For a long time, carbide sintering has been extremely time-consuming. Traditional manufacturing processes require hours of heat preservation at high temperatures. However, a new technology called “ultrafast high-temperature sintering (UHS)” is rewriting this situation with “second-level sintering” speed. This article will show you how UHS technology accelerates cemented carbide production by 100 times and the innovative breakthroughs behind it.

 

What is UHS?

Ultrafast high-temperature sintering (UHS) is an innovative carbide sintering technology that subverts tradition, with its core secrets lying in “speed” and “precision”. Using carbon felt with extremely low heat capacity as the heat source, it can rapidly heat materials to a high temperature of 1000-1500℃ in just a few seconds through Joule heating and radiant heating. After carbide sintering, it can cool down quickly, and the entire process (including heating, heat preservation, and cooling) takes only about 1 minute.

More ingeniously, carbon felt is not only a “fast heater” but also creates a low oxygen partial pressure environment, which is perfectly suitable for the sintering of oxidation-prone materials such as tungsten carbide. Previously, UHS technology has been successfully applied to the rapid preparation of zirconia ceramics, pure tungsten carbide and other materials, but this is the first attempt to apply it to WC-Co cemented carbide—similar to cooking a dish that requires “slow stewing” with the “quick stir-fry” method, which tests both temperature control and precise detail management.

The UHS experimental device and temperature control curve used in the study are shown in Figure 1, which clearly shows the carbide sintering temperature changes corresponding to different carbon felt currents, providing an intuitive basis for precise temperature control.

Figure 1 (a) Schematic diagram of the UHS experimental device; (b) Carbon felt current-time curve, marking the estimated temperature of the sample under different carbon felt currents.

UHS vs. Traditional Carbide Sintering

To intuitively feel the advantages of UHS, let’s compare its core differences with traditional sintering:

Comparison Dimension	Traditional Carbide Sintering	Ultrafast High-Temperature Sintering (UHS)
Sintering Time	Hours (heat preservation stage only)	Approximately 65 seconds in total (including heating)
Heating Rate	Slow (℃/minute level)	Ultra-fast (10,000 ℃/minute level)
Grain State	Prone to coarsening due to long heat preservation	Maintains ultra-fine grains through rapid cooling
Energy Consumption	High (long-time heating)	Low (local instantaneous heating)
Process Efficiency	Low, suitable for batch slow production	Extremely high, suitable for rapid preparation

In simple terms, traditional carbide sintering is like “slowly simmering soup”, relying on long-time high temperature to densify materials but easily leading to grain growth; while UHS is like “quick stir-frying”, completing carbide sintering in the moment when material grains have no time to grow. It not only ensures density but also retains fine grains, resulting in better performance.

Research Findings on UHS Preparation of WC-Co Cemented Carbide

The research team took WC-11% Co cemented carbide as the research object and conducted systematic investigations on UHS process parameters, microstructure, and defect solutions, ultimately achieving a series of key breakthroughs:

The “Golden Window” for High Densification in 65 Seconds

The sintering effect of UHS is not “the higher the temperature and the longer the time, the better”, but there is an optimal parameter combination. The study found that when the carbon felt current is 20A (corresponding to a carbide sintering temperature of 1400℃, just higher than the eutectic temperature of WC-Co at 1320℃) and the heat preservation time is 60 seconds, a dense sintered body with a density of 14.3g/cm³ can be prepared, with a density close to complete densification.

If the parameters deviate from this “window”, the effect will be greatly reduced: for example, when the current increases to 22A or the heat preservation time extends to 90 seconds, a large number of pores will appear in the sintered body; while when the current is too low (18A, corresponding to 1300℃), the cobalt phase cannot fully penetrate, making it difficult to densify the material.

This rule is intuitively reflected in Figure 2: (a) The optical photos clearly show the appearance differences of samples under different processes, and the samples marked with white dots are used for subsequent phase and microstructure analysis; (b) The density curve quantifies the impact of carbon felt current on density, reaching a peak at 20A. This indicates that the core of UHS technology is “precise temperature control + short-time heat preservation”, which must not only reach the sintering temperature but also avoid overheating.

Figure 2 (a) Optical photos of UHS alloys under different heat preservation times and carbon felt currents (sintering temperatures); (b) Density change curves of UHS alloys under different carbon felt currents (carbide sintering temperatures). The alloy samples marked with white dots in the optical photos in (a) are used for XRD testing and SEM analysis, and the relevant results are shown in Figures 3 and 4.

UHS Enables Grain Refinement and Pure Composition

Observation with a high-power microscope found that the cemented carbide prepared by UHS has a unique microstructure: WC grains present a regular truncated hexagonal column shape, like exquisite small prisms; the (0001) basal plane has an atomic-level step structure, while the {10-10} side faces are atomically flat. This structure can balance the hardness and toughness of the material.

More importantly, all UHS sintered bodies only contain WC and cobalt phase (γ phase), with no excess impurity phases or free graphite detected. This is confirmed by the XRD patterns in Figure 3—all diffraction peaks of the samples correspond only to WC and γ phases, with no impurity phase peaks appearing, proving that the carbon felt heat source does not contaminate the material and that the rapid carbide sintering of UHS can avoid component segregation, making the alloy composition purer.

The SEM images and EDS element distribution in Figure 4 further show the microstructure: (a-d) clearly present the grain arrangement and pore conditions under different processes, and the 20A-60s sample has the finest grains and no obvious pores; (e-f) The element distribution map shows that W and Co elements are evenly distributed without local enrichment or deficiency. Just like quick freezing can lock in food nutrients, rapid carbide sintering can also lock in the excellent performance of materials.

Figure 3 XRD patterns of UHS alloys under different carbon felt currents and heat preservation times.

Figure 4 (a~d) Cross-sectional SEM images of UHS alloys under different carbon felt currents and heat preservation times; (e~f) EDS mapping images of W and Co elements taken from the area marked by the white square in (d).

To further explore the microstructure, the research team conducted refined STEM analysis on the optimal 20A-60s sample (Figure 5): (a) The bright-field STEM image shows the overall microstructure; (b-c) The high-angle annular dark-field (HAADF) STEM images clearly present the interface structure between WC and γ phase, with no common relaxation phase WC₁₋ₓ found; (d) It is a schematic diagram of truncated hexagonal WC grains to help understand the crystal structure; (e-g) The element distribution again verifies the composition uniformity. These details fully illustrate that the ultra-fast cooling rate of UHS can not only refine grains but also regulate the interface structure, resulting in better material performance.

Figure 5 STEM microstructure analysis results of UHS alloy under 20A carbon felt current and 60s heat preservation time: (a) Bright-field scanning transmission electron microscope (BF-STEM) image; (b, c) High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images of WC (0001)/γ phase and WC {10-10}/γ phase interfaces; (d) Schematic diagram of common truncated hexagonal WC grains; (e~g) BF-STEM image and EDS mapping images of W and Co elements. The observation direction of the STEM images in (a~c) is perpendicular to the crystal plane marked by the shadow in (d).

UHS Can “Add an Insulation Layer” to Materials to Eliminate Pores

In the early stage of the study, a tricky problem appeared in the UHS sintered body: when the process parameters exceeded the optimal window, a large number of pores would be generated. Analysis found that WC-Co cemented carbide is a good conductor. After carbide sintering and densification, it forms good electrical contact with the carbon felt, leading to concentrated current flow into the alloy, causing volatilization of cobalt elements and ultimately forming pores.

The resistance change curve in Figure 6 also confirms this mechanism: as carbide sintering proceeds, the carbon felt resistance changes, and current concentration will aggravate element volatilization.To solve this problem, the research team proposed a simple yet effective solution: inserting a thin alumina single crystal plate between the carbon felt and the alloy green body to play an electrical insulation role.

Figure 7 shows the optimized effect:

(a) It is a schematic diagram of sample assembly, clearly marking the position of the alumina insulation layer;

(b) The optical photo shows that the sample surface is flat with no pores;

(c) The resistance change curve indicates that the insulation layer effectively stabilizes the carbon felt resistance;

(d) The XRD pattern and (e) SEM image confirm that the optimized sample still maintains a pure phase and fine microstructure. This small modification completely blocks the current flow into the alloy. Even under extreme conditions of 22A current and 90-second heat preservation, a pore-free sintered body with a density of 14.4g/cm³ can be prepared, and the average WC grain size is only 1.1μm, which is finer than that of traditional carbide sintering.

Figure 6 (a) Carbon felt current and carbon felt resistance change curves during UHS process; (b) Rt/R20 change curve during UHS process, where Rt is the carbon felt resistance and R20 is the carbon felt resistance at 20s in (a).

Figure 7 Analysis results of UHS alloy under 22A carbon felt current and 90s heat preservation time with thin alumina plate insulation: (a) Schematic diagram of sample assembly; (b) Optical photo of UHS alloy; (c) Rt/R20 change curve with heat preservation time; (d) XRD pattern; (e) SEM image. Figure (c) also shows the 22A-60s condition without thin alumina plate insulation (data same as Figure 6 (b)) and the Rt/R20 change data of pure carbon felt without green body.

UHS Achieves Hardness and Toughness Comparable to Traditional Products

Many people worry that “rapid preparation” will sacrifice performance, but test results show that the Vickers hardness of cemented carbide prepared by UHS is about 1330kgf/mm², and the fracture toughness is about 11.2MPam¹/², which is comparable to products prepared by traditional 1380℃ heat preservation for 1 hour. More notably, the oxygen content of the UHS sintered body is less than 0.005wt%, which is much lower than that of the green body, indicating that the low oxygen environment of the carbon felt effectively inhibits oxidation and makes the material purer.

Prospects of UHS Technology

The success of this research not only confirms the feasibility of UHS technology in preparing WC-Co cemented carbide for the first time but also provides a complete process plan: by precisely controlling the carbon felt current and heat preservation time (20A-60s is optimal) and cooperating with an alumina insulation layer to inhibit pores, the efficient preparation of cemented carbide can be completed in 1 minute.

The breakthrough of UHS technology is of great significance: in terms of efficiency, it compresses the carbide sintering time from “hour-level” to “second-level”, greatly improving production efficiency; in terms of performance, it can obtain finer grains and purer composition, providing possibilities for high-end cemented carbide tools; in terms of energy consumption, instantaneous heating significantly reduces energy consumption, conforming to the trend of green manufacturing.

Of course, to realize industrial application, UHS technology still needs to further optimize insulation material selection and process parameters, but this does not affect it becoming the “future direction” of cemented carbide preparation. With the improvement of technology, perhaps in the future, the cutting tools and wear-resistant parts around us will be produced through this “second-level sintering” technology, making the “industrial teeth” both sharp and efficient, and injecting new impetus into high-end manufacturing.