JPH03274220A - Blend soft-carburizing heat treatment and product thereby - Google Patents

Abstract

PURPOSE:To produce a steel product excellent in toughness and having a high hardness carburized layer in the internal part and at the surface in a short time by applying carburizing treatment to a worked product prepared by using, as a stock, a medium carbon steel with specific carbon content to a specific depth from the surface and then applying hardening treatment to the above product to a depth beyond the carburized layer. CONSTITUTION:Carburizing treatment is applied to the part requiring high hardness at the surface of machine parts composed of a medium-carbon structural carbon steel or alloy steel containing 0.30-0.53% C by a gas carburizing method by means of high-frequency heating or by using a pasty carburizing material so that C content is regulated to 0.6-0.9% at a depth of <=0.8mm from the surface. Subsequently, the surface of the above parts is subjected to hardening treatment to a depth beyond the carburizing depth of 0.8mm, by which the machine parts where surface hardness is remarkably improved and no abrupt changes are seen in hardness distribution in the noncarburized tough part in the inner part can be produced by means of short-time carburizing treatment.

Description

[Detailed Description of the Invention] [Industrial Application Scope 2] This invention relates to a heat treatment method for surface hardening of processed products made of steel, and products based thereon. This is related to this.

[Prior Art] Various heat treatment methods have been put into practical use for surface hardening of gears, shafts, cams, and other mesh mechanical parts. Among these methods, the coupled method uses case-hardened steel with a low carbon content as a material, so while the carbon content increases and hardens on the surface due to coupling, the internal carbon content remains low and improves toughness. It is widely used because it is easy to maintain and allows accurate heat treatment. However, the coupling method is
Although furnace heating is possible and suitable for mass production,
Normally, the treatment often requires several hours or more, and the process is a long process, which is a significant disadvantage.

[Problems to be Solved by the Invention] This invention addresses the weaknesses of the above-mentioned coupled method by further shortening the treatment time in decarburizing treatment, while maintaining high wear resistance and toughness and excellent mechanical properties. The aim is to make it easier to obtain products that

[Means for Solving the Problems] In order to solve the above problems, in the present invention, a material of structural carbon steel or alloy steel other than case hardening steel is machined and heat treated by high frequency heating. When hardening, the carbon content of the material is 0.30% or more and 0.53%
By carrying out carburization treatment using high-frequency heating, the surface depth is shallower than the normal coupling depth.
The carbon content in a depth range of up to 8 millimeters is increased to the degree required for the hardening treatment, and then the part is heated to a depth exceeding said increased carbon content by radiofrequency heating. The object of the present invention is to provide a heat treatment method characterized by performing a quenching treatment to increase surface hardness, and to obtain a product using the heat treatment method.

Conventionally, the carbon content of steel for mechanical structures used for induction hardening has been kept to about 0.6% (equivalent to JIS G 3102558C) or less from the viewpoint of processing and mechanical properties. Also, due to the characteristics of high-frequency heating, there is a tendency for the hardness of the outer surface to decrease slightly, and it is thought that the surface hardness of about Hs80 is the limit with normal hardening technology.
For this reason, the applications are often limited and disadvantageous.

On the other hand, when we investigated the relationship between carbon content and quench cracking deformation, we found that if the carbon content in the main body part is suppressed, even if the carbon content in the surface part is slightly high, if high-frequency heating is used, the internal residual stress will be negligible. It has been found through numerous test experiences that it does not cause problems such as quench cracking damage. For this reason, attempts are made to increase the amount of carbon only in the surface portion by carrying out decarburization treatment to the amount of carbon necessary for quenching to the desired hardness. Generally, the amount is often used in the range of 0.6% to 0.9%. For example, when hardness Hs85 is required,
If the carbon content reaches 0.7 to 0.8%, the necessary hardness can be achieved by sufficient induction hardening treatment.

As mentioned above, the amount of carbon increase required depends on the carbon content of the material and the final required carbon content, but the amount of carbon increase is greater than the amount of carbon increase required when case hardening steel is used. Since only a very small amount is required and high-temperature heating can be concentrated on the surface layer due to the characteristics of high-frequency heating, the time required for coupling may be much shorter. As a result, the carbon content of the material was 0.53% (JIS G 310255
Even if the temperature is less than 0C equivalent), Hs80~85 in a short time
The carbon content required for heat-treated products with a certain degree of hardness can be easily obtained. The means for obtaining the required carbon amount corresponding to the required cross-sectional hardness can be freely selected depending on the combination of the carbon amount of the material to be used and the carbon increase amount, as necessary.

In addition, structural carbon steel or alloy steel whose material has a carbon content of 0.53% or more is excluded from the scope because it is technically possible to achieve a surface hardness of Hs 80 or more with induction hardening alone.

On the other hand, if the carbon content of the material is less than 0.30%, the amount of carbon required for hardening must be significantly increased, which increases the time required for processing, and even after hardening, the surface and internal parts The hardness difference between the carbon and the uncoupled parts becomes sharp and becomes a structural weakness, which is of course economically disadvantageous. Therefore, we aim to keep the carbon increase within 0.30%, and those with less than 0.30% are targeted. shall be excluded from The depth range for coupling is set to 0.8 mm because the time required for coupling is 3.
This is to shorten the hardness to within about 0 minutes.On the other hand, since induction hardening is performed even on areas with a depth of 0.8 mm or more, the hardness does not drop sharply and draws a gentle curve. This is because, if the heat treatment depth is within this range, it can be applied without any particular problem in the final polishing of gears and other gears under severe conditions, and in actual use.

Of course, the coupling method itself is not limited to gas coupling; it is also possible to use paste-like carbon-absorbing materials, and induction hardening can be used if stable high-quality products can be obtained. There is basically no problem in using flame heating instead.

[Example] As an example of the present invention, high frequency decharring treatment was performed under the following conditions.

Material: 535G (carbon content 0.37%) Dimensions
・50Φ×50 Combined gas: Carrier gas (in this case, CO7°Hs
, N t mixed gas) Enriched gas (in this case, propane gas is used) Heating power source: 8KHz inverter decoaling time 1000℃ 1O minute induction hardening; 900℃ heating Rapid cooling tempering from 900℃
After injecting N2 gas into the container for 2 hours at 180°C for about 10 minutes,
The carrier gas was filled and high frequency heating was started, the heating temperature reached 1000° C., the same amount of carrier gas was reduced while injecting the enrich gas, and the heating temperature was maintained for 10 minutes. Thereafter, the heating was stopped, the material was replaced with nitrogen and gas, and after cooling, necessary induction hardening was performed and tempering was performed. FIG. 1 shows the heat treatment pattern in this case, where the solid line shows the temperature change and the broken line shows the change in the amount of gas for heat treatment.

As a result, we measured the carbon content and hardness of each part after tempering, and found that the carbon content in the thickness direction of this product was as shown in Figure 2.
The cross-sectional hardness showed a distribution as shown in FIG. 3, as shown by a solid line.

As a result, the hardness of the carbon piece remains at Hs 85 on the surface, and as it goes inside, it decreases depending on the degree of charring, but within a depth of 0.3 mm, the carbon piece maintains a hardness of 0.85%.
75%, hardness equivalent to Hs80 or higher was maintained, and the internal uncoupled portion gradually transitioned to hardness equivalent to original induction hardening. Further, the amount of carbon increase in this case is about 0.4%, which corresponds to what is called a mild soft carbon treatment.

For comparison, the yield obtained when 5Cr22, which is a normal case hardening steel, is decarburized for about 3 hours and then quenched is shown as a broken line in Figure 3 for reference. The characteristics of this invention are illustrated by the situation.

Note that FIG. 4 shows one of the microstructures after the decharring treatment of this example.
This is a micrograph taken at a magnification of 00x.

[Effects of the invention] This invention applies mild soft carburizing treatment by high-frequency heating to products made of mechanical structural steel with a medium carbon content, and then quench hardening by high-frequency heating. By doing this, it is possible to easily manufacture products that have an appropriate hardness distribution throughout the interior and have relatively little residual sagging force while sufficiently shortening the coupling time, and the total processing time is significantly reduced. As a result of the shortening, an effective cost reduction was realized, which can be widely used in the future for manufacturing machine parts with excellent mechanical properties.

[Brief explanation of drawings]

Fig. 1 shows the decarburizing treatment pattern according to the example of the present invention, Fig. 2 shows the analysis results of the carbon content of the cross section after coupling, and Fig. 3 shows the cross-sectional hardness after quenching and tempering of the present invention and comparative example. FIG. 4 shows a micrograph of a cross section after coupling according to the present invention. Between Daiichi Koshuha Kogyo Co., Ltd. (sin,) Distance from the outer surface (lI Peng) Figure 2 Procedural amendment (method) November 1990) 31 Yo

Claims (1)

[Claims] 1. A method of hardening a processed product made of steel such as structural carbon steel or alloy steel by heat treatment using high-frequency heating, wherein the carbon content of the material is 0.30%. The carbon content is within the range of 0.53% or more, and by performing carburization treatment by high frequency heating on the part to be quenched and hardened, the carbon content is within a depth of 0.8 mm from the surface. The carbon content is increased to an extent necessary for a later quenching treatment, and then the portion is quenched by high-frequency heating to a depth exceeding the range in which the carbon content has increased. Heat treatment method 2 Materials made of steel such as structural carbon steel or alloy steel with a carbon content in the range of 0.30% or more and less than 0.53% should be subjected to machining and quench hardening. The part is decarburized by high-frequency heating to increase the carbon content within a depth of 0.8 mm from the surface to the extent necessary for the subsequent quenching treatment, A steel product manufactured by subsequently subjecting the portion to a quenching treatment using high-frequency heating over a depth exceeding the portion where the carbon content has increased.