A gas carburising process

Abstract: A material to be treated (W) is preheated to a carburising temperature of 750-950°C and is heat-treated in a carburising atmosphere directly supplied with hydrocarbon and oxidizing gases, in which it is heated to 1000-1100°C. Then, the material being treated (W) is forcibly cooled to a temperature below 600°C, the material (W) is reheated to 750-850°C, and then it is hardened by quenching in oil (20), preferably using a laminar flow quenching technique.

Description: This invention relates to a gas carburising process In a conventional gas carburising process being commonly employed today, the heat treatment is conducted at 900-930°C using carburising gas formed in a transforming furnace. A new gas carburising process has been proposed by the applicant of this invention (see, for example, JP-A-89/38870, JP-A-94/51904, etc.) intending to improve economics by eliminating manufacturing process of the carburising gas in the transforming furnace and by directly supplying both hydrocarbon and oxidizing gases as raw gas into the furnace. The treatment temperature of 900-930°C used in the conventional gas carburising process was set having consideration to the prevention of coarse crystal grain formation in the material being treated and the efficiency of the treatment time. That is, when the treatment temperature is set at a temperature exceeding the upper limit of 900-930°C, even though the required carburised layer can be obtained in a short time, obtaining a satisfactorily carburised structure is very difficult due to the formation of coarse crystal grains in the material being treated. On the other hand, when the treatment temperature is below the lower limit of 900-930°C, it takes a long time to obtain a required carburised depth although a good carburised structure is obtained. Shortening of the treatment time in gas carburisation contributes greatly to cost saving by reducing the consumption of both power and gas. Consequently, it would be of great benefit to provide a gas carburising process and an apparatus therefor which has improved efficiency without reducing the quality of the product. US-A-3950192, US-A-4495004 and EP-A-0105138 disclose further gas carburising processes. High temperature carburisation techniques are also discussed in an article entitled: "Gasaufkohlen bei Temperaturen oberhalb 950°C in konvertionellen Öfen und in Vakuumöfen", (Podiumsdiskussion beim 36. Härtereikolloquim, 1980), by J. Grosch, et al. The present invention provides a gas carburising process comprising the sequential steps of: preheating the material to be treated (W) to a temperature of 750-950°C in a pre-heating chamber; heating the material to be treated (W) up to 1000-1100°C in a directly supplied carburising atmosphere of hydrocarbon and oxidizing gases in a carburising chamber; forcibly cooling the material being treated (W) to a temperature below 600°C in a cooling chamber; reheating the material (W) to 750-850°C in a reheating chamber; and hardening the material (W) in a hardening chamber wherein each of said chambers has its own transfer means; and are connected in series through opening/closing doors, to transfer said material to be heated (W). Thus the present invention provides the advantage that the treatment time is shortened with the associated benefits to efficiency. Furthermore, a transforming furnace or formation of carburising gas in a carburising furnace is not required in the present invention. According to the present invention, unlike the prior art methods, the heat-treatment is carried out in a carburising atmosphere supplied with the carburising gas and heated to 1000-1100°C, the carburising atmosphere being directly produced in the furnace. The carburising atmosphere produced directly in the furnace is highly reductive. Thus, grain boundary oxidation is very low. Further, heating energy (gas sensible heat) can be saved due to elimination of the carburising gas. Furthermore, variation of the carburised layer and the carburising time can be reduced. In addition, coarse crystal grains formed by the high-temperature carburisation can be regulated to specified grain sizes during the cooling and reheating steps so as to further reduce grain boundary oxidation. Furthermore, it can be easily attained to crystallize carbides homogeneously in order to improve the wear resistance and fatigue strength etc., and a product having equal or even higher quality than prior art products can be achieved. Further, because the laminar flow hardening is used, a superior quality product having less hardening distortion can be produced in a short time. Preferably, The hardening chamber used in the method is constructed as a laminar flow hardening chamber. Preferred embodiments of the present invention will now be descnbed by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is a side elevation showing in section the main parts of an apparatus for the gas carburising process; Fig. 2 is a schematic sectioned elevation of the hardening chamber; Fig. 3 is a graph showing temperature of the material being treated and the temperature of the quench oil during hardening; Fig. 4 is a chart showing a preferred processing sequence for carburising in accordance with the present invention; and Fig. 5 is a chart showing a typical processing sequence for carburising in accordance with the prior art. In the Figure 1, numeral 1 is a preheating chamber, 2 is a carburising chamber, 3 is a cooling chamber, 4 is a reheating chamber, 5 is a hardening chamber, and 6 is a purge chamber. Further, in Figure 1, numeral 7 is an inlet door, 8 to 12 are opening/closing doors, respectively, 13 is an exit door, 14 refers to the transfer means provided to each of the chambers, and W is a material being treated. In the preheating chamber 1, the material to be treated is preheated from room temperature up to the carburising temperature, for example, 750-950°C, preferably 930°C. Construction of the preheating chamber 1 is basically similar to a heating chamber of a conventional batch furnace. In the preheating chamber 1, it is possible to stop a fan 15 at initial supply phase or to shot-purge to protect initial atmosphere. Further, the preheating chamber 1 is constructed so as to enable control of the heating gradient so that no distortion due to thermal stresses occurs in the material being treated W during the preheating process. In the carburising chamber 2, the material being treated is transferred from the preheating chamber 1 by the transfer means 14 through opened opening/closing door 8, is heated up to a suitable temperature of greater than 1000°C, in particular to 1050°C, and is carburised simultaneously by supplying hydrocarbon gas (methane, propane, butane etc.) and oxidizing gas (pure oxygen, air, carbon dioxide etc). All the apparatus installed in the carburising chamber 2, such as the transfer means 14, a fan 16, a fan shaft 17, opening/closing doors 8 and 9 etc. are constructed of high temperature resistant materials. In the carburising chamber 2, the carburisation can reach to a targeted effective depth in a short time because the diffusion coefficient of carbon is twice as high as that of the prior art, due to a higher carburising temperature than that of the prior art. In the cooling chamber 3, the material W, which has been heated up to 1050°C in the carburising chamber 2, is forcibly cooled to a temperature below 600°C, preferably to 500°C. In the cooling chamber 3, a cooling method utilizing the latent heat of boiling of water (refer to JP-A-89/255619), a gas cooling method utilizing highly pressurised (about 5 kg/cm 2 ) nitrogen or carbon dioxide gas flow, a convection cooling method by cooled scirocco fan etc. are used jointly. In the reheating chamber 4, the material being treated W, which has been cooled to 500°C in the cooling chamber 3, is reheated up to an austenitising temperature of 850°C. When it is necessary, ammonia gas can be fed into the reheating chamber 4 to reduce surface irregular layer and to improve resistance to tempering softening. Also, similar to the preheating chamber 1, the reheating chamber 4 is constructed so as to allow control of the heating gradient so that no distortion from thermal stresses in the material being treated W occurs during the reheating process. Coarse crystal grains formed during the high temperature carburisation in the carburising chamber 2 are regulated to the specified size during the cooling process in the cooling chamber 3 and by the reheating process in the reheating chamber 4. In the hardening chamber 5, there is provided a quenching vessel 18 and an elevator 19 as with conventional methods. Instead of using agitated quench oil 20, however, laminar flow hardening shown in Fig. 2 is preferably utilized in the hardening chamber 5. As shown in Figure 2, a hardening frame 21 to receive a descending elevator 19 is disposed in approximately the middle of the quenching vessel 18. A horizontal dynamic pressure eliminating plate 22 is disposed slightly below the middle of the outer periphery of the quenching frame 21. A vertical partition 23 is disposed between the peripheral rim of the dynamic pressure eliminating plate 22 and the bottom of the quenching vessel 18. The vertical partition 23 supports the quenching frame 21 through the dynamic pressure eliminating plate 22. The lower end of the quenching frame 21 does not contact the bottom of the quenching vessel 18. A sub-chamber 24 is formed under the quenching frame 21 by the vertical partition 23 and the dynamic pressure eliminating plate 22. A suitable number of guide pipes 25 penetrate through the vertical partition 23 at regular intervals. The inner openings of the guide pipes 25 are bent towards the dynamic pressure eliminating plate 22, ie. upwards. The quench oil 20 in the quenching vessel 18 is supplied equally to the guide pipes 25 through an upwardly discharging pump 26. Numeral 27 in Fig. 2 is a circulation pump to circulate the quench oil 20 in the upper and lower portions of the quenching frame 21, and 28 is a circulating pipe therefor. In the aforementioned construction, the quench oil 20 in the quenching vessel 18 is supplied into the sub-chamber 24 through the guide pipes 25 by operation of the upwardly discharging pump 26. The quench oil 20 supplied into the sub-chamber 24 collides with the dynamic pressure eliminating plate 22, and converts into laminar flow, ie. the oil flows in layers without any eddies (laminar flow), and then flows into the quenching frame 21 from its lower end. The material being treated W descends into the inside of the quenching frame 21 by the elevator 19. The material W is cooled there by the quench oil 20 flowing into the quench frame 21. It is said that the principle of the hardening is to perform quickly but slowly. Particularly in order to perform hardening perfectly with less distortion, the material being treated W should be cooled down rapidly until the temperature of the material W reaches a so-called nose point of the S-curve (the Time-Temperature-Transformation curve) and kept thereafter at the Ms point (the martensitic start point, at about 210°C) for a while to equalise the temperature throughout the material being treated W before proceeding to martensitic transformation. Homogeneous hardening without any irregularity can be attained in the laminar flow hardening chamber 5 because, unlike the methods used in the prior art using blades to agitate the quench oil, no bubbles are generated in the quench oil, and in addition there is no turbulent flow. Fig. 3 shows an example of the temperature variation of the material being treated W (curve X) and the temperature variation of the quench oil (curve Y) during the actual hardening process in the hardening chamber 5 having the above described construction. In Fig. 3, the range between O and A along the time axis is a so-called critical range in which the material being treated W is to be cooled quickly by operating the upwardly discharging pump 26. The range between A and B is a relatively slow cooling stage of the material W once the pump 26 has been stopped. That is, when the upwardly discharging pump 26 is stopped, the temperature of the quench oil 20 rises due to heat transferred from the material being treated W. Consequently, the material W is cooled down slowly. The range between B and C is a stage to decrease the temperature difference between the upper and lower parts of the material being treated W by operating the circulation pump 27. The circulation pump 27 supplies the quench oil in the quenching frame 21, sucking from the upper part and supplying to the lower part. Thus, the quench oil in the quenching frame 21 circulates vertically and reduces the temperature difference between the upper and lower parts of the material being treated W. The range between C and D is a stage to enhance martensitic transformation by decreasing the temperature of the material W and the temperature of the quench oil 20 by restarting the pump 26. The range between D and E is a slinger process. An invertor is used to operate the upwardly discharging pump 26 to enable changing flow velocity by setting its frequency at a suitable value. Operation time of the pump 26 can be set at predetermined intervals using a timer. In the purge chamber 6 adjacent to the hardening chamber 5, nitrogen or carbon dioxide gas can be purged so as to form a shielded environment during transportation of the treated material W. Fig. 4 shows a processing sequence during a carburising treatment using the aforementioned gas carburising apparatus. In one example, a gross weight of 300kg of material to be treated W was preheated up to 930°C in 1.2 hours in the preheating chamber 1. In the initial stage following charging of the material to be treated W, heating was controlled by stopping the fan 15 and shot purging with butane. Then, the material W, which had been preheated to 930°C, was transferred to the carburising chamber 2 to be heated up to 1050°C in 0.43 hour and to carry out a carburisation treatment in 1.18 hours in a carburising atmosphere comprising butane supplied at the flow rate of 1-5 l/min. as hydrocarbon gas and carbon dioxide at the flow rate of 0.5-2.0 l/min. as oxidizing gas. Thereafter, the material being treated W was cooled down to 500°C in 0.17 hour in the cooling chamber 3, then reheated to the preferable hardening temperature of 850°C in 0.6 hour in the reheating chamber 4, followed by hardening with the laminar flow method resulting in a carburised layer of thickness greater than 1.3 mm. Total time required for the carburisation including hardening was 3.35 hours, and so-called cycle time corresponds to the longest staying time, that is, the pre-heating time of 1.2 hours. Therefore, hourly production rate can be as much as 300 kg/1.2 hour = 250 kg/hour. A processing sequence for a common carburising treatment of the prior art (carburising temperature: 930°C) is shown in Fig. 5, for comparison with the preferred carburising process of the present invention. In this reference, a carburising treatment using 550 kg of material was performed in a batch furnace. The material being treated W and the carburising atmosphere in this reference were the same as those used in the present invention. Total time required for the carburisation process to hardening in this reference was 7.5 hours, and the hourly production rate was 550 kg/7.5 hours = 73 kg/hour. Comparing the hourly production rates of the two processes, gives 250 kg/73 kg = 3.4, and therefore the hourly production efficiency for the preferred carburising process of the present invention was 3.4 times higher. That means that the treatment time can be reduced. Since gas consumption is decreased by reducing the treatment time, the illustrated carburising process according to the present invention is more economical. By increasing tray components in the preheating chamber, carburising chamber and reheating chamber in the case of the present invention, the hourly production rate can be further increased. As heating media, either electric power or gas can be used. Further, grain boundary oxidation of SCM420-type material was 20-25 µm in the reference shown in Fig. 5, but it could be reduced to less than 15 µm in the illustrated example of the present invention shown in Fig. 4.

Claims: A gas carburising process comprising the sequential steps of: preheating the material to be treated (W) to a temperature of 750-950°C in a pre-heating chamber (1); heating the material to be treated (W) up to 1000-1100°C in a directly supplied carburising atmosphere of hydrocarbon and oxidizing gases in a carburising chamber (2); forcibly cooling the material being treated (W) to a temperature below 600°C in a cooling chamber (3); reheating the material (W) to 750-850°C in a reheating chamber (4); and hardening the material (W) in a hardening chamber (5), wherein each of said chambers has its own transfer means (14), and are connected in series through opening/closing doors (8-12), to transfer said material to be heated (w). A gas carburising process as claimed in claim 1, wherein the material to be treated (W) is preheated to 930°C. A gas carburising process as claimed in claim 1 or 2, wherein the material to be treated (W) is carburised at 1050°C. A gas carburising process as claimed in any of claims 1, 2 or 3, wherein the material (W) is forcibly cooled to a temperature of 500°C after carburisation. A gas carburising process as claimed in any preceding claim, wherein the material is reheated to 850°C. A gas carburising process as claimed in any preceding claim, wherein hardening is achieved by quenching the material (W) in a laminar flow of quench oil. A gas carburising process as claimed in any preceding claim, wherein the material (W) is hardened at 210°C.

Inventors: GOI HITOSHI, KAMISUGI HIROFUMI, KAWAMURA TOSHIYUKI, MURAYAMA ATSUSHI, GOI, HITOSHI, KAMISUGI, HIROFUMI, KAWAMURA, TOSHIYUKI, MURAYAMA, ATSUSHI, Goi, Hitoshi, c/o Dowa Mining Co., Ltd., Kamisugi, Hirofumi, c/o Dowa Mining Co., Ltd., Kawamura, Toshiyuki, c/o Dowa Mining Co., Ltd., Murayama, Atsushi, c/o Dowa Mining Co., Ltd.
Applicants: DOWA MINING CO, DOWA MINING CO., LTD.
Application year: 1995
Main classification: C23C8
IPC codes: C21D9, C21D1, C23C8