JPH05140726A - Manufacture of driving system machine parts having high fatigue strength - Google Patents

Abstract

PURPOSE:To provide the method for remarkably improving fatigue properties required for machine parts represented by a gear used as the driving system parts of an automobile and construction equipment. CONSTITUTION:Since a soft layer called as a slack quenched layer generated on the surface layer of a carburized layer generally deteriorates fatigue strength, the development for reducing this deterioration had been executed heretofore. But, the method for utilizing this deterioration was found. Namely, at the time of subjecting 0.1 to 0.3% C steel to carburizing or carbonitriding treatment, a slack quenched layer having 400 to 700 HV hardness is allowed to exist to the range of 10 to 50mum depth from the surface, and after that, high hardness shot peening treatment is executed by using a projecting material having >=500 HV hardness to remarkably increase the compressive residual stress of the surface layer. The increase of the compressive residual stress remarkably improves the fatigue properties.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended for machine parts represented by gears used as drive system parts for automobiles and construction machines, and these drive systems aiming at improvement in use performance, particularly fatigue characteristics. The present invention relates to a method of manufacturing a mechanical component.

[0002]

2. Description of the Related Art Among mechanical parts such as gears of automobiles and construction machines, drive system mechanical parts which require particularly high fatigue strength are usually subjected to a carburizing process of steel parts processed into a required shape. Being manufactured. Carburizing is, for example, as shown in “Heat Treatment of Steel, Revised 5th Edition”, pages 85-97, issued by Maruzen Co., Ltd. on October 1, 1969, after carburizing at a temperature of about 900 ° C., then quenching and necessary The tempering is carried out in accordance with the above, and only the surface layer is made of high carbon martensite, which is a treatment for improving the fatigue strength.

In such carburizing treatment, for example, "Heat Treatment Vol. 24, No. 3" issued by the Japan Heat Treatment Technical Association in June 1984.
As can be seen on pages 128 to 136, since the grain boundary oxidation layer along the austenite grain boundaries in the carburized surface layer portion and the incompletely hardened layer (generally collectively referred to as surface abnormal layer) are generated, the outermost surface layer Hardness and compressive residual stress decrease. Therefore, it has been a problem that sufficient mechanical strength cannot be imparted to carburized machine parts.

On the other hand, JP-A-61-253346
In the publication, Si: 0.10% or less, Mn: 0.05%
It is reduced to the following to suppress the generation of the abnormal surface layer, and P:
A steel material for carburizing treatment in which the grain boundary strength is improved by suppressing the content to 0.010% or less is shown. However, even if such a material is used, it is impossible to eliminate the abnormal surface layer by the current carburizing technology, and in particular, it is not possible to guarantee the fatigue strength of carburized machine parts that require high fatigue strength. , Still not enough.

On the other hand, since the compressive residual stress of the surface layer has a great influence on the fatigue strength of steel products, it has been implemented mechanically by shot peening or the like in order to improve the fatigue strength. For example, as shown in “Casting and Forging and Heat Treatment”, pages 15 to 20, published in February 1991, the shot peening treatment has a remarkable effect of improving fatigue strength. It's not clear if it's good.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for dramatically improving fatigue characteristics required for mechanical parts represented by gears used as drive system parts for automobiles and construction machines. Is what you are trying to do.

[0007]

The gist of the present invention is as follows. (1) Forming a machine part using steel containing 0.1 to 0.3% of carbon, and carburizing or carbonitriding to form an incompletely hardened layer with a Vickers hardness of HV 400 or more and less than 700 10 μm or more from the surface. A method of manufacturing a mechanical component for a drive system having high fatigue strength, characterized by performing shot peening treatment with a projection material having a Vickers hardness of HV500 or more after being made to exist to a depth of 50 μm or less. (2) An incompletely hardened layer having a Vickers hardness of HV 400 or more and less than 700 after being formed into a machine part using a steel containing 0.35 to 0.75% carbon, austenitized at 800 to 950 ° C, and then quenched. 10 μm or more from the surface 5
A method for producing a mechanical component for a drive system having high fatigue strength, which is characterized by performing tempering after being present to a depth of 0 μm or less, and then performing shot peening with a shot material having a hardness of Vickers hardness HV500 or more. (3) For the mechanical component manufactured by the method described in 1 or 2 above, in the final step, 10 μm or more from the surface 10
A method of manufacturing a drive train mechanical component having high fatigue strength, which comprises removing a surface layer portion having a depth of 0 μm or less.

[0008]

The present inventors systematically studied materials that can effectively utilize the shot peening treatment in order to realize high fatigue strength of mechanical parts. As a result, under a certain kind of shot peening treatment conditions, a new phenomenon that the fatigue strength is rather increased when a soft layer generated by some incomplete quenching in the surface layer of the carburized material, that is, an incompletely quenched layer is present. I found it. For example, the above-mentioned Japanese Patent Laid-Open No. 61-25
As seen in Japanese Patent No. 3346, there is a large number of ideas that the incompletely hardened layer that promotes the occurrence of fatigue cracks should be thoroughly suppressed, and this phenomenon is completely contrary to this. ..

Therefore, this phenomenon was examined in more detail.
The effect of improving the fatigue strength by the shot peening treatment is to reduce the tensile stress applied in actual use by increasing the compressive residual stress in the surface layer. As shown in Condition A of FIG. 2, when a soft layer is present in the surface layer, it is plastically deformed relatively easily as compared with the internal hard layer, and as a result, as shown in FIG. It was found that the stress was high. However, it was also found that if the shot peening condition is not appropriate, the easiness of fatigue crack initiation due to the presence of the soft layer affects the fatigue strength improvement effect due to the increase in the compressive residual stress.

From the results of these examinations, the conditions of the material and the conditions of the shot peening treatment which make the fatigue strength extremely high were found out concretely. The limitation of the manufacturing method and the specific reason will be described below. First, regarding the steel material to be used, the carbon content is 0.1% or more and 0.3% or less.
The carbon content of steel determines the hardness of the internal material after carburization (or carbonitriding), and thus has a great influence on the residual stress distribution of the product. If it is less than 0.1%, the desired compressive residual stress cannot be achieved. On the other hand, if it exceeds 0.3%, the hardness of the inner material becomes high, and the desired compressive residual stress cannot be obtained.

The hardness of the hardened layer of carburized (or carbonitrided) steel basically depends on the amount of carbon, and the depth of the hardened layer depends on the carburizing atmosphere and the carburizing time. Although not limited, considering the hardenability when carburizing (or carbonitriding) quenching, Cr 0.3-1.2%, Mo 0.1-1%, Ni
It is desirable to contain 0.2 to 2% of one kind or two or more kinds. Further, in order to prevent the segregation to the grain boundary that is the starting point of fatigue fracture and the reduction of the grain boundary strength, P0.05
% Or less, S 0.05% or less, and nitrogen 0.03% or less is desirable. In order to make the crystal grains finer, V, T
One or more of i and Nb is 0.005 to 0.
Addition of about 02% brings about a better effect.

Next, regarding the condition of the incompletely hardened layer after carburizing (or carbonitriding), which is important in the present invention, the hardness is Vickers hardness HV400 or more 70
It is less than 0. If the HV is less than 400, the compressive residual stress after shot peening will be high, but fatigue cracking will become so easy that improvement in fatigue strength cannot be expected. Also HV
Is 700 or more, the compressive residual stress after shot peening does not become high, and the fatigue strength also deteriorates. The depth of this incompletely hardened layer is 10 μm or more and 50 μm or less from the surface. When the depth is less than 10 μm, there is almost no effect of increasing the compressive residual stress after shot peening, and the thickness is 50 μm.
If it exceeds, fatigue cracking becomes too easy.

In order to obtain the effect of the present invention, the incompletely hardened layer after the carburizing (or carbonitriding) treatment is specified, but the conditions of the carburizing or carbonitriding treatment are not particularly limited, and the carburizing treatment or carburizing atmosphere is not limited. The effect of the present invention can be obtained by any surface hardening treatment such as carbonitriding treatment in which ammonia gas or the like is added as long as the condition of the incompletely hardened layer is satisfied. The hardness of the carburized (or carbonitrided) surface hardened layer is preferably HV 800 or more and the depth is 0.3 mm or more, but such a carburized surface hardened layer can be easily obtained under ordinary carburizing conditions. Further, the method of forming the incompletely hardened layer limited to the above may be a method which is completely opposite to the conventional suppressing method. That is, for example, JP-A-61-2
Without intentionally adding Si or Mn, which is a technique for suppressing the formation of an abnormal surface layer, which is disclosed in Japanese Patent No. 53346, without intentionally reducing the oxygen content in a carburizing (or carbonitriding) atmosphere. It can be achieved by a method such as appropriate adjustment.

The carburized (or carbonitrided) material having such an incompletely hardened layer is subsequently shot peened. The hardness of the shot material used at this time must be 500 or more in Vickers hardness HV. This is a balance with the hardness of the incompletely hardened surface layer, and the effect of improving the fatigue strength peculiar to the present invention cannot be expected in a shot material having a hardness of less than 500. In the present invention, the fatigue strength improving effect can be obtained without particularly limiting the shot peening treatment conditions other than the hardness of the shot material, but the weight of the shot material is 0.2 mg or more per piece, and the shot speed is 50 m or more per second. It is desirable to perform shot peening processing at the projection speed.

The effects of the present invention are not limited to those obtained by carburizing or carbonitriding, but can also be obtained when quenching steel having a large amount of carbon under appropriate conditions. That is, 0.35 to 0.
Molded into machine parts using steel containing 75% carbon,
After austenitizing at 800 to 950 ° C., quenching is performed, and an incompletely hardened layer having a Vickers hardness HV of 400 or more and less than 700 is made to exist from the surface to a depth of 10 μm or more and 50 μm or less, and then tempered, and then a Vickers hardness of HV500 or more. The effect of the present invention can be obtained even when shot peening is performed with a projection material having If the carbon content is less than 0.35%, sufficient hardness of the internal quality cannot be obtained after quenching, and therefore the fatigue strength also deteriorates. If it exceeds 0.75%, the toughness deteriorates and the fatigue strength also deteriorates. Again 8
If the heating temperature is less than 00 ° C, it is difficult to form austenite sufficiently for quenching, and if it exceeds 950 ° C, the austenite becomes coarse and the toughness deteriorates, and in any case, the fatigue strength deteriorates. Also in this case, the formation of an incompletely hardened layer can be achieved by positively adding Si or Mn, or by appropriately adjusting the oxygen potential in the heating atmosphere during the hardening treatment without reducing it.

Further, by any of these methods, it is more effective to remove the surface soft layer to the extent that the compressive residual stress is not reduced after applying the compressive residual stress to the surface layer by shot peening. If the depth of the surface soft layer to be removed is less than 10 μm, the effect of removing the surface layer is not remarkable, and if it exceeds 100 μm, most of the compressive residual stress region of the surface layer is removed, and the fatigue strength deteriorates. At this time, the surface layer can be removed by any method such as electrolytic polishing, mechanical grinding, or liquid honing.

The effects of the present invention will be more specifically described below with reference to examples.

[0018]

EXAMPLES In the following examples, a steel material that was melted in a 150 t converter and hot-rolled into a steel bar having a diameter of 25 mm was cut in a groove with a curvature of 1 mm in the center so as to reproduce the fatigue breakage condition of a gear. Ono type rotary bending fatigue test piece with a diameter of 10 mm, a length of 30 mm and a total length of 350 mm was made on the notched parallel part, and after carburizing and shot peening the
A fatigue test was carried out and the fatigue limit strength (maximum stress value that does not cause fatigue fracture) was evaluated. In each of the following tables, the conditions surrounded by thick frames are examples satisfying the present invention, and the other conditions are comparative examples.

Effect of Steel Material Components (In the Case of Low C Steel Carburized Material) Steel materials having the components A to C shown in Table 1 were used. Among them, A is a target steel of the present invention, B is an excessively small carbon amount, and C is an excessively large amount. Carburization is carried out at a temperature of 920 ° C., a carbon potential of 0.80, and a dew point of −3 ° C. for 8 hours and then 120 ° C.
Quenched in oil of No. 1 and held at 180 ° C. for 1 hour.
As a result, an incompletely hardened layer having a Vickers hardness HV of 400 or more and less than 700 was generated in any of the materials within a depth range of 10 μm or more and 50 μm or less from the surface. Then H
Shot peening treatment was performed using a steel ball having a hardness of V620 to 750 and a diameter of 0.6 mm as a shot material under the condition of a shot speed of 60 m / sec. The carburizing-shot peening treatment conditions were the same for all test pieces. As a result, the fatigue limit strength of the component A of the present invention example is increased by 40% or more as compared with the comparative example.

[0020]

[Table 1]

Effect of Incompletely Hardened Layer The effect of the incompletely hardened layer was examined using the steels having the components shown in Table 1-A, which is the target steel of the present invention. As shown in Table 2, the incompletely hardened layer was adjusted by changing the dew point during carburization and the in-furnace time. A change in the dew point means a change in the oxygen potential (oxygen amount) in the atmosphere, and the lower the dew point, the smaller the oxygen potential. The carburizing conditions were the same except for the dew point and the in-furnace time, the carburizing temperature was 920 ° C., the carbon potential was 0.80, and the carburizing temperature was maintained for 8 hours.

The hardness of the incompletely hardened layer is indicated by the minimum hardness of the surface layer portion, and the depth is indicated by the depth at the point of returning to HV700. Among the conditions, the condition D is the condition of the present invention.
Condition E has a high dew point, that is, the oxygen potential of the carburizing atmosphere is high, so that the hardenability-improving elements are oxidized and disappeared and the hardness of the incompletely hardened layer is lowered. Condition F is a dew point as low as possible and less than HV700. This is a comparative example in which no incompletely hardened layer is observed. Conditions G and H are comparative examples in which the depth of the incompletely hardened layer is made too small or too large by adjusting the time in the furnace. After removing these from the carburizing furnace, quenching them in oil at 120 ° C
Furthermore, it hold | maintained at 180 degreeC for 1 hour.

Next, shot peening was carried out using a steel ball having a hardness of HV620 to 750 and a diameter of 0.6 mm as a shot material under the conditions of a shot speed of 60 m / sec. As a result, the fatigue limit strength of only the incompletely hardened layer D of the present invention was 910 MPa, and that of the comparative example was at most 700.
Only MPa.

[0024]

[Table 2]

Effect of Shot Material Hardness Next, carburizing is carried out under the carburizing conditions of Table 2-D which satisfy the conditions of the incompletely hardened layer of the present invention, which are the steels of the present invention having the composition of Table 1-A. The effect of shot material hardness during shot peening was examined using a material that had been low temperature tempered at 180 ° C. for 1 hour. As shown in Table 3, shot materials having two types of hardness were used. All shot materials were steel balls having a diameter of 0.6 mm, and shot peening was performed under the conditions of a projection speed of 60 m / sec.

Table 3-I is an example of shot peening with a shot material having a hardness satisfying the conditions of the present invention. On the other hand, in Table 3-J, the hardness of the shot material is low, and as a result, the fatigue limit strength is as low as about 70% of the inventive examples.

[0027]

[Table 3]

Influence of Steel Material Composition (In Case of Medium C Steel Quenched and Tempered Material) An example of a medium carbon steel quenched product according to claim 2 of the present invention is shown below. First, the effect of the carbon content of steel will be shown.
Steels having the components of Table 4-KO were used. Of these, K ~
M is a comparative steel of the present invention, N is an excessively small carbon amount, and O is an excessively large amount. These materials were heated to 870 ° C. in an argon gas atmosphere with a dew point of −5 ° C., held for 1 hour, and then
Quench into oil at 20 ° C. As a result, generation of an incompletely hardened layer having a Vickers hardness HV of 400 or more and less than 700 was observed in any of the materials within a depth range of 10 μm or more and 50 μm or less from the surface. Then, after tempering these materials at 500 ° C., they have a diameter of 0.1 mm with a hardness of HV620-750.
Shot peening was performed using a 6 mm steel ball as a shot material under the condition of a shot speed of 60 m / sec. This quenching and tempering-
The shot peening treatment conditions were the same for all the test pieces. As a result, the components of K to M, which are examples of the present invention,
The fatigue limit strength is increased by 40% or more as compared with the comparative example.

[0029]

[Table 4]

Effect of Quenching Temperature Next, an example of studying the quenching temperature in the quenched medium carbon steel product according to claim 2 of the present invention will be described. Table 4 which is the target steel of the present invention
Using a steel having a component of -L, the steel was heated to the quenching temperature shown in Table 5 in an argon gas atmosphere with a dew point of -5 ° C, held for 1 hour, and then quenched into oil at 120 ° C. As a result, generation of an incompletely hardened layer having a Vickers hardness HV of 400 or more and less than 700 was observed in any of the materials within a depth range of 10 μm or more and 50 μm or less from the surface. These materials are then added to 500
After tempering at 0 ° C., shot peening was performed under the condition of a projection speed of 60 m / sec using a steel ball having a hardness of HV620 to 750 and a diameter of 0.6 mm as a projection material.

In contrast to the conditions shown in Table 5-P, which is the condition of the present invention, in the condition Q, the quenching temperature is too low and austenitization is not sufficiently performed, and in the condition R, conversely, it is too high and austenite coarsening occurs. It is a comparative example of conditions. The examples of the present invention are excellent in fatigue limit strength by 20% or more as compared with the comparative examples.

[0032]

[Table 5]

Effect of Removal of Surface Layer Finally, an example in which the improvement of the fatigue limit strength due to the addition of the removal of the surface layer shown in claim 3 of the present invention was examined using two kinds of steel materials which are the objects of the present invention will be shown. One is Table 1-
Carburizing was carried out under the carburizing conditions shown in Table 2-D using the components shown in A, and the other was carried out by quenching and tempering under the hardening conditions shown in Table 5-P using the components shown in Table 4-L. It is a thing. All of these are then shot peened under the conditions of a projection speed of 60 m / sec using a steel ball having a hardness of HV620-750 and a diameter of 0.6 mm as a projection material.
After removing the depths shown in Tables 6-S to Z by electrolytic polishing, a fatigue test was performed.

Among the respective conditions, those satisfying the conditions of the present invention for the amount of surface layer removal are the conditions S and W. Under the U and Y conditions where the surface layer removal amount is less than 10 μm, the surface layer removal effect is not recognized because the fatigue strength is almost the same as the T and X conditions where the surface layer was not removed, and V and Z where the surface layer removal amount exceeds 100 μm. On the other hand, the fatigue limit strength rather deteriorates under the condition of. The fatigue limit strength is further improved by adding the surface layer removal only under the conditions of S and W where the surface layer removal amount is within the conditions of the present invention.

[0035]

[Table 6]

From the above results, it is understood that the present invention has an excellent fatigue limit as compared with the fatigue limit of the conventional method.

[0037]

As described above, the present invention reduces the conventional fatigue strength when the shot peening treatment is applied to the carburized or carbonitrided material of low C steel or the quenched and tempered material of medium C steel to improve the fatigue strength. It is a completely new method of manufacturing mechanical parts in which fatigue properties are improved by intentionally forming a surface soft layer called incompletely hardened layer. It is a very effective method that can significantly improve the

[Brief description of drawings]

FIG. 1 is a comparison diagram of compressive residual stress distribution curves of products according to the method of the present invention and a conventional method.

FIG. 2 is a comparison diagram of hardness distribution curves of products according to the method of the present invention and the conventional method.

Claims (3)

[Claims] 1. An incompletely hardened layer having a Vickers hardness of HV 400 or more and less than 700 is formed from a surface by molding into a machine part using steel containing 0.1 to 0.3% of carbon and carburizing or carbonitriding. A method of manufacturing a mechanical component for a drive system having high fatigue strength, which comprises performing shot peening with a projection material having a hardness of Vickers hardness HV500 or more after being present to a depth of 10 μm or more and 50 μm or less. 2. A steel part containing 0.35 to 0.75% carbon is used to form a mechanical part, which is austenitized at 800 to 950 ° C. and is then quenched to obtain a Vickers hardness HV4.
Incomplete quenching layer of 00 or more and less than 700 is 10μ from the surface
A method for producing a mechanical component for a drive system having high fatigue strength, which comprises: performing a tempering process after having been present to a depth of m or more and 50 μm or less, and then performing shot peening with a shot material having a Vickers hardness of HV500 or more. 3. A machine part manufactured by the method according to claim 1 or 2 is further subjected to 10 μm from the surface in the final step.
A method for manufacturing a drive system mechanical component having high fatigue strength, which comprises removing a surface layer portion having a depth of m or more and 100 μm or less.