INTRODUCTION TO HARDMETALS
1.02 Fundamentals and General Applications of Hardmetals
1.02.5 Key Properties of Hardmetals
The various attributes of hardmetals can be summarized as follows:
Compressive strength:Compressive strength values of 4–8 kN mm2make hardmetals one of the strongest materials available, witness their use in diamond manufacture as anvils and in hot rolls for metallic materials.
Tensile strength:Usually measured in a bend test. Surface condition (grinding stresses) can have a significant effect on measured values. Less than compressive strength, typically 2.4 kN mm2, but still much stronger than most other materials. Processing route is very important. HIPping or SINTERHIP required for highest values of strength (Chapter 1.10).
Hardness: Defined as the resistance to indenter penetration. It correlates directly with the strength of the cutting tool material. It is measured using either a Vickers test (HV), or the Rockwell test (HRA). It can easily be varied between 1000 and 2000 HV30 by changing cobalt content and WC grain size. Like many other properties of hardmetal, there is theflexibility to design specific hardness values by changing composition (Chapter 1.09).
The ability to maintain high hardness at elevated temperatures is calledhot hardness(Figure 18).
Figure 17 Carbon differential to tailor hardmetal gradient (EPMA, 2006).
Fundamentals and General Applications of Hardmetals 43
Toughness: Defined as the ability of a material to absorb energy before fracture. The greater the fracture toughness of a tool material, the better it resists shock load, chipping and fracturing, vibration, misalignments, runouts and other imperfections in the machining system.
The fracture toughness of a hard tool material is measured using the Palmqvist Vickers indentation test. It is difficult to measure due to difficulties in introducing cracks with a known geometry. Typical values of plane strain fracture toughness lie in the range 7–25 m3/2MPa m0,5. These values are much higher than for similar hard materials like ceramics which have fracture toughnesses of 2–8 MN m3/2. Unfortunately this test is not truly characteristic since the testing conditions of the tool materials is not determined using loading conditions (stress state, strain rate and temperature) similar to that occurring in machining. The validity of such a test is discussed in Chapter 1.10.
Fatigue strength: Limited data available to judge properties but likely to be quite good in the absence of strength-limiting defects, as this property usually correlates with strength (Chapter 1.11).
High-temperature strength: Excellent, although decreases steadily with increasing temperature. Sensitive to choice of hard phase. Cubic carbide additions such as (Ta,Nb)C and (Ta,Ti)C increase elevated temperature strength significantly.
Low temperatures:Hardmetals retain a good combination of properties at temperatures down to as low as that of liquid nitrogen. Hardness increases by about 20–30% with a concomitant decrease in fracture toughness.
Impact strength:Not well understood, probably related to tensile strength and fracture toughness. No standard test method available.
Wear resistance:In general, wear resistance is defined as the attainment of acceptable tool life before tools need to be replaced. Although seemingly very simple, this characteristic is the least understood. Wear resistance is not a defined characteristic of the tool material and the methodology of its measurement. The nature of tool wear, unfortunately, is not yet sufficiently clear despite numerous theoretical and experimental studies. Cutting tool wear is a result of complicated physical, chemical, and thermomechanical phenomena. Because various simple mechanisms of wear (adhesion, abrasion, diffusion, oxidation, etc.) act simultaneously with a pre- dominant influence of one or more of them in different situations, identification of the dominant mechanism is far from simple, and most interpretations are subject to controversy (Figure 19).
Abrasion and erosion resistance: In severe abrasion and erosion environments, it is the most wear-resistant material available. This is because it has a unique combination of strength, hardness and fracture toughness compared with other hard materials (Figure 20) (Chapter 1.12).
Stiffness:It is one of the stiffest materials known, after diamond, with a Young’s modulus about three times that of steel and six times that of aluminum (Chapter 1.09).
Figure 18 Palmqvist fracture toughness relationship to hardness (Gee et al., 2007).
Density:Hardmetals are very dense if they contain significant amounts of WC; up to 15 Mg m3is possible.
Lower densities down to 4–5 Mg m3can be tailored by using higher Co contents or additions of cubic carbides and carbonitrides for the hard phase. The high density endows hardmetal with a very satisfying feel when handling products but if specific properties (i.e. properties per unit weight) are required, it is a disadvantage.
Surfacefinish:Hardmetal can take a superb mirrorfinish, a consequence of its defect-freefine-scale structure. It has a deep gray luster, which is particularly attractive.
Coefficient of friction:Hardmetals have a relatively low coefficient of friction, about 0.2, in unlubricated tests in contact with steel, as compared with typical steel against steel values of about 0.4. There is little systematic data of the effects of surface roughness, composition variations and temperature effects.
Coatings:Hardmetals easily take a variety of coatings, e.g. electroless, electroplate, chemical or physical vapor or spray deposited. Both wear resistance and cosmetic coatings of many types are cheap and effective.
Shape:Since hardmetals are manufactured by a powder metallurgy route, there are few restrictions on shape and mass production technologies are possible. However, postmanufacture shape changes can be expensive if extensive diamond grinding is required.
Dimensional stability:Their use as high-quality slip gauges indicates exceptional dimensional stability, pro- vided grinding stresses are stress-relieved. The microstructures are inherently stable since they are formed by an equilibrium process at very high temperatures and are cooled to room temperature relatively slowly.
Figure 19 Variation of wear volume with hardness for ASTM B611 tests on range of Hardmetals (Gee, Gant, & Roebuck, 2007).
Figure 20 Dependence of abrasive wear volume (ASTM B611 tests) on Palmqvist toughness. The different colored points are different sets of WC–Co (Gee et al., 2007).
Fundamentals and General Applications of Hardmetals 45
Edge retention and sharpness:Thefine scale of the microstructure and the high strength of the hardmetal enable very sharp edges and corners to be manufactured which have myriad uses in cutting and shaping processes.
Corrosion resistance:Corrosion resistance in nonacids is excellent for all hardmetals. However, if a cobalt binder phase is used, they can be prone to dissolution in even mildly acidic environments. This can be mitigated by using Ni-base or alloy binder-phases which are inherently corrosion resistant (Table 10).
Joining:Brazing is relatively straightforward as hardmetals are readily wetted if clean. However, care has to be taken to minimize thermal residual stresses because the thermal expansion coefficient is quite low, about half that of high-speed steel. Welding is not always possible but adhesives can be used in some circumstances if care is taken with surface preparation (Figure 21) (Chapter 1.19).
Thermal expansion and conductivity:The thermal conductivity of standard WC/Co hardmetals is about twice as high as that of high-speed steel. Both conductivity and expansion can be tailored by changing the volume fraction of binder phase and type of carbide/carbonitride hard phase.
Table 10 Corrosion resistance of cemented carbides with Co, Ni, and Ni–Cr binders (Kny et al., 1986)
Medium
Corrosion resistanceb
Temperature (C)a Concentration (mol/l) WC–6Co WC–12Co WC–6Nic WC–9Nic TiC–16Mo2C–18Ni
Acetic acid RT 1 2 2 1 1 1
100 1 3 3 2 2 2
Hydrochloric acid RT 6 2 3 2 2 2
100 6 4 4 4 4 4
Nitric acid RT 6 2 3 1 1 4
100 6 4 4 4 4 4
Phosphoric acid RT 0.67 3 3 2 2 2
100 0.67 4 4 3 3 2
Sodium chloride RT 0.5 2 2 1 1 1
Sodium hydroxide RT 1 1 1 1 1 1
Seawater RT 2 2 1 1 1
Sulfuric acid RT 1 3 3 2 2 2
100 1 4 4 3 4 3
Water, tap RT 1 2 1 1 1
Water, pure RT 1 1 1 1 1
100 1 1 1 1 1
aRT, room temperature.
b1¼negligible attack; 2¼light attack; 3¼medium attack; 4¼strong attack.
cNi–Cr alloy with 5% Cr.
Braze alloy
Cobalt coating
Figure 21 Hardmetal with cobalt surface coating and braze alloy (Konyashin, Hlawatschek, Ries, 2014).
Electrical and magnetic properties:Hardmetals usually have transition metal binder phases and consequently are slightly ferromagnetic, with coercive force values between 5 and 30 kA m1, depending on the binder- phase content. Nonmagnetic hardmetals can be made using Ni or alloy binder-phases as an alternative to the more common cobalt. Electrical conductivity is good with values of resistivity in the range of 150–250 nm. Higher values of up to 1000 nm can be produced if cubic carbides such as TiC substitute for the WC (Roebuck, 2012).
1.02.5.1 Hardmetal Standards
The ISO subcommittee on hardmetal standards, TC119/SC4, has in recent years developed a number of new standards as well as reviewing some of the more long-standing documents.
1.02.5.1.1 Metallographic Determination of MicrostructuredISO 4499: 2008
Thefirst two parts of this new and revised standard were published as ISO 4499-1 and -2 in 2008.
l Part 1: specifies the methods of metallographic determination of the microstructure.
l Part 2: gives the guidelines for the measurement of hardmetal grain size by metallographic techniques using optical or electron microscopy. It is intended for sintered WC/Co hardmetals containing primarily WC as the hard phase. It is also intended for measurement of the grain size by the linear intercept technique.
Further parts to the standard are under consideration:
ISO 4499-3 hardmetalsdmetallographic determination of microstructure
l Part 3: measurement of microstructural features in Ti(C,N) and WC/cubic carbide-based hardmetals.
ISO 4499-4 hardmetalsdmetallographic determination of microstructure l Part 4: characterization of porosity, carbon defects and eta-phase content.
ISO 4499-5 hardmetalsdmetallographic determination of microstructure
l Part 5: characterization and measurement of miscellaneous microstructural features.
1.02.5.1.2 Palmqvist Toughness Test for HardmetalsdISO 28079: 2009
This standard describes a method for measuring the Palmqvist toughness of hardmetals and cermets at room temperature by an indentation method. The standard recommends good practice to minimize levels of uncertainty in the measurement process. The procedure was validated through underpinning technical work within an interlaboratory exercise conducted to generate underpinning technical information.
1.02.5.1.3 Abrasion Tests for HardmetalsdISO 28080: 2011
This ISO standard provides new and improved methods for testing the abrasion characteristics of hardmetals using rotating wheel systems. There has been a number of abrasion test methods that have been developed that use this type of geometry including the ASTM G65 dry sand rubber wheel test, the ASTM G105 wet rubber wheel test and the ASTM B611 steel wheel test. Other variants of these tests have also been developed for specific applications in various institutes. Because of a fundamental commonality, much of the methodology is the same for the different tests. However, they do differ in the details of how the abrasive is fed to the interface between the wheel and the test sample if the test can be carried out in the presence offluids and if the abrasive is only used once and passes through the test system, or is reused multiple times. This new standard gives results that indicate how comparable the different tests are, and also gives information on their reproducibility and repeatability.
This new standard is in three parts. Part 1 updates the ASTM B611 test method for abrasion wear with information on testing uncertainties and a common reporting format with Parts 2 and 3. In Part 2, a section is included for the measurement of abrasion wear bases on the ASTM G65 test method, again with new infor- mation on testing uncertainties, while Part 3 arises from new studies in Germany that provide a combination of the principles of B611 and G65 with additional instrumentation.
1.02.5.1.4 Determination of Transverse Rupture StrengthdISO 3327: 2009
Historically the TRS standard allowed two sample geometries to be tested in 3pt bend: type A and type B. Type A is 55 mm in cross-section and 35 mm long, whereas type B is 6.5 mm wide by 5.25 mm high and 20 mm Fundamentals and General Applications of Hardmetals 47
long. Recent work by SC4 has revised the standard to allow a round geometry, 25 mm long and 3.30.5 mm diameter, type C to be used as well. This revision was supported by an international interlaboratory exercise to compare the properties of various geometries (Roebuck, 1997).
1.02.5.1.5 Vickers and Rockwell Hardness TestsdISO 3878 and ISO 3738-1
ISO/TC 119/SC4 has decided that an NWIP for Revision of ISO 3878 and ISO 3738-1 will be instigated in order to provide supporting data for annexes regarding the uncertainties of measurement data. An intercomparison is planned between Rockwell A and Vickers, HV30, for basic WC/Co hardmetals with hardness >1600 HV30.
(NBdgood data already exists for hardmetal grades with HV30<1600 and>1100.) 1.02.5.1.6 Knoop Hardness TestdISO 22394: 2011
A Knoop hardness test method for hardmetals has been introduced because many metallurgical problems require the determination of hardness over very small areas. The special shape of the Knoop indenter makes it possible to place indentations much closer together than with a square Vickers indentation, e.g. to measure a steep hardness gradient. For a given long diagonal length, the depth and area of the Knoop indentation are known to be only 15% of what they would be for a Vickers indentation with the same diagonal length.
1.02.5.1.7 Other Topics
In addition, several other topics are being considered by the committee, including magnetic saturation measurements; characterization of surface condition and semiquantitative identification of binder type. Also, progress is being made with standards for chemical analysis (i.e. total carbon contentdgravimetric method;
insoluble carbon contentdgravimetric method; atomic absorption spectrometric methods and determination of lead and cadmium).
1.02.5.2 Future NeedsdCharacterization of Hardmetals
The performance and reliability of hardmetal components will, in the future, certainly benefit from combining modelling and experimental approaches through a wide length scale range. Economic drivers and health concerns are continually putting demands on producers and users to optimize, or even consider the replace- ment, of current binder phase constituents. The majority of the world’s production of hard tool materials are multiphase with the predominant hard carbide or carbonitride phase toughened by a secondary binder-phase.
For the most part, this binder phase is based on a transition metal or alloy using Fe, Co and Ni as the major constituents. Detailed knowledge of the in-service degradation mechanisms will clearly lead to improved performance. Effective implementation of relevant microstructural effects on critical design parameters requires further research and applied work,both experimental and modelling, ondamage and damage evolution, particularly as related to behavior of intrinsic (short)flaws and surface behavior. The criteria for material selection, in terms of binder content and/or carbide grain size, for many applications with respect to wear, fracture and fatigue mechanisms need further clarification. Cemented carbides are heterogeneous in nature, and modeling of their mechanical behavior (especially in the presence of damage) will require amultiscale approach. At the microscale, the initial processes relate to the size dependence and discrete behavior of plastic deformation (often under constraint), strain hardening and fracture within individual phases. At the macroscale, continuum mechanical models are based on homogenized material parameters. At all levels, molecular dynamics, crystal plasticity models andfinite element approaches may be effective tools for quantifying the relevance of representative size effects. Capturing microscopic phenomena and bringing them to the macroscale will be essential for the correct simulation of wear, fracture and fatigue degradation phenomena in hardmetals. It is particularly important to take advantage of advanced characterization techniques such as in situ scanning electron microscopy (SEM) testing, nanoindentation, electron backscatter diffraction, transmission electron microscopy (TEM), focused ion beam, and 3-D tomography to provide equivalent information to that yielded by electron microscopy (mainly by means of fractographic analysis) to current knowledge. Extensive and detailed use of these techniques will help to characterize the deformation behavior and operative mechanisms at the microstructural scale for cemented carbides (Figure 22).
A major part of the world’s production of hardmetals is manufactured with a coating or an engineered surface. Coatings and surface layers are generally thin (of the order 10mm) and usually multiphased with
microstructural features in the nano range (20–200 nm). Characterization of these zones, whether it be by phase analysis or through mechanical behavior, provides significant challenges for the science and engineering community. Coatings are multiphase and can contain carbides, carbonitrides, ceramic phases, transition metals and alloy binders. To assess performance and confirm processing quality and consistency, it is necessary to be able to measure the size, shape, composition and distribution of each constituent, as well as characterizing the scale dependence of their physical and mechanical properties in combination. Clearly no one technique is sufficient for this purpose and it is necessary to use a range of complementary methods. For microstructural purposes, the science community has used Transmission Electron Microscopy (TEM), Field Emission Gun Scanning Electron Microscopy (FEGSEM), optical, X-ray, Glow Discharge optical emission spectroscopy (GDS) and other advanced instruments such as Secondary Ion Mass Spectroscopy (SIMS) and Electron Energy Loss Spectroscopy (EELS) for examination of these small regions of material. Each instrument provides unique in- formation on a specific attribute; for example, TEM for internal defects/dislocations/boundaries, FEGSEM for high-resolution measurements of phase size, shape and distribution, X-ray for phase composition and optical for more macroscopic issues, like reproducibility over large areas and gross defect characterization.
Mechanical properties are determined by the material microstructure and WC grain size is significant in this respect. Mechanical properties can be usefully assessed by methods such as hardness and toughness for bulk properties, but is there sufficient knowledge to mechanically interrogate near-surface regions with the same confidence? For mechanical characterization there are, again, many options open to the researcher. For example, indentation methods for plastic and fracture behavior (often on a veryfine scale and depth sensing, such as microscratch and nanoindentation testing methods), surface acoustic waves for elastic property measurement, beam bending for joint substrate/coating characterization and, in principle, micro hot hardness for high- temperature studies (although this is not often reported due its challenging nature). Also, knowledge of residual stresses is vital in interpreting the performance of coatings.
Magnetic coercivity provides a characterization tool for indirectly checking the WC grain size and magnetic saturation measurements can be used to investigate differences in binder phase chemistry. But, how do these techniques cope with more complex (i.e. hybrid, gradient or composite), or finer or coarser structures with perhaps alloyed binder phases? Structure–property relations allow the effects of differences in structure and properties to be compared and provide a baseline for the evaluation of new materials and coatings. Measurements on materials with a low Co content have indicated that current understanding is probably suitable only for specified composition ranges. New models are needed for materials outside these limits, and these should be underpinned by further work investigating the nature of the cobalt distribution (Roebuck & Prakash, 2013).
Figure 22 EBSD image of WC grain structure with superimposed lines for intercept measurements. The different colors of each grain correspond to differences in crystal orientation and thus provide a mechanism for automation of the grain recognition process (Roebuck, 2012).
Fundamentals and General Applications of Hardmetals 49