High-speed steels owe their exceptional cutting performance and wear resistance to a microstructure built from a hardenable steel matrix studded with a uniform dispersion of carbides. The desirable phases are M6C carbides, rich in tungsten and molybdenum, and the extremely hard MC vanadium carbides that also carry smaller amounts of tungsten and molybdenum. Undesirable carbide stoichiometries, such as M2C, M3C, M7C3, M23C6 and various eutectic forms, degrade properties and are precisely what metallurgists try to avoid. Powder metallurgy press-and-sinter processing allows alloying levels that conventional wrought routes cannot reach, but sintering such concentrated compositions demands a precise handle on the thermodynamics of the liquid phase that forms near the optimum sintering temperature.

Nitrogen has long been known to help. Earlier work showed that sintering vanadium-rich high-speed steels in nitrogen-containing atmospheres lowers the optimum sintering temperature and widens the temperature window in which successful densification occurs, a breakthrough that enabled continuous belt-furnace processing on an industrial scale. Yet the underlying absorption mechanism remained murky. The new research set out to identify which alloying elements and processing variables govern nitrogen uptake, using experimental grades deliberately designed beyond conventional composition limits and beyond the validated range of the widely used Thermo-Calc software and its TCFe13 database.

The team produced water-atomized powders at their research centre, including high-vanadium grades containing only vanadium carbides and complex alloys additionally loaded with cobalt, molybdenum and tungsten. Powders were sieved below 100 micrometres, hydrogen-annealed to push oxygen below 0.1 weight percent, mixed with a small amount of organic binder and graphite, uniaxially pressed at 700 megapascals into small cylinders at roughly 70 percent of theoretical density, and sintered for one hour under atmospheres of differing nitrogen potential, each containing 1 percent methane to prevent decarburization. Nitrogen and carbon contents of the sintered samples were then measured with dedicated fusion and combustion analysers, while scanning electron microscopy with energy-dispersive X-ray spectroscopy tracked the microstructural consequences.

The absorption mechanism that emerges is a two-step gas-solid reaction. At sintering temperatures, nitrogen molecules dissociate into atomic nitrogen on the steel surface, and the adsorbed atoms enter solid solution in the austenitic matrix and diffuse inward. When a dissolved nitrogen atom reaches an MC vanadium carbide, an exchange reaction takes place: nitrogen substitutes for carbon in the carbide lattice, converting it into an M(C,N) carbonitride and releasing carbon into the matrix. Because the substoichiometric vanadium carbides typical of these steels, close to V8C7 in composition, contain lattice vacancies, nitrogen initially occupies those vacancies before displacing carbon, which explains why earlier researchers observed nitrogen uptake without the expected immediate carbon release.

Diffusion rates might seem a bottleneck, since nitrogen diffuses far more slowly in austenite than in ferrite, even though its solubility is greater in austenite thanks to larger interstitial sites. But the porous network between powder particles offers a shortcut. Below the onset of liquid formation, the maximum diffusion distance is only about half a particle size, under 50 micrometres, meaning nitrogen can reach carbides at the centre of a particle in less than ten seconds. This also means high heating rates can starve the process: they limit the time available for dissociation, diffusion and carbon substitution before liquid forms, densification closes off the pore network and further nitrogen transport is choked. The team therefore used a comparatively gentle heating rate of 20 degrees Celsius per minute to promote controlled absorption.

Thermodynamic calculations confirmed that the MX carbonitride phase is overwhelmingly the nitrogen sink, containing roughly three orders of magnitude more nitrogen than any other phase and accounting for about 90 percent of the nitrogen in these steels. Vanadium nitrides show the largest drop in Gibbs free energy among the candidate nitrides, which is why vanadium carbonitrides are the only nitrogen-bearing phase observed. The study also found that nitrogen uptake increases markedly once liquid forms through the eutectic reaction between austenite and M6C carbides, because the liquid dissolves less nitrogen and releases it to the carbonitrides. The extent of the carbide-to-carbonitride transformation depends on the nitrogen activity of the atmosphere: higher nitrogen partial pressure consistently drove greater uptake, though the atmospheres tested, below 90 percent nitrogen, were insufficient to convert vanadium carbide fully into vanadium nitride.

The alloying chemistry proved equally decisive. At constant vanadium content, higher carbon suppresses nitrogen absorption, because carbon pushes the exchange reaction back toward the carbide side of the equilibrium. Tungsten and molybdenum, which behave almost interchangeably in these alloys, act as a hindrance through a molybdenum-equivalent parameter that showed a nearly linear decline in nitrogen uptake as their combined content rose. These strong carbide formers demand more carbon to satisfy carbide stoichiometry and partially substitute for vanadium in the carbonitride lattice, forming complex (V,W,Mo)(C,N) phases that reduce the effective vanadium available for nitrogen binding. Comparisons between nearly identical compositions differing only in carbon or in tungsten and cobalt isolated these effects cleanly.

The practical payoff lies in the sintering window. Increasing the nitrogen potential lowered the optimum sintering temperature by 20 to 40 degrees Celsius and expanded the window from roughly 40 to about 100 degrees Celsius, an almost threefold widening, while the liquid fraction at the optimum temperature remained essentially constant at around 12 weight percent. Phase diagrams showed that nitrogen, a strong austenite stabilizer comparable to carbon, widens the temperature interval between the solidus and the austenite disappearance line, enlarging the liquid-plus-austenite-plus-carbide region that defines supersolidus liquid phase sintering. A flatter liquid-fraction-versus-temperature slope means the process is less sensitive to small temperature fluctuations, reducing the risk of undersintering or dimensional distortion.

The authors caution that their calculations should be read as qualitative trends rather than absolute predictions, since some compositions exceeded the database limits and kinetic factors such as diffusion are not captured by equilibrium thermodynamics. Even so, the guidelines are clear: tailor vanadium content and atmospheric nitrogen activity to promote uptake, manage carbon and molybdenum-equivalent levels to avoid tipping the equilibrium the wrong way, and exploit the widened processing window for energy-efficient, more controllable sintering of next-generation tool steels, including alloys that could eventually feed additive manufacturing routes beyond current compositional limits.

Subject of Research: Nitrogen absorption mechanisms in powder metallurgy high-speed steels sintered in nitrogen-rich atmospheres

Article Title: Nitrogen absorption mechanism in powder metallurgy high-speed steels sintered in nitrogen-rich atmospheres

Article References: Iraola-Arregui, I., Lozada, L., Mancisidor, A. M., & Iturriza, I. (2026). Nitrogen absorption mechanism in powder metallurgy high-speed steels sintered in nitrogen-rich atmospheres. Journal of Materials Science: Metallurgy, 1(1), Article 9. https://doi.org/10.1007/s44492-026-00009-x

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00009-x

Keywords: high-speed steel, powder metallurgy, sintering, nitrogen absorption, vanadium carbide, carbonitrides, Thermo-Calc, liquid phase sintering, alloy design, sintering window, thermodynamics, tool steels