JPH04362124A - Residual stress improvement by water jet peening - Google Patents

Abstract

PURPOSE:To execute a residual stress improvement by striking the surface of a metallic material by the impact of the cavitation generated by a water jet under water or the underwater environment created in the atm. air, thereby imparting the residual stresses of surface compression to this metallic material. CONSTITUTION:The high-pressure water fed via a hose 2 and a conduit 15 from a high-pressure pump 1 is ejected into the water from a horn type nozzle 3 toward the metallic material 4 placed in the water in a tank 16 to positively generate the cavitation. The metallic material 4 is striken by the impact force generated by the collapse of the air bubbles to locally generate the strain above the yield point, by which the residual stresses of the surface compression are imparted to the metallic material. The nozzle 3 is moved along the material 4 by a moving mechanism 44. In another embodiment, the metallic material is placed in the atm. air and high-pressure water flow is ejected from the central ejection port of the nozzle into the local underwater environment formed by the water flow ejected from the ejection port in the outer peripheral part of the nozzle to generate the cavitation in same way, by which the residual stress improvement is similarly executed.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for improving residual stress in metal materials using water jet peening to improve tensile residual stress existing in metal materials and preventing stress corrosion cracking, and a method suitable for use therein. related to water jet nozzles.

0002

2. Description of the Related Art It is generally known that when metal materials such as austenitic stainless steel are placed in high-temperature water, stress corrosion cracking occurs at or near welds. Stress corrosion cracking is caused by material, stress,
It is said to occur under conditions where environmental factors are combined. The material factor is sensitization due to the precipitation of Cr carbide at grain boundaries and the formation of a Cr-depleted layer with poor corrosion resistance around it, and the stress factor is tensile residual stress that remains inside the material due to welding and processing. Examples of environmental factors include the amount of dissolved oxygen in high-temperature water. Since stress corrosion cracking occurs under conditions where these three factors are superimposed, it is possible to prevent it by removing one of these three factors.

[0003] As a means for improving residual stress, Japanese Patent Application Laid-Open No. 62-63
The prior art described in Japanese Patent No. 614 involves inserting a high-pressure water shot peening device having a rotating nozzle part that sprays a high-pressure liquid jet into the inside of a tube of a heat exchanger or the like that is an object to improve residual stress. A high-pressure liquid jet is ejected from the nozzle, and the axial dynamic pressure energy of the jet itself (axial dynamic pressure energy of the jet flow) peens the inner surface of the tube, thereby compressing the tensile residual stress that originally existed in the tube. This is converted into residual stress.

0004

[Problems to be Solved by the Invention] The above-mentioned prior art is an effective method for improving the residual stress on the inner surface of tubes of heat exchangers, etc., but this method does not allow the liquid jet ejected from the nozzle in the atmosphere to be improved. The impact energy is used to peen the surface of a metal member, and this technology utilizes the axial dynamic pressure of a water jet in the atmosphere, relying only on the ejection pressure of the water jet. For example, when using this technology underwater, the axial dynamic pressure of the jet will attenuate due to the resistance of the surrounding water, the rapid diffusion of the water jet because it is in phase with the surrounding water, etc. It is difficult to effectively obtain a peening effect, and in order to obtain a shaft dynamic pressure equivalent to that of an atmospheric jet, a water jet is required at an extremely high pressure, which is disadvantageous in terms of the cost of the pump and related equipment.

An object of the present invention is to provide a residual stress improvement method that effectively imparts compressive residual stress by peening the surface of a metal material using a water jet in water or in an atmosphere equivalent to water. .

[0006]

[Means for Solving the Problems] In order to achieve the above object, the method for improving residual stress of the present invention actively causes cavitation phenomenon induced by the ejection of a water jet in water or in an environment equivalent to water. This method actively generates cavitation bubbles, and uses the impact energy generated when the cavitation bubbles collide with the surface of the metal material and collapse to peen the surface of the metal material to impart surface compressive residual stress to the metal material.

[0007] In order to efficiently produce the cavitation phenomenon, the metal material is immersed in water, and a water jet is ejected from a nozzle toward the metal material under water to reduce the pressure difference between the surrounding water and the ejected water stream. and causes cavitation phenomenon due to shearing action, etc.

In the case of metal materials placed in the atmosphere, in order to locally create an environment equivalent to that in water in the atmosphere and cause cavitation, a peening jet is placed in the center of the nozzle. Then, a water jet is provided on the outer periphery to surround the peening jet, and by ejecting water from the water jet on the outer periphery, a water jet is created in a virtual aquarium around the peening jet in the center. The water flow locally creates an underwater environment in which water is present in the water flow, and a high-pressure water jet is ejected from the peening spout at the center into the water flow at a higher speed than the water flow, so that the surrounding water flow is mixed with the surrounding water flow. The cavitation phenomenon occurs due to the speed difference and pressure difference.

[0009]

[Operation] According to the method for improving residual stress of the present invention described above, cavitation bubbles generated by the cavitation phenomenon caused by the water jet jet grow, collide with the surface of the metal material, and collapse. At this time, the cavitation bubble collapses at an extremely high speed, and an extremely high water hammer pressure is generated at the core where the bubble has collapsed. The water hammer pressure caused by the collapse of the cavitation bubbles causes a localized high pressure distribution and strain greater than yield to occur on the surface of the metal material, and the surface of the metal material is stretched, plastically deformed, and laterally expanded. At this time, a compressed elastic strain region exists in the metal material around the stretched surface,
The stretched surface is pushed back by the surrounding elastic strain region, creating compressive residual stress. Note that the axial dynamic pressure impact force of the water jet itself also contributes to the above action.

By improving residual stress to compressive residual stress in this manner, stress corrosion cracking can be prevented from occurring.

[0011]

[Embodiment] An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. FIG. 1 is a diagram of the water jet peening equipment used in this example, and shows a situation in which water jet peening of a metal material is performed underwater. The water stream is pressurized by a high-pressure pump 1, passes through a hose 2 and a water conduit 15, and is ejected from a nozzle 3 onto the surface of the metal material 4 as a water jet. The metal material 4 is immersed in water in a tank 16, and the nozzle 3 spouts a water jet into the water. At this time, the nozzle is moved by the nozzle moving mechanism 14 while maintaining a constant distance from the metal material 4, and its moving speed is controlled to an optimum value by the high-pressure pump 1 and the control device. The nozzle 3 is a horn type nozzle (see Japanese Patent Application Laid-Open No. 60-18554) as shown in FIG.
The water flow passing through this increases the flow velocity at the orifice portion 5, and is ejected into the water from the horn portion 6 tapered toward the axis at the downstream of the orifice portion 5. At this time, the speed and pressure of the water jet change greatly at the orifice section 5 and the horn section 6, inducing a cavitation phenomenon and generating a large number of cavitation bubbles 7. The water hammer pressure generated when the cavitation bubbles 7 collide with the surface of the metal material 4 and collapse can peen the surface of the metal material 4 and impart compressive residual stress. Note that the dynamic pressure impact force of the water jet itself also contributes to the above action.

The graph shown in FIG. 3 shows the residual stress as a result of water jet peening in water, in which a 700 kg load was applied to a SUS304 plate metal test piece immersed in water at a depth of 0.3 m using the horn type nozzle. /c
This figure shows the results of peening the test piece by ejecting a water jet of m2 to generate cavitation bubbles. In the graph, d is the nozzle diameter, L is the distance from the nozzle to the test piece, and the vertical axis of the graph is the residual stress value obtained as a result of water jet peening (compressive stress is shown as a negative value). ). As shown in FIG. 3, the maximum residual stress improvement result (approximately 50 kg/mm2 of compressive residual stress) was achieved at L/d=20 to 30.

For comparison, an experiment was conducted in which water jet peening was applied to a SUS304 specimen in the atmosphere using a conventional jet nozzle. In this case, since it is in the atmosphere, the effect of water hammer pressure due to the collapse of cavitation bubbles cannot be expected, so it is necessary to increase the axial dynamic pressure energy of the water jet, and the injection pressure is increased to 3000 k.
It was set at a high value of g/cm2. In addition, L/d is set to 300 to avoid erosion of the test piece.
was set. 5.2 at jet pressure 3000kg/cm2
A residual compressive stress of kg/mm2 was obtained. This residual compressive stress value is lower than that in the case of water jet peening using underwater cavitation according to the embodiment of the present invention. This shows that water jet peening using cavitation according to the present invention is an effective technique for improving residual stress.

FIGS. 4 and 5 are diagrams showing equipment and nozzles used therein according to an embodiment of the present invention when water jet peening is performed in the atmosphere to improve residual stress. The water flow is pressurized by an ultra-high pressure pump 17,
It passes through the hose 2 and the water conduit 15 and is sprayed onto the surface of the metal material 4 from the nozzle 8 in the atmosphere. At this time, the nozzle 8 is moved by the nozzle moving mechanism 14 while maintaining a constant distance from the metal material 4, and its moving speed is controlled to an optimum value by the controller together with the ultra-high pressure pump 17. Above nozzle 8
As shown in FIG. 5, the nozzle has a peening nozzle 9 at its center, and a plurality of water jet nozzles 10 surrounding the peening nozzle 9 at its outer periphery. By ejecting the water stream 11, an underwater environment is created locally around the peening spout 9, as if water exists in a virtual aquarium, and this water stream 1
1, a high-pressure water jet 12 is ejected from the peening nozzle 9 at a higher speed than the water stream 11. Since the peening water jet 12 has a smaller diameter at the outlet, the peening water jet 12 vibrates resonantly and generates cavitation bubbles 13. Similar to water jet peening, compressive residual stress can be applied to the surface of the metal material 4 by peening.

In the nozzle shown in FIG. 5, the plurality of water jet ports 10 surrounding the peening port 9 at the center are combined into a ring-shaped jet port surrounding the peening port 9. You may.

[0016]

According to the present invention, a water jet is ejected to generate cavitation, and the impact energy generated by the collapse of cavitation bubbles peens the metal material to generate surface compressive residual stress, thereby preventing stress corrosion cracking. It can be prevented.

[0017] Furthermore, since the cavitation caused by water jet ejection is effectively used, the attenuation of energy is smaller than in the conventional residual stress improvement method that uses the impact energy of the water jet itself, and energy can be used effectively.

[0018] Furthermore, the present invention makes it possible to perform peening to improve residual stress using only a water jet using water, and does not use shots unlike conventional shot peening, so that it does not adversely affect the environment. can be prevented.

[Brief explanation of drawings]

FIG. 1 is a diagram showing water jet peening of a metal material according to an embodiment of the present invention when performed underwater.

[
Figure 2: Cross-sectional view of a horn-shaped nozzle that generates cavitation by water jet in the same example

[Figure 3
] Graph showing residual stress after peening based on the above example

FIG. 4 is a diagram showing water jet peening of a metal material according to an embodiment of the present invention when performed in the atmosphere.

5 shows the nozzle used in FIG. 4, (a) is a cross-sectional view,
(b) is its bottom view.

[Explanation of symbols]

1...High pressure pump
2...Hose 3...Horn type nozzle
4... Metal material 5... Orifice part
6...Horn portion 7...Cavitation bubbles 8...Nozzle 9...Peening spout 10...Outer circumference spout 11...Water flow
12...Water jet 13...Cavitation bubbles 14...Nozzle moving mechanism 15...Water pipe
16...Tank 17...Ultra high pressure pump

Claims (6)

[Claims] Claim 1: A high-speed water jet ejected from a nozzle is caused to collide with the surface of a metal material in an underwater environment, and the high-speed water jet ejected from the nozzle generates cavitation on the surface of the metal material. A method for improving residual stress by water jet peening, characterized in that surface compressive residual stress is generated in the metal material by generating a localized high pressure distribution and strain exceeding the yield point. 2. A high-speed water jet ejected from a nozzle in water collides with the surface of a metal material immersed in water, and cavitation is generated by the high-speed water jet ejected from the nozzle. A method for improving residual stress by water jet peening, the method comprising generating surface compressive residual stress in the metal material by generating a localized high pressure distribution and strain at or above the yield point on the surface of the metal material. 3. The method for improving residual stress by water jet peening according to claim 2, wherein the nozzle is a horn-type nozzle having an orifice portion and a horn portion having a tapered end that extends downstream of the orifice portion. 4. Using a nozzle having a central nozzle for spouting a peening water stream and an outer circumferential nozzle surrounding it, the nozzle is applied to the surface of a metal material placed in the atmosphere in the atmosphere. A local underwater environment is formed by a water stream ejected from an outer circumferential outlet, and cavitation is generated by ejecting a peening water stream faster than the water stream from a central outlet of the nozzle.
A method for improving residual stress by water jet peening, characterized in that a surface compressive residual stress is generated in the metal material by generating a local high pressure distribution and a strain higher than the yield point on the surface of the metal material. 5. A water jet characterized by having a central spout that spouts a high-speed peening water stream, and an outer peripheral spout that spouts a water stream for forming a local underwater environment surrounding the central spout. Water jet nozzle for improving residual stress through peening. 6. The water jet nozzle for improving residual stress by water jet peening according to claim 5, wherein the outer peripheral jet port of the nozzle comprises a plurality of jet ports arranged so as to surround the central jet port of the nozzle. .