Tungsten carbide melting point, oxidation and heat limits

Tungsten carbide (WC), the compound of tungsten and carbon that makes up most of a carbide tool, melts at about 2,785-2,800°C (5,045-5,072°F). The US National Toxicology Program (NTP) lists 2,785°C, with a boiling point of 6,000°C (10,832°F), from the HSDB 2010 table. A 2022 paper rounds it to 2,800°C, and a 2023 paper puts pure WC at more than 2,800°C.

In the tungsten-carbon phase diagram described in a UCLA paper, WC melts incongruently, meaning it comes apart as it melts instead of turning into a liquid of the same makeup. The other carbide of tungsten, W₂C, melts congruently.

A thermodynamic model in the same paper computed a clean (congruent) melt of WC only as a metastable case, at 3106 K, which is about 2,833°C (5,131°F). So 2,785°C is the table value to use; the 3106 K figure is a model result for a metastable case, not a measured melting point.

Sources: NTP Report on Carcinogens: cobalt-tungsten carbide profile (HSDB 2010 table); Materials (Basel) 2022;15(3):1187; Materials (Basel) 2023;16(4):1347; UCLA paper on the metastable W-C system.

Tungsten carbide melts about 640°C lower than pure tungsten, which ITIA says has the highest melting point of all metals, 3,422°C (6,192°F). The cobalt binder, the metal that holds the WC grains together in cemented carbide (the sintered WC-and-metal material tools are made of), melts about 1,290°C below the carbide.

Some grades, meaning carbide recipes set mainly by cobalt content and grain size, use nickel as the binder instead, which a 2023 paper says is well known to improve corrosion resistance.

Cast, or fused, carbide is the kind made from a melt on purpose. ITIA describes melting tungsten and WC together into a eutectic of WC and W₂C, cast and quenched fast into very hard particles for surface hardening. A 2010 welding journal paper says the ingots were traditionally melted at 3,100°C (5,612°F) and crushed.

Sources: ITIA Newsletter, December 2007; NTP Report on Carcinogens: cobalt-tungsten carbide profile; Materials (Basel) 2020;13(6):1377; NTP Report on Carcinogens: nickel profile (HSDB 2009 table); Materials (Basel) 2023;16(12):4480; ITIA: properties of tungsten intermediates; The Paton Welding Journal 2010, no. 4.

Cemented carbide parts are made by sintering, not by melting: pressed powder is heated until it bonds into a solid (the whole route is on how tungsten carbide is made). NTP's background document gives 1,350-1,550°C (2,462-2,822°F) for the WC and cobalt mix, and says the solid forms by diffusion of cobalt, which can happen even below cobalt's 1,495°C melting point.

A 1999 University of Cape Town thesis says sintering is usually done in vacuum, at about 1,350°C for high-cobalt grades up to about 1,550°C for low-cobalt grades.

Cobalt is what makes the low temperature possible. With WC it forms a eutectic, a mixture that melts lower than either part alone, and a 2023 paper says that drops the processing temperature from more than 2,800°C for pure WC to 1,300-1,500°C (2,372-2,732°F) with enough cobalt. At the eutectic temperature about 12% of the WC dissolves in the cobalt, and as the part cools that falls to about 1% between 800 and 600°C.

Sources: NTP RoC background document: cobalt-tungsten carbide (2009/2010); University of Cape Town MSc thesis, May 1999; Materials (Basel) 2023;16(4):1347; Materials (Basel) 2023;16(6):2450.

Yes. Carbide oxidizes in air at a few hundred degrees, far below its melting point. A 2020 review of cutting-tool materials says cemented carbides tend to oxidize already above 400°C (752°F).

The cobalt goes first. A 2021 paper in Frontiers in Materials cites studies showing that WC-Co oxidizes selectively at 500°C (932°F), with the cobalt phase oxidizing before the WC. In its own tests on binderless WC and on YG3, a grade of 97% WC and 3% cobalt, the rate of weight gain from oxidation at 700°C (1,292°F) was 10 to 100 times the rate at 600°C (1,112°F).

What forms on the oxidized YG3 surface is a scale of tungsten trioxide (WO₃) and cobalt tungstate (CoWO₄). Its grains differ in size and give "a loose oxidation layer with cracks and faults", so the scale gives the carbide beneath it little protection.

A 2018 recycling study cites earlier reports that oxidation can run to completion at 900°C (1,652°F). In its own runs in air at that temperature, broken coated inserts (replaceable cutting tips) swelled where the coating had come off, and after 180 minutes their volume reached more than 200% of the start.

For oxidation-resistant work, ITIA names nickel-chromium binders.

Sources: Materials (Basel) 2020;13(6):1377; Frontiers in Materials 8:645612 (2021); Materials (Basel) 2018;11(10):1796; ITIA: Cemented carbides (newsletter, June 2010).

Carbide keeps more of its hardness when hot than high-speed steel (HSS), which a 2020 review ranks second to carbide in the 2018 cutting-tool market. ITIA says hardened and tempered HSS keeps most of its hardness up to about 550°C (1,022°F) and regains it when it cools. A 2021 review cites authors who put the safe limit for HSS tools at a contact temperature of 600-650°C (1,112-1,202°F), at about 60 HRC.

The trade is toughness: a University of Pennsylvania lecture lists HSS as high in toughness and cemented carbide as very high in hot hardness but poor in toughness.

A 2023 paper says WC-Co is valued for its red hardness, meaning it holds its hardness at high temperature. It still softens: a Fraunhofer IKTS study measured four grades from room temperature to 1,000°C (1,832°F), and every one lost hardness, at different rates.

Cobalt content made the difference. Two grades with similar WC grain size, 0.3 and 0.4 microns, and 5% and 12% cobalt kept similar shares of their hardness up to 500°C, but from 600°C the 12% grade kept clearly less. The authors blame the binder turning highly plastic in that range, and conclude that a low binder content is very favorable for hot hardness.

A 2020 conference paper's property table sets the two side by side. Its hardness scale is not labeled clearly, so compare these numbers only with each other.

Sources: ITIA: Tungsten in Steel (file dated 28 May 2024); Materials (Basel) 2021;14(24):7779; Materials (Basel) 2020;13(6):1377; Materials (Basel) 2023;16(4):1347; Materials (Basel) 2020;13(20):4571 (Fraunhofer IKTS); IConETech-2020, University of the West Indies; University of Pennsylvania IPD 501 lecture, Speeds and Feeds.

The sources put the hottest point on a cutting tool at roughly 600-1,000°C (1,112-1,832°F), depending on the cut. Each figure below holds only for its own conditions.

• 600-1,000°C (1,112-1,832°F): the highest tool/chip interface temperature, from work cited by the 2021 Frontiers paper, which says this is why carbide oxidizes in dry, high-speed cutting and wears out sooner.
• 671-715°C (1,240-1,319°F), calculated: the Fraunhofer IKTS model of grooving AISI 4140 steel with a 5% cobalt grade and a 30-micron edge rounding, as cutting speed rose from 120 to 180 m/min (about 394 to 591 ft/min). The hottest point was always at the cutting edge.
• 906°C (1,663°F), measured: the peak time-averaged chip-tool temperature NIST and Wichita State researchers imaged while cutting the titanium alloy Ti-6Al-4V at 1 m/s (about 197 ft/min), through transparent YAG tools rather than carbide ones.

In the same Fraunhofer model, raising the carbide's thermal conductivity from 48 to 72 W/m·K lowered the maximum edge temperature by about 7%.

Sources: Frontiers in Materials 8:645612 (2021); Materials (Basel) 2020;13(20):4571 (Fraunhofer IKTS); NIST and Wichita State, Proceedings of NAMRI/SME vol. 42 (2014).

No source gives one temperature limit for tungsten carbide, because it depends on the grade, the air around the part and the job. These are the marks the sources give, from hottest to coolest:

• About 2,785-2,800°C: WC melts, or more exactly comes apart as it melts.
• 1,350-1,550°C: the sintering range.
• 600°C up to 1,200°C (2,192°F) at local points: the working temperatures a 2024 study of a 6% cobalt grade says carbide's properties allow.
• 600°C and up: a 12% cobalt grade loses hardness faster than a 5% cobalt grade.
• Above 400-500°C: oxidation in air begins, cobalt first.

A tool retired by heat or wear still holds its tungsten. ITIA describes a recycling route that oxidizes clean WC-Co scrap on purpose, then reduces it in hydrogen and carburizes it back into WC-Co powder, and a 2021 review says less pure tungsten scrap can be made into cast eutectic carbide for hardfacing. So send photos and a rough weight of worn inserts and end mills (rotary milling cutters), or read how we buy carbide; we pay the shipping from anywhere in the US.

Sources: NTP Report on Carcinogens: cobalt-tungsten carbide profile; UCLA paper on the metastable W-C system; NTP RoC background document: cobalt-tungsten carbide (2009/2010); Materials (Basel) 2024;17(24):6032; Materials (Basel) 2020;13(20):4571 (Fraunhofer IKTS); Materials (Basel) 2020;13(6):1377; Frontiers in Materials 8:645612 (2021); ITIA: Recycling of tungsten products, part 2 (2024); Int. J. Refractory Metals and Hard Materials 98 (2021) 105546 (ITIA-hosted).