JP3023322B2 - Iron-based material nitriding method and iron-based material product obtained thereby - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for nitriding an iron-based material and an iron-based material product obtained by the method.

[0002]

2. Description of the Related Art Generally, cast iron and carbon steel are used as various sliding members for compressors such as air conditioners and coolers. As measures to improve wear resistance, quenching and hardening such as carburizing and induction hardening, and PVD
(Physical vapordepositio
n) and the like.

[0003] In the above quenching curing method, transformation point of steel since it is necessary rapid cooling from the high temperature exceeding (steel A ₃ or A _cm transformation point becomes an austenite phase), before and after the treatment dimensional change or distortion of Outbreak is large. For this reason, there is a problem in that the final product needs to be ground and finished, and the finishing process requires a large cost. On the other hand, the dry plating method has problems in that the adhesion between the base material and the coating layer is difficult, the mass productivity is low, and the processing cost is high.

Accordingly, recently, as a method of hardening the surface of an iron-based material, a nitriding method, which can be treated at a relatively low temperature and has a relatively small dimensional change and distortion, has been used.

[0005]

However, even in the case of nitriding, since heating is carried out at a temperature of 540 to 580 ° C. for a long time, thermal deformation due to release of residual stress or recrystallization of steel occurs. Inevitable. Further, when the steel is nitrided, a nitride layer is formed on the outermost surface, and a diffusion layer in which nitrogen diffuses and penetrates and fine nitride is dispersed is formed below the nitride layer. These are causes of a decrease in accuracy such as dimensional swelling, surface roughness, and poor roundness. On the other hand, the demand for higher precision of products accompanying higher rotational speeds of equipment has become more and more severe, and even for nitrided products, finish processing such as grinding finish is required like hardened products. It is a fact.

[0006] Further, in order to reduce thermal deformation due to steel recrystallization or the like, the recrystallization temperature (iron recrystallization temperature is about 45
If the nitriding treatment is performed at a low temperature of 0 ° C. or less, there is a problem that the diffusion and infiltration of nitrogen is not sufficiently performed and the formation of the nitrided hardened layer becomes extremely unstable. In particular, in a member that has been subjected to mechanical processing such as cutting and grinding, the formation of a nitrided hardened layer is likely to be further unstable due to the influence of a work-affected layer generated on the surface. For this reason, conventionally, gas nitriding has not been performed at such a low temperature.

The present invention has been made in view of such circumstances, and a method of nitriding an iron-based material, which has a small dimensional change and distortion before and after processing, and has excellent dimensional accuracy, surface roughness, and the like, is provided. The aim is to provide iron-based material products obtained thereby.

[0008]

In order to achieve the above object, the present invention provides a method for nitriding an iron-based material, which comprises heating and holding the iron-based material in a fluorine-based gas atmosphere prior to nitriding. And then the temperature at the time of nitriding is set to 300
４５０450 ° C., and nitriding the fluorinated iron-based material, so that at least a part of the surface layer of the fluorinated iron-based material has a thickness of 1-6 μm ε-Fe
and summarized in that to form a nitride layer composed of _x N.

Further, in the iron-based material product of the present invention, the base material is cast.
Tetsudea is, at least a portion of the surface layer portion thickness. 1 to 6 [mu] m
It is summarized as the nitride layer consisting of ε-Fe _x N of is formed.

The present inventors have conducted a series of studies in order to reduce dimensional changes and distortions before and after the nitriding process. Further, it has been found that prior to the nitriding treatment, the surface of the iron-based material is activated and easily nitrided by heating and holding the iron-based material in a fluorine-based gas atmosphere to perform the fluorination treatment. And, as a result of repeating various experiments, nitriding in a low temperature range of 300 to 450 ° C., which was considered impossible in the past, becomes possible by performing fluorination treatment.
Further, at least a portion of the surface layer portion by nitriding treatment at the low temperature range, by forming a nitride <br/> oxide layer consisting of ε-Fe _x N of thickness. 1 to 6 [mu] m, the processing dimensional change Ya longitudinal The inventors have found that the distortion is extremely reduced, and that a product excellent in dimensional accuracy, surface roughness, and the like can be obtained.

[0011]

Next, an embodiment of the present invention will be described.

In the nitriding method of the present invention, prior to the nitriding treatment, the iron-based material is heated and held in a fluorine-based gas atmosphere,
Then, by nitriding by setting the temperature in the nitriding treatment 300 to 450 ° C., at least a portion of the surface layer portion to form a nitride layer composed of thick 1~ 6 μm ε-Fe _x N .

The iron-based material is not particularly limited, and various materials can be used. For example, nitrided steel,
Nickel-chromium steel, nickel-chromium-molybdenum steel, chrome steel, chromium-molybdenum steel, etc., carbon steel for mechanical structures, manganese-chromium steel, chromium-vanadium steel, spring steel such as silicon-manganese steel, high-carbon chromium steel , Tungsten-chromium steel, tool steel such as tungsten-vanadium steel, high-speed steel such as tungsten-chromium-vanadium steel, manganese steel, H steel, high-strength steel, free-cutting steel, die steel, bearing steel, heat-resistant Examples include various alloy steels such as steel and boron steel, various cast irons such as gray cast iron, white cast iron, mottled cast iron, ferrite cast iron, pearlite cast iron, spheroidal graphite cast iron, inoculated cast iron, and malleable cast iron, and cast steel. Further, not only steel but also industrial pure iron containing almost no carbon may be used, or pure iron or carbon steel may be subjected to carburizing treatment. further,
Not limited to ingot steel, but a sintered alloy obtained by powder metallurgy may be used. These iron-based metals may be subjected to heat treatment such as tempering and stress relief annealing in advance.

Prior to nitriding, the iron-based material is first subjected to fluorination by heating and holding it in a fluorine-based gas atmosphere.

As the fluorine-based gas used for the fluorination treatment, NF ₃ , BF ₃ , CF ₄ , HF, SF ₆ , C ₂ F
_6, WF ₆ , CHF ₃ , SiF and other fluorine compound gas and F ₂ gas, which may be used alone or in combination. In addition, other fluorine compound gases containing fluorine in the molecule can also be used as the fluorine-based gas. Such a fluorine-based gas can be used alone, but is usually used after being diluted with an inert gas such as N ₂ gas. The concentration of the fluorine-based gas in such a diluted gas is, for example, 10,000 to 100
2,000 ppm, preferably 20,000-7000
0 ppm, more preferably 30,000 to 50,000p
pm. NF ₃ is the most practical as this kind of fluorine-based gas. This is because NF ₃ is gaseous at room temperature, has high chemical stability, and is easy to handle.

The fluorination treatment is carried out by creating a fluorine-based gas atmosphere having the above concentration, placing the iron-based material in this atmosphere, and keeping the iron-based material in a heated state. The heating temperature in this case is set to, for example, a temperature of 250 to 450 ° C. The heating holding time may be set to an appropriate time according to the type, shape and size of the product, the heating temperature, and the like, and is usually set in the range of ten minutes to several tens of minutes. By fluorinating the iron-based material, “N” atoms can easily penetrate from the surface of the iron-based material to the inside. The reason is that an oxide film such as FeO, Fe ₃ O ₄ , Cr ₂ O ₃ or the like is formed on the surface of the iron-based material. When the fluorination treatment is performed as described above, the above oxide reacts with the fluorine-based gas to produce FeF ₂ , FeF ₃ , CrF
_It is considered that the surface is converted into a thin fluorinated film containing a compound such as ₂ , CrF ₄ and activated, and becomes a surface state in which “N” atoms in the nitriding gas easily penetrate. As described above, since the surface of the iron-based material is in a state where "N" atoms can easily penetrate,
It is considered that even in a relatively low temperature range of 450 ° C. or lower, “N” atoms diffuse uniformly at a constant depth, and a uniform nitride layer is formed.

Next, as described above, the iron-based material which is in a state where "N" atoms easily penetrate by the fluoridation treatment is held in a heating state in a nitriding atmosphere, and the nitriding treatment is performed.

In this case, as the nitriding gas for creating the nitriding atmosphere, a simple gas consisting of only NH ₃ or a mixture of the above simple gas with an inert gas such as N ₂ is used. In some cases, these gases may be mixed with H ₂ gas for use.

In the nitriding atmosphere, the fluorinated iron-based material is kept in a heated state. The heating temperature in this case is 3 which is much lower than the conventional nitriding treatment.
It is set at 00-450 ° C. That is, when the temperature is 4
If the temperature exceeds 50 ° C., recrystallization or the like of the iron-based material occurs and thermal deformation or the like occurs. However, by performing the treatment at 450 ° C. or less, the thermal deformation due to the recrystallization or the like is suppressed. The nitriding time is usually set to 2 to 48 hours. In addition, 3
At a nitriding temperature of 00 ° C. or lower, it is not practical because a nitrided hardened layer having a sufficient thickness cannot be obtained even if the nitriding is performed for a long time.

The above fluorination treatment and nitridation treatment are performed, for example, in a metal muffle furnace 1 as shown in FIG. The muffle furnace 1 includes an inner container 4 heated by a heater 3 and an outer shell 2 that accommodates the inner container 4. A gas introduction pipe 5 for introducing an atmospheric gas and an exhaust pipe 6 communicate with the inner container 4. A fluorine gas cylinder 16, an NH ₃ gas cylinder 15, a NH ₃ gas cylinder 15,
The N ₂ gas cylinder 30 is connected. On the other hand, the exhaust pipe 6 has a vacuum pump 13 for exhausting gas in the inner container 4.
And an exhaust gas treatment device 14 are connected. In the figure, reference numeral 8 denotes a fan which is rotated by a motor 7 to stir the atmosphere gas in the inner container 4, and reference numeral 11 denotes a wire mesh basket for filling the iron-based material 10.

The temperature inside the muffle furnace 1 is raised to 250 to 450 ° C., an iron-based material 10 is charged, and a fluorine-based gas cylinder 1 is charged.
6, a fluorine-based gas such as NF ₃ is introduced into the inner container 4 and heated and held for fluorination treatment. Then, the gas in the inner container 4 is extracted from the exhaust pipe 6 by the action of the vacuum pump 13, and exhaust gas treatment is performed. It is detoxified in the device 14 and released to the outside. Next,
The nitriding gas (NH) is introduced into the inner container 4 from the NH ₃ gas cylinder 15.
₃ gas, NH ₃ gas + N ₂ gas, etc.)
The nitriding process is performed by setting the temperature to 450 ° C. and heating and holding, and then the gas in the inner container 4 is discharged to the outside via the exhaust pipe 6 and the exhaust gas treatment device 14. By this series of operations, the fluorination treatment and the nitridation treatment are performed.

By performing the nitriding treatment at a low temperature range as described above, a dense and uniform thickness of 1 to
A 6 μm nitride layer is formed. This nitride layer has an ε-
_{Fe x N (2 <x ≦} 3: including a wide range of concentrations of N 7-11%) of which consists of almost single phase. According to the nitriding treatment of the present invention, the dimensional change and surface roughness of the iron-based material due to the nitriding treatment hardly occur.

That is, in the conventional nitriding treatment, ε-Fe
_x N and gamma prime-Fe ₄ nitrides of two kinds of nitride that N layer is formed, the nitride layer, depending on the nitriding condition, usually, the thickness of about 10 to 30 [mu] m.
The nitride layer obtained by the above-mentioned conventional method has a concentration gradient of “N”, and the surface layer has a relatively large “N” content of ε-
An Fe _x N phase is formed, and a γ′-Fe ₄ N phase having a relatively small “N” content is formed on the base material side. The nitride layer has many pores (average diameter: 0.3 to 0.6 μm) reaching 20 to 40% in terms of the area ratio of the cross section. In particular, γ′-Fe ₄ N has a large pore diameter (presumably because the dissociation pressure of γ′-Fe ₄ N is low).
Moreover, the conventional nitride layer, ε-Fe _x N is only present very thinly on the outermost surface, 70 to 75% of the total thickness,
epsilon-Fe _x N and gamma prime-Fe ₄ or N and are mixed, Ganma'-
It consists only of Fe ₄ N. And γ′-Fe ₄ N
Has coarse atmospheric holes, the nitride layer obtained by the above-mentioned conventional nitriding method has γ′-Fe ₄ N
Is formed thickly, and the outer dimensions of the product expand, causing dimensional changes, and the surface roughness and roundness of the product are deteriorated, resulting in a decrease in dimensional accuracy. For this reason, a final processing finish is required, and a large cost is required. Further, in a high-speed sliding member such as a compressor component, high-speed sliding is performed in a severe lubricating environment. Therefore, when a thick γ′-Fe ₄ N is formed in the nitride layer as described above, γ ′ −Fe ₄
The roughness of the surface is deteriorated by the influence of the coarse pores of N, causing pitting wear and abrading of a sliding partner material, resulting in poor durability.

On the other hand, in the present invention, 300 to 450
Nitriding at a low temperature of ℃, the maximum thickness of the nitride layer is 6μm
Up to. In this way, γ'-Fe ₄ N is not generated, the nitride layer is formed becomes consisting ε-Fe _x N. Note that, as the thickness of the nitride layer exceeds 6 μm, the amount of γ′-Fe ₄ N greatly increases. The ε-Fe _x N phase has extremely small pores and is dense as compared with the γ′-Fe ₄ N phase. Thus, in the present invention does not generate the γ'-Fe ₄ N phase with coarse pores, nitride layer is formed consisting of ε-Fe _x N,
The dimensional change is very small, and the surface roughness is less deteriorated.

The iron-based material product obtained as described above is suitably used as a high-speed sliding member, such as a part for a compressor. Minimum i.e., nitride layer consisting of ε-Fe _x N formed on the surface of the iron-based material products, the presence of fine and dense pores, lubricating liquid effectively retains the, the wear even at high sliding Can be minimized. Also, ε-Fe
_x N, since coarse pores as γ'-Fe ₄ N is little, or cause pitting wear nitride layer is destroyed, never or to wear the sliding counterpart material. In addition,
The thickness of the nitride layer consisting of ε-Fe _x N is if more than 1 [mu] m, the lubricating effect as described above can be obtained. On the other hand, when the thickness is less than 1 μm, the thickness of the nitride layer itself is small and the nitrogen diffusion layer formed immediately below the nitride layer is also shallow, so that the hardness of the surface layer portion is not so high and sufficient wear resistance cannot be obtained.

In the nitriding method of the present invention, 450
Since the nitriding treatment is performed in a low temperature range of not more than ℃, the penetration potential of [N] in the nitriding atmosphere (hereinafter referred to as “[N] potential”) becomes extremely high. This is because the [N] potential when steel is generally held in ammonia gas is expressed by the following equation (1). As the nitriding temperature becomes lower, the dissociation rate of NH ₃ becomes smaller and H ₂ becomes lower. This is because the concentration becomes low. High [N] at such low temperatures
Under a potential atmosphere, ε is larger than γ′-Fe ₄ N.
Who -fe _x N is easy nucleation. On the other hand, 450 ° C
[N] potential decreases at high temperatures exceeding
Formation of thermodynamically stable γ'-Fe ₄ N is increased than -fe _x N. Then, once γ′-Fe ₄ N is formed, the thickness of the entire nitride layer rapidly increases.

[0027]

[Formula 1] pNH ₃ / pH ₂ ^3/2 ... (1) Partial pressure of pNH ₃ : NH ₃ Partial pressure of pH ₂ : H ₂

In the nitriding method of the present invention, it is preferable to use an iron-based material in which a large amount of carbon is present in the material. That, ε-Fe _x N, since a large amount dissolved carbon, in the case of using a ferrous material present many carbon,
This is because nucleation and growth occur more stably. Therefore, in materials with high carbon such as cast iron and carburized products, γ'-
Fe ₄ N is the hardly occurs nucleation, nitride layer consisting of ε-Fe _x N is formed. For example, FCD600
In (spheroidal graphite cast iron), γ′-Fe ₄ N is not formed until the thickness of the entire nitride layer becomes about 6 μm. On the other hand, S
In a medium carbon steel such as CM435, when the thickness of the entire nitride layer becomes about 2 to 3 μm, γ′-F
e ₄ N is formed.

In particular, it is preferable to use cast iron as the iron-based material. In other words, cast iron has various excellent properties such as easy formation of a complex-shaped product by casting, slidability by precipitated graphite, good vibration damping properties, excellent corrosion resistance and low cost. It is a very good material for a sliding member. However, cast iron has high thermal deformation due to large residual stress during casting, and even if subjected to stress relief annealing etc., it has problems such as expansion due to precipitation of secondary graphite. Could not get. Moreover, since cast iron contains a large amount of Si, nitriding treatment itself is difficult, and low-temperature nitriding in particular has been impossible. According to the present invention, the surface of the cast iron is easily permeable to [N] atoms by performing fluorination treatment prior to nitriding, and nitriding treatment at a relatively low temperature of 450 ° C. or less is enabled.

Next, examples will be described together with comparative examples.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A spheroidal graphite cast iron (FCD600) is cast, molded and then cut, as shown in FIG. 2, comprising a cylindrical shaft portion 2 and a crank portion 3 provided slightly closer to the end than the center. The crankshaft 1 was formed.

This crankshaft was charged into the muffle furnace shown in FIG. 1 and heated in an N ₂ gas atmosphere at 350 ° C.
After reaching ₃ % in an N ₂ + 10% by volume NF ₃ gas atmosphere.
Fluorination treatment was performed at 50 ° C. for 20 minutes. Next, an NH ₃ gas was introduced, the temperature was raised to 400 ° C., and nitriding was performed at 400 ° C. for 6 hours in a 100% by volume NH ₃ gas atmosphere to obtain an iron-based material product of the present invention.

[0033]

Comparative Example A crankshaft formed in the same manner as in the above example was nitrided at 570 ° C. for 2 hours in an atmosphere of 70% by volume NH ₃ gas + 30% by volume RX gas to obtain a cast iron material product.

FIGS. 3 and 4 show cross-sectional metal micrographs of the crankshafts of the above Examples and Comparative Examples, respectively. In the embodiment, a nitride layer of about 2 μm is formed. On the other hand, in the comparative example, it can be seen that a nitride layer of about 12 μm was formed. 5 and 6 show cross-sectional electron micrographs of each crankshaft. In the example, the nitride layer is dense and has almost no pores.
On the other hand, in the comparative example, it can be seen that a large number of coarse pores exist.

The surface of each of the above crankshafts is represented by X
The results of the line diffraction are shown in FIGS. In the example,
A large peak of ε-Fe ₃ N is clearly seen, and γ′-F
Almost no e ₄ N peak is seen. On the other hand, in the comparative example, a large peak of γ′-Fe ₄ N is clearly seen, and the peak of ε-Fe ₃ N is very weak. The peak of ε-Fe ₃ N in the examples indicates that the diffraction line is broader than the comparative example, the half width (peak width at half the height of the peak) is wide, and the crystal is fine. I have.

Table 1 below shows the results of measurement of the surface hardness, shaft runout, surface roughness, and external dimensions of each of the crankshafts. In addition, the surface roughness was measured using RA-100 manufactured by Mitutoyo Corporation, and the external dimensions and shaft runout were measured using a dial gauge manufactured by Mitutoyo Corporation.

[0037]

[Table 1]

As can be seen from Table 1 above, in the embodiment,
The surface hardness is slightly higher than the comparative example, and the roundness, surface roughness,
Dimensional swelling, both show better values than the comparative example,
It can be seen that high accuracy has been obtained.

[0039]

As described above, according to the nitriding method of the present invention, prior to the nitriding treatment, the iron-based material is heated and held in a fluorine-based gas atmosphere to perform the fluorination treatment.
Nitriding at a low temperature of 450 ° C. becomes possible. Therefore, thermal deformation due to release of residual stress or recrystallization is reduced. Further, the nitriding treatment in the low temperature range, at least a portion of the surface layer portion, the thickness 1~ 6 μm ε-Fe _x N
Since a nitride layer is formed, the dimensional change and distortion before and after the treatment are extremely reduced, and a product excellent in dimensional accuracy, surface roughness and the like can be obtained. Therefore, the product can be maintained with high precision, finishing work such as grinding finish is not required, and cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a furnace used for a nitriding treatment method of the present invention.

FIG. 2 is a view showing a high-speed sliding member (crankshaft) of the present invention, wherein (a) is a front view, (b) is a right side view,
(C) is a rear view.

FIG. 3 is a metal micrograph showing a metal structure of a surface layer portion of a high-speed sliding member of an example.

FIG. 4 is a metal micrograph showing a metal structure of a surface portion of a comparative example.

FIG. 5 is an electron micrograph showing a metal structure of a surface layer portion of the high-speed sliding member of the example.

FIG. 6 is an electron micrograph showing the metal structure of the surface layer of the comparative example.

FIG. 7 is a diagram showing an X-ray diffraction result of the high-speed sliding member of the example.

FIG. 8 is a diagram showing an X-ray diffraction result of a comparative example.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-145951 (JP, A) JP-A-7-179985 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 8/26

Claims (4)

(57) [Claims] 1. Prior to the nitriding treatment, an iron-based material is heated and held in a fluorine-based gas atmosphere to perform a fluoridation treatment.
By setting the temperature at the time of the nitriding treatment to 300 to 450 ° C. and nitriding the fluorinated iron-based material, at least a part of the surface layer of the fluorinated iron-based material has a thickness. nitriding method of an iron-based material, which comprises forming a nitride layer composed of ε-Fe _x N of. 1 to 6 [mu] m. 2. A nitriding process of iron-based material according to claim 1 Symbol placement iron-based material is cast iron. Wherein Ri preform cast iron der, at least a portion of the surface layer portion, an iron-based material, characterized in that the nitride layer consisting of thick 1~ 6 μm ε-Fe _x N is formed Product. 4. The iron-based material article according to claim 3 Symbol placement is used as a high-speed sliding member.