Decarburization superfine hard metal gas phase carburization sintering repair process

Abstract

The application discloses a kind of decarburization superfine hard alloy gas phase carburizing sintering repair processes, belong to powder metallurgy technical field.The application is directed to the problem that superfine grain hard alloy product appears decarburization after sintering can only be scrapped, provide a kind of post-processing carburizing process: the decarburized superfine hard alloy product is placed in multi-atmosphere high-temperature sintering furnace, and the gas medium that can be cleaved to generate carbon element is introduced, and the gas medium is cleaved at high temperature, and a uniform controllable carbon-rich atmosphere is formed in the furnace, and the decarburized product is carburized.The gas medium is preferably CH4 or a mixture of CH4 and H2, and the cleavage temperature is 500-1380 DEG C.The application uses gas carburizing medium instead of traditional solid filler, avoids the problem of uneven distribution of filler and uncontrollable reaction, can restore the decarburized superfine hard alloy product to normal performance qualified product, greatly reduces the scrap rate, saves tungsten resources, and has good industrial application value.

Description

Technical Field

[0001] This invention relates to the field of powder metallurgy technology, specifically to a decarburized ultrafine hard alloy vapor phase carburizing sintering repair process. Background Technology

[0002] Ultrafine cemented carbide is a high-hardness, high-strength cemented carbide material obtained through powder metallurgy using ultrafine WC (tungsten carbide) as the main raw material. Due to the high reactivity of ultrafine WC particles, control during production is challenging, and decarburization occasionally occurs in alloy products after sintering. For cemented carbide products produced using medium-coarse WC particles (above 1μm), decarburization can be corrected by re-sintering in a hydrogen sintering furnace using fillers that have either increased or decreased carbon content. For example, burying decarburized products in fillers and re-sintering at around 1400℃ under hydrogen protection can achieve carbon increase. However, this process suffers from uneven carbon content in the fillers and uneven sintering temperature, resulting in poor product stability. Nevertheless, due to the lower reactivity of medium-coarse WC particles, their insensitivity to carbon reactions, and relatively lower usage requirements, this process remains acceptable.

[0003] For ultrafine-grained (below 0.9μm) cemented carbide products, decarburization after sintering is currently the standard practice in the industry, resulting in scrapping. This is because ultrafine WC is extremely reactive and highly sensitive to changes in carbon content, making it difficult to control using traditional filler reheating processes. This can easily lead to overcarburization or uneven reaction, resulting in products that fail to meet performance requirements. With the tightening supply and demand of tungsten resources and continuously rising product prices, scrapping costs are increasing daily. Therefore, there is an urgent need to develop a process that can effectively post-treat decarburized ultrafine cemented carbide products, restoring them to acceptable performance, in order to reduce scrap rates, conserve resources, and ensure production delivery schedules. Summary of the Invention

[0004] The purpose of this invention is to provide a vapor-phase carburizing sintering repair process for decarburized ultrafine cemented carbide, in order to solve the problem that ultrafine grain cemented carbide products can only be scrapped when decarburization occurs after sintering.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A sintering process for ultrafine cemented carbide includes the following steps: The decarburized ultrafine cemented carbide products are placed in a high-temperature sintering furnace that can be sintered with one or more atmospheres. A gaseous medium capable of being decomposed to generate carbon is introduced into the high-temperature sintering furnace; The gaseous medium is subjected to a cracking reaction under high temperature conditions to form a carbon-rich atmosphere in the furnace, and the decarburized ultrafine cemented carbide products are then subjected to carburizing treatment.

[0006] Furthermore, the gaseous medium that can be decomposed to generate carbon is CH4 gas or a mixture of CH4 and H2.

[0007] Furthermore, the temperature of the pyrolysis reaction is 500–1380 °C.

[0008] As a preferred specific process step: The decarburized ultrafine cemented carbide product was placed in a high-temperature sintering furnace and vacuum sintered at a heating rate of 9℃ / min to 750℃. CH4 gas is introduced at a flow rate of 4–5 L / min; Heat to 1200–1350℃ at a rate of 3℃ / min and hold for 60–90 min; Then raise the temperature to 1400-1410℃ at a rate of 5℃ / min and hold for 50min; Remove from the furnace when cooled to below 100°C.

[0009] The WC grain size of the ultrafine cemented carbide product is ≤0.9μm, preferably <0.6μm.

[0010] The cobalt magnetic properties of the ultrafine cemented carbide product after carburizing treatment are restored, and the metallographic structure changes from decarburized structure (such as A02B00E04, A02B00E06, etc.) to normal structure (A02B00C00E00).

[0011] The present invention has the following beneficial effects: This invention solves the problem of localized over- or under-carburization caused by uneven packing distribution by using a decomposable gaseous medium instead of traditional solid carburizing filler to create a uniform and controllable carbon-rich atmosphere in the sintering furnace. Simultaneously, precise control of gas flow and temperature effectively prevents runaway reactions caused by excessively high activity in ultrafine tungsten carbide, achieving gentle and uniform carburization of decarburized products. Based on this, the cobalt magnetic properties of the treated product are restored, the decarburized characteristic phases are completely eliminated, and the metallographic structure transforms into a normal two-phase structure. This transforms previously scrapped decarburized ultrafine cemented carbide products back into qualified products, significantly reducing the scrap rate, conserving valuable tungsten resources, lowering production costs, and providing reliable guarantees for production delivery. Attached Figure Description

[0012] Figure 1 Metallographic photograph of the decarburized rod before treatment in Example 1; Figure 2 Metallographic photograph of the decarburized rod before treatment in Example 2. Detailed Implementation

[0013] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments. It should be noted that the scope of protection of this invention is not limited to the following specific embodiments; any equivalent substitutions or improvements made based on the inventive concept shall fall within the scope of protection of this invention.

[0015] This invention addresses the decarburization problem that occurs after sintering of ultrafine-grained (WC grain size ≤ 0.9 μm) cemented carbide products, proposing a vapor-phase carburizing sintering repair process for decarburized ultrafine-grained cemented carbide. The core principle is: in a multi-atmosphere high-temperature sintering furnace, a gaseous medium capable of decomposing to generate active carbon atoms (such as CH4 or...) is introduced. A mixture of CH4 and H2 gases undergoes a cracking reaction at high temperatures of 500–1380°C (e.g., CH4→C+2H2), forming a uniform and controllable carbon-rich atmosphere in the furnace. Activated carbon atoms diffuse into the surface and interior of the decarburized alloy product, compensating for the missing carbon and restoring the alloy's phase structure to a normal WC+Co two-phase structure. This eliminates the η phase (a characteristic phase of decarburization) and restores key performance indicators such as cobalt magnetism and coercivity.

[0016] The present invention will be verified and illustrated through specific embodiments below.

[0017] Example 1 (9% cobalt, WC grain size 0.3μm ultrafine grain cemented carbide rod) Raw materials and initial properties Ultrafine cemented carbide rods with a cobalt content of 9% (mass fraction) and a WC grain size of 0.3 μm were selected. These rods exhibited decarburization defects after initial sintering. Testing was performed using a cobalt magnetometer and a coercivity magnetometer (according to GB / T 3848-2017 standard), and the results are as follows: Cobalt magnet (Ms): 5.73% (normal target is 7.1%~8.0%), significantly low, indicating insufficient carbon content in the alloy. Coercivity (Hc): 36.95 kA / m Metallographic structure (evaluated according to GB / T 3488-2014 standard): A02B00E06, where E06 indicates the presence of a large amount of η phase (decarburization characteristic phase), and the structure is unqualified.

[0018] Equipment and process parameters Equipment: A laboratory-type high-temperature sintering furnace capable of handling various atmospheres, equipped with vacuum sintering and atmosphere flow control functions.

[0019] Specific process steps: The bars to be processed are loaded into the sintering furnace and vacuumed until the pressure inside the furnace is below 10 Pa.

[0020] Vacuum sintering is performed at a heating rate of 9℃ / min, raising the temperature to 750℃. This stage is for preheating and degassing, and its purpose is to remove adsorbed gases and residual forming agents from the workpiece surface to avoid contamination during subsequent carburizing processes.

[0021] Once the temperature reaches 750℃, high-purity CH4 gas (purity ≥99.9%) is introduced into the furnace at a flow rate of 5 L / min. 750℃ was chosen as the initial gas introduction temperature because at this temperature, CH4 begins to undergo significant decomposition, and the alloy surface has sufficient activity to adsorb carbon atoms, but without causing excessive carburization of the surface due to excessively rapid decomposition.

[0022] Under a CH4 atmosphere, the temperature was further increased to 1350℃ at a heating rate of 3℃ / min, and held at 1350℃ for 90 min. This stage is the main carburizing stage, where the continuous decomposition of CH4 provides active carbon atoms, which diffuse into the alloy interior to compensate for carbon deficiency. The slower heating rate (3℃ / min) and longer holding time (90 min) are conducive to the uniform diffusion of carbon atoms and avoid local enrichment.

[0023] After the main carburizing process, the temperature is increased to 1400℃ at a rate of 5℃ / min and held for 50 min. This stage is the high-temperature homogenization stage, which further homogenizes carbon atoms in the alloy and promotes residual stress release and alloy densification.

[0024] Cool in the furnace and remove from the furnace after the temperature drops below 100℃ (natural cooling in the atmospheric environment).

[0025] Post-processing performance testing Visual inspection after baking: The product surface has a normal metallic luster, with no peeling, bubbling or discoloration, indicating that no carburizing or uncontrolled surface reaction has occurred.

[0026] The same bar stock (marked and tracked) was tested again for performance: Cobalt magnetism (Ms): 7.53%, returning to the normal range (the theoretical cobalt magnetism corresponding to 9% cobalt content is about 7.1% to 8.0%), indicating that the carbon content has basically met the standard.

[0027] Coercivity (Hc): 33.67 kA / m. The coercivity decreased slightly, which is related to slight grain growth, but it is still within the acceptable range (generally ≥30 kA / m is required).

[0028] Figure 1 Example 1: Metallographic photographs of decarburized rods before treatment (left × 100x, right × 1600x), showing the presence of the η phase (decarburized structure). Metallographic structure: rated as A02B00C00E00, where E00 indicates that the η phase has completely disappeared, C00 indicates that there is no free carbon, and the structure is a normal WC+Co two-phase structure, which meets the standard microstructure requirements of cemented carbide.

[0029] Example 1 demonstrates that for decarburized ultrafine cemented carbide rods with a cobalt content of 9% and a WC grain size of 0.3 μm, the process of this invention (CH4 flow rate 5 L / min, holding at 1350℃ for 90 min + holding at 1400℃ for 50 min) can successfully restore the cobalt magnetism from 5.73% to 7.53%, transforming the metallographic structure from a decarburized state to a normal state, and turning the product from scrap to a qualified product. This verifies the effectiveness of this invention in the decarburization repair of ultrafine alloys with high cobalt content.

[0030] Example 2 (6% cobalt, 0.3μm WC ultrafine cemented carbide rod) Raw materials and initial properties Ultrafine cemented carbide rods with a cobalt content of 6% (mass fraction) and a WC grain size of 0.3 μm were selected. Decarburization occurred after the initial sintering. Initial test results: Cobalt magnet (Ms): 4.46% (normal target value is approximately between 4.8% and 5.4%, significantly low) Coercivity (Hc): 35.97 kA / m Metallographic structure: A02B00E04, η phase is present, the structure is unqualified.

[0031] Compared to Example 1, the alloy with lower cobalt content (6% vs 9%) is more sensitive to changes in carbon content. Therefore, the carburizing process needs to be appropriately adjusted to reduce carburizing strength and prevent over-carburizing. Specific adjustments are as follows: The CH4 flow rate was reduced from 5 L / min to 4 L / min to decrease the supply rate of active carbon atoms.

[0032] The main carburizing temperature was reduced from 1350℃ to 1200℃, and the holding time was shortened from 90 min to 60 min, in order to reduce the diffusion depth and total amount of carbon atoms.

[0033] The high-temperature homogenization temperature was slightly increased from 1400℃ to 1410℃, while the holding time remained at 50 min, to ensure final densification and tissue uniformity.

[0034] Specific process steps: Evacuate the vacuum and heat to 750℃ at a rate of 9℃ / min.

[0035] CH4 gas is introduced at a flow rate of 4 L / min.

[0036] Heat to 1200℃ at a rate of 3℃ / min and hold for 60 minutes.

[0037] Increase the temperature to 1410℃ at a rate of 5℃ / min and hold for 50 minutes.

[0038] Cool in the furnace and remove from the furnace at a temperature below 100°C.

[0039] Post-processing performance testing Visual inspection: The product surface is normal.

[0040] Test results for the same bar: Cobalt magnet: 5.35% (normal range is between 4.8% and 5.4%, this is close to the upper limit, and is considered a qualified product) Coercivity: 39.45 kA / m (an increase compared to before treatment, indicating tissue improvement) Figure 3: Metallographic photographs of the decarburized rods before treatment in Example 2 (left × 100x, right × 1600x), showing the η phase; Metallographic structure: It changes from A02B00E04 before treatment (see Figure 3) to A02B00E00 (see Figure 4), the η phase is completely eliminated, and the structure is a normal WC+Co two-phase structure.

[0041] Example 2 demonstrates that by appropriately reducing the CH4 flow rate, lowering the main carburizing temperature, and shortening the holding time, the process of the present invention is also applicable to the repair of decarburized ultrafine cemented carbide products with low cobalt content. After treatment, the cobalt magnetic flux density of the product increased from 4.46% to 5.35%, and the metallographic structure returned to normal, verifying the adjustability of the process and its adaptability to alloys with different compositions.

[0042] Comparative Example (Traditional Packing Method) To demonstrate the superiority of this invention over the prior art, a comparative test was conducted on the same batch of decarburized ultrafine cemented carbide rods (6% cobalt, 0.3μm WC) using the industry's traditional filler reheating process.

[0043] Traditional process steps: The decarburized rods are embedded in the carbon-reinforcing filler (the main components are carbon black + alumina + trace amounts of carbonate, a standard formula in the industry).

[0044] It is placed in a hydrogen-protected sintering furnace, and hydrogen gas (flow rate 15 L / min) is introduced as a protective atmosphere.

[0045] Increase the temperature to 1400℃ at a rate of 10℃ / min and hold for 120min.

[0046] Cool in the furnace to below 100°C before unloading.

[0047] The product exhibited the following problems after processing: Appearance: The surface color is uneven, with some areas appearing grayish-black (overcarburization) and others light gray (still decarburized), indicating that the carbon distribution in the packing and the temperature field inside the furnace are extremely uneven.

[0048] Cobalt magnetic field testing: The cobalt magnetic values ​​of products at different locations within the same furnace fluctuate greatly (4.2% to 5.5%), and the difference in cobalt magnetic values ​​at different parts of the same bar also exceeds 1.3 percentage points.

[0049] Metallographic structure: Free carbon (C-type pores) appears in some areas, and η phase (E-type pores) is still present in some areas, making it impossible to obtain a uniform A00B00C00E00 structure.

[0050] Mechanical properties: Bending strength test (according to GB / T 3851-2015) shows that the average bending strength of the treated product is only 60% to 70% of that of the normal product before treatment, and the data has large dispersion, which cannot meet the usage requirements.

[0051] Comparative results show that traditional filler reheating processes are completely unsuitable for decarburization post-treatment of ultrafine-grained cemented carbides due to uneven carbon content distribution in the filler, uneven furnace atmosphere, and uncontrollable carburizing process. Ultrafine WC is highly reactive and extremely sensitive to minute changes in carbon content; traditional processes easily lead to localized overcarburizing or continued decarburization. This invention, however, uses a gaseous carburizing medium and achieves a uniform and controllable carburizing process through precise control of gas flow rate, temperature, and time, thus successfully solving this technical problem.

[0052] Reproducibility tests were conducted on no fewer than five batches of each of Examples 1 and 2 above. The results showed that, under the same process conditions, the cobalt magnetic value fluctuation of the treated products did not exceed ±0.3 percentage points, and the metallographic structure consistently reached A02B00C00E00 (or A02B00E00). The product qualification rate increased from 0% (decarburization meant scrap) to over 95% (some products with severe edge decarburization may not be fully repairable). This process has been successfully pilot-scaled in the applicant's production line, demonstrating good industrial scalability and economic value.

[0053] In summary, the ultrafine cemented carbide sintering process provided by this invention, by replacing traditional solid fillers with gas carburizing media, achieves precise, controllable, and uniform carburizing of decarburized ultrafine cemented carbide products, successfully restoring products that would otherwise be scrapped to qualified products, significantly reducing the scrap rate, saving valuable tungsten resources, and possessing outstanding substantive features and significant progress.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, without departing from the principles of the present invention, various adjustments can be made to the process parameters based on the actual degree of decarburization, alloy composition, and product dimensions; these adjustments should also be considered within the scope of protection of the present invention.

Claims

1. A decarburized ultrafine hard alloy vapor phase carburizing sintering repair process, characterized in that, Includes the following steps: The decarburized ultrafine cemented carbide product is placed in a high-temperature sintering furnace that can be sintered in one or more atmospheres; a gaseous medium that can be decomposed to generate carbon is introduced into the high-temperature sintering furnace. The gaseous medium is subjected to a cracking reaction under high temperature conditions to form a carbon-rich atmosphere in the furnace, and the decarburized ultrafine cemented carbide products are then subjected to carburizing treatment.

2. The decarburized ultrafine hard alloy vapor phase carburizing sintering repair process according to claim 1, characterized in that, The gaseous medium that can be decomposed to generate carbon is CH4 gas or a mixture of CH4 and H2.

3. The decarburized ultrafine hard alloy vapor phase carburizing sintering repair process according to claim 1 or 2, characterized in that, The pyrolysis reaction occurs at temperatures ranging from 500 to 1380°C.

4. The decarburized ultrafine hard alloy vapor phase carburizing sintering repair process according to claim 1, characterized in that, Specifically, the following steps are included: The decarburized ultrafine cemented carbide product was placed in a high-temperature sintering furnace and vacuum sintered at a heating rate of 9℃ / min to 750℃. CH4 gas is introduced at a flow rate of 4–5 L / min; Heat to 1200–1350℃ at a rate of 3℃ / min and hold for 60–90 min; Then raise the temperature to 1400-1410℃ at a rate of 5℃ / min and hold for 50min; Remove from the furnace when cooled to below 100°C.

5. The decarburized ultrafine hard alloy vapor phase carburizing sintering repair process according to claim 1, characterized in that, The WC grain size of the ultrafine cemented carbide product is ≤0.9μm.

6. The decarburized ultrafine cemented carbide vapor phase carburizing sintering repair process according to claim 1, characterized in that, The cobalt magnetic properties of the ultrafine cemented carbide products after carburizing treatment are restored, and the metallographic structure changes from decarburized structure to normal structure.

7. The decarburized ultrafine hard alloy vapor phase carburizing sintering repair process according to claim 4, characterized in that, The ultrafine cemented carbide product is a fine-grained cemented carbide bar with a cobalt content of 3% to 15% and a WC grain size of <1.0 μm.

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