Tungsten carbide grades: cobalt, grain size and ISO codes

A grade is one recipe of cemented carbide: hard tungsten carbide (WC) grains held in a metal binder, usually cobalt, and sintered: heated so the pores between the grains close and the part shrinks while keeping its shape. ITIA's typical example is 90% WC with 10% cobalt, a so-called straight grade: WC and cobalt with only minor additions.

A 2010 US government (NTP) background document says commercial grades generally run from 3% to 16% cobalt and 50% to 97% WC, some grades with other carbides such as titanium carbide or tantalum carbide. This page covers four things that separate one grade from another:

• Cobalt content, in weight percent (wt%).
• Grain size, the mean WC grain size in micrometres (µm).
• Added carbides such as TiC and TaC.
• Grain growth inhibitors, small additions, under 1 wt%, that hold back WC grain growth.

ITIA names grain size and cobalt content as the two main settings.

Sources: ITIA, Cemented carbides (newsletter, June 2010); ITIA, Applications and markets; ITIA, Properties and intermediates; NTP Report on Carcinogens background document, hard metals (2010); LLNL manuscript on carbide binders (OSTI, 2018); Grain growth inhibitors in WC-6Co, Materials (2021).

More cobalt makes a grade tougher and less hard, and less cobalt does the reverse: in ITIA's words, the hardness comes from the tungsten carbide and the toughness from the cobalt.

ITIA's 6% cobalt example reads 1450 HV10 with a fracture toughness of 12 MPa·m½; its 20% cobalt example reads 800 HV10 with 25 MPa·m½. HV is Vickers hardness (resistance to a diamond indenter); toughness is how well a grade absorbs impact energy.

NTP notes that rough-machining grades carry more binder for toughness and finishing grades less. ITIA (2010) gives these ranges by field:

• Metal cutting: 3 to 12 wt% cobalt, WC grains of 0.5 to 5 µm.
• Mining and construction: commonly 6 to 10 wt% cobalt, coarse grains up to 20 µm.
• Extra-coarse grades for surface mining or hot rolling: typically 6 to 30 wt% cobalt, or a cobalt-nickel-chromium binder.

Sources: ITIA, Cemented carbides (newsletter, June 2010); ITIA, Applications and markets; NTP Report on Carcinogens background document, hard metals (2010); Eclética Química review of cemented carbides (2024).

Grain size is the mean size of the WC grains in the finished part, and finer grains give a harder, less tough grade. ISO 4499-2:2008 recommends these names, measured by the mean-linear-intercept method:

The standard admits the words are loose: it says "none of these terms have commonly agreed or well defined size ranges among users and producers". Its measuring uncertainty is about 10% when 200 to 300 grains are counted, so it says results near a class boundary need care and gives them two names: 0.75 µm is submicron/fine.

ITIA (2010) says industry calls grades above 5 µm extra-coarse, where ISO draws the line at 6.0 µm.

Sources: ISO 4499-2:2008 (preview text); ITIA, Cemented carbides (newsletter, June 2010).

This section covers two reasons other carbides go into a grade: to change what it can cut, and to keep the WC grains small.

For cutting steel. ITIA says additions such as TiC, Ti(C,N) and TaC widened where carbide can work, steel machining among them (TiC, TaC). Its so-called steel cutting grades combine WC with (W,Ti,Ta,Nb)(C,N) and cobalt for steel, above all long-chipping alloys, while straight grades machine cast irons, hardened steel and stainless steels.

For holding grain size. A 2021 paper calls additions under 1 wt% of inhibitors such as vanadium carbide (VC), chromium carbide (Cr₃C₂), TaC, NbC, TiC or Mo₂C the most successful method so far of controlling WC grain growth, with VC and Cr₃C₂ the most effective.

Sources: ITIA, Cemented carbides (newsletter, June 2010); Grain growth inhibitors in WC-6Co, Materials (2021).

Every grade trades toughness for wear resistance, or the reverse. ITIA's caption for its two 12% cobalt grades reads: "the finer the carbide the higher is the hardness, but the lower the toughness, and contrariwise". ASTM says the same of hardness within a compositional group: lower-hardness grades usually have less wear resistance but greater toughness.

The 12% cobalt pair changes only grain size, and that alone doubles toughness, from 10 to 20. The lab rows are one study of WC-6% Co sintered in argon at 5 MPa and 1450 °C for 1 hour; more inhibitor there meant more hardness and less toughness. Test loads are covered on the carbide hardness page.

Sources: ITIA, Cemented carbides (newsletter, June 2010); ASTM B294-17 scope; Grain growth inhibitors in WC-6Co, Materials (2021).

ISO 513:2012 sorts hard cutting materials, carbide among them, by the work they machine rather than by recipe, and marks each group with a letter and a colour.

The number after the letter places a grade inside its group. P runs from P01 to P50, M and K to 40, and N, S and H to 30. Lower numbers mean higher cutting speed and more wear resistance; higher numbers mean heavier feed and more toughness.

NTP's 2010 table of representative machining grades, citing Brookes (1998), shows the same pattern in hardness: P grades run from 1900 HV at P01 down to 1250 HV at P50, and K grades from 1850 HV at K01 to 1320 HV at K40.

NTP's documents describe M differently, as multiple purposes, steel and nickel-based superalloys included, and put non-ferrous metals under K. ISO 513:2012, the fourth edition, puts stainless steel under M and has a separate N group for non-ferrous metals.

Sources: ISO 513:2012 (preview text); NTP Report on Carcinogens background document, hard metals (2010); NTP Report on Carcinogens profile, cobalt-tungsten carbide.

The ANSI C system numbers carbide grades from C1 to C8. A Tooling U-SME course description names it beside the ISO letters as one of two ways carbide grades are classified, and university machining slides (2010) list it as the other system.

This handbook prints no C-to-ISO chart, because its sources do not define each C number.

Sources: Tooling U-SME course description (Purdue WHIN); Cutting tool materials, university course slides (2010).

An ISO 513:2012 designation is a material code plus an application group, as in the standard's examples HW-P10 and HC-K20.

• HW: uncoated carbide, mainly WC, with a grain size of 1 µm or more.
• HF: the same, with a grain size under 1 µm, which covers the ISO 4499-2 nano, ultrafine and submicron classes and the lower part of fine.
• HT: uncoated carbide based on TiC, TiN or both, also called cermet.
• HC: any of these carbides with a coating.

So HC-K20 is coated carbide for cast iron, in the middle of the K range. HW-P10 is uncoated WC-based carbide of 1 µm grain or coarser, near the wear-resistant end of the steel group.

The code is not the recipe. ISO 513 classifies by application only, and NTP notes the coding is at each maker's discretion, so the code does not fix the cobalt content, and HW or HF tells you grain size only as above or below 1 µm. Coatings have their own page: tungsten carbide coatings.

Mixed grades are fine to send: we buy carbide in any form, coated or not. If the boxes still show grade codes, add them in the Anything else? box of the quote form, with the forms, a rough weight and photos. Selling tungsten carbide covers shipping, pickup and payment.

Sources: ISO 513:2012 (preview text); NTP Report on Carcinogens background document, hard metals (2010); ISO 4499-2:2008 (preview text).