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

Подождите немного. Документ загружается.

CONCLUSION 173

6.10 CONCLUSION

High speed steel tools, first demonstrated in 1900, were the product of an intense technological

research program to determine the most efficient tool steel composition and heat treatment for

the rough machining of steel. They made possible the cutting of steel at about four times the rate

of metal removal achieved with carbon steel tools. This advance was made possible by retention

of compressive strength of the order of 1500 MPa to temperatures in the range 550-650° C.

These properties are the result of precipitation hardening within the martensitic structure of the

chromium, tungsten, molybdenum, and vanadium tool steels after a very high temperature heat

treatment. The critical strength is retained at the same temperatures generated at the cutting edge

of tools when used to cut a wide range of engineering steels over a range of cutting speeds and

feeds useful in industrial machining.

The most important development has been the coating of tools with thin layers (

∼ 10 μm) of

TiN by a PVD process. Such coatings prolong tool life in most situations and in others give a

smoother surface finish on the machined part. Figures 3.25 to 3.30 are worth revisiting at this

point. The evidence indicates that the TiN coatings have several beneficial effects. They keep the

all-important cutting edge smooth and blemish free; they reduce friction on the rake face and

allow a smoother chip flow process - consequently both forces and temperatures are lower; they

reduce any heat due to rubbing on the clearance face. All these effects reduce the amount of sub-

surface strain going into the machined surface.

Correctly heat-treated high speed steel has adequate toughness to resist fracture in machine

shop conditions, and it is the combination of high-temperature strength and toughness that has

made this class of tool material a successful survivor for 100 years in the evolution of cutting

tool materials. It is significant that in those 100 years no new class of tool steel has superseded it.

The loss of strength and permanent changes in the structure of high speed steel when heated

above about 650° C limit the rate of metal removal when cutting high melting-point metals and

alloys.

6.11 REFERENCES

1. ‘Tool Steels’, Iron & Steel, Special Issue, (1968)

2. Brookes, K.J.A., World Directory and Handbook of Hard Metals, Engineers Digest (1987)

3. Osborn, F.M., The Story of the Mushets, Nelson (1952)

4. Taylor, F. W., Trans. A.S. M. E.,

28, 31 (1907)

5.American National Standards Institute B212.4-86, B212.12-91

6. Mukherjee, T., I.S.I. Publication,

126, 80 (1970)

7.Kirk, F.A., Child, H.C., Lowe, E.M. and Williams, T.J., I.S.I. Publication,

126, 67(1970)

8. Weaver, C., Unpublished work

9.Trent, E.M., Proc. Int. Conf M.T.D.R., Manchester, 1967, 629 (1968)

10. Hellman, P. and Wisell, H., Bulletin du Cercle d’Etudes des Metaux, p. 483 (Nov. 1975)

11. Ekelund, S., Fagersta High Speed Steel Symposium, p. 3 (1981)

12. Wright, P.K. and Trent, E.M., Metals Technology,

1, 13 (1974)

174 CUTTING TOOL MATERIALS I: HIGH SPEED STEELS

13. Opitz, H. and Konig, W., I.S.I. Publication, 126, 6 (1970)

14. A.S.M. Handbook (1948), also Metal Progress, 15 July,

141, (1954)

15. Amini, E. and Winterton, R.H.S., Proc. Inst. Mech. Eng.,

195 (21), 241 (1981)

16. Horton, S.A. and Child, H.C., Metals Technol.,

10, 245 (1983)

17. El-Rakayby, A.M. and Mills, B., Mat. Sci. & Tech.,

2, 175 (1986)

18. Hoyle, G., High Speed Steels, Butterworths, London (1988)

19. Machining Data Handbook, Vol. 1 Machinability Data Center, Cincinnati (1990)

20. ASM Handbook, 9th Edition, Vol. 16 (1989)

21. Leatham, A., Ogilvy, A., Chesney, P. and Wood, J. V., Metals & Materials, 140 (March

1989)

22. Tarkan, S.E., and Mal, M.K., Metal Progress, 105, 99 (1974)

23. Sandvik Coromant Company, Modern Metal Cutting- A Practical Handbook, page III-41,

(1994)

24. Barrell, R. and Rickerby, D.S., Metals & Materials, (August 1989)

25. Chow, J.G., Ph.D. Dissertation, Carnegie Mellon University, Pittsburgh PA., (1985)

CHAPTER 7 CUTTING TOOL

MATERIALS II:

CEMENTED

CARBIDES

7.1 CEMENTED CARBIDES: AN INTRODUCTION

Many substances harder than quenched tool steel have been known from ancient times. Dia-

mond, corundum and quartzite, among many others, were natural materials used to grind metals.

These could be used in the form of loose abrasive or as grinding wheels, but were unsuitable as

metal-cutting tools because of inadequate toughness. The introduction of the electric furnace in

the last century, led to the production of new hard substances at the very high temperatures made

available. The American chemist Acheson produced silicon carbide in 1891 in an electric arc

between carbon electrodes. This can be used loose as an abrasive and, when bonded with porce-

lain, is very important as a grinding wheel material, but is not tough enough for cutting tools.

Many scientists, engineers and inventors in this period explored the use of the electric furnace

with the aim of producing synthetic diamonds. In this they were not successful, but Henri

Moissan at the Sorbonne made many new carbides, borides and silicides - all very hard materials

with high melting points. Among these was tungsten carbide which was found to be exception-

ally hard and had many metallic characteristics. It did not attract much attention at the time, but

there were some attempts to prepare it in a form suitable for use as a cutting tool or drawing die.

There are, in fact, two carbides of tungsten - WC which decomposes at 2,600°C, and W

which melts at 2,750°C. Both are very hard and there is a eutectic alloy at an intermediate com-

position and a lower melting point (2,525°C). This can be melted and cast with difficulty, and

ground to shape using diamond grinding wheels, but the castings are coarse in structure, with

many flaws. They fracture easily and proved to be unsatisfactory for cutting tools and dies.

In the early 1900s the work of Coolidge led to the manufacture of lamp filaments from tung-

sten, starting with tungsten powder with a grain size of a few micrometers (microns).

176 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

The use of the powder route of manufacture eventually solved the problem of how to make

use of the hardness and wear resistant qualities of tungsten carbide. In the early 1920s Schröter,

working in the laboratories of Osram in Germany, heated tungsten powder with carbon to pro-

duce the carbide WC in powder form, with a grain size of a few microns.

1,2

This was thoroughly

mixed with a small percentage of a metal of the iron group - iron, nickel or cobalt - also in the

form of a fine powder.

Today, the raw materials used in carbide cutting tools are made by melting and subsequent

particle-size reduction by ball-milling (comminution) to a fine powder. The comminution pro-

cess limits the final size of the powder to about a size of one micron. Newer, chemical routes,

such as the sol-gel technique can produce ultrafine materials in nanocrystalline size. These mate-

rials are also relatively freer from defects. Cost permitting, they will find a niche for high-speed

milling.

Cobalt was found to be the most efficient metal for bonding WC. The mixed powders were

pressed into compacts which were sintered by heating in hydrogen to above 1,300°C. Unlike

many powder-metal products, tungsten carbide/cobalt mixtures can be sintered to full density -

free from porosity - in a single heating cycle.

This is because a liquid phase solution of WC in cobalt is formed at about 1,300°C. This wets

and pulls together all the remaining WC particles. On cooling to room temperature, the liquid

phase solidifies and the product is fully dense. The liquid phase sintering process, introduced by

Schröter, became the technological basis for the cemented carbide industry and is used univer-

sally, not only for alloys of WC and cobalt, but for many other combinations of metal carbides

and nitrides with cobalt, nickel and iron.

The cemented carbides have a unique combination of properties which has led to their devel-

opment into the second major “genus” of cutting tool materials in use today.

7.2 STRUCTURES AND PROPERTIES

Tungsten carbide is one of a group of compounds including the carbides, nitrides, borides and

silicides of transition elements of Groups IV, V and VI of the Periodic Table.

1-4

Of these, the car-

bides are important as tool materials, and the dominant role has been played by the mono-car-

bide of tungsten, WC.

Table 7.1 gives the melting point and room temperature hardness of some of the carbides. All

the values are very high compared with those of steel.

The carbides of tungsten and molybdenum have hexagonal structures, while the others of

major importance are cubic. These rigid and strongly bonded compounds undergo no major

structural changes up to their melting points, and their properties are therefore stable and unal-

tered by heat treatment, unlike steels which can be softened by annealing and hardened by rapid

cooling.

These carbides are strongly metallic in character, having good electrical and thermal conduc-

tivities and a metallic appearance. Although they have only slight ability to deform plastically

without fracture at room temperature, the electron microscope has shown that they are deformed

by the same mechanism as are metals by movement of dislocations.

TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co) 177

TABLE 7.1 Properties of carbides (There are large differences in reported values for all of the

carbides. The values given here are representative.)

They are sometimes included in the category of ceramics, and the cemented carbides have

been referred to as ‘cermets’, implying a combination of ceramic and metal, but this term seems

quite inappropriate, since the carbides are much closer in character to metals than to ceramics.

Figure 7.1 shows the hardness of four of the more important carbides measured at tempera-

tures from 15°C to over 1,000°C.

5,6

All are much harder than steel, though not as hard as dia-

mond (6,000-8,000 HV in this temperature range).

The hardness of the carbides drops rapidly with increasing temperatures, but they remain

much harder than steel under almost all conditions. This hardness, and the stability of the prop-

erties under a wide range of thermal treatments, encourage the use of carbide cutting tools.

In cemented carbide alloys, the carbide particles constitute about 55-92% by volume of the

structure, and those alloys used for metal cutting normally contain at least 80% carbide by vol-

ume. Figure 7.2 shows the structure of one alloy, the angular gray particles being WC and the

white areas cobalt metal.

In high speed steels the hard carbide particles of microscopic size constitute only 10-15% by

volume of the heat-treated steel (Figure 6.1b), and play a minor role in the performance of these

alloys as cutting tools, but in the cemented carbides they are the decisive constituents. The pow-

der metal production process makes it possible to control accurately both the composition of the

alloy and the size of the carbide grains.

7.3 TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co)

This group, technologically the most important, is considered first. These are available com-

mercially with cobalt contents between 4% and 30% by weight - those with cobalt contents

between 4% and 12% being commonly used for metal cutting - and with carbide grains varying

in size between 0.5 μm and 10 μm across. The performance of carbide cutting tools is very

dependent upon the composition and grain size: Tables 7.1 to 7.3 include the range of the prop-

erties and usage of WC-Co grades.

Carbides Melting point Diamond indentation

hardness

(°C) (HV)

TiC 3,200 3,200

2,800 2,500

TbC 3,500 2,400

TaC 3,900 1,800

WC 2,750 2,100

178 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

FIGURE 7.1 Hot hardness tests on mono carbides of four transition elements (After Atkins and Tabor

;

Miyoshi and Hara

)

FIGURE 7.2 Structure of a cemented carbide (WC-Co) of coarse grain size

TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co) 179

The structure of tungsten carbide cobalt alloys should show two phases only - the carbide

WC and cobalt metal (Figure 7.2). The carbon content must be controlled within very narrow

limits. The presence of either free carbon (too high a carbon content) or ‘eta phase’ (a carbide

with the composition Co

C, the presence of which denotes too low a carbon content) results

in reduction of strength and performance as a cutting tool. The structures should be very sound

showing very few holes or non-metallic inclusions.

Table 7.2 gives properties of a range of WC-Co alloys in relation to their composition and

grain size, and Figure 7.3 shows graphically the influence of cobalt content on some of the

properties. Both hardness and compressive strength are highest with alloys of low cobalt content

and decrease continuously as the cobalt content is raised.

For any composition the hardness is

higher the finer the grain size and over the whole range of compositions used for cutting, the

cemented carbides are much harder than the hardest steel.

As with high speed steels, the tensile strength is seldom measured, and the breaking strength

in a bend test or ‘transverse rupture strength’ is often used as a measure of the ability to resist

fracture in service. Figure 7.3 shows that the transverse rupture strength varies inversely with

the hardness, and is highest for the high cobalt alloys with coarse grain size.

FIGURE 7.3 Mechanical properties of WC-Co alloys of medium-fine grain size

In tensile or bend tests, fracture occurs with no measurable plastic deformation, and

cemented carbides are, therefore, often characterized as ‘brittle’ materials in the same category

180 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

TABLE 7.2 Properties of WC-Co alloys

Mean WC

grain size

Hardness Transverse

rupture strength

Compressive

Strength

Young’s

modulus

Fracture

toughness

Specific

gravity

(μm) (HV 30) (MPa) (tonf/

)

(M Pa) (tonf/

)

(G Pa) (tonf/in

x 10

)

(MPa m

-1/2

)

3 0.7 2,020 1,000 65 - - - - - -

1.4 1,820- - ---

-8 -

6 0.7 1,800 1,750 113 4,550 295 - - - -

1.4 1,575 2,300 148 4,250 275 630 40.7 10 14.95

0.7 1,670 2,300 148 - - - - - -

9 1.4 1,420 2,400 156 4,000 260 588 38.0 13 14.75

4.0 1,210 2,770 179 4,000 260 - - - -

15 0.7 1,400 2,770 179 - - 538 34.8 - -

1.4 1,160 2,600 168 3,500 225 - - 18 14.00

TUNGSTEN CARBIDE-COBALT ALLOYS (WC-Co) 181

as glass and ceramics. This is not justified because, under conditions where the stress is largely

compressive, cemented carbides are capable of considerable plastic deformation before failure.

Figure 7.4 shows stress-strain curves for two cemented carbide alloys in compression, in

comparison with high speed steel. Both cemented carbides have much higher Young’s modulus

(E), and the 6% Co alloy has higher yield stress than the steel. Above the yield stress, the curve

departs gradually from linearity, and plastic deformation occurs accompanied by strain harden-

ing raising still further the yield stress. The amount of plastic deformation before fracture

increases with the cobalt content.

8,9

The ability to yield plastically before failure can also be

demonstrated by making indentations in a polished surface.

Indentations in a carbide surface, made with a carbide ball, are shown in Figure 7.5, the con-

tours of the impressions being demonstrated by optical interferometry. Indentations to a measur-

able depth can be made before a peripheral crack appears, and the higher the cobalt content the

deeper the indentation before cracking. It is this combination of properties - the high hardness

and strength, together with the ability to deform plastically before failure under compressive

stress - which makes cemented carbides based on WC so well adapted for use as tool materials

in the engineering industry (Table 7.3).

FIGURE 7.4 Compression tests on two WC-Co alloys compared with high speed steel

There is no generally accepted test for toughness of cemented carbides and the terms tough-

ness and brittleness have to be used in a qualitative way. This is unfortunate because it is on the

basis of toughness that the selection of the optimum tool material must often be made.

Fracture toughness tests on cemented carbide have now been reported from a number of lab-

oratories and there is reasonably good agreement on the values of K

. Table 7.2 shows that K

values for WC-Co alloys increase with cobalt content, varying from 10 MPa m

-1/2

for a 6% Co

182 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

TABLE 7.3 Classification of carbides according to use

Symbol

Broad categories of

material to be

machined Designation Material to be machined Use and Working Conditions

P 01

Steel, steel castings Finish turning and boring; high cutting speeds,

small chip section, accuracy of dimensions and

fine finish, vibration-free operation.

P 10

Steel, steel castings Turning, copying, threading and milling, high cut-

ting speeds, small or medium chip sections.

P 20 Steel, steel castings

Malleable cast iron with long

chips

Turning, copying, milling, medium cutting speeds

and chip sections, planing with small chip sec-

tions.

Ferrous metals with

long chips

P 30 Steel, steel castings

Malleable cast iron with long

chips

Turning, milling, planing, medium or low cutting

speeds, medium or large chip sections, and

machining in unfavorable conditions

P 40 Steel

Steel castings with sand inclu-

sion and cavities

Turning, planing, slotting, low cutting speeds,

large chip sections with the possibility of large

cutting angles for machining in unfavorable con-

ditions and work on automatic machines

P 50

Steel

Steel castings of medium or

low tensile strength, with sand

inclusion and cavities

For operations demanding very tough carbide:

turning, planing, slotting, low cutting speeds,

large chip sections, with the possibility of large

cutting angles for machining in unfavorable con-

ditions and work on automatic machines.

Ferrous metals with

long or short chips and

non-ferrous metals

M 10 Steel, steel castings, manga-

nese steel, gray cast iron, alloy

cast iron

Turning, medium or high cutting speeds. Small or

medium chip sections

M 20 Steel, steel castings, austenitic

or manganese steel, gray cast

iron

Turning, milling. Medium cutting speeds and chip

sections