JPS62185826A - Production of high-strength gear - Google Patents

Abstract

PURPOSE:To produce a gear having improved strength and toughness by forming residual austenite on the surface layer by carburization hardening in which carbon potential is specified then transforming only the extreme surface part of the residual austenite to martensite by strong working. CONSTITUTION:The carbon potential is controlled to a 1.0-1.2% range to form a large amt. of the residual austenite in the surface layer (about 0.4-0.5mm depth) of the gear when the gear is held at the hardening temp. in the carburization hardening treatment in the stage of subjecting the gear to carburization hardening. The gear is then subjected to physically strong working such as heavy-duty shot peening or heavy cutting (strong cutting) to transform only the residual austenite in the extreme surface layer part (about 0.1-0.2mm depth) to the strain-induced transformation type martensite. A large amt. of the austenite layer is thereby made to remain in the lower part (about 0.3-0.5mm depth) of the martensite layer so that said layer functions effectively to prohibit the progression of fatigue crack.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to gears used in drive transmission systems such as vehicle transmissions, and more particularly to a method for manufacturing gears that effectively utilizes retained austenite.

(Prior art) In order to improve the pitting resistance life of gears used in drive systems, etc., as disclosed in Japanese Patent Publication No. 17699/1983, an appropriate amount of residual austenite is added to the hardened surface layer of the gear. It is effective to generate them. Conventionally, in order to control the amount of retained austenite and generate a large amount, a treatment method by carburizing and nitriding has been widely used.

(Problems to be Solved by the Invention) However, even if a large amount of retained austenite is produced using the conventional technology, the tooth bending fatigue strength of gears cannot be improved, but rather decreases. The reason for this is that if the residual austenite in the outermost layer of the gear is simply increased, the compressive residual stress in the outermost layer will decrease as the retained austenite increases.

In addition, retained austenite produced using conventional technology has an unstable structure, and before cracks occur due to gear use,
Due to repeated stress, a considerable amount becomes deformation-induced transformed martensite, and as a result, it does not work effectively in inhibiting crack propagation.

The present invention has been made in view of the above points, and aims to improve the strength and toughness of gears by effectively utilizing the outermost surface portion of retained austenite.

(Means for Solving the Problems) According to the present invention, this object is achieved by carburizing and quenching at a carbon potential of 1.0 to 1.2%, thereby creating a large amount of residual austenite in the surface layer. This is achieved by a method of manufacturing a gear in which only the outermost surface of the retained austenite layer is made into a martensitic structure by generating the residual austenite layer and then performing all the hard working.

The present invention applies carburizing and quenching to gears after gear cutting. In this carburizing and quenching process, when maintaining the quenching temperature and controlling the carbon potential within the range of 1.00 to 1.20%, the surface layer of the gear (depth L
4 to α5), a large amount of retained austenite is generated.

Thereafter, by subjecting the gear to strong physical processing such as strong shot peening and heavy cutting, only the retained austenite in the outermost layer (depth 0.1 to α2 rrrm) is transformed into a process-induced transformation type. Use martensite. For example, in the case of heavy-duty shot peening, it can be carried out under the conditions of arc height [L8A and coverage of 300 mm, and in the case of heavy cutting, continuous cutting should be performed with a depth of cut of α1 to α4a on the tooth surface. Just go.

When processed in this way, a layered region containing a large amount of retained austenite remains in the lower part (depth 13 to αS=) of the process-induced transformed martensite layer. This retained austenite layer is stable and effectively functions to inhibit the growth of fatigue cracks.

Therefore, the products treated according to the present invention have significantly improved fatigue strength and toughness compared to the products treated according to the prior art.

(Function) In other words, according to the present invention, when the surface layer of a gear containing a large amount of retained austenite is subjected to strong processing such as strong shot peening, the outermost layer of the gear contains the residual austenite introduced by the strong processing. Compressive residual stress is generated, and at the same time, compressive residual stress generated when residual austenite in the outermost layer undergoes deformation-induced transformation is further added. For this reason, the compressive residual stress in the outermost layer of the product is significantly higher than that of conventional products, and the crack initiation strength is improved.

However, many retained austenite layers that exist below the strain-induced transformation martensite layer are high carbon parts and are extremely stable. Therefore, when a crack develops into a region of this retained austenite layer, the retained austenite transforms into martensite due to stress concentration at the tip of the crack. Therefore, the crack propagation energy increases dramatically compared to the conventional one, which contributes to improving fatigue strength.

(Example) Hereinafter, the present invention will be described in detail based on the accompanying drawings.

The following four types of surface hardening treatments A, B, C, and D were applied to the transmission spur gear 1 shown in FIG. 4.

Here, excessive carburization means that the carbon potential is (18
Carburization was carried out at 930°C to 950°C for 3 hours, and then the carbon potential was 1.0 to 1.2%.
Carburized and quenched at 360°C for 2 to 3 hours, tempered with 130°C oil, tempered at 170°C for 1 hour, and air cooled. In addition, normal carburizing is carburizing diffusion at 930°C for 3 hours with a carbon potential of a8ts, then carburizing and quenching at 860°C for 40 minutes with a carbon potential of cL8%, martempering with oil at 130°C, and then It was tempered at ℃ and air cooled. In addition, heavy working refers to surface hardening processes such as strong carbon beaning or heavy cutting that are carried out to transform the outermost surface layer of a large amount of retained austenite generated by excessive carburization into deformation-induced transformation martensite. It is.

FIG. 1 is a process diagram of heat treatment and strong working according to the present invention.

Residual stress distribution was measured for the spur gears subjected to the above treatments A to D. The results are shown in FIG.

As is clear from this figure, unlike treatments A, B, and C, the spur gear treated according to the method of the present invention exhibits strong compressive residual stress near the surface and weak tensile residual stress in the lower layer. It showed a characteristic residual stress distribution.

Next, a fatigue test was conducted on the spur gears subjected to the treatments A to D for one tooth curve. The results are shown in FIG. As is clear from FIG. 3, those treated with the present invention had a higher
It has high time strength and fatigue limit, and fatigue strength has been significantly improved.

This is because, as shown in Fig. 5, deformation-induced transformation type martensite 3 is generated on the outermost surface N (depth IIL1 to α2rMn) of product 2, and a large amount of martensite is generated below it (depth 13 to (15 dark)). Retained austenite 4 exists 17, and as a result, the compressive residual stress in the outermost layer decreases in treatment A.

This is understood to be due to a marked improvement compared to B and C. Note that the reference numeral 5 in FIG. 5 is a hardened martensite layer.

As shown in FIG. 6, considering the case where a crack 7 occurs in the root portion 6 of the spur gear 1 of the present invention, the propagation of the crack 7 is effectively prevented in the spur gear 1 of the present invention. This is because the retained austenite layer 4 existing under the strain-induced transformation martensite layer 3 is high in carbon and stable, and as shown in FIG. 7, the crack 7 has developed into the area of the retained austenite layer 4. In this case, stress concentration at the tip of the crack 7 causes deformation-induced transformation of the retained austenite 4 into martensite 8, which is extremely effective in inhibiting crack propagation.

Therefore, in the treatment of the present invention, coupled with the high compressive residual stress in the outermost layer of the product, the crack propagation energy is much greater than that in treatments A, B, and C.
It is thought that this has a positive effect on fatigue strength.

(Effects of the Invention) As described above, according to the method of the present invention, by subjecting only the outermost layer of a large amount of retained austenite to deformation-induced transformation into martensite, the crack propagation energy increases and the compression residual in the outermost layer is reduced. Since the stress is absorbed, the fatigue strength and toughness against applied stress are significantly improved compared to those produced by conventional methods.

[Brief explanation of drawings]

Figure 1 is a process diagram showing heat treatment and heavy working according to the method of the present invention, Figure 2 is a graph showing the residual stress distribution of gears according to the treatment methods of the present invention and comparative examples, and Figure 3 is single tooth bending fatigue of the same gear. A graph showing the test results, Figure 4 is a cross-sectional view of the gear,
Fig. 5 is a cross-sectional view showing the surface structure of the product according to the present invention, Fig. 6 is a partial front view showing the state of cracks in the dedendum of the same gear, and Fig. 7 is the surface of the cracked part of the gear shown in Fig. 6. FIG. 3 is an enlarged cross-sectional view of the structure. 1... Spur gear 3... Deformation-induced transformation martensite layer 4... Retained austenite layer 7... Crack patent applicant Toyota Motor Corporation representative Patent attorney Yumi Sakai (and 1 other person)
-ノ'Ni27 Figure 4 Figure 6 Figure 5 Tube 7 Figure

Claims (1)

[Claims] A large amount of retained austenite is generated in the surface layer by carburizing and quenching at a carbon potential of 1.0 to 1.2%, and then strong processing such as strong shot peening or heavy cutting is performed to remove the residual austenite layer. Only the outermost part is transformed into martensite,
A method for manufacturing a high-strength gear, characterized by leaving a large amount of austenite under the martensite layer.