JP2009001863A - Nitriding treatment method, mechanical component having different materials jointed, method for manufacturing engine valve, and engine valve - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing an engine valve with a shaft part having different materials jointed in its middle, which forms a uniform nitride layer on the shaft and simultaneously minimizes a difference of shaft diameters, which is formed in the jointed part after nitriding treatment. <P>SOLUTION: The nitriding treatment method of an engine valve comprises: a fluorine treatment step of heating and holding the engine valve in a gas atmosphere containing a fluorine source gas to form a fluoride on the surface; a carbon-diffusing treatment step of heating and holding the engine valve in a gas atmosphere which mainly contains a carbon source gas to make carbon preferentially diffused in the surface; and a nitriding treatment step of heating and holding the engine valve in a gas atmosphere which mainly contains a nitride source gas. Even when the engine valve is a jointed valve of which the valve stem and the valve umbrella are made from each different material, the difference of the shaft diameters in both sides of the jointed part can be minimized after the nitriding treatment, by holding the engine valve in the atmosphere containing the carbon source gas of 5% by volume or more at 300 to 600°C for 10 minutes or longer after the fluorine treatment step. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nitriding method, a dissimilar material joining machine component, an engine valve manufacturing method, and an engine valve.

  Surface hardening treatments such as carburizing treatment and nitriding treatment are mainly used for various machine parts such as automobile parts and compressors as a means to improve mechanical properties such as wear resistance, seizure resistance and fatigue strength of steel materials. It is used in a wide range of fields. Of these, nitriding is performed at a temperature lower than the transformation point of steel, so parts that require precision in various shapes, mainly sliding parts, due to the fact that there is little distortion. It is also applied to.

At this time, the nitrogen compound layer formed on the surface of the steel material is high in altitude, and has a property of being particularly excellent in wear resistance and seizure resistance because the reactivity with the sliding metal material is lower than that of the base material. Therefore, regardless of the nitriding treatment method such as gas nitriding treatment, salt bath nitriding treatment, ion nitriding treatment, etc., a nitrogen compound mainly composed of Fe ₃ N and Fe ₄ N is formed on the surface of the steel material to satisfy the durability performance of the parts. Things have been done.

  Among mechanical parts to which nitriding is applied, there are engine valves used as automobile parts, for example. Since engine valves are used in harsh environments, they are components that require not only wear resistance but also heat resistance and impact resistance. In particular, the exhaust valve is exposed to high-temperature exhaust gas, and the maximum temperature of the face portion reaches about 700 ° C. For this reason, austenitic heat-resistant steel such as SUH35 and SUH36 containing a large amount of Cr is often used for the portion on the valve umbrella side where the temperature becomes higher even in the exhaust valve. For the shaft side part, heat-resistant steels such as SUH3 and SUH11, which have lower Cr and Ni contents and are less expensive, are used. As illustrated in FIG. A joining valve joined in the middle has been mainly used.

  In recent years, there has been a tendency to further increase the combustion temperature for the purpose of improving the combustion efficiency of the engine. In such a high temperature environment, the above heat resistant steel has sufficient wear resistance and impact resistance. May not be able to get. Therefore, an engine valve using a corrosion-resistant heat-resistant alloy such as NCF750 or NCF751 containing a large amount of Ni, which is superior in heat resistance at a higher temperature, on the valve umbrella side has begun to be used (Patent Document 1 below).

  However, since corrosion-resistant and heat-resistant alloys such as NCF750 and NCF751 contain a large amount of Ni, an oxide film that is very difficult to reduce, such as NiO that inhibits penetration, is formed on the surface thereof. Of course, it is difficult to form a uniform nitride layer even by a method such as salt bath nitriding or ion nitriding with strong nitriding power.

As a method for nitriding the corrosion-resistant and heat-resistant alloy that is a hardly nitrided material as described above, there is a method of performing a fluorination treatment as a pretreatment of the nitriding treatment. This method uses the property that fluoride is more easily reduced than oxide in the reducing atmosphere, while fluoride is more stable than oxide. After replacement with a fluoride film, the above fluoride film can be easily reduced with hydrogen generated by the decomposition of NH ₃ by supplying a nitrogen source gas having a reducing property in the nitriding temperature range, for example, NH _3. Simultaneously with the removal, nitrogen generated by the decomposition of NH ₃ can enter and diffuse into the treated product. This method makes it possible to form a uniform nitrided layer even with a corrosion-resistant and heat-resistant alloy that is normally difficult to nitride (Patent Document 2 below).
JP-A-9-256821 JP-A-5-195193

  However, when a nitriding treatment is performed to form a nitride layer on the material surface, the object to be processed expands due to the formation of nitrogen compounds and the intrusion of nitrogen atoms. For example, when the valve shaft side material is composed of steel types mainly composed of ferrite and martensite, and the valve head side material is composed of a corrosion resistant heat resistant alloy mainly composed of austenite, nitriding treatment is performed, respectively. It was extremely difficult to form a predetermined nitride layer on both of the two types of materials, because the diffusion rate of nitrogen atoms was different for these materials.

  In addition, since the state of the formed nitride layer is also different, the expansion amount is also different, resulting in a difference in the expansion amount after the nitriding treatment, and a shaft diameter difference is generated on both sides of the joint portion. When such a shaft diameter difference occurs, problems such as abnormal noise or gas leakage occur when the engine is driven. As a countermeasure, after the nitriding treatment, a method of reducing the shaft diameter difference of the joint portion by grinding or polishing the shaft portion including the joint portion may be considered, but the nitride layer thickness is reduced by the above grinding and polishing. There is a possibility that sufficient wear resistance may not be obtained, and at the same time, the number of processes increases, leading to an increase in cost.

  The present invention has been made in view of such circumstances, and in a joining member of different materials, a predetermined nitriding layer is formed on both materials, and a nitriding treatment method in which a dimensional change of a joint portion in nitriding treatment is reduced and A first object is to provide a dissimilar material joining machine part, and a predetermined nitrided layer is formed on both materials of an engine valve composed of different materials on the valve shaft side and the valve head side, and a joining part by nitriding treatment A second object of the present invention is to provide an engine valve manufacturing method and an engine valve with a reduced shaft diameter difference.

  In order to achieve the above object, the first nitriding method of the present invention is a bonding member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are bonded. On the other hand, before performing the nitriding treatment for heating and holding in the nitriding gas atmosphere to form a nitride layer on the surface layer portion, the fluorination treatment by heating and holding in the fluorine-based gas atmosphere is performed in advance, and in the carbon source gas atmosphere. The gist is to reduce the difference in the dimensional change between the first material and the second material in the nitriding treatment by performing a carbon diffusion treatment in which a carbon diffusion layer is formed on the surface layer by heating and holding.

  In order to achieve the above object, the second nitriding method of the present invention provides a joining member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are joined. On the other hand, before performing the nitriding treatment for heating and holding in the nitriding gas atmosphere to form a nitride layer on the surface layer portion, the fluorination treatment by heating and holding in the fluorine-based gas atmosphere is performed in advance, and in the carbon source gas atmosphere. A carbon diffusion treatment for forming a carbon diffusion layer on the surface layer portion by heating and holding is performed, and the carbon potential of the atmospheric gas in the carbon diffusion treatment is controlled, whereby a difference in dimensional change between the first material and the second material in the nitriding treatment is achieved. The gist is to control.

  In order to achieve the above object, the dissimilar metal joining machine component of the present invention includes a first material mainly composed of a ferrite phase or a martensite phase, and a heat resistant steel or Ni composed mainly of an austenitic phase containing 20 mass% or more of Ni. The gist is that the second material of the base alloy is joined to form a nitride layer of 5 μm or more on the surface of both the first material and the second material.

  In order to achieve the above object, the method for manufacturing an engine valve according to the present invention is such that the material on the valve shaft side is a steel material mainly composed of a ferrite phase or a martensite phase, and the material on the valve head side contains 20% by mass or more of Ni. Before performing nitriding treatment to form a nitride layer on the surface layer by heating and holding the engine valve composed of heat-resistant steel or Ni-base alloy mainly composed of austenite phase to form a nitride layer on the surface layer portion While performing the fluorination treatment that is heated and held in the gas atmosphere, the carbon diffusion treatment is performed in which the carbon diffusion layer is formed in the surface layer portion by heating and holding in the carbon source gas atmosphere. The gist is to hold at a temperature of 300 to 600 ° C. for 10 minutes or more in an atmosphere of 5% by volume or more.

  In order to achieve the above object, the engine valve of the present invention is a first steel material mainly composed of a ferrite phase or a martensite phase on the valve shaft side, and mainly composed of an austenitic phase containing 20% by mass or more of Ni on the valve head side. A heat-resistant steel or Ni-based alloy second material, which is an engine valve formed by joining the first material and the second material at a shaft portion, the surfaces of both the first material and the second material In summary, a nitride layer of 5 μm or more is formed.

  In the present invention, as the carbon source gas atmosphere, any of a gas containing a carbon source gas, a gas containing both a reducing gas and a carbon source gas, and a reducing carbon source gas can be used.

  In the first nitriding method of the present invention, a nitriding treatment is performed on a bonding member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are bonded. Further, by performing the fluorination treatment and the carbon diffusion treatment in advance, the difference in dimensional change between the first material and the second material in the nitriding treatment is reduced.

  By doing so, carbon diffuses preferentially on the surface of the joining member that is the object to be processed, and in the first material mainly composed of ferrite phase or martensite phase, nitrogen is removed from the surface by subsequent nitriding treatment. Even when trying to penetrate and diffuse, nitrogen atoms penetrate between the carbon atoms invading into the surface layer part, or penetrate while pushing the invading carbon atoms deeper. Become. Therefore, since the intrusion and diffusion of nitrogen atoms are suppressed as compared with the case where the carbon diffusion process is not performed, the growth of the nitride layer is also suppressed to some extent, and as a result, excessive expansion due to the nitriding process can be suppressed.

  On the other hand, in the second material whose material is mainly austenite phase, the diffusion rate of nitrogen and carbon itself is slower than the material whose material is mainly ferrite phase and martensite phase, but nitrogen and carbon can be dissolved. Since the amount is large, even if carbon is diffused preferentially on the surface by the carbon diffusion treatment, the effect of preventing the penetration of nitrogen in the subsequent nitriding treatment is slight.

  Even if it is the 1st material which mainly has a ferrite phase or a martensite phase, if it does not carry out fluorination treatment, it will invade and diffuse a carbon atom uniformly by the influence of the oxide film formed in the surface. Although difficult, by performing the fluorination treatment before the above-described carbon diffusion treatment, carbon atoms can penetrate and diffuse, and a uniform nitride layer can be formed even in the nitridation treatment performed thereafter. .

  As described above, by performing the fluorination treatment and the carbon diffusion treatment before the nitriding treatment, even if the joining member is obtained by joining the first material and the second material, predetermined nitriding is performed on both of the two types of materials. A layer can be formed. In addition, it is possible to form a nitride layer having excellent wear resistance on both materials without generating a large dimensional difference in the joint and without requiring a post-process such as polishing after nitriding. In addition, the growth of the nitride layer due to the nitriding treatment in the first material is suppressed and excessive expansion is suppressed. On the other hand, the action of preventing the nitrogen intrusion in the nitriding treatment in the second material is slight. The difference in dimensional change due to nitriding between the two materials is reduced.

  According to the second nitriding method of the present invention, before the nitriding treatment is performed on the bonding member in which the first material mainly composed of the ferrite phase or the martensite phase and the second material mainly composed of the austenite phase are bonded. In addition, the difference in dimensional change between the first material and the second material in the nitriding treatment is controlled by performing the fluorination treatment and the carbon diffusion treatment in advance and controlling the carbon potential of the atmospheric gas in the carbon diffusion treatment.

  As described above, in the first material mainly composed of the ferrite phase or martensite phase by the carbon diffusion treatment, intrusion and diffusion of nitrogen atoms in the subsequent nitriding treatment are suppressed. By controlling the carbon potential, the degree of expansion of the first material in the subsequent nitriding treatment can be controlled. Thereby, it becomes possible to control the difference in dimensional change between the first material and the second material in the nitriding treatment.

  The dissimilar material joining machine part of the present invention is a second material of a heat-resisting steel or Ni-based alloy mainly composed of a first material mainly composed of a ferrite phase or a martensite phase and an austenite phase containing Ni of 20% by mass or more. A nitride layer of 5 μm or more is formed on the surface of both the first material and the second material, and a nitride layer having excellent wear resistance is formed on both of the two materials. -ing

  In the dissimilar material joining machine part of the present invention, when the dimensional difference between the first material and the second material after the nitriding treatment for forming the nitride layer is 2 μm or less on one side, the dimensions of the joint The difference is small, a post-process such as polishing is not required, and a nitride layer having excellent wear resistance is formed.

  In the method for producing an engine valve of the present invention, the material on the valve shaft side is a steel material mainly composed of a ferrite phase or a martensite phase, and the material on the valve head side is mainly composed of an austenite phase containing 20% by mass or more of Ni. An engine valve made of heat-resistant steel or Ni-based alloy is subjected to fluorination treatment and carbon diffusion treatment before nitriding treatment, and the carbon diffusion treatment is performed at a carbon source gas concentration of 5% by volume or more. In an atmosphere to be maintained at a temperature of 300 to 600 ° C. for 10 minutes or more.

  In this way, carbon diffuses preferentially on the surface of the engine valve, which is the object to be treated, and in the case of steel materials mainly composed of ferrite phase or martensite phase, nitrogen enters the surface by the subsequent nitriding treatment. Even if it is intended to diffuse, nitrogen atoms penetrate between the carbon atoms entering the surface layer part, or enter while pushing the penetrated carbon atoms deeper. Therefore, the growth and growth of the nitride layer can be suppressed by suppressing the intrusion and diffusion of nitrogen atoms as compared with the case where the carbon diffusion treatment is not performed.

  On the other hand, in the case of materials mainly composed of austenite phase, especially corrosion-resistant heat-resistant alloys containing a large amount of Ni, the diffusion rate of nitrogen and carbon itself is slower than materials mainly composed of ferrite phase and martensite phase. However, since nitrogen and carbon can be dissolved in a large amount, even if carbon is preferentially diffused on the surface by the carbon diffusion treatment, the effect of preventing the penetration of nitrogen in the subsequent nitriding treatment is slight.

  Even if the material is a material mainly composed of a ferrite phase or a martensite phase and containing a relatively small amount of Cr or the like, if the fluorination treatment is not performed, the material is uniformly applied due to the influence of the oxide film formed on the surface. Although it is difficult to intrude and diffuse carbon atoms, it is possible to intrude and diffuse carbon atoms by performing the fluorination treatment before the above-described carbon diffusion treatment, and also in the nitriding treatment process performed thereafter. A uniform cured layer can be formed.

  As described above, by performing the fluorination treatment and the carbon diffusion treatment before the nitriding treatment, a predetermined nitrided layer is formed on both of the two types of materials even in the joining valve in which different materials are joined. be able to. In addition, it is possible to form a nitride layer having excellent wear resistance on both materials without generating a large shaft diameter difference at the joint and without requiring a post-process such as polishing after nitriding. . In addition, the growth of the nitrided layer due to nitriding treatment is suppressed in materials mainly composed of ferrite phase or martensite phase, and excessive expansion is suppressed. On the other hand, the material mainly composed of austenite phase prevents nitrogen penetration during nitriding treatment. Therefore, the difference in dimensional change due to the nitriding treatment between both materials is small.

  The engine valve of the present invention is a first material of steel mainly composed of a ferrite phase or a martensite phase on the valve shaft side, and a heat resistant steel mainly composed of an austenitic phase containing 20% by mass or more of Ni on the valve head side. A second Ni-based alloy material, wherein the first material and the second material are joined at the shaft portion, and a nitride layer of 5 μm or more is formed on the surfaces of both the first material and the second material. A nitride layer having excellent wear resistance is formed on both types of materials.

  In the engine valve of the present invention, when the axial diameter difference between the first material and the second material after the nitriding treatment for forming the nitride layer is 4 μm or less, the axial diameter difference between the joint portions is small. Thus, a nitride layer having excellent wear resistance is formed without requiring a post-process such as polishing.

  Next, the best mode for carrying out the present invention will be described.

  In the nitriding method of the present invention, a bonded member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are heated and held in a nitriding gas atmosphere. Then, before performing the nitriding treatment to form a nitride layer on the surface layer portion, the fluorination treatment is performed by heating and holding in a fluorine-based gas atmosphere in advance, and the carbon diffusion layer is formed on the surface layer portion by heating and holding in the carbon source gas atmosphere. A carbon diffusion treatment is performed to form the carbon.

  As the first material constituting the target joining member, a steel material mainly composed of a ferrite phase or a martensite phase is used. For example, high-tensile steel, steel for machine structure, free-cutting steel, carbon tool steel, alloy tool Steel, high speed steel, bearing steel, spring steel, ferritic heat resistant steel, martensitic heat resistant steel and the like can be used. These can be subjected to a predetermined heat treatment in advance after being formed into a predetermined part shape by plastic processing or cutting.

  As the second material constituting the joining member, a heat-resistant steel or Ni-based alloy mainly composed of an austenite phase containing 20 mass% or more of Ni can be suitably used.

  For example, as the heat-resistant steel, Cr-Ni-based austenitic heat-resistant steel, Cr-Ni-Mn-based austenitic heat-resistant steel, 18-8 steel added with Nb, Ti, Mo, 18-8 Nb steel, 18- 8Ti steel, 18-8Mo steel, 18-8TiNb steel, 23-13 steel, 25-20 steel, 16-30 steel and the like can be used. In addition, Cr—Ni—Mn heat resistant steel, Cr—Ni heat resistant steel and the like used for engine valves can also be suitably used.

  As the Ni-based alloy, various nickel-based heat-resistant alloys such as Inconel, Hastelloy, Nimonic, Udimet, Rene41, Astroy, Waspaloy (all are registered trademarks), D979, M252, IN738, IN100, and the like can be used.

  Examples of the joining member include an engine valve for automobiles. As the engine valve, the first material, which is a steel material mainly composed of a ferrite phase or a martensite phase, is used as a material on the valve shaft side, and an austenite phase containing 20% by mass or more of Ni is mainly used as a material on the valve head side. It is possible to use a joint valve obtained by joining the first material on the valve shaft side and the second material on the valve head side at the shaft portion using the second material which is the heat-resistant steel or Ni-based alloy.

The joint member is charged into a predetermined heat treatment furnace,
(1) Perform fluorination treatment by heating and holding in a fluorine-based gas atmosphere in advance.
(2) A carbon diffusion treatment is performed by heating and holding in a carbon source gas atmosphere to form a carbon diffusion layer on the surface layer portion,
(3) Thereafter, a nitriding treatment is performed in which a nitride layer is formed on the surface layer portion by heating and holding in a nitriding gas atmosphere.
Hereinafter, each step will be described.

(1) Fluorination treatment step In the fluorination treatment step, first, a joining member such as an engine valve is heated and held in an atmosphere containing a fluorine source gas to remove an oxide film formed on the surface of the engine valve to remove a fluoride film. To form.

The fluorine source gas used for the fluorination treatment is a fluorine-based gas that is a halogen-based substance having a stronger affinity than oxygen for Fe, Cr, Ni, etc., which are base material components forming an oxide film. (Fluorine compound gas or gas containing fluorine gas) is used. Examples of the fluorine-based gas include a gas mainly containing a fluorine compound, for example, NF ₃ , BF ₃ , CF ₄ , SF ₆ , and a gas mainly containing F ₂ . Usually, this main component gas is diluted with a diluent gas such as nitrogen gas and used as a fluorine-based gas. Of the main component gases used for these fluorine-based gases, NF ₃ is the most excellent in terms of reactivity and handling properties and is practical.

By holding the engine valve in a nitrogen gas atmosphere containing, for example, NF ₃ in a temperature range of 200 to 600 ° C. for 10 to 60 minutes in the above fluorine-based gas atmosphere, NF ₃ is decomposed to generate active F, and the engine The oxide on the valve surface is replaced to form a fluoride film that is more stable than the oxide. Since this fluoride film is easily reduced and removed when exposed to a reducing atmosphere, this method will result in the appearance of a surface that does not have an oxide film that becomes a barrier when nitrogen and carbon atoms enter and diffuse. Therefore, this process is extremely suitable as a pretreatment for gas nitriding and gas soft nitriding. In addition, about said process temperature and process time, suitable conditions are set so that the oxide film of a surface can be substituted by a fluoride film according to the material, surface state, etc. of the engine valve which processes.

  For the reasons described above, it is possible to easily obtain a uniform nitride layer on the surface of the engine valve by performing nitriding after fluorination. Moreover, since the fluoride film is reduced and the surface without the oxide film is in a state where not only nitrogen but also carbon easily permeates and diffuses, it is extremely pretreatment for diffusing carbon in the subsequent carbon diffusion treatment process. It is a suitable process.

  At this time, the concentration of the fluorine compound or fluorine in the fluorine-based gas atmosphere is preferably 1000 to 10,000 ppm.

  As for this fluorination treatment step, it is possible to carry out the carbon diffusion treatment step and the nitridation treatment step as they are using the same furnace. For example, the fluorination treatment chamber and the subsequent steps are carried out in a continuous furnace. A method in which processing chambers are separated and implemented is also possible.

(2) Carbon diffusion treatment step At the same time as or after the fluorination treatment step, the fluoride film on the surface of the engine valve formed by the fluorination treatment step is reduced and removed, and carbon is further penetrated and diffused from the surface. Therefore, in an atmosphere containing a gas containing a carbon source gas such as CO, in an atmosphere containing a reducing carbon source gas such as a hydrocarbon gas such as acetylene or methane, or H ₂ gas or NH ₃ gas, etc. A carbon diffusion treatment step of heating and holding the joining member such as the engine valve is performed in an atmosphere containing both a reducing gas and a carbon source gas such as CO.

At this time, it is not necessary to supply the reducing gas and the carbon source gas at the same time, and it is also possible to supply the carbon source gas after supplying the reducing gas and reducing and removing the fluoride film first. It is more preferable to use the method of supplying them at the same time because the process time can be shortened. However, when supplying NH ₃ gas as a reducing gas for reducing the above-mentioned fluoride film at this time, it is desirable to adjust the concentration so that carbon diffusion occurs preferentially, and it is optimal depending on the processing temperature. Although the amount is different, it is preferably 30% by volume or less, more preferably 20% by volume or less.

The carbon source gas supplied at this time is not particularly limited as long as it is a carburizing gas such as a hydrocarbon gas such as methane or acetylene or carbon monoxide, but is not limited to propane gas or butane gas. Since RX gas, which is a gas, contains both carbon monoxide, which is a carbon source gas, and H ₂ gas, which is a reducing gas, it has both carburizing and reducing properties and is low in cost. It can be used suitably.

  As described above, by performing the fluorination treatment and the carbon diffusion treatment before the nitriding treatment, in the first material mainly composed of the ferrite phase or the martensite phase, the penetration and diffusion of nitrogen in the subsequent nitriding treatment due to the carbon diffusion. In the second material mainly composed of the austenite phase, the penetration and diffusion of nitrogen in the nitriding treatment is not so suppressed even when carbon diffuses. In addition, the fluorination treatment allows carbon atoms to enter and diffuse into the first material mainly composed of a ferrite phase or a martensite phase, and a uniform nitride layer can be formed in the subsequent nitridation treatment.

  As described above, by performing the fluorination treatment and the carbon diffusion treatment before the nitriding treatment, even if the joining member is obtained by joining the first material and the second material, predetermined nitriding is performed on both of the two types of materials. A layer can be formed. Moreover, the difference in the dimensional change of the joint can be reduced.

  The concentration of the carbon source gas supplied in the carbon diffusion treatment step is preferably 5% by volume or more. When the concentration of the carbon source gas is less than 5% by volume, it takes a long time to diffuse a sufficient amount of carbon to suppress the diffusion of nitrogen in the next nitriding treatment step with respect to the material on the valve shaft side. This is because it is not efficient.

  At this time, the concentration of the carbon source gas in the atmospheric gas during the carbon diffusion treatment is controlled to be increased or decreased, and the carbon potential in the carbon diffusion treatment is controlled, so that the nitrogen in the nitriding treatment for the first material The degree of dimensional change between the first material and the second material in the nitriding process can be controlled by controlling the carbon potential of the atmospheric gas in the carbon diffusion process. Can be controlled.

  Moreover, it is preferable that the temperature which performs this carbon diffusion process process shall be 300 degreeC or more and 600 degrees C or less. If the treatment temperature is less than 300 ° C., it takes a long time to diffuse a sufficient amount of carbon to suppress the diffusion of nitrogen in the next nitriding treatment step for the material on the valve shaft side. This is because the temperature is more preferably 400 ° C. or more. Further, when the processing temperature exceeds 600 ° C., it becomes higher than the normal nitriding temperature, and it is necessary to lower the temperature in the next nitriding step, which is not efficient.

  In addition, the time for performing this carbon diffusion treatment step is preferably 10 minutes or more. This is because if the treatment time is less than 10 minutes, it is difficult to diffuse a sufficient amount of carbon to suppress the diffusion of nitrogen in the next nitriding treatment step with respect to the valve shaft side material.

  The processing temperature and time are preferably optimized in consideration of the material on the valve shaft side, the next nitriding step, and the like. As a result, even if the nitriding treatment process of the next process is performed on a joint member such as an engine valve which is made of different materials and is joined at the shaft part, the shaft diameter difference generated on both sides of the joint part Can be made minute.

  As described above, the carbon diffusion treatment is performed before the nitriding treatment on the dissimilar metal joining member in which the first material and the second material are joined, so that the difference in the dimensional change between the first material and the second material in the nitriding treatment. Can be reduced. Further, by controlling the carbon potential of the atmospheric gas in the carbon diffusion treatment, it is possible to control the difference in dimensional change between the first material and the second material in the nitriding treatment.

(3) Nitriding treatment step Due to the effects of the fluorination treatment step and the carbon diffusion treatment step described above, the nitriding treatment step to be subsequently carried out is not particularly limited in terms of its gas composition, treatment temperature, etc. It is desirable to carry out at a processing temperature of 300 to 600 ° C., more preferably 400 to 600 ° C. in an atmosphere containing 5% by volume or more, more preferably 10% by volume or more of NH ₃ gas as a gas. However, in order to obtain sufficient wear resistance, it is desirable to form a hardened layer of 5 μm or more on both the valve shaft side and the valve head side shaft, and it is nitrided depending on the material of the joint member such as the engine valve to be processed. The processing conditions need to be adjusted as appropriate.

  In the present embodiment, the nitride layer may be a nitride layer consisting of only a nitrogen diffusion layer depending on the state of the surface layer portion of the material to be treated after nitriding treatment, or a nitrogen compound layer may be provided on the surface portion of the nitrogen diffusion layer. In some cases, the nitride layer is formed from a nitrogen diffusion layer and a nitrogen compound layer. In the case of this embodiment, a nitride layer composed of a nitrogen compound layer and a nitrogen diffusion layer is often formed on the surface portion of the first material mainly composed of ferrite phase or martensite phase, and mainly composed of austenite phase. A nitride layer composed of a nitrogen diffusion layer is often formed on the surface portion of the second material.

  The nitriding method of the present invention can be performed by, for example, a heat treatment furnace as shown in FIG.

  This heat treatment furnace includes a process gas supply port 3 for supplying a process gas such as a nitriding gas and a carburizing gas to a furnace body 1 provided with a heater 2, and a stirring fan for stirring the process gas introduced into the furnace. 6 and an agitation fan motor 5 and an exhaust gas discharge port 4 are provided.

The process gas supply port 3 is connected to a gas modification device 11 that generates a carburizing gas such as RX gas, an NF ₃ gas cylinder 12, an N ₂ gas cylinder 13, an NH ₃ gas cylinder 14, and an H ₂ gas cylinder 15 as process gases. Denatured gas, NF ₃ gas, N ₂ gas, NH ₃ gas, and H ₂ gas are supplied. A propane gas cylinder 16 for supplying propane gas as an enriched gas serving as a carbon source is connected to the gas denaturing device 11. In the figure, 7 is a gas supply line main valve, 8 is a pneumatically operated valve, 9 is a pressure reducing valve, 10 is a gas flow meter, and 17 is an exhaust gas abatement device.

Using the above heat treatment furnace, NF ₃ gas diluted with nitrogen gas is supplied to perform fluorination treatment, and gas having both reducing and carburizing properties with RX gas as the main component is supplied to perform carbon diffusion treatment. Then, nitriding is performed by supplying a nitrogen source gas mainly composed of NH ₃ gas alone or NH ₃ gas added with RX gas or N ₂ gas as required. The RX gas is a mixed gas such as N ₂ gas, H ₂ gas, CO gas, CO ₂ gas, etc., and is a gas containing 20 to 25% by volume of carburizing CO gas.

That is, first, a joining member such as an engine valve, which is an object to be processed, is placed in the furnace, and the atmosphere in the furnace is sufficiently replaced with N ₂ gas or the like to prevent oxidation during temperature rise, and then 200 to 600 ° C. Heat to. When the workpiece in the furnace is soaked, a gas containing NF ₃ or the like is introduced into the furnace, and a fluoride treatment is performed to form a fluoride film on the surface of a joining member such as an engine valve.

Next, when the supply of NF ₃ gas is stopped and the main component is RX gas in a furnace heated to 300 to 600 ° C., more preferably 400 to 600 ° C., and it is necessary to increase the reducing power, an appropriate amount of H _A reducing gas added with ₂ gas, NH ₃ gas, or the like is introduced, and the reduction treatment of the fluoride film and the carbon diffusion treatment are performed on the joining member such as the engine valve that has been subjected to the fluorination treatment.

  The fluoride formed on the surface by the fluorination treatment is easily reduced and removed from the surface by the mixed gas. This exposes the active metal surface. Carbon generated from the CO gas contained in the RX gas enters and diffuses into the steel material on the active metal surface to form a carbon concentrated layer.

  At this time, if the nitriding process is performed without performing the above carbon diffusing process, the diffusion of nitrogen is given priority even if the atmosphere during the nitriding process includes a carbon source gas such as RX gas. In the material mainly composed of the ferrite phase or martensite phase that is the material on the valve shaft side, carbon can be added to the nitrogen compound layer formed on the surface, but almost all carbon atoms are diffused in the internal diffusion layer. I can't. This is because the space between the metal crystal lattices, which is the diffusion path of carbon atoms, is first occupied by nitrogen atoms. As a result, a thick nitride layer is formed, resulting in the austenite phase that is the material on the valve stem side. The difference in the thickness of the nitrided layer from the material mainly composed of Ni, especially the so-called Ni-based alloy containing a large amount of Ni, such as a material with a slow diffusion rate of nitrogen and carbon, becomes significant, and the expansion amount of both, that is, the shaft diameter on both sides of the joint There will be a big difference.

  However, by performing the carbon diffusion treatment step before the nitriding treatment step, the amount of expansion of the material on the valve shaft side on which a thick nitride layer is easily formed in the nitriding treatment step can be greatly suppressed. This is because a carbon-enriched layer is formed on the valve shaft side material prior to the nitriding process, so that even when nitriding is performed thereafter, nitrogen penetrates between the carbon invading the surface layer. This is because it is necessary to intrude while intruding or pushing the invading carbon deeper. Therefore, invasion and diffusion of nitrogen atoms are suppressed as compared with the case where there is no carbon enriched layer, and a predetermined nitrided layer is formed on both the material on the valve umbrella side and the material on the valve shaft side, Even when the amount of expansion of the material on the valve umbrella side is very small, there is no significant difference between the amounts of expansion of the shaft diameters of the two.

  At this time, in order to minimize the difference between the shaft diameters of the materials on both sides of the joint, that is, the difference in expansion due to the nitriding treatment, it is necessary to set an appropriate time for the carbon diffusion treatment process in consideration of both materials. However, in order to develop the effect of the carbon diffusion treatment step, it is desirable to set the time to at least 10 minutes or more, more preferably 20 minutes or more.

After carrying out the carbon diffusion treatment step, heating or holding at 300 to 600 ° C., more preferably 400 to 600 ° C. is performed in order to obtain a desired nitride layer thickness by supplying a nitrogen source gas containing NH _3. Then, the nitriding treatment is performed by holding for a necessary time, and a hardened layer of at least 5 μm or more is formed on the material on the valve shaft side and the valve head side.

  At this time, the nitrogen compound layer formed on the outermost surface of the material mainly composed of ferrite phase or martensite phase has high hardness and excellent wear resistance and seizure resistance, but also has brittle properties. It is desirable to adjust the atmosphere of the nitriding process so as to have not only wear resistance but also toughness by adding RX gas or the like.

  In the case of an engine valve, when the material on the valve head side is an alloy containing 20% by mass or more of Ni with an emphasis on heat resistance, it is usually compared with 550 to 600 ° C. to obtain a hardened layer of 5 μm or more. High nitriding temperature must be applied, but the above-mentioned carbon diffusion treatment conditions are optimized even under such conditions that the above-mentioned material on the valve shaft side forms a thick nitride layer and the amount of expansion tends to increase. By doing so, the required hardened layer can be formed and a large difference between the expansion amounts of the two is not generated, so that the nitriding method of the present invention can be used more suitably.

  Thus, in this embodiment, after performing the fluorination treatment and removing the dense oxide film formed on the material surface, the fluoride film is removed by reduction, and before the nitriding treatment, the carbon is added. By performing the carbon diffusion treatment in the source gas atmosphere, it is possible to make the difference in the shaft diameters on both sides of the joint portion minute even in the case of dissimilar metal joining machine parts such as engine valves joined with different materials.

  Furthermore, in the nitriding method of this embodiment, the material on the valve shaft side of the engine valve or the like is a steel material based on a ferrite phase or martensite phase, and the material on the valve head side contains 20% or more of Ni from the viewpoint of heat resistance. This is a more effective nitriding method when it is made of an alloy whose base material is an austenite phase.

  In the dissimilar metal bonding machine part subjected to the nitriding treatment of the present invention, a nitride layer of 5 μm or more is formed on the surfaces of both the first material and the second material. The dimensional difference between the first material and the second material after the nitriding treatment for forming the nitride layer is 2 μm or less on one side, preferably 1.5 μm or less.

  Further, the engine valve manufactured as described above has a nitride layer of 5 μm or more formed on the surface of both the first material and the second material, and exhibits excellent heat resistance and wear resistance. Furthermore, since the difference in the axial diameter of the joint portion between the first material and the second material after the nitriding treatment for forming the nitride layer is as small as 4 μm or less, preferably 3 μm or less, it has excellent performance. It can be used more suitably as an exhaust valve having a high temperature.

  Next, examples will be described.

The material on the valve shaft side is composed of SUH11, which is martensitic heat-resistant steel, and the material on the valve head side is composed of austenitic heat-resistant steel, which contains approximately 25% Ni, both of which are shown in FIG. joined at the valve stem as shown, the valve shaft including the joint use abrasive-finished engine valve, arranged in the furnace body 1 shown in FIG. 2, NF ₃ was heated to 400 ° C. A gas for fluorination treatment containing gas was introduced into the furnace body 1 and held for 15 minutes.

Thereafter, the temperature is raised to 500 ° C., and a mixed gas having a composition of 50% by volume of RX gas, 15% by volume of H ₂ gas, 15% by volume of NH ₃ gas, and 20% by volume of N ₂ gas is contained in the furnace body 1. The carbon diffusion treatment was performed by holding for 10 minutes as Example (a) and 20 minutes as Example (b).

Thereafter, the temperature was raised to 540 ° C., and a mixed gas having a composition of NH ₃ gas 50% by volume and RX gas 50% by volume was introduced into the furnace body 1 and held for 3 hours for nitriding.
In addition, as a comparative example (c), the above nitriding treatment was performed without performing the carbon diffusion treatment after the above fluorination treatment, and as the comparative example (d), the above fluorination treatment was not performed. Then, the above carbon diffusion treatment was carried out for 10 minutes to produce a material subjected to the above nitriding treatment.

Table 1 below summarizes these cured layer thicknesses, shaft diameter differences, and the like.

  As shown in Table 1 above, if the carbon diffusion treatment is 10 minutes or longer, the thickness of the hardened layer on the valve shaft side, which is easily expanded due to the thickened hardened layer, that is, the amount of expansion due to the nitriding treatment is suppressed. It can be seen that the difference in the shaft diameter is 3 μm or less. A more preferable carbon diffusion treatment time is 20 minutes or more.

  In Table 1, the hardened layer is a nitride layer (a nitride layer composed of only a nitrogen diffusion layer or a nitride layer composed of a nitrogen diffusion layer and a nitrogen compound layer), and is formed immediately below the nitride layer. The hardened layer combined with the carbon diffusion layer formed.

On the other hand, in the case of the comparative example (c) in which the fluorination treatment is carried out and the carbon diffusion treatment is not carried out, a uniform nitride layer is formed on both the valve shaft side and the valve umbrella side as in the case of the embodiment (a) and the embodiment (b). However, the post-processing such as re-polishing is necessary because of the large difference in the shaft diameters on both sides of the joint.
In the case of Comparative Example (d) in which no fluorination treatment is performed, formation of a hardened layer is not recognized, and thus it is understood that application of the fluorination treatment is indispensable for the nitriding treatment method of the present invention.

  Next, the material on the valve shaft side is composed of SUH3, which is martensitic heat resistant steel, and the material on the valve head side is composed of austenitic heat resistant steel and NCF751 containing approximately 70% Ni. As shown in FIG. 1, an engine valve is used which is joined at the valve shaft portion and the valve shaft including the joint portion is polished and disposed in the furnace body main body 1 shown in FIG. The temperature was raised to 0 ° C., and a fluorination gas containing NF 3 gas was introduced into the furnace body 1 and held for 10 minutes.

Thereafter, the temperature was raised to 590 ° C., and a mixed gas having a composition of 70% by volume of RX gas and 30% by volume of H ₂ gas was introduced into the furnace body 1 and held for 15 minutes for carbon diffusion treatment.

After that, a mixed gas having a composition of NH 3 gas of 70% by volume and RX gas of 30% by volume at 590 ° C. was introduced into the furnace body 1 and held for 3 hours to perform nitriding treatment.
Moreover, what carried out the salt bath nitriding process at 590 degreeC for 3 hours as a comparative example (f) was also produced.

A summary of these hardened layer thicknesses and shaft diameter differences is shown in Table 2 below.

  As shown in Table 2 above, the material on the leaflet side is a material containing 70% or more of Ni, and even when a high temperature treatment is applied in order to obtain a necessary hardened layer in as short a time as possible, In Example (e), it can be seen that the axial diameter difference on both sides of the joint is within 3 μm. Due to the effect of the fluorination treatment, a uniform hardened layer is formed not only on the valve shaft side where a hardened layer is relatively easily formed but also on the valve umbrella side containing a large amount of Ni. Was about 10 μm.

  As in Table 1, the hardened layer in Table 2 is also a nitride layer (a nitride layer composed only of a nitrogen diffusion layer or a nitride layer composed of a nitrogen diffusion layer and a nitrogen compound layer), and nitrided. It is a hardened layer combined with a carbon diffusion layer formed immediately below the layer.

  On the other hand, in Comparative Example (f) in which salt bath nitriding was performed at the same temperature and for the same time as Example (e), a relatively uniform hardened layer was formed on the valve shaft side, It was confirmed that almost no cured layer was formed. Therefore, in the salt bath nitriding treatment, which has a relatively strong nitriding power and is widely used as a nitriding treatment method for engine valves, a uniform nitriding treatment cannot be performed on a heat-resistant alloy containing a large amount of Ni, and the shaft diameters on both sides of the joint portion In addition to the problem of difference, it can be seen that when the material on the valve head side contains a large amount of Ni, there is a high possibility of a problem in wear resistance.

It is an external view which shows an example of the engine valve in which the dissimilar material was joined by the axial part. It is sectional drawing which shows an example of a processing furnace.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Furnace body 2 Heating heater 3 Process gas supply port 4 Exhaust gas discharge port 5 Stirrer fan motor 6 Stirrer fan 7 Gas supply line main valve 8 Pneumatic pressure valve 9 Pressure reducing valve 10 Gas flow meter 11 Gas denaturing device 12 NF ₃ Gas cylinder 13 N ₂ gas cylinder 14 NH ₃ gas cylinder 15 H ₂ gas cylinder 16 Propane gas cylinder 17 Exhaust gas abatement system

Claims (7)

  A bonding member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are bonded by heating in a nitriding gas atmosphere to form a nitride layer on the surface layer portion. Before performing the nitriding treatment to be formed, the fluorination treatment is performed by heating and holding in a fluorine-based gas atmosphere, and the carbon diffusion treatment is performed by heating and holding in the carbon source gas atmosphere to form a carbon diffusion layer in the surface layer portion. Thus, the difference in dimensional change between the first material and the second material in the nitriding treatment is reduced.   A bonding member in which a first material mainly composed of a ferrite phase or a martensite phase and a second material mainly composed of an austenite phase are bonded by heating in a nitriding gas atmosphere to form a nitride layer on the surface layer portion. Before performing the nitriding treatment to be formed, a fluorination treatment is performed by heating and holding in a fluorine gas atmosphere in advance, and a carbon diffusion treatment is performed by heating and holding in a carbon source gas atmosphere to form a carbon diffusion layer on the surface layer portion. A nitriding treatment method characterized by controlling the difference in dimensional change between the first material and the second material in the nitriding treatment by controlling the carbon potential of the atmospheric gas in the carbon diffusion treatment.   A first material mainly composed of a ferrite phase or a martensite phase and a second material made of a heat-resistant steel or Ni-based alloy mainly composed of an austenite phase containing 20 mass% or more of Ni are joined to each other. A dissimilar-material joining machine part, wherein a nitride layer of 5 μm or more is formed on both surfaces of the material and the second material.   The dissimilar-material joining machine component according to claim 3, wherein a dimensional difference between the joining portions of the first material and the second material after nitriding for forming the nitrided layer is 2 µm or less on one side.   The material on the valve shaft side is a steel material mainly composed of a ferrite phase or a martensite phase, and the material on the valve head side is composed of a heat-resistant steel or a Ni-based alloy mainly composed of an austenite phase containing 20 mass% or more of Ni. Prior to performing nitriding to heat and hold the engine valve in a nitriding gas atmosphere to form a nitride layer on the surface layer portion, the engine valve is preliminarily heated and held in a fluorine-based gas atmosphere, and carbon source gas A carbon diffusion treatment is performed by heating and holding in the atmosphere to form a carbon diffusion layer on the surface layer portion. The carbon diffusion treatment is performed at a temperature of 300 to 600 ° C. in an atmosphere where the carbon source gas concentration is 5% by volume or more. A method for producing an engine valve, characterized by holding for at least minutes.   The valve stem side is the first material of steel mainly composed of ferrite phase or martensite phase, and the valve head side is the second material of heat resistant steel or Ni base alloy mainly composed of austenite phase containing Ni of 20 mass% or more. An engine valve configured by joining the first material and the second material at a shaft portion, characterized in that a nitride layer of 5 μm or more is formed on the surfaces of both the first material and the second material. Engine valve to be used.   The engine valve according to claim 6, wherein a shaft diameter difference between the first material and the second material after the nitriding treatment for forming the nitride layer is 4 µm or less.

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