Nitrogen-containing sintered hard alloy

Abstract: A nitrogen-containing sintered hard alloy includes at least 75 percent by weight and not more than 95 percent by weight of a hard phase containing Ti, a group 6A metal and WC in a prescribed composition, and at least 5 percent by weight and not more than 25 percent by weight of a binder phase containing Ni, Co and unavoidable impurities, and contains at least 5 percent by weight and not more than 60 percent by weight of Ti in terms of a carbide or the like, and at least 30 percent by weight and not more than 70 percent by weight of a metal belonging to the group 6A of the periodic table in terms of a carbide, while the atomic ratio of nitrogen/(carbon + nitrogen) in the hard phase is at least 0.2 and less than 0.5, and the nitrogen-containing sintered hard alloy is provided with a soft layer containing a binder phase metal and WC in its outermost surface, and has a layer which is hardly provided with the hard phase containing WC in a portion immediately under the soft layer in a thickness of at least 3 µm and not more than 30 µm. According to this composition, it is possible to provide a nitrogen-containing sintered hard alloy which can be employed as a cutting tool having high reliability with no surface coating also in working under conditions bringing a strong thermal shock.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a nitrogen-containing sintered hard alloy, and more particularly, it relates to a nitrogen-containing sintered hard alloy which is improved in thermal shock resistance, wear resistance and strength for serving as a material for a cutting tool and enabling application to wet cutting. Description of the Background Art A nitrogen-containing sintered hard alloy having a hard phase of a carbo-nitride mainly composed of Ti, which is bonded by a metal containing Ni and Co, has already been put into practice as a cutting tool. This nitrogen-containing sintered hard alloy is widely applied to a cutting tool similarly to the so-called cemented carbide which is mainly composed of WC, since the hard phase is extremely fined as compared with a conventional sintered hard alloy which is free from nitrogen to remarkably improve high-temperature creep resistance as the result. In this nitrogen-containing sintered hard alloy, however, resistance against a thermal shock is reduced for the following reasons: (i) The thermal conductivity of this nitrogen-containing sintered hard alloy is about half that of the cemented carbide since the thermal conductivity of Ti which is the main component of the carbo-nitride is extremely smaller than that of WC which is the main component of the cemented carbide, and (ii) the thermal expansion coefficient of the nitrogen-containing sintered hard alloy is about 1.3 times that of the cemented carbide, since this coefficient also depends on the characteristic value of the main component similarly to the thermal conductivity. Therefore, the nitrogen-containing sintered hard alloy is disadvantageously inferior in reliability to a coated cemented carbide or the like in cutting under conditions bringing a particularly strong thermal shock such as milling, cutting of a square timber with a lathe or wet copying with remarkable variation in depth of cut, for example. In order to solve such problems of the conventional nitrogen-containing sintered hard alloy, various improvements have been attempted as follows: For example, Japanese Patent Laying-Open No. 2-15139 (1990) proposes means of improving surface roughness of a material containing at least 50 percent by weight of Ti in terms of a carbide or the like and less than 40 percent by weight of an element belonging to the group 6A (the group VIB in the CAS version) in terms of a carbide and having an atomic ratio N/(C + N) of 0.4 to 0.6 with a high nitrogen content by controlling the sintering atmosphere, for forming a modified part having high toughness and hardness in a surface layer part. On the other hand, Japanese Patent Laying-Open No. 5-9646 (1993) discloses a cermet which is prepared by sintering a material, which is mainly composed of Ti, containing less than 40 percent by weight of W, Mo and Cr in total in terms of a carbide, and thereafter controlling a cooling step for providing a surface part with a region having a smaller amount of binder phase as compared with the interior, to leave compressive stress on the surface. However, each of the cermets disclosed in the aforementioned gazettes is insufficient in chipping resistance as compared with the coated cemented carbide, although wear resistance and toughness are improved. Further, the cermet is so inferior in thermal shock resistance that sudden chipping is easily caused by occurrence of thermal cracking or crack extension resulting from both thermal and mechanical shocks in particular, and sufficient reliability cannot be attained. Although the manufacturing cost for such prior art is reduced due to omission of a coating step, the performance cannot be sufficiently improved. This suggests that improvement in strength against chipping is naturally limited in the category of the so-called cermet which is prepared on the premise that the same contains Ti in excess of a certain degree of amount. The inventors have made deep study on analysis of cutting phenomenons such as temperature distributions in various cutting operations and arrangements of material components in tools, to obtain the following recognition: During cutting, a cutting portion is partially exposed to high-temperature environment in a surface part of an insert which is in contact with a workpiece, a part of a rake face which is fretted by chips, and the like. Comparing the cermet with the cemented carbide, the thermal conductivity of the former is about half that of the latter as hereinabove described, and hence heat which is generated on the surface of the cermet is so hardly diffused into the interior that the temperature is abruptly reduced in the interior although the surface is at a high temperature. Once cracking is caused in such a state, the cermet is extremely easily chipped. When the cermet is rapidly quenched with water-soluble cutting oil from a high temperature state or cooled with cutting in lost motion, further, only an extremely small part of its surface is quenched. Comparing the cermet with the cemented carbide, further, the thermal expansion coefficient of the former is about 1.3 times that of the latter as hereinabove described, and hence tensile stress is caused on a surface layer part to extremely easily cause thermal cracking. In relation to either characteristic, the cermet is inferior in thermal shock resistance to the cemented carbide. Comparing the cermet and the cemented carbide having the same grain sizes and the same amounts of binder phases, further, the fracture toughness of the former is reduced by about 30 to 50 % as compared with the latter, and hence crack extension resistance is also reduced in the interior of the alloy. In the conventional nitrogen-containing sintered hard alloy, as hereinabove described, there are limits to improvement of thermal conductivity, reduction of the thermal expansion coefficient and improvement of crack extension resistance with a large content of Ti which can bring an excellent machined surface and is advantageous in view of the resource. SUMMARY OF THE INVENTION An object of the present invention is to provide a nitrogen-containing sintered hard alloy which can be employed as a cutting tool in high reliability with no surface coating also in a working region under conditions bringing a strong thermal shock with no requirement for the high-priced coated cemented carbide which has been employed in general. This object is solved by a nitrogen-containing sintered hard alloy as defined in claims 1 and 3. The nitrogen-containing sintered hard alloy according to the present invention is provided in its interior with a larger amount of WC as compared with the conventional nitrogen-containing sintered hard alloy in structure, to be improved in resistance against crack extension. When a large amount of WC is blended, WC particles toward the alloy surface appear in the conventional nitrogen-containing sintered hard alloy to provide a tool material called a P-type material, while this tool material is inferior in smoothness of the machined surface. Therefore, this material is also remarkably inferior in abrasive wear resistance to the so-called cermet or coated cemented carbide. However, it has been proved possible to eliminate WC particles from a soft layer which is present in the outermost surface of the tool, i.e., a surface part up to a specific depth from a portion immediately under the so-called exudation layer, deciding smoothness of the machined surface. Thus, abrasive wear resistance and crater wear resistance can be remarkably improved, while the amount of a binder phase is reduced in the vicinity of the surface layer and a group 6A metal such as W is solidly solved in hard phase particles at the same time when cooling is carried out in a decarburizing atmosphere such as a vacuum. Further, the alloy surface is hardened and toughness can be improved by such an effect that compressive stress against the surface part is caused by difference in thermal expansion coefficient due to a gradient in the amount of the binder phase, whereby wear resistance and thermal shock resistance can be remarkably improved. According to the first aspect of the present invention, the nitrogen-containing sintered hard alloy includes: at least 75 percent by weight and not more than 95 percent by weight of a hard phase containing (Ti·W x M y )(C u N 1-u ) (M represents at least one of metals belonging to the group 6A of the periodic table excluding W, 0 < x < 1, 0 ≤ y ≤ 0.9, and 0 ≤ u < 0.9) and WC, and at least 5 percent by weight and not more than 25 percent by weight of a binder phase containing Ni, Co and unavoidable impurities, and contains: at least 5 percent by weight and not more than 60 percent by weight of Ti in terms of a carbide, a nitride or a carbo-nitride, and at least 30 percent by weight and not more than 70 percent by weight of a metal belonging to the group 6A of the periodic table in terms of a carbide, the atomic ratio of nitrogen/(carbon + nitrogen) in the hard phase is at least 0.2 and less than 0.5, and the nitrogen-containing sintered hard alloy is provided with a soft layer containing a binder phase metal and WC in its outermost surface, and has a layer which is hardly provided with the hard phase containing WC in a portion immediately under the soft layer in a thickness of at least 3 µm and not more than 30 µm. According to this nitrogen-containing sintered hard alloy, the content of the hard phase is set in the range of at least 75 percent by weight and not more than 95 percent by weight. This is because wear resistance and plastic deformation resistance are remarkably reduced if the content of the hard phase is less than 75 percent by weight while strength and toughness are insufficient if the content exceeds 95 percent by weight. The Ti content is set in the range of at least 5 percent by weight and not more than 60 percent by weight in terms of a carbide or the like since wear resistance cannot reach a desired level if the Ti content is less than 5 percent by weight while toughness is deteriorated if the Ti content exceeds 60 percent by weight. The Ti content is preferably at least 5 percent by weight and not more than 50 percent by weight, and particularly preferably at least 20 percent by weight and not more than 50 percent by weight. The content of the metal belonging to the group 6A of the periodic table is set in the range of at least 30 percent by weight and not more than 70 percent by weight in terms of a carbide since desired toughness cannot be attained if the content is less than 30 percent by weight while a large amount of WC particles remain in the surface to disadvantageously result in insufficient wear resistance if the content exceeds 70 percent by weight. The content of the metal belonging to the group 6A of the periodic table is preferably at least 40 percent by weight and not more than 70 percent by weight, and particularly preferably at least 40 percent by weight and not more than 60 percent by weight in terms of the carbide. The atomic ratio of nitrogen/(carbon + nitrogen) in the hard phase is set in the range of at least 0.2 and less than 0.5, since both toughness and wear resistance cannot reach desired levels if the atomic ratio is less than 0.2 while the degree of sintering is reduced and toughness is deteriorated if the atomic ratio exceeds 0.5. This atomic ratio is preferably at least 0.2 and less than 0.4. Further, the thickness of the layer hardly provided with the hard phase containing WC, in an amount of not more than 1 percent by volume in more concrete terms, is set in the range of at least 3 µm and not more than 30 µm in the portion immediately under the soft layer consisting of the binder phase metal and WC located on the outermost surface since desired abrasive wear resistance and crater wear resistance cannot be attained if the thickness is less than 3 µm while no effect of facilitating crack extension resistance is attained and toughness is reduced as the result if the thickness exceeds 30 µm. In a preferred embodiment of the nitrogen-containing sintered hard alloy according to the present invention, it is preferable that the abundance of the hard phase containing WC is gradually increased toward the interior from the layer hardly provided with the hard phase containing WC up to a maximum depth of 1 mm from the outermost surface in the aforementioned composition. According to this structure, the abundance of the hard phase containing WC is gradually increased toward the interior from the layer provided with not more than 1 percent by volume of the hard phase containing WC up to the maximum depth of 1 mm from the outermost surface, whereby abrupt change of the WC content distribution is prevented in the boundary between the regions provided and not provided with WC, so that occurrence of residual stress is relieved in this boundary. According to the second aspect of the present invention, on the other hand, the nitrogen-containing sintered hard alloy includes: at least 75 percent by weight and not more than 95 percent by weight of a hard phase containing (Ti·W x M y )(C u N 1-u ) (M represents at least one of metals belonging to the groups 4A, 5A and 6A of the periodic table, corresponding to the groups IVB, VB and VIB in the CAS version respectively, excluding Ti and W, 0 < x < 1, 0 ≤ y ≤ 0.9, and 0 ≤ u < 0.9) and WC, and at least 5 percent by weight and not more than 25 percent by weight of a binder phase containing Ni, Co and unavoidable impurities, and contains: at least 5 percent by weight and not more than 60 percent by weight of Ti in terms of a carbide, a nitride or a carbo-nitride, at least 30 percent by weight and not more than 70 percent by weight of a metal belonging to the group 6A of the periodic table in terms of a carbide, at least 2 percent by weight and not more than 15 percent by weight of Ta and Nb in total in terms of a carbide, a nitride or a carbo-nitride, and not more than 5 percent by weight of V, Zr and Hf in total in terms of a carbide, a nitride or a carbo-nitride, the atomic ratio of nitrogen/(carbon + nitrogen) in the hard phase is at least 0.2 and less than 0.5, and the nitrogen-containing sintered hard alloy is provided with a soft layer containing a binder phase metal and WC in its outermost surface, and has a layer which is hardly provided with the hard phase containing WC in a portion immediately under the soft layer in a thickness of at least 3 µm and not more than 30 µm. Also when the nitrogen-containing sintered hard alloy contains a metal belonging to the group 4A of the periodic table excluding Ti and/or the group 5A in addition to the metal belonging to the group 6A of the periodic table excluding W, at least 2 percent by weight and not more than 15 percent by weight of Ta and Nb in total in terms of a carbide, a nitride or a carbo-nitride, and not more than 5 percent by weight of V, Zr and Hf in total in terms of a carbide, a nitride or a carbo-nitride, a functional effect which is similar to that of the composition according to the first aspect can be attained. Crater wear resistance is not improved if the total content of Ta and Nb is less than 2 percent by weight in terms of a carbide or the like, while chipping resistance is reduced if the content exceeds 15 percent by weight. The alloy preferably contains V, Zr and Hf to be improved in strength and hardness under a high temperature, while the degree of sintering is reduced and chipping resistance is also reduced as the result if the total content of V, Zr and Hf exceeds 5 percent by weight in terms of the carbide etc. In the inventive nitrogen-containing sintered hard alloy, it is preferable that the abundance of the hard phase containing WC is gradually increased toward the interior from the layer hardly provided with the hard phase containing WC up to a maximum depth of 1 mm from the outermost surface in the aforementioned composition. DESCRIPTION OF THE PREFERRED EMBODIMENTS Concrete Examples of the present invention are now described. Example 1 45 percent by weight of (Ti 0.85 Ta 0.04 Nb 0.04 W 0.07 )(C 0.56 N 0.44 ) powder of 2 µm in mean particle size having a cored structure including an outer portion appearing pure white and a core portion appearing jet-black in a reflecting electron microscopic image, 40 percent by weight of WC powder of 0.7 µm in mean particle size, 7 percent by weight of Ni powder of 1.5 µm in mean particle size and 8 percent by weight of Co powder of 1.5 µm in mean particle size were wet-blended with each other, and thereafter the mixture was stamped and degassed in a vacuum of 10 -2 Torr at 1200°C. Thereafter the mixture was sintered under a nitrogen gas partial pressure of 30 Torr at 1450°C for 1 hour, and then cooled in a vacuum at 5°C/min., to form a sample 1. The sample 1 had a Ti content of 34 percent by weight in terms of TiCN, a W content of 45 percent by weight in terms of WC, and a Ta and Nb content of 6 percent by weight in terms of TaC + Nb. The atomic ratio N/(C + N) was 0.3. Absolutely no WC particles were present in a region of 10 µm in thickness located immediately under a soft layer. For the purpose of comparison, samples 2 to 4 were prepared by conventional methods respectively. The sample 2 was prepared by sintering a stamped compact which was identical to that of the sample 1 under a nitrogen partial pressure of 5 Torr at 1400°C. The sample 3 was prepared by cooling a sintered body which was identical to that of the sample 2 under a CO partial pressure of 200 Torr after sintering. The sample 4 was prepared by cooling a sintered body which was identical to that of the sample 2 under a nitrogen partial pressure of 180 Torr after sintering. In the samples 2 to 4, the abundances of hard phases containing WC located immediately under soft layers were 10 percent by volume, 15 percent by volume and 5 percent by volume respectively. In addition to raw materials which were identical to those for the sample 1, further, TaC, NbC, ZrC and VC of 1 to 3 µm in mean particle size were blended in weight ratios shown in Table 1 to form sintered alloys through steps similar to those for the sample 1, thereby preparing samples 5 to 10 having reduced contents shown in Table 1. Ni, Co, ZrC and VC were omitted from Table 1 since the reduced contents thereof were substantially identical to the blending compositions. Table 2 shows atomic ratios N/(C + N), thicknesses of layers provided with not more than 1 percent by volume of hard phases containing WC located immediately under soft layers in alloy surface parts. Sample No. Blending Composition (weight %) Reduced Content (weight %) (TiTaNbW)CN WC TaC NbC ZrC VC Ni Co TiCN WC TaC + NbC 1 2* 3* 45 40 - - - - 7 8 34 45 6 4* 5 30 40 4 4 2 - 5 15 22 45 11 6 60 20 3 - - 2 10 5 44 30 9 7* 80 2 2 - 2 - 7 7 58 15 11 8* 89 - - - - - 5 6 65 14 10 9* 50 40 - 2 - - 4 4 37 48 7 10* 45 25 2 - 2 - 13 13 3 32 7 Note) Asterisked numerals indicate comparative samples, and underlined numeric values are out of the inventive ranges. Sample No. N/C+N (Atomic Ratio) Thickness of Region Provided with Not More Than 1 vol.% of Hard Phase Containing WC (µm) 1 0.30 10 2* 0.30 0 3* 0.30 0 4* 0.30 0 5 0.27 5 6 0.41 15 7* 0.44 200 8* 0.44 Overall Alloy Region 9* 0.34 10 10* 0.36 60 Note) Asterisked numerals indicate comparative samples, and underlined numeric values are out of the inventive ranges. The nitrogen-containing sintered hard alloys of the aforementioned samples 1 to 10 were employed for cutting work under cutting conditions 1 to 3 shown in Table 3, to consequently obtain results shown in Table 4. Cutting Condition 1 (Wear Resistance Test) Cutting Condition 2 (Toughness Test) Cutting Condition 3 (Thermal Shock Resistance Test) Tool Shape TNMG332 SNMG432 SNMG432 Workpiece SCM435 (H B = 350) Round Bar SCM435 (H B = 250) Round Bar with Four Longitudinal Flutes SCM435 (H B = 220) Round Bar Cutting Speed 150 m/min. 100 m/min. 250 m/min. Feed Rate 0.36 mm/rev. 0.24 mm/rev. 0.20 mm/rev. Depth of Cut 1.5 mm 2.0 mm Changed from 2.5 to 0.2 mm Cutting Oil Water Soluble Not Used Water Soluble Cutting Time 30 min. 30 sec. 15 min. Determination Flank Wear Width (mm) Number of Chipped Ones among 20 Inserts Number of Chipped Ones among 20 Inserts Sample No. Cutting Condition 1 Flank Wear Width (mm) Cutting Condition 2 Number of Chipped Ones among 20 Inserts Cutting Condition 3 Number of Chipped Ones among 20 Inserts Inventive Sample 1 0.14 4 2 Comparative Sample 2 0.25 10 20 3 0.34 10 18 4 0.35 12 6 Inventive Sample 5 0.16 3 5 6 0.11 6 3 Comparative Sample 7 0.15 16 20 8 0.28 15 16 9 0.11 18 9 10 0.58 6 10 As understood from the results shown in Table 4, the samples 1, 5 and 6 having compositions etc. satisfying the conditions according to the first or second aspect of the present invention are superior in wear resistance, toughness and thermal shock resistance to the samples 2 to 4 and 7 to 10 having compositions etc. which are out of the inventive conditions. Example 2 Raw powder materials shown in Table 5 were blended and mixed/crushed to attain respective reduced contents, thereby forming samples 11 to 23. Each TiCN powder material had a mean particle size of 2 µm and an atomic ratio C/N of 5/5, while the remaining powder materials were 1 to 3 µm in mean particle size. The sample 12 was prepared with a Ta and Nb source of (TaNb)C powder (TaC:NbC = 2:1 (weight ratio)) of 1.5 µm in mean particle size, while the sample 17 was prepared with a Ti and W source of (Ti 0.8 W 0.2 )(C 0.7 N 0.3 ) of 2 µm in mean particle size. Table 5 shows the amounts of blending of these solid solution raw powder materials in terms of single compounds. Blending compositions of the respective samples were omitted from Table 5 since the same were substantially identical to the reduced contents. Sample No. Reduced Content (wt.%) TiCN TiC TiN WC Mo 2 C TaC NbC ZrC HfC Ni Co 11 45 - - 35 5 - - - - 5 10 12 40 - - 30 5 4 2 2 2 5 10 13 - 15 21 44 - - - - - 10 10 14 - 10 16 44 - - 7 3 - 10 10 15 - 23 12 50 - - - - - 8 7 16 - 10 25 50 - - - - - 8 7 17 - 26 17 37 3 3 - - - 6 8 18* 55 - - 25 - 4 - - - 8 8 19* 18 - - 72 - - - - - 5 5 20* - 25 10 50 - - - - - 8 7 21* - 7 28 50 - - - - - 8 7 22* 40 - 5 35 - - - 3 3 6 8 23* 30 - 5 35 - 10 6 - - 6 8 Note) Asterisked numerals indicate comparative samples, and underlined numeric values are out of the inventive ranges. The samples 11 to 23 were heated in a vacuum of 10 -2 Torr at 3°C/min., degassed at 1200°C for 15 minutes, thereafter sintered under a nitrogen gas partial pressure of 15 to 40 Torr at 1450°C for 1 hour, thereafter control-cooled in a vacuum to 1200°C at 3°C/min., and thereafter nitrogen-quenched. As to the samples 11 and 12, samples 11A to llC and 12A to 12C were formed after sintering under the same conditions, under various cooling conditions. The samples 11A and 12A were cooled under a CO partial pressure of 150 Torr after sintering under the same conditions as the samples 11 and 12 respectively, the samples 11B and 12B were cooled under a nitrogen partial pressure of 200 Torr, and the samples 11C and 12C were heated to 1530°C, thereafter sintered for 1.5 hours, and thereafter control-cooled. Table 6 shows atomic ratios N/(C + N), thicknesses of regions provided with not more than 1 percent by volume of hard phases containing WC located immediately under soft layers in alloy surface parts. Sample No. N/C+N (Atomic Ratio) Thickness of Region Provided with Not More Than 1 vol.% of Hard Phase Containing WC (µm) 11 039 8 11A* " 2 11B* " 35 11C* " 8 12 0.37 15 12A* " 0 12B* " 50 12C* " 15 13 0.42 10 14 0.35 5 15 0.23 17 16 0.48 8 17 0.29 15 18* 0.43 Overall Alloy Region 19* 0.22 0 20* 0.19 23 21* 0.54 3 22* 0.43 8 23* 0.38 9 Note) Asterisked numerals indicate. comparative samples, and underlined numeric values are out of the inventive ranges. The samples shown in Table 6 were employed for cutting work under cutting conditions 4 to 6 shown in Table 7, to obtain results shown in Table 8. For the purpose of comparison, a commercially available coated cemented carbide (grade P10) was also subjected to a cutting test. Cutting Condition 4 (Wear Resistance Test) Cutting Condition 5 (Toughness Test) Cutting Condition 5 (Milling Cutter Thermal Shock Resistance Test) Tool Shape TNMG332 SNMG432 SDKN42 Workpiece SCM435 (H B = 250) Round Bar SCM435 (H B = 250) Round Bar with Four Longitudinal Flutes SCM435 (H B = 240) Plate with Three Longitudinal Flutes (Flute of 5mm in Width Every 20mm) Cutting Speed 180 m/min. 150 m/min. 160 m/min. Feed Rate 0.3 mm/rev. 0.2 mm/rev. 0.28 mm/rev. Depth of Cut 1.5 mm 2.0 mm 0.2 mm Cutting Oil Water Soluble Not Used Water Soluble Cutting Time 20 min. 30 sec. 5 Passes Determination Flank Wear Width (mm) Number of Chipped Ones among 20 Inserts Number of Inserts Chipped by Thermal Cracking etc. among 20 Ones As understood from the results shown in Table 8, the samples 11, 12 and 13 to 17 having compositions etc. satisfying the conditions according to the first or second aspect of the present invention are superior in wear resistance, toughness and thermal shock resistance to the samples 11A to 11C, 12A to 12C and 18 to 23 having compositions etc. which are out of the inventive conditions. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only.

Claims: A nitrogen-containing sintered hard alloy, including: at least 75 percent by weight and not more than 95 percent by weight of a hard phase containing (Ti . W x M y ) (C u N 1-u ) (M represents at least one of metals belonging to the group 6A of the periodic table excluding W, 0 < x < 1, 0 ≤ y ≤ 0.9 and 0 ≤ u < 0.9) and WC, and at least 5 percent by weight and not more than 25 percent by weight of a binder phase containing Ni, Co and unavoidable impurities, and containing: at least 5 percent by weight and not more than 60 percent by weight of Ti in terms of a carbide, a nitride or a carbo-nitride, and at least 30 percent by weight and not more than 70 percent by weight of a metal belonging to the group 6A of the periodic table in terms of a carbide, the atomic ratio of nitrogen/(carbon + nitrogen) in said hard phase being at least 0.2 and less than 0.5, said nitrogen-containing sintered hard alloy being provided with a soft layer containing a binder phase metal and WC in its outermost surface, with a layer of not more than 1 % by volume of said hard phase in a portion immediately under said soft layer in a thickness of at least 3 µm and not more than 30 µm. The nitrogen-containing sintered hard alloy in accordance with claim 1, wherein the abundance of said hard phase is gradually increased toward the interior from said layer being hardly provided with said hard phase up to a maximum depth of 1 mm from said outermost surface. A nitrogen-containing sintered hard alloy, including: at least 75 percent by weight and not more than 95 percent by weight of a hard phase containing (Ti . W x M y )(C u N 1-u ) (M represents at least one of metals belonging to the groups 4A, 5A and 6A of the periodic table excluding Ti and W 0 < x < 1, 0 ≤ y ≤ 0.9, and 0 ≤ u < 0.9) and WC, and at least 5 percent by weight and not more than 25 percent by weight of a binder phase containing Ni, Co and unavoidable impurities, and containing: at least 5 percent by weight and not more than 60 percent by weight of Ti in terms of a carbide, a nitride or a carbo-nitride, at least 30 percent by weight and not more than 70 percent by weight of a metal belonging to the group 6A of the periodic table in terms of a carbide, at least 2 percent by weight and not more than 15 percent by weight of Ta and Nb in total in terms of a carbide, a nitride or a carbo-nitride, and not more than 5 percent by weight of V, Zr and Hf in total in terms of a carbide, a nitride or a carbo-nitride, the atomic ratio of nitrogen/(carbon + nitrogen) in said hard phase being at least 0.2 and less than 0.5, said nitrogen-containing sintered hard alloy being provided with a soft layer containing a binder phase metal and WC in its outermost surface, with a layer of not more than 1 % by volume of said hard phase in a portion immediately unter said soft layer in thickness of at least 3 µm and not more than 30 µm. The nitrogen-containing sintered hard alloy in accordance with claim 3, wherein the abundance of said hard phase is gradually increased toward the interior from said layer being hardly provided with said hard phase up to a maximum depth of 1 mm from said outermost surface.

Inventors: IKEGAYA AKIHIKO, ISOBE KAZUTAKA, KITAGAWA NOBUYUKI, TSUDA KEIICHI, IKEGAYA, AKIHIKO, ISOBE, KAZUTAKA, KITAGAWA, NOBUYUKI, TSUDA, KEIICHI, IKEGAYA, AKIHIKO, C/O ITAMI WORKS OF SUMITOMO, ISOBE, KAZUTAKA, C/O ITAMI WORKS OF SUMITOMO ELEC., KITAGAWA, NOBUYUKI, C/O ITAMI WORKS OF SUMITOMO, TSUDA, KEIICHI, C/O ITAMI WORKS OF SUMITOMO ELEC.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1995
Main classification: C22C29
IPC codes: C22C1, B22F7, C22C29