JPS61124521A - Thermo-mechanical processing - Google Patents

Abstract

PURPOSE:To improve the fatigue strength of an alloy steel contg. a prescribed percentage of C by austempering and shot-peening the steel under prescribed conditions so as to make the mechanically processed layer thick as well as to shorten the time required to heat treat the material. CONSTITUTION:An alloy steel contg. 0.5-1% C is austempered to increase the hardness HRC to 50-60. The steel is subjected to warm shot peening at the austempering temp., and it is cooled once to room temp. The steel is then subjected to secondary shot peening with shot having a smaller diameter than shot used in the primary shot peening.

Description

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

Conventional technology To improve the fatigue strength of copper products such as shafts and gears, treatments to strengthen the surface layer (rounding, shot peening) are widely used. For example, it can be applied to carburized and quenched parts, austenburized spring members, etc., and it can be used to remove the negative effects caused by surface decarburization layers during heat treatment, increase the hardness of one surface layer, and impart large compressive residual stress. This improves the fatigue strength of parts.

Problems to be Solved by the Invention However, the surface treatment layer formed by the above method has a thickness of about 0.2 to 0.3 layers at most, and as the size of one part increases, the effect decreases. Become. Therefore, it is necessary to make the surface treatment layer even larger so that the effect can be exhibited even in large parts.

Therefore, in order to increase the number of surface-treated layers, the present inventors developed an ausforming treatment (a processing heat treatment method in which copper is rapidly cooled to an appropriate temperature range after being turned into austenite, and the appropriately cooled austenite is subjected to & mechanical processing) and a shot bean treatment. We have developed a method for carburizing and quenching products by combining it with a carburizing process, and have filed separate patent applications (%Application No. 18392/1983, Japanese Patent Application No. 59/1983).
No. 00507). However, this processing heat treatment method uses a carburized and quenched material, and there is a problem in that the treatment takes a long time.

Therefore, an object of the present invention is to provide a processing heat treatment method that can further improve the compressive residual stress on the surface, which contributes greatly to improving fatigue strength, in a short time, and that can yield a product with good surface roughness. It's about doing.

Means and Effects for Solving the Problems The method of the present invention reduces the heat treatment time of the material by using austenbur treatment (which creates a bainitic structure with high hardness and 1 toughness), which is widely used in spring steels. The aim is to improve fatigue strength by shortening the heat treatment (heat treatment is completed in 1 to 2 hours for alloy steel containing about 0.6% C) and combining this with two-stage shot peening treatment.

That is, shot peening is performed at room temperature after austenbur treatment on spring steel for the purpose of improving the deterioration of the surface seam.
After austenitizing the alloy steel containing 5 to 1.0% C, it is austenitized at a temperature of 300°C above the &JF point of the steel, resulting in a hardness of H! After obtaining a bainite structure of lC5Q~60, shot peening is subsequently started from that temperature range to form a deep surface treatment layer.

After that, once cooled, a second shot peening is performed for a short time using a shot having a smaller shot diameter than that of the first shot peening to further improve the compressive residual stress on one surface. It is characterized by obtaining a product with good roughness.

Aspects of the Invention An example of the pattern of the treatment method of the present invention is shown in FIG. First, the steel to be treated is heated to m degrees in the austenite region in a neutral salt path or neutral atmosphere.

For example, after heating to 830 to 870°C for a certain period of time to austenite, it is rapidly cooled in a hot bath at a temperature range of 300°C above the Hz point of copper, and kept at this temperature for a certain period of time to perform austenite treatment, resulting in a hardness of RRC50. ~60 bainite structures are obtained. Subsequently, warm shot peening is performed from this temperature range, and then, after cooling once, a second shot peening is performed.

The shot peening conditions are as follows: A centrifugal device is used, and the shot diameter is 0.6 to 0.8 sm.

It is preferable to set the projection speed to 35 to 50 nr/I and the projection time to 5 to 40 nr/I. 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 carry out shot peening under optimal conditions. The influence layer of compressive residual stress in the axial direction is shallow, and therefore the increase in fatigue strength is small.On the other hand, if it is too large, it may cause damage to the stressed center of complex shaped objects such as power transmission shafts (for example, peening into grooves, etc.) becomes impossible. Therefore, preferably 0.6
-0. It is better to use a shot (ball) with a g-th diameter.

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. For this reason.

Considering the cooling by the shot, the projection time is 5 to 40 minutes.
A range of minutes is preferred.

Furthermore, if the shot projection speed (corresponding to the rotation speed of the pulley in a centrifugal device) is too small, 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.

A range of 35 to 50 rn/I is preferred, and most preferred is a range of 45 to 5 Q m/z.

After performing peening at medium temperature as described above, a second shot peening is performed according to the present invention. In this case.

Basically, the conditions for shot peening from the medium temperature range can be applied as they are, but shot peening at room temperature not only further increases compressive residual stress, which greatly contributes to improving fatigue strength. 6. Since this is done to improve the surface roughness, a shot having a smaller shot diameter than that used for shot peening from a medium temperature range is used.

Preferably, a shot with a diameter of φ0.3 to φ0.5- is used.In the following examples, the present invention will be specifically explained by showing *flow side and comparative examples, but the present invention is limited to the following flow side. Of course not.

Comparative Example 1 A test piece of m 2 a x 2401 of 0.6 Cl Cr -0,5Ni -0,2No steel (MI point: 210°C) was austenitized at 850°C for 1 hour, and then austenitized at 230°C. After austenburizing for 2 hours, this was cooled to room temperature, and the shot diameter was 0.8 square meters at room temperature.

The current rotational speed of the centrifugal device is 2500 ypya, and the projection time is 1.
0 Comparative Example 2 in which shot peening was carried out under conditions of 0 minutes The same test piece used in Comparative Example 1 above was austenitized at 850°C for 1 hour, and then austenitized at 230°C for 2 hours. Shot peening was performed under exactly the same conditions as in Comparative Example 1 above.

Example The same test piece used in Comparative Example 1 above was austenitized at 850°C for 1 hour, and then austenitized at 230°C for 2 hours. After performing the first shot peening and then cooling, the second shot peening was performed at room temperature with a shot diameter of 0.3 mm and a separate car rotation speed of 250 Orp.
The test was conducted under the conditions of m and projection time of 5 minutes.

FIG. 2 shows the compressive residual stress distribution of the test pieces treated according to Comparative Examples 1 and 2 and Examples.

As is clear from FIG. 2, the material subjected to shot peening at room temperature after austempering (Comparative Example 1) has a very high compressive residual stress, but its influence layer is very shallow. On the other hand, after austempering, the first
The sample subjected to shot peening for the second time (Comparative Example 2) had a very deep processed layer, although the value of compressive residual stress was inferior. For adjusting one-sided roughness and applying new compressive residual stress according to the present invention.

The residual stress distribution after the second shot peening is performed in addition to the first shot peening is an intermediate distribution between the two.

In Comparative Examples 1.2 and Examples above, austempering was carried out at various temperatures. The relationship between the resulting austempering temperature, hardness, and maximum surface roughness after shot peening is shown in Table 1 below.

0.6C-1cr-0,5Ni-0,2M My points: 2
10°C Comparative Example 1 Comparative Example 2 Example 3° Hardness 1st after room temperature SP*), sF*)
Second sf temperature 3 HRC surface roughness Surface roughness 230°C 57.8 4.0 2
3 5.1260℃ 55.5
g, 2 25 9.3300℃
52.1 14.3 2 g
10.5350℃ 49.0 184
31 15 As mentioned above, after the first shot peening from a medium temperature range, the machining depth becomes larger, but as shown in the table above, the surface roughness is very rough. It is inferred that this is not preferable from the viewpoint of strength as well. On the other hand, when the second shot peening is further performed according to the present invention, the surface roughness, which causes a decrease in fatigue strength, is significantly improved. However, as the austempering temperature increases, the hardness of the bainite phase decreases, so the final surface roughness tends to remain rough. Therefore, the austenbur m degree before shot peening should be 300℃ or less, and the hardness HRC
50 or more is required.

3 of the test specimens treated according to Comparative Example 1 and Examples
Figure 3 shows the results of the point bending fatigue test. The results for test pieces that were only austempered at 230°C are also shown. From Figure 3.

If treated according to the method of the present invention, L! 14 Effects of Fatigue Strength Invention As mentioned above, the processing heat treatment method of the present invention has a fatigue strength of 0.5
~ +, alloy steel containing O*C, hardness HRC50~6
0, warm shot peening is performed from that temperature range, and once cooled to the temperature, the shot diameter is /J% smaller than the shot diameter used in the first shot peening. Since the second shot peening is performed using a shot, the time for heat treatment of the material can be greatly shortened, and the processed layer and compressive residual stress layer can be sufficiently peened, and the compressive residual stress can be increased.
Furthermore, since the surface roughness can be sufficiently improved, fatigue strength can be further improved.

[Brief explanation of the drawing]

Fig. 1 is a cycle diagram showing an example of the pattern of the treatment method of the present invention, Fig. 2 is a compressive residual stress distribution diagram of each treated material, and Fig. 3 is a graph showing the results of a three-point bending fatigue trial sale. . 83°・. ~87°・. 811, Figure 1 Figure 2/11+plane j'pubo1 (mm+

Claims (1)

[Claims] Alloy steel containing 0.5 to 1.0% C has a hardness of H_RC
50 to 60, warm shot peening is performed from that temperature range, and once cooled to room temperature, using shot having a smaller shot diameter than the shot diameter used in the first shot peening. A processing heat treatment method characterized by performing a second shot peening.