JPH04256577A - Prevention of delayed fracture for static load bolt - Google Patents

Abstract

PURPOSE:To remarkably reinforce, miniaturize and lighten a bolt receiving a static load, by executing a shot peening treatment by the shot in smaller diameter than the diameter of curvature of the arc like face of the groove bottom of the bolt, thereon. CONSTITUTION:A shot peening treatment by the shot in a smaller diameter than the diameter of curvature of the arc like face of the groove bottom 14 of a bolt 13 is executed on the bolt 13 on which the specified screw groove 12 is formed. Consequently, sudden fracture on the bolt 13 on which a static load is loaded for a long period, namely a delayed fracture is prevented.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to architectural or automotive applications.
The present invention relates to a method for preventing delayed fracture of statically loaded bolts, which prevents bolts for various structural materials such as machines, which are subjected to statically loaded materials, from suddenly breaking after a certain period of time.

0002

2. Description of the Related Art For bolts that are subjected to static tensile loads without load fluctuations, the material and bolt diameter are generally selected in consideration of their tensile strength and safety factor. However, if a static tensile load is applied for a long period of time, the bolt may suddenly break even if the tensile load is less than the strength of the bolt. This is different from the creep phenomenon that has been often discussed in the past, and is called delayed fracture. The characteristics of this delayed fracture are that it fractures under a constant load below the static breaking load without any load fluctuations such as fatigue, and that it does not occur with low-strength materials such as mild steel; , which occurs in high-strength steel such as high-tensile bolts for construction. It is certain that this delayed fracture phenomenon is caused by hydrogen in the atmosphere, water, aqueous solutions, etc., but the details of the fracture mechanism have not yet been elucidated, and several theories have been proposed. Broadly speaking, there are (1) surface pressure theory, (2) interatomic cohesive force reduction theory, (3) dislocation theory, and (4) lattice deterioration theory, etc., but the general mechanism behind delayed fracture is that hydrogen It is explained that the gas penetrates and diffuses into the steel material and gathers in a specific place, generating a large gas pressure, where cracks occur under the action of an external force, resulting in the destruction of the entire steel material.

0003

[Problems to be Solved by the Invention] However, in the past, baking processing (15
There has been a method of diffusing hydrogen in steel by heating the steel to 0 to 250°C, but this baking method required a long processing time of about 10 hours, resulting in extremely low productivity. Therefore, no effective method for preventing the above-mentioned delayed fracture has been found in the past. Conventionally, the only way to deal with this problem was to increase the safety factor, but this method did not meet the demands for smaller and lighter automobiles to improve fuel efficiency, for example.

On the other hand, by colliding small metal balls (hereinafter referred to as shots) with the workpiece to create minute impressions on the workpiece surface and work-hardening the surface layer, the corrosion resistance of the workpiece can be improved. Shot peening, which improves mechanical properties such as wear resistance, has traditionally been applied to mechanical parts such as gears and springs that are subject to repeated dynamic loads. However, there has never been an example of shot peening applied to steel parts such as bolts that are subject to only static loads.

[0005]

[Means for Solving the Problems] The present invention aims to provide an extremely effective method for preventing the delayed fracture of bolts as described above.
A shot peening treatment is performed using a shot having a diameter smaller than the curvature diameter of the arcuate surface of the groove bottom.

[0006]

[Function] This bolt has a shot peening effect on the surface layer up to the arcuate surface at the bottom of the thread groove, preventing hydrogen from entering and causing delayed fracture.

[0007]

[Example] In order to measure the degree of reinforcement of the high-strength bolt according to the present invention, the present inventors prepared a test piece having a diameter of 6 mm as shown in FIG. 1. This test piece was made of SCM435 steel, which is used as a steel for high-strength bolts, and its chemical composition is shown in Table 1.

[Table 1] This cold-rolled round bar with a diameter of 6 mm was held at 870°C for 30 minutes and normalized by air cooling, then held at 855°C for 30 minutes, quenched by oil cooling, and then heated to 400°C and 440°C for 1 hour. After holding and water-cooling tempering, a groove having a size and shape with a radius of curvature of 0.2 mm and an opening angle of 60 degrees as shown enlarged in FIG. 2 was machined.

FIG. 3 shows a cantilever constant load bending tester used for the delayed fracture test of the above-mentioned test piece, in which 2 is a column supporting the test piece 1 horizontally in a cantilever manner, and 3 is the support column for supporting the test piece 1 horizontally. A moment arm is connected horizontally to the free end of the During the test, 0.1N hydrochloric acid sent from the pump 4 was dropped into the notch of the test piece 1 as shown in FIG. 5 is a receiver for the hydrochloric acid, and 6 is a recovery tank. A weight 7 is suspended from the tip of the moment arm 3. 8 is a timer that measures the time when the weight 7 falls.

FIG. 5 shows a comparison of the fracture strength of test specimens of 400° C. tempered material subjected to shot peening treatment and untreated specimens. In the same figure, 〇 indicates hardness Hv5
50 indicates the strength of a test piece irradiated with shot grains having a diameter of 150 μm for 10 minutes. Similarly, □△ is the result of changing the hardness and size of shot grains as indicated in the figure. × indicates unprocessed results.

Further, FIG. 6 shows the results of a strength test similar to that described above using a 440° C. tempered material as a test piece. As a result, it was found that the delayed fracture life due to shot peening was significantly improved compared to untreated material.

Reinforcement against such delayed fracture is shown in FIG.
This can be achieved by uniformly making shots with a diameter smaller than the radius of curvature (0.2 mm) of the groove bottom shown in Figure 2, all the way to the groove bottom of this test piece. 7 and 8 show the results of measuring the residual stress in the surface layer of the test piece used in FIGS. 5 and 6 by X-ray diffraction method. It is presumed that the effect of shot peening to prevent delayed fracture occurs because the surface layer is provided with compressive residual stress in advance, making it difficult for hydrogen to penetrate.

The above are the test results for the test piece in which the grooves shown in FIG. It is clear that by performing peening treatment using a shot with a diameter smaller than the radius of curvature of the arc-shaped surface, it is possible to achieve equivalent reinforcement against delayed fracture.

[0013]

As described above, according to the present invention, bolts subjected to static loads are significantly strengthened by shot peening treatment, which is beneficial in reducing their size and weight.

[Brief explanation of the drawing]

FIG. 1 is a side view of a test piece according to the present invention.

FIG. 2 is an enlarged sectional view of section A in FIG. 1.

FIG. 3 is a front view of the testing machine.

FIG. 4 is an enlarged view of section A in FIG. 3.

FIG. 5 is a graph showing a comparison of delayed fracture strength of test pieces tempered at 400°C.

FIG. 6 is a graph showing a comparison of delayed fracture strength of test pieces tempered at 440°C.

7 is a graph showing residual stress distribution from the surface of the test piece used in FIG. 5. FIG.

8 is a graph showing residual stress distribution from the surface of the test piece used in FIG. 6. FIG.

FIG. 9 is a side view of the bolt.

[Explanation of symbols]

12 Thread groove 13 bolt 14 Groove bottom

Claims (1)

[Claims] [Claim 1] A bolt in which a predetermined thread groove is formed,
A method for preventing delayed fracture of static load bolts characterized by performing shot peening treatment with a shot having a diameter smaller than the curvature diameter of the arcuate surface of the groove bottom.