Burnt surface sintered alloy with and without a rigid surface film coating and process for producing the alloy

Abstract: There are disclosed a surface refined sintered alloy with a burnt surface, comprising 75 to 95 % by weight of a hard phase containing Ti, C and N as the essential components and otherwise comprising at least one of Zr, Hf, V, Nb, Ta, Cr, Mo and W and the balance of the alloy comprising a binder phase composed mainly of Co and/or Ni and inevit­ able impurities, wherein the sintered alloy satisfies at least one conditions selected from the group consisting of the following (1) to (3): (1) the average grain size of the hard phase in a surface layer to the inner portion of 0.05 mm from the burnt surface of the sintered alloy is 0.8 to 1.2-fold of the average grain size of the hard phase in the inner portion of the sintered alloy excluding the surface layer; (2) the average content of the binder phase in the surface layer to the inner portion of 0.05 mm from the burnt surface of the sintered alloy is 0.7 to 1.2-fold of the average content of the binder phase in the inner portion of the sintered alloy; and (3) the average hardness in the surface layer to the inner portion of 0.05 mm from the burnt surface of the sintered alloy is 0.95 to 1.10-fold of the average hard­ ness in the inner portion of of the sintered alloy; and a process for producing the same.

Description: BACKGROUND OF THE INVENTION This invention relates to a burnt surface sintered alloy suitable primarily as a construction material, including parts for cutting tools, parts for abrasion resistant tools, parts for impact resistant tools or parts for decoration, and a process for producing the same and a coated burnt surface sintered alloy comprising a rigid film coated on the burnt surface sintered alloy. N-containing TiC-based sintered alloy comprising a basic composition of TiC-TiN-Ni tends to have superior strength and plastic deformation resistance compared with a non-N-containing TiC based sintered alloy having a basic composition of TiC-Ni. Because of this, the N-containing TiC based sintered alloy has wide application, and can even be used in the fields of heavy cutting or high feed cutting when employed as part of a cutting tool. In these fields, in order to make the tool parts economically, the sintered alloy is sometimes used without polishing or grinding the surface of the sintered alloy. In this case, the surface state after sintering without substantially further surface treatment is termed a burnt surface. The N-containing Tic-based sintered alloy, when used as a burnt surface alloy, has the problem that fracturing or chipping is more liable to occur as compared with the case when it is used with a polished or ground surface. As a representative example of an attempt to solve such problems of the surface layer in N-containing Tic-based sintered alloy, there is Japanese Provisional Patent Publication No. 101704-1979. Japanese Provisional Patent Publication No. 101704/1979 discloses a TiC-based sintered alloy having a hardness in the region 0.005 to 0.2 mm from its surface which has been made 1.02-fold or less of the hardness at 1.0 mm from the surface. The process described in Japanese Provisional Patent Publication No. 101704/1979 inhibits the oozing of the metal binder phase by making the amount of oxygen in the surface portion larger than in the inner portion. This is achieved by increasing the CO gas partial pressure during the cooling process compared to the CO partial pressure during the temperature elevation and sintering processes during the whole sintering process. This makes the hardness in the surface portion and the inner portion of the alloy uniform, thereby solving the hardness embrittlement at the surface portion. However, because a concentration gradient of oxygen is used in the sintered alloy, oxygen must be used as an essential component and therefore the problem still arises that the results obtained are still unsatisfactory with respect to strength and fracturing resistance. Patent Abstracts of Japan, Vol. 11, No. 361 (C-459)(2808) discloses the production of a cermet consisting of 50-95% by weight of a hard phase containing Ti, C, N and at least one element of Ta, W, Mo and the balance being a binder phase consisting of Ni and Co. The relative atomic ratio of Co in the binder phase is 0.1-0.9. In the manufacture of the cermet, compacted material powders are sintered at 1350-1600°C, then an Ar-N₂ gas mixture is admitted at a pressure of 0.66-93.3 kPa and finally HIP-treatment is applied. SUMMARY OF THE INVENTION The present invention has solved the problems as mentioned above. More specifically its object is to provide a N-containing TIC-based sintered alloy and a process for producing the same and also a coated burnt surface alloy comprising a rigid film coated on the sintered alloy, by making uniform the average content of binder phase in the surface portion and the inner portion of the N-containing TiC-based sintered alloy according to a method entirely different from Japanese Provisional Patent Publication No. 101704/1979, making uniform the hardness in the surface portion and the inner portion, or making uniform both the contents of binder phases and hardnesses in the surface portion and the inner portion. The present inventors have studied the reason for the inferior strength and plastic deformation resistance of an N-containing TiC-based sintered alloy having a burnt surface as compared with an N-containing TiC-based alloy comprising a polished surface or a ground surface. The inventors found that the grain side of the hard phase at the surface portion of the burnt surface of a conventional N-containing TiC-based sintered alloy is remarkably roughened as compared with the grain size of the hard phase in the inner portion. The inventors also found that when the grain size at the surface portion of the burnt surface and the inner portion of the sintered alloy is made uniform, the strength and plastic deformation resistance of the sintered alloy become excellent, and also, when both the content of the binder phase and the grain size of the hard phase at the surface portion of the burnt surface and the inner portion of the sintered alloy are made uniform, then the strength and plastic deformation resistance of the sintered alloy become remarkably excellent. The present inventors have further studied the reason for the inferior strength and fracturing resistance of an N-containing TiC-based sintered alloy having a burnt surface as compared with an N-containing TiC-based alloy comprising a polished surface or a ground surface. The inventors found that the binder phase is indeed oozed out on the surface, and a layer having a higher hardness than the inner portion of the alloy is formed immediately below the binder phase enriched layer. The binder phase enriched layer was found to be at most about 10 µm thick, while the rigid layer has a thickness of about 0.5 mm. In other words, the formation of the hard layer in the surface portion is not caused mainly by oozing of the binder phase, but mainly by the phenomenon of denitrification during the temperature elevation and sintering process. Based on such findings, the present inventors considered that the strength and fracturing resistance of the sintered alloy can be improved by making uniform the hardness in the surface portion and the inner portion of the sintered alloy, and also that further strength and fracturing resistance can be improved by making uniform both the binder phase content in the surface portion and the inner portion, and the hardness. The present invention has been accomplished on the basis of the above considerations. According to one aspect, the present invention provides a burnt surface sintered alloy consisting essentially of 75 to 95% by weight of a hard phase containing Ti, C (carbon), N (nitrogen) and at least one element selected from Zr, Hf, V, Nb, Ta, Cr, Mo and W, and the balance being a binder phase composed of Co and/or Ni, or at least 50% by volume of Co and/or Ni, and at least one metal element selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Al, and Mn, and inevitable impurities; the sintered alloy having a surface region defined to be a region extending from the alloy surface to 0.05 mm below the alloy surface, and an inner portion defined to be that portion of the alloy which does not form part of the surface region; wherein said sintered alloy satisfies at least two conditions of the following conditions (1) to (3): (1) the average grain size of the hard phase in the surface region is 0.8 to 1.2-fold of the average grain size of the hard phase in the inner portion; (2) the average volume content of the binder phase in the surface region is 0.7 to 1.2-fold of the average volume content of the binder phase in the inner portion; and (3) the average hardness in the surface region is 0.95 to 1.10-fold of the average hardness in the inner portion. The above coated burnt surface sintered alloy of the present invention may further comprise a rigid film having a higher hardness than the burnt surface sintered alloy covered on the surface of the burnt surface sintered alloy. According to a further aspect, the present invention provides a process for producing the above burnt surface sintered alloy comprising the steps of: subjecting a powdery mixture comprising at least one carbide and at least one nitride of a metal from the group 4a, 5a, or 6a of the periodic table or mutual solid solutions of these, and a powder of Co and/or Ni for forming the binder phase, to a first sintering step in either a vacuum or an atmosphere of an inert gas at a temperature of up to 1300°C, and subjecting the mixture to a second sintering step in which the temperature is increased from 1300°C and simultaneously subjecting the mixture to a nitrogen atmosphere of gradually increasing pressure in the range 0.013 kPa (0.1 Torr) - 2.7 kPa (20 Torr). DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the present invention will be described in more detail. The sintered alloy in the burnt surface sintered alloy of the present invention can include all of the component compositions of TiC-based sintered alloys containing N of the prior art, for example, the component compositions described in Japanese Provisional Patent Publication No. 101704/1979, although the presence of oxygen is non-essential. Among them, the hard phase constituting the sintered alloy comprises, for example, specifically at least one of (Ti,M)C , (Ti,M)N, (Ti,M)(C,N) (wherein M represents at least one of Zr, Hf, V, Nb, Ta, Cr, Mo and W), and the binder phase constituting the sintered alloy comprises at least 50% by volume of Co and/or Ni of the binder phase, containing otherwise, for example, the metal elements in the compounds forming the hard phase and Fe, Al, Mn, etc. The burnt surface of the sintered alloy of the present invention includes the surface state after sintering, the surface state after washing with water or an organic solvent and drying after sintering, or the surface state from which the attached matters on the burnt surface are removed by sand blast treatment, etc. after sintering, as representative surfaces. The burnt surface sintered alloy of the present invention has the alloy structure in the surface region (defined to be a region extending from the alloy surface to 0.05 mm below the alloy surface) approximated to the alloy structure in the inner portion (defined to be that portion of the alloy which does not form part of the surface region), and among said alloy structure, by making the average grain size of the hard phase present in the surface region approximate to the average grain size of the hard phase present in the inner portion by controlling it to 0.8 to 1.2-fold of that of the inner portion, whereby the strength and plastic deformation resistance of the sintered alloy have been improved. In addition to the grain size of the hard phases in the surface region and the inner portion, by controlling the average volume content of the binder phase presented in the surface region to 0.7 to 1.2-fold of that of the inner portion, the strength and plastic deformation resistance can be further improved. Furthermore, it is more preferred that in addition to the grain size of the hard phases and the average volume contents of the binder phase in the surface region and the inner portion, by controlling the average hardness in the surface region to 0.95 to 1.10-fold of that in the inner portion, strength and stability to plastic deformation resistance of the sintered alloy can be heightened. The burnt surface sintered alloy of the present invention has an average volume content of the binder phase in the surface region (as defined above) approximated to the average volume content in the binder phase in the inner portion, by controlling it to 0.7 to 1.2-fold of that of the inner portion, whereby strength and fracturing resistance of the sintered alloy have been improved. Other than the contents in the surface region and the inner portion, by controlling the average hardness in the surface region to 0.95 to 1.10-fold of that in the inner portion, strength and fracturing resistance of the sintered alloy can be further improved. The burnt surface sintered alloy of the present invention an the average hardness in the surface region (as defined above) approximated to the average hardness in the inner portion, by controlling the average hardness in the surface region to 0.95 to 1.10-fold of that in the inner portion, whereby the strength and fracturing resistance of the sintered alloy have been improved. In the burnt surface sintered alloy of the present invention, if the average grain size of the hard phase in the surface region (as defined above) is less than 0.8-fold, the average volume content of the binder phase thereof is less than 0.7-fold or the average hardness thereof exceeds 1.10-fold, deterioration in fracturing resistance becomes remarkable. To the contrary, if the average grain size thereof exceeds 1.2-fold, the average volume content thereof exceeds 1.2-fold or the average hardness thereof is less than 0.95-fold, deterioration in abrasion resistance becomes remarkable. The ranges of the average grain size of the hard phase, the average volume content of the binder phase and the average hardness of the sintered alloy in accordance with the burnt surface sintered alloy of the present invention may be those which have been employed in conventional N-containing TiC-based sintered alloys. In order to heighten both of the abrasion resistance and the fracturing resistance of the sintered alloy, it is particularly preferred that the average grain sizes of the hard phases, the average volume contents of the binder phases or the average hardnesses of the sintered alloy at the surface region and the inner portion are substantially equal to each other, respectively. In producing the burnt surface sintered alloy of the present invention, it is important to control the carbon content and the nitrogen content contained in the powdery mixture used as the starting material. It is further important to control the temperature in the sintering step of the production steps and the atmosphere at that time. Particularly, by carefully controlling the nitrogen pressure in the second temperature region where sintering proceeds together with the generation of a liquid phase compared to the first temperature region in the sintering step, the content of the binder phase and the hardness in the surface region of the sintered alloy can be controlled. Also, as described above, since the formation of the hard layer at the surface portion is caused by the N-eliminating phenomenon during the temperature elevation and sintering processes, it is effective to make the sintered alloy a low carbon alloy from which N can be eliminated with difficulty. It is also preferred that the burnt surface sintered alloy thus obtained may be coated according to, for example, the physical vapor deposition method (PVD method) or the chemical vapor deposition method (CVD method) conventionally practiced in the art. The rigid film formed by these coating methods has higher hardness than the burnt surface sintered alloy. The coating may be formed specifically of carbides, nitrides, carboxides, nitroxides of the metals of the group 4a, 5a and 6a of the periodic table or mutual solid solution of these and single layer or multi-layer coatings comprising at least one of silicon nitride, silicon carbide, aluminum oxide, aluminum nitride, aluminum oxynitride, cubic boron nitride and diamond. Particularly, if the coated burnt surface sintered alloy is obtained by forming a rigid film comprising a nitride film on its surface by maintaining further its surface after completion of sintering in the second temperature region in the process for producing the surface refined sintered alloy as described above under an atmosphere of high nitrogen pressure for a prolonged period of time, the steps can be simplified and also no additional installation of equipment is required. The thickness of the rigid film in the coated burnt surface sintered alloy is selected depending on the material, use and shape of the rigid film, but is preferably about 0.1 to 10 µm. In the burnt surface sintered alloy of the present invention, by making the grain size of she hard phase in the surface region (as defined above) finer in comparison with the sintered alloy of the prior art, stress to the hard phase in the surface layer is dissipated, whereby it has the action of enhancing the strength and plastic deformation resistance of the sintered alloy. The burnt surface sintered alloy of the present invention has the action of enhancing strength and fracturing resistance of the sintered alloy by making the average content of binder phase in the surface region (as defined above) greater as compared to the sintered alloy of the prior art. Also, the process for producing the burnt surface sintered alloy of the present invention has the action of inhibiting denitrification in the surface region of the sintered alloy simultaneously with inhibition of grain growth of hard phase by changing over the atmosphere in the first temperature region to the atmosphere in the second temperature region during sintering and increasing gradually the nitrogen pressure with temperature elevation in the second temperature region. In the present specification, metals of the groups 4a, 5a and 6a of the periodic table mean that metals of the group 4a are Ti, Zr and Hf, those of the group 5a are V, Nb and Ta and those of the group 6a are Cr, Mo and W, respectively. EXAMPLES In the following, the present invention will be explained in more detail by referring to Examples. Example 1 By use of respective powders of commercially available TiC, TiN, Mo₂C and Ni having average grain sizes falling within 1 to 2 µm, a composition comprising 40 wt% TiC-30 wt% TiN-15 wt% Mo₂C-15 wt% Ni was formulated, and the formulated powder, acetone and balls were placed in a mixing vessel to perform wet mixing and pulverization for 72 hours. To the mixed powder thus obtained, a small amount of paraffin was added, and the mixture was press molded so as to obtain SNMN120408 (shape of JIS standard). After the paraffin was removed by heating from the pressed powder obtained from the press molding, it was sintered by elevating the temperature from room temperature to 1200 °C in vacuum of 0.007 kPa (0.05 torr) over 4 hours, then at 3 °C/min in the atmosphere shown in Table 1 from 1200 °C to 1450 °C, and further maintaining the temperature at 1450 °C for one hour. After sintering, the sintered product was cooled at 50 °C/min to obtain the sintered alloys 1 to 10 of the present invention and comparative sintered alloys 1 to 4 corresponding to the sintered step of the prior art. The products 1 to 4 and 8 to 10 of the present invention and the comparative products 1 to 4 thus obtained were subjected to examination of the surface layer and the inner portion by means of a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA) and a Vickers hardness meter to obtain the results shown in Table 2. The grain size of the hard phase shown in Table 2 was measured from an alloy structure photograph of 5000-fold according to SEM. The binder phase content was determined by polishing the sintered alloy to a tilted angle of 10° and measuring the polished surface by use of EPMA under the plane analysis conditions of an acceleration voltage of 20 kV and 20 x 30 µm² from average value of 5 points. Particularly, binder phase content and hardness were determined as average value of 5 points at equidistance from the surface toward the inner portion, because they are greately flucutuated within the surface layer. Next, by use of the products 1 to 4 and 8 to 10 of the present invention and the comparative products 1 to 4, cutting tests were conducted under the conditions (A) and (B) shown below, and the results are shown in Table 3. (A) Cutting conditions for wear resistance test: Workpiece S48C (H B 250) 250 mm⊘ Tip shape SNMN120408 (0.1 x -30° chamfer type honing) Cutting speed 160 m/min Depth of cut 1.5 mm Feed 0.3 mm/rev Cutting time 20 min (B) Cutting conditions for test for tool life by fracturing: Workpiece S48C (H B 230) 120 mm⊘ with 4 slots Tip shape SNMN120408 (0.1 x -30° chamfer type honing) Cutting speed 100 m/min Depth of cut 1.5 mm Feed 0.3 mm/rev Cutting time cutting for 10 minutes was repeated for 10 times, and the ratio of fractured tips within 10 minutes was evaluated. Sample (A) Wear resistant cutting test (B) Ratio of fractured tip to all tips used in the cutting test Average flank wear (mm) Face wear (mm) Product of the present invention 1 0.14 0.18 5/10 2 0.15 0.20 3/10 3 0.12 0.02 1/10 4 0.10 None 0/10 8 0.13 0.18 3/10 9 0.12 0.20 2/10 10 0.11 0.18 3/10 Comparative product 1 0.13 0.16 10/10 2 0.13 0.17 10/10 3 0.14 0.17 9/10 4 0.14 0.18 8/10 The burnt surface sintered alloy of the present invention is equal in wear resistance to N-containing TiC-based sintered alloys of the prior art, but since it is more excellent in strength and plastic deformation resistance, it has two or three times higher fracturing resistance in cutting test. Also, the coated burnt surface sintered alloy of the present invention comprising a rigid film coated on the burnt surface sintered alloy has excellent abrasion resistance and excellent fracturing resistance. From these facts, the sintered alloy of the present invention has a wide scope of uses including those of N-containing TiC-based sintered alloys of the prior art and in addition those where impact resistance and fracturing resistance are required and is also high in stability. Thus, the present invention provided an industrially useful material and a process for producing the same.

Claims: A burnt surface sintered alloy consisting essentially of 75 to 95% by weight of a hard phase containing Ti, C (carbon), N (nitrogen) and at least one element selected from Zr, Hf, V, Nb, Ta, Cr, Mo and W, and the balance being a binder phase composed of Co and/or Ni, or at least 50% by volume of Co and/or Ni, and at least one metal element selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Al, and Mn, and inevitable impurities; the sintered alloy having a surface region defined to be a region extending from the alloy surface to 0.05 mm below the alloy surface, and an inner portion defined to be that portion of the alloy which does not form part of the surface region; wherein said sintered alloy satisfies at least two conditions of the following conditions (1) to (3): (1) the average grain size of the hard phase in the surface region is 0.8 to 1.2-fold of the average grain size of the hard phase in the inner portion; (2) the average volume content of the binder phase in the surface region is 0.7 to 1.2-fold of the average volume content of the binder phase in the inner portion; and (3) the average hardness in the surface region is 0.95 to 1.10-fold of the average hardness in the inner portion. A burnt surface sintered alloy according to Claim 1, wherein the hard phase comprises at least one of (Ti,M)C, (Ti,M)N and (Ti,M)(C,N); wherein M represents at least one of Zr, Hf, V, Nb, Ta, Cr, Mo and W. A coated surface sintered alloy, comprising the burnt surface sintered alloy according to Claim 1 and a rigid film having greater hardness than said burnt surface sintered alloy covering the surface of said burnt surface sintered alloy. A coated surface sintered alloy according to Claim 3, wherein said rigid film has a thickness of 0.1 to 10 µm. A process for producing a burnt surface sintered alloy according to claim 1 comprising the steps of: subjecting a powdery mixture comprising at least one carbide and at least one nitride of a metal from the group 4a, 5a, or 6a of the periodic table or mutual solid solutions of these, and a powder of Co and/or Ni for forming the binder phase, to a first sintering step in either a vacuum or an atmosphere of an inert gas at a temperature of up to 1300°C, and subjecting the mixture to a second sintering step in which the temperature is increased from 1300°C and simultaneously subjecting the mixture to a nitrogen atmosphere of gradually increasing pressure in the range 0.013 kPa (0.1 Torr) - 2.7 kPa (20 Torr). A process according to Claim 5, further comprising the step of forming a rigid nitride containing film on the surface of the burnt surface sintered alloy by maintaining further the surface of the surface refined sintered alloy after completion of the said second sintering step in a high pressure nitrogen atmosphere. A process according to Claim 6, wherein the rigid film has a thickness of 0.1 to 10 µm.

Inventors: KOBORI KEIICHI, SAITO TUYOSHI, SAITOH TAKESHI, SUZUKI HISASHI, UEKI MITSUO, KOBORI, KEIICHI C/O TOSHIBA TUNGALOY CO., LTD., SAITO, TUYOSHI C/O TOSHIBA TUNGALOY CO., LTD., SAITOH, TAKESHI C/O TOSHIBA TUNGALOY CO., LTD., SUZUKI, HISASHI, UEKI, MITSUO C/O TOSHIBA TUNGALOY CO., LTD.
Applicants: TOSHIBA TUNGALOY CO LTD, TOSHIBA TUNGALOY CO., LTD.
Application year: 1989
Main classification: C22C29
IPC codes: C22C29, C23C30, C22C32, C22C1