Sintered hard alloys, crushing tools, kneading tools, wear-resistant tools and dies

Abstract

A sintered hard alloy comprising a hard phase mainly composed of NbC, a first hard phase made of a carbide containing at least one of Groups 4 to 6 elements of the periodic table (however, Nb is an essential component), a second hard phase made of a boride containing at least one of Groups 4 to 6 elements of the periodic table, and a binder phase containing at least one of Ni, Co and Fe, the hard phase containing at least one of Groups 4 to 6 elements of the periodic table (excluding Nb contained in the first hard phase) in an amount of 3 to 30 volume % based on the total amount of the hard phase in terms of carbide, at least one of Ni, Co and Fe in terms of 4 to 40 volume % as a binder phase component, and 0.3 to 8 mass % of B in terms of the binder phase component.

Description

[Technical field]

[0001] The present invention relates to a sintered hard alloy, and to a crushing tool, a kneading tool, a wear-resistant tool, and a die each using the same. [Background technology]

[0002] In recent years, ceramics and cemented carbide are often used for wear-resistant parts and dies used in crushing, mixing, and kneading resins and magnetic materials. However, ceramics have low toughness and are prone to chipping due to interference between wear-resistant parts. Screws and other parts are large in size, and when made from cemented carbide, the specific gravity is high, making the parts heavy, which can cause deflection in cantilever screws and makes it difficult to increase the rotation speed.

[0003] On the other hand, when sintering large parts with cermets, there is a problem that they are prone to cracking during sintering. Therefore, in recent years, there has been a demand for large cermet parts that are both lightweight and tough for such wear-resistant parts and dies, and that are less susceptible to sintering cracks.

[0004] As such a material, NbC has been attracting attention for a long time, because it has a hardness and specific gravity intermediate between WC and TiC. In recent years, research and development of alloys with NbC as the hard phase has been progressing, but NbC alloys sintered with Ni or Co as a binder phase tend to have low mechanical properties due to the coarsening of NbC, and are therefore not strong enough for use in actual tools and dies, so there has been a demand for improving their strength. In addition, most of the prototypes are made by vacuum sintering test pieces or by hot pressing or SPS, and no examples of actual commercialization have been reported.

[0005] Non-Patent Document 1 discloses that by using NbC as the main raw material and adding one or more of Mo2C, WC, VC, etc., NbC forms a solid solution phase, and furthermore, grain growth of the NbC solid solution phase can be suppressed, so that mechanical properties such as hardness and toughness can be improved. It also discloses that mechanical properties can be improved by changing the type of binder phase. On the other hand, strength such as transverse rupture strength is low, and strength improvement has been a challenge.

[0006] Patent Document 1 discloses a composite cylinder having a structure in which a layer of corrosion-resistant, wear-resistant Ni-based or Co-based sintered alloy containing 10 to 90% by weight of one or more kinds of carbide particles is sintered and integrated on the inner surface of a metal cylinder, and at least 30% by volume of the carbide particles are carbides with a specific gravity of 6.5 to 8.5. In a composite cylinder in which the inner surface of a metal cylinder is made of a wear-resistant sintered alloy, the sintered alloy on the inner surface is replaced from WC-Co to NbC-Ni or the like, and a cylinder that solves the manufacturing problems caused by the high specific gravity of WC-Co is obtained. However, the strength and wear resistance are insufficient, and it cannot be applied to applications that require them.

[0007] Patent Document 2 discloses a cermet alloy consisting of a hard phase mainly composed of carbides, carbonitrides, etc. of transition metal elements of the 4a, 5a, and 6a groups and W-Ni-B compounds, and a binder phase mainly composed of Ni, which can increase hardness without decreasing toughness compared to, for example, TiC-WC-Co alloys and can be sintered at high density by reduced or normal pressure means, and discloses a cermet alloy (sample No. 6) having a raw powder composition of NbC-30 vol% WB-10 vol% Ni in Example 2. However, since sample No. 6 in Example 2 contains a large amount of WB of 30 vol%, most of the Ni contained in the 10 vol% forms a complex boride such as a W2NiB2 phase, and the binder phase remaining in the sintered body is 1 mass% or less, so it cannot be said that the toughness is high.

[0008] Thus, a major feature of NbC-based alloys is their low specific gravity compared to WC-based alloys, but there is still room for improvement in terms of strength and wear resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 2527078 [Patent Document 2] Japanese Patent Application Publication No. 5-78776 [Non-patent literature]

[0010] [Non-Patent Document 1] Hubler and Gradt: Forsch Ingenieurwes, 86(2022), 197-211 Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, an object of the present invention is to provide a sintered hard alloy which is lightweight, strong, and tough, and which is less likely to crack during sintering even in thick-walled products and has excellent grindability, and to provide a crushing tool, a kneading tool, a wear-resistant tool, and a mold which use the same. [Means for solving the problem]

[0012] Therefore, the sintered hard alloy according to one embodiment of the present invention has a hard phase mainly composed of NbC, the hard phase including a first hard phase made of a carbide containing at least one element of Groups 4 to 6 of the periodic table (however, Nb is an essential component), and a second hard phase made of a boride containing at least one element of Groups 4 to 6 of the periodic table; a binder phase containing at least one of Ni, Co, and Fe; At least one of Groups 4 to 6 of the periodic table elements (excluding Nb) is contained in an amount of 3 to 30% by volume in terms of carbide relative to the total amount of the hard phase; At least one of Ni, Co, and Fe is contained in an amount of 4 to 40% by volume as a binder phase component; The binder phase component contains 0.3 to 8% B by mass.

[0013] It is preferable that at least one of W, Mo and V is contained as a metallic element constituting the hard phase.

[0014] The first hard phase is preferably made of a carbide phase containing Nb and at least one element of Groups 4 to 6 of the periodic table other than Nb.

[0015] The second hard phase preferably further contains at least one of Ni, Co and Fe.

[0016] The second hard phase preferably contains W and / or Mo, and more preferably contains Ni.

[0017] The first hard phase is composed of carbide and / or carbonitride, and the carbon content C C and nitrogen content C N C N / (C C +C N )<0.5.

[0018] The first hard phase preferably has an average grain size of 0.3 to 4 μm.

[0019] A crushing tool, a kneading tool, a wear-resistant tool, and a metal mold according to one embodiment of the present invention are characterized in that they use the above-mentioned sintered hard alloy. Effect of the Invention

[0020] According to the present invention, a sintered hard alloy can be obtained that is lightweight, strong, and tough, and that is less likely to crack during sintering even in thick-walled products, and has excellent grindability. This makes it suitable for use in wear-resistant parts that are large in size and rotate at high speed, such as screws and crushing blades, and can dramatically improve production efficiency in crushing, mixing, and kneading. For example, it is suitable for use as a tool or crushing blade for crushing, mixing, and kneading resins and magnetic materials. Also, because it has similar characteristics, it is suitable for use as a peripheral part for molding dies and lenses. [Brief description of the drawings]

[0021] [Figure 1] 1 is an SEM photograph showing a polished cross section of Invention Product 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] [1] Sintered hard alloys A sintered hard alloy according to one embodiment of the present invention is characterized in that it comprises a hard phase mainly composed of NbC, a first hard phase made of a carbide containing at least one of Groups 4 to 6 elements of the periodic table (however, Nb is an essential component), a second hard phase made of a boride containing at least one of Groups 4 to 6 elements of the periodic table, and a binder phase containing at least one of Ni, Co and Fe, and contains 3 to 30 volume % of at least one of Groups 4 to 6 elements of the periodic table (excluding Nb) in terms of carbide relative to the total amount of the hard phase, contains 4 to 40 volume % of at least one of Ni, Co and Fe as a binder phase component, and contains 0.3 to 8% B by mass relative to the binder phase component.

[0023] The hard phase contains NbC as a main component. The hard phase contains NbC as a "main component" means that the content of NbC is greater than the total content of other contained compounds in terms of volume ratio. The content of NbC is preferably 1.5 to 5 times, more preferably 2 to 4 times, the total content of other contained compounds in terms of volume ratio.

[0024] The first hard phase is a carbide phase containing NbC as an essential component and at least one of Groups 4 to 6 elements of the periodic table. The first hard phase is preferably a carbide phase containing Nb and at least one of Groups 4 to 6 elements of the periodic table other than Nb. The first hard phase is preferably a carbide solid solution phase. By forming a solid solution containing Nb and an element of Groups 4 to 6 of the periodic table other than Nb, wear resistance can be imparted to the first hard phase mainly composed of NbC. The hard phase preferably contains at least one of W, Mo, and V as a metal element. The first hard phase preferably contains at least one of W, Mo, and V.

[0025] The hard phase contains at least one element of Groups 4-6 of the periodic table (excluding Nb) in an amount of 3-30% by volume in terms of carbide relative to the total amount of the hard phase. The content of at least one element of Groups 4-6 of the periodic table (excluding Nb) means the total amount of elements of Groups 4-6 of the periodic table other than Nb contained in the hard phase. If the content of elements of Groups 4-6 of the periodic table other than Nb contained in the hard phase is less than 3% by volume, the content of elements of Groups 4-6 of the periodic table other than Nb contained in the first hard phase is insufficient, and therefore the hard phase cannot obtain sufficient wear resistance. If the content of elements of Groups 4-6 of the periodic table other than Nb contained in the hard phase exceeds 30% by volume, the sintered hard alloy cannot obtain sufficient toughness. The content of elements of Groups 4-6 of the periodic table other than Nb contained in the hard phase is preferably 5-30% by volume, more preferably 6-25% by volume, and even more preferably 6-20% by volume.

[0026] The first hard phase preferably has an average grain size of 0.3 to 4 μm. The average grain size of the first hard phase is determined by Fulman's formula based on the SEM structure of any cross section of the sintered hard alloy. If the average grain size of the first hard phase is less than 0.3 μm, the toughness decreases and chipping occurs depending on the usage conditions. If the average grain size of the first hard phase is more than 4 μm, the strength is insufficient. The average grain size of the first hard phase is more preferably 0.5 to 3.5 μm, and even more preferably 0.7 to 3 μm.

[0027] The first hard phase may be a carbonitride phase, or may contain both a carbide phase and a carbonitride phase. In this case, the carbon content C C and nitrogen content C N C N / (C C +C N It is preferable that the carbon content of the sintered hard alloy is greater than the nitrogen content, so that the sinterability is not reduced and the material strength can be maintained. The carbon content of the sintered hard alloy is C C and nitrogen content C N C N / (C C +C N It is more preferable that the relationship be satisfied:

[0028] The second hard phase is made of a boride containing at least one of elements of Groups 4 to 6 of the periodic table. The presence of the second hard phase in the hard phase can refine the grains of the first hard phase and improve the strength. In particular, a boride phase containing W and / or Mo is preferable because it has a large grain refinement effect. Furthermore, the presence of the boride phase can improve the sliding properties of the sintered hard alloy. The average grain size of the second hard phase is preferably about 4 μm or less, more preferably 0.1 to 4 μm, and even more preferably 0.2 to 2 μm.

[0029] The second hard phase is preferably made of a composite boride X2YB2 (X is at least one of the elements in Group 6 of the periodic table, and Y is at least one of Ni, Co, and Fe) containing at least one element in Group 6 of the periodic table, at least one metal element in Group 6 of Ni, Co, and Fe, and boron. This allows the first hard phase to be further refined and improves strength. This is believed to be because the inclusion of at least one metal element in Group 6 of Ni, Co, and Fe suppresses grain growth due to a pinning effect, and also because the presence of the composite boride reduces the liquid phase appearance temperature, suppressing growth in the solid phase. Examples of composite borides include W2NiB2, WNiB, W3CoB3, NiW2B 15 etc.

[0030] The sintered hard alloy of the present invention has a binder phase containing at least one of Ni, Co and Fe, and contains at least one of Ni, Co and Fe as the binder phase component in an amount of 4 to 40% by volume. Here, the binder phase component refers to Ni, Co and Fe contained in the sintered hard alloy, and the content of the binder phase component means the content of all Ni, Co and Fe in the sintered hard alloy including Ni, Co and Fe present other than the binder phase. At least one of Ni, Co and Fe is preferably contained in an amount of 5 to 30% by volume, more preferably 6 to 20% by volume, as the binder phase component. The hard phase component can be dissolved in the binder phase.

[0031] When the sintered hard alloy contains boron, it forms a composite boride of the binder phase component and the group 6 element, and the content of the binder phase is accordingly reduced. Therefore, if a large amount of composite borides are formed, the hardness of the sintered hard alloy increases, but the toughness and strength decrease. For this reason, the amount of boron contained in the sintered hard alloy is 0.3 to 8% by mass relative to the binder phase component. If it is less than 0.3%, a sufficient grain refinement effect cannot be obtained, and if it exceeds 8%, sufficient strength and toughness cannot be obtained. The amount of boron contained in the sintered hard alloy is preferably 0.4 to 7% by mass relative to the binder phase component, more preferably 0.6 to 5%, and even more preferably 0.8 to 4%.

[0032] The sintered hard alloy of the present invention may further contain a fourth phase consisting of an intermetallic compound of a binder phase component and Nb. Examples of intermetallic compounds include Ni3Nb, Ni6Nb7, Fe2Nb, Fe7Nb6, Fe2Nb3, Co2Nb, and Co7Nb6. The presence of the fourth phase does not particularly adversely affect the characteristics and performance of the sintered hard alloy at room temperature, but it can suppress a decrease in hardness when the temperature of the usage environment increases. The fourth phase may be contained in an amount of 40% by volume or less, preferably 30% by volume or less, and more preferably 20% by volume or less, based on the total amount of the binder phase and the fourth phase.

[0033] [2] Manufacturing method of sintered hard alloys One example of a method for producing the sintered hard alloy of the present invention is a method in which powders of the constituent particles of the above-mentioned sintered hard alloy are blended, wet-mixed and pulverized in an organic solvent, dried, and then the powder to which a binder such as paraffin has been added is pressed to form a green body, and the green body is sintered to obtain a sintered hard alloy.

[0034] Boron, which is a constituent of the second hard phase, may be added as boron powder or as a powder of a boride such as TiB2 or WB.

[0035] The mixed powder compact may be formed into a near-finished state (near net shape) by press molding, and may be further machined to give a desired shape, or may be machined after preliminary sintering to give a desired shape.

[0036] The atmosphere during sintering may be a vacuum or an inert gas. When sintering a compact of a nitrogen-containing mixed powder, nitrogen or a mixed gas containing nitrogen may be used, or CO gas may be used. The introduction temperature and gas pressure of these atmospheric gases may be changed depending on the purpose. The heating rate, sintering temperature and holding time, as well as the temperature holding and gas pressure on the way thereto, can be selected arbitrarily depending on the purpose, such as degreasing, improving sinterability, or improving surface properties.

[0037] The sintering temperature is preferably 1250 to 1500°C, and more preferably 1280 to 1450°C. The normally sintered sintered body may be subjected to HIP treatment. The pressure of the HIP treatment is preferably 0.5 MPa or more and 200 MPa or less. Sintering may be performed by hot press sintering, or by electromagnetic energy assisted sintering such as electric current sintering or SPS sintering. Sintering may also be performed by sinter-HIP depending on the application.

[0038] [3] Sintered hard alloy components The sintered hard alloy of the present invention is suitable for use in crushing tools, kneading tools, wear-resistant tools, dies, etc., which are large in size and rotate at high speed, such as screws and crushing blades. Therefore, the sintered hard alloy member using the sintered hard alloy of the present invention can exhibit excellent performance, for example, when used as a crushing, mixing, or kneading member. In addition, the sintered hard alloy of the present invention is not limited to these applications, and can be used for punching punches, room temperature, warm and hot molding dies, extrusion dies, dies, and forging punches, since cracks and the like are unlikely to occur during use of the tool, and it is effective to use it for lens molding peripheral members such as barrel dies for special lenses, which have a large thermal expansion coefficient, since chipping and cracks are unlikely to occur during handling of the member.

[0039] The sintered hard alloy member of the present invention may be effective not only in wear-resistant tools and members as described above, but also in cutting tools such as insert tips, end mills, and drills. Unlike TiCN-based cermets, the sintered hard alloy of the present invention has grindability almost equivalent to that of WC-based cemented carbide, and even members having complex shapes can be easily shaped, and cutting costs are low. In addition, the sintered hard alloy member of the present invention may have a hard coating on the surface by DLC or PVD, or may have a hard coating on the surface by CVD depending on the application. EXAMPLES

[0040] The present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[0041] Example 1 As raw material powders, NbC (1.6 μm), WC (0.8 μm), VC (2.0 μm), Mo2C (3.5 μm), TaC (1.8 μm), TiC (1.7 μm), B (0.8 μm), TiB2 (2.8 μm), WB (4.5 μm), NbB (2.0 μm), Ni (2.4 μm), Co (1.4 μm), and Fe (3.0 μm) were prepared. [The numbers in parentheses are the average particle sizes measured by the Fisher particle size measurement method (FSSS method)]. Mixed powders were prepared by wet mixing to have the composition shown in Table 1, compacted under a pressure of 98 MPa, and sintered at 1300 ° C (Ar atmosphere) to produce invention products 1 to 21 and comparison products 1 to 3. Invention product 12 uses Nb (C) instead of NbC as raw material powder. 0.5 N 0.5 ) (1.8 μm) was used. The carbon content C C The nitrogen content C N When the oxygen and nitrogen were measured using an oxygen / nitrogen analyzer (manufactured by LECO), the relationship between the two was as follows: N / (C C +C N ) was 0.47.

[0042] [Table 1]

[0043] The constituent phases of invention products 1-21 and comparison products 1-3 were investigated for the ratio (volume %) of elements from Groups 4-6 of the periodic table other than Nb in the metal elements constituting the hard phase, calculated as carbide, the ratio (%) of boron (mass %) to the binder phase components (mass %), the amount of boron (mass %) contained in the sintered alloy, and the metal elements contained in the boride phase. The metal elements contained in the boride phase were measured by elemental analysis using SEM-EDX. The amount of boron contained in the sintered alloy was measured by ICP atomic emission spectrometry. The results are shown in Table 2.

[0044] [Table 2] Note *1. A: The content (volume %) of elements in Groups 4 to 6 of the periodic table other than Nb among the metallic elements that make up the hard phase, calculated as carbide. Note *2. B: The ratio (%) of the boron content (mass%) to the binder phase component content (mass%). Note *3. C: The amount of boron contained in the sintered alloy (mass%). Note *4.D: Metal elements contained in the boride phase. Note ※5. First hard phase. Note ※6.Second hard phase. Note *7. The first phase is a carbonitride phase.

[0045] The average grain size, Vickers hardness, and flexural strength of the first hard phase of each sample were investigated. The average grain size of the first hard phase was determined by the Fullman formula based on the SEM structure of an arbitrary cross section of each sample, the Vickers hardness was measured (HV30) using a Vickers hardness tester, and the flexural strength was measured by a three-point bending test based on JISR 1601, with the #140 flat-ground surface of each sample as the tensile surface.

[0046] In order to evaluate the wear resistance (resistance to wear as a tool) of each sample assuming its use as a kneading tool, a rubber wheel test (standard: ASTM G65) was conducted, and the degree of wear was evaluated from 0 to 5. When the wear amount of invention product 1 was 0.9 times or more but less than 1.0 times, it was rated as "3," when it was 0.8 times or more but less than 0.9 times, it was rated as "4," when it was less than 0.8 times, it was rated as "5," when it was 1.0 times or more but less than 1.1 times, when it was 1.1 times or more but less than 1.2 times, it was rated as "1," and when it was 1.2 times or more, it was rated as "0."

[0047] In order to evaluate the chipping resistance of each sample (resistance to chipping as a tool; toughness), a diamond wheel grindstone (#140) was used to perform surface grinding at a cutting depth of 5 μm, and a sharp edge of 20° was processed. The size of the chipping at the tip of the sharp edge of each sample was evaluated from 0 to 5. When the chipping width was 0.9 times or more and less than 1.1 times the chipping width of invention product 3, it was rated as "4", when it was less than 0.9 times, when it was rated as "5", when it was 1.1 times or more and less than 1.2 times, when it was rated as "3", when it was 1.2 times or more and less than 1.3 times, when it was rated as "2", when it was 1.3 times or more and less than 1.4 times, when it was rated as "1", and when it was rated as "0" or more. Table 3 shows the evaluation results and the average grain size, Vickers hardness, and transverse strength of the first hard phase of each prototype.

[0048] For comparison, a sample made of powdered high speed steel was prepared as Comparative Sample 4, and similar tests were carried out on the wear resistance and chipping resistance.

[0049] [Table 3] Note *1: Powdered high speed steel.

[0050] Comparison samples 1 and 2 did not contain a secondary hard phase made of boride, and the grain size of the primary hard phase was large, so the wear resistance was low, and the flexural strength was low, so the chipping resistance was also poor. Comparison sample 3 had a high boron content and a large amount of the secondary hard phase, so the wear resistance was excellent, but at the same time, the flexural strength was low, so the chipping resistance was poor.

[0051] Comparative product 4 was rated "5" for excellent chipping resistance, but its wear resistance was 4.5 times that of invention product 3, demonstrating its extremely poor wear resistance.

[0052] Next, in order to evaluate the wear resistance assuming that the sintered hard alloy of the present invention is used as a crushing tool, a stainless steel sample was separately prepared, and using a blasting device, SiC powder (grain size: #500) was collided with the stainless steel sample and invention product 3 under conditions of a projection angle of 30°, a projection pressure of 0.6 MPa, and a projection time of 90 seconds. After the wear resistance test, the stainless steel sample had twice the amount of wear compared to invention product 3, and it was found that the wear resistance as a crushing tool was significantly inferior.

[0053] The cut surface of the sintered body of Invention Product 3 was mirror-polished and photographed with a scanning electron microscope (Regulus8100, Hitachi High-Technologies Corporation) to obtain an SEM structure (observation magnification: 5,000 times) shown in Figure 1. At the same time, composition analysis of each phase was performed using EDS analysis, and it was confirmed that the first hard phase (gray phase) is a carbide phase containing Nb, W, Mo, and V, and the second hard phase (white phase) is a boride phase containing W, Mo, Ni, and Co. In addition, the crystal structure was examined using XRD, and it was confirmed that the second hard phase is (W,Mo)2(Ni,Co)B2.

[0054] When used as a kneading tool, invention product 3 had a five-fold longer life than the powdered high-speed steel used in comparison product 4. When used as a crushing tool, the existing stainless steel tool was subject to significant wear, but invention product 3 had twice the life of the stainless steel tool.

Claims

1. A hard phase mainly composed of NbC, comprising a first hard phase consisting of carbides containing at least one element from groups 4 to 6 of the periodic table (however, Nb is an essential component), and a second hard phase consisting of borides containing at least one element from groups 4 to 6 of the periodic table, It consists of a bonded phase containing at least one of Ni, Co, and Fe, It contains at least one element from groups 4 to 6 of the periodic table (excluding Nb) in 3 to 30 volume percent relative to the total amount of the hard phase, in terms of carbide. The binding phase component contains at least one of Ni, Co, and Fe in an amount of 4 to 40% by volume. A sintered hard alloy characterized by containing 0.3 to 8% by mass ratio of B relative to the aforementioned binder phase component.

2. The sintered hard alloy according to Claim 1, characterized in that the second hard phase further comprises at least one of Ni, Co, and Fe.

3. The sintered hard alloy according to claim 2, characterized in that the second hard phase contains Ni.

4. The sintered hard alloy according to claim 1, characterized in that the metal element constituting the hard phase includes at least one of W, Mo, and V.

5. The sintered hard alloy according to claim 1, characterized in that the first hard phase comprises a carbide phase containing Nb and at least one element from Groups 4 to 6 of the periodic table other than Nb.

6. The sintered hard alloy according to claim 2, characterized in that the first hard phase comprises a carbide phase containing Nb and at least one element from Groups 4 to 6 of the periodic table other than Nb.

7. The sintered hard alloy according to claim 1, characterized in that the second hard phase contains W and / or Mo.

8. The sintered hard alloy according to claim 2, characterized in that the second hard phase contains W and / or Mo.

9. The first hard phase consists of carbides and / or carbonitrides, and the amount of carbon contained in the sintered hard alloy is C C and nitrogen amount C N C N / (C C +C N A sintered hard alloy according to any one of claims 1 to 8, characterized in that it satisfies the relationship < 0.

5.

10. The sintered hard alloy according to any one of claims 1 to 8, characterized in that the average particle size of the first hard phase is 0.3 to 4 μm.

11. A grinding tool, a mixing tool, a wear-resistant tool, or a mold using the sintered hard alloy described in any one of claims 1 to 8.

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