JPH03229852A - Production of high strength parts - Google Patents

Abstract

PURPOSE:To improve fatigue strength and to produce high strength parts by exerting carburizing treatment at specific carbon potential, performing specific hardening and tempering while reducing carbon potential, and further carrying out shot peening treatment. CONSTITUTION:Carburizing treatment is applied to parts composed of carburiz ing steel, such as Cr steel, by setting carbon potential at 1.0-1.2%, by which the amount of equilibrium carbon in the position at about 100mum depth from the parts surface is regulated to about 1.0%. Subsequently, carbon potential is lowered to 0.4-0.6%, and the above parts are held at 830-870 deg.C for 2-15min and hardening and tempering are carried out. By the above heat treatment, the structure in the outermost surface part at about 70mum depth from the surface part is formed into martensitic structure and the amount of retained austenite in the position at about 100mum depth from the surface part is regulated to >=about 30%. Then, shot peening treatment is applied to these parts to establish high residual compressive stress. By this method, the fatigue strength and tough ness of the carburized parts can be improved, and high strength parts can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method of manufacturing high-strength mechanical structural parts by carburizing and quenching and shot peening.

[Prior Art] Recently, as the performance of mechanical devices has improved, mechanical structural parts have come to be used under harsh conditions such as high speeds and high loads. For this reason, increasing the strength of parts has become an important issue in industry. For example, in automobile engines, with the advent of high-output engines such as turbocharged engines and 4-valve engines, the load stress acting on transmission gears, etc. tends to increase, and conventional carburizing Parts may lack tooth bending fatigue strength.

Therefore, the invention disclosed in JP-A No. 62-70512 has been proposed, in which the outermost layer is carburized and quenched at a carbon potential (equilibrium carbon concentration) of 1.00 to 1.20%. This method of surface hardening a carburized product is characterized by generating a large amount of retained austenite, and then subjecting the retained austenite to a shot peening process to transform the retained austenite into martensite. In this invention, in addition to the generation of compressive residual stress due to shot peening, further compressive residual stress is added due to deformation-induced transformation of retained austenite due to shot peening, which significantly increases the compressive residual stress and increases the fatigue strength of carburized parts. and improved toughness.

Also, Mazda Technical Report (No. 5.1987, P165~
According to 173), by carbonitriding, which performs gas nitriding for a very short time after carburizing, only the outermost layer is nitrided, and during this time, by controlling the carbon potential and the 9-element potential, the amount of retained austenite can be reduced to 30 to 35%. After controlling the fatigue strength within this range, hard shot peening is applied to successfully increase the fatigue strength of gears.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional technology, in order to enhance the effect of shot peening, the main focus is placed on optimizing the amount of retained austenite in the surface area, and it is difficult to achieve an effective carbon concentration distribution. A deep review including this has not yet been carried out.

In addition, in order to improve the performance of mechanical equipment, it has been desired to provide a method for manufacturing parts that can obtain higher fatigue strength than conventional methods.The present invention uses carburizing treatment and shot peening. This was done in view of the above-mentioned problems in the method of manufacturing high-strength parts, and the object is to provide a method of manufacturing high-strength parts that can obtain even higher fatigue strength than conventional methods.

[Means for Solving the Problems] In the method for manufacturing high-strength parts of the present invention, carbon potential is set at 1.0 to 1.2% and carburizing treatment is performed, and then the carbon potential is set at 0.4 to 0.0%. The gist is to reduce the temperature to 6% and hold it at 830 to 870°C for 2 to 15 minutes to perform quenching and tempering, and further to perform shot peening treatment.

Carburizing steels to which the present invention is applied include, for example, Cr steel, Cr-Mo steel, Ni-Cr steel, and Ni-Cr steel, which are specified in JIS as machine structural alloy steels used for case hardening steel.
-In addition to Mo steel, M n -Cr steel and Ni -
It is Mo steel.

In the method of the present invention, a gas carburizing method using a carburizing atmosphere is used. The carburizing atmosphere can be, for example, an endothermic gas that has been modified by mixing a theoretical amount of air with the raw material gas and using charcoal or a nickel catalyst filled in an externally heated retort, or a liquid raw material that is directly introduced into the furnace. It is possible to use a drip-type decomposition gas that is dropped and uses the decomposed gas.

As carburization or decarburization progresses in a carburizing atmosphere, an equilibrium is reached in which the amount of carbon in the steel neither increases nor decreases. The amount of carbon at this time is called the equilibrium carbon concentration (carbon potential), and the carburizing ability of the carburizing atmosphere is expressed in this form. There is a certain relationship between the dew point of the carburizing gas and the carbon potential, so once the treatment temperature and desired carbon content are determined, the specified enrichment gas can be used to obtain the carburizing gas with this dew point. Just add it.

In the method of the present invention, the carbon potential at the beginning of carburizing treatment is set at 1.0 to 1.2%.
The equilibrium carbon content at the position around 0μ is less than 1.0%,
This is because the amount of retained austenite, which is said to provide high compressive residual stress, is less than 30%, and if the carbon potential exceeds 1.2%, compressive residual stress decreases and fatigue strength decreases. It is.

In addition, after that, the carbon potential was increased to 0.4 to 0.6%.
The reason for specifying the heat treatment conditions of quenching and tempering by holding at 830-870°C for 2-15 minutes is as follows.

Carbon potential: 0.4 to 0.6 When the carbon potential is less than 0.4, the carbon concentration on the surface becomes too low, and the hardness of the surface portion significantly decreases, so that the fatigue strength conversely decreases.

When the carbon potential exceeds 0.6, it takes a long time (more than 15 minutes) to reduce the carbon concentration on the surface, and the carbon concentration at a position approximately 100μ from the surface decreases, so shot peening is The effectiveness of this will be reduced.

Set temperature = 830 to 870°C If the set temperature is less than 830°C, the austenite phase region will not be uniform, and quench hardening will be incomplete. If the set temperature exceeds 870° C., the diffusion rate of C becomes too fast, making it difficult to obtain the desired carbon concentration. In addition,
The timing to set this temperature does not necessarily have to be at the same time as reducing the carbon potential to 0°4-0.6%, but it should be selected at an appropriate time before that so that the optimum carbon concentration can be obtained. be able to. An example of a heat treatment cycle according to the method of the present invention is shown in the second UJ! Shown in J.

In addition, in FIG. 2, CP represents carbon potential.

If the holding time is 2 to 15 minutes or less than 2 minutes, the carbon concentration at the outermost surface portion will not be reduced sufficiently, and the effects of the present invention will not be obtained. If the holding time exceeds 15 minutes, the surface will deteriorate], 007
The carbon concentration at the position of the Jto front paper decreases, and the effect of shot peening decreases.

Conventional equipment can be used for shot peening. As a shot projection device, either a centrifugal projection device that is accelerated by the blades of a rotating impeller, or a pneumatic shot projection device that utilizes the air velocity when compressed air is ejected from a nostle can be used. can. The shot may be cast iron shot, cast steel shot, steel wire shot, or the like. Shot diameter, shot I
- Shot peening conditions such as hardness, blasting speed, blasting time, etc. are appropriately selected depending on the material of the part, the size of the part, etc.

[Function] In the method for manufacturing high-strength parts of the present invention, by performing carburizing treatment with the carbon potential set at 1.0 to 12%, the carbon concentration at a position approximately 100μ from the surface is reduced to 1.0%.
It can be around 0.

Therefore, the residual austenite after carburizing and quenching can be increased to 30% or more, and it is possible to secure the amount of residual austenite that allows high compressive residual stress to be obtained by shot peening.

In addition, after that, the carbon potential was reduced to 0.4-06%, held at 830-870°C for 2-15 minutes, and quenched.
The outermost surface of the μ-cells has a low carbon concentration, suppressing the formation of retained austenite, and at the same time providing a martensitic structure with high toughness.

Subsequently, the retained austenite at a position around 00 μm from the surface is 309. When shot peening is applied to a part on iIJ, in addition to the generation of compressive residual stress due to shot peening, compressive residual stress is further added due to deformation-induced transformation of retained austenite due to shot peening. Moreover, the outermost surface has a low carbon concentration and a martensitic structure with high toughness. It is now possible to produce parts with high fatigue strength never before possible.

[Example] An example of the present invention will be explained in comparison with a conventional example, and the effects of the present invention will be clarified.

5Cr, which is currently widely used in automotive drivetrain parts
420tI4 was melted, forged, and gear cut to produce a Mofunir 2.55 spur gear (pressure angle 225°, standard pitch circle diameter 795 teeth + 11o, base circle diameter 73033mm, number of teeth 31). .

The manufactured gear was carburized, quenched and tempered using an all-case carburizing and quenching furnace and a Matsufuru furnace according to the heat treatment cycle shown in FIG.

Further, as Conventional Example 1, the manufactured gear was carburized, quenched, and tempered according to the heat treatment cycle shown in FIG. 4, with the carbon potential at the time of carburizing treatment set to 10. Furthermore, as Conventional Example 2, the carbon potential shown in Fig. 4 and the quenching temperature after carburizing in the heat treatment cycle (85
15% of the total flow rate of ammonia gas at 0°C).
Carburizing, nitriding, and quenching were carried out by adding carbon dioxide for a minute.

Subsequently, the gears of the present invention example, conventional example 1, and conventional example 2, which had been heat treated as described above, were subjected to shot peening under the following conditions.

Shot grains Average particle diameter 0.8 mm Average hardness Hv750 Equipment Air blast type shot peening machine Air pressure 7,0 kg/cm2 Projection time
4-minute projection amount: 80 kg/min The arc height value at that time was measured with a piece of Almen strip A and was 0.52 mm.

For gears that have undergone shot peening and become finished products, fatigue tests are conducted on individual gears using the so-called pulsator method. The number of retardations is shown on the axis.

From the results shown in Figure 1, the carbon potential is set to 1.
After carburizing the residual austenite to around 30% by controlling it to 0 to 1.2%, shot peening the gear (conventional example 1) and carburizing and nitriding the residual austenite to a specific range (30%). Compared to the gear (conventional example 2) which was subjected to shot peening after being controlled to 35%), the gear of the present invention has a higher durability limit of about 10%, and according to the method of the present invention, It was confirmed that it was possible to manufacture gears with high strength, which was not possible with conventional methods.

[Effects of the Invention] As explained above, in the method for manufacturing high-strength parts of the present invention, the carbon potential is set to 1.0 to 1.2% and carburizing treatment is performed, and then the carbon potential is set to 0.4%.
It is characterized by reducing the carbon content to ~0.6% and holding it at 830 to 870°C for 2 to 15 minutes to perform quenching and tempering, and then subjecting it to shot peening treatment, so that the outermost surface part has a low carbon concentration. By suppressing the formation of retained austenite and obtaining a martensitic structure with high toughness, and by controlling the retained austenite at a position around 100μI from one surface to an optimal range that provides high fatigue strength, the shot after carburizing and quenching and tempering is In addition to the generation of compressive residual stress due to peening, additional compressive residual stress is added due to the deformation-induced transformation of retained austenite due to shot peening.Furthermore, the outermost surface has a low carbon concentration and a martensitic structure with high toughness. , it is possible to manufacture parts with high fatigue strength that could not be obtained using conventional methods.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between stress and repetition rate in fatigue tests of gears of the present invention and conventional examples, Fig. 2 is a diagram showing the heat treatment cycle of carburizing, quenching, and tempering of the present invention. FIG. 4 is a diagram showing the heat treatment cycle of the embodiment, and FIG. 4 is a diagram showing the heat treatment cycle of the conventional example. Agent for patent applicant Toyota Motor Corporation

Claims (1)

[Claims] (1) Set the carbon potential to 1.0 to 1.2% and perform carburizing treatment, then reduce the carbon potential to 0.
．． 2 to 1 at 830 to 870°C by reducing it to 4 to 0.6%.
A method for manufacturing high-strength parts, characterized by performing quenching and tempering by holding for 5 minutes, and further performing shot peening treatment.