Sintering furnace

Abstract

The invention relates to a sintering furnace (100) for sintering green compacts, comprising a kettle (9); and i) an inlet for in-furnace atmosphere gas, said inlet being located at one end of said kettle (9); ii) a stack of sintering trays (1, 2, 3, 4, 5, 6) arranged in the interior of the vessel (9), the sintering trays comprising a central opening (11) having an area A for conveying the furnace atmosphere gas introduced through the inlet, in which adjacent sintering trays in the stack are spaced apart from each other by a predetermined distance, the central opening (11) having an area A for conveying the furnace atmosphere gas introduced through the inlet; the invention relates to a sintering device (1) for sintering in-furnace gases, comprising a plurality of sintering trays (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13 wherein the ratio between the area A of each individual central opening (11) and the total area of the distributed holes (10) between adjacent sintering trays of each individual pair is in the range of 1: 10 to 10: 1; iii) an outlet for withdrawing the furnace atmosphere gas, the outlet being located at an end of the kettle (9) opposite the end of the inlet, the inlet and the outlet being arranged at opposite ends of the stack of sintering trays (1, 2, 3, 4, 5, 6). The invention also relates to a method for sintering green compacts in said sintering furnace (100) and to sintered compacts obtainable by said method.

Description

Technical Field

[0001] The present invention relates to a sintering furnace, a method for sintering a green compact in a sintering furnace, and a sintered compact obtainable by this method. Background Art

[0002] Sintering is a process for treating a solid material mass (such as a green compact), which generally forms a sintered product by heating and / or pressurizing without melting the solid material mass to the liquefaction point. A sintering furnace is used in the sintering process of a powder compact to improve the mechanical strength, density, and translucency of the product.

[0003] A necessary feature of a sintering furnace is to provide a uniform temperature distribution in the crucible of the furnace, especially for the sintering of green compacts. During the sintering step, large temperature gradients may occur when the furnace atmosphere gas passes through the crucible of the furnace. Due to the non-uniform temperature distribution, especially in small furnaces (such as test furnaces), but equally in medium and large production furnaces, the metallurgical parameters of the sintered material (such as coercivity, magnetic saturation, and shape deformation) will be non-uniform.

[0004] The present invention aims to solve the problems raised above. Another object of the present invention is to provide a compact and energy-efficient sintering furnace with high production capacity. Summary of the Invention

[0005] The present invention relates to a sintering furnace for sintering a green compact, which includes a crucible; and: i) an inlet for the furnace atmosphere gas, which is located at one end of the crucible distiller; ii) a stack of sintering trays, which is arranged inside the crucible. The sintering trays include a central opening with an area A for transmitting the furnace atmosphere gas introduced through the inlet. The adjacent sintering trays in the stack are spaced apart by a predetermined distance, thereby providing a gap between the adjacent sintering trays. The gap is enclosed by a peripheral wall with distributed holes for transmitting the furnace atmosphere gas between the gap inside the peripheral wall and the space outside the peripheral wall. The ratio between the area A of each individual central opening and the total area of the distributed holes between each individual pair of adjacent sintering trays is in the range of 1:10 to 10:1, preferably in the range of 1:5 to 5:1 or 1:3 to 3:1, or in the range of 1:3 to 1:10; iii) an outlet for extracting the furnace atmosphere gas, which is located at the end of the crucible opposite to the end where the inlet is located. The inlet and the outlet are arranged at opposite ends of the stack of sintering trays.

[0006] The term "retort" refers to an airtight chamber within which the green compacts are exposed to a sintering cycle that includes, for example, a vacuum step and a step of treating with furnace atmosphere gases, such as nitrogen in a nitriding process step. Preferably, the retort is made of graphite to withstand the inevitable high temperatures in the sintering cycle.

[0007] The term "furnace atmosphere gas" refers to the gas that passes through the retort during the sintering cycle to treat the green compacts.

[0008] The term "opposite ends" refers to the ends that extend beyond the volume occupied by the stack of sintering trays (which extends a height H in the axial direction within the retort), as Figure 3 further shown. Thus, the inlet and outlet are arranged in the volume HE at the opposite ends of the stack and of the heat exchanger (if the heat exchanger forms part of the sintering furnace).

[0009] The "green compact" refers to a compact that includes an unsintered compact, preferably produced by: i) mixing and grinding powders forming a binder phase and powders forming a hard component in a slurry, and subsequently spray-drying it into a ready-to-press (RTP) powder; and ii) pressing the RTP powder into a green compact. Subsequently, the compact can be sintered according to the present invention to ultimately form a sintered compact, such as a sintered cemented carbide or cermet substrate.

[0010] As used herein, the term "vacuum" refers to a pressure range below 10⁻¹ mbar during which the vacuum step of the sintering cycle is carried out, and this pressure range is preferably between 10⁻ 4 and 10⁻¹ mbar or between 10⁻² and 10⁻¹ mbar.

[0011] Preferably, the sintering trays, the peripheral walls, and any other elements exposed to the temperatures used within the retort are made of graphite. The sintering trays are preferably coated with yttrium oxide, yttrium oxide - zirconium oxide, or other metal oxides, which act as a barrier between the green compacts and the graphite material in the sintering trays, as is commonly known in the art.

[0012] The sintering furnace of the present invention provides a uniform temperature distribution inside the retort and thus improves the uniformity of the metallurgical parameters (such as coercivity, magnetic saturation, and shape deformation) of the sintered product.

[0013] According to one embodiment, the heat exchanger for heating the furnace atmosphere gas entering the retort: i) is arranged inside the retort and at the inlet such that the furnace atmosphere gas is directed to the heat exchanger; or ii) forms an integral part of the end of the retort (9) where the inlet is arranged.

[0014] According to one embodiment, a heat exchanger for heating the furnace atmosphere gas entering the autoclave is arranged inside the autoclave and at the inlet.

[0015] According to one embodiment, the inlet and the outlet form an integral part of the heat exchanger. The inlet and the outlet may also form separate parts. In such an embodiment, the inlet and the outlet are connected to the respective heat exchangers such that the furnace atmosphere gas can be transferred to the stack of sintering trays and can be transferred away from the stack of sintering trays. For example, the furnace atmosphere gas entering the autoclave via the inlet is transferred to the heat exchanger via a passage of the heat exchanger fluidly arranged to the inlet in a manner allowing fluid flow. The furnace atmosphere gas is heated while flowing through the passage and then transferred to the stack of sintering trays. Thus, the introduced furnace atmosphere gas is heated with the heat inside the autoclave and then reaches the stack of sintering trays. By using the heat exchanger located at the inlet, the furnace atmosphere gas obtains a higher temperature when reaching the nearest sintering tray compared to the case without a heat exchanger.

[0016] According to one embodiment, a heat exchanger for recovering the heat of the furnace atmosphere gas withdrawn from the autoclave: i) is arranged inside the autoclave and at the outlet such that the furnace atmosphere gas is directed to the outlet; or ii) forms an integral part of the end of the autoclave where the outlet is arranged.

[0017] According to one embodiment, the heat exchanger is arranged at the end of the autoclave, preferably within a volume of the autoclave extending axially along a length HE, as Figure 3 shown.

[0018] Since the inlet can be used as the outlet and vice versa during different sintering steps, the transfer of the gas can be carried out in the direction opposite to the following: the inlet / outlet, the stack of sintering trays, and the heat exchanger are arranged in a manner allowing fluid flow. However, a sintering cycle where the inlet and the outlet are always located at the same position is also possible. The term "arranged in a manner allowing fluid flow" means that the furnace atmosphere gas is directed to: i) flow from the inlet to the heat exchanger (if any) and then to the stack of sintering trays; or ii) flow from the stack of sintering trays to the heat exchanger (if any) and then to the outlet. If the inlet and the outlet respectively form an integral part of the heat exchanger, the heat exchanger is arranged in the same manner as the stack of sintering trays in a manner allowing fluid flow.

[0019] According to one embodiment, heat exchangers are arranged at both the inlet and the outlet of the autoclave.

[0020] With the above-described embodiments including at least one heat exchanger, a more uniform temperature can be achieved inside the kettle. Since the heat exchanger can be designed to fit smoothly with the design and volume of the kettle, a more uniform average temperature of the furnace atmosphere gas is achieved without significantly reducing the volume of the kettle. The heat exchanger can have a radial extension substantially the same as that of the sintering tray or the inner wall of the kettle. For example, in the case where the sintering tray is square or circular, or the kettle is square or circular, the heat exchanger can have corresponding dimensions to utilize the volume as efficiently as possible.

[0021] According to one embodiment, the heat exchanger can include channels adapted for substantially radial flow through which the furnace atmosphere gas flows before reaching the central opening of the sintering tray, as Figure 4a and 4b further described therein. When the gas travels through such channels, the gas is gradually heated, resulting in a more uniform temperature distribution in the stack of sintering trays. Other designs of the heat exchanger can also be used for a similar volume or length so that the gas travels through. This also applies to the case where the heat exchanger can be used as both an inlet and an outlet in different steps of the sintering cycle.

[0022] Preferably, the heat exchanger is arranged removably on top of the uppermost sintering tray, for example, by means of removable fixing means. By the removable attachment of the heat exchanger arranged on top of the uppermost sintering tray, it is convenient to load the sintering tray before the sintering cycle runs and unload the sintering tray after the sintering cycle runs. According to one embodiment, the heat exchanger located at the inlet or outlet according to the flow direction and arranged below the lowermost sintering tray in the stack is arranged to the kettle in a fixed manner or in a removable manner and is located below the stack of sintering trays.

[0023] According to one embodiment, the total travel length of the channels of the heat exchanger located at the inlet or outlet of the kettle (with a volume of 1 dm 3 of ) can be in the range of, for example, 150 to 1000 mm, for example, in the range of 300 to 600 mm. The corresponding length of the production furnace can be sized based on the volume of the kettle, the flow rate, etc. to provide accurate heating and heat recovery.

[0024] According to one embodiment, the volume of the kettle is in the range of 200 to 800 dm 3 , preferably in the range of 300 to 600 dm 3 , or in the range of 400 to 500 dm 3 .

[0025] According to one embodiment, the volume of the kettle is in the range of 1 to 10 dm 3within the range of, preferably within 2 to 5 dm 3 within the range of, or within 2 to 4 dm 3 within the range of.

[0026] According to one embodiment, the volume of the heat exchanger is within the range of 10 to 100 dm 3 within the range of, for example within 30 to 80 dm 3 within the range of.

[0027] According to one embodiment, the volume of the heat exchanger is within the range of 0.1 to 10 dm 3 within the range of, for example within 0.5 to 5 dm 3 within the range of, or within 0.5 to 2 dm 3 within the range of.

[0028] According to one embodiment, depending on the type of the furnace, the stack of sintering trays includes 4 to 100 sintering trays, for example 5 to 60, or 4 to 15, or 4 to 10 sintering trays.

[0029] According to one embodiment, the stack of sintering trays includes 60 to 100 sintering trays.

[0030] According to one embodiment, the distance h between adjacent sintering trays is within the range of 5 to 200 mm, for example within the range of 5 to 100 mm, such as within the range of 5 to 15 mm, or within the range of 5 to 10 mm. According to one embodiment, the distance h between adjacent sintering trays is within the range of 20 to 100 mm.

[0031] According to one embodiment, each individual central opening of the sintering tray has an area A, and the area A is within the range of 100 to 100000 mm 2 within the range of, for example within the range of 100 to 10000, or within the range of 1000 to 10000 mm 2 within the range of.

[0032] According to one embodiment, the height H of the stack of sintering trays is within the range of 100 to 2000 mm, for example within the range of 150 to 1000 mm, such as within the range of 400 to 700 mm.

[0033] According to one embodiment, the holes in the peripheral wall are substantially uniformly distributed. The so-called "substantially uniformly distributed" means that these holes are distributed such that the radial flow of gas through the peripheral wall is substantially uniform, so as to minimize the temperature difference caused by the flow conditions, for example, symmetrically distributed holes, or holes distributed at a predetermined distance from each other.

[0034] The peripheral wall surrounding the gap between adjacent sintering trays is designed such that the peripheral wall encloses the gap, except for the holes in the peripheral wall. The peripheral wall can be stacked between adjacent sintering trays, but can also be arranged at the periphery of the sintering trays by other means (preferably by removable fixing means). In such an embodiment, for example, a single peripheral wall can be used to enclose the gaps of several pairs of adjacent sintering trays in a stack.

[0035] According to one embodiment, the sintering trays and the peripheral walls are stacked alternately and there is no fixed connection between them.

[0036] According to one embodiment, the inlet and the outlet are arranged at opposite ends of the autoclave and at the position where the axes extending through all the sintering trays intersect.

[0037] The area of the holes in the peripheral wall can vary according to, for example, the size of the sintering trays, the number of holes in the peripheral wall, the diameter of the central opening of the sintering trays, and the volume of the autoclave, to provide a flow pattern such that the gas flows axially through the central opening and radially through the holes, which is optimized to provide a uniform temperature distribution. The shape of the holes can be circular, but other shapes can also be adopted, such as an oval shape.

[0038] According to one embodiment, the sintering trays are of circular shape, square shape, or have a rectangular shape, preferably circular or square shape. The peripheral wall is adapted to the shape of the sintering tray to enclose the periphery of the tray.

[0039] Preferably, depending on the type of sintering furnace used, the diameter or side length of the sintering trays is in the range of 100 to 2000 mm, for example in the range of 100 to 1000 mm, or in the range of 100 to 700 mm, or in the range of 100 to 300 mm, or in the range of 100 to 200 mm.

[0040] According to one embodiment, the sintering furnace includes a jacket surrounding the autoclave, such as a steel jacket. According to one embodiment, the jacket surrounds at least one heating element arranged outside the autoclave. According to one embodiment, the sintering furnace includes a heat insulation package arranged between the jacket and the at least one heating element.

[0041] Preferably, when installing the stack of sintering trays, the sintering trays are aligned by inserting a rod-shaped element (whose external dimensions correspond to the size of the central opening) through the central openings of the stacked sintering trays, where the peripheral wall serves as a spacer element to separate the stacked sintering trays. Subsequently, when the alignment is completed, the rod-shaped element is removed from the stack.

[0042] According to one embodiment, the autoclave can have any suitable shape, such as a cylindrical shape, a cube, or the shape of a rectangular tank.

[0043] According to one embodiment, at least one heating element (e.g., a single heating element) is arranged outside the crucible. According to one embodiment, two or three heating elements are arranged outside the crucible in a surrounding arrangement. The heating element preferably has a shape corresponding to the crucible, such as a cubic or cylindrical shape adapted to be evenly separated from the crucible.

[0044] According to one embodiment, no powered heating element, such as an electric heating element, is arranged inside the wall of the crucible. Arranging a heating element inside the crucible will result in uneven heating, thus leading to poor eutectic liquid phase binder development and thus uneven and unfavorable results in terms of the properties of the sintered product.

[0045] According to one embodiment, in the case of a cylindrical heating element and a cylindrical crucible, the outer diameter of the crucible is 0.3D HT to 0.99D HT e.g., 0.7D HT to 0.8D HT where D HT is the inner diameter of the heating element. The wall thickness of the crucible can be, for example, 5 to 20 mm, such as 5 to 10 mm.

[0046] According to one embodiment, the furnace is a vertical cylindrical furnace, such as a furnace for experimental use or for small-scale production.

[0047] According to one embodiment, the furnace is a horizontal cylindrical furnace, such as a production furnace.

[0048] Preferably, at least one thermocouple is arranged outside the crucible to control the temperature, preferably arranged near the wall of the crucible. The at least one heating element is preferably controlled in a conventional manner, such as by means of at least one thermocouple connected to a control unit. According to one embodiment, several thermocouples are arranged outside the crucible to monitor the temperature.

[0049] The present invention also relates to a method of sintering a green compact in a sintering furnace as disclosed herein, the method comprising performing a sintering cycle, the sintering cycle involving at least one process step during which an in-furnace atmosphere gas is supplied to the crucible at a pressure in the range of 0.001 to 10 mbar. Certain sintering steps are typically carried out under vacuum without the in-furnace atmosphere gas flowing through the crucible.

[0050] At the end of the sintering cycle, a high-pressure gas in the range of 20 to 100 bar can be introduced to avoid undesirable defects and enhance the densification of the compact.

[0051] In a small furnace (<10 dm 3In ( ), the flow rate of the furnace atmosphere gas can vary according to the steps of the sintering cycle being performed, and the range of this flow rate can be from 0 (if it is a vacuum step) to 50, for example from 0.01 to 50, such as from 0.1 to 30, or from 0.1 to 25, or from 0.1 to 5, or from 0.1 to 1 standard liter per minute, while the flow rate of the production furnace can be proportional to its volume. Preferably, a conventional mass flow meter is used to control the flow rate of the furnace atmosphere gas through the autoclave.

[0052] According to one embodiment, the temperature at the sintering tray is monitored by means of a ceramic ring having a calibrated shrinkage behavior that is a function of temperature and time, preferably a ceramic ring (https: / / www.ferro.com / - / media / files / resources / industrial-specialty-materials / technical / ferro-industrial-specialty-materials-process-temperature-control-rings-ptcr.pdf?la=en&hash=8FD87135E2C3363D07C57D799F8674C6A9405040) that can be obtained from, for example, Ferro Corporation.

[0053] Before starting any cycle (such as a sintering cycle), the ceramic ring is positioned on the sintering tray to allow for a retrospective analysis of the temperature distribution. Since the shrinkage of the ceramic ring is a function of the heat absorbed, the so-called ring temperature RT (which is not a measurement in °C or K) as cited herein can be obtained from the measurement of the shrinkage of the diameter of the ceramic ring by means of a micrometer (as disclosed by Ferro Corporation (Microsoft Word – PTCR Manual English Revised Edition 6_30.07.2019.docx (pxdental.com))) after the sintering cycle has ended.

[0054] Therefore, the heat Q cited in the previous paragraph is proportional to ΔT, i.e., Q ~ ΔT * t, (1) where ΔT is the firing temperature and t is the firing time. The shrinkage ΔL / L (which can be measured by a micrometer) is further proportional to the sintering temperature (T) and the sintering time (t), where L corresponds to the original diameter of the ring and ΔL is the change in diameter, i.e., ΔL / L ~ F(T) * t, (2) where the function F(T) includes the absolute temperature as well as material-specific and geometric factors. Over the entire operating range of the ring, the degree of contraction is almost linear. Combining equations (1) and (2) gives ΔL / L ~ Q.

[0055] Thus, the constriction ΔL / L, i.e., the amount of shrinkage of the ring diameter, is proportional to the total amount of heat Q absorbed and can be measured by a micrometer as further described in the Ferro guidelines cited above.

[0056] The present invention also relates to a sintered compact that can be obtained by the process described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 A schematic view of a crucible arranged in a sintering furnace according to the prior art is shown.

[0058] Figure 2a A side view of a crucible, a stack of sintering trays, and a schematic heat exchanger located at the outlet of the crucible ( Figure 6b ) is shown.

[0059] Figure 2b A side view of a crucible, a stack of sintering trays, and a heat exchanger located at the inlet is shown.

[0060] Figure 2c A cross-sectional view of two sintering trays with a gap therebetween is shown.

[0061] Figure 3 A side view of a crucible and heat exchangers located at the inlet and outlet is shown.

[0062] Figure 4a and 4b A different embodiment of the heat exchanger is shown.

[0063] Figure 5 Steps of a sintering cycle performed in a sintering furnace according to the present invention are generally illustrated.

[0064] Figure 6a and 6b A top view of an embodiment of the prior art is shown, in which a sintering tray ( Figure 6a ) and the top plate of the crucible with openings (inlet / outlet) are shown.

[0065] Figure 7 The final ring temperature of the PTCR (Process Temperature Control Ring) on the sintering tray after a mapping program (thermal cycle) performed in crucibles according to the prior art and the present invention is shown.

[0066] Figure 8 The ring temperature (heat value) after completion of the sintering cycle performed in crucibles according to the prior art and the present invention is shown.

[0067] Figure 9 is shown fromFigure 8 COM values obtained with the same sintering cycle as described in

[0068] Figure 10 Shows the final H Figure 8 obtained with the same sintering cycle as described in c value.

[0069] Figure 11 Shows the correlation curve of the ring temperature T (heat value) with the diameter of the PTCR (Process Temperature Control Ring). Detailed Description of the Invention

[0070] Figure 1 Shows a schematic view of the autoclave 9 according to the prior art, which has a bottom plate 13 and a top plate 14, and the bottom plate 13 and the top plate 14 limit the space of the autoclave 9 in the axial direction. Openings 7 (used as gas inlets) and openings 8 (used as gas outlets) are provided at both ends of the autoclave 9. The graphite element 20 is adhered to the top plate 14 and the opening 8. The sintering tray arranged in the autoclave 9 is not shown.

[0071] Figure 2a Shows a side view of the stacking of sintering trays (numbered 1-6) in the autoclave 9 of the sintering furnace 100 according to the present invention. The furnace atmosphere gas enters the autoclave 9 at the inlet 7 and leaves the autoclave 9 at the outlet 8 and via the schematically shown heat exchanger 12b. In the shown design, the diameters of the inlet 7 and the outlet 8 (also collectively referred to as openings hereinafter) are 14 mm. In Figure 2b the shown alternative embodiment, a reverse flow pattern is shown, where the opening 8 is used as the inlet and the opening 7 is used as the outlet. As Figure 2a and 2b shown, the flow pattern can be used in different sintering steps during the sintering cycle. Figure 2a and 2b The arrows in show the axial flow through the central opening 11 of the sintering trays 1-6. In the tests conducted, the diameter of the central opening 11 was 19 mm. The central opening is used for axial and radial flow through the gap 16 between adjacent sintering trays 1-6, and the gap 16 is enclosed by a peripheral wall 15 provided with holes 10 having a diameter of 5 mm. In the tests, 24 holes were used between each adjacent sintering tray. In Figure 2a and 2b , depending on the sintering step using the furnace atmosphere gas, the flow is radially outwardly directed to the outside of the gap 16 or radially inwardly towards the central opening 11. Of course, Figure 2a and 2b the inlets and outlets of the embodiment can be used as both inlets and outlets in certain sintering cycles, or always used as inlets and outlets respectively in other types of sintering cycles.

[0072] Figure 2c Figure 2 and Figure 3 show sintering trays 2 and 3 that form an adjacent pair of sintering trays, and each sintering tray is provided with a central opening 11. A peripheral wall 15 with a height of h encloses the gap 16 between the adjacent sintering trays 2 and 3. In this embodiment, the peripheral wall 15 arranged between the adjacent trays also functions as a spacer element. The shape of the peripheral wall 15 enclosing the circular sintering trays is a cylindrical shape. Additional peripheral walls 15 (not shown) enclose other adjacent pairs of sintering trays (not shown). In an alternative embodiment, a single peripheral wall 15 covers all pairs of adjacent sintering trays (not shown). In such an embodiment, additional spacer elements may be required to keep the sintering trays at a predetermined distance. Holes 10 ( Figure 2c only two holes are shown in the figure) are provided in the peripheral wall 15. According to the flow pattern, the furnace atmosphere gas can flow radially outward or radially inward through the holes 10, as Figure 2c indicated by the arrows in the figure. The distance d between the kettle wall 9a of the kettle 9 and the peripheral wall 15 allows the flow between the peripheral wall 15 and the kettle towards the outlet.

[0073] Figure 3 Figure 10 shows a bottom heat exchanger 12a and a top heat exchanger 12b, which are arranged to allow the furnace atmosphere gas to reach the stack of sintering trays 1 - 6 arranged between the heat exchangers 12a and 12b and to leave from the stack. The heat exchanger 12a allows the preheating of the furnace atmosphere gas entering the inlet 7, so that the temperature of the furnace atmosphere gas rises from room temperature before reaching the inlet of the kettle to a temperature as high as about 500 to 700 °C, depending on the flow rate of the furnace atmosphere gas and the temperature inside the kettle. The top heat exchanger 12b is used to recover the remaining heat in the gas leaving the kettle 9. If the furnace atmosphere gas is changed to enter through an opening 8 (used as an inlet, Figure 3 not shown in the figure) arranged at the top of the heat exchanger 12b and leave through an opening 7 (used as an outlet) arranged at the bottom heat exchanger 12a, then by preheating the incoming gas with the heat exchanger 12b and recovering the heat from the leaving gas with the heat exchanger 12a, the temperature in the kettle 9 will be homogenized in the same way.

[0074] Before starting the sintering cycle, the green compacts (not shown) are positioned on the sintering trays 1 - 6 in a conventional manner. The gap 16 between the adjacent sintering trays is enclosed by the peripheral wall 15 provided with uniformly distributed holes 10, and the furnace atmosphere gas existing inside the peripheral wall 15 can pass through these holes and through the enclosed gap 16. In Figure 3 the shown design, the height of the stack of sintering trays is H = 122 mm. The central openings 11 provided in the sintering trays 1 - 6 allow the incoming gas to flow axially from the inlet 7 towards the outlet 8 arranged on the top of the stack of sintering trays. InFigure 3 The opposite flow direction can also be applied, for example, as Figure 2b shown. Thus, both flow directions can be applied in the different steps when performing the sintering cycle.

[0075] Figure 4a and 4b illustrate the flow pattern of the furnace atmosphere gas through the corresponding bottom heat exchanger 12a ( Figure 4a ) and the top heat exchanger 12b ( Figure 4b ), as indicated by the arrows.

[0076] In Figure 4a , the opening 7 is the inlet, and the opening 8 is the outlet. If the furnace atmosphere gas enters via the opening 8 (inlet) and exits via the opening 7 (outlet) in another sintering step, the opposite flow direction will be adopted. The letters A, B, C, and D represent the elements of the channel design of the heat exchangers 12a and 12b. In all embodiments of these working examples, the heat exchanger 12a used has a width (radial extension) of 150 mm and a height of 38 mm. The diameter of the inlet is 14 mm. The heat exchanger 12a consists of channels suitable for radial gas flow, and the total heat exchange volume is 0.672 dm 3 . The width of the heat exchanger 12b is 165 mm, and the height is 36 mm. The heat exchanger 12b with a volume of 0.77 dm 3 also consists of channels for gas flow.

[0077] Figure 5 Generally shows a typical sintering cycle performed in the sintering furnace 100 according to the present invention, including the following steps: 1. Vacuum and pressure vessel testing; 2. Filling the autoclave with argon; 3. Debinding pressure regulation controlled by the combustion tower; 4. Converting from filling the autoclave with argon to flushing the autoclave with hydrogen; 5. Debinding by filling the autoclave from the top heat exchanger, which preheats the incoming gas and then conveys the furnace atmosphere gas to the stack of sintering trays; 6. Vacuum heating; 7. Filling with N 2 to the debinding pressure (overpressure) to perform the nitriding step. Nitrogen enters the autoclave from the top heat exchanger to equalize the temperature, whereby the gas is heated, and then the gas is conveyed to the stack of sintering trays for solid-state reaction; 8. N 2Enter the autoclave via the bottom heat exchanger (at reduced pressure - partial pressure) and flow from tray 1 through the stack of sintering trays to tray 6. Thus, the gas flows in the opposite direction compared to step 7. The bottom heat exchanger equalizes the temperature before the nitrogen reaches the stack of sintering trays; 9. Vacuum heating; 10. Heating to the sintering temperature with the furnace atmosphere gas; 11. Sintering; 12. Cooling (by free cooling or by gradual cooling).

[0078] Figure 6a is an embodiment of the prior art, showing a sintering tray on which conventional spacer elements 21 are stacked to provide a gap 16 between adjacent sintering trays. A PTCR (Process Temperature Control Ring) 22 is also positioned on the sintering tray. Figure 6b Shows the top plate with an opening 8 which serves as the inlet / outlet for the gas.

[0079] Figure 7 Shows the loop temperatures of the individual sintering trays measured after completion of the mapping, which is carried out by running a thermal cycle with the furnace atmosphere gas passing through under i) vacuum and ii) at reduced pressure respectively.

[0080] In Figures 7 to 10 ,"Original Hardware" represents the design according to Figure 1 and Figure 6a and 6b (reference example), and "New Hardware" represents the design according to Figure 3 (the present invention).

[0081] Figure 8 Shows the loop temperatures measured after completion of the entire sintering cycle (25-step cycle).

[0082] Example

[0083] The sintering cycle according to Table 1 was carried out in a sintering furnace COV 131 R (which can be commercially obtained from PVA TePla) to produce a functional grade material (cemented carbide) from an initial RTP (Ready to Press) powder, with a nominal composition of 5.3 wt% Co and 2.2 wt% Ta / Nb / Ti (weight ratio 3.4 / 2.0 / 2.1 in terms of the binder phase content), and the rest being tungsten carbide (WC). In all examples, the diameter of the sintering trays was 150 mm.

[0084] In the furnace being tested, the design of the kettle 9 and the arrangement of the sintering trays are as shown in Figure 1 , Figure 6a and 6b (reference example) and Figure 3 (the present invention).

[0085] In the design of the reference example, six sintering trays are stacked in the kettle 9, and these sintering trays are separated by separating elements, as shown in Figure 6a . As shown in Figure 6b , the bottom plate 13 and the top plate 14 limit the kettle in the axial direction. As is clear from Figure 3 , in the embodiment according to the present invention, the same number of sintering trays are arranged, and heat exchangers 12a and 12b are arranged at the inlet and the outlet. Instead of the spacer elements in the reference example, a peripheral ring 15 with holes 10 is stacked between these sintering trays, which serves as an enclosure wall and as a spacer element.

[0086] In both the reference embodiment and the embodiment of the present invention, the thermocouples are arranged outside the kettle 9 and near the sintering trays 3 and 4.

[0087] Functional test

[0088] Before running the sintering cycle in Table 1, a functional test consisting of two tests is carried out in the furnace: i) Leakage rate test; ii) Overpressure test.

[0089] In the leakage rate test i), the pressure in the furnace is evacuated to 10⁻² to 10⁻³ mbar for 30 minutes. Subsequently, the pumping unit for the furnace atmosphere gas is closed, and all valves for controlling the gas flow in the furnace are closed to monitor the increase in pressure over time. The estimated leakage rate is 2.3×10⁻ 4 mbar·liter / second.

[0090] The overpressure test ii) is carried out as follows: The system is filled with argon to 1200 mbar, and then all valves are closed. The monitored pressure drop is less than 5 mbar / hour. Table 1 – Sintering cycle

[0091] The steps of the sintering cycle in Table 1 are further described below:

[0092] Steps 1 to 2 :

[0093] First, the autoclave is filled with argon, which is done before step 1 of the sintering cycle. After the set point of 1020 mbar has been reached, the argon filling is replaced with hydrogen.

[0094] Steps 3 to 7 :

[0095] Debinding of the green compacts positioned on the sintering trays in the autoclave is carried out.

[0096] Steps 8 to 11 :

[0097] The debound green compacts are heated in a vacuum.

[0098] Step 12 :

[0099] The green compacts are subjected to solid state nitriding to increase the nitrogen content in the cemented carbide green compacts.

[0100] Steps 13 to 16 :

[0101] A nitrogen back pressure is applied to avoid nitrogen loss before reaching liquid phase sintering.

[0102] Steps 17 to 18 :

[0103] Heating steps are carried out in which the green compacts start to reach the liquid phase sintering temperature at which further shrinkage occurs.

[0104] Steps 19 to 21 :

[0105] A sintering atmosphere (Ar / CO) is added to avoid evaporation of cobalt in the cemented carbide.

[0106] Steps 22 to 25 :

[0107] Cooling step 22 - 25 involves free cooling.

[0108] Example 1

[0109] Before starting the sintering cycle according to Table 1, the temperature inside the autoclave of the reference example is mapped. Shrinkage PTCR rings (commercially available from Ferro Corporation, batch number 332) are positioned on 6 sintering trays, with 3 rings evenly distributed on each sintering tray (placed at 120° intervals, similar to Figure 6a the prior art embodiment shown, but with only 3 rings per tray).

[0110] According to the standardized method of PTCR supplier Ferro Corporation, the mapping is carried out according to the following thermal cycle: i) Vacuum treatment was carried out at 10⁻¹ to 10⁻² mbar and heated from room temperature to the sintering temperature of 1470°C at a rate of 3°C / minute in a gradually increasing temperature manner; ii) Immersion (holding time) for 90 minutes at the same pressure; iii) Free cooling in vacuum from 1470°C to 1200°C at the same pressure; iv) Free cooling with argon at 600 mbar until room temperature was reached.

[0111] In the autoclave design of the reference example, reference points (zero points) were obtained by mapping using PTCR.

[0112] The results of Example 1 (reference example) are presented in Table 2 and Figure 7 in.

[0113] Table 2 shows the individual loop temperatures and the average loop temperature of the PTCR loops located at three positions (spaced 120° apart from each other) on trays 1 - 6.

[0114] The average loop temperature calculated based on the average loop temperatures of trays numbered 1 - 6 was 1431, which differed by 4 degrees from the target temperature (desired temperature) of 1435. Therefore, a deviation of 4 degrees from the target was recorded, which was still within the supplier's specification limits (±7K). The recorded gradient was ΔT = 11, which corresponded to the maximum difference between the highest loop temperature and the lowest loop temperature during the test (1425 and 1436 respectively, as is evident from Table 2). Table 2 T* represents the loop temperature of the loops (these loops are T1, T2, T3). 0°* represents the position of T1 relative to the other loops (for detecting changes around the tray).

[0115] The ring temperature (RT) is retrieved by measuring the diameter of the PTCR (Process Temperature Control Ring) at room temperature using a micrometer after the sintering cycle has terminated, according to the guidelines of the supplier Ferro Corporation (https: / / www.ferro.com / - / media / files / resources / industrial-specialty-materials / technical / ferro-industrial-specialty-materials-process-temperature-control-rings-ptcr.pdf?la=en&hash=8FD87135E2C3363D07C57D799F8674C6A9405040, (Microsoft Word – PTCR Manual English Revised Edition 6_30.07.2019.docx (pxdental.com))). The ring temperature is retrieved by correlating the measured diameter with the ring temperature (RT), as shown by the ring temperature (RT)-diameter curve in Figure 11 as shown.

[0116] Example 2

[0117] Using Figure 3 the autoclave design (of the present invention), the thermal cycle performed in Example 1 was repeated. The results of Example 2 are shown in Table 3 and Figure 7 as shown. In Figure 7 although both Example 1 (reference example) and Example 2 (the present invention) were carried out under vacuum treatment, it can be noted that there are significant differences in the temperature distribution between Example 1 and Example 2. Thus, the results show that the design according to the present invention also does not allow the furnace atmosphere gas to pass through the heat exchanger (see Figure 3 ), but results in a more uniform temperature distribution. Table 3

[0118] Example 3

[0119] The thermal cycle of Example 3 was carried out as in Example 1 (reference design), with the difference that during the soaking step ii), 1 standard liter per minute of an inert gas (usually Ar, possibly CO) flowed through the autoclave at a pressure of 40 mbar, which led to a decrease in temperature uniformity, especially at the sintering tray 1 near the gas inlet. The results of Example 3 are shown in Table 4 and Figure 7 as shown. Table 4

[0120] Example 4

[0121] The thermal cycle of Example 4 is carried out according to Example 3 and using Figure 3 the design (of the present invention). The results are presented in Table 5 and Figure 7 in. As can be seen in Figure 7 it, in the steps carried out at a reduced pressure (partial pressure), significantly smaller temperature differences can be noted for Example 4 according to the present invention compared to Example 3 (reference example), especially at tray 1 near the gas inlet and the heat exchanger. Thus, the results show that the heat exchanger contributes to a more uniform temperature distribution (especially at trays 1 and 2). Table 5

[0122] Example 5

[0123] Using the autoclave design of the reference example ( Figure 1 ) and the design of the present invention ( Figure 3 ), the sintering cycle according to Table 1 is run. It can be seen that, when running the sintering cycle, the design of the present invention is superior to the design of the reference example. The results using the design of the reference example and the results using the design of the present invention are shown in Table 6a (reference example), Table 6b (present invention) and Figure 8 in. It can be noted that the design of the present invention is superior to the reference example in terms of its uniform temperature distribution. Table 6a (Reference example) Table 6b (This invention)

[0124] Reference example (Table 6a) This invention (Table 6b)

[0125] By sintering the green compacts (cemented carbides), the design of the present invention has more uniform metallurgical properties compared to the design of the reference example in terms of magnetic properties such as coercive force (H c ).

[0126] Table 7a shows the H for the reference example cand the results of Com, and Table 7b shows the corresponding results of the present invention. It can be concluded that the design according to the present invention improves the uniformity of metallurgical properties over the entire sintering cycle (including the nitriding step) compared to the reference example, as Figure 9 and Figure 10 further shown in Table 7a (Reference example) Table 7b (This invention)

Claims

1. A sintering furnace (100) for sintering green compacts, the sintering furnace (100) comprising a kettle (9); and: i) An inlet for furnace atmosphere gas, the inlet being located at one end of the kettle (9); ii) A stack of sintering trays (1, 2, 3, 4, 5, 6), the stack being arranged inside the kettle (9), the sintering trays comprising a central opening (11), the central opening (11) having an area A for transmitting the furnace atmosphere gas introduced through the inlet, wherein adjacent sintering trays in the stack are spaced apart from each other by a predetermined distance, thereby providing a gap (16) between the adjacent sintering trays, the gap (16) being enclosed by a peripheral wall (15), the peripheral wall (15) being provided with distributed holes (10) for transmitting the furnace atmosphere gas between the gap (16) inside the peripheral wall (15) and the space outside the peripheral wall (15), wherein the ratio between the area A of each individual central opening (11) and the total area of the distributed holes (10) between each individual pair of adjacent sintering trays is in the range of 1:10 to 10:1; iii) An outlet for extracting the furnace atmosphere gas, the outlet being located at the end of the kettle (9) opposite to the end where the inlet is located, the inlet and the outlet being arranged at opposite ends of the stack of sintering trays (1, 2, 3, 4, 5, 6).

2. The sintering furnace (100) according to claim 1, wherein, A heat exchanger (12a, 12b) for heating the furnace atmosphere gas entering the kettle (9) i) Is arranged inside the kettle (9) and at the inlet such that the furnace atmosphere gas is guided to the heat exchanger (12a, 12b); or ii) Forms an integral part of the end of the kettle (9) where the inlet is arranged.

3. The sintering furnace (100) according to claim 1 or 2, wherein a heat exchanger (12a, 12b) for recovering the heat of the furnace atmosphere gas extracted from the kettle (9): i) Is arranged inside the kettle (9) and at the outlet such that the furnace atmosphere gas is guided to the outlet; or ii) Forms an integral part of the end of the kettle (9) where the outlet is arranged.

4. The sintering furnace (100) according to claim 2 or 3, wherein, The heat exchanger (12a, 12b) is arranged at the inlet and the outlet of the kettle (9).

5. The sintering furnace (100) according to any one of claims 1 to 4, wherein, The stack of sintering trays (1, 2, 3, 4, 5, 6) comprises 4 to 100 sintering trays.

6. The sintering furnace (100) according to any one of claims 1 to 5, wherein, The distance h between adjacent sintering trays is in the range of 5 to 200 mm.

7. The sintering furnace (100) according to any one of claims 1 to 6, wherein, The area of each individual central opening (11) of the sintering tray is in the range of 100 to 100000 mm².

8. The sintering furnace (100) according to any one of claims 1 to 7, wherein, at least one heating element is arranged outside the kettle (9).

9. The sintering furnace (100) according to any one of claims 1 to 8, wherein, no electrically heated element is arranged inside the kettle (9).

10. The sintering furnace (100) according to any one of claims 1 to 9, wherein, the holes (10) are distributed substantially uniformly.

11. The sintering furnace (100) according to any one of claims 1 to 10, wherein, the inlet and the outlet are arranged at opposite ends of the kettle (9) and at a position where the axes extending through all the sintering trays intersect.

12. The sintering furnace (100) according to any one of claims 1 to 11, wherein, heat exchangers (12a, 12b) are arranged inside the kettle (9), wherein the inlet and the outlet form an integral part of the heat exchangers (12a, 12b).

13. A method for sintering a green compact in a sintering furnace (100) according to any one of claims 1 to 12, the method comprising performing a sintering cycle, the sintering cycle including at least one process step during which an in-furnace atmosphere gas is supplied to the inlet of the kettle (9) at a pressure in the range of 0.001 to 10 mbar.

14. A sintered compact obtainable by the method according to claim 13.

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