ION-NITRIDED CERMET TIP AND METHOD FOR PRODUCING THE SAME

Abstract: An ion-nitrided cermet tip comprising a nitrogen-containing layer formed only on the surfaces of the cermet material grains existing in the outermost surface region of the cermet tip. The cermet tip is produced by charging a cermet tip in an ion-nitriding decompression oven, and conducting an ion-nitriding treatment using said cermet tip as a negative electrode and the oven wall as a positive electrode, at a treating temperature of 350 to 600 DEG C for a treating time of 1 to 120 minutes, within a treating atmosphere constituted by nitrogen or a nitrogen-containing mixed gas maintained at a pressure of 0.1 to 10 Torr., thereby forming a nitrogen-containing layer only on the surfaces of the cermet material grains existing in the outermost surface region of said cermet tip.

Description: ION-NITRIDED CERMET TIP AND METHOD FOR PRODUCING THE SAME BACKGROUND OF THE INVENTION The present invention relates to an ion-nitrided cermet tip and a method for producing the same. More particularly, the present invention is concerned with a method for producing a cermet tip having a very thin outermost nitrided compound layer and a method for producing such a cermet tip. In recent years, conventional one-piece cutting tools such as drills and cutters are being replaced with tools of the type having tips which are attached to the cutting ends of the body portions of the tools. Such tips are generally referred to as throw-away tips. The throw-away tips are generally sorted into several types according to the material and production method: namely, hard metal tips, coating-type tips, cermet-type tips, cramics-type tips and super-high pressure sintered tips. Among these tip materials, cermet is a sintered alloy made by powder metallurgical method mainly from titanium carbide (TiC), titanium nitride (TiN) and molybdenum, and is finding a spreading use as a tip material because of its superior workability and high toughness. It is a known method to further improve the strength of cermet tips by adding a nitride and/or carbide to the SϋSSTOTTE SHEET cermet material when it is produced. Methods also have been proposed for increasing the nitride content of cermet. These known methods, however, are unsatisfactory in that the nitride and/or carbide content is increased not only in the surface region which requires improved strength but also other portions of the tip, with the result that the cost of the cermet tip is raised undesirably. Ion-nitriding, gas-nitriding and salt-bath nitriding methods have been known as surface-treating methods for improving properties of the sintered alloy products such as automotive and other machine parts. No attempt has been made, however, for applying such surface treating methods to cermet material which has been developed as a material of cutting tools. In general, a sintered alloy is formed by compacting and shaping powdered alloy material followed by firing, as shown in Fig. 2. The metallurgical structure of the sintered alloy, therefore, is "coarse" as compared with metallurgical structures of ordinary metals such as iron and steel. If the sintered alloy is treated by a nitriding method other than ion nitriding, nitrides or nitride layers are formed not only on the surface region but also along the grain boundaries. In consequence, toughness is decreased to make the product fragile, though the desired SUBSTITUTE SHEET improvement in the hardness at the surface region is achieved. In contrast, ion nitriding method is effective in providing improved hardness to the surface region. In this case, however, a compound layer (I) of several microns to ten and several microns in thickness from the surface and an underlying diffusion layer (nitrided layer) (3) of 0.1 mm to 0.5 mm are formed as shown in Fig. 3. The compound layer and the diffusion layer are inevitably formed in all the known nitriding methods. The cermet material, which is intended for use as a material of cutting tools, is required to have a specifically high level of toughness. The thick compound layer, which is hard and fragile, tends to be exfoliated together with the nitrides in the diffusion layer. The presence of the thick compound layer in cermet tips, therefore, is quite undesirable. As will be understood from the foregoing description, hitherto, attempts and studies on ion nitriding technic have been focused mainly on improvement in the hardness at the surface region and increase in the depth of the nitriding layer. Thus, no approach has been made toward improvement in the durability of cermet tips through elimination of problems such as brittlenesε due to the SU3S7!TϋraSEιΞET presence of the compound layer and exfoliation at the diffusion layer. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a cermet tip which exhibits, through elimination of compound layer and diffusion layers of substantial thicknesses, an improved durability. Another object of the invention is to provide a method for producing a highly durable cermet tip having high durability by an ion nitriding process which is comparatively inexpensive and easy to carry out. To these ends, according to one aspect of the present invention, there is provided an ion-nitrided cermet tip comprising a nitrogen-containing layer formed only on the surfaces of the cermet material grains existing in the outermost surface region of the cermet tip. According to another aspect of the present invention, there is provided a method for producing a cermet tip comprising: charging a cermet tip in an ion-nitriding vacuum furnace; and conducting an ion-nitriding treatment using said cermet tip as a negative electrode and the furnace wall as a positive electrode, at a treating temperature of 350 to 600°C for a treating time of 1 to 120 minutes, within a treating atmosphere constituted by nitrogen or a nitrogen-containing gas maintained at a SUBSTitUT! SHΞE1 pressure of 0.1 to 10 Torr., thereby forming a nitrogen- containing layer only on the surfaces of the cermet material grains existing in the outermost surface region of said cermet tip. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged view of the outermost surface structure of a cermet tip after an ion nitriding treatment conducted in accordance with the method of the present invention, in which a nitrogen-containing layer is denoted by a numeral 1; Fig. 2 is an enlarged view of the metallurgical structure of a sintered alloy; Fig. 3 is an enlarged view of the metallurgical structure of a sintered alloy produced by a conventional ion nitriding method, in which numerals 2 and 3 denote, respectively, a compound layer and a diffusion layer; Fig. 4 is a graph showing results of cutting tests according to Example 2; Fig. 5 is a light microscopic photographic representation of the metallurgical structure of a cermet tip before treatment; Fig. 6 is a light microscopic photographic representation of the metallurgical structure of a cermet tip after ion-nitriding treatment conducted in accordance with the method of the present invention; and SUBSTITUTE -ε W t l !__. __. > Fig. 7 is an electron microscopic photographic representation of the metallurgical structure of a cermet tip after ion-nitriding treatment conducted in accordance with the method of the present invention DESCRIPTION OF THE PREFERRED EMBODIMENTS The ion nitriding treatment in accordance with the method of the invention has no limitation in regard to the material. Namely, the method of the invention can be applied to any type of known cermet tip materials. The ion-nitrided cermet tip in accordance with the present invention is devoid of any compound layer and diffusion layer having substantial thicknesses which cause exfoliation etc. The reason why the ion-nitriding treatment conducted in accordance with the invention improves the hardness at the surface of the cermet tip and eliminates the undesirable exfoliation has not been theoretically determined yet. However, considering that no substantial difference exists between the metallurgical structure before the ion-nitriding treatment and that obtained after the ion-nitriding treatment as will be explained later in connection with microscopic photographic representation, it is deemed that these advantageous effects are attributable to the fact that a very thin nitrogen-containing layer is formed only on the outermost surface of the cermet tip, i.e., only on the surfaces of SUESTJT TgBH ΞT the cermet material grains which exist in the outermost surface region of the cermet tip, as shown in Fig. 1. The ion-nitrided cermet tip of the present invention exhibits improved hardness at the surface region, as well as higher toughness, thus improving the durability of cermet tips without suffering from problems such as exfoliation. An embodiment of the method of the present invention for producing an ion-nitrided cermet tip will be described hereinafter. A cermet tip to be treated in accordance with the method of the invention is charged in an ion-nitriding vacuum furnace which may be of any one of furnaces ordinarily used for the purpose of ion-nitriding treatment. Usually, the wall of the ion-nitriding furnace constitutes an electrode of positive polarity, while the article to be treated constitutes an electrode of negative polarity. In operation, a high-density glow discharge is made to occur between these electrodes, thereby ion-nitriding the article. The ion-nitrided cermet tip of the present invention does not have any compound layer and diffusion layer of large grain boundary thicknesses. To this end, the ion- nitriding treatment is carried out under specific conditions as follows. During the ion-nitriding treatment, the nitriding atmosphere in the ion-nitriding vacuum furnace is maintained within a range of between 0.1 and 10 Torr. The optimum pressure varies depending on the voltage applied but the pressure within the above-specified range is generally desirable for causing a high-density glow discharge suitable for carrying out the present invention. When the vacuum furnace is operated at a pressure which is higher than 0.1 Torr, it is preferred that the pressure in the oven is temporarily reduced to 0.1 Torr. and then gradually increased again to a predetermined set pressure. The treating temperature generally ranges between 350 and 600°C, preferably between 450 and 520°C, while the treating time generally ranges between 1 and 120 minutes, preferably between 1 and 60 minutes and more preferably between 1 and 15 minutes. The treating time is the time which is measured after the cermet temperature has reached a lower limit treating temperature which is usually 350°C, preferably 450°C. The treatment may be conducted such that the heating is suspended to maintain the article at a constant temperature near the above-mentioned lower limit treating temperature. In such a case, a long treating time is required. The treatment also may be carried out in such a manner that the heating is continued even after the lower limit treating temperature has reached, so as to SUBSTITUTE SHEET enable the treatment to be effected at a higher treating temperature. In such a case, a shorter treating time is required. The atmosphere maintained in the ion-nitriding vacuum furnace is a nitrogen-gas atmosphere or a mixture gas atmosphere containing nitrogen. When a nitrogen- containing mixture gas is used, the nitrogen content of the gas should be at least 20%. Substances which may be used together with nitrogen in forming the nitrogen-containing mixture gas are hydrogen, CH 4 and so forth. The most preferred treating atmosphere is provided by a nitrogen/hydrogen mixture gas having a nitrogen-to-hydrogen ratio of 1 : 3 in terms of mol ratio. The ion-nitriding treatment may be carried out such that different atmospheres are used during heating and during treating. Preferably, substitution of the atmosphere in the ion- nitriding vacuum furnace is carried out before the commencement of the heating of the oven. There is no limitation in the method of operating the ion-nitriding vacuum furnace. Thus, an automatic furnace control may be adopted in which electric current and voltage are automatically applied once the above-described conditions are set. Heating of the cermet tip in the furnace may be effected by any known method. For instance, the heating may be effected by a high-density glow discharge or by means of a heater. When the heating is carried out by a high-density glow discharge, the treatment may be made such that the ion- nitriding ' operation is commenced when the above-mentioned lower limit treating temperature is reached and, while the ion-nitriding operation is continued, the temperature of the cermet tip is further raised to a predetermined set treating temperature, followed by ion-nitriding to a desired degree at the constant set treating temperature. When the heating is effected by means of a heater, the ion nitriding process may be carried out by commencing the ion-nitriding treatment when the above-mentioned lower limit treating temperature is reached, maintaining the ion- nitriding treatment while the temperature is being increased to a predetermined set temperature and continuing the ion-nitriding treatment at the set temperature until the desired degree of treatment is attained. When a heater is used as heating means, the method also may be such that, the article is heated to a desired treating temperature and, while the temperature is maintained at this level, high-density glow discharge is commenced to effect ion-nitriding. After the ion-nitriding treatment under the described conditions is finished, the discharge is ceased and the SUBSTITUTE SHEET cermet tip in the furnace is cooled. The cooling is made by leaving the article while maintaining the treating gas atmosphere or by forcibly cooling the article by circulating a cooling gas. The cooling also may be made by introducing atmospheric air into the furnace thus breaking the vacuum and leaving the article in the air. The cermet tip thus cooled to the normal temperature is then taken out of the furnace. Figs. 4 and 5 show the metallurgical structures of the cermet tips as observed through light microscopy before and after the ion-nitriding treatment. Although no significant difference is found between these structures, the hardness of the cermet tip has been remarkably improved as a result of the ion-nitriding treatment. The remarkable improvement in the hardness is attributed to the fact that a very thin compound layer is formed only on the outermost surfaces of the cermet grains existing in the outermost surface region of the cermet tip. As will be understood also from Fig. 5, the ion-nitrided cermet tip of the present invention does not have any compound layer and diffusion layer which would be recognized through light microscopic observation. Thus, the ion-nitrided cermet tip of the invention is free from the degradation which has been encountered with known cermet tips due to exfoliation ■ SuL »lUUi£-5riC£f of compound layer and diffusion layer as a result of application of conventional ion-nitriding technique. The advantage of the ion-nitrided cermet tip of the present invention will be realized from the following description of Examples. EXAMPLE 1 Sample cermet tips (Toshiba Tungalloy N 308 and Sumitomo Igetalloy T25A) were charged in a combination-type ion-nitriding vacuum furnace made of Klδckner-Ionon GMBH. The atmosphere in the furnace was substituted by a nitrogen/hydrogen mixture gas having a nitrogen-to-hydrogen ratio of 1/3 in terms of mol ratio. The temperature in the furnace was then raised by a high-density glow discharge from 20°C to a set treating temperature of 500°C in 2 hours. After maintaining the cermet tip at 500°C for a period of 1 minute, the ion-nitriding treatment was finished to allow the cermet tips to be cooled to normal temperature, and then thus obtained ion-nitrided cermet tips in accordance with the present invention were taken out. Hardness Test of a Cermet Tip Surface hardnesses of the cermet tips before and after the ion-nitriding treatment were measured by Vickers hardness meter under a load of 300 g, the results of which are shown below. SUBSTITUTE_StøEET IS Vickers Hardness Toshiba Tunqalloy (N308) Before ion-nitriding After nitriding 1607 1816 1590 1716 1573 1735 Sumitomo Igetalloy (T25A) Before ion-nitriding After nitriding 1607 1716 1590 1755 1540 1716 As will be seen from the table showing the test results, the ion-nitrided cermet tips of the present invention showed surface hardness levels improved on the order of 100 to 200 in terms of Vickers hardness, as compared with the state before the ion-nitriding. The hardness of the samples was measured also at the core portions of these samples but no substantial difference was observed between the cermet tips before the ion-nitriding and the cermet tips after the ion nitriding. EXAMPLE 2 Cermet tips (Sumitomo Igetalloy T25A) were ion-nitrided according to the method in Example 1. The thus obtained SUBSTITUTE-SBEE " tips were tested by turning lathe together with untreated tips for the purpose of comparison. Cutting conditions: Material: S48C (JIS,carbon steel) Cutting speed: 200 m/min depth: 1.0 mm feed : 0.2 mm/rev The results of the test are shown in Fig. 4. As can be seen from the figure, less flank wear land was observed in ion-nitrided tips than those of non-treated tips proving that the former is superior in resistivity for abrasion to the latter. EXAMPLE 3 Cermet tips (Toshiba Tungalloy) were ion-nitrided according to the method in Example 1. The thus obtained tips were tested by turning lathe together with untreated tips for the purpose of comparison. Cutting conditions: work piece: disc brake material/hardness: boss (S35C, carbon steel) Hb200 : plate (S45C, carbon steel) Hb250 cutting speed : boss 216 m/min : plate 208 m/min feed : boss 0.15 mm/rev SUBSTSTUTE-SHEET : plate 0.12 mm/rev depth : boss 0.5 mm : plate 0.3 + 0.5 mm Test Result Number of Discs Cut Non-treated cermet tips Treated cermet tips 13 38 11 44 14 43 As can be seen from the above result, ion-nitrided tips can cut much more number of discs than non-treated ones. EXAMPLE 4 The depth of nitrogen containing layer in an ion nitrided cermet tip (Nippon Hard Metal NN20) was investigated by Glow Discharge Emission spectroscopy (ANALYMAT: RSV CO., West Germany). Discharge Condition: constant voltage mode, 600V, 40 mA Argon pressure: 9 Torr The result is shown in the following Table. Depth from surface ( μm) (discharge hour: sec) 0.08μm(2sec) 0.2(5) 0.4(10) 4(100) Intensity of N 11 7.1 ' 6.0 5.8 As can be seen from the above Table, nitrogen was concentrated in an area very close to the surface of the cermet and in the portion deeper than 0.2μm there is no appreciable difference in nitrogen content. Figs. 5 and 6 show, respectively, light microscopic photographs at 400X magnification of the metallurgical structure of the cermet tip in the state before the ion- nitriding treatment and that in the state after the ion- nitriding treatment. As will be seen from these Figures, no significant changes was observed in the metallurgical structure by the ion-nitriding treatment. In the microscopic photographs shown in Figs. 5 and 6, the dark portions appearing in upper portions of these photographs are images of a resin used for fixing the metal samples. Fig. 7 is a photograph of the surface of the cermet tip after ion-nitriding treatment in accordance with the present invention observed through an electron microscopy magnified 3500 times. It can be assumed that the white line just visible on the very top surface is an ion- SUBSTITUTESHEET nitrided layer, but there is not a trace of diffusion of the nitrided layer into the internal portion. This is in accordance with the result of the glow discharge emission spectroscopy in Example 4. Thus, according to the invention, it is possible to increase the hardness at the surface of the cermet tip without being accompanied by formation of a thick compound layer and a diffusion layer. Although the invention has been described through its specific forms, it is to be understood that the described embodiments are only illustrative and various changes and modifications may be imparted thereto without departing from the scope of the invention. ' SUBSTITUTE; IT

Claims: WHAT IS CLAIMED IS 1. An ion-nitrided cermet tip comprising a nitrogen- containing layer formed only on the surfaces of the cermet material grains existing in the outermost surface region of the cermet tip. 2. A method for producing a cermet tip according to Claim 1, comprising: charging a cermet tip in an ion-nitriding decompression oven; and conducting an ion-nitriding treatment using said cermet tip as a negative electrode and the oven wall as a positive electrode, at a treating temperature of 350 to 600°C for a treating time of 1 to 120 minutes, within a treating atmosphere constituted by nitrogen or a nitrogen-containing mixed gas maintained at a pressure of 0.1 to 10 Torr., thereby forming a nitrogen- containing layer only on the surfaces of the cermet material grains existing in the outermost surface region of said cermet tip. SUBSTITUTI ∑ SHEET

Inventors: MIYAZAWA TOSHIO, MIYAZAWA, TOSHIO
Applicants: HAMAMATSU NETSUSHORI KOGYO CO, KLOECKNER IONON, NIPPON IONON KK, HAMAMATSU NETSUSHORI KOGYO CO., LTD., KLOECKNER IONON GMBH, NIPPON IONON CO., LTD.
Application year: 1988
Main classification: C23C8
IPC codes: B22F3, B23P15, C23C8, C23C30