is coating the cemented carbide

substrate. Cemented carbide layers seal the substrate surface. This prevents direct

contact between the substrate and the material, thus reducing or suppressing diffu-

sion phenomena. As the line scans of annealing tests in Fig. 3.42 show, diffusion

processes between the cemented carbide substrate and the material can by all means

still occur in the case of TiN or TiCN layers. In contrast, no more diffusion takes

place in the case of the coated cemented carbide with Al

as an intermediate layer.

The aluminium oxide layer thus functions as a diffusion barrier. The elements iron,

nickel and chrome are found in the ZrN surface layer, while tungsten and cobalt are

in the TiN boundary layer (Fig. 3.42). In the aluminium oxide intermediate layer

Coated cemented carbide, t

= 20 min

Uncoated cemented carbide, t

= 40s

= 120 m/min, f = 0.32 mm, a

= 2 mm v

= 200 m/min, f = 0.2 mm, a

= 2 mm

Material: X6CrNiMoTi17–12–2

Fig. 3.41 Formation of crater wear on an uncoated and a coated cemented carbide, acc. to

ERSCHWILER [Gers04]

3.7 Wear 83

X6CrNiMoTi

17-12-2

WC-7,5Co-

6,8MK

TiN

(C,N)

O3 ZrN

X6CrNiMoTi17-12-2WC-7,5Co- 6,8MK

(C,N)

TiN

Fig. 3.42 Veriﬁcation of the diffusion phenomena between coated cemented carbide and steel

dependant on the coating system on the basis of annealing tests

the traceable amount of these elements is reduced to 0 mass-%. There is above all

no indication that iron, chrome, tungsten or cobalt have diffused through the Al

intermediate layer on the opposite side of that layer.

The observation that, excluding air or water, electrically isolated oxide layers

function as diffusion barriers on cemented carbides as opposed to semiconducting

nitride or carbon nitride layers can be explained as follows. In electrically isolating

oxides like Al

or HfO

, only ion diffusion is possible, no migration of elec-

trons or holes. The result of this is that “component diffusion” becomes impossible.

“Component” refers to an atom or iron, which theoretically can be split into an ion

and an electron. For reasons of electron neutrality, the iron atom cannot “release”

its electrons outside the layer and then migrate through the layer as an ion. The

consequence is that these oxide layers act as barriers as long as there i s no way for

electrons to get around it, and the transport of oxygen ions is also slow enough (in

oxides this transport is empirically slower than cation transport by several orders of

magnitude.) In semiconductive or conductive l ayers however, electrons can migrate

through the layer together with the ions, leading ﬁnally to the iron element migrat-

ing through the layer to alloy with the substrate. Conversely, conveyance of Co or W

into the chip is also possible. Diffusive transport is already possible at the tempera-

tures prevalent on the tool during machining of > 800

◦

C. The diffusion coefﬁcients

are indeed still small, but the necessary diffusion paths are very short.

While diffusion of iron through the cemented carbide layer into the cemented

carbide substrate is indeed possible in the case of cemented carbides coated with

TiN and TiCN, this diffusion is not as signiﬁcant as is the case for uncoated sub-

strates. As opposed to uncoated cemented carbides, the coat provides the substrate

84 3 Fundamentals of Cutting

with additional protection from the abrasive action of the ﬂowing chip. Especially

because they are much harder than the substrate, cemented carbide layers help

to reduce abrasive wear to a signiﬁcant extent. Cemented carbide layers thereby

ensure the high performance level of coated tools and allow for much higher cutting

speeds when cutting steel materials i n comparison with uncoated cemented carbides

(Fig. 3.41). But as soon as the hard material l ayer in the contact zone of the rake face

has been rubbed off and the substrate exposed, diffusion processes now once again

taking place between the substrate and the chip contribute, together with the abrasive

effect of the chip on the substrate, to an acceleration of crater wear.

3.7.2.5 Scaling

If we examine a tool after cutting, we can recognize several annealing colours near

the contact zones, indicating scaling (oxidation) of the cutting tool material. Scaling

varies in importance depending on the cutting tool material alloy and cutting temper-

ature (Fig. 3.43). Cemented carbides already begin scaling at 700–800

◦

C, whereby

Annealing time / h

Increase in mass / %

HW-K10

HW-P25

HW-P10

HT-P05

12 3 4

HT-P10

HT-P20

Atmosphere: air

Temperature: 1000 °C

Annealing

temperature:

500 °C

Annealing time:

3h exp. to air

HW-K10

HW-K10 HW-P25 HW-P10 HT-P10

Annealing temperature: 1000 °C; Annealing time: 3 h exp. to air

Fig. 3.43 Gain of weight during the annealing process in air dependant on the used cutting tool

material

3.7 Wear 85

Flank face

Oxidation

zones

Rake face

Oxidation

Cutting tool material: HW-P30

Cutting velocity:

Cutting time:

Feed:

Depth of cut:

Material: C53E

=125 m/min

= 20 min

f = 0.25 mm

= 3 mm

Fig. 3.44 Oxidation zones at a cemented carbide turning tool

those made of pure tungsten carbide and cobalt oxidize more than those alloyed with

titanium carbide or other carbides [Kief65].

Even under the usual cutting conditions, an oxide ﬁlm is formed on WC tools in

the area of the cutting edge under the effect of the cutting temperatures and atmo-

spheric oxygen. This ﬁlm covers the areas to which atmospheric oxygen has free

access, i.e. the ends of the contact zones on the ﬂank face, minor ﬂank face and rake

face (Fig. 3.44).

The destructive effect of oxidation on the structure of cemented carbide can

be observed especially clearly on the minor ﬂank face. A complex tungsten-

cobalt-iron oxide is formed that, as a result of its larger molar volume compared

with cemented carbide, develops in a wart-like fashion and can lead to corner

breaking [Köni75].

Scaling is of no practical importance for tool steels and high speed steels since

their heat resistance is exceeded before their surfaces are oxidized more strongly.

3.7.3 Forms and Dimensions of Wear

Wear phenomena that are generated on the cutting part during cutting vary greatly

depending on the type and duration of stress. Figure 3.45 shows wear forms that

occur primarily on turning tools. The cutting part is worn on the rake face (crater

wear) as well as on the major and minor ﬂank faces (ﬂank face wear) (Fig. 3.46).

Depending on the cutting parameters and the combination of workpiece and cutting

tool material, ﬂank face wear tends to be strongest on the edge of the contact zone

on the major and minor cutting edge of the tool, which continues on the rake face.

Such “notch wear” is caused by the abrupt transitions of mechanical and thermal

stress prevalent on the end of the contact area, the abrasive character of the sharp

86 3 Fundamentals of Cutting

Notch wear on minor cutting edge

Cratering on rake face

Plastic deformation of corner of

cutting edge to the point of

breaking

Layer flaking beneath width of

flank wear land

Layer flaking due to plastic

deformation of cutting edge

Notch wear on flank face

Wear on flank and minor

flank face

Delamination on cutting edge

beyond contact zone

Smeared material, partly

overlapping the cutting edge in a

palisade-like manner

Spalling of cutting edge

due to chip impact or

clamped chips

Spalling of cutting edge along

major cutting edge

Notch wear on rake face

Fig. 3.45 Characteristic wear forms at the cutting part during the turning process

Cutting tool material: Ceramic

Material: Nickel-based alloy

Cutting parameters:

= 220 m/min

f = 0.16 mm

= 2.3 min

= 0.3 mm

Insert geometry:

RCGX090700T00515

Cutting tool material: Cermet

Material: 42CrMo4+QT

Cutting parameters:

= 400 m/min

f = 0.1 mm

= 12 min

= 0.5 mm

Insert geometry:

SPGN120308

Cutting tool material: PCBN

Material: Nickel-based alloy

Cutting parameters:

= 300 m/min

f = 0.16 mm

= 1.6 min

= 0.3 mm

Insert geometry:

VBMW160412

Notch wear on:

Minor cutting

edge

Major cutting

edge

Fig. 3.46 Examples for the formation of crater wear, ﬂank face wear and notch wear dependent

on the used cutting tool material and the machined material

3.8 Cutting Theory 87

KB: Crater width

KM: Distance betw. crater

centers

KT: Crater depth

: Displacement of cutting

edge toward flank face

: Displacement of cutting

edge toward rake face

Wear notches on

minor cutting edge

Sectional view

A-A

crater

Wear notches on

major cutting

edge

Width of flank wear land VB

max.

CB N

b/4

Plane P

Fig. 3.47 Wear forms and measured quantities at the cutting part, acc. to DIN ISO 3685

chip edge or the feed comb and the tool edge as well as direct contact with the

atmosphere (Fig. 3.46).

Figure 3.47 is a schematic representation of the dimensions of wear. In particular,

we distinguish the width of ﬂank wear land VB, the displacement of cutting edge

toward ﬂank face SV

and rake face SV

, crater depth KT and the crater centre

distance KM from which the crater ratio K = KT/KM is formed.

3.8 Cutting Theory

3.8.1 Shear Plane Theory

The model representation of the shear plane is based on ideal cutting conditions.

Required are:

• ideally sharp cutting edges,

• cutting edges that are not subject to wear,

• the representation of mechanical loads as substitute forces and

• ideal plastic material behaviour.

88 3 Fundamentals of Cutting

Trace of cutting

edge plane

Trace of tool reference plane

Tool

≡

Selected point of cutting edge

Workpiece

Trace of shear plane P

Fig. 3.48 Model representation of the shear plane

The chip formation theory derived from this model (shear plane theory) assumes

that deformation takes place on only one plane, the shear plane. By deﬁnition, the

shear plane passes through the selected point of cutting edge just like the tool-in-

hand and tool-in-use reference system (Fig. 3.48).

In the following the shear plane P



will be considered in the tool-in-hand refer-

ence system. This is a plane which is inclined toward the cutting edge plane P

the shear angle F.

The requirements made on process kinematics are fulﬁlled when:

• tool cutting edge angle κ

= 90

◦

• tool inclination λ

= 0

◦

We are thus assuming a free, orthogonal cut. The shear plane model has the

advantage of making complicated cutting processes highly comprehensible.

If the effects of the corner radius and minor cutting edge are negligible in com-

parison with that of the major cutting edge, the theory of the shear plane can also be

used for the bound cut.

With the shear angle we can macroscopically consider material and cutting tool

material properties as well as thermal loads and friction conditions which pre-

dominate during contact of the chip with the rake face. Moreover, the shear angle

establishes the relation between undeformed chip thickness h and chip thickness

cos

(

 − γ

)

sin

· h (3.14)

3.8 Cutting Theory 89

By means of the shear angle w e can now calculate the surface area of the shear

area A



in the shear plane:



= b ·

sin

(3.15)

The ratio of undeformed to deformed magnitudes is designated in forming tech-

nology as deformation. If the material is compressed and only the change in height

is described, the term degree of compression is also used. This concerns volume

consistency. In cutting theory, the term chip compression λ

is used analogously

[Krys39].

cos

(

 − γ

)

sin

(3.16)

The shear angle  is a purely theoretical quantity and should not be confused

with the actual slip direction of the material, even if the shear surface and slip surface

are nearly identical under certain marginal conditions.

In conclusion, to show that the theory of the shear plane makes it possible to

calculate various quantities with complicated relations, three classical applications

of shear plane theory will be described.

3.8.1.1 Calculation of Chip Speed

From simple trigonometric relations (law of sine) in the velocity plan, M

ERCHANT

calculated the chip speed v

with the help of the shear angle F (Fig. 3.49)[Merc45].

|v

sin

cos

(

− 

)

·|v

| (3.17)

Trace of cutting

edge plane

Trace of tool refernce plane

Tool

≡

Selected point of cutting edge

Workpiece

Trace of shear plane P

Fig. 3.49 Velocity plan, acc. to MERCHANT [Merc45]

90 3 Fundamentals of Cutting

The continuity equation can also be used to calculate chip speed v

. The idea

behind this is that a mass ﬂow ﬂows through a control surface, in this case the shear

surface. Assuming that the density of the material is not altered by mechanical and

thermal load, we obtain:

˙m = const. ⇒|v

|·A =|v

|·A

(3.18)

From which follows:

|v

·|v

| (3.19)

3.8.1.2 Calculation of Resultant Force

It is assumed that the shear angle is a function of the shear stress in the shear plane,

which will be shown in the following steps. In the ﬁrst place we have:







(3.20)

The component of the resultant force in the shear plane, the shear force F



, can

be represented by the r esulting resultant force F

. With the help of the angle relations

in the circle of T

HALES we obtain according to Fig. 3.50:





|=|



|·cos

(

 + ρ − γ

)

(3.21)

Trace of cutting

edge plane

Trace of tool reference plane

≡

Trace of shear plane

Tool

Φn

γn

− γ

Fig. 3.50 Circle of THALES for the force diagram, acc. to MERCHANT