Trent E.M., Wright P.K. Metal Cutting

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TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co) 183

M 30 Steel, steel castings, austenitic

steel, gray cast iron, high tem-

perature resistant alloys

Turning, milling, planing. Medium cutting speeds,

medium or large chip sections

M 40

Mild free cutting steel, low ten-

sile steel

Non-ferrous metals and light

alloys

Turning, parting off, particularly on automatic

machines

K Ferrous metals with

short chips, non-ferrous

metals and non-metallic

materials

K 01 Very hard gray cast iron,

chilled castings of over 85

Shore, high silicon aluminium

alloys, hardened steel, highly

abrasive plastics, hard card-

board, ceramics

Turning, finish turning, boring, milling, scraping

K 10 Gray cast iron over 220 Brinell,

malleable cast iron with short

chips, hardened steel, silicon

aluminium alloys, copper

alloys, plastics, glass, hard rub-

ber, hard cardboard, porcelain,

stone

Turning, milling, drilling, boring, broaching,

scraping

K 20

Gray cast iron up to 220

Brinell, non-ferrous metals:

copper, brass, aluminium

Turning, milling, planing, boring, broaching,

demanding very tough carbide

K 30

Low hardness gray cast iron,

low tensile steel, compressed

wood

Turning, milling, planing, slotting, for machining

in unfavorable conditions and with the possibility

of large cutting angles

K 40 Soft wood or hard wood, Non-

ferrous metals

Turning, milling, planing, slotting, for machining

in unfavorable conditions and with the possibility

of large cutting angles

Symbol

Broad categories of

material to be

machined Designation Material to be machined Use and Working Conditions

184 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

alloy to 18 MPa m

-1/2

for a 15% Co alloy. Considering the scatter in individual test results, this is

a relatively small difference, while in practice, there is a very large difference between the per-

formances of 15% Co and 6% Co alloys subjected to applications with severe impact. This may

be because the test measures the energy required to propagate a crack under tensile stress, while

in many practical applications performance depends on the energy required to initiate a crack

under localized compressive stress.

FIGURE 7.5 Ball indentations in surface of cemented carbide, revealed by optical interferometry

More research should be directed towards understanding the quality of toughness of cemented

carbides as observed in practice and towards developing adequate tests for its measurement. Fre-

quently the best tool material for a particular application is the hardest combined with adequate

toughness to resist fracture. In practice the solution to this problem has been achieved by the

‘natural selection’ process of industrial experience.

Just as for high speed steels, where a general purpose grade M2, has become established for

general machining work, so with the WC-Co alloys, a grade containing 6% Co and a grain size

near 2 μm (Figure 7.6) has become acknowledged as the grade for the majority of applications

for which this range of alloys is suitable, and this grade is made in much larger quantities than

the others. If this grade proves too prone to fracture in a particular application, a grade with a

higher cobalt content is tried, while, if increased wear resistance is needed, one with a finer grain

size or lower cobalt content is recommended.

Trial by industrial experience is a very prolonged and expensive way of establishing differ-

ences in toughness. It is difficult for the user to assess the claims of competing suppliers regard-

ing the qualities of their carbide grades. The user must rely on classifications such as that of the

International Organization for Standardization (ISO).

Table 7.3 shows the performance of cut-

ting tools as related to the relative toughness and hardness of the different WC-Co alloys,

arranged in a series from K01 to K40. It is the responsibility of the individual manufacturer to

decide in which category their products shall be placed for guiding the end-user.

Both hardness and compressive strength of cemented carbides decrease as the temperature is

raised (Figures 7.7 and 6.9). The comparison of compressive strength at elevated temperatures

TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co) 185

with that of high speed steel

(Figure 6.9) shows that a WC-Co alloy with 6% Co withstands a

stress of 750 MPa (50 tonf/in

) at 1000°C, while the corresponding temperature for high speed

steel is 750°C.

FIGURE 7.6 Structure of medium fine grained alloy of WC with 6% Co

With cemented carbides, the temperature at which this stress can be supported drops if the

cobalt content is raised or the carbide grain size is increased.

The coefficient of thermal

expansion is low - about half that of most steels. Thermal conductivity is relatively high: a 6%

Co-94% WC alloy has a thermal conductivity, of 100 W/mK compared with 31 W/mK for high

speed steel. Oxidation resistance at elevated temperatures is poor, oxidation in air becomes

rapid over 600°C and at 900°C is very rapid indeed. Fortunately this rarely becomes a serious

problem with cutting tools because the surfaces at high temperature are usually protected from

oxidation (see Chapter 10).

FIGURE 7.7 Hot hardness of cemented carbides

186 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

7.4 TOOL LIFE AND PERFORMANCE OF TUNGSTEN

CARBIDE-COBALT TOOLS

Cemented tungsten carbide tools were introduced in the early 1930s and inaugurated a revolu-

tion in productive capacity of machine tools as great at that which followed the introduction of

high speed steels. Over a period of years the necessary adaptations have been made to tool

geometry, machine tool construction, and operation methodology, enabling these tools to be used

efficiently. Having learned how to avoid premature fracturing, machinists could use carbide

tools to cut many metals and alloys at much higher speeds and with much longer tool life than

had previously been possible. There are limits to the rate of metal removal at which one can

maintain a reasonable tool life.

As with high speed steel tools, the shape of the edge gradually changes with continued use

until the tool no longer cuts efficiently. The mechanisms and processes which change the shape

of the edge of tungsten carbide-cobalt tools when cutting cast iron, steel and other high melting-

point alloys are now considered, looking first at those which set the limits to the rate of metal

removal.

7.4.1 Superficial plastic deformation by shear at high temperature

Similar to the situation that was examined with high speed steel tools, with cemented tungsten

carbide-cobalt tools the work material is seized to the tool over much of the worn rake and flank

surfaces when cutting cast iron and most steels at medium and high cutting speeds. A flow-zone

or built-up edge is formed at the interface, of the same character as described for steel tools.

Nevertheless, the significantly higher hot-strength of carbides seems to protect them from the

wear mechanism due to superficial plastic deformation by shear at high temperature (compare

Figure 6.13 and the top left of Figure 6.24 for high speed steel.)

7.4.2 Plastic deformation under compressive stress

Even though there is yet no experimental evidence for superficial plastic deformation by

shear, carbides are nonetheless prone to deformation by compression under the influence of high

temperatures and compressive stress. Thus, the rate of metal removal as cutting speed or feed are

raised is often limited by deformation of the tool under compressive stress on the rake face. Car-

bide tools can withstand only limited deformation, even at elevated temperature, and cracks

form which lead to sudden fracture. Figure 7.8 shows such a crack in the rake face of a tool, this

surface being stressed in tension as the edge is depressed.

Failure due to deformation is more

probable at high feed rates, and when cutting materials of high hardness. Carbide grades with

low cobalt content can be used to higher speed and feed rate because of increased resistance to

deformation.

Deformation can be detected at an early stage in a laboratory tool test by lapping the clearance

face optically flat before the test, and observing this face after the test using optical interferome-

try. Any deformation in the form of a bulge on the clearance face can be observed and measured

as a contour map formed by the interference pattern. Figure 7.9 shows the interference fringe

pattern on the clearance face of a carbide tool used to cut steel at high speed and feed. The max-

TOOL LIFE AND PERFORMANCE OF TUNGSTEN CARBIDE-COBALT TOOLS 187

imum deformation is seen to be at the nose, a common feature when cutting steel and iron. A

tool with a sharp nose, or a very small nose radius, starts to deform at the nose and thus fails at

much lower speed than tools with a large nose radius. In many cutting operations the precise

form of the nose radius is critical for the performance of the tool. The ability of tungsten car-

bide-cobalt tools to resist deformation at high temperatures is the most important property per-

mitting higher rates of metal removal compared with steel tools.

FIGURE 7.8 Cracks across the nose of cemented carbide tool deformed during cutting

FIGURE 7.9 Deformation on clearance face of cemented carbide tool, after cutting at high speed and

feed, revealed by optical interferometry

188 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

7.4.3 Diffusion wear

When cutting steel at high speed and feed, a crater is formed on the rake face of tungsten car-

bide-cobalt tools, with an unworn flat at the tool edge (Figure 7.10). Dearnley’s

measurement

of temperature in carbide tools (using iron-bonded carbide) shows the same pattern of tempera-

ture as with steel tools (Figure 7.11). This was predictable since the heat source is of the same

character: a thin flow-zone in metallic contact with the rake face of the tool. The crater is in the

same position as on steel tools, the deeply worn crater being associated with the high tempera-

ture regions, and the unworn flat with the low temperatures near the edge. Sections through the

crater in WC-Co tools show no evidence of plastic deformation by shear in the tool material, in

contrast to the deformation observed in high speed steel tools (Figures 6.11 and 6.13). The

grains are smoothly worn (Figure 7.12) with no evidence that particles are broken away from the

surface.

FIGURE 7.10 Crater on rake face of WC-Co alloy tool after cutting steel at high speed and feed

FIGURE 7.11 Temperature distribution in cemented carbide tool used to cut 0.4% C steel at a speed of

183 m min

-1

(600 ft/min); estimated from structural change in iron bonded carbide tool

TOOL LIFE AND PERFORMANCE OF TUNGSTEN CARBIDE-COBALT TOOLS 189

FIGURE 7.12 Section through crater surface in WC-Co tool and adhering steel, showing interface

characteristics of diffusion wear

The wear process in the cratering of WC-Co tools is one of diffusion wear. Metal and carbon

atoms of the tool material, diffuse into the work material that is seized to the rake face surface.

These atoms are then carried away in the body chip. An extreme example of this sort of process

was observed when studying sections through WC-Co bullet cores embedded in steel plates.

These showed a layer in which WC and Co were dissolved in the steel which had been melted at

the interface.

Figure 7.13 shows such a section with steel at the top, the cemented carbide at the bottom,

and the white, fused layer in between containing partially dissolved, rounded WC grains. The

melting point of the eutectic between WC and cobalt or WC and iron is about 1300°C. Evi-

dently, this temperature had been reached at the thin layer at the interface. In this case, the speed

on impact exceeded 1000 ms

-1

and a very large amount of energy had been converted into heat

in a very short interval of time.

In a paper published in 1952,

Trent suggested that the cratering of WC-Co tools when cut-

ting steel was also the result of fusion at the interface at a temperature of 1300°C. With cutting

tools, however, the speeds involved are more than 100 times lower than in the case of the bullet

core.

Evidence of temperature deduced from observations of worn high speed steel (Figure 5.12)

and iron-bonded carbide tools (Figure 7.11) suggests that the maximum sustained temperatures

at the interface, for conditions where cratering of WC-Co carbide tools occurs, are in the range

850° to 1200 °C. While these temperatures are too low for fusion, they are high enough to allow

considerable diffusion to take place in the solid state, and the characteristics of the observed sur-

faces are consistent with a wear process based on solid phase diffusion.

190 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

FIGURE 7.13 Section through interface of WC-Co bullet core embedded in steel plate

Worn carbide tools can be treated in HCl or other mineral acids to remove adhering steel or

iron so that the worn areas of the tool surface can be studied in detail. While the acids dissolve

the cobalt binder to some extent they do not attack the carbides, and wear on carbide tools can

thus be examined by a method not possible for high speed steel tools.

Figures 7.14 and 7.15 show the worn crater surfaces of WC-Co alloy tools after cutting steel.

The carbide grains are mostly smoothly worn through, but sometimes have an etched appear-

ance, as in the large grain in Figure 7.15 where steps etched in the surface are parallel to one of

the main crystallographic directions. Smooth ridges are often seen on the surface, originating

from large WC grains and running in the direction of chip flow, as in Figure 7.14. The smooth

polished surfaces are characteristic of areas where the work material flows rapidly immediately

adjacent to the surface, while the etched appearance is seen where the temperature is high

enough, but the flow is less rapid or there is a stagnant layer at the interface.

TOOL LIFE AND PERFORMANCE OF TUNGSTEN CARBIDE-COBALT TOOLS 191

FIGURE 7.14 Worn crater surface in WC-Co tool after cutting steel at high speed. Very smooth surface

with ridge characteristic of diffusion wear

FIGURE 7.15 Worn crater surface on WC-Co tool, showing large WC grain ‘etched’ by action of the hot

steel at the interface

192 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

Cratering wear showing these features occurs only at relatively high rates of metal removal,

where a flow-zone exists at the interface. This and other features of wear on cemented carbide

tools can be conveniently summarized by ‘machining charts’ such as Figure 7.16.

FIGURE 7.16 ‘Machining chart’ for WC + 6% Co tool cutting 0.4% steel with hardness 200 HV

The coordinates on the chart are the cutting speed and feed plotted on a logarithmic scale, and

the diagonal dashed lines are lines of equal rate of metal removal. On each chart are plotted the

occurrence of the main wear features for one combination of tool and work material using a stan-

dard tool geometry. Two lines show the conditions under which cratering wear was first

detected, and the conditions under which it became so severe as to cause tool failure within a

few minutes of cutting. Figure 7.16 is for a WC-6% Co tool cutting a medium carbon steel.

The occurrence of crater wear by diffusion is a function of both cutting speed and feed and an

approximately straight line relationship is found to exist over a wide range of cutting conditions.

For example, the limit to cutting speed caused by rapid cratering was 90 m min

-1

(300 ft/min) at

a feed of 0.25 mm rev

-1

(0.01 in/rev), while the critical speed is reduced to 35 m min

-1

(120ft/

min) at a feed of 0.75 mm rev

-1

(0.030 in/rev). These charts and the figures for maximum cutting

speeds, illustrate the importance of the cratering-diffusion wear in limiting the rates of metal

removal when using WC-Co alloys to cut steel. The maximum rates of metal removal are not