JP2007239880A - Power transmission shaft and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power transmission shaft capable of obtaining high compression residual stress upon quenching and improving fatigue strength, and its manufacturing method. <P>SOLUTION: The power transmission shaft includes a surface-hardened layer 4. Particles are mixed in a coolant for high frequency hardening, and this admixture is sprayed onto carbon steel containing 0.30 to 0.48 wt.% of carbon. The surface-hardened layer 4 is thereby produced. The residual stress of the surface-hardened layer 4 is set to -800 MPa or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a power transmission shaft used for power transmission in automobiles and various industrial machines, and more particularly to a power transmission shaft used for a constant velocity universal joint and a manufacturing method thereof.

  For example, in a power transmission shaft used for a drive shaft or a propeller shaft incorporated in a power transmission system of an automobile, carbon steel is usually used as a material, and the surface is hardened by heat treatment to ensure a predetermined strength. . Further, the power transmission shaft includes a solid integrated shaft and a joint shaft using welding or pressure welding.

  In recent years, it has been desired to further increase the strength of the power transmission shaft in response to an increase in the output of an automobile or an increase in vehicle weight due to safety. In addition, from the viewpoint of improving fuel efficiency, demands for reducing the weight of the power transmission shaft have become apparent, and in order to achieve this, it is urgent to increase the strength of the power transmission shaft. Therefore, various proposals have been made for the purpose of increasing the strength of the power transmission shaft (see, for example, Patent Documents 1 to 3).

  In the power transmission shaft disclosed in Patent Document 1, by adding B to the carbon steel material, the hardenability is improved, the grain boundary is strengthened, and the cracking susceptibility is reduced. Nb, V, Zr Is added to improve toughness by refining crystal grains.

  In the power transmission shaft disclosed in Patent Document 2, by adding Mo or B to the steel material, the base metal structure is bainite, and the hardenability is extremely improved. The fatigue strength is improved.

  As described above, in the power transmission shafts described in Patent Document 1 and Patent Document 2, the strength of the power transmission shaft is increased by adding a special alloy element that has an effect of improving hardenability and refinement of crystal grains. ing.

By the way, such heat treatment of the power transmission shaft is generally performed by tempering after induction hardening or carburizing and quenching. Therefore, in the power transmission shaft disclosed in Patent Document 3, after the quenching and tempering, a shot peening process is further added to increase the strength.
JP 2000-234141 A JP-A-2005-060718 JP 2003-307111 A

  However, adding a large amount of special alloy elements as in the prior art has a problem of increasing the cost of the product. In addition, the bainite of the material structure for increasing the strength of the power transmission shaft has a problem in that the workability is lowered due to the increase in material hardness and the cost of the product is increased. Furthermore, in the heat treatment, adding a shot peening treatment after performing a normal quenching and tempering step leads to an increase in cost.

  Further, as described in Patent Document 1, when B is added, it is necessary to add Ti together in order to sufficiently bring out the effect of addition of B. There was also a problem that the hard and large inclusions of TiN produced when Ti was added induced chipping in turning and the tool life was reduced. Further, as described in Patent Document 2, improving the hardenability by adding Mo or the like generally has a problem that the weldability is deteriorated. Further, when S increases, there is a problem that jointability (welding / friction pressure weldability) decreases.

  Therefore, the present invention has been proposed in view of the above-described problems, and the object of the present invention is to provide a power transmission capable of obtaining high compressive residual stress simultaneously with quenching and improving fatigue strength. It is to provide a shaft and a manufacturing method thereof.

  The power transmission shaft of the present invention has a hardened surface layer formed by induction hardening in which a mixture in which particles are mixed in a coolant is blown.

  In the power transmission shaft of the present invention, since particles are sprayed together with the coolant in the induction hardening, shot peening is performed during the quenching.

  Carbon steel having a carbon content of 0.30 to 0.48 wt% is used. If the amount of carbon is less than 0.30 wt%, sufficient hardness and depth cannot be obtained after quenching and tempering, and there is a problem of strength reduction. On the other hand, if the carbon content exceeds 0.48 wt%, weldability is remarkably impaired, and further, machinability is inhibited and fire cracking is likely to occur.

  The residual stress is set to -800 MPa or less. Thereby, an improvement in strength is obtained. If the residual stress exceeds -800 MPa, sufficient strength (for example, strength by a torsional fatigue test) cannot be obtained. Moreover, ceramic particles can be used as the particles to be sprayed.

  The method for manufacturing a power transmission shaft according to the present invention is a method for manufacturing a power transmission shaft having a surface hardened layer, in which carbon particles having a carbon content of 0.30 to 0.48 wt% are used as coolant in induction hardening. And the mixture is sprayed to produce the surface hardened layer.

  In the method for manufacturing a power transmission shaft of the present invention, particles are sprayed together with a coolant in induction hardening, so shot peening is performed simultaneously with quenching.

  The power transmission shaft of the present invention can obtain a peening effect simultaneously with cooling by mixing particles (high hardness particles) in the coolant of induction hardening. For this reason, a high compressive residual stress is obtained simultaneously with quenching, thereby improving the fatigue strength. In addition, it is not necessary to further perform a shot peening process after the tempering process, and the manufacturing time and cost can be reduced.

  Embodiments of a power transmission shaft and a manufacturing method thereof according to the present invention will be described in detail below.

  FIG. 1 illustrates a power transmission shaft as an embodiment, for example, when used for a drive shaft of an automobile. The drive shaft has a sliding type constant velocity universal joint (not shown) connected to one shaft end 2 of the intermediate shaft 1 serving as a power transmission shaft by spline fitting and the like, and is fixed to the other shaft end 3. A constant velocity universal joint (not shown) is connected by spline fitting or the like.

  Spline portions 5 and 6 are formed at the end portions 2 and 3 of the shaft 1, respectively. The intermediate shaft 1 may be either a solid shaft machined from a solid bar, a hollow shaft machined from a steel pipe, or a joint shaft using welding or friction welding.

  The intermediate shaft 1 uses carbon steel having a carbon content of 0.3 to 0.48 wt%. That is, a material made of carbon steel containing C: 0.3 to 0.48 wt% is heated at high frequency, and the hardened layer 4 is formed on the surface by the high frequency heating. In this high-frequency heat treatment, the quenching temperature is set to 1000 ° C., for example.

  The reason why the components of the material are defined in the above-mentioned range is as follows. That is, if the amount of C is less than 0.3 wt%, sufficient hardness and hardening depth cannot be obtained after quenching and tempering, and there is a problem of strength reduction. On the other hand, if the amount of C exceeds 0.48 wt%, there is a problem that weldability is remarkably impaired, and further, machinability is inhibited and burn cracking is likely to occur.

  By the way, induction hardening is a heat treatment method in which a high-frequency coil wound in a cylindrical shape is energized to rapidly heat the steel surface temperature to the transformation point or higher and rapidly cool before the internal temperature rises. For this reason, it cools by spraying a coolant with an injection device. In this case, in the present invention, particles (a shot material used for shot peening, which is a hard small sphere) are mixed in this coolant, and this mixture is sprayed. In addition, water, oil, etc. can be used as a coolant.

  That is, a peening effect can be obtained simultaneously with cooling by mixing high-hardness particles in the induction hardening coolant. For this reason, a high compressive residual stress is obtained simultaneously with quenching, thereby improving the fatigue strength. At this time, the compressive residual stress is set to −800 MPa or less. This compressive residual stress can be changed by the spraying pressure of the particles, the particle diameter, and the like. When the compressive residual stress exceeds -800 MPa, sufficient strength (for example, strength by a torsional fatigue test) cannot be obtained.

  Further, various materials such as metals and ceramics can be adopted as the material of the particles. The hardness of the particles is preferably Rockwell hardness (HRC) of 58 or more. If the HRC is less than 58, the hardness of the workpiece is low, so that a sufficient peening effect cannot be obtained. Furthermore, the ratio of the effective hardening depth t to the radius r (t / r) is 0.40 or more. When the ratio of the effective hardening depth to the radius is less than 0.40, there is a problem that the breakage due to the internal origin occurs and the strength is remarkably lowered.

  According to the present invention, a high compressive residual stress can be obtained simultaneously with quenching, whereby the fatigue strength can be improved. In addition, it is not necessary to further perform a shot peening process after the tempering process, and the manufacturing time and cost can be reduced.

  Since carbon steel having a carbon amount of 0.30 to 0.48 wt% is used, the hardened layer 4 can obtain sufficient hardness and depth, and can exhibit stable strength. Moreover, it is excellent in weldability, and also can improve machinability and prevent cracking.

  By setting the residual stress to −800 MPa or less, strength improvement (particularly strength against torsional fatigue test) can be obtained. Various particles such as metals and ceramics can be used for the particles mixed in the coolant, and the cost can be reduced.

  As shown in Table 1, a torsion test is performed on each invention product and the conventional product by forming an invention product that is the intermediate shaft 1 having the shape shown in FIG. 1 and a conventional product having the same shape as the intermediate shaft 1. Went. SAE1041 was used for each shaft material of the invention and the conventional product. Each material was subjected to induction quenching at a quenching temperature of 1000 ° C., and the conventional product was tempered at 180 ° C. for 1 hour by furnace tempering. Moreover, about the invention goods, particle | grains were mixed in the coolant and shot peening was performed in induction hardening.

図１に示すように、発明品及び従来品ではスプライン部の硬化比（ｔ／ｒ）を０．４とし、平滑部の硬化比（ｔ／ｒ）を０．６としている。ここで、「平滑部」とは、中間シャフト１の中央部分である。また、残留応力は、発明品では−８００ＭＰａとなり、従来品では−５００ＭＰａとなった。

As shown in FIG. 1, the invention product and the conventional product have a spline portion curing ratio (t / r) of 0.4 and a smooth portion curing ratio (t / r) of 0.6. Here, the “smoothing part” is a central part of the intermediate shaft 1. Further, the residual stress was -800 MPa for the inventive product and -500 MPa for the conventional product.

  The results of conducting a torsional fatigue strength test on these products of the present invention and conventional products are shown in Table 2 below.

  As shown in Table 2, when the conventional product was used as a reference, the product of the present invention had a 10-fold increase in fracture life. Note that τ is a stress calculated using the reference diameter of the test piece.

It is a front view which shows embodiment of the power transmission shaft of this invention.

Explanation of symbols

1 Power transmission shaft 4 Surface hardened layer

Claims (5)

  A power transmission shaft comprising a hardened surface layer formed by induction hardening in which a mixture in which particles are mixed in a coolant is sprayed.   The power transmission shaft according to claim 1, wherein the power transmission shaft is carbon steel having a carbon content of 0.30 to 0.48 wt%.   The power transmission shaft according to claim 1, wherein the particles are ceramic particles.   The power transmission shaft according to any one of claims 1 to 3, wherein a residual stress of the surface hardened layer is set to -800 MPa or less.   In the method of manufacturing a power transmission shaft having a hardened surface layer, carbon steel having a carbon content of 0.30 to 0.48 wt% is mixed with particles in an induction hardening coolant and sprayed with this mixture. A method of manufacturing a power transmission shaft, wherein the surface hardened layer is generated.

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