JPH0339459A - Surface hardened parts and their production - Google Patents

Abstract

PURPOSE:To produce surface hardened steel parts excellent in bending fatigue strength by applying carburizing and quenching treatments to the surface of a low-or medium-carbon carbon steel or alloy steel, irradiating the above surface with laser beam, and successively performing tempering treatment. CONSTITUTION:Gas carburizing treatment is applied to the surface of parts made of carbon steel, alloy steel, etc., containing 0.1-0.5wt.% C to form a gas carburized layer, and martensite is formed by means of quenching from a temp. forming austenite solid solution to carry out hardening. Subsequently, the carburized and quenched surface part mentioned above is irradiated with laser beam having high energy density and then heated and held, e.g. at 150-250 deg.C for 1-2hr to undergo tempering treatment, by which machine parts made of carbon steel or alloy steel having high surface hardness and excellent in bending fatigue strength can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a surface-hardened component, such as a steel component, having excellent bending fatigue strength, and a method for manufacturing the same.

(Conventional Technology) In recent years, there has been an increasing expectation in industry for higher strength materials. Under these circumstances, parts that are subjected to repeated stress during use, such as steel parts such as gears, various shafts, and pinions, have traditionally been required to have higher bending fatigue strength.

By the way, carburizing treatment is one of the most typical methods for increasing the bending fatigue strength of steel parts. Carburizing treatment causes carbon to penetrate and diffuse from the surface of the steel in the austenite region of the steel.
Thereafter, the steel is quenched and tempered from the austenite region to make the surface of the steel a high-strength, high-carbon martensite. The effect of carburizing on bending fatigue strength is
This is largely due to the high carbon martensite on the steel surface.

Usually, in order to obtain such high carbon martensite, the carbon potential in carburizing treatment is set to 0.8 to 1.
The amount should be about 2% by weight.

However, there is a limit to the increase in fatigue strength by relying only on the strength of such high carbon martensite.

Therefore, as a result of numerous studies and investigations, a new carburizing method has been proposed that does not rely solely on the strength of high carbon martensite, but skillfully incorporates other factors for increasing fatigue strength. The following are listed below.

■ Utilization of residual stress Residual stress generated during heat treatment has a large effect on bending fatigue strength. In other words, since compressive residual stress increases the bending fatigue strength, this compressive residual stress is actively introduced during carburizing and quenching, especially during quenching, to increase the fatigue limit (the applicant first proposed Patent application filed 1986-143298
(see issue).

■High carbon carburizing process During carburizing, the carbon potential is iron-carbon L! The carbide is precipitated in a spherical shape by maintaining the temperature above the ^cs point in the i diagram.
The bending fatigue strength is increased by dispersing the high carbon martensite in the base and the spherical hard precipitates (Japanese Patent Publication No. 1983-
See Publication No. 35630>.

On the other hand, numerous studies and surveys have been carried out not only on carburizing methods but also on the materials of steel used for carburizing, and the following new steels for carburizing have been proposed.

■Carburized abnormal layer reduction steel Normal case hardening steel (for example, JIS standard, 5CR420, S
When CM420, etc.) is gas carburized, a layer consisting of grain boundary oxidation and incomplete quenching & IIIa, called an abnormal carburization layer, is generated near the surface of the steel. Since this abnormal carburized layer has a negative effect on the bending fatigue strength, the steel is a case-hardened steel with a reduced abnormal carburized layer (see Japanese Patent Publication No. 32777/1983).

■Carburized layer grain boundary strengthened by lowering P Due to bending fatigue fracture of carburized steel parts, the fracture surface morphology of the carburized layer becomes prior austenite grain boundary fracture. Therefore, in order to strengthen the prior austenite grain boundaries in the carburized layer, this is a case hardening steel that reduces the grain boundary segregation of P, which weakens the prior austenite grain boundaries.
(Refer to Japanese Patent Application Laid-Open No. 60-243252).

(Problem to be solved by the invention) As described above, various carburizing methods and various carburizing steels have been proposed to increase the bending fatigue strength of steel parts. However, any of the above-mentioned methods and any of the steels for carburizing have the following common problems.

That is, as mentioned above, in order to increase the bending fatigue strength, it is first necessary to make the surface of the steel high carbon martensite, and therefore, any of the above carburizing methods and any carburizing steel During the carburizing process, it is necessary to set the carbon potential to about 0.8 to 1.2% by weight in order to make the surface of the sea bream high-carbon martensite.

However, conventionally, when carburizing is carried out at such a high carbon potential to obtain high carbon martensite, the matrix becomes high carbon martensite and high hardness is obtained, but the grain boundaries become relatively significantly weakened. There is a problem.

Therefore, bending fatigue fracture occurs due to intergranular fracture in the carburized portion, and the strength of the high carbon martensite that constitutes the matrix is not sufficiently reflected in the bending fatigue strength.

The present invention is aimed at solving the above problem, and is aimed at reducing the carbon potential during carburizing to 0.
Even at 8 to 1.2% by weight, grain boundaries do not become brittle,
The object of the present invention is to provide a surface-hardened component that sufficiently reflects the strength of high-carbon martensite in the matrix against bending fatigue, and a method for manufacturing the same.

(Means for Solving the Problems) As a result of various studies in order to solve the above problems, the present inventors found that the following method for carburizing steel and any carburizing method shown in the prior art was found. We focused on the fact that there are basic facts. That is, in order to sufficiently strengthen the matrix strength, the carbon potential is 0.8 to 1.2.
When carburized at % by weight, intergranular fracture becomes dominant at the end face of the carburized part due to bending fatigue fracture.

Based on this fact, the present inventors conducted a detailed investigation and obtained the following new findings.

(a) The reason why the carburized part is dominated by grain boundary fracture due to bending fatigue fracture is that film-like carbides precipitate at the prior T grain boundaries during carburizing, which weakens the grain boundaries.

(b) Reheating and quenching of the carburized layer is effective in countering grain boundary embrittlement due to film-like carbides precipitated at the former T grain boundaries of the carburized layer. (y) Heating transformation (α゛→T ) Dissolution of the film-like carbide into one grain by (a) Formation of new T grain boundaries where no film-like carbide exists due to thermal transformation (α° → T), reducing the film-like carbide at the grain boundary, The former T grain boundaries after reheating and quenching can be strengthened.

(c) In addition, the reheating and quenching treatment shown in (2) above can significantly reduce grain boundary segregated substances (P, grain boundary oxides) that cause grain boundary embrittlement other than film-like carbides. can.

(d) High-energy surface hardening treatment using a laser beam is effective for reheating the carburized layer, followed by tempering treatment and providing a laser beam surface hardening and tempering layer. ), (C) can be achieved, and steel parts with excellent bending fatigue can be manufactured by strengthening the prior T grain boundaries.

Based on such knowledge, the present inventors conducted further studies, and as a result, completed the present invention.

Here, the gist of the present invention is to apply laser beam surface hardening to a carburized and hardened surface portion formed on the surface of carbon steel or alloy steel containing 0.1 to 0.5% C by weight. This is a hardened surface part with a tempered layer and excellent bending fatigue strength.

In addition, from another point of view, C is 0.1 to 0.5% by weight.
When hardening the surface of carbon steel or alloy steel containing carbon steel, bending fatigue strength is characterized by first performing carburizing and quenching, then irradiating the carburized surface with a laser beam, and then performing tempering. This is a method for manufacturing surface-hardened parts with excellent properties.

(Operation) First, the constituent elements of the present invention will be explained. In this specification, "1%" means "wt%" unless otherwise specified.

The reason why C is set at 0.1 to 0.5% by weight is as follows.

C has the effect of increasing the strength of the core, which is a non-hardened layer, and decreasing the toughness of the core.

However, if the content is less than 0.1%, the strength of the core portion will not be sufficient, and the strength of the entire material including the hardened layer will be insufficient. Therefore, the lower limit is set to 0.1%.

On the other hand, if more than 0.5% is added, the noodle skin of the core increases, but conversely the toughness of the core decreases and the toughness of the entire material becomes insufficient. Therefore, the upper limit is set at 0.5%.

Also, elements other than C, such as S ball, MII% 3%
P etc. may be used as long as it is normally used for this type of surface hardened parts, and is not particularly limited, but for example, SI: 0 to 2.0.
%, Mn: 0.2-2.0%, S: O-0.10%, P
:O~0.03% is acceptable.

Next, the reason why the target was carbon steel or alloy steel is because of the following reasons (]) and (2).

(1) Depending on the application, some carburized steel parts that require bending fatigue strength are large. In the case of such large parts, many alloy steels are used to ensure hardenability.

Therefore, the present invention is applicable not only to carbon steel but also to so-called alloy steel.

(2) This is because the present invention has the effect of increasing bending fatigue strength regardless of the type of alloying element added to alloy steel.

Furthermore, examples of the carburizing treatment performed in the present invention include gas carburizing. Although the conditions for this gas carburizing treatment are not particularly limited, it is desirable to keep the following in mind.

As mentioned before, in order to increase the bending fatigue strength, it is necessary to make the steel surface high carbon martensite, and for this purpose, the carbon potential of the atmosphere during gas carburizing should be set to 0.8 It is desirable to set the content to about 1.2%.

There is no need to particularly limit the quenching method after gas carburizing. The quenching at this time has the effect of generating a supersaturated ferrite solid solution (martensite) by cooling the austenite solid solution and hardening the carburized layer and core. Generally, in order to increase bending fatigue, it is necessary to sufficiently harden the steel material to increase both surface hardness and core hardness, and the hardening method should be considered accordingly. However, the applications of steel parts (precision parts where it is necessary to minimize thermal strain and transformation strain due to carburizing and quenching, such as automotive transmission gears)
In some cases, hardening may be intentionally insufficient in order to reduce hardening distortion. However, the carbides in the carburized layer that have an adverse effect on the fatigue limit, which is the problem of the present invention, are generated only during carburizing, and depending on the method of quenching treatment performed after carburizing, the generation may be suppressed or
Therefore, the hardening process is not specified in order to make the present invention applicable even to dense parts that have been moderated in hardening as shown above.

Next, according to the present invention, a laser beam surface hardening and tempering treatment layer is provided on the carburized and quenched surface portion obtained as described above, but this is a surface treatment layer obtained by laser beam irradiation and subsequent tempering treatment. It is.

In the laser treatment in the present invention, a laser beam with high energy density (>10'll/cm") is irradiated onto the steel surface for a short period of time to raise the temperature above the T temperature, and after irradiation with the laser beam, heat diffusion ( This is a surface hardening treatment that turns only the steel surface into martensite by self-cooling.

Laser treatment performed after hardening has the following effects.

(1) Due to heating transformation (α°→T), film-like carbides at the grain boundaries of the carburized layer, which cause a decrease in bending fatigue strength, are dissolved into the γ grains.

(2) New and clean γ grain boundaries free of film-like carbides are formed by heating If (α゛→γ).

(3) Re-quenching of the Ti carbon layer by self-cooling of the steel (γ→
α゛) The surface portion has a high carbon martensitic structure.

Processing conditions for laser processing are not particularly specified, power density: 104 to 105-1 cm, which is optimal for heat treatment (quenching).
, irradiation time: 10-' to 10-' (S) are also suitable in the present invention.

The tempering treatment performed after carburizing and quenching and laser treatment has the effect of increasing the toughness of the carburized layer and prevents cracking and peeling of the carburized layer. Tempering conditions are not particularly specified, but in order to fully exhibit the above effects, it is desirable to hold the tempering at 150 to 250°C for 1 to 2 hours.

Note that Japanese Patent Publication No. 60-13045 is a similar patent whose structure is seemingly similar to the present invention. The problem that this patent seeks to solve is the large residual strain caused by the hardening of the entire part by conventional carburizing and tempering processes. As a solution, the residual strain was successfully reduced by hardening and hardening only the surface using a laser after carburizing. According to this solution, since the entire part is not hardened after carburizing, the hardness of the core of the part is clearly lower than that of a part that is hardened as a whole.

By the way, the purpose of the present invention is to obtain high bending fatigue strength, and for this purpose, it is necessary to harden the core to make it martensite and obtain high hardness. Special Public Service 1986-1
Although the treatment described in the claims of Publication No. 3045 has excellent wear resistance, the core hardness is low, and therefore no improvement in bending fatigue strength can be expected.

In this regard, unlike the invention described in Japanese Patent Publication No. 60-13045, the present invention stipulates immersion R11k and hardening of the entire part in order to improve bending fatigue. The hardness is consciously increased.

That is, the present invention and the invention described in Japanese Patent Publication No. 60-13045 are completely different in terms of the method and the effects obtained by the method.

(Example) The present invention will be explained in detail by way of specific examples.

Example 1 A 30φ forged material of JIS standard 5C11420 steel shown in Table 1 was normalized under the conditions of 930°C Post-laser treatment was performed, and the other was only carburized and quenched.

Both samples in Table 1 were then tempered under the conditions of 170°CX 2 hr-+AC, and then subjected to the Ono 2 rotary bending fatigue test. The carburizing and quenching conditions are 930°C x 2hr→
0 (20°C), C, P, (carbon potential)
−0.8%, and the laser treatment has a power density of −1
0' and irradiation time -10'' (S), only the notch of the test piece was irradiated.

The results are shown in Table 2.

By cleaning the prior γ grain boundaries of the carburized layer by laser treatment,
Fatigue limit increased by about 20%.

Example 2 - Carburized abnormal layer reduced steel (B steel) shown in Table 3 as test steel,
A low P type carburized bed grain boundary strengthened sea bream (C steel) was prepared.

From these, test pieces were manufactured in the same manner as in Example 1, and the influence of laser treatment after carburizing on the fatigue limit was investigated.

The test steel A shown in Table 3 and Table 1 was subjected to high carbon carburizing treatment, and the influence of the laser treatment after carburizing on the fatigue limit was investigated.

High-carbon carburization was performed using the following method to spheroidize excessively precipitated cementite.

-OQ, 180℃X2hr-+AC Table 4 shows the results.

As shown in Table 4, the fatigue limit of any steel is improved by laser treatment. All three steel types aimed at improving fatigue limits have a common problem of weakening of prior γ grain boundaries due to carburizing and quenching, and it is thought that this problem was solved by laser treatment after carburizing.

Table 4 Example 3 According to Example 1, after standardizing the 30φ forged and drawn material of JIS standard 5CR420 steel, one side was carburized and air-cooled to have a ferrite/pearlite structure with less distortion, and then laser treatment was performed to only the carburized part. martensai) [woven. The other piece was carburized and quenched in the same manner as in Example 1, and then subjected to laser treatment. In this case, unlike the former, the latter had a martensitic structure not only in the carburized part but also in the core.

These samples were subjected to tempering treatment in the same manner as in Example 1, and a fatigue test was conducted.

The results are shown in Table 5.

After carburizing, the core part is made of ferrite + pearlite mm by air cooling, and the fatigue limit is low because the core part has low hardness.The purpose of the present invention is to provide air cooling treatment after carburizing to reduce such distortion. It is inaccurate to improve the bending fatigue strength, and it is not possible to provide the desired bending fatigue strength.

That is, from this example, as detailed in this specification, it is clear that in order to improve bending fatigue, it is necessary to harden the core to improve the core hardness.

Table 5 (Effects of the Invention) By having the structure as explained above, the present invention solves the problem of intergranular cracking in the carburized layer due to bending fatigue, which is a problem in carburized steel parts, and improves the bending fatigue strength. It can be applied to parts such as gears, various shafts, and binions that are subjected to repeated stress, and has the extremely useful effect of being able to achieve higher stress loads than conventional ones.

Claims (2)

[Claims] (1) Bending fatigue with a laser beam surface quenching and tempering treatment layer on the carburized and quenched surface portion formed on the surface of carbon steel or alloy steel containing 0.1 to 0.5% C by weight Hardened surface parts with excellent strength. (2) When surface hardening carbon steel or alloy steel containing 0.1 to 0.5% C by weight, first perform carburizing and quenching treatment, then irradiate the carburized surface with a laser beam. , followed by a tempering treatment. A method for manufacturing surface hardened parts with excellent bending fatigue strength.