High temperature and high pressure synthesis device and method for diamond carbide composite sheet

Abstract

The present invention relates to a high-temperature and high-pressure synthesis device and method for diamond carbide composite sheets, comprising an outer salt tube, an inner salt tube and pyrophyllite, the pyrophyllite having a through hole for fitting the outer salt tube and a conductive steel ring, the outer salt tube being installed therein, the outer salt tube being equipped with a carbon tube, the carbon tube being equipped with the inner salt tube, the metal cup being placed in the inner salt tube, the upper and lower ports being encapsulated with salt sheets, the upper and lower ends of the through hole of the pyrophyllite being loaded with carbon sheets and a conductive steel ring, the carbon sheets being fitted with the outer salt tube, the carbon tube, the inner salt tube and the salt sheets. When used, the carbon tube heats the inner salt tube and the outer salt tube at the same time, and simultaneously generates two isostatic pressure areas formed by molten salt, namely, an inner isostatic pressure tank and an outer isostatic pressure tank, the inner isostatic pressure tank provides a hot isostatic pressure environment with equal pressure in all directions suitable for sintering of diamond carbide composite sheets, and the outer isostatic pressure tank provides a pressure environment with uniform force on the carbon tubes therein, and the carbon tubes are balanced in force; the sintering quality of diamond carbide composite sheets is significantly improved.

Description

Technical Field

[0001] The invention relates to a synthesis device and method for a diamond carbide composite sheet, and in particular to a high-temperature and high-pressure synthesis device and a synthesis process under double-cell hot isostatic pressing conditions. Background Art

[0002] Diamond carbide composite sheets are sintered from diamond powder and carbide base under high temperature and high pressure conditions. They have the high hardness and wear resistance of diamond and the impact resistance of carbide. They are used to manufacture diamond drill bits and ultimately used in the exploration and exploitation of oil, natural gas and shale gas.

[0003] At present, the production process of diamond carbide composite sheets is as follows: ① acid and alkali purification of diamond powder; ② sandblasting the surface of the carbide base to remove impurities; ③ loading the purified carbide base and diamond powder into a high melting point metal cup; ④ placing the aforementioned metal cup loaded with carbide base and diamond powder into a vacuum furnace for high temperature purification; ⑤ buckling the aforementioned metal cup after purification and loading the synthetic block. ⑥ placing the synthetic block into a six-sided top press for high temperature and high pressure sintering; ⑦ finally processing the sintered diamond carbide composite sheet into a product that meets commercial requirements. When the six-sided top press is working, 60-100MPa high-pressure oil is injected into the oil cylinder with a diameter of 500-850mm, pushing the six pistons to move toward the center. When the front top hammer contacts the six faces of the synthetic block (hexahedron), the synthetic block is squeezed. The high pressure required for synthesis is transmitted through pyrophyllite, and the heating current is transmitted to the carbon sheet and carbon tube through the upper and lower conductive steel rings to make them hot, and the salt and high-melting-point metal cup in the carbon tube are heated to 1400-1500℃. The salt melts at this high temperature and changes from solid phase to liquid phase, forming an isostatic pressure transmission environment, so that the diamond micropowder and cemented carbide base wrapped in the high-melting-point metal cup inside are sintered in a hot isostatic pressing environment with a temperature of 1400-1500℃ and a pressure of 6-7GPa. Since the external pressure transmitted through talc is very high, the carbon sheets and carbon tubes that are responsible for heating will be deformed, causing the heating field to be distorted and deformed, affecting the spatial uniformity of the thermal field, and then affecting the heating uniformity of the metal cup, and ultimately causing the diamond carbide composite sheet in the cup to be heated unevenly during the sintering process, reducing its sintering quality. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a high temperature and high pressure synthesis device and method for a diamond carbide composite sheet, aiming to improve the uniformity of high temperature heating of the diamond carbide composite sheet.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] The synthesis device of diamond carbide composite sheet is characterized by comprising an outer salt tube, an inner salt tube and pyrophyllite, the pyrophyllite having a through hole for fitting the outer salt tube and the conductive steel ring, the outer salt tube is installed in the pyrophyllite, the outer salt tube is fitted with a carbon tube in the outer salt tube, the carbon tube is fitted with the inner salt tube in the carbon tube, a metal cup is placed in the inner salt tube, the upper and lower ports are encapsulated with salt sheets, the upper and lower ends of the through hole of the pyrophyllite are then loaded with carbon sheets and the conductive steel ring, the carbon sheets are fitted with the outer salt tube, the carbon tube, the inner salt tube and the salt sheets.

[0007] Furthermore, in the synthesis device of the diamond carbide composite sheet, the inner salt tube uses high-purity salt with a NaCl purity higher than 99.5%.

[0008] Furthermore, in the synthesis device of the diamond carbide composite sheet, the inner salt tube is formed by cold isostatic high pressure molding of high purity salt with a NaCl purity higher than 99.5%, and the density of the inner salt tube is higher than 2.15 g / cm 3 .

[0009] Furthermore, in the above-mentioned synthesis device of diamond carbide composite sheets, the radial thickness of the inner salt tube is 2 to 5 mm.

[0010] Furthermore, in the synthesis device of the diamond carbide composite sheet, the outer salt tube is formed by pressing NaCl and thermal insulation powder material.

[0011] Furthermore, in the above-mentioned synthesis device of diamond carbide composite sheet, the mass ratio of the thermal insulation powder material is 5-30%.

[0012] Furthermore, in the above-mentioned synthesis device of diamond carbide composite sheet, the thermal insulation powder material is Si3N4, CrO2 or Al2O3.

[0013] Furthermore, in the synthesis device of the diamond carbide composite sheet, the radial thickness of the outer salt tube is 2 to 5 mm.

[0014] The method for synthesizing a diamond carbide composite sheet of the present invention comprises the following steps:

[0015] ① Treat diamond powder with acid and alkali for purification;

[0016] ②Sandblasting the surface of the carbide base to remove impurities

[0017] ③Put the purified carbide base and diamond powder into the metal cup;

[0018] ④ The metal cup equipped with the cemented carbide base and diamond powder is placed in a vacuum furnace for high-temperature purification;

[0019] ⑤ Put the cover cup on the purified metal cup, insert the outer salt tube into the hole of pyrophyllite, and then install the carbon tube, inner salt tube and metal cup in sequence, encapsulate the salt sheet, and put the carbon sheet and conductive steel ring at the upper and lower ends of the pyrophyllite hole;

[0020] ⑥After assembly, put it into the six-sided top press for high temperature and high pressure sintering. The sintering process is:

[0021] First, slowly pressurize to 2-4 GPa;

[0022] Then, the heating is started, and the current is introduced into the carbon sheet and carbon tube through the upper and lower conductive steel rings, causing them to heat up and generate a high temperature of 1400-1500℃;

[0023] The carbon tube heats the inner salt tube and the outer salt tube at the same time. The inner and outer salt tubes melt and generate two isostatic pressure areas formed by molten salt, forming two inner and outer hot isostatic pressure pools, namely the inner isostatic pressure pool and the outer isostatic pressure pool. The inner isostatic pressure pool provides a hot isostatic pressure environment with equal pressure in all directions suitable for sintering of diamond carbide composite sheets. The outer isostatic pressure pool provides a pressure environment with uniform force for the carbon tube inside. The carbon tube is balanced in force and has small deformation, so that the inner isostatic pressure pool produces a more balanced high temperature and pressure environment. It also provides heat preservation for the carbon tube and the space inside to reduce heat loss.

[0024] Finally, the pressure is continued to be increased to 6-7 GPa while maintaining the high temperature to sinter the diamond and cemented carbide in the high melting point metal cup;

[0025] ⑦ Release the pressure, cool down to normal pressure and room temperature, and take out the sintered diamond carbide composite sheet.

[0026] Compared with the prior art, the present invention has significant advantages and beneficial effects, which are specifically embodied in the following aspects:

[0027] The present invention provides an inner salt tube on the inner side of the carbon tube and an outer salt tube on the outer side. When current passes through the carbon tube, the carbon tube generates heat and heats the inner salt tube and the outer salt tube at the same time, and generates two isostatic pressing areas formed by molten salt, namely, an inner isostatic pressing pool and an outer isostatic pressing pool. The inner isostatic pressing pool provides a hot isostatic pressing environment with equal pressure in all directions suitable for sintering diamond carbide composite sheets; the outer isostatic pressing pool provides a pressure environment with uniform force for the carbon tubes therein, greatly reduces the geometric deformation of the carbon tubes, thereby improving the uniformity of heating in the inner isostatic pressing pool, and finally significantly improves the sintering quality of the diamond carbide composite sheets; the outer salt tube also provides heat preservation for the heat-generating carbon tube and the inner space, reduces heat loss, and reduces heating energy consumption.

[0028] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or be understood by practicing the specific embodiments of the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 : A schematic diagram of the cross-sectional structure of the device of the present invention;

[0031] Figure 2 : Microstructure photo of the composite sheet of the comparative example;

[0032] Figure 3 : Microstructure photograph of the composite sheet of Example 1. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

[0034] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. At the same time, in the description of the present invention, directional terms and order terms are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0035] like Figure 1As shown, the synthesis device of the diamond carbide composite sheet comprises an outer salt tube 8, an inner salt tube 6 and pyrophyllite 1. The pyrophyllite 1 has a through hole for fitting the outer salt tube 8 and the conductive steel ring 2. The outer salt tube 8 is installed therein. The carbon tube 5 is installed in the outer salt tube 8. The inner salt tube 6 is installed in the carbon tube 5. The metal cup 7 is placed in the inner salt tube 6. The upper and lower ports are encapsulated with salt sheets 4. The carbon sheet 3 and the conductive steel ring 2 are then loaded into the upper and lower ends of the through hole of the pyrophyllite 1. The carbon sheet 3 fits the outer salt tube 8, the carbon tube 5, the inner salt tube 6 and the salt sheet 4.

[0036] The inner salt tube 6 is formed by cold isostatic high pressure molding of high purity salt with a NaCl purity higher than 99.5%, and the density of the inner salt tube is higher than 2.15 g / cm 3 The radial thickness of the inner salt tube 6 is 2 to 5 mm, ensuring that the liquid salt pool formed after the salt melts fully wraps the high melting point metal cup therein.

[0037] The outer salt tube 8 is formed by pressing NaCl and heat-insulating powder material, the mass ratio of the heat-insulating powder material is 5-30%, the heat-insulating powder material is Si3N4, CrO2 or Al2O3, and the radial thickness of the outer salt tube 8 is 2-5 mm, so that the liquid salt pool formed after the outer salt tube is melted can greatly reduce the geometric deformation of the carbon tube inside. The outer salt tube has good heat insulation performance and can reduce heating energy consumption by 5-10%.

[0038] The synthesis process steps of diamond carbide composite sheet are as follows:

[0039] ① Treat diamond powder with acid and alkali for purification;

[0040] ②Sandblasting the surface of the carbide base to remove impurities

[0041] ③Put the purified carbide base and diamond powder into the metal cup;

[0042] ④ The metal cup equipped with the cemented carbide base and diamond powder is placed in a vacuum furnace for high-temperature purification;

[0043] ⑤ Put the cover cup on the purified metal cup, insert the outer salt tube 8 into the hole of pyrophyllite 1, and then install the carbon tube 5, inner salt tube 6 and metal cup 7 in sequence, encapsulate the salt sheet 4, and put the carbon sheet 3 and conductive steel ring 2 at the upper and lower ends of the hole of pyrophyllite 1;

[0044] ⑥After assembly, put it into the six-sided top press for high temperature and high pressure sintering. The sintering process is:

[0045] First, slowly pressurize to 2-4 GPa;

[0046] Then, the heating is started, and the current is introduced into the carbon sheet 3 and the carbon tube 5 through the upper and lower conductive steel rings 2, causing them to heat up and generate a high temperature of 1400-1500°C;

[0047] The carbon tube 5 heats the inner salt tube 6 and the outer salt tube 8 at the same time, and the inner and outer salt tubes melt and simultaneously generate two isostatic pressure areas formed by molten salt, forming two inner and outer hot isostatic pressure pools, namely the inner isostatic pressure pool and the outer isostatic pressure pool. The inner isostatic pressure pool provides a hot isostatic pressure environment with equal pressure in all directions suitable for sintering of diamond carbide composite sheets, and the outer isostatic pressure pool provides a pressure environment with uniform force for the carbon tubes therein. The carbon tube 5 is subjected to balanced force and has small deformation, so that the inner isostatic pressure pool generates a more balanced high temperature and pressure environment; it also provides heat preservation for the carbon tube 5 and the space inside, reducing heat loss;

[0048] Finally, the pressure is continuously increased to 6-7 GPa while maintaining the high temperature to sinter the diamond and cemented carbide in the high melting point metal cup 7;

[0049] ⑦ Release the pressure, cool down to normal pressure and room temperature, and take out the sintered diamond carbide composite sheet.

[0050] Comparative Example

[0051] The high temperature and high pressure sintering process of a single salt tube is adopted. A high melting point metal cup filled with diamond powder and cemented carbide is placed in the salt tube, and then a heating carbon tube is placed on the outer jacket of the inner salt tube, and carbon sheets, conductive steel rings and pyrophyllite blocks for pressure transmission and sealing are placed above and below. Sintering is carried out at 1500℃ and 6.5GPa for 300 seconds, and then the heat is stopped, the temperature is lowered and the pressure is slowly reduced to room temperature and normal pressure. The sintered body is taken out and then mechanically processed and shaped to meet the commercial requirements, and finally a diamond cemented carbide composite sheet is obtained.

[0052] like Figure 2 The results show the inhomogeneity of the microstructure of the composite sheet. On the one hand, it shows that more catalyst Co is concentrated in the high temperature area during sintering. On the other hand, it shows that abnormal WC grains grow abnormally near the interface between cemented carbide and diamond. The uneven temperature causes the segregation of multiple components, which reduces the impact resistance and heat resistance of the diamond cemented carbide composite sheet.

[0053] Example 1

[0054] The double salt tube structure of the present invention is adopted, the inner salt tube is 5 mm thick, high-purity salt is used, the purity of NaCl is not less than 99.5%, and the density of the salt tube is 2.15 g / cm3; the outer salt tube is 2 mm thick, and its components are 85% NaCl and 15% Cr2O3. The carbon tube is energized and heated. After reaching a high temperature, the inner salt tube and the outer salt tube melt to form an inner isostatic pressure tank and an outer isostatic pressure tank respectively; the outer isostatic pressure tank effectively controls the deformation of the heated carbon tube, thereby making it heat evenly, and the high-melting-point metal cup is evenly heated through the inner isostatic pressure tank, so that the organizational structure of the diamond carbide composite sheet obtained after high-temperature and high-pressure sintering is uniform, such as Figure 3As shown, there is no segregation of catalyst Co, no abnormal growth of WC grains, the performance is improved, and the static pressure strength of the composite sheet reaches Q2. As shown in Table 1, the heating power used is 6700W, which is 5% lower than the heating power used for the previous single carbon tube.

[0055] Example 2

[0056] The double salt tube structure described in the present invention is adopted, the inner salt tube is 3.5mm thick, high-purity salt is used, the purity of NaCl is not less than 99.5%, and the salt tube density is 2.15g / cm3; the outer salt tube is 4mm thick, and its components are 90% NaCl and 10% AL2O3. The carbon tube is energized to generate heat, and after reaching a high temperature, the inner salt tube and the outer salt tube melt to form an inner isostatic pressure tank and an outer isostatic pressure tank respectively. The outer isostatic pressure tank effectively controls the deformation of the heated carbon tube, thereby making it heat evenly, and the high melting point metal cup is evenly heated through the inner isostatic pressure tank, so that the diamond carbide composite sheet obtained after high temperature and high pressure sintering has a uniform structure, no segregation of catalyst Co, no abnormal growth of WC grains, and improved performance. The static pressure strength of the composite sheet reaches Q3, as shown in Table 1, and the heating power used is 6560W, which is 7% lower than the heating power used by the previous single carbon tube.

[0057] Example 3

[0058] The double salt tube structure described in the present invention is adopted, the inner salt tube is 2.5mm thick, high-purity salt is used, the purity of NaCl is not less than 99.5%, and the salt tube density is 2.15g / cm3; the outer salt tube is 4.8mm thick, and its components are 92% NaCl and 8% Si3N4. The carbon tube is energized to generate heat, and after reaching a high temperature, the inner salt tube and the outer salt tube melt to form an inner isostatic pressure tank and an outer isostatic pressure tank respectively. The outer isostatic pressure tank effectively controls the deformation of the heated carbon tube, thereby making it heat evenly, and the high melting point metal cup is evenly heated through the inner isostatic pressure tank, so that the diamond carbide composite sheet obtained after high temperature and high pressure sintering has a uniform structure, no segregation of catalyst Co, no abnormal growth of WC grains, and improved performance. The static pressure strength of the composite sheet reaches Q4, as shown in Table 1, and the heating power used is 6350W, which is 10% lower than the heating power used by the previous single carbon tube.

[0059] The magnitude of the force applied when the composite sheet is destroyed under the same conditions is shown in Table 1. The larger the value, the higher the strength.

[0060] Table 1

[0061]

[0062] In summary, the present invention is provided with an inner salt tube on the inner side of the carbon tube and an outer salt tube on the outer side. When current passes through the carbon tube, the carbon tube generates heat and heats the inner salt tube and the outer salt tube at the same time, and generates two isostatic pressing areas formed by molten salt, namely the inner isostatic pressing pool and the outer isostatic pressing pool. The inner isostatic pressing pool provides a hot isostatic pressing environment with equal pressure in all directions suitable for sintering of diamond carbide composite sheets; the outer isostatic pressing pool provides a pressure environment with uniform force for the carbon tubes therein, greatly reducing the geometric deformation of the carbon tubes, thereby improving the uniformity of heating in the inner isostatic pressing pool, and ultimately improving the sintering quality of the diamond carbide composite sheets; the outer salt tube also provides insulation for the heat-generating carbon tubes and the space therein, reducing heat loss and reducing heating energy consumption.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention. It should be noted that similar numbers and letters represent similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings.

[0064] The above is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

[0065] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

Claims

1. A method for synthesizing a diamond carbide composite sheet, characterized in that: A synthesis device for a diamond carbide composite sheet comprises an outer salt tube (8), an inner salt tube (6) and pyrophyllite (1), wherein the pyrophyllite (1) has a through hole for fitting the outer salt tube (8) and a conductive steel ring (2), the outer salt tube (8) is installed in the pyrophyllite, a carbon tube (5) is installed in the outer salt tube (8), the inner salt tube (6) is installed in the carbon tube (5), a metal cup (7) is placed in the inner salt tube (6), the upper and lower ports are encapsulated with a salt sheet (4), the upper and lower ends of the through hole of the pyrophyllite (1) are then filled with a carbon sheet (3) and a conductive steel ring (2), and the carbon sheet (3) is fitted with the outer salt tube (8), the carbon tube (5), the inner salt tube (6) and the salt sheet (4); the steps include: ① Treat diamond powder with acid and alkali for purification; ②Sandblasting the surface of the carbide base to remove impurities ③Put the purified carbide base and diamond powder into the metal cup; ④ The metal cup equipped with the cemented carbide base and diamond powder is placed in a vacuum furnace for high-temperature purification; ⑤ Put the cover cup on the purified metal cup, insert the outer salt tube (8) into the hole of the pyrophyllite (1), and then install the carbon tube (5), the inner salt tube (6) and the metal cup (7) in sequence, encapsulate the salt sheet (4), and put the carbon sheet (3) and the conductive steel ring (2) at the upper and lower ends of the hole of the pyrophyllite (1); ⑥After assembly, put it into the six-sided top press for high temperature and high pressure sintering. The sintering process is: First, slowly pressurize to 2-4 GPa; Then, the heating is started, and the current is introduced into the carbon sheet (3) and the carbon tube (5) through the upper and lower conductive steel rings (2), causing them to heat up and generate 1400-1500 o C high temperature; The carbon tube (5) heats the inner salt tube (6) and the outer salt tube (8) at the same time. The inner and outer salt tubes melt and simultaneously generate two isostatic pressure areas formed by molten salt, forming two inner and outer hot isostatic pressure pools, namely the inner isostatic pressure pool and the outer isostatic pressure pool. The inner isostatic pressure pool provides a hot isostatic pressure environment with equal pressure in all directions suitable for sintering the diamond carbide composite sheet. The outer isostatic pressure pool provides a pressure environment with uniform force for the carbon tubes therein. The carbon tubes (5) are subjected to balanced force and have little deformation, so that the inner isostatic pressure pool generates a more balanced high temperature and pressure environment. It also provides a heat preservation effect for the carbon tubes (5) and the space therein, thereby reducing heat loss. Finally, the pressure is continuously increased to 6-7 GPa while maintaining the high temperature to sinter the diamond and cemented carbide in the high melting point metal cup (7); ⑦ Release the pressure, cool down to normal pressure and room temperature, and take out the sintered diamond carbide composite sheet.

2. The method for synthesizing a diamond carbide composite sheet according to claim 1, characterized in that: The inner salt pipe (6) uses high-purity salt with a NaCl purity higher than 99.5%.

3. The method for synthesizing a diamond carbide composite sheet according to claim 2, characterized in that: The inner salt tube (6) is formed by cold isostatic high pressure molding of high purity salt with a NaCl purity higher than 99.5%, and the density of the inner salt tube is higher than 2.15 g / cm 3 .

4. The method for synthesizing a diamond carbide composite sheet according to claim 1 or 2, characterized in that: The radial thickness of the inner salt tube (6) is 2 to 5 mm.

5. The method for synthesizing a diamond carbide composite sheet according to claim 1, characterized in that: The outer salt pipe (8) is formed by pressing NaCl and thermal insulation powder material.

6. The method for synthesizing a diamond carbide composite sheet according to claim 5, characterized in that: The mass ratio of the thermal insulation powder material is 5 to 30%.

7. The method for synthesizing a diamond carbide composite sheet according to claim 5 or 6, characterized in that: The thermal insulation powder material is Si3N4, CrO2 or Al2O3.

8. The method for synthesizing a diamond carbide composite sheet according to claim 1 or 5, characterized in that: The radial thickness of the outer salt pipe (8) is 2 to 5 mm.

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