KR20100115637A - Eco friendly nitrocarburizing process at high temperature - Google Patents

Abstract

PURPOSE: An environmentally-friendly high-temperature nitrocarburizing method is provided to reduce soot generation using a small amount of hydrocarbon gas besides ammonia and nitrogen. CONSTITUTION: An environmentally-friendly high-temperature nitrocarburizing method comprises steps of: loading a steel material into a furnace of 500~600°C and preheating the steel material while injecting inert gas, nitrocarburizing the steel material by maintaining it at 850~1100°C in the atmosphere of a mixed gas consisting of ammonia, hydrocarbon gas and nitrogen gas for 1~100 hours, and rapid-cooling the treated material.

Description

Eco friendly nitrocarburizing process at high temperature}

The present invention is an environmentally friendly high temperature that heats a steel material to the austenite zone and adsorbs and diffuses nitrogen, which is the main hardening element, and carbon, which is an additional hardening element, on the surface using a mixture of ammonia, a small amount of hydrocarbons and nitrogen It relates to a nitriding method.

Surface modification treatment methods to improve the wear resistance and fatigue strength of iron or iron alloys are typically carburizing process by diffusing and infiltrating carbon and carbonitriding process by diffusing and infiltrating carbon and nitrogen. There is.

The carburizing heat treatment and carburizing nitriding heat treatment include gas method, plasma method, salt bath method and vacuum method depending on the medium containing carbon and nitrogen, and the most widely used gas method is endothermic type produced by modifying hydrocarbon gas. A small amount of propane, methane, butane, etc. is added to the gas (RX gas) as an enrichment gas, or a hydrocarbon-based solution such as methanol-nitrogen-hydrocarbon gas or toluene or benzene is added dropwise to the carburizing atmosphere. Carburization is formed, and the carburization-nitriding heat treatment is performed by carburizing heat treatment at 910 ° C. or higher and injecting 5-7% ammonia for 30 minutes to 1 hour in a diffuser at 850 ° C.

This carburizing heat treatment was carried out at a high temperature of 880 ° C to 940 ° C and the martensite structure containing about 0.8% to 1.0 wt.% Of carbon as the main hardening element was obtained on the surface to obtain high surface hardness and deep cure depth. Since martensitic structure can be obtained, it has the advantages of excellent wear resistance and high strength, and is widely applied to heat treatment processes such as power transmission gears of automobiles or industrial machines.

However, in such carburizing and carburizing and nitriding processes, the gas formed by endothermic gas or methanol decomposition contains more than 40% of carbon monoxide, depending on the composition of the gas used. Therefore, gas carburization or gas carburization is a process that generates a large amount of carbon dioxide and is not environmentally friendly.

In addition, as shown in Fig. 1, the gas carburizing method and the carburizing nitriding method have a problem in that abrasion resistance is lowered or fatigue life is lowered due to the formation of an abnormal layer on the carburized surface, which is troublesome to remove it. In addition, carburizing parts, depending on the use environment, the temperature of the parts rises, carburizing heat treatment (including carburization nitriding) has a problem that the hardness of the high temperature tempering softening resistance is low, the sharp drop at a high temperature.

On the other hand, gas nitrocarburizing or gas nitriding is mainly performed at endothermic gas, nitrogen, ammonia, etc. at a temperature of 590 ° C. or lower, which is the A ₁ transformation point (vacancy temperature) of Fe-N. A process that adsorbs and diffuses nitrogen or nitrogen and carbon on the surface by using a mixed gas. This process is applicable to most steel materials. Since it is a low temperature process, there is almost no phase change due to quenching. It has the advantage of low deformation and low coefficient of friction.

However, since it is a low temperature process, the diffusion depth of nitrogen and carbon is small and the martensite transformation does not occur at the core part, so it is hard to be hardened. The surface hardness is about Hv 600 and Hv 200 ~ 300 at depth of 0.5㎛, and it is applied to parts requiring high strength. Not enough to do it.

Moreover, this technique makes the iron-nitrogen compound layer of the outermost surface of steel parts, and there exists a problem of peeling this compound layer, and it cannot use it for the use which may be impacted on the surface. In addition, the problem of nitriding or surface defect may occur due to the process oil or rust preventive oil in the whole process beyond carburizing hardening.

In addition, the vacuum material which charges and diffuses nitrogen on the surface by inserting steel parts into a furnace at room temperature and heating to 600-800 ° C. under reduced pressure, using ammonia gas or ammonia gas and a gas that does not adversely affect nitriding. Speech is known. This method has the advantage of obtaining high strength and hardening even in deeper parts compared to gas nitriding or gas nitriding, but it is impossible to obtain surface hardening effect by carbon other than nitrogen, and the apparatus is expensive, maintenance costs are high, and treatment is performed. The disadvantage is small capacity.

Accordingly, the present invention has been made in view of the above problems of the prior art, and the present invention uses a nitriding-carburizing gas to adsorb and diffuse nitrogen and carbon on the surface of a material heated to an austenite region, and then rapidly cools the surface. An object of the present invention is to provide a martensite structure having an ideal layer and having a solid solution strengthening effect of nitrogen and carbon, and to provide a method for producing a metal material having high wear resistance, tempering softening resistance and high strength characteristics.

In addition, the present invention, using only a small amount of hydrocarbon gas in addition to ammonia and nitrogen, there is no soot generation, and the surface of the material to be processed is activated through high temperature treatment than conventional gas nitriding or gas nitriding, which is hardly affected by the entire process. It is an object of the present invention to provide a method for producing a metal material having high wear resistance, tempering softening resistance, and high strength characteristics, as a result of which a good surface is obtained.

Another object of the present invention is to provide an environment-friendly surface modification process by performing a surface hardening treatment with little carbon dioxide in the waste gas, unlike conventional carburizing heat treatment which discharges a large amount of carbon dioxide in the waste gas.

Other objects and advantages of the invention will be described below and will be appreciated by the embodiments of the invention.

In order to achieve the above object, according to the present invention, the step of charging the steel material in the furnace maintained at 500 ℃ to 600 ℃ in a heat treatment furnace and preheating while injecting inert gas; Nitriding and carburizing the steel material by maintaining the steel material at a temperature of 850 to 1100 ° C. for 1 to 100 hours in a mixed gas atmosphere of ammonia, a nitrocarburizing gas, and a hydrocarbon gas and nitrogen gas; Subsequently, the step of lowering the temperature to 800 to 880 ° C and maintaining the same furnace for 1 to 10 hours to perform diffusion heat treatment; And it provides an environmentally friendly high temperature nitriding carburizing method comprising the step of quenching the material in which the diffusion heat treatment is complete.

The present invention charges the steel material in the furnace maintained at 500 ℃ to 600 ℃ and preheated for 1 to 5 hours while injecting nitrogen gas while introducing a mixed gas of ammonia, a nitrocarburizing gas and a small amount of hydrocarbon gas and nitrogen gas Nitrocarburizing while controlling the temperature to be in the austenite region at 850 to 1100 ° C and controlling the nitrogen concentration, which is the main hardening element, to be in the range of 0.05 to 1.0% in the workpiece for 1 to 100 hours while maintaining a certain temperature of this temperature range. After the temperature is lowered to 800 to 880 ° C., and subjected to diffusion heat treatment of nitrogen and carbon for 1 to 10 hours while maintaining a certain temperature according to the composition of the treated material, it is quenched.

 The present invention heats the entire material to a single region of austenite and uses a high diffusion coefficient of nitrogen and carbon atoms at high temperatures to sufficiently diffuse nitrogen and carbon atoms, which are the main hardening elements, and then crack at lower temperatures. By treating and quenching the nitrogen concentration of the surface, the surface is made by solid solution strengthening and martensitic transformation of nitrogen as the main hardening element and carbon as the additional hardening element, and the core is high due to high wear resistance and high nitrogen employment due to the martensite transformation. It is possible to provide a steel material having tempering softening resistance and high strength characteristics, and is an environmentally friendly nitride carburizing process in which carbon dioxide is hardly generated in the treatment process.

In addition, the present invention does not form a compound layer that can be peeled off, so that the present invention can also be applied to a part that is likely to be impacted. Furthermore, since the surface abnormality layer is not formed, there is no need to remove it.

In addition, since only a small amount of hydrocarbon gas is used in addition to ammonia and nitrogen, there is no soot, and since it is treated at a higher temperature than gas nitriding or gas nitriding, the surface of the material to be treated is activated and hardly affected by the entire process. Material can be provided.

The present invention thus provides a sufficiently high surface hardness nitride-carburized quenched product, free of iron or iron alloys, in particular no carbon or iron alloys of 0.45 wt. The penetrating treatment of nitrogen and carbon in a shorter time has a hardened layer of sufficiently high hardness than in the case of the conventional nitriding treatment, carburizing or nitriding treatment, etc., thereby providing an economical manufacturing method.

As described above, the present invention provides the following effects.

First, the process of the present invention is a high temperature treatment process is a martensite structure formed by the solid solution of nitrogen and carbon on the surface, the surface hardness is high, and sufficient hardening depth can be obtained by diffusion penetration of nitrogen and carbon, the core part of the martensite Since a single phase appears, it is possible to provide a high strength material.

Second, it is possible to provide a material that maintains high strength even when used at a high temperature for a long time by improving tempering softening resistance of nitrogen, which is a major surface hardening element.

Third, the decarburized or oxidized layer formed by traces of oxygen or moisture during preheating or treatment is reduced again by hydrocarbon gas or by hydrogen produced by ammonia decomposition, so that no surface abnormality layer as in gas carburizing heat treatment occurs. .

Fourth, compared with the conventional gas carburizing heat treatment, the amount of carbon dioxide which causes global warming in the exhaust gas is extremely small, thereby providing an environment-friendly surface modification process.

Hereinafter, the present invention will be described in more detail.

First, the to-be-processed material (material) is charged to a furnace maintained at 500 ° C to 600 ° C and preheated for 1 to 5 hours while injecting an inert gas. During preheating, the treatment time is determined by the thickness of the material, that is, the mass effect and the amount of material to be treated.The processing temperature does not cause the phase transformation of the material at this temperature, so that the inside and outside are uniformly heated to minimize thermal deformation. It is the proper temperature to reduce deformation by transformation when heated at temperature.

The preheating time is preferably 30 minutes to 1 hour per inch of thickness of the workpiece. In this case, in order to prevent oxidation, nitrogen gas is preferably injected.

Thereafter, the temperature is raised to a predetermined nitriding carburization temperature of 850 ° C to 1100 ° C, and a nitriding carburizing gas is injected into the furnace. The nitride carburizing gas may be a gas having the same composition or a gas having a different composition from that used for the nitride carburizing at 850 ° C to 1100 ° C.

Subsequently, nitriding is carried out for 1 to 100 hours while injecting a nitrocarburizing gas at a predetermined temperature of 850 ° C to 1100 ° C. Limiting the nitrification temperature to 850-1100 ° C is related to the diffusion rates of nitrogen and carbon.

That is, the higher the treatment temperature, the higher the hardness and the sufficient curing depth can be obtained in a short time. However, if the temperature is higher than this, the decomposition rate of ammonia is so large that the source of nitrogen atoms is small, and the grains of austenite are coarsened, which may lower the strength and fatigue strength. On the other hand, if the temperature is lower than this, the time required for nitriding and carburizing is excessively long. In consideration of this, it is preferable to treat it at about 930 ° C.

In addition, the lower limit of the nitrification time is limited to 1 hour because at least 1 hour of treatment time is required when treating small weights and small volumes, and if the treatment time is shorter, sufficient curing depth cannot be obtained. The reason for making 100 hours is that the carburizing depth obtained at the heat treatment time longer than 2.5 mm is not required.

 As said nitriding-carburizing gas composition, ammonia gas is 20 to 95%, hydrocarbon is 2 to 10%, and the remainder is nitrogen gas. If the ratio of the hydrocarbon gas is lower than this, the surface hardness may be lowered due to insufficient supply of carbon atoms required for carburization, and soot may occur if higher than this ratio.

In this case, as the hydrocarbon gas, propane (C ₃ H ₈ ), methane (CH ₄ ), ethylene (C ₂ H ₄ ), acetylene (C ₂ H ₂ ), butane (C ₄ H ₁₀ ) or ethane (C ₂ H ₆ ) Gas can be used.

The nitriding-carburizing gas is introduced while controlling to diffuse nitrogen in the concentration of 0.05-1.0 wt.% In the workpiece, depending on the required characteristics of the workpiece. The method of introducing the nitriding-carburizing gas in the furnace to produce such a result in the workpiece can be determined experimentally and empirically.

Basically, it feeds back from the process result about the to-be-processed object, and determines the introduction method of nitrocarburizing gas in a furnace. It is advisable to prepare several patterns for each type of heat treatment furnace and to temporarily determine the introduction method of nitrocarburizing gas, and to fine-tune the results of the treatment.

Alternatively, the detection may be performed based on various detectable indicators of the furnace state. For example, the concentration of undecomposed residual ammonia remaining in the furnace is detected, or gas components in the furnace are detected using gas chromatography, and the respective concentrations are fed back from the treatment result generated in the object to be processed. It is also possible to determine the method of introducing the nitrocarburizing gas.

In addition, in consideration of the amount of the material to be loaded, its shape, the size of the furnace, and the like, the amount of introduction introduced per unit time is appropriately set, or the nitriding and carburizing time or temperature can be adjusted therein to assist auxiliary control.

In the carburizing nitride of the present invention, the decarburized layer formed by the trace amount of oxygen or moisture during preheating or treatment is recoagulated by carbon monoxide gas and the oxide layer is reduced again by hydrogen generated by ammonia decomposition, as shown in FIG. 2. As a result of observing the microstructure of the surface, the surface abnormality layer as in gas carburization heat treatment does not occur.

Although it may be quenched immediately after the treatment at 850 to 1100 ° C., when quenched at such a high temperature, too much residual austenite may be generated to lower the maximum surface hardness or increase the dimensional deformation, thereby lowering the temperature to 800 to 880 ° C. 1 It is preferable to carry out diffusion heat treatment of nitrogen and carbon for ˜10 hours to control the concentration of nitrogen and carbon at the outermost surface, and to quench the entire material to a uniform temperature.

That is, in order to reduce the deformation of the workpiece and to control the amount of retained austenite, the temperature is lowered to 800 to 880 ° C, the lowest possible quenching temperature, and then subjected to quenching, preferably to be treated at about 850 ° C. good.

At this time, the gas is introduced while controlling nitrogen to be diffused to a concentration of 0.05 to 1.0 wt.%, Which is the main hardening element, in the product to be treated. Although it is desirable to use a gas having the same composition as the gas used for nitriding and carburizing at 850 to 1100 ° C from an economical point of view, a gas having a different composition may be used as long as the above object can be achieved.

Cooling may be carried out by injecting nitrogen gas or inert gas into the furnace for gas cooling or by using a cooling medium composed of water, oil or a suitable polymer quench liquid mixed with water by taking the material out of the furnace.

According to the manufacturing method of the nitriding-carburized article of the present invention, the penetration depth of nitrogen and carbon to the to-be-processed article can be freely controlled by the extremely simple operation of adjusting the time of nitriding-carburizing process.

In addition, since the nitriding carburization temperature is controlled in the above range and the introduction of the nitriding carburizing gas is controlled as described above to generate a thick nitrogen diffusion layer, an unnecessary compound layer is not generated even if a long treatment time is required.

In addition to controlling the nitriding time, it is possible to more efficiently control the penetration depth of nitrogen and carbon by adjusting the nitriding temperature within the temperature range.

The nitrogen diffusion produced through the nitriding and carburizing treatment is 130 ° C lower than the austenitic temperature of Fe-C steel, and the Ms point, which is the diffusion diffusion of nitrogen, is higher than that of carburized carbon and has good hardenability. . In addition, by adjusting the quenching temperature, it is possible to martensite only the whole of the workpiece or the diffusion layer of nitrogen produced by nitriding, or to obtain a cured layer having a necessary depth and hardness.

 Therefore, according to the present invention, by producing nitrogen martensite of Hv700 or more in the surface layer of the workpiece and forming martensite up to the core portion or the required depth, a material of high wear resistance and high strength can be made, and no abnormal layer is formed on the surface layer. Do not.

In addition, as described above, in the diffusion region of nitrogen produced through nitriding and carburizing, the austenitization temperature range is lower than that of Fe-C steel, so that the quenching temperature decreases, consequently contributing to the reduction of quenching strain.

 In the conventional carburizing nitriding heat treatment, carbon is the main hardening element, while nitrogen is the additional hardening element, whereas in the high temperature austenite nitride carburizing according to the present invention, nitrogen is the main hardening element and carbon is the additional hardening element.

Nitrogen-containing martensite structure obtained in accordance with the present invention has a higher temper softening resistance than carbon-containing martensite obtained in carburizing heat treatment, so that it can maintain high strength even at high temperature for a long time, so that the temperature rises to about 400 ° C during high-speed driving. It is good to use for a gear.

The heat treatment furnace for the nitriding carburizing treatment may be a fit furnace, a sealed quench furnace, a fluidized bed furnace or a continuous furnace, but is not particularly limited to these furnaces as long as the nitriding carburizing conditions are satisfied. Do not.

In the case of nitrification, the composition of gas is 20 to 95% for ammonia, 2 to 10% for hydrocarbon, and nitrogen gas for the rest.

In the conventional carburizing heat treatment, the ratio of carbon monoxide in the injection gas is in the range of 40%, whereas in the gas composition according to the present invention, the ratio of hydrocarbons in the entire injection gas is less than 10%, and the concentration of carbon dioxide in the exhaust gas is extremely small.

For example, in a pit-type heat treatment furnace with a diameter of 500Φ and a height of 1000 mm held at 935 ° C., a mixed gas (volume%) having a total flow rate of 2.6 m ³ / hr., 90% ammonia, 5% nitrogen, and 5% propane. As a result of analyzing the components of the exhaust gas using gas chromatography while injecting, as shown in Table 1, the carbon dioxide concentration was only about 0.02%.

(vol.%) ingredient Hydrogen Oxygen nitrogen carbon monoxide carbon dioxide moisture Propane content(%) 70.7494 0.124 25.61 1.1732 0.0214 0.2038 0.2139

According to the present invention, in the case of nitriding for a relatively long period of time, in order to diffuse nitrogen into the entire workpiece, the surface layer does not form a compound layer or a surface abnormality that may be peeled off. It can raise enough.

Therefore, after determining the introduction method and nitriding temperature of nitrification carburizing gas under the predetermined conditions or ranges, the nitriding time is set based on the penetration rate of nitrogen and carbon determined accordingly. Depth can be set freely.

In addition, the introduction method of nitrocarburizing gas in each condition and the treatment at the nitriding carburizing temperature in the above range enable the diffusion and penetration of nitrogen and carbon at a high speed. Depth can be obtained respectively. In addition, when the workpiece is a material having a thickness of about 3 to 4 mm, even when carburizing diffusion to the core portion is required, it is possible in a short time.

Iron or various iron alloys can be used as the material of the workpiece to be hardened. In particular, an iron alloy having a carbon content of 0.45% or less is suitable, and there is no problem even if aluminum, chromium, silicon or nickel are not included. An expensive material containing chromium or the like may also be used as the object. The present invention is characterized by obtaining the same surface hardness as using an alloy steel containing expensive chromium or the like by using inexpensive iron or low carbon steel.

(Example 1)

The SCM415 and SCr420HB materials were charged into a pit furnace with a diameter of 500 mmφ and a height of 1000 mm maintained at 550 ° C., and preheated for 2 hours while injecting nitrogen at 1 m ³ per hour, followed by a total flow rate of 2.0 m ³ / hr, 88% ammonia, 6.5 Nitrogen carburizing treatment at 920 ° C. for 3.5 hours while injecting a process gas (% by volume), which is% butane and the remainder, followed by diffusion treatment at 850 ° C. for 1.5 hours, followed by oil-cooled high-temperature nitride carburizing material and SCr420HB In the same furnace as above, the total flow rate was 1.8㎥ per hour, gas composition was 23.7% carbon monoxide, 31.7% hydrogen, 1.0% propane, and the rest was nitrogen-carburized heat treatment for 3 hours at 930 ° C and diffusion for 2 hours. As a result of comparing the surface hardness distribution of the material obtained by heat-cooling after heat treatment, as shown in Fig. 4, the effective hardening depth (based on Hv 550) was 0.42 mm for the carburized heat treated product (previous product), nitrification Tan treatment material could be seen that the effective hardening depth of product high temperature carburizing nitriding of the present invention to 0.81mm excellent.

(Example 2)

The S20C, SCM415 and SCr420HB materials were charged into a pit furnace with a diameter of 500 mmφ and a height of 1000 mm maintained at 550 ° C, and preheated for 1.5 hours while injecting 1 m ^{3 of} nitrogen per hour, followed by a total flow rate of 2.3 m ³ / hr, 85% ammonia , Nitrogen carburization treatment at 930 ° C for 3.5 hours while injecting process gas with 7% ethylene and the rest, followed by diffusion treatment at 850 ° C for 1.0 hour, followed by water-cooled high-temperature nitrided carburizing material and SCM420 material. 2.0㎥, gas composition: carbon monoxide 25%, hydrogen 30%, propane 0.8%, the rest is nitrogen carburizing gas atmosphere at 3 hours by carburizing heat treatment at 930 ℃ for 3 hours, diffusion heat treatment for 1.5 hours, the tempering temperature of the material obtained by cooling the oil As a result of comparing the hardness, the hardness of the high temperature nitride product of the present invention was high at all tempering temperatures, as shown in FIG. 5, and from this, the tempering softening resistance of the high temperature nitride carburized product of the present invention. It was found that the sex is excellent.

Example 3

The high temperature nitride carburized product obtained by treating the SCr420HB material under the same conditions as in Example 1 and the carburized heat treated product obtained by the conventional method were subjected to an oil temperature of 50 ° C. and 1000 rpm using a roller fitting wear tester (KOMATSU, RPT 201 model). As a result of measuring the rotation speed until the occurrence of fitting, the high temperature nitride carburized product of the present invention was found to be at least 20% higher according to the load, as shown in FIG. It can be seen that it has the same endurance life even under 20% higher load.

As described above, although the present invention has been described by means of a limited embodiment, the present invention is not limited thereto and will be described below by the person skilled in the art and the technical spirit of the present invention. Of course, various modifications and variations are possible within the scope of the claims.

Figure 1 is an optical micrograph showing that the surface abnormal layer appears by the conventional carburizing heat treatment.

Figure 2 is an optical micrograph showing the surface texture by the method of the present invention.

Figure 3 shows an example of the surface modification process according to the present invention.

Figure 4 is a graph showing the surface hardness change of the treated material obtained according to the present invention in comparison with the conventional one.

5 is a graph showing the hardness change with respect to the tempering temperature of the treated material obtained according to the present invention in comparison with the conventional one.

6 shows the relationship between the fitting life and the load of the treated material obtained according to the present invention together with the conventional one.

Claims (5)

Charging the steel material to a furnace maintained at 500 ° C. to 600 ° C. in a heat treatment furnace and preheating the inert gas; Nitriding and carburizing the steel material by maintaining the steel material at a temperature of 850 to 1100 ° C. for 1 to 100 hours in a mixed gas atmosphere of ammonia, a nitrocarburizing gas, and a hydrocarbon gas and nitrogen gas; Then, the temperature is lowered to 800 to 880 ° C. and maintained for 1 to 10 hours to perform diffusion heat treatment; And Eco-friendly high temperature nitriding carburizing method comprising the step of quenching the material after the diffusion heat treatment. The method according to claim 1, The nitride carburizing gas is volume%, 20 to 95% ammonia, 2 to 10% hydrocarbons, and the rest is nitrogen gas. The method according to claim 1, The hydrocarbon gas is methane (CH ₄ ), propane (C ₃ H ₈ ), ethylene (C ₂ H ₄ ), acetylene (C ₂ H ₂ ), butane (C ₄ H ₁₀ ), ethane (C ₂ H ₆ ) Environmental friendly high temperature nitriding carburizing method, characterized in that at least one gas selected. The method according to claim 1, The heat treatment furnace subjected to the nitriding carburization treatment is any one selected from a pit furnace, a sealed quench furnace, a fluidized bed furnace, and a continuous furnace. The method according to claim 1, The inert gas is an environmentally friendly high temperature nitriding carburizing method, characterized in that the nitrogen gas.

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