Process for producing a corrosion-resistant austenitic stainless steel

Abstract

A process for producing a corrosion-resistant austenitic stainless steel, comprising: preparing a workpiece made of austenitic stainless steel; and applying a compressive residual stress to a surface layer of the workpiece without subjecting the surface layer to plastic machining.

Description

Title of the invention: Process for producing a corrosion-resistant austenitic stainless steel. Technical field

[0001] The present exposition relates to a process for producing a corrosion-resistant austenitic stainless steel. Background

[0002] As a method for improving the corrosion resistance of an austenitic stainless steel, a method for applying a compressive residual stress by shot peening or burnishing is known. During shot peening or burnishing, a deformation-induced martensitic transformation is generated by collision with a medium or tool pressure, and an austenitic phase is transformed into a martensitic phase with poor corrosion resistance. Consequently, the corrosion resistance is relatively reduced.

[0003] Japanese patent application left open for public inspection No. 1978-104520 discloses a process in which the surface of an austenitic stainless steel is plastically machined to generate at least a residual compressive stress on the surface. In this process, the plastic machining is carried out at a temperature above the upper limiting temperature at which a strain-induced martensitic transformation occurs. As a result, a residual compressive stress can be applied to the surface without generating a martensitic phase. Summary

[0004] In the above process, since plastic machining is carried out at a high temperature, a sufficient compressive residual stress cannot be applied.

[0005] Therefore, one aim of the present exposition is to make available a process for producing a corrosion-resistant austenitic stainless steel capable of applying sufficient compressive residual stress.

[0006] A process for producing a corrosion-resistant austenitic stainless steel according to one aspect of this disclosure comprises the following steps (1) and (2).

[0007] (1) Preparation of a workpiece made of austenitic stainless steel.

[0008] (2) Application of a compressive residual stress to the surface layer of the workpiece without subjecting the surface layer to plastic machining.

[0009] In the manufacturing process involving plastic machining such as shot peening and burnishing, a high compressive residual stress is applied, but a martensitic transformation is generated by deformation. In the process for producing a corrosion-resistant austenitic stainless steel according to one aspect of this exposition, the surface layer of the workpiece is not subjected to plastic machining. Consequently, a compressive residual stress can be applied without generating a deformation-induced martensitic transformation in the workpiece surface layer. Because the martensitic phase with poor corrosion resistance is eliminated, sufficient compressive residual stress can be applied while maintaining corrosion resistance.

[0010] The application can be carried out by means of a shock wave generated by at least one of the following methods: laser ablation and cavitation. In this case, the plastic deformation due to the shock wave causes plastic deformation within the crystalline grains, but does not cause deformation or refining of the crystalline grains because it is not plastic machining. Therefore, sufficient compressive residual stress can be applied while corrosion resistance is maintained.

[0011] The application may include generating laser ablation by establishing a laser light power density of 1 GW / cm² or more and 20 GW / cm² or less. In this case, by establishing the power density of 1 GW / cm² or more, laser ablation can be reliably generated. By establishing the power density of 20 GW / cm² or less, surface damage to the workpiece is eliminated.

[0012] The application may include generating laser ablation by establishing a laser light pulse width of 150 fs or more and 30 ns or less. In this case, laser ablation can be reliably generated.

[0013] The application can be carried out while the workpiece is being cooled. In this case, since an increase in the temperature of the workpiece is eliminated, a decrease in the applied compressive residual stress is eliminated.

[0014] The application can be carried out while the workpiece is placed in a liquid. In this case, the workpiece can be easily cooled.

[0015] The application can be carried out in such a way that the amount of martensitic phase change in the surface layer is ±10% or less. In this case, since the amount of martensitic phase change with poor corrosion resistance is eliminated, corrosion resistance can be maintained.

[0016] The compressive residual stress applied during application can be 500 MPa or more. In this case, a sufficient compressive residual stress can be applied.

[0017] The application may include the application of compressive residual stress to the surface layer without shrinkage of the surface layer.

[0018] The application can be carried out in such a way that the workpiece is plastically deformed without changing the crystalline state of the workpiece. In this case, there is no generation of an induced martensitic transformation, and a compressive residual stress can be applied while the material structure remains in the austenitic phase.

[0019] The application may include the generation of laser ablation by establishing a laser light pulse width of 150 fs or more and less than 10 ps. In this case, laser ablation can be generated more reliably. Brief description of the drawings

[0020] [Fig-1] The [Fig.1] is a configuration diagram showing a device laser irradiation.

[0021] [Fig.2] Fig.2 is a configuration diagram showing a device of Airborne cavitation.

[0022] [Fig.3] Fig.3 is a configuration diagram showing a device Submerged cavitation.

[0023] [Fig.4] [Fig.4] is a SEM image obtained using a microscope scanning electronics with field emission (SEM-EC).

[0024] [Fig.5] [Fig.5] is a phase map obtained from an analysis result based on a process of electron backscatter diffraction (in English electron backscatter diffraction or EBSD).

[0025] [Fig.6] The [Fig.6] is a graph showing a residual stress distribution. Detailed description

[0026] Embodiments will be described in detail below with reference to the accompanying drawings. In the drawing descriptions, identical or corresponding elements are designated by identical reference numbers, and redundant descriptions are omitted.

[0027] A process for producing a corrosion-resistant austenitic stainless steel according to an embodiment is a process for producing a corrosion-resistant austenitic stainless steel to which a compressive residual stress is applied by applying the compressive residual stress to a surface layer of an austenitic stainless steel without subjecting the surface layer to plastic machining. The production process according to this embodiment comprises a preparation step and a residual stress application step.

[0028] The preparation step is a preparation step for a workpiece W ([Fig. 1]) made of austenitic stainless steel. The austenitic stainless steel used as For example, the workpiece W is made of SUS304 (JIS standard). SUS304 is a relatively inexpensive steel material among austenitic stainless steels.

[0029] The residual stress application step is a step in which a compressive residual stress is applied to the surface layer Wa (see [Fig. 1]) of the workpiece W without subjecting the surface layer Wa to plastic machining and removing the surface layer Wa. Here, the surface layer Wa is a region having a depth, for example, of 100 µm or less from the surface of the workpiece W. Plastic machining is machining that causes deformation or refinement of the crystalline grains. The residual stress application step is carried out such that the amount of change in the martensitic phase in the surface layer Wa is ±10% or less. The compressive residual stress applied to the surface layer Wa in the residual stress application step is 500 MPa or more and can be 550 MPa or more.

[0030] The residual stress application step is carried out using a shock wave generated by at least one of the following methods: laser ablation and cavitation. Laser peening and cavitation peening are processes for applying compressive residual stress within a metallic material in the same way as shot peening and burnishing. In shot peening and burnishing, media and tools are brought into physical contact with a surface of the metallic material, whereas in laser peening and cavitation peening, there is no such physical contact.

[0031] In laser hammering and cavitation hammering, it is possible to plastically deform the workpiece W without changing its crystalline state by using a shock wave. Since the plastic deformation caused by shock hammering is not plastic machining, no deformation or refining of the crystalline grains occurs. The plastic deformation caused by shock hammering generates plastic deformation within the crystalline grains. Therefore, compressive residual stress can be applied while the material structure remains in the austenitic phase without inducing an induced martensitic transformation.

[0032] The residual stress application step is performed while the workpiece W is being cooled. Examples of the cooling method include water cooling and air cooling. Cooling can be carried out using a liquid other than water and a gas other than air. The residual stress application step is performed, for example, while the workpiece W is placed in a liquid. The residual stress application step is carried out, for example, at ambient temperature.

[0033] Figure 1 is a configuration diagram showing a laser irradiation device used in the residual stress application step. As shown in Figure 1, the laser irradiation device 10 comprises a laser oscillator 11, reflecting mirrors 12 and 13, a condensing lens 14, a reservoir 15, a support 16, and a control device 17. The laser oscillator 11 is a device that oscillates the laser light L. The reflecting mirrors 12 and 13 transmit the laser light L, oscillated by the laser oscillator 11, to the condensing lens 14. The condensing lens 14 condenses the laser light L at a workpiece shaping position W with a high density. The reservoir 15 is filled with a transparent liquid 18, such as water. The support 16 supports the workpiece W and places the workpiece W in the tank 15. The support 16 is an actuator or a robot.

[0034] The laser irradiation device 10 is controlled by the control device 17. The control device 17 is configured as a motion controller such as a programmable logic controller (PLC) or a digital signal processor (DSP), for example. The control device 17 can be configured as a computer system comprising a processor such as a central processing unit (CPU), memory such as random access memory (RAM) and read-only memory (ROM), input / output devices such as a touchscreen, mouse, keyboard, and display, and a communication device such as a network card. The control device 17 operates each piece of hardware under the control of the processor so that it performs the function of the control device 17. The processor's control is based on the computer program stored in memory.

[0035] When the residual stress application step is performed using the laser irradiation device 10, first, the workpiece W is placed on the support 16. Then, the workpiece W is moved into the tank 15 and placed in the liquid 18 by the support 16. Next, while the workpiece W is being 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 fs or more and 30 ns or less. The pulse width of the laser light L can be 4 ns or more and 10 ns or less. The pulse width of the laser light L can be 150 fs or more and less than 10 ps.

[0036] The laser light L is oscillated by the laser oscillator 11 and then transmitted to the condensing lens 14 by the optical system comprising the reflecting mirrors 12 and 13. The laser light L is condensed to a high density by the condensing lens 14 and applied to the surface of the workpiece W via the liquid 18. The power density of the laser light L is established at 1 GW / cm² or more and 20 GW / cm2 or less. The power density of the laser light L can be established at 3 GW / cm2 or more and 15 GW / cm2 or less.

[0037] In the surface layer Wa of the workpiece W corresponding to the laser light irradiation point L, the hammering effect of laser hammering is generated as follows. First, when the laser light L is irradiated onto the surface of the workpiece W, laser ablation occurs on the surface of the workpiece W, and a plasma is generated. In the atmosphere, the material at the irradiation point is vaporized. Since the irradiation point on the workpiece W is covered by the liquid 18, the expansion of the plasma is suppressed. As a result, the plasma has a high pressure, and a shock wave is generated by the plasma pressure. Through the propagation of the shock wave, a region of plastic deformation is generated within the workpiece W. In the region of plastic deformation, a compressive residual stress appears due to restraint from an undeformed portion.As described above, since plastic deformation caused by impact melting is not plastic machining, no deformation or refining of the crystalline grains occurs.

[0038] The irradiation of the laser light L corresponds to the operation of the support 16, and is carried out during the movement of the irradiation point on the workpiece W. Each time the workpiece W is irradiated by the laser light L, the support 16 moves the workpiece W so that the irradiation point moves on the workpiece W. Thus, an area of ​​the workpiece W irradiated by the laser light L can be ensured.

[0039] Figure 2 is a configuration diagram showing an air cavitation device used in the residual stress application step. As shown in Figure 2, the air cavitation machine 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 located inside the first nozzle 21. The first nozzle 21 and the second nozzle 22 are positioned on the workpiece W so that the nozzle tips face the surface of the workpiece W. The control device controls the air cavitation device 20. The control device has, for example, the same configuration as the control device 17 of the laser irradiation device 10.

[0040] 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 on the shear layer 25 between the low-speed jet of liquid 23 and The high-speed jet of liquid 24. By spraying the cavitation jet stream, accompanied by such bubbles, onto the surface of the workpiece W, a shock wave generated during the collapse of the cavitation bubbles is transmitted to the workpiece W. As a result, a hammering effect occurs on the surface layer Wa of the workpiece W. Namely, the crystalline structure undergoes high-density dislocation, and a compressive residual stress is imparted. In cavitation hammering, the generation, growth, and collapse of the bubbles strongly influence the cavitation effect. The underside surface of the workpiece W, onto which liquids 23 and 24 are not sprayed, can be air-cooled.

[0041] Figure 3 is a configuration diagram showing a submerged cavitation device used in the residual stress application step. As shown in Figure 3, the submerged cavitation device 30 comprises a tank 31, a nozzle 32, and a control device (not shown). The tank 31 is filled with a liquid 33 such as water. A workpiece W is disposed inside the tank 31. The tip of the nozzle 32 is disposed in the tank 31 and faces the surface of the workpiece W. The control device controls the submerged cavitation device 30. The control device has, for example, the same configuration as the control device 17 of the laser irradiation device 10.

[0042] The nozzle 32 sprays a liquid 34, such as water, onto the surface of the workpiece W at a high speed. Cavitation bubbles are generated at the tip of the nozzle 32. These cavitation bubbles grow on the shear layer 35 between the high-speed jet of liquid 34 and the liquid 33. By spraying the cavitation jet stream, along with these bubbles, onto the surface of the workpiece W, a shock wave generated during the collapse of the cavitation bubbles is transmitted to the workpiece W. As a result, a hammering effect occurs on the surface layer Wa of the workpiece W.

[0043] As described above, in the manufacturing process according to the embodiment, the surface layer Wa of the workpiece W is not subjected to plastic machining. Consequently, the compressive residual stress can be applied without generating a deformation-induced martensitic transformation in the surface layer Wa of the workpiece W. Since the martensitic phase, which has poor corrosion resistance, is eliminated, sufficient compressive residual stress can be applied while maintaining corrosion resistance.

[0044] When a steel is used in a region exposed to hydrogen, hydrogen enters the steel and hydrogen embrittlement occurs. In the martensitic phase, the dislocation density is high and hydrogen embrittlement is likely to occur. In the austenitic phase, the dislocation density is low, and hydrogen embrittlement is unlikely to occur. For example, SUS304 is fabricated as a workpiece W by the fabrication process according to the embodiment, and a residual stress is applied without degradation of the hydrogen embrittlement. Thus, a steel material having a corrosion resistance corresponding to SUS316 (JIS standard) can be obtained.

[0045] As described above, in the manufacturing process disclosed in Japanese Patent Application No. 1978-104520, it is necessary to carry out the manufacturing at a temperature higher than the upper limiting temperature at which a strain-induced martensitic transformation occurs. Consequently, in addition to the problem that a compressive residual stress cannot be applied, there is the problem that heating takes time, or that the residual stress varies due to uneven heating. On the other hand, since the residual stress application step according to the present embodiment is carried out at normal temperature, the problem described above can be resolved.

[0046] The residual stress application step is performed using shock melt generated by at least one of laser ablation and cavitation. Although the plastic deformation due to shock melt causes plastic deformation within the crystalline grains, since plastic deformation is not plastic machining, no deformation or refining of the crystalline grains occurs. Therefore, it is possible to apply sufficient compressive residual stress to the surface layer Wa of the workpiece W while maintaining corrosion resistance.

[0047] In the residual stress application step, laser ablation is generated by setting the laser light power density to 1 GW / cm² or more and 20 GW / cm² or less. By setting the power density to 1 GW / cm² or more, laser ablation can be reliably generated. By setting the power density to 20 GW / cm² or less, surface damage to the workpiece W is eliminated. The laser light power density can be 3 GW / cm² or more and 15 GW / cm² or less. By setting the power density to 3 GW / cm² or more, laser ablation can be generated more reliably. By setting the power density to 15 GW / cm² or less, surface damage to the workpiece W is further eliminated.

[0048] In the residual stress application step, laser ablation is generated by establishing the pulse width of the laser light LA to be 150 fs or more and 30 ns or less. Thus, laser ablation can be generated reliably. The pulse width of the laser light LA can be 4 ns or more and 10 ns or less. Thus, laser ablation can be generated even more reliably.

[0049] The residual stress application step is performed while the workpiece W is being cooled. The residual stress application step using the laser irradiation device 10 is performed while the workpiece W is being cooled by liquid 18. The residual stress application step using the air cavitation device 20 is performed while the workpiece W is being cooled by at least liquid 23 and liquid 24. The residual stress application step using the submerged cavitation device 30 is performed while the workpiece W is being cooled by liquid 33 and liquid 34. Consequently, an increase in the temperature of the workpiece W is prevented.

[0050] The residual stress application step using the laser irradiation device 10 is carried out while the workpiece W is placed in the liquid 18. The residual stress application step using the immersed cavitation device 30 is carried out while the workpiece W is placed in the liquid 33. Thus, the workpiece W can be easily cooled.

[0051] The residual stress application step is carried out such that the amount of martensitic phase change in the surface layer Wa is ±10% or less. Since the amount of martensitic phase change with poor corrosion resistance is suppressed as described above, the corrosion resistance of the workpiece W can be maintained.

[0052] The compressive residual stress applied by the residual stress application step is 500 MPa or more. Thus, sufficient compressive residual stress can be applied to the workpiece W.

[0053] The present invention is not necessarily limited to the embodiment described above, and various modifications can be made without departing from the scope of the present invention.

[0054] Examples will be described below.

[0055] By way of example, a sample to which a compressive residual stress has been imparted by laser hammering was produced. Specifically, firstly, a sample made of SUS304 was prepared as a workpiece and placed in a tank filled with water. Laser hammering was performed using a laser irradiation device with a spot size of 0.7 mm, a power density of 4.2 GW / cm², a pulse energy of 100 mJ, and an irradiation density of 0.3 pulses / mm².

[0056] By way of comparison, a sample was produced to which a compressive residual stress was imparted by shot peening instead of laser peening. Shot peening was carried out using a medium (RCW06PM) composed of amorphous round metallic spheres, with an injection pressure of 0.2 MPa, a injection quantity of 9.0 kg / min, coverage of 300% or more, and arc height of 0.361 mmA.

[0057] (Measurement of the volume quantity of residual austenite)

[0058] To verify the occurrence of an induced martensitic transformation, the volumetric amount of residual austenite in the surface layer of each of the following samples was measured: the sample according to the example, the sample according to the comparative example, and the sample in an untreated state. The measurement was performed using a cos α method with a p-X360 residual stress measuring device manufactured by Pulstec Industrial Co., Ltd. A Cr bulb was used with an irradiation radius θ of 1.0 mm, a collimator radius θ of 1.0 mm, and a measurement angle of 0 degrees. The measurement results are presented in Table 1. [Tables 1] No treatment. Comparative example. Example. Quantity by volume of residual austenite (% by volume). 95.9 56.5 96.7

[0059] As shown in Table 1, in the comparative example after shot peening, approximately 40% induced martensitic transformation occurred, compared to the untreated state. On the other hand, in the examples after laser peening, no induced martensitic transformation occurred compared to the untreated state. The slight increase in austenite volume in the example, compared to the untreated state, is assumed to be due to a measurement error.

[0060] (Tissue observation)

[0061] In order to visually verify the occurrence of an induced martensitic transformation, the structure of the sample as described in the example was observed using a field emission scanning electron microscope (FES-SEM). Specifically, the sample was cut, and its surfaces and cross-sections were observed using a JSM-7200F Schottky field emission scanning electron microscope manufactured by JEOL Ltd.

[0062] Figure 4 is a scanning electron microscopy (SEM) image obtained using an electron microscopy (EMS) instrument. On the left side of the dashed line in Figure 4 are shown a laser-punched surface and a cross-section of tissue beneath the laser-punched surface. On the right side of the dashed line in Figure 4 are shown an untreated surface and a cross-section of the tissue beneath the untreated surface. As Figure 4 shows, it can be confirmed that there is no difference between the laser-punched portion and the untreated portion in the SEM image.

[0063] Figure 5 is a phase map obtained from an analysis result based on the EBSD process. In Figure 5, the FCC structure is shown in light color, and the Cementite, the BCC structure, and chromium carbide are shown in dark color. Here, the FCC structure is an austenitic phase, and the BCC structure is a martensitic phase. As shown in [Fig. 5], a large portion of the analysis area is occupied by the austenitic phase in both the laser-hammered and untreated portions. Consequently, it is understood that no induced martensitic transformation occurs even when laser hammering is performed. Furthermore, it can be confirmed that there is no difference in the size of the crystalline grains 41 and in the state of the crystalline grain boundaries 42, indicated by black lines, between the laser-hammered and untreated portions.

[0064] (Residual stress measurement)

[0065] The residual stress of the sample surface layer was measured for the sample according to the example. The measurement was carried out by a cos α method using a p-X360 residual stress measuring device manufactured by Pulstec Industrial Co., Ltd. A Cr bulb was used, the irradiation radius was q > 1.0 mm, the collimator radius was q > 1.0 mm, and the measurement angle was 35 degrees.

[0066] Figure 6 is a graph showing the residual stress distribution. In Figure 6, the horizontal axis represents the depth (pm) from the surface and the vertical axis represents the residual stress (MPa). The compression side is indicated by a negative value and the tension side by a positive value. As shown in Figure 6, a residual stress of -550 MPa or less, i.e., a compressive residual stress of 550 MPa or more, is generated in the vicinity of the surfaces. This value is greater than the residual stress applied by the manufacturing process described in Japanese Patent Application No. 1978-104520, which is 100 MPa or more on the compression side. This value is considered to have a strong effect on stress corrosion cracking and can suppress a new generation of stress corrosion cracking.

Claims

Demands

1. A method for producing a corrosion-resistant austenitic stainless steel, the method comprising: preparing a workpiece made of austenitic stainless steel; and applying a compressive residual stress to a surface layer of the workpiece without subjecting the surface layer to plastic machining, wherein the application is carried out in such a way that the amount of martensitic phase change in the surface layer is ±10% or less, the compressive residual stress applied during the application being 500 MPa or more.

2. A method according to claim 1, wherein the application is carried out by means of a shock wave generated by at least one of laser ablation and cavitation.

3. A method according to claim 2, wherein the application comprises the generation of laser ablation by establishing a laser light power density of 1 GW / cm2 or more and 20 GW / cm2 or less.

4. A method according to claim 2 or 3, wherein the application comprises the generation of laser ablation by establishing a laser light pulse width of 150 fs or more and 30 ns or less.

5. A method according to any one of claims 1 to 4, wherein the application is carried out while the workpiece is being cooled.

6. A method according to any one of claims 1 to 4, wherein the application is carried out while the workpiece is disposed in a liquid.

7. A method according to any one of claims 1 to 6, wherein the application comprises the application of compressive residual stress to the surface layer without shrinkage of the surface layer.

8. A method according to any one of claims 1 to 7, wherein the application is carried out in such a way that the workpiece is plastically deformed without changing the crystalline state of the workpiece.

9. A method according to any one of claims 2 or 3, wherein the application comprises the generation of laser ablation by establishing a laser light pulse width of 150 fs or more and less than 10 ps.

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