Hard sintered body for tool

Abstract: A hard sintered body for a tool contains 25 to 47 vol.% of cBN, 40 to 70 vol.% in total of a carbo-nitride, in which the ratio of carbon to nitrogen is 60:40 to 30:70, and a boride of Ti, and 2 to 20 vol.% in total of a boride and a nitride of Al. In this hard sintered body, cBN particles are bonded to each other through binder phases. Thus obtained is a hard sintered body for a tool which is excellent in wear resistance and chipping resistance in high-speed cutting of hardened steel.

Description: The present invention relates to a hard sintered body for a tool employing cubic boron nitride (hereinafter referred to as cBN). In general, cBN which is the hardest material following diamond is widely known. Sintered bodies of cBN, having lower reactivity with ferrous materials as compared with diamond, are applied to various cutting tools. Japanese Patent Laying-Open No. 53-77811 (1978) discloses examples of such cBN sintered bodies. This gazette discloses a sintered body consisting of a material containing 80 to 40 % of cBN in volume ratio and a rest mainly composed of a carbide, a nitride, a boride or a silicide of a transition metal belonging to the group IVa, VA or VIa of the periodic table or a mixture or a mutual solid solution compound thereof, and a sintered body prepared by adding Al and/or Si to this material. However, a tool prepared from either sintered body disclosed in Japanese Patent Laying-Open No. 53-77811 is remarkably worn or abruptly chipped in high-speed working of hardened steel, for example. The tool is worn conceivably because the temperature of its cutting edge is increased in high-speed cutting of hardened steel to thermally deteriorate binders or drop cBN particles. Further, the tool is abruptly chipped conceivably because the binders are thermally deteriorated and reduced in holding power for the cBN particles. The present invention has been proposed in order to solve the aforementioned problems. An object of the present invention is to provide a hard sintered body for a tool which is excellent in wear resistance as well as in chipping resistance for causing no abrupt chipping in high-speed cutting of hardened steel. The hard sintered body for a tool for cutting hardened steel according to the present invention contains 25 to 47 vol.% of cBN, 40 to 58 vol.% in total of a carbo-nitride and a boride of Ti, the ratio of carbon to nitrogen in said carbo-nitride of titanium being 60:40 to 30:70, and 8 to 18 vol.% in total of a boride and a nitride of Al, particles of said cubic boron nitride being bonded to each other through binder phases. The sintered body according to the present invention contains cBN particles as hereinabove described, and can maintain its hardness and strength at high levels. Further, the carbo-nitride and the boride of Ti contained in the inventive sintered body serve as binder phases strongly bonding the cBN particles with each other, and improve wear resistance of the sintered body due to excellent heat resistance and low reactivity with iron. In addition, the nitride and the boride of Al serve as binder phases strongly bonding the cBN particles with each other. The inventors have made deep study for attaining the aforementioned object, to succeed in improving wear resistance and chipping resistance of the sintered body under conditions increasing a cutting edge temperature in high-speed cutting of hardened steel or the like by optimizing the combination of the contents of cBN, the carbo-nitride and the boride of Ti and the nitride and the boride of Al having the aforementioned functions and the ratio of carbon to nitrogen in the carbo-nitride of Ti thereby increasing heat resistance of the binders and bonding strength of the cBN particles. The sintered body may contain alumina, WC, a transition metal/element such as a ferrous metal or W belonging to the group IVa, Va or VIa of the periodic table, and/or oxygen as unavoidable impurities. The inventive sintered body is prepared by sintering fine particles of cBN and binders. Surfaces of such fine particles inevitably adsorb oxygen, and the adsorbed oxygen is dissolved in the binder phases such as the carbo-nitride of Ti or bonded with Al having high reactivity with oxygen to form alumina remaining in the sintered body. However, the performance of the sintered body is not remarkably deteriorated by such presence of oxygen. While the sintered body may contain a transition metal such as a ferrous metal or W belonging to the group Iva, Va or VIa of the periodic table as an impurity from raw material powder or WC as an impurity resulting from a mixing step, its performance is not remarkably deteriorated by the presence of such an impurity. The hard sintered body according to the present invention preferably contains from 27 to 45 vol.% of cubic boron nitride. The hard sintered body preferably contains from 45 to 58 vol.% in total of a carbo-nitride and a boride of Ti. The ratio of carbon to nitrogen in said carbo-nitride of Ti is preferably from 58:42 to 33:67. The present invention also provides a process of producing a hard sintered body for a tool for cutting hardened steel as herein described, the process comprising the step of sintering cubic boron nitride, at least one or two binder powder materials selected from a carbide, a nitride, a carbo-nitride and a boride of Ti, and at least one or two binder powder materials selected from Al, an alloy of Al and Ti, a nitride of Al, a nitride of Ti and Al and a boride of Al, wherein the sintering is performed under a very high pressure of at least 4 GPa and a temperature of at least 1200°C. Sintering is preferably performed at a temperature of at least 1300°C, more preferably at a temperature of at least 1500°C. Sintering is preferably performed at a pressure of at least 5GPa. As hereinabove described, the cBN content in the inventive sintered body is 25 to 47 vol.%. If the cBN content is less than 25 vol.%, the sintered body is reduced in hardness, strength and chipping resistance, leading to abrupt chipping of a tool prepared from the sintered body. If the cBN content exceeds 47 vol.%, on the other hand, the quantity of the binders for improving wear resistance is reduced, to reduce the wear resistance of the sintered body. As hereinabove described, further, the total content of the carbo-nitride and the boride of Ti must be 40 to 58 vol.%, the ratio of carbon to nitrogen in the carbo-nitride of Ti must be 60:40 to 30:70, and the total content of the boride and the nitride of Al must be 8 to 18 vol.%. If the total contents of the carbo-nitride and the boride of Ti and the boride and the nitride of Al serving as binder phases are less than 40 vol.% and 8 vol.% respectively, bonding strength of the particles forming the sintered body becomes insufficient, to reduce the strength of the sintered body. If the total content of the carbo-nitride and the boride of Ti exceeds 58 vol.%, on the other hand, the cBN content is reduced, to reduce the hardness and the strength of the sintered body. If the carbo-nitride of Ti contains carbon in excess of the aforementioned range, toughness and the bonding strength between the carbo-nitride and the cBN particles are weakened to reduce the chipping resistance, although heat resistance of the binder phases is improved. If the carbo-nitride of Ti contains nitrogen in excess of the aforementioned range, on the other hand, the heat resistance of the binder phases is reduced, to result in insufficient wear resistance. The boride of Al is fragile as compared with the remaining binder phase compounds, while the nitride of Al is inferior in heat resistance. If the total content of the boride and the nitride of Al exceeds 18 vol.%, therefore, the strength, heat resistance, chipping resistance and wear resistance of the sintered body are reduced. The aforementioned hard sintered body for a tool is preferably obtained by sintering the aforementioned cBN, at least one or two binder powder materials selected from a group consisting of a carbide, a nitride, a carbo-nitride and a boride of Ti, and at least one or two binder powder materials selected from a group consisting of Al, an alloy of Al and Ti, a nitride of Al, a nitride of Ti and Al and a boride of Al under a very high pressure of at least 4 GPa and a temperature of at least 1200°C. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 illustrates a result of X-ray analysis of a sintered body sample No. 2 in Table 1. Examples of the present invention are now described with reference to Tables 1 to 3 and Fig. 1. (Example 1) Sintered bodies having compositions shown in Table 1 were prepared by sintering cBN, at least one or two binder powder materials selected from a group consisting of a carbide, a nitride, a carbo-nitride and a boride of Ti, and at least one or two binder powder materials selected from a group consisting of Al, an alloy of Al and Ti, a nitride of Al, a nitride of Ti and Al and a boride of Al under a very high pressure of 4.5 GPa and a temperature of 1300°C. Fig. 1 shows a result of X-ray analysis (Rad. CuKa 1) of the sintered body sample No. 2. Then, tools were prepared from these sintered bodies for cutting carburized and quenched chromium steel workpieces (SCr 420) of HRC 60 in hardness for 30 minutes under conditions of a cutting speed of 200 m/min., a feed rate of 0.1 mm/rev., and a depth of cut of 0.1 mm in wet cutting, and frank wear width of the tools were measured. No. cBN Content (vol.%) Carbo-Nitride and Boride of Ti (vol%) Ratio of Carbon to Nitrogen in Carbo-Nitride of Ti Nitride and Boride of Aℓ (vol.%) Frank Wear Width of Tool (mm) 1 44 48 52:48 8 0.092 2 31 58 50:50 11 0.088 3 21 64 55:45 15 chipped in 22 min. 4 50 44 45:55 6 0.232 It is understood from Table 1 that the tools prepared from the samples Nos. 3 and 4 having cBN contents of 21 vol.% and 50 vol.% were chipped and remarkably worn as compared with the remaining tools respectively. Thus, it is inferred that frank wear width of the tool can be suppressed to the minimum when the cBN content is 25 vol.% to 47 vol.%. Referring to Table 1, underlined values are out of the scope of the present invention. This also applies to Tables 2 and 3. (Example 2) Sintered bodies having compositions shown in Table 2 were prepared by sintering cBN, at least one or two binder powder materials selected from a group consisting of a carbide, a nitride, a carbo-nitride and a boride of Ti, and at least one or two binder powder materials selected from a group consisting of Al, an alloy of Al and Ti, a nitride of Al, a nitride of Ti and Al and a boride of Al under a very high pressure of 5 GPa and a temperature of 1500°C. Then, tools were prepared from these sintered bodies for cutting carburized and quenched chromium molybdenum workpieces (SCM 415) of HRC 62 in hardness for 40 minutes under conditions of a cutting speed of 220 m/min., a feed rate of 0.08 mm/rev., and a depth of cut of 0.15 mm in dry cutting, and frank wear width of the tools were measured. No. cBN Content (vol.%) Carbo-Nitride and Boride of Ti (vol.%) Ratio of Carbon to Nitrogen in Carbo-Nitride of Ti Nitride and Boride of Aℓ (vol.%) Frank Wear Width of Tool (mm) 5 45 45 58:42 10 0.092 6 27 57 33:67 16 0.088 7 45 37 49:51 18 0.210 8 26 71 35:65 3 chipped in 22 min. 9 27 62 75:25 11 chipped in 25 min. 10 39 47 20:80 14 0.253 It is understood from Table 2 that the tools prepared from the samples Nos. 7 and 8 containing 37 vol.% and 71 vol.% in total of carbo-nitrides and borides of Ti were remarkably worn and chipped respectively. It is also understood that the tools prepared from the samples Nos. 9 and 10 containing carbon and nitrogen in the ratios 75:25 and 20:80 in carbo-nitrides of Ti were chipped and remarkably worn respectively. Thus, it is inferred that frank wear width of the tool can be suppressed to the minimum when the total content of the carbo-nitride and the boride of Ti is 40 to 70 vol.% and the ratio of carbon to nitrogen in the carbo-nitride of Ti is 60:40 to 30:70. (Example 3) Sintered bodies having compositions shown in Table 3 were prepared similarly to Example 2. Then, tools were prepared from these sintered bodies for cutting hardened alloy tool steel workpieces (SKD 11) of HRC 63 in hardness for 20 minutes under conditions of a cutting speed of 180 m/min., a feed rate of 0.06 mm/rev., and a depth of cut of 0.1 mm in wet cutting, and frank wear width of these tools were measured. No. cBN Content (vol.%) Carbo-Nitride and Boride of Ti (vol%) Ratio of Carbon to Nitrogen in Carbo-Nitride of Ti Nitride and Boride of Aℓ (vol.%) Frank Wear Width of Tool (mm) 11 39 51 35:65 10 0.098 12 29 53 53:47 18 0.091 13 34 65 57:43 1 chipped in 15 min. 14 29 47 58:42 24 0.298 It is understood from Table 3 that the tools prepared from the samples Nos. 13 and 14 containing 1 vol.% and 24 vol.% in total of nitrides and borides of Al were chipped and remarkably worn as compared with the remaining tools respectively. Thus, it is inferred that frank wear width of the tool can be suppressed to the minimum when the total content of the nitride and the boride of Al is 2 to 20 vol.%. As hereinabove described, it is possible to obtain a hard sintered body for a tool which is excellent in wear resistance and chipping resistance in high-speed cutting of hardened steel by properly adjusting the contents of cBN, a carbo-nitride and a boride of Ti, and a nitride and a boride of Al in accordance with the present invention. 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 and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A hard sintered body for a tool for cutting hardened steel, the body containing 25 to 47 vol.% of cubic boron nitride, 40 to 58 vol.% in total of a carbo-nitride and a boride of Ti, the ratio of carbon to nitrogen in said carbo-nitride of Ti being 60:40 to 30:70, and 8 to 18 vol.% in total of a boride and a nitride of Al, particles of said cubic boron nitride being bonded to each other through binder phases. A hard sintered body as claimed in claim 1, which contains from 27 to 45 vol.% of cubic boron nitride. A hard sintered body as claimed in claim 1 or claim 2, which contains from 45 to 58 vol.% in total of a carbo-nitride and a boride of Ti. A hard sintered body as claimed in any one of claims 1 to 3, wherein the ratio of carbon to nitrogen in said carbo-nitride of Ti is from 58:42 to 33:67. A process of producing a hard sintered body for a tool for cutting hardened steel as claimed in any one of claims 1 to 4, the process comprising the step of sintering cubic boron nitride, at least one or two binder powder materials selected from a carbide, a nitride, a carbo-nitride and a boride of Ti, and at least one or two binder powder materials selected from Al, an alloy of Al and Ti, a nitride of Al, a nitride of Ti and Al and a boride of Al, wherein the sintering is performed under a very high pressure of at least 4 GPa and a temperature of at least 1200°C. A process as claimed in claim 5, wherein sintering is performed at a temperature of at least 1300°C. A process as claimed in claim 5 or claim 6, wherein sintering is performed at a temperature of at least 1500°C. A process as claimed in any one of claims 5 to 7, wherein sintering is performed at a pressure of at least 5GPa.

Inventors: FUKAYA TOMOHIRO, NAKAI TETSUO, FUKAYA, TOMOHIRO, NAKAI, TETSUO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1997
Main classification: C04B35
IPC codes: C04B35, C22C29