JPH0676654B2 - Nitrogen gas base soft nitriding method - Google Patents

Description

The present invention relates to a soft nitriding method using nitrogen gas as a base atmosphere.

[Conventional technology]

As a nitriding method for hardening a surface layer of steel or the like in a high nitrogen state, Japanese Patent Publication No. 49-30339 discloses a method in which 0.5 to 5% by volume of air or 0.1% ammonia (NH ₃ ) gas is used.
A gas soft nitriding method is shown in which nitriding is performed in an atmosphere to which oxygen of 1% by volume is added.

In this gas soft nitriding method, by adding air or oxygen to the ammonia gas, the decomposition of the ammonia gas is promoted, the nitrogen concentration in the atmosphere is increased, the nitriding treatment time is shortened, and at the same time, Fe ₂ easily peels off. It is intended to prevent the generation of N.

[Problems to be solved by the invention]

However, in the conventional nitriding method, as shown in the processing step diagram of FIG. 5, not only during the soaking step of ammonia gas,
Although it is added during the entire heating process at a low temperature, the diffusion rate of nitrogen in the steel to be treated is slow and the utilization of ammonia gas is low during the heating process at a low temperature. . Therefore, a large amount (95% by volume)
However, the expensive ammonia gas is not effectively used, and the nitriding treatment cost is high.

Further, during the temperature raising step, since the surface temperature of the heater arranged in the heat treatment furnace is higher than that of the processed product, ammonia is preferentially decomposed on the heater surface, and ammonia acts violently on the heater surface. Therefore, there is a problem that the life of the heater is shortened.

Furthermore, since ammonia gas is added also in the temperature raising step, the oxygen concentration during the temperature raising step is extremely low. For this reason, if the processed product that has been machined before the nitriding process is not sufficiently washed, the cutting oil remaining and adhering to the processed product decomposes and carbonizes during the temperature raising process, Therefore, there is a problem that uneven nitridation occurs in the processed product.

The present invention has been made in order to solve the above problems in the conventional gas nitrocarburizing method, and makes a significant cost reduction by effectively using expensive ammonia gas, and stable nitriding without uneven nitriding. An object of the present invention is to provide a nitrogen gas-based soft nitriding method capable of performing treatment.

[Means and Actions for Solving Problems]

In order to solve the above problems, the present invention uses an inexpensive nitrogen gas as a base gas, adds only air or oxygen during the temperature raising step, performs an oxidation treatment to purge the generated gas, and raises the temperature. Immediately before the completion of the above or in the soaking step after completing the temperature raising operation, ammonia gas is added to perform the nitriding treatment.

By performing the nitriding treatment in this way, carbonization can be prevented by oxidizing and removing the cutting oil adhering to the treated product by the added air or oxygen during the temperature raising step. Therefore, it is possible to prevent the occurrence of uneven nitriding in the nitriding process immediately before the completion of the temperature raising process or after the transition to the soaking process.

Also, since the nitriding treatment is performed by adding the ammonia gas just before the completion of the temperature raising process or only in the soaking process, the expensive ammonia is effectively used to reduce the consumption amount, and the cost is remarkably reduced. It can be measured.

〔Example〕

Examples will be described below. FIG. 1 is a diagram showing an example of a treatment process of a nitrogen gas-based soft nitriding method according to the present invention,
An embodiment of the present invention will be described based on the process chart.

First, in the temperature raising step before entering the soaking step for performing nitriding treatment, nitrogen gas is introduced into the heat treatment furnace to make this a base atmosphere, to which 0.5 to 5% by volume of air or 0.1 to 1% by volume of oxygen is added. To do. By the addition of the air or oxygen, the cutting oil attached in the machining step, which is a pre-process of the processed product, reacts with the added oxygen, and mainly CO gas is removed to remove the cutting oil. The amount of air or oxygen added is appropriately set depending on the shape and surface area of the processed product, the amount of cutting oil brought into the processing furnace, the type of cutting oil, and the like.

In this way, during the heating process, while completely gasifying the cutting oil, the temperature rising process of the processed product is completed, CO gas generated by the reaction of the cutting oil and oxygen, etc. is always in the furnace as the base gas. It is mixed with the introduced nitrogen gas and purged by exhausting the mixed gas. Then, it transfers to the next soaking process.

In the soaking process, while maintaining the treated product at a predetermined temperature T, a mixed gas of ammonia nitrogen gas and ammonia gas is introduced based on nitrogen gas. The added ammonia gas is decomposed to generate nitrogen, and the nitrogen permeates and diffuses on the surface of the treated product to promote nitriding. After performing nitriding treatment for a predetermined time, it is cooled, and the following steps such as cleaning treatment are performed.

FIG. 2 shows a structure 400 of the surface portion of the product subjected to the nitriding treatment in the above-mentioned embodiment of the nitrogen gas-based soft nitriding method according to the present invention.
It is a magnification micrograph. This treated product is a carbon steel for machine structure (S50C) at 570 ℃ with a mixed gas of 75% N ₂ gas and 25% NH ₃ gas.
Was processed for 4 hours at a constant temperature of
A uniform compound layer of 20 μm is formed. As a result, it is clear that carbonization is prevented during the temperature raising step and uneven nitriding does not occur.

In order to compare FIG. 3 with the present invention, the carbon steel for machine structural steel (S50C), which was subjected to the nitriding treatment (4 hours at 570 ° C.) in the conventional treatment process shown in FIG. It is a magnification micrograph. As is clear from this figure, according to the conventional method, only a non-uniform thin compound layer was obtained.

In the above-mentioned embodiment, the nitriding treatment is performed by adding the ammonia gas after the temperature raising process is completed.
As shown in the drawing, even if the ammonia gas is added immediately before the completion of the temperature raising step, the same effect as that of the above-described embodiment can be obtained.

〔The invention's effect〕

As described above based on the embodiments, according to the present invention, the cutting oil adhering to the processed product is gasified by adding air or oxygen during the temperature raising step using nitrogen gas as the base gas. It is possible to remove and prevent the carbonization of the cutting oil that causes uneven nitriding, whereby the pre-cleaning step before the nitriding step can be omitted.

In addition, since expensive ammonia gas is added just before the completion of the temperature raising step of high temperature at which the diffusion rate of nitrogen in the steel is fast or only in the soaking step, the effective use of ammonia gas can be measured, resulting in a significant cost reduction. It can be reduced. Further, since ammonia gas is not added during the temperature raising step of low temperature, the heater of high temperature is not attacked by the ammonia gas, so that the life of the heater can be extended.

[Brief description of drawings]

FIG. 1 is a diagram showing a treatment step of an embodiment of a nitrogen gas-based soft nitriding method according to the present invention, and FIG. 2 is a surface of a treated product obtained by the nitriding method by the treatment step shown in FIG. Fig. 3 is a diagram showing a metallographic structure of a portion of the treated product obtained by a conventional nitriding method, Fig. 3 is a diagram showing a metallographic structure of a surface of a treated product obtained by a conventional nitriding method, and Fig. 4 is another embodiment of the present invention. FIG. 5 is a diagram showing an example treatment process, and FIG. 5 is a diagram showing a treatment process of a conventional nitriding method.

Claims (1)

[Claims] 1. Nitrogen gas is used as a base gas, an oxidizing treatment is performed in an atmosphere to which 0.5 to 5% by volume of air or 0.1 to 1% by volume of oxygen is added during the temperature raising step, and the generated gas is purged, followed by rising. A nitrogen gas-based soft nitriding method characterized by performing nitriding treatment by adding ammonia gas immediately before the completion of the warming step or in the soaking step.