JPH0364500A - Method for strengthening formed product of carburizing steel - Google Patents

Abstract

PURPOSE:To strengthen a formed product of alloy steel having a specified concn. of Ni by carburizing the product and removing the surface by electropolishing by a specified thickness or more. CONSTITUTION:A formed product of alloy steel having >=0.5% Ni content is carburized and the surface is removed by electropolishing or other method by >=20mum thickness. The abnormal surface layer formed by surface hardening is removed and the static bending strength of the product can be considerably enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a strengthening treatment method for improving the static bending strength of a carburized steel molded product.

(Prior technology) Gears for automobile transmissions currently mass-produced use case-hardened steel, which is generally formed by blanking, hobbing, and shaving, followed by carburizing and quenching, and polishing of bearings and sliding surfaces. It is manufactured through a sequential processing process.In recent years, however, turbocharged engines,
With the advent of the four-Hartz engine, the output of automobile engines has become higher and higher, and as a result, the load stress acting on the gears and shafts, which are the power transmission parts of automobiles, has increased, and static breaking strength, bending fatigue strength, etc. have become less than the allowable strength. It often exceeds.

JP-A No. 61-117014 discloses that a shaved gear is subjected to surface hardening treatment such as carburizing, and then the root fillet including the shaving step is ground (f-J-, shaved and shaved). Is there a method for manufacturing high-strength gears that eliminates steps? This method eliminates the steps that occur at the root of the gear tooth and smoothes the shape of the root to reduce the concentration of stress at the root of the tooth fillet. This is intended to improve the bending fatigue strength of gears.

In addition, the applicant has applied shot peening processing to a gear that has been carburized, quenched and tempered, and then the surface of the gear is
A patent application was filed for the invention of a method for reinforcing gears that improves bending fatigue strength by reducing the thickness by 60 mm.
-164352).

[Invention or problem to be solved]

When the load stress acting on gears and shafts, which are the power transmission parts of automobiles, increases and exceeds the static breaking strength, tlh fatigue strength, etc., the load stress must be reduced by significantly designing and revising the floats and shafts. It would be necessary to lower the transmission weight, which would increase the weight of the transmission and require a great deal of effort.

The effect of shot peening is that the strength decrease due to the abnormal layer generated by carburization is caused by the deformation of the surface layer due to the addition of L by shot peening, and hardening and compressive residual stress are added. remains as it is, which may have a considerable negative effect on bending fatigue strength.

An object of the present invention is to provide a treatment method for improving the static bending strength of an alloy steel molded product that has been subjected to surface hardening treatment such as carburization, regardless of whether or not shot peening treatment is performed.

(Means and Effects for Solving the Problems) The present invention provides that after carburizing a molded product using an alloy steel material with a nickel content of 0.5% or more, at least 20 JL of the surface of the molded product is carburized.
This is a method of strengthening a carburized steel molded product by removing a thickness of m or more by electrolytic polishing or the like.

According to the results of static bending tests, compared to conventional alloy steels, there is a significant improvement in strength for steels containing nickel of 0.5% or more. The surface carbon content at this time is lower than that of conventional products, and it is thought that the surface carbon content affects the strength.

Furthermore, static bending strength is significantly improved by removing the W normal layer formed on the surface due to hardening treatment by electrolytic polishing or the like.

(Example) The present inventors focused on static breaking strength among gear strengths,
Material composition of alloy steel, heat treatment method, surface treatment such as shot peening and electrolytic polishing
A test was conducted.

The results of the static bending test are shown in FIG. 1, and FIG. 2 shows the shape of the test piece subjected to the static bending test.

The shape of the test piece is, as shown in Figure 2, diameter D = 15
mm, L = 105 mm rod, r = 2.5n +
Form an annular groove of I11, and set the diameter of the annular groove to d-IOIl.
llll. The annular groove was provided because the shape of the test piece was approximated to the base of the tooth Bli, and the stress concentration coefficient α of this test piece was r/d=0.25 D/d.
Since α=1.5, α=],48.

In Fig. 1, test piece A is made of nickel-chromium-molybdenum steel, and its chemical composition percentage is as follows:
S: 0. IO,N,:2. Old, Cr: 0.28゜M
o: 0.77.

The material of test piece B was the material standard SN of the structural alloy steel standard.
CM 42 (nickel-chromium-molybdenum steel belonging to IH), the material chemical composition % is S: 0.
27. Ni: ], 61. C: 0.60. M,
:0.19.

Test pieces A and B were carburized, and the depth of the surface hardening layer was set to 0.5 to 1. It was set to 0mm.

The material of test piece C was material standard SC of the structural alloy steel standard.
It is made of M420H chromium-molybdenum steel, and its material chemical composition % is S: 0.23. NH:0.0
8. C, :1.15. Mo: 0.15. The chromium-molybdenum steel of this SCM 420H does not contain nickel according to the standard, but as mentioned above, a material containing slightly stitched nickel was used.

Test piece C1 is obtained by applying a low concentration of carburization to test piece C made from the above material, and test piece C2 is obtained by applying 23% carburization to test piece C+.
Test piece C3, which was tempered at 0°C, also had a temperature of 16
Test piece C4, which was tempered at 0°C, is a test piece made from the above-mentioned material and carburized to a high degree.

For the above test pieces A, B and C1 to C1, prepare one with shot peening and the other without shot peening.
,,C3, a static bending test was conducted on the samples with a thickness of about 401 Lm removed from their surfaces by electropolishing and those without electropolishing.

In FIG. 1, the surface treatment status of each test piece is indicated as follows.

O show 1 ~ No beaning, no electrolytic polishing.

・：With shot peening, without electrolytic polishing.

△: No shot peening, electrolytic polishing N right side.

Mu: 1 shot pinion kui, with electric M polishing.

In Fig. 1, the solid broken line ■ indicates the average strength of each test piece that has undergone heat treatment such as carburization, and the range of ■ surrounded by a dashed-dotted line indicates the strength of the test piece that has been subjected to electrolytic polishing. Indicates intensity range.

As is clear from the figure, the average value of the strength of test piece C:l as heat treated is 1714
If electrolytic polishing is performed, the strength will be 2569 kg as shown in (2), and the strength will be improved by about 50%.

Similar strengths were observed for the other test specimens as well.

The average breaking load when electrolytic polishing is performed on test piece C7 is 2633 kg as shown in ■, or is it electrolytic polishing Wj?
The test piece B used was 35:14Kg as shown in ■.
In addition, as shown in ■, the test piece A that underwent electrolytic polishing weighed 3757 kg, which was 34% to 43%, respectively.
An improvement in strength can be seen.

The strength of test specimens A and B subjected to electropolishing is approximately twice as high as the average strength (3) of test specimen C3 which was not subjected to electropolishing and was heat treated.

From the above static bending test results, material standard SCM 42
Compared to specimen C made of 0H chromium-molybdenum steel, specimens A and B containing 0.5% or more of nickel show a significant improvement in strength.

This is probably because the surface carbon content in the carburized layer of test specimens A and B is lower than that of test specimen C, and the surface carbon base has an effect on the strength.

It was also found that although the presence or absence of shot peening does not significantly affect the strength, if the heat-treated surface layer is removed by electrolytic polishing, the strength can be improved by about 50%. Removal of this surface layer can be achieved by grinding, pressure, etc., or electrolytic polishing improves surface roughness, which is preferable in terms of improving strength.

Next, other tests will be explained.

Figure 3 shows the relationship between the depth from the surface of a carburized gear subjected to shot peening and the distribution of residual stress.1.
There is -c.

The residual stress on the root surface is about -30 Kg/n+m2, or the maximum value of the residual stress appears at about 50p+a from the surface and is about -100 Kg/lll112. The cause of the insufficient bending fatigue strength of carburized gears is thought to be that an incompletely quenched layer called an abnormal layer of 5 to 20 gm is generated on the surface of the carburized layer, and the strength of this surface layer is low.

When a thickness of about 40 pn from the surface is removed by electrolytic polishing, a surface with large residual stress due to shot peening appears on the root surface of the gear.

The following table shows the test results of bending fatigue tests regarding root stress of carburized gears. (Unit: Kg/mm2) As is clear from this table, by applying the sit-pe-ninta process to the carburized state, 1111
However, the bending fatigue strength at the root of the tooth is hardly improved by removing 10 E of J-dah from the surface. This is considered to be because the abnormal surface layer generated by carburization remains as it is.

By removing 20 pm or more of thickness from the surface, bending fatigue strength is significantly improved, increasing by 39% when 40 pm is removed from shot-pinned gears. As is clear from Figure 3, 40 to 50 pm from the surface
By removing the thickness of 20 to 60 pLm, it is appropriate to remove the thickness of 20 to 60 pLm, considering whether it appears on the surface with the maximum residual stress or the variation in the thickness of the abnormal layer due to carburization.

It is thought that the decrease in bending fatigue strength of gears is largely due to an increase in surface compressive residual stress at the root of the tooth, and is also influenced by improvement in surface roughness by electrolytic polishing.

Summarizing the results of each test described above, heat treatment is applied to molded products such as gears and shafts using materials containing 0.5% or more of nickel to conventional alloy 0 steel, and the surface of the molded products is Removing only 60 μl of thickness increases the static bending strength and also improves the bending fatigue strength against repeated loads.

The present invention is applicable not only to gears and shafts, but also to other molded products that undergo surface hardening treatment, and induction hardening, electron beam hardening, laser hardening, etc. can also be used as the surface hardening treatment.

〔Effect of the invention〕

The present invention reduces the surface carbon content during carburizing by using a conventional alloy steel material containing nickel at a specified level or more, and also reduces the surface carbon content by removing the abnormal surface layer due to surface hardening treatment. It has the effect of improving static bending strength and, by extension, bending fatigue strength.

[Brief explanation of drawings]

Figure 1 is a diagram showing the results of the static bending test, Figure 2 is a plan view showing the shape of the test piece that was subjected to the static bending test, and Figure 3 is a carburized gear subjected to shot peening and J2. FIG. 3 is a diagram showing the relationship between the depth from the surface of the gear and the distribution of residual stress. A, B, C: Test pieces with different material compositions C5-C4: Test pieces with different surface treatments C○: Shots 1-
No peening, no electrolytic polishing ・With rainbow target peening, without electrolytic polishing △ Without rainbow target peening, with electrolytic polishing, with shot peening, with electrolytic polishing■
: 8 Average strength value of the test piece Cal as-treated■・Average strength value of each test piece as heat treated■・Strength range of the test piece subjected to electrolytic polishing■～■: Each test piece subjected to electrolytic polishing The average value of the intensity of

Claims (1)

[Claims] After carburizing a molded product using alloy steel with a nickel content of 0.5% or more, at least 20% of the surface of the molded product is carburized.
A method for strengthening a carburized steel molded product, characterized by removing a thickness of μm or more by electrolytic polishing or the like.