JPS6179719A - Thermo-mechanical treatment - Google Patents

Abstract

PURPOSE:To improve the fatigue strength of machine parts or a structural member by quenching a low-carbon low-alloy steel after reheating, shot-peening the steel under cooling and shot-peening it again at room temp. CONSTITUTION:A carburized, hardened and tempered low-carbon low-alloy steel having 0.7-0.9% surface concn. of carbon is reheated to a temp. in the austenite range of 800-850 deg.C and quenched to 300-550 deg.C with a hot bath, or a low-carbon low-alloy steel is quenched to 300-550 deg.C with a hot bath immediately after carburization. The steel is then shot-peened again at room temp. with shot having a diameter which is equal to or larger than the diameter of shot used in the first shot peening. By this treatment, a mechanically treated layer and a layer having compressive residual stress can be formed up to a sufficiently large depth each, and the surface roughness of the steel as well as the fatigue strength can be improved.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a processing heat treatment method that can significantly increase the fatigue strength of mechanical parts and structural members, such as power transmission components such as power transmission shafts and gears. be.

Conventional technology Conventionally, in order to improve the fatigue strength of mechanical parts and structural members against repeated loads, methods of generating compressive residual stress in the surface layer and hardening the surface layer at the same time include (α) carburizing, nitriding, tuftride treatment, Various heat treatments such as induction hardening, (b) surface cold treatments such as surface rolling and shot peening, and (C) processing heat treatments such as ausforming are employed.

However, the above (as represented by ausforming)
Processing heat treatment (C) involves rolling, etc. to perform strong working, so it has the disadvantage that it is difficult to process products with complex shapes such as power transmission shafts. The disadvantage is that it is expensive because it uses

On the other hand, the surface hardening methods (α) and (A) are widely used because they do not pose any problems in terms of processing.

For example, Japanese Patent Application Laid-Open No. 54-164079 discloses that a metal workpiece is tightened to a mandrel in order to generate tensile stress in the workpiece, and then compressive stress is induced on the surface of the workpiece by shot peening. Methods have been proposed to create residual compressive stress on the outer surface of the workpiece. However, in the surface cold processing of <b> above, such as shot peening, and the heat treatment method of (α) above,
Rotational bending fatigue strength (100 times): 50 to 90 kg/a
100 when using the processing heat treatment method of (C)
~l l There is a problem that it is lower than OkgAj.

Background of the Invention or Basic Knowledge In order to improve the fatigue strength of ferrous metal parts, the present inventors have developed a method that combines the heat treatment (α) and the shot peening (h), that is, shot peening is performed after the heat treatment. discovered a treatment method that could improve fatigue strength by work-hardening the surface layer and imparting compressive residual stress, and filed a separate patent application (Japanese Patent Application No. 59-729).
No. 97 and Japanese Patent Application No. 59-72998). That is, when shot peening is applied to a carburized part under optimal conditions, a hardness of about 1000 in maximum Hv is obtained due to work hardening, and the compressive residual stress reaches a maximum of about 120 to 9. -, the fatigue strength is significantly improved to 1.5 times that of carburizing alone, and the surface roughness is also quite good at about 3.2 to 5.5S.

However, the processed layer and compressive residual stress layer obtained by shot peening are at most 0.2 to 0.31111
The problem is that it is only 1. Therefore, as the size of the workpiece increases, the rate of improvement in fatigue strength decreases due to size effects.

Therefore, in order to increase the processed layer and the compressive residual stress layer, the present inventors further developed an ausforming treatment (mechanical heat treatment method in which the austenite is rapidly cooled to an appropriate temperature range after being turned into austenite, and the appropriately cooled austenite is mechanically processed. ) and shot peening treatment, and has separately filed a patent application (Japanese Patent Application No. 18392/1983). That is,
This treatment method belongs to the above-mentioned (c) heat treatment category, and ausforming, which is a typical example, heats medium-carbon and high-carbon alloy steel to an austenitizing temperature and forms 5EllllI wires. Austenite Bay (40
0 to 7oooC), then subjected to plastic deformation by rolling etc. at this temperature and then rapidly cooled to room temperature, whereas low carbon low alloy steel is carburized and quenched and then tempered. was reheated to an austenitizing temperature of 800 to 8501°C, rapidly cooled in a heat bath of 300 to 550°C, held for a certain period of time, shot peening was started, and the surface layer was processed and shot to an austenitizing temperature of +IO to 130°C/(each This method is characterized by performing processing while cooling the material to near room temperature.

By employing such a treatment method, the processed layer and the compressive residual stress layer can be made 0.5 to Q, 6 brocade, which is approximately twice that in the case of ordinary shot peening treatment. However, in the case of this treatment, since processing is performed at a medium temperature range (300 to 550° C.), the surface roughness is about 20 to 275, and the dimensional accuracy is worse than that of normal shot peening treatment. Further, the compressive residual stress distribution maintains a high level up to about twice the depth as described above, but its absolute value is -90 to -+00#/j, which is lower than that of normal shot peening.

Problems to be Solved by the Invention Accordingly, the general object of the present invention from the perspective of the prior art is to improve the drawbacks of the prior art methods described above, to facilitate processing of parts with complex shapes, and to improve power transmission. The object of the present invention is to provide a processing heat treatment method that can significantly increase the fatigue strength of mechanical parts and structural members such as power transmission components such as shafts and gears.

Further, the direct object of the present invention when viewed from the above-mentioned related inventions of the present inventors is to improve surface roughness, which is a drawback of the method for improving fatigue strength by shot peening treatment, and to improve fatigue strength. The object of the present invention is to provide a processing heat treatment method that can further increase compressive residual stress, which greatly contributes to improving strength.

Means and Function for Solving the Problems In order to achieve the above-mentioned object, the processing heat treatment method according to the present invention provides low carbon, low alloy steel that is carburized to have a surface carbon concentration of 0.7 to 0.9%. is reheated to the austenite region of 800 to 850°C, then rapidly cooled in a heat bath of 300 to 550°C,
After holding the carburized layer for a certain period of time, shot peening is performed from this 'tMW range, and processing is performed while cooling the carburized layer to below the no-motion point. Shot peening is performed at room temperature using a shot with a smaller shot diameter, and this improves the surface roughness to about 3.9 to 5.9S and reduces the maximum value of compressive residual stress to -110" 9/, j or more.

Another processing heat treatment method according to the present invention is that in the above treatment method, instead of the step of reheating to F300 to 850°C after carburizing and quenching and tempering, and then rapidly cooling in a 300 to 550°C hot bath, carburizing is performed. This is followed by a step of rapidly cooling in a 300 to 550° C. heat bath, and the subsequent steps are similar to the above treatment method, and the same effects as the above treatment method can be obtained.

Aspects of the Invention The material to which the processing heat treatment method according to the present invention is applied is low-carbon, low-alloy copper (such as SCMAI5), which is subjected to ordinary carburizing and quenching treatment and then low-temperature tempering at 150 to 200°C. to reduce the surface carbon concentration to 0.7-0.9%.
Use hardened copper. By using such a material, the transformation start curve of the S curve of the surface layer shifts to the long time side due to the high carbon concentration in the surface layer, which can be used to suitably It is possible to perform mechanical heat treatment on the surface layer in a cold austenitic state.

According to the present invention, the above material is reheated to an austenite range of 800 to 550 gsoc, then rapidly cooled in a hot bath of 300 to 550 t::, held for a certain period of time, and then subjected to shot peening and plastic deformation on the surface in the above state. The reason why shot peening is used as a method for applying this is that it can easily process parts with complex shapes such as power transmission shafts, and the cooling rate after plastic deformation of the surface layer can be increased to a rate higher than that of air cooling (110 to +3
This is to ensure hardenability of the surface layer.

In addition, since shot peening is a process that also serves as a cooling process, it is possible to process properly cooled austenite, process martensite during and after transformation, and perform complex process heat treatment, resulting in noticeable fatigue. Strength can be improved.

According to another aspect of the present invention, the low carbon low alloy Δ is carburized in a conventional manner, for example, at 900 to 950°C for 6 to 20 hours, then further carburized at 850°C for a predetermined time, and then immediately carburized at 300 to 550°C. After cooling rapidly in a hot bath and holding for a certain period of time, shot peening is applied from this temperature range.

Shot peening from the medium temperature range starts from a temperature of 300 to 550°C as described above. The reason for setting this starting temperature to 300 to 550°C is to reduce the axial compression residual near the surface, which has a large effect on fatigue strength. In the case of processing from this temperature range, the stress distribution is at a very high level of -90 to -1009/j, whereas in the case of processing from below 300°C, the compressive residual stress level at this position is -30
This is because the fatigue strength is reduced by half compared to treatment from the above-mentioned temperature range of -40 -, and no significant improvement in fatigue strength can be expected compared to ordinary carburized materials.

The conditions for the above shot peening are: ``A centrifugal device is used, the shot diameter is 0.6 to 0.8, and the projection speed is 35.
~50ml! , the projection time is preferably set to 5 to 4O-1=. If shot peening conditions are inappropriate, a significant improvement in fatigue strength may not be expected, or conversely, fatigue strength may decrease, so it is necessary to perform shot peening under optimal conditions.

First, if the shot grain size is too small, the influence layer of compressive residual stress in the axial direction will be shallow, and therefore the increase in fatigue strength will be small.
On the other hand, if it is too large, it becomes practically impossible to peen stress concentration parts (for example, various grooves) of a complex-shaped object such as a power transmission shaft. For this reason, it is best to use a shot with a bait diameter of 0.3 to +, o, preferably 0.6 to 0.8.

Further, as for the shot projection time, the longer the shot projection time is, the better the fatigue strength will be, but after that the fatigue strength tends to be saturated. Therefore, considering the cooling by shots, the projection time is preferably in the range of 5 to 40 minutes.

Furthermore, if the shot projection speed (corresponding to the rotation speed of another car in a centrifugal device) is too low, the effect will not be apparent.
Also, if it is too large, the surface roughness will increase or micro-cracks will occur on the surface, so it is preferably in the range of 35 to 50 m/I, and most preferably 45 to 50 m/I! is within the range of

After performing shot peening from a medium temperature range as described above, shot peening is further performed at room temperature according to the present invention. The shot peening conditions in this case can basically be applied as they are for shot peening from the medium temperature range, but this 7-shot peening at room temperature reduces compressive residual stress, which greatly contributes to improving fatigue strength. In addition to further improving the surface roughness, it is preferable to use shot having a shot diameter that is the same as or smaller than that used in shot peening from a medium temperature range. By this second shot peening, the surface roughness was improved to about 3.9 to 5.9I, and the maximum value of compressive residual stress was -110K.
g. reach more than employment.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples and Comparative Examples, but it goes without saying that the present invention is not limited to the following Examples.

Comparative Example 1 Low carbon, low alloy steel SCM415 was subjected to carburizing, quenching and tempering according to a conventional method. That is, at 930℃ for 6 hours (
Carburizing was performed for 3.5 hours with a carburizing gas containing 0.13% CO, then for 2.5 hours with a carburizing gas containing 0.2% CO7, and then at 850°C for an additional 0.5 hours (0.5% CO2
Carburizing gas) and oil quenching, 150 ~
It is tempered at 200℃. In this way, a normal carburized material C1 was obtained.

Comparative Example 2 A normal carburized material that had been carburized, quenched and tempered as in Comparative Example 1 above was shot with a shot diameter of Q and 8 m@.

Separate wheel rotation speed of centrifugal device 250 Orpm, projection time 1
Shot peening was performed under conditions of 0 minutes to obtain a normal shot peening material C2.

Comparative Example 3 All of the normal carburized materials that had been carburized and quenched and tempered as in Comparative Example 1 above were further reheated to 800 to 850 C,
This was rapidly cooled in a 300° C. heat bath, held for a predetermined time, and shot peened from this temperature under the same conditions as in Comparative Example 2 to obtain (ausforming + shot peening) treated material C3.

Example After carrying out the same treatment as in Comparative Example 3 above, further 2 treatments were carried out at room temperature.
The second shot peening was performed to obtain the material P of the present invention.

Fig. 1 shows the axial compressive residual stress distribution of the treated materials p, c, , c, , c, and Fig. 2 shows the average surface roughness after various treatments. From Figures 1 and 2, it can be seen that according to the treatment method of the present invention, the processed layer and the compressive residual stress layer can be sufficiently deep, and there is a closer relationship with fatigue strength than in ausforming + shot peening treatment. It can be seen that it is possible to further increase a certain compressive residual stress and at the same time improve the surface roughness.

Effects of the Invention As described above, according to the processing and heat treatment method of the present invention, a carburized, quenched and tempered low carbon low alloy lid can be heated to 800 to 850
300~550℃ after reheating to austenite range of ℃
Alternatively, immediately after carburizing the low-carbon low-alloy lid, quickly cool it in a heat bath of 300 to 550°C, perform shot peening from this temperature range, and then perform a second shot peening treatment at room temperature. Therefore, the processed layer and the compressive residual stress layer can be sufficiently deep and the compressive residual stress can be further increased, and the fatigue strength can be significantly improved, and at the same time, the surface roughness can be sufficiently improved. Furthermore, there is no problem with conventional ausforming that it is difficult to process into complex shapes.

[Brief explanation of drawings]

Figure 1 is a graph showing the axial compressive residual stress distribution of various treated materials, and Figure 2 is a distribution diagram showing the average surface roughness after various treatments.

Claims (2)

[Claims] (1) Carburizing the surface carbon concentration to 0.7 to 0.9
% low carbon low alloy steel is reheated to the austenitic region at 800 to 850°C, then rapidly cooled in a 300 to 550°C heat bath, held for a certain period of time, and shot peened from this temperature range to near room temperature. A processing heat treatment method characterized in that processing is performed while cooling to a temperature of 100.degree. C., and then shot peening is performed at room temperature using shot having a shot diameter that is the same as or smaller than that used in the shot peening. (2) 300 to 550 immediately after carburizing low carbon low alloy steel
After cooling rapidly in a hot bath at ℃ and holding for a certain period of time, shot peening is performed from this temperature range while cooling to near room temperature. A processing heat treatment method characterized by performing shot peening at room temperature using shot having a diameter.