Tool materials: HSS, HSS-Co, carbide, cermet, ceramic, CBN and PCD

The first factor that determines the performance of a cutting tool is not the geometry but the cutting material itself. The same geometry can run in the low tens of m/min with HSS, at several times that with coated carbide, and higher still with PCD. This article covers the hardness, toughness, hot hardness and thermal conductivity characteristics of the seven main cutting material families from HSS to PCD, the ISO 513 classification, and the application limit of each family. Since coating layers are a separate subject, only the substrate is examined here.

1. The four properties that define a cutting material

The selection of a cutting material rests on the balance of four properties, and these properties conflict with one another.

Hardness is the resistance of the cutting edge to being scratched and abraded by the workpiece. It is measured in Vickers (HV) or, for sintered hard materials, in Rockwell A (HRA). As hardness rises, resistance to abrasive wear increases.

Toughness is the ability of the edge to deform under impact and variable load without fracturing. There is an inverse relationship between hardness and toughness: the hardest materials are at the same time the most brittle. For this reason a tougher and therefore softer grade has to be chosen for interrupted cutting, vibration-prone clamping and irregular chip cross-sections.

Hot hardness is the retention of hardness at the temperature of the cutting zone. This is the property that actually sets the upper limit of the cutting speed. HSS tempers and softens above roughly 600 °C, whereas cemented carbide sustains its cutting ability in the 800–1,000 °C band; ceramics and CBN remain stable at even higher temperatures.

Thermal conductivity determines how fast the heat generated in the cutting zone is dissipated into the tool body. High thermal conductivity lowers the edge temperature but also means that part of the heat is carried into the tool. In ceramics with low thermal conductivity the heat is removed largely with the chip; in return, the material becomes sensitive to thermal shock. How the wear mechanisms relate to these properties is covered in detail under tool wear mechanisms and tool life.

2. Classification according to ISO 513

Hard cutting materials are identified by two-letter codes in the ISO 513 standard. The first letter indicates the material family, the second the sub-type. The same standard also defines the application groups (P, M, K, N, S, H); how these groups match workpiece materials is explained in the article ISO material groups P, M, K, N, S, H.

Critical point: the ISO 513 code is not a grade name but a definition of a material family. Two carbide grades carrying the same HC code can be entirely different in grain size, cobalt content and coating.

3. HSS and HSS-Co: the effect of cobalt content

High speed steel (HSS) is an alloyed tool steel containing tungsten, molybdenum, chromium and vanadium, hardened by heat treatment. Its hardness is typically in the 62–67 HRC band. Its greatest advantage is toughness: since the cutting edge can deform plastically to a certain extent before fracturing, it behaves reliably on weak connections, low-speed machines and vibration-prone setups.

HSS-Co grades are obtained by adding cobalt to the structure. Dissolving in the matrix, cobalt increases the resistance of martensite to tempering at high temperature; in practical terms this corresponds to an increase in red hardness. In return, the matrix becomes more brittle.

Thanks to its increased cobalt content, M42 retains its hardness up to a higher cutting zone temperature than M35. In stainless steels and heat-resistant alloys, where heat generation is intense, this difference produces a visible tool life advantage in favour of M42. On the other hand, the M42 grade is more prone to edge chipping in interrupted cutting and hand-fed operations; M35 is preferred under these conditions.

A frequent misconception: The difference between HSS and HSS-Co is not a speed difference of an order of magnitude. Both grades work in the same order of magnitude of cutting speed band; cobalt essentially increases tool life and heat resistance.

4. Cemented carbide

Cemented carbide is produced by sintering tungsten carbide grains with a cobalt binder using powder metallurgy. Its hardness is typically in the 1,300–1,800 HV, 89–93 HRA band, and its hot hardness is far above that of HSS. Its thermal conductivity is also high; the values reported in the literature for WC-Co compositions vary with cobalt content and grain size, but are markedly above those of tool steel.

The real advantage of carbide is its speed capacity. Manufacturers' technical publications state that cemented carbide permits cutting speeds several times higher than HSS; with coated grades and a suitable workpiece material this ratio widens further. The reason is directly hot hardness: at the temperature where HSS softens, carbide is still able to cut.

The behaviour of carbide depends on two variables. As the cobalt content rises, toughness increases and hardness falls. As the grain size decreases, hardness and edge sharpness increase. Grades with a higher cobalt content and greater toughness are preferred for roughing passes, while fine-grain, low-cobalt grades are preferred for finishing and hard materials. The contribution of the coating layer is covered under end mill coatings.

5. Cermet

Cermet is a material whose hard phase is predominantly titanium carbonitride (TiCN), sintered with a nickel- and cobalt-based binder. It is designated by the code HT in ISO 513. Its hardness is slightly above that of tungsten carbide based grades, and since the chemical affinity of TiCN for steel is low, built-up edge formation is markedly reduced.

This property makes cermet the material of finishing operations: low surface roughness, stable dimensions and long tool life are achieved. Its toughness, compressive strength and thermal shock resistance are, however, below those of cemented carbide. For this reason cermet is not recommended for roughing passes, interrupted cutting, variable chip cross-sections and interrupted cooling conditions. Its application area is the finishing passes of steel, ferritic-pearlitic cast iron and low-carbon steels.

6. Ceramics: Al2O3, Si3N4 and SiAlON

Ceramic cutting materials offer chemical stability at high temperature; the literature states that they behave stably up to the order of 1,200 °C. Their common weaknesses are low toughness and low thermal conductivity. There are three main groups.

Al2O3-based oxide ceramics (CA) have the highest chemical stability but are markedly sensitive to thermal shock. They are therefore mostly used in dry machining and uninterrupted cutting. The typical application area is high-speed turning of grey cast iron. Mixed ceramics (CM) are improved in terms of toughness and thermal conductivity by adding TiC or TiCN to the structure; they can also be used on hardened steel. Reinforced ceramics (CR) are grades whose fracture toughness has been increased with SiC whisker reinforcement, and they are applied on nickel-based alloys.

Si3N4-based nitride ceramics (CN) show higher fracture toughness and thermal shock resistance than oxide ceramics. They are widespread in high-speed roughing of grey cast iron.

SiAlON is a type of nitride ceramic obtained by dissolving aluminium and oxygen into the Si3N4 lattice. Because it combines high-temperature stability with medium-level toughness, it is used for high-speed machining of nickel-based superalloys. Academic publications report that, in the turning of Inconel 718, the most favourable band for SiAlON grades in terms of tool life is approximately 200–300 m/min; this value is many times the speed used with carbide on the same material. The general approach to machining superalloys is covered in the article milling titanium and heat-resistant alloys.

Important: because of the brittleness of ceramic tools, machine rigidity, clamping quality and the entry-exit strategy are far more decisive than with carbide. In a setup that is not rigid, the gain expected from a ceramic grade cannot be obtained.

7. CBN

Cubic boron nitride (CBN) is the hardest material after diamond; its Knoop hardness at room temperature is of the order of 45 GPa. Its difference from diamond is that it is chemically stable towards ferrous materials. It is therefore the only practical superhard cutting material for hardened steel.

As a cutting insert it is generally used in polycrystalline form (PCBN), as a tip brazed onto a carbide base. There are two basic grade families:

• Grades with low CBN content: the ceramic binder proportion is high; they are used in uninterrupted finishing of hardened steel. Chemical wear resistance is to the fore.
• Grades with high CBN content: these have a metallic binder and higher toughness and thermal conductivity; they are preferred for grey cast iron and interrupted cutting conditions.

Manufacturers' technical publications define the application area of CBN inserts as hardened steel above 45 HRC; the upper limit is 65 HRC and, in some applications, higher still. Below this hardness, coated carbide remains economically superior. In die steels, grade selection is determined not only by the hardness band but also by whether the cut is interrupted and by machine rigidity.

8. PCD and the ferrous material limit

Polycrystalline diamond (PCD) is produced by sintering fine diamond grains with a cobalt binder under high pressure and temperature, and is the hardest known cutting material; the Knoop hardness of diamond is of the order of 88 GPa. Its thermal conductivity is also extremely high; published data state that it is of the order of several times that of tungsten carbide. This allows the heat in the cutting zone to be removed rapidly and an extremely sharp, wear-resistant edge to be maintained.

PCD has two basic limits.

The first is temperature. Diamond loses its stability above roughly 700–800 °C; it reacts with oxygen and graphitises. For this reason, when working with PCD the cutting zone temperature has to be kept under control, with copious cooling or a suitable MQL application.

The second is that it cannot be used on ferrous materials. The chemical affinity of carbon for iron is very high. At cutting temperature the tetrahedral carbon structure at the diamond surface converts to graphite; since the released carbon atoms are much smaller than iron atoms, they diffuse rapidly into the iron lattice of the workpiece. The result is a chemical wear that is independent of mechanical wear and progresses extremely fast. The same affinity applies to cobalt and nickel as well, so PCD is likewise not used on nickel-based superalloys.

The application area of PCD is therefore non-ferrous and non-metallic materials: high-silicon aluminium alloys, copper and brass, carbon fibre reinforced composites, graphite, plastics and wood-based panels. In high-silicon aluminium alloys the abrasive silicon phase wears the carbide edge rapidly, so the tool life advantage of PCD emerges most clearly in this group.

9. Comparison table

The following table summarises a qualitative comparison compiled from common manufacturer and literature data. The cutting speed bands are an indication of order of magnitude; the actual value varies with the workpiece material, the coating, machine rigidity and the cooling condition.

The selection logic can be reduced to two questions. The first is whether the workpiece material is chemically compatible with the cutting material; the PCD-iron pairing is the sharpest example of this incompatibility. The second is whether the rigidity of the setup can carry the brittleness of the chosen material. Once these two conditions are met, the cutting speed band is determined according to the hot hardness of the cutting material. The setup of the cutting speed and spindle speed calculation is explained in the article cutting speed, spindle speed and feed calculation.

In general purpose milling, coated fine-grain carbide continues to offer the widest field of use between speed capacity and toughness. The size and type options of the carbide end mills in the ATH Tools catalogue can be examined in the square end mill category.

Sources

• ISO 513:2012 — Classification and application of hard cutting materials for metal removal with defined cutting edges — Designation of the main groups and groups of application (iso.org)
• ISO 3685:1993 — Tool-life testing with single-point turning tools (iso.org)
• ISO 8688-2:1989 — Tool life testing in milling — Part 2: End milling (iso.org)
• Sandvik Coromant — Cutting tool materials (sandvik.coromant.com)
• Kennametal — PcBN turning inserts KBH10B & KBH20B, grade description and application range (kennametal.com)
• ASM International — ASM Handbook, Volume 16: Machining, "Cutting Tool Materials" chapter
• Machinery's Handbook, Industrial Press — "Cutting Tool Materials" and "Cemented Carbides" chapters
• Zong, W. J. et al. — A critical review on the chemical wear and wear suppression of diamond tools in diamond cutting of ferrous metals, International Journal of Extreme Manufacturing (iopscience.iop.org)