JPH07136863A - Manufacture of gear made of austemper ductile cast iron - Google Patents

Abstract

PURPOSE:To improve toughness and tensile strength by carrying out austenite formation of a gear rough member and upper benite formation and then carrying out quenching. CONSTITUTION:After a gear rough member 12 is formed by carrying out the gear cutting work such as hobbing for a gear member 12 made of the spheroidal graphite casted iron in the first process 14, the gear rough member is subjected to the ADI processing (austemper processing) in the second process 16, and the shaving finishing work is applied for the gear surface in the final third process 18. Since the gear member 12 before the ADI processing possesses the excellent ground performance, the gear cutting work in the first process can be carried out easily. In the second process 16 for applying the ADI processing for the gear rough member after the gear cutting work, the second prescribed temperature for the benite formation is set slightly higher than the ordinary temperature, and the temperature difference for the first prescribed temperature for austenite formation is reduced. Accordingly, the upper benite structure for securing the excellent toughness and the high tensile strength can be formed, and the thermal processing deformation quantity can be reduced sufficiently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an austempered ductile cast iron gear.

[0002]

2. Description of the Related Art Austempered ductile cast iron gears used for vehicle engine timing gears, etc.
It was manufactured by one of the manufacturing methods shown in FIGS. 4 and 5.

In the conventional manufacturing method shown in FIG. 4, the appropriate spheroidal graphite cast iron (for example, JIS G 5502) is used.
Step 1 of manufacturing a gear material by FCD500) specified in 1), and step 2 of performing gear cutting on the same material by bob cutting or other suitable gear cutting method and then shaving the tooth surface to finish. It comprises a third step of subjecting the machined gear to a heat treatment (hereinafter referred to as ADI treatment).

The ADI process is a so-called austempering process, in which the gear material is heated to an appropriate temperature (hereinafter referred to as a first predetermined temperature) above the Ac3 transformation point or the Ac1 transformation point for an appropriate time (hereinafter referred to as the first prescribed temperature). By holding it, the base structure of the material is changed to an austenite structure, and then the transformation is prevented as it is, and the ferrite and pearlite formation temperature Ar 'or less and the martensite formation temperature A
By immersing in a metal bath or a salt bath at an appropriate temperature (hereinafter referred to as a second predetermined temperature) or higher (r second point) for a suitable time (hereinafter referred to as a second predetermined time) to cool the ground structure of the material. This is a treatment method of forming a bainite structure and then cooling in air.

In the ADI process, when the second predetermined temperature is close to Ar ', a soft upper bainite structure is formed, and when it is close to Ar ", a hard lower bainite structure is formed.

In the conventional method for manufacturing a granular graphite cast iron gear shown in FIG. 4, gears that have been precisely finished by shaving after gear cutting in step 2 are heat treated (ADI treatment). However, there is a drawback that the deformation due to heat treatment is large and the accuracy is deteriorated.

Next, in the conventional manufacturing method shown in FIG. 5, a step 1'of casting a gear material with the same granular graphite cast iron as described above is used.
And a step 2 ′ of subjecting the cast gear material to the same ADI treatment as above, and a machining step 3 of performing gear cutting such as bob cutting on the gear material after heat treatment and then shaving to perform precision finishing of the tooth surface 3 'Is composed of.

In this manufacturing method, since the machining allowance when hobbing and cutting the gear material after the ADI treatment is extremely large, work hardening is caused and the machinability is greatly deteriorated. In the worst case, it becomes impossible to cut, and there is a problem that it is difficult to perform cutting within a practically acceptable cost range.

[0009]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a manufacturing method capable of manufacturing a gear made of austempered ductile cast iron having excellent strength and toughness at low cost while ensuring high accuracy. It is what

[0010]

The present invention was devised in order to achieve the above-mentioned object, and a first step of gear cutting a raw material made of spheroidal graphite cast iron to form a rough gear material, The gear rough material is first austenitized at the first predetermined temperature.
Hold for a certain period of time and then use the second for upper bainite
It comprises a second step of holding at a predetermined temperature for a second predetermined time and then gradually cooling, and a third step of shaving the tooth surface of the gear rough material subjected to the heat treatment of the second step to form a gear. The present invention proposes a method for manufacturing an austempered ductile cast iron gear.

[0011]

According to the present invention, in the first step, gear raw material made of spheroidal graphite cast iron is subjected to gear cutting such as hobbing to form a gear rough material, and then in the second step, AD is applied to the gear rough material.
I treatment is applied, and in the final third step, the tooth surface is shaving finished. Since the gear material before ADI processing has good machinability, the gear cutting process in the first step is easily performed. In the second step of subjecting the gear rough material after gear cutting to the ADI treatment, the second predetermined temperature for the bainitization is set slightly higher than usual, and the temperature difference from the first predetermined temperature for the austenitization is made small. By doing so, an upper bainite structure capable of ensuring excellent toughness and high tensile strength is generated, and the heat treatment deformation amount can be made sufficiently small. In the third step, the gear tooth flanks are shaving finished. At this time, the shaving finishing allowance is the sum of the finishing allowance for the gear cutting in the first step and the heat treatment deformation amount in the second step. Moreover, since the amount of heat treatment deformation is suppressed to be small, the shaving process is smoothly performed and a highly accurate gear can be obtained.

[0012]

Embodiments of the present invention will be specifically described below with reference to FIGS. First, in the process diagram of FIG. 1, reference numeral 10 generally indicates a manufacturing process of the method of the present invention, 12 is a cast gear material, 14 is a first step of performing gear cutting such as hobbing on the gear material 12, Reference numeral 16 denotes a second step of subjecting the gear rough material having undergone gear cutting to ADI treatment, 18 denotes a third step of subjecting the tooth surface of the gear which has undergone ADI treatment to shaving, and 20 denotes shaving as desired. The 4th process of performing shot peening on the tooth surface of the finished gear is shown.

The gear material 12 is an appropriate granular graphite cast iron, preferably F specified in JIS G 5502.
Manufactured by CD700. The gear material 12 in an as-cast state usually has a microstructure composed of granular graphite, ferrite mainly deposited around graphite, and pearlite, and has a tensile strength σ _{B of} about 60 to 70 kg / mm ² ,
The hardness H _B is about 250 and the machinability is excellent.

Next, the gear material 12 is processed in the first step 14
In the above, gear cutting such as hobbing is carried out, but since gear cutting is carried out before the gear material is heat-treated in the as-cast state, the machining is carried out smoothly and easily, and a rough gear material is formed.

As an example, the gear rough material that has undergone gear cutting is subjected to the AD of the second step according to the temperature-treatment time diagram of FIG.
I processing is performed. That is, the gear coarse material, first A ₁
The temperature is heated to a first predetermined temperature T ₁ equal to or higher than the point (which is appropriately set within a temperature range of 830 ° C. to 900 ° C.), and the temperature T
₁ for a first predetermined time h ₁ (for example, 1 hour). In the illustrated case, the gear rough material is gradually heated from room temperature to the first predetermined temperature T ₁ in 0.5 hour. By holding at the first predetermined temperature T ₁ for the first predetermined time, the ground structure made of the coarse gear material composed of ferrite and pearlite becomes an austenite structure.

The coarse gear material having the austenite structure as the ground structure as described above prevents the transformation, and as it is, the second temperature of the ferrite and pearlite generation temperature Ar 'or lower and the marsite formation temperature Ar "(MS) or higher. Predetermined temperature T
After being immersed in a metal bath or salt bath maintained at ₂ (for example, 370 ° C. close to Ar ′) for a second set time h ₂ (for example, 1 h) and cooled, it is taken out of the bath and left to cool in the air. By the second set time h ₂ held by the second predetermined temperature T _2, the ground structure of the gear coarse material is changed to the upper base Knight tissue, comprising the toughness of the steel material comparable, and the tensile strength sigma _{B =} 90 to Excellent mechanical properties of 110 kg / mm ² and hardness H _a = about 300 can be obtained. As an example of the salt bath, a mixed salt of potassium nitrate and sodium nitrate having a melting point of about 140 ° C. and a density (at 300 ° C.) of 1.8 g / cm 3
^{3. A} temperature range of 160 ° to 550 ° C. is advantageously used.

The tooth surface of the gear rough material which has been subjected to the ADI treatment is subjected to shaving finishing to obtain a high precision gear of grades 2 to 3 or higher according to JIS. The finishing allowance of the shaving is of course the sum of the finishing allowance remaining after the gear cutting in the first step and the heat treatment deformation caused by the ADI treatment in the second step.
In the I process, following the austenitizing process at the first predetermined temperature T ₁ , the second predetermined temperature T ₂ for quenching into a bainite structure is set to a temperature close to Ar ′, and the difference from the first predetermined temperature T ₁ is set. By making it smaller, the residual stress of the gear material can be made smaller and the amount of deformation can be reduced. In particular, the as-cast structure has a low ferrite ratio and a large pearlite structure F
In the case of the gear material composed of CD600 to FCD700, in the austenitization at the first predetermined temperature T ₁ , the volume increase rate accompanying the transformation from the ferrite structure to the austenite structure causes the transformation from the pearlite structure to the austenite structure. Since the volume increase rate is sufficiently small, it is possible to further suppress the heat treatment deformation amount during the ADI treatment. As a result, the shaving finishing allowance can be set within the range of 0.05 to 0.1 mm, and the finishing allowance in this range ensures excellent productivity without impairing the durability of the shaving tool. However, it is possible to obtain a highly accurate gear at low cost. By this finishing, residual stress due to work hardening is generated on the surface, the pitting resistance is improved, and the fatigue strength of the gear is improved. Therefore, when this gear is adopted as a timing gear of a vehicle engine, for example, there is an advantage that the original damping characteristics of cast iron and high accuracy are combined, and meshing noise can be effectively reduced.

Next, shot peening in the fourth step is performed on the tooth surface of the gear that has been shaving finished, if necessary. The shot peening is a workability that can increase the surface hardness without lowering the accuracy of the tooth surface to improve the wear resistance and the fatigue strength, and preferably the arc height is in the range of 0.15 to 0.30 mm. To do. At this time, the meshing tooth surface other than the tooth root may be appropriately masked, and shot peening may be applied only to the tooth root portion where fatigue strength is particularly required.

Furthermore, before gear cutting is performed in the first step, the gear material 12 can be pre-normalized (hereinafter referred to as N treatment), if necessary. Above N
As shown in FIG. 3, the treatment is performed by heating the gear material 12 to an appropriate temperature T ₃ (for example, 860 ° C.) which is equal to or higher than the transformation point A _{1 and} an appropriate time h ₃ (for example, 1 minute to 1 hour). This is for holding for a time) and then cooling to remove the strain and residual stress of the gear material. By performing this treatment, the machinability of the gear cutting in the first step can be further improved. Further, by performing this treatment, the ferrite structure in the base structure is transformed into an austenite structure and almost all becomes a pearlite structure by cooling. It can be made smaller. This means that, as described above, in the ADI treatment, the volume increase amount when the ferrite structure included in the ground structure of the gear material 12 in a considerable amount is transformed into the austenite structure is large, and the N process causes the ground increase. By changing the ferrite structure in the structure to the pearlite structure in advance, the pearlite structure is transformed into the austenite structure in the ADI treatment,
This is because the volume change amount can be suppressed small. As a result, the finishing allowance in the shaving process of the third step can be further reduced, and there is an additional advantage that the processing speed can be increased and the processing cost can be further reduced.

[0020]

As described above, the method for manufacturing an austempered ductile cast iron gear according to the present invention comprises the first step of gear cutting a spheroidal graphite cast iron material to form a gear rough material, the above gear. A second step of holding the crude material at a first predetermined temperature for a first predetermined time for austenitizing, holding it at a second predetermined temperature for an upper bainite for a second predetermined time, and then slowly cooling it, and the second step Forming a gear by shaving the tooth surface of the gear rough material that has been heat-treated
It has the advantage that it is possible to provide a highly accurate ductile cast iron gear with excellent toughness and tensile strength at low cost.

[Brief description of drawings]

FIG. 1 is a flowchart showing a gear manufacturing process of the present invention.

FIG. 2 is a diagram showing a second step (ADI processing) of the present invention.

FIG. 3 is a diagram showing N treatment which is applied to the gear material of the present invention as needed.

FIG. 4 is a flowchart showing a manufacturing process of a conventional ductile cast iron gear.

FIG. 5 is a flowchart showing another manufacturing process of a conventional ductile cast iron gear.

[Explanation of symbols]

 12 Gear material 14 1st process 16 2nd process 18 3rd process

Claims (1)

[Claims] 1. A first step of forming a gear rough material by gear cutting a material made of spheroidal graphite cast iron, holding the gear rough material at a first predetermined temperature for a first predetermined time for austenitizing,
Further, a second step of holding for a second predetermined time at a second predetermined temperature for upper bainite and then gradually cooling, and shaving the tooth surface of the gear rough material subjected to the heat treatment of the second step to form a gear A method for manufacturing an austempered ductile cast iron gear, comprising a third step of forming.