Processing method for corrosion-resistant austenitic stainless steel

Abstract

To provide a method for working corrosion-resistant austenitic stainless steel which can impart sufficient compressive residual stress.SOLUTION: A method for working corrosion-resistant austenitic stainless steel includes the steps of: preparing a material W to be worked composed of austenitic stainless steel; and imparting compressive residual stress to a surface layer Wa of the material W to be worked without plastically working the surface layer Wa.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a method for processing corrosion-resistant austenitic stainless steel. [Background technology]

[0002] One method for improving the corrosion resistance of austenitic stainless steel is to impart compressive residual stress through shot peening or burnishing. In shot peening and burnishing, processing-induced martensitic transformation occurs due to media impact and tool indentation, causing the austenite phase to transform into the less corrosion-resistant martensite phase. Therefore, the corrosion resistance is relatively reduced.

[0003] Patent Document 1 discloses a method for generating compressive residual stress on at least the surface of an austenitic stainless steel by plastic deformation. In this method, plastic deformation is performed at a temperature higher than the upper limit temperature at which strain-induced martensitic transformation occurs. This makes it possible to impart compressive residual stress to the surface without the formation of a martensite phase. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 53-104520 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In the processing method described in Patent Document 1, plastic deformation is performed at a high temperature, making it impossible to impart sufficient compressive residual stress.

[0006] Therefore, the object of this disclosure is to provide a processing method for corrosion-resistant austenitic stainless steel that can impart sufficient compressive residual stress. [Means for solving the problem]

[0007] A method for processing corrosion-resistant austenitic stainless steel according to one aspect of the present disclosure includes the steps of preparing a workpiece made of austenitic stainless steel and applying compressive residual stress to the surface layer of the workpiece without plastic deformation of the surface layer.

[0008] In processes involving plastic deformation, such as shot peening and burnishing, large compressive residual stress is imparted, but work-induced martensitic transformation occurs. In a method for processing corrosion-resistant austenitic stainless steel according to one aspect of this disclosure, the surface layer of the workpiece is not plastically deformed. Therefore, compressive residual stress can be imparted without causing work-induced martensitic transformation in the surface layer of the workpiece. Since the martensite phase, which has poor corrosion resistance, is suppressed, sufficient compressive residual stress can be imparted while maintaining corrosion resistance.

[0009] The process of imparting stress may be carried out using shock waves generated by at least one of laser ablation and cavitation. In this case, although the plastic deformation caused by the shock waves generates plastic strain within the crystal grains, it is not a plastic processing, and therefore no deformation or refinement of the crystal grains occurs. Thus, sufficient compressive residual stress can be imparted while maintaining corrosion resistance.

[0010] The process involves increasing the power density of the laser light to 1 GW / cm². 2 More than 20GW / cm 2 Laser ablation may be generated as follows. In this case, the power density is 1 GW / cm². 2 By doing so, laser ablation can be reliably generated. Power density of 20 GW / cm² 2 By doing the following, surface damage to the workpiece is suppressed.

[0011] The application process may involve generating laser ablation by setting the pulse width of the laser beam to 150 fsec or more and 30 nsec or less. In this case, laser ablation can be reliably generated.

[0012] The applying step may be performed with the workpiece in a cooled state. In this case, since the temperature rise of the workpiece is suppressed, a decrease in the applied compressive residual stress is suppressed.

[0013] The applying step may be performed with the workpiece disposed in a liquid. In this case, the workpiece can be easily cooled.

[0014] The applying step may be performed such that the change amount of the martensite phase in the surface layer is ±10% or less. In this case, since the change amount of the martensite phase, which is inferior in corrosion resistance, is suppressed, the corrosion resistance can be maintained.

[0015] The compressive residual stress applied by the applying step may be 500 MPa or more. In this case, a sufficient compressive residual stress can be applied.

Advantages of the Invention

[0016] According to the present disclosure, it is possible to provide a method for processing a corrosion-resistant austenitic stainless steel capable of applying a sufficient compressive residual stress.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a configuration diagram showing a laser irradiation device. [Figure 2] FIG. 2 is a configuration diagram showing an air cavitation device. [Figure 3] FIG. 3 is a configuration diagram showing a liquid cavitation device. [Figure 4] FIG. 4 is a SEM image obtained by a field emission scanning electron microscope (FE-SEM). [Figure 5] FIG. 5 is a phase map obtained from an analysis result based on the electron backscatter diffraction (EBSD) method. [Figure 6] FIG. 6 is a graph showing the residual stress distribution.

Embodiments for Carrying Out the Invention

[0018] Embodiments of the present invention will be described in detail below with reference to the attached drawings. In this description, the same reference numerals will be used for the same element or element having the same function, and redundant explanations will be omitted.

[0019] The processing method for corrosion-resistant austenitic stainless steel according to the embodiment is a method for producing corrosion-resistant austenitic stainless steel with compressive residual stress applied to the surface layer of the austenitic stainless steel without plastic deformation of the surface layer. The processing method according to the embodiment includes a preparation step and a residual stress application step.

[0020] The preparation process involves preparing the workpiece W (see Figure 1), which is made of austenitic stainless steel. An example of the austenitic stainless steel used as the workpiece W is SUS304 (JIS standard). SUS304 is a relatively inexpensive steel among austenitic stainless steels.

[0021] The residual stress application process is a process of applying compressive residual stress to the surface layer Wa (see Figure 1) of the workpiece W without plastic deformation of the surface layer Wa. Here, the surface layer Wa is a region with a depth from the surface of the workpiece W that is, for example, 100 μm or less. Plastic deformation is a process that causes deformation or refinement of crystal grains. The residual stress application process is carried out so that the change in the martensite phase in the surface layer Wa is ±10% or less. The compressive residual stress applied to the surface layer Wa by the residual stress application process is 500 MPa or more, and may be 550 MPa or more.

[0022] The residual stress imparting process is carried out using shock waves generated by at least one of laser ablation and cavitation. Laser peening and cavitation peening are methods of imparting compressive residual stress to the interior of a metal material, similar to shot peening and burnishing. In shot peening and burnishing, the media or tool is brought into physical contact with the surface of the metal material, whereas in laser peening and cavitation peening, there is no such physical contact.

[0023] In laser peening and cavitation peening, shock waves are used to plastically deform the workpiece W without changing its crystalline state. Since plastic deformation by shock waves is not plastic processing, no deformation or refinement of crystal grains occurs. Plastic deformation by shock waves generates plastic strain within the crystal grains. Therefore, compressive residual stress can be applied without inducing martensitic transformation, while the material structure remains in the austenite phase.

[0024] The residual stress application process is carried out while the workpiece W is cooled. Examples of cooling methods include water cooling and air cooling. Cooling may also be carried out using liquids other than water and gases other than air. The residual stress application process is carried out, for example, while the workpiece W is placed in a liquid. The residual stress application process is carried out, for example, at room temperature.

[0025] Figure 1 is a configuration diagram showing a laser irradiation device used in the residual stress application process. As shown in Figure 1, the laser irradiation device 10 comprises a laser oscillator 11, reflective mirrors 12 and 13, a focusing lens 14, a water tank 15, a holding unit 16, and a control device 17. The laser oscillator 11 is a device that emits laser light L. The reflective mirrors 12 and 13 transmit the laser light L emitted by the laser oscillator 11 to the focusing lens 14. The focusing lens 14 concentrates the laser light L at a high density at the processing position of the workpiece W. The water tank 15 is filled with a transparent liquid 18 such as water. The holding unit 16 holds the workpiece W and places the workpiece W in the water tank 15. The holding unit 16 is an actuator or a robot.

[0026] The laser irradiation device 10 is controlled by a control device 17. The control device 17 is configured as a motion controller, such as a PLC (Programmable Logic Controller) or a DSP (Digital Signal Processor). The control device 17 may also be configured as a computer system including a processor such as a CPU (Central Processing Unit), memory such as RAM (Random Access Memory) and ROM (Read Only Memory), input / output devices such as a touch panel, mouse, keyboard, and display, and communication devices such as a network card. The control device 17 realizes its functions by operating each piece of hardware under the control of the processor based on a computer program stored in memory.

[0027] When performing the residual stress application process using the laser irradiation device 10, first, the workpiece W is placed in the holding unit 16. Next, the workpiece W is moved into the water tank 15 by the holding unit 16, and the workpiece W is placed in the liquid 18. Then, with the workpiece W cooled by the liquid 18, the workpiece W is irradiated with laser light L. The laser light L is a short-pulse laser light. The pulse width of the laser light L is 150 fsec or more and 30 nsec or less. The pulse width of the laser light L may be 4 nsec or more and 10 nsec or less.

[0028] The laser beam L is emitted by the laser oscillator 11 and then transmitted to the focusing lens 14 via an optical system consisting of reflective mirrors 12 and 13. The laser beam L is focused to high density by the focusing lens 14 and irradiated onto the surface of the workpiece W through the liquid 18. The power density of the laser beam L is 1 GW / cm². 2 More than 20GW / cm 2 The following settings are applied: The power density of the laser beam L is 3 GW / cm². 2 More than 15GW / cm 2 The following settings may also be used.

[0029] At the surface layer Wa of the workpiece W corresponding to the irradiation point of the laser beam L, a peening effect occurs due to laser peening as follows. First, when the laser beam L is irradiated onto the surface of the workpiece W, laser ablation occurs on the surface of the workpiece W, generating plasma. In the atmosphere, the material at the irradiation point vaporizes. Since the irradiation point on the workpiece W is covered with liquid 18, the expansion of the plasma is suppressed. As a result, the plasma becomes high pressure, and a shock wave is generated by the pressure of the plasma. As the shock wave propagates, a plastic deformation region is created inside the workpiece W. In the plastic deformation region, compressive residual stress is generated due to constraint from the undeformed area. As described above, plastic deformation due to the shock wave is not plastic processing, so no deformation or refinement of crystal grains occurs.

[0030] The irradiation of the laser beam L is performed in accordance with the operation of the holding unit 16, shifting the irradiation point on the workpiece W. Each time the laser beam L is irradiated, the holding unit 16 moves the workpiece W and shifts the irradiation point on the workpiece W. This ensures that an area of ​​the workpiece W is irradiated with the laser beam L.

[0031] Figure 2 is a configuration diagram showing an air cavitation apparatus used in a residual stress application process. As shown in Figure 2, the air cavitation apparatus 20 comprises a first nozzle 21, a second nozzle 22, and a control device (not shown). The second nozzle 22 has a smaller diameter than the first nozzle 21 and is positioned inside the first nozzle 21. The first nozzle 21 and the second nozzle 22 are positioned on the workpiece W such that their nozzle tips face the surface of the workpiece W. The control device controls the air cavitation apparatus 20. The control device has a configuration similar to, for example, the control device 17 of the laser irradiation apparatus 10.

[0032] The first nozzle 21 sprays a liquid 23, such as water, onto the surface of the workpiece W at a low speed. The second nozzle 22 sprays a liquid 24, such as water, onto the surface of the workpiece W at a high speed. Cavitation bubbles are generated at the tip of the second nozzle 22. The cavitation bubbles grow in the shear layer 25 between the low-speed jet of liquid 23 and the high-speed jet of liquid 24. By spraying such a cavitation jet with bubbles onto the surface of the workpiece W, shock waves generated when the cavitation bubbles collapse are transmitted to the workpiece W. As a result, a peening effect occurs in the surface layer Wa of the workpiece W. That is, the crystal structure undergoes a high-density transition, and compressive residual stress is imparted. In cavitation peening, the generation, growth, and collapse of bubbles greatly affect the peening effect. The back surface of the workpiece W, where liquids 23 and 24 are not sprayed, may be air-cooled.

[0033] Figure 3 is a configuration diagram showing a liquid cavitation apparatus used in a residual stress application process. As shown in Figure 3, the liquid cavitation apparatus 30 comprises a water tank 31, a nozzle 32, and a control device (not shown). The water tank 31 is filled with a liquid 33 such as water. The workpiece W is placed inside the water tank 31. The tip of the nozzle 32 is positioned inside the water tank 31 and faces the surface of the workpiece W. The control device controls the liquid cavitation apparatus 30. The control device has a configuration similar to, for example, the control device 17 of the laser irradiation apparatus 10.

[0034] The nozzle 32 sprays a liquid 34, such as water, at high speed onto the surface of the workpiece W. Cavitation bubbles are generated at the tip of the nozzle 32. The cavitation bubbles grow in the shear layer 35 between the high-speed jet of liquid 34 and liquid 33. By spraying such a cavitation jet with bubbles onto the surface of the workpiece W, shock waves generated when the cavitation bubbles collapse are transmitted to the workpiece W. As a result, a peening effect occurs on the surface layer Wa of the workpiece W.

[0035] As described above, in the processing method according to the embodiment, the surface layer Wa of the workpiece W is not plastically processed. Therefore, compressive residual stress can be applied without causing processing-induced martensitic transformation in the surface layer Wa of the workpiece W. Since the martensite phase with poor corrosion resistance is suppressed, sufficient compressive residual stress can be applied while maintaining the corrosion resistance.

[0036] When steel is used in a hydrogen bomb explosion area, hydrogen embrittlement occurs where hydrogen penetrates into the steel. In the martensite phase, the dislocation density is large and hydrogen embrittlement is likely to occur. In the austenite phase, the dislocation density is small and hydrogen embrittlement is less likely to occur. For example, when SUS304 is processed as the workpiece W by the processing method according to the embodiment and residual stress is applied without deteriorating hydrogen brittleness, a steel material having corrosion resistance equivalent to that of SUS316 (JIS standard) can be obtained.

[0037] As described above, in the processing method described in Patent Document 1, it is necessary to perform processing at a temperature higher than the upper limit temperature at which strain-induced martensitic transformation occurs. Therefore, in addition to the problem that sufficient compressive residual stress cannot be applied, there are problems such as the heating taking a long time and the residual stress varying due to uneven heating. In contrast, since the residual stress application step according to the present embodiment is carried out at room temperature, the above problems can be solved.

[0038] The residual stress application step is carried out by utilizing a shock wave generated by at least one of laser ablation and cavitation. Although plastic deformation by the shock wave causes plastic strain inside the crystal grains, it is not plastic processing, so deformation and refinement of the crystal grains do not occur. Therefore, sufficient compressive residual stress can be applied to the surface layer Wa of the workpiece W while maintaining the corrosion resistance.

[0039] The residual stress application step generates laser ablation by setting the power density of the laser beam L to be 1 GW / cm 2 or more and 20 GW / cm 2 or less. The power density is 1 GW / cm 2By doing so, laser ablation can be reliably generated. Power density of 20 GW / cm² 2 By doing the following, surface damage to the workpiece W is suppressed. The power density of the laser beam L is 3 GW / cm². 2 More than 15GW / cm 2 The following is also acceptable: Power density of 3 GW / cm² 2 By doing so, laser ablation can be generated more reliably. Power density of 15 GW / cm² 2 By doing the following, surface damage to the workpiece W is further suppressed.

[0040] The residual stress application process generates laser ablation by setting the pulse width of the laser beam to 150 fsec or more and 30 nsec or less. This ensures reliable laser ablation. The pulse width of the laser beam L may be 4 nsec or more and 10 nsec or less. This allows for even more reliable laser ablation.

[0041] The residual stress application process is performed with the workpiece W cooled. The residual stress application process using the laser irradiation device 10 is performed with the workpiece W cooled by the liquid 18. The residual stress application process using the air cavitation device 20 is performed with the workpiece W cooled by at least the liquids 23 and 24. The residual stress application process using the liquid cavitation device 30 is performed with the workpiece W cooled by the liquids 33 and 34. Therefore, the temperature rise of the workpiece W is suppressed. The higher the temperature of the workpiece W, the lower the compressive residual stress applied. Since the temperature rise of the workpiece W is suppressed, the decrease in the applied compressive residual stress is suppressed.

[0042] The residual stress application process using the laser irradiation device 10 is performed with the workpiece W placed in the liquid 18. The residual stress application process using the liquid cavitation device 30 is performed with the workpiece W placed in the liquid 33. This allows the workpiece W to be easily cooled.

[0043] The residual stress application process is carried out so that the change in the martensite phase in the surface layer Wa is ±10% or less. By suppressing the change in the martensite phase, which has poor corrosion resistance, the corrosion resistance of the workpiece W can be maintained.

[0044] The compressive residual stress imparted by the residual stress imparting process is 500 MPa or higher. This allows sufficient compressive residual stress to be imparted to the workpiece W.

[0045] The present invention is not necessarily limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention.

[0046] The following describes some examples.

[0047] As an example, a sample was prepared in which compressive residual stress was imparted by laser peening. Specifically, first, a sample made of SUS304 was prepared as the workpiece and placed in a water bath. A laser irradiation device was used with a spot diameter of 0.7 mm and a power density of 4.2 GW / cm². 2 The pulse energy is 100 mJ, and the irradiation density is 0.3 pulses / mm². 2 Laser peening was performed as a treatment.

[0048] As a comparative example, samples were prepared in which compressive residual stress was imparted by shot peening instead of laser peening. Shot peening was performed using a medium (RCW06PM) consisting of amorphous round metal spheres, with a jet pressure of 0.2 MPa, a jet volume of 9.0 kg / min, a coverage of 300% or more, and an arc height of 0.361 mmA.

[0049] (Measurement of residual austenite volume) To verify the occurrence of induced martensitic transformation, the volume of retained austenite on the surface of the sample was measured for the sample used in the example, the sample used in the comparative example, and the untreated sample. The measurement was performed using the cosα method with a residual stress measuring device μ-X360 manufactured by PulseTech Industries Co., Ltd. A Cr tube was used, with an irradiation diameter of φ1.0 mm, a collimator diameter of φ1.0 mm, and a measurement angle of 0 degrees. The measurement results are shown in Table 1. [Table 1]

[0050] As shown in Table 1, in the comparative example after shot peening, approximately 40% induced martensitic transformation occurred compared to the untreated state. In contrast, in the example after laser peening, no induced martensitic transformation occurred compared to the untreated state. The slight increase in the austenite volume in the example compared to the austenite volume in the untreated state is thought to be due to measurement error.

[0051] (Tissue observation) To visually verify the occurrence of induced martensitic transformation, the samples used in the examples were subjected to microscopic observation using a field emission scanning electron microscope (FE-SEM). Specifically, the samples were cut, and their surfaces and cross-sections were observed using a Schottky field emission scanning electron microscope JSM-7200F manufactured by JEOL Ltd.

[0052] Figure 4 shows an SEM image obtained using FE-SEM. To the left of the dotted line in Figure 4, the cross-section of the laser-peened surface and the tissue beneath it is shown. To the right of the dotted line in Figure 4, the cross-section of the untreated surface and the tissue beneath it is shown. As shown in Figure 4, the SEM image confirms that there is no difference between the laser-peened area and the untreated area.

[0053] Figure 5 is a phase map obtained from the analysis results based on the EBSD method. In Figure 5, the FCC structure is shown in light colors, while cementite, BCC structure, and chromium carbide are shown in dark colors. Here, the FCC structure is the austenite phase, and the BCC structure is the martensite phase. As shown in Figure 5, in both the laser-peened and untreated areas, the majority of the analysis range is occupied by the austenite phase. This indicates that induced martensitic transformation does not occur even after laser peening. Furthermore, it can be confirmed that there is no difference in the size of the crystal grains 41 and the state of the crystal grain boundaries 42, shown by the black lines, between the laser-peened and untreated areas.

[0054] (Measurement of residual stress) Residual stress measurements were performed on the surface of the samples used in the examples. The measurements were performed using the cosα method with a residual stress measuring device μ-X360 manufactured by PulseTech Industries Co., Ltd. A Cr tube was used, with an irradiation diameter of φ1.0 mm, a collimator diameter of φ1.0 mm, and a measurement angle of 35 degrees.

[0055] Figure 6 is a graph showing the residual stress distribution. The horizontal axis of Figure 6 represents the depth from the surface (μm), and the vertical axis represents the residual stress (MPa). The compression side is shown as a negative value, and the tension side is shown as a positive value. As shown in Figure 6, residual stress of -550 MPa or less, i.e., compressive residual stress of 550 MPa or more, occurs near the surface. This value is more than 100 MPa greater on the compression side than the residual stress imparted by the processing method described in Patent Document 1, and is considered to have a strong effect on stress corrosion cracking, thereby suppressing the new occurrence of stress corrosion cracking. [Explanation of Symbols]

[0056] 18,23,24,33,34...liquid, L...laser light, W...workpiece material, Wa...surface layer.

Claims

1. A process for preparing a workpiece made of austenitic stainless steel, The process includes a step of applying compressive residual stress to the surface layer of the workpiece such that, without performing plastic deformation that causes deformation or refinement of the crystal grains, plastic strain is generated within the crystal grains, and the amount of change in the martensite phase on the surface layer of the workpiece is ±10% or less. Processing methods for corrosion-resistant austenitic stainless steel.

2. The aforementioned imparting process is carried out using shock waves generated by at least one of laser ablation and cavitation. A method for processing corrosion-resistant austenitic stainless steel according to claim 1.

3. The aforementioned process involves increasing the power density of the laser light to 1 GW / cm². ２ More than 20GW / cm ２ The laser ablation is generated as follows: A method for processing corrosion-resistant austenitic stainless steel according to claim 2.

4. The aforementioned step involves generating the laser ablation by setting the pulse width of the laser light to 150 fsec or more and 30 nsec or less. A method for processing corrosion-resistant austenitic stainless steel according to claim 2 or 3.

5. The aforementioned step of applying the coating is performed while the workpiece is cooled. A method for processing corrosion-resistant austenitic stainless steel according to any one of claims 1 to 4.

6. The aforementioned imparting step is performed with the workpiece placed in the liquid. A method for processing corrosion-resistant austenitic stainless steel according to any one of claims 1 to 4.

7. The compressive residual stress imparted by the aforementioned imparting process is 500 MPa or more. A method for processing corrosion-resistant austenitic stainless steel according to any one of claims 1 to 6.

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