JPH05202464A - Method for partially nitriding parts - Google Patents

Abstract

PURPOSE:To attain efficient partial nitriding of machine parts, etc., by coating parts to be nitrided with a nitriding agent and carrying out heating in a nonoxidizing atmosphere. CONSTITUTION:When machine parts, etc., made of iron-based stock are partially nitrided, parts to be nitrided are coated with a cyanic acid-based nitriding agent and heating is carried out in a nonoxidizing atmosphere or an atmosphere regulated to a prescribed nitrogen potential. When machine parts, etc., made of steel stock contg. >=5% Cr are partially nitrided, parts to be nitrided are coated with chemicals for destroying oxidized films on the surfaces of the parts and heating is carried out in an atmosphere regulated to a prescribed nitrogen potential. Nitriding is applied to an extended range and the performance of the machine parts, etc., can be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selectively nitriding a part of a material containing iron as a main component or steel containing 5% or more of Cr.

[0002]

2. Description of the Related Art In the nitriding of steel machine parts, it is often a problem that only a part of the parts is nitrided. For example, in finishing machining performed after the nitriding treatment, when a certain portion is threaded, the portion cannot be threaded even if the portion is nitrided. In such a case, nitriding prevention treatment is applied to that portion. In most cases, copper plating, nickel plating, or tin plating is applied only to the nitriding prevention portion. Since such a conventional nitriding preventive treatment requires a special device for it and becomes large-scale, various nitriding preventive agents have been recently developed.

Such a conventional partial nitriding method is a partial nitriding method for performing a nitriding prevention treatment on a part of the surface of a target mechanical component to form a non-nitrided portion. However, as a practical problem, contrary to these conventional methods, the non-nitrided portion is mainly used, and the portion requiring nitriding may be a very small part of the target mechanical component. In order to meet such demands, the conventional method of performing the nitriding prevention treatment on the non-nitrided portion as described above is not an efficient method.

The present invention is based on the opposite idea to the conventional partial nitriding method, in which nitriding prevention treatment is applied to a portion which should not be nitrided.
It is an object of the present invention to provide a method capable of efficiently nitriding only a portion of a target mechanical component to be nitrided.

[0005]

According to the present invention, when nitriding a part of a part made of a material containing iron as a main component, a nitriding agent containing cyanic acid as a main component is applied to a portion to be nitrided, When heating in a non-oxidizing atmosphere or in an atmosphere controlled to a predetermined nitrogen potential and nitriding a part of a part made of steel containing 5% or more of Cr, the surface of the non-nitrided part of the part is passivated, Alternatively, the above-mentioned problems can be solved by applying a chemical for destroying the oxide film on the surface of the nitriding portion without heating for passivation and heating in an atmosphere controlled to a predetermined nitrogen potential. is there.

In the present invention as described above, the processing differs depending on the material of the target mechanical component. That is, for a part made of a material containing iron as a main component, a chemical containing cyanic acid as a main component is applied to a predetermined portion of the part and heated to a normal nitriding temperature of 500 to 600 ° C. As a result, only the portion coated with the drug is nitrided.
Here, the chemicals containing cyanic acid as the main component were used as the nitriding agent to be applied because these chemicals have a high nitrogen potential at a general nitriding temperature and have a high nitriding ability for steel. More specifically, these agents are mixed salts, in which cyanate is mixed with various salts to increase the melting point and stability suitable for nitriding parts. Since these mixed salts melt at the nitriding temperature,
In order to retain the nitriding agent at the application position of the initial parts, a mixture of these mixed salts mixed with a chemical retention agent such as kaolin was applied, or a resin and a solvent were further added to adjust the viscosity. You may apply a thing. Since these chemicals are largely consumed in the air, the heating atmosphere must be a non-oxidizing atmosphere. Further, depending on the required depth of the nitrided layer, the amount of the applied chemical agent may cause a shortage of the depth of the nitrided layer. Therefore, some ammonia is added to the non-oxidizing atmosphere to adjust the nitrogen potential of the heating atmosphere. This nitrogen potential is negligible as the nitriding ability of the non-nitriding portion of the treated component, but is sufficient to supply nitrogen as a nitriding agent.

When the material of the target machine part is steel containing 5% or more of Cr such as alloy tool steel and stainless steel, a stable oxide film is formed on the surface of the material. Almost no nitriding is performed by the usual gas nitriding method mainly containing ammonia. Therefore, in such a case, a treatment for destroying the stable oxide film is performed only on the portion to be nitrided, and a completely normal gas nitriding treatment is performed in an atmosphere in which the nitrogen potential is controlled. The point of the procedure for destroying the oxide film is that under normal gas nitriding treatment conditions, the oxide film is destroyed immediately before the nitriding starts, and when the nitriding starts, the nitriding is not hindered. In an extreme case, as a pretreatment for the gas nitriding treatment, even if the portion to be nitrided is previously pickled, even the pickled portion is no longer nitrided.

A specific measure for destroying a stable oxide film only on the surface of the portion to be nitrided is to perform the treatment at a temperature before nitriding starts.
This is to apply a complex salt containing hydrochloric acid, sulfuric acid, phosphoric acid, potassium, sodium, ammonia, etc., which has the effect of destroying this film, to predetermined places. In order to make it easier to apply these chemicals to the parts, it is desirable to use those in which a suitable resin or fat and a solvent are added and the viscosity is adjusted.
The function of the resin or fat here is to facilitate the uniform application of the drug to predetermined parts of the part, and to hold the drug stably at the predetermined parts to facilitate the handling of the part before nitriding treatment. . Further, the resin or fat helps the above-mentioned destruction of the film just before the nitriding starts and does not prevent the nitriding when the nitriding starts. Therefore, the type of resin may be a very general type such as epoxy, and the fat may be head or lard. on the other hand,
Some resins or fats have both of the above functions only. One example is vinyl chloride, an acidified fat. That is, the purpose of vinyl chloride or acidified fat can be achieved by applying only these.

In the case where a large number of parts are processed at one time by the above method, the surface of the part which is not applied by the residual amount of the chemical applied to the nitrided part of the part, that is, the non-nitrided part is partially On the other hand, in order to avoid a trouble such as nitriding, it is possible to take a measure that the surface of the component other than the portion to be nitrided is not actively nitrided. As an example of such a treatment, a treatment (passivation treatment) for forming a stable surface film on the non-nitrided portion is performed, and a treatment for destroying the oxide film as described above is performed on the nitrided portion. A specific method of the passivation treatment is to immerse it in a strong oxidizing agent such as nitric acid, or to heat it at a low temperature in the atmosphere.

[0010]

As described above, according to the present invention, only the local parts such as the mechanical parts can be efficiently nitrided, so that the range of application of the nitriding treatment is expanded and the performance of the mechanical parts is improved. .

[0011]

Example 1 JIS standard nitrided steel SCM435 5 × 4
A 0 × 50 mm plate was used, and only the 20 mmφ portion at the center of one surface thereof was nitrided. The chemicals applied here were potassium cyanate, sodium carbonate, and kaolin, 35, 62, and 3, respectively.
%, Water was added and kneaded, and a ball-shaped product having a diameter of about 20 mm was attached to the portion to be nitrided and dried.
Ammonia is adjusted so that the steel plate thus prepared has an atmosphere of only nitrogen and a nitrogen potential of about 0.3% (the nitrogen potential in the atmosphere is the amount of nitrogen in the steel surface layer in equilibrium with this). 54 in an atmosphere with
Heated at 0 ° C. for 90 minutes. The steel plate thus treated was cut at the center, the cut surface was polished, and the nitriding depth and nitriding width were measured with an optical microscope. As a result, the nitriding depth: 30 μm and the nitriding width: about 20 mm in the case of nitrogen only, and the nitriding depth: 40 in the atmosphere of adding ammonia to nitrogen.
μm, nitriding width: about 21 mm. In each case, a predetermined partial nitriding was possible. The hardness distribution from the surface of the non-nitrided portion and the nitrided portion in the latter case is shown in FIG.

[0012]

[Example 2] JIS-SUS420 stainless steel 5 ×
Use a plate of 40 x 50 mm, 20 mmφ at the center of one side
Only part was nitrided. A test piece was prepared by applying several kinds of chemicals for destroying the oxide film on the surface to be nitrided to the portion to be nitrided, and these were adjusted to a nitrogen potential of about 10% in an ammonia + nitrogen atmosphere at 570 ° C, 60 It was denitrified. These test pieces after nitriding were cut at the center, the cut surfaces were polished, and the nitriding depth and nitriding width were measured with an optical microscope.
The results are shown in Table 1 together with the types of drugs used. Each of these agents decomposes at a temperature of 570 ° C. or lower, and the decomposition products destroy the oxide film on the surface of the test piece. No. of this embodiment. FIG. 1 shows hardness distributions from the surfaces of the non-nitrided portion and the nitrided portion in the case of 1.

[0013]

[Table 1]

[0014]

Example 3 A test piece similar to that of Example 2 was used, and an attempt was made to nitride only the 20 mmφ portion at the center of one surface. A resin containing fluorine is used as a chemical for destroying the oxide film on the surface to be nitrided, and the resin is applied to the portion to be nitrided and dried, and then the whole is made to pass 70% in order to passivate the non-nitrided portion.
A test piece was dipped in a 50% nitric acid solution at 60 ° C. for 60 minutes. Ten pieces of this were stacked at intervals of about 1 mm between each test piece, and subjected to nitriding treatment at 570 ° C. for 180 minutes in an ammonia + nitrogen atmosphere in which the nitrogen potential was adjusted to about 10%. These nitrided test pieces were cut at the center, the cut surface was polished, and the nitriding depth and nitriding width were measured with an optical microscope. As a result, FIG. 2 shows the hardness distribution from the non-nitrided part to the nitrided part at a depth of about 50 μm only in the part coated with the chemical in all the test pieces.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the distance from the surface of a test piece and Vickers hardness in Examples 1 and 2 of the present invention.

FIG. 2 is a diagram showing a relationship between a distance from a boundary between a non-nitrided portion and a nitrided portion and Vickers hardness in Example 3 of the present invention.

Claims (3)

[Claims] 1. When nitriding a part of a part made of a material containing iron as a main component, a nitriding agent containing cyanic acid as a main component is applied to a portion to be nitrided to control a non-oxidizing atmosphere or a predetermined nitrogen potential. Partial nitriding method for parts, characterized by heating in a selected atmosphere. 2. When nitriding a part of a part made of steel containing 5% or more of Cr as a raw material, an atmosphere in which a chemical for destroying an oxide film on the surface of the portion to be nitrided is applied and the nitrogen potential is controlled to a predetermined value. A method for partially nitriding a part, which comprises heating in a room. 3. When nitriding a part of a part made of steel containing 5% or more of Cr as a raw material, the non-nitrided surface of the part is passivated and an oxide film is destroyed on the surface of the nitrided part. A partial nitriding method for parts, which comprises applying a chemical for heating and heating in an atmosphere controlled to a predetermined nitrogen potential.