Metal Machining Episode 14 docx

Figure A6.1a shows how the room temperature Vickers Hardness HV of M2, T15 and M42, and the room temperature tensile rupture stress TRS of M2 and M42, typically vary with tempering tempe

A5.1 Tool yielding

The required tool hardnesses to avoid the yielding shown in Figure 3.19 have been obtained by a method due to Hill (1954).The requirement that the tool does not yield at its apex, together with force equilibrium in the tool, limits the difference between the rake face contact stress and the zero stress on the clearance face and hence places a maximum value on the allowable rake face contact stress.

With the cylindrical polar coordinate system shown in Figure A5.1(a), in which the

origin is at the tool apex and the angular variable q varies from 0 on the rake face to b on the clearance face, and in which the stresses sr, sqand t are positive as shown, the radial

and circumferential equilibrium equations are

sides of equation A5.2b by kwork

The largest value of sn/kworkis obtained when the integral takes its largest negative value.

Figure (A5.2) shows the variation of f with q that gives that largest negative value: at q =

b, f = 0; and at q = 0, f is determined by the friction contact stress on the rake face In Chapter 3 (Figure 3.18) extreme examples of friction stress were considered, up to kwork

during steady chip creation, but zero at the start of a cut:

5kworkor 2.5kwork, a minimum ratio of tool to work shear yield stress to avoid yield can be

derived Taking the tool’s Vickers Hardness HV to equal 5kt, relations between tool

hard-ness, kworkand b to avoid tool yielding can be derived Thus, the HV/b relations dependent

on kworkshown in Figure 3.19 are obtained.

A5.2 Tool fracture

Figure A5.1(b) shows a wedge-shaped tool with a line force R per unit length acting at a friction angle l at a distance d from the apex of the wedge This force is equivalent to a force R acting at the apex, with a moment M = Rd A classical result of stressing a wedge (Coker and Filon, 1931) is that on the rake face the tensile stress at a distance r from the

substituting these in equation (A5.8) and differentiating with respect to r to obtain the

posi-tion and hence the value of the maximum tensile stress It is supposed that a tool will ture when the maximum tensile stress is the TRS The results presented in Figure 3.19 are

frac-for the case of a tool entering a cut, assuming that tf= 0 and snis constant and equal to

5kworkover the contact length l between the work and tool It is found for this example that the maximum tensile stress occurs at r ≈ l To replace the distributed stress by the equiva-

lent line force and moment is only marginally justifiable: the treatment is only mate.

approxi-Tool fracture 385

Fig A5.2 Variations of φ with θ that maximize σn/kwork

Appendix 6

Tool material properties

More detail is given here than in Chapter 3 of the materials that make up the main tool groupings.

A6.1 High speed steels

The high speed steels are alloy steels with about 0.75% to 1.5% carbon (C), 4% to 4.5% chromium (Cr), between 10% and 20% tungsten (W) and molybdenum (Mo); they can also have vanadium (V), up to 5%, and cobalt (Co), up to 12% They are strengthened by heat- ing to high temperature (around 1150 to 1250˚C), just below the solidus; then quenching

in two stages (to avoid thermal cracking) – to the range 500˚C to 600˚C and then to room temperature; and then tempering typically between 500˚C and 560˚C Tempering causes hardening by the precipitation of fine carbides More details may be found in metallurgi- cal texts such as those by Trent (1991) and Hoyle (1988).

There are two series of materials, the T series which is based on W (with no Mo), and the M series which substitutes Mo for some of the W There are no major technical advan- tages of one series over the other The choice is one of cost, varying with the availability

of these two elements The basic grades in each series contain 0.75% to 0.85% C and 4%

to 4.5% Cr, with a small amount of V (<2%) but no Co The addition of extra V, with extra

C as well, results in the formation of hard vanadium carbides on tempering These increase the alloy’s room temperature hardness and abrasion resistance but at the expense slightly

of its toughness The addition of Co improves hot hardness, also at the expense of ness Table A6.1 gives the nominal compositions of a range of grades.

tough-Table A6.1 Sample compositions of some high speed steels

Grade Composition (wt %, balance Fe)

Figure A6.1(a) shows how the room temperature Vickers Hardness (HV) of M2, T15 and M42, and the room temperature tensile rupture stress (TRS) of M2 and M42, typically vary with tempering temperature after quenching from the recommended austenitizing temperatures for these alloys Figure A6.1(b) shows, for M2, how HV and TRS vary with austenitizing temperature after tempering at 560˚C The data have been derived mainly from Hoyle (1988), converting from Rockwell to Vickers Hardness, with additional data from other sources The data are presented to show the sensitivity of mechanical proper- ties to composition and heat treatment.

Traditionally, high speed steels have been shaped by hot working Now, powder lurgy technology is used to make high speed steel indexable inserts HV and TRS values

metal-are not much changed but there is evidence that fracture toughness (KICvalues) can be higher for powder metallurgy than wrought products Sheldon and Wronski (1987) give

KICat room temperature for sintered T6 as 30 MP m1/2whereas wrought T6 heat treated

in the same way has KIC= 15 to 20 MP m1/2 This paper also gives the temperature dence of TRS quoted in Chapter 3 (Figure 3.22).

depen-A6.2 Cemented carbides and cermets

Cemented carbide and cermet cutting tools consist of hard carbide (or carbo-nitride) grains, bonded or cemented together by up to around 20% by weight of cobalt or nickel,

388 Appendix 6

Fig A6.1 Variations of room temperature HV and TRS with (a) tempering and (b) austenitizing temperature, for a

range of high speed steels as indicated

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with minor additions of other metals (such as molybdenum or chromium) possible The hardness of the tools reduces and the toughness increases as the proportion of the metal binder phase is increased.

Cemented carbides and cermets are manufactured by sintering The reactions that take place during sintering are extremely complex and the creation of good cutting tool grades requires a close attention to detail A comprehensive monograph has been published (Schwarzkopf and Keiffer, 1960) and since then research reviews have appeared at regular intervals (Exner, 1979; Gurland, 1988) However, from a user’s point of view, the elements

of cemented carbide tool development are quite clear.

The earliest cemented carbides, developed in the 1920s, consisted of tungsten carbide (WC) cemented together by cobalt (Co) It soon became clear that this material was not suitable for machining steels at high cutting speeds The WC dissolved in the steel at the temperatures generated by cutting, leading to rapid cratering of the rake face of the cutting tool It was found that the system titanium carbide (TiC)-Co was more chemically resistant to steel, although cemented carbides based on TiC alone were more brittle than WC-Co Toughness could be recovered by adding tantalum carbide (TaC) During the 1930s, cemented carbides based on WC-TiC-TaC-Co started to be developed Tools based

on WC-Co, suitable for cutting non-ferrous metals (and also cast iron, which does not get hot enough in machining to trigger rapid dissolution of WC, so tool life remains deter- mined by flank wear) are now known as K-type carbides and those based on WC-TiC- TaC-Co, for steel cutting, as P-type (In practice, the tantalum carbide often includes niobium; one should then refer to Ta(Nb)C.) During the 1950s, an alternative system for steel cutting began to be studied, based on TiC cemented mainly by nickel (Ni) These have developed to titanium carbo-nitrides (Ti(C,N)) bonded by Ni (with minor amounts

of WC and Co), and are known as cermets Much more detailed data are available on the composition and properties of the K- and P-type carbides (and M-type as well – see later) than on the cermets The remainder of this section will concentrate mainly on the carbide grades.

The description K-, P- and M-type carbides, although it closely relates to carbide composition, in fact refers not to composition but to performance An international Standard (ISO 513, 1991) classifies cemented carbide cutting tools by type and grade Type refers to suitability for steel cutting (P) or non-ferrous materials (K) or to a compro- mise between the two (M) Grade refers to whether the tool material’s mechanical proper- ties have been optimized for hardness and hence abrasive wear resistance, or for toughness Wear resistance is more important than toughness for low feed, finishing cuts Toughness is more important for high feed, roughing or interrupted cuts Grades run from

01 to 50, as properties change from hard to tough.

Different manufacturers achieve a particular tool performance by minor differences of the processing route, so that there is not a one-to-one relation between a tool’s type and grade on the one hand and its composition on the other This is illustrated in Figure A6.2 Each row of the figure presents data on composition, hardness and transverse rupture stress (at room temperature) for one manufacturer’s range of tool materials, according to infor- mation published by Brookes (1992) The first row is data from a German manufacturer, the second is from a major international company and the third is from a Japanese producer Each data point in the left hand column represents the TiC-TaC and Co weight

% of one tool material (the balance is WC) What type and grade is assigned to the ial is indicated by the solid and dashed lines The ranges of compositions giving P-,

mater-Cemented carbides and cermets 389

M- and K-types are slightly different for each producer So are the ranges of compositions giving the different grades.

The right-hand column shows the relation between transverse rupture stress and ness for all the grades It can be seen that the relation depends on the carbide grain size.

hard-All three manufacturers produce tool materials of 1 to 2 mm grain size These have the

same relation between transverse rupture stress and hardness, independent of K-, M- and P-type However, one set of data, in the first row, is for material of sub-micrometre

390 Appendix 6

Fig A6.2 Composition and mechanical property differences of cemented carbide cutting tools classified according to

ISO 513 (1991) by three different manufacturers

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grain size: it shows a greater transverse rupture stress for a given hardness than the coarser grained material Such a fine grain size is only achievable with WC-Co (K-type) materials.

The mechanical and physical properties of commercial cemented carbide cutting tools broadly depend on the wt % of Co, the wt % of TiC-TaC and the grain size of the mater- ial Rather than describe the material by type and grade, the remainder of this section will describe it by these quantities For convenience, the classification by amount of TiC-TaC will be by whether the amount of this by weight is in the range 0–3%, 8–15% or 19–35% The data presented in Brookes (1992) show that very few cutting tool materials have amounts of TiC-TaC outside these ranges.

Figure A6.3 shows that the room temperature hardness of a cemented carbide depends mainly on cobalt content and grain size Figure A6.4 shows that quantities such as thermal

conductivity, K, heat capacity, rC, thermal expansion coefficient, ae, Young’s modulus, E, and thermal shock resistance, (TRS.K)/(Eae), are most influenced by the type of carbide present Figures A6.2 to A6.4 are the main source of information for the cemented carbide data presented in Chapter 3.

Such detailed information on the properties of cermets is not available in the open ature Table A6.2 presents data for one manufacturer’s products TiC and TiN are the major hard phase, with WC as a minor part Ni is the major binder metal, with Co as a minor part Less complete or differently presented data from other manufacturers, extracted from Brookes (1992) are gathered in Table A6.3.

liter-The densities of the cermets are almost half those of the cemented carbides (the ties of which, because of the high specific weight of tungsten, are around 14 000 to 15 000 kg/m3for the WC-Co types and 10 000 to 13 000 kg/m3for the high TiC-TaC-Co types) The cermets are mainly described as P-types, although some manufacturers also recom- mend them as K-types, but because of their limited toughness (TRS < 2.5 GPa, compared with up to 4 GPa for fine grained WC-Co materials), none of them are recommended for heavy duty use, above 30-grade.

densi-Cemented carbides and cermets 391

Fig A6.3 Hardness dependence on % Co and grain size, for cemented carbides

392 Appendix 6

Fig A6.4 Composition dependence of some properties of cemented carbides

Table A6.2 One manufacturer’s range of cermet tool materials

Wt %

A6.3 Ceramics and superhard materials

Even less systematically detailed information than for cermet tools is available for the composition and properties of ceramic and superhard materials.

Data for tools based on alumina, extracted from Brookes (1992), are gathered in Table A6.4 There are three sub-groups of material The first, called white alumina because of its colour, is pure alumina together with minor additions (headed ‘other’ in the table) to promote sintering These sintering aids can be either magnesium oxide (MgO) or zirconia (ZrO2): for tool grade aluminas, ZrO2is predominantly used The second group is the black aluminas: alumina to which is added TiC The third group is SiC whisker reinforced alumina The data demonstrate that the black aluminas are harder but no tougher than the white aluminas Silicon carbide whisker reinforcement increases toughness without improving hardness, relative to the black aluminas All the materials are developed, according to their ISO classification, for finishing duties.

The data in Table A6.4 were all collected before 1992 Recently, a new handbook has appeared which uprates the maximum toughness of whisker reinforced aluminas to 1.2

GPa (Japanese Carbide Manufacturers Handbook, 1998) Manufacturers’ data in the

authors’ possession also show maximum hardness of the black aluminas has been enhanced up to 22 GPa; and other information suggests room temperature thermal conduc- tivity can be higher than given, up to 35 W/m K These extended ranges of data have been included in the construction of Figures 3.20 and 3.21.

Data for silicon nitride based tools, also from Brookes (1992), are collected in Table A6.5 The fact that there is less information for these than for alumina tools reflects the more recent development of these materials for cutting There are two groups: straight sili- con nitrides and sialons Silicon nitride, without modifications, requires hot pressing for its manufacture It is also susceptible to contamination by silica (SiO2) This may segregate

at grain boundaries to form silicates which soften at around 1000˚C This is fatal to the performance of cutting tools One way to prevent these glassy grain boundary phases is by the addition of yttria (Y O ) Thus, almost all silicon nitride based cutting tools have some

Ceramics and superhard materials 393

Table A6.3 Cermet tool materials’ data from a range of other manufacturers

Wt %

*: data not provided.

addition of Y2O3 If Y2O3 is added in greater quantities, and also alumina and/or aluminium nitride, an alloy of Si, Al, O and N (sialon) is formed, also containing yttrium The benefit is that this material can be manufactured by pressureless sintering and main- tains its mechanical properties in use up to about 1300˚C The table shows that the bene- fits of one group over the other are entirely in the ease of manufacture There is little to choose between their room temperature mechanical properties (although the sialon mater- ials are likely to have a more reliable high temperature strength) As with the alumina materials, there has been some materials development over the last 10 years More recent

transverse rupture stress data are more commonly in the range 0.95 to 1.2 GPa (Japanese Carbide Manufacturers’ Handbook, 1998).

Finally, Table A6.6 summarizes the small amount of available information on PcBN and PCD tools These tools are manufactured in a two-stage process First, synthetic diamond

or cubic boron nitride grits are created at high temperature and pressure These are then cemented together by binders Each class of tool has two types of binder, ceramic-based

*: material present, but composition not given.

Table A6.5 Compositions and properties (pre-1992) of Si3N4based tool materials

*: material present, but composition not given; 1 : HRA.

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for ultimate hardness or metal-based for toughness For PcBN, the ceramic base is Al2O3and the metal base is sintered carbide or cermet For PCD, the ceramic is based on SiC and the metal on Co.

References

Brookes, K J A (1992)World Directory and Handbook of Hardmetals and Hard Materials, 5th edn.

East Barnet, UK: International Carbide Data

Exner, H E (1979) Physical and chemical nature of cemented carbides Int Metals Revs, 24,

149–173

Gurland, J (1988) New scientific approaches to development of tool materials Int Mats Revs, 33,

151–166

Handbook (1998) Japanese Cemented Carbide Manufacturers’ Handbook Tokyo: Japanese

Cemented Carbide Tool Manufacturers’ Association

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

ISO 513 (1991) Classification of Carbides According to Use Geneva: International Standards

Organisation

Schwarzkopf, P and Keiffer, R (1960) Cemented Carbides New York: MacMillan.

Shelton, P W and Wronski, A S (1987) Strength, toughness and stiffness of wrought and directly

sintered T6 high speed steel at 20–600˚C Mats Sci Technol 3, 260–267.

Trent, E M (1991) Metal Cutting, 3rd edn London: Butterworths.