JP2007024250A - Pinion shaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pinion shaft having excellent rolling fatigue resistance. <P>SOLUTION: The pinion shaft 5 is preferably used in a planetary gear mechanism of an automatic transmission for an automobile or the like. The pinion shaft 5 includes on its outer peripheral surface a hardened layer 5a containing residual austenite of 20 to 50 vol.% and containing no pearlite nor bainite. The hardened layer 5a is formed by such induction hardening process as to cool to a temperature higher than the Mf point after high-frequency heating so as to maintain a temperature for a predetermined period of time prior to cooling substantially to a room temperature. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a pinion shaft used for a pinion gear of a planetary gear device such as an automatic transmission for automobiles.

  When the roller rolls on the outer peripheral surface of the pinion shaft, a high contact stress of several GPa is generated. For this reason, the material used for the pinion shaft is required to be hard and able to withstand a load, to have excellent rolling fatigue life, and to have good wear resistance against sliding. In order to satisfy such a requirement, conventionally, for example, case-hardened steel such as SCr420 or induction-hardened material such as SK5 is used.

Further, since the pinion shaft is repeatedly subjected to shear stress under high surface pressure, it is required to withstand the shear stress and to have excellent rolling fatigue life. In order to satisfy these requirements, conventionally, for example, SUJ2 and SUJ3 are subjected to heat treatment such as carburizing and carbonitriding, followed by high-frequency heat treatment to ensure surface hardness and improve rolling fatigue life. ing.
However, since the usage environment of rolling bearings has become increasingly severe in recent years, pinion shafts using conventional materials, such as rolling bearings used in automotive transmissions, can be used under lubricated conditions and high temperatures. Satisfactory durability could not be obtained when used below.

Therefore, as countermeasures against the above problems, Patent Document 1 discloses that high carbon chromium bearing steel is subjected to nitriding treatment or carbonitriding treatment to apply residual compressive stress to the surface layer portion, and simultaneously form nitride. An example of improving the heat resistance and rolling fatigue life with high hardness is disclosed. Further, Patent Document 2 discloses a rolling bearing that is made of high silicon and high manganese steel equivalent to SUJ3 and further suppresses the decomposition of austenite by increasing the amount of retained austenite.
U.S. Pat. No. 3,216,869 Japanese Patent Laid-Open No. 7-190072 Japanese Patent Laid-Open No. 2002-4003

However, when performing high frequency heat treatment on the rolling surface of the pinion shaft, it is difficult to form a stable amount of retained austenite on the rolling surface if the shape, such as the diameter and length of the pinion shaft, is different. There was a problem in terms of quality assurance.
On the other hand, when quenching the pinion shaft, cooling is performed by injecting cooling water onto the outer peripheral surface of the pinion shaft that is heated at high frequency, so that the surface side of the pinion shaft is cooled first and then cooled toward the core. Will go. Therefore, heat treatment transformation stress is generated in the surface layer portion of the pinion shaft in the temperature range where martensite transformation starts (200 to 300 ° C. in the case of SUJ2), and heat due to the difference in thermal expansion from the temperature difference between the surface layer portion and the core portion. Internal stress will occur.

In general, pinion shafts used in planetary gear units are provided with oil holes for supplying lubricating oil to the axially central portion and the axial end surface of the outer peripheral surface. When cooling water is sprayed onto a stress concentrated part, a thermal internal stress is instantaneously induced, which may reduce the strength of the material.
Accordingly, an object of the present invention is to solve the above-described problems of the prior art and provide a pinion shaft having an excellent rolling fatigue life.

  In order to solve the above problems, the present invention has the following configuration. That is, the pinion shaft of claim 1 according to the present invention includes a sun gear located at the center, a ring gear concentrically arranged with the sun gear, a pinion gear that meshes with the sun gear and the ring gear and revolves around the sun gear, A pinion shaft that is inserted into a center hole of the pinion gear and rotatably supports the pinion gear via a needle bearing is provided with a hardened layer that satisfies the following three conditions on the outer peripheral surface thereof: It is characterized by that.

Condition 1: It is formed by induction hardening treatment in which after high-frequency heating, it is cooled to a temperature higher than the Mf point, held at that temperature for a predetermined time, and cooled to room temperature.
Condition 2: Perlite and bainite are not present.
Condition 3: The amount of retained austenite is 20% by volume or more and 50% by volume or less.
Induction hardening is a process of cooling a metal member that has been induction-heated. In this cooling step, if the temperature is maintained at a temperature higher than the temperature at which the martensitic transformation ends (Mf point), the temperature difference between the surface layer portion and the core portion. Therefore, thermal deformation that occurs when cooling without holding the temperature is suppressed. In addition, when the cooling water is injected and cooled, thermal internal stress is induced in the stress concentration portion of the pinion shaft (for example, the edge portion at the tip of the oil hole) and the strength of the material is also prevented from being lowered.

  Furthermore, since the austenite stabilizes during the temperature holding and the amount of retained austenite after induction hardening increases, the rolling fatigue life is reduced by suppressing the formation of indentations, reducing the stress concentration at the indentations, improving the cracking resistance, etc. improves. In order to obtain an excellent rolling fatigue life, the amount of retained austenite needs to be 20% by volume or more, but if it exceeds 50% by volume, the hardness may decrease.

  In addition, if pearlite and bainite are present, hardness and toughness may decrease and rolling fatigue life may be significantly deteriorated, so an appropriate cooling rate is set so that pearlite transformation and bainite transformation do not occur during induction hardening. There is a need. In addition, the steel structure is composed of martensite and austenite in the previous stage of the temperature holding, but if the temperature holding time is too long, the steel structure changes to a mixture of martensite, austenite and lower bainite. The temperature holding time is preferably 1 hour or less (about several tens of minutes).

  The temperature must be maintained at a temperature higher than the Mf point and at a temperature at which pearlite or bainite does not occur, and may be maintained at a temperature higher than the temperature at which martensite starts to be generated (Ms point). However, it may be held at a temperature below the Ms point and higher than the Mf point. As an example, 43 degreeC or more and 65 degrees C or less are mention | raise | lifted. Further, after the temperature holding is completed, the temperature may be cooled to about room temperature. As an example, 10 degreeC or more and 30 degrees C or less are mention | raise | lifted. Since the Mf point may be lower than room temperature, it is not always necessary to cool below the Mf point.

A pinion shaft according to a second aspect of the present invention is characterized in that in the pinion shaft according to the first aspect, a carbonitriding process is performed before the induction hardening process.
The pinion shaft that has been carbonitrided prior to induction hardening has austenite remaining in the surface layer portion due to the enrichment of nitrogen, and therefore has excellent rolling fatigue life due to this retained austenite. Moreover, since fine carbonitride is dispersed in the surface layer portion by adding nitrogen, wear resistance and toughness are excellent.

Furthermore, the pinion shaft according to claim 3 of the present invention is the pinion shaft according to claim 2, wherein a high-temperature tempering treatment is performed between the carbonitriding treatment and the induction hardening treatment, and after the induction hardening treatment. A low-temperature tempering treatment is performed.
When a high temperature tempering treatment is performed after the carbonitriding treatment, the retained austenite inherent in the steel disappears. And when an induction hardening process is performed after that, only a surface layer part will harden | cure and a hardened layer will be formed. Since the retained austenite remains in the hardened layer at 20 volume% or more and 50 volume% or less, the rolling fatigue life is excellent even under the contamination with foreign matter. The temperature of the high temperature tempering treatment is preferably 300 ° C. or more and 700 ° C. or less, and the treatment time is preferably about several hours.
The low-temperature tempering process is a general tempering process, and has effects of removing strain and improving toughness.

Furthermore, the pinion shaft of Claim 4 which concerns on this invention is a pinion shaft as described in any one of Claims 1-3. 0.8 mass% or more and 1.2 mass% or less of carbon, 0.1 mass % To 1.2% by mass of silicon, 0.7% to 1.8% by mass of chromium, and 0.1% to 1.5% by mass of manganese. Features.
In order to make the hardened layer sufficiently hard, the carbon content is preferably 0.8% by mass or more and 1.2% by mass or less. In addition, it is preferable that the hardness (surface hardness) of a hardened layer is Hv750 or more.

Further, in order to stabilize the amount of retained austenite in the hardened layer to 20% by volume or more and to provide temper softening resistance and to ensure heat resistance, the silicon content is preferably 0.1% by mass or more. However, if it exceeds 1.2% by mass, enrichment of nitrogen and carbon in the surface layer portion may be hindered in the carbonitriding process.
Furthermore, if the chromium content is less than 0.7% by mass, the amount of carbide formed may be reduced and the hardness of the cured layer may be insufficient. If it exceeds 1.8% by mass, the carbide is coarse. It becomes a starting point of peeling and tends to have a short life.

Further, manganese is an element that ensures hardenability, but in the present invention, it has an action of stabilizing retained austenite in the quenching step and the tempering step, so that the amount of retained austenite in the hardened layer is increased. However, if added in a large amount, it causes a decrease in cold workability, burn cracking, and embrittlement, so the upper limit of the content is preferably 1.5% by mass.
Examples of such alloy steel include SUJ1, SUJ2, SUJ3, SUJ4, and SUJ5. In addition, in order to improve hardenability, SUJ4 and SUJ5 are added with 0.3% by mass or less of molybdenum.

  The pinion shaft of the present invention has an excellent rolling fatigue life.

  An embodiment of a pinion shaft according to the present invention will be described in detail with reference to the drawings. The planetary gear device shown in FIG. 1 is suitably used for a planetary gear mechanism such as an automatic transmission for automobiles, and includes a sun gear 1 through which a shaft (not shown) is inserted, and a ring gear 2 arranged concentrically with the sun gear 1. And one or more (three in FIG. 1) pinion gears 3 that mesh with the sun gear 1 and the ring gear 2 and revolve around the sun gear 1, and are arranged concentrically with the sun gear 1 and the ring gear 2, and rotatably support the pinion gear 3. And a carrier 4.

A pinion shaft 5 fixed to the carrier 4 by caulking or the like is inserted into the center hole of the pinion gear 3, and a plurality of unillustrated pins are provided between the outer peripheral surface of the pinion shaft 5 and the inner peripheral surface of the pinion gear 3. Needle rollers are arranged, and the pinion gear 3 is thereby rotatable about the pinion shaft 5.
This pinion shaft 5 is made of 0.8 mass% or more and 1.2 mass% or less carbon, 0.1 mass% or more and 1.2 mass% or less silicon, 0.7 mass% or more and 1.8 mass% or less chromium. , 0.1 to 1.5% by mass of manganese, the balance being iron and inevitable impurities. Heat treatment is performed in the order of carbonitriding, high temperature tempering, induction hardening, and low temperature tempering.

  In this induction hardening process, after heating to 800-900 degreeC by high frequency induction heating and making the surface layer part of an outer peripheral surface into an austenite structure, the surface layer part is cooled and hardened with a hardening liquid. First, in the first stage cooling, the temperature of the quenching liquid is set to a temperature higher than the Mf point, and the temperature is cooled to the temperature. And it hold | maintains for this predetermined time (temperature holding process) at this temperature. Next, in the second stage cooling, the temperature of the quenching liquid is set to room temperature and cooled to the Mf point or lower. When the Mf point is below room temperature, it is not necessary to cool below the Mf point. The MJ point of SUJ is generally below room temperature.

As a result of such heat treatment, the hardened layer 5a and the core portion of the surface layer portion are formed on the pinion shaft 5 (see FIG. 2), and the amount of retained austenite of the hardened layer 5a is 20% by volume or more and 50% by volume. Hereinafter, the surface hardness is Hv750 or more. The hardened layer 5a is formed only on the outer peripheral surface of the pinion shaft 5 that serves as the rolling surface of the needle rollers. Moreover, since the cooling rate in the first stage cooling is set to such a rate that pearlite transformation and bainite transformation do not occur, the hardened layer 5a does not have pearlite and bainite.
Since such a pinion shaft 5 includes the hardened layer 5a as described above, it has an excellent rolling fatigue life. Moreover, if it is an induction hardening process as mentioned above, an internal crack and the fall of material strength will be suppressed.

〔Example〕
The present invention will be described more specifically with reference to the following examples. The pinion shafts of Examples and Comparative Examples were manufactured in the same manner as in the case of the above-described pinion shaft 5 except that the types of steel and the contents of the heat treatment were variously changed as shown in Table 1. The contents of the heat treatment are the same as in the case of the pinion shaft 5 in Examples 1 to 3 and Comparative Example 3, and the difference in Example 4 is that there is no carbonitriding treatment. Further, Comparative Examples 1 and 2 are different in that there is no temperature holding process for induction hardening, and Comparative Example 4 is different in that there is no temperature holding process for carbonitriding and induction hardening.

Table 1 shows the surface hardness of the outer peripheral surface of each pinion shaft obtained, the amount of retained austenite of the hardened layer, and the hardness of the core.
These pinion shafts have an outer diameter of 16.7 mm and an inner diameter of 3.5 mm, and have oil holes as shown in FIG. Further, although SUJ2 and SUJ3 are used as the material of the pinion shaft, SUJ1, SUJ4 and SUJ5 may be used.

Each pinion shaft was mounted on a planetary needle testing machine manufactured by Nippon Seiko Co., Ltd., and a rotation test was performed to evaluate the rolling fatigue life and the presence or absence of internal cracks of the full roller bearing provided with the pinion shaft. The test conditions are as follows.
・ Basic dynamic load rating C: 20500N
・ Basic static load rating C ₀ : 21000N
・ Radial load: 10000N
-Spinning speed of pinion gear: 8000rpm
- calculated life L _10: 46 hours, lubricants Type: Automatic Transmission Fluid lubricant temperature: 90 ° C.

  The test results are shown in Table 1. The numerical value of the rolling fatigue life is shown as a relative value when the rolling fatigue life of Comparative Example 1 is 1. In addition, for the presence or absence of internal cracks, the pinion shaft is broken along a plane along the axial direction, and the periphery of the oil hole, particularly the part where stress concentration tends to occur, such as the edge part at the tip of the oil hole, is observed using an optical microscope. It was evaluated by.

As can be seen from Table 1, Examples 1-4 were superior in rolling fatigue life compared to Comparative Examples 1-4. The reason for this is that the amount of retained austenite is large. From the comparison between Example 2 and Comparative Example 2, it can be seen that the amount of retained austenite is increased by the temperature holding step. This is presumably because austenite was stabilized in the temperature holding step, and untransformed austenite remained. Moreover, it turns out from the comparison with Example 4 and the comparative example 4 that even if there is no carbonitriding process, the same result is obtained.
Further, Comparative Examples 2 and 4 were inferior in rolling fatigue life because of internal cracks. Further, in Comparative Example 3, since the retained austenite amount exceeds 50% by volume, the surface hardness and the yield stress are lowered, and the rolling fatigue life is insufficient.

It is a disassembled perspective view of the planetary gear apparatus provided with the pinion shaft which is one Embodiment of this invention. It is sectional drawing of a pinion shaft.

Explanation of symbols

1 Sun Gear 2 Ring Gear 3 Pinion Gear 4 Carrier 5 Pinion Shaft 5a Hardened Layer

Claims (4)

A sun gear located in the center, a ring gear concentrically arranged with the sun gear, and a pinion gear that meshes with the sun gear and the ring gear and revolves around the sun gear. A pinion shaft that is inserted through a needle bearing and rotatably supports the pinion gear via a needle bearing, and has a hardened layer that satisfies the following three conditions on its outer peripheral surface:
Condition 1: It is formed by induction hardening treatment in which after high-frequency heating, it is cooled to a temperature higher than the Mf point, held at that temperature for a predetermined time, and cooled to room temperature.
Condition 2: Perlite and bainite are not present.
Condition 3: The amount of retained austenite is 20% by volume or more and 50% by volume or less.   The pinion shaft according to claim 1, wherein a carbonitriding process is performed before the induction hardening process.   The pinion shaft according to claim 2, wherein a high-temperature tempering process is performed between the carbonitriding process and the induction-quenching process, and a low-temperature tempering process is performed after the induction-quenching process.   0.8% by mass to 1.2% by mass of carbon, 0.1% by mass to 1.2% by mass of silicon, 0.7% by mass to 1.8% by mass of chromium, 0.1% by mass The pinion shaft according to any one of claims 1 to 3, wherein the pinion shaft is made of steel containing 1.5% by mass or less of manganese.

Images