Laser surface melting hardening method

Abstract

To provide a laser surface melting hardening method that enables high-speed processing while eliminating the need for pretreatments such as blasting and forming a stable hardened layer on a surface of a workpiece.SOLUTION: A laser surface melting hardening method includes the following steps: irradiating a surface of a workpiece 20 made of martensitic stainless steel with near-infrared laser light 11 without performing a pretreatment for roughening the surface; melting the surface of the workpiece 20 by the irradiation of the laser light 11; forming a hardened layer by rapidly cooling, through self-cooling, a region that has reached the quenching temperature of the martensitic stainless steel from the melted surface; and removing a surface portion of the hardened layer that exhibits tensile residual stress to expose a surface exhibiting compressive residual stress.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a laser surface melting hardening method for martensitic stainless steel. [Background technology]

[0002] The sliding surfaces of thrust disks in internal pumps used in nuclear power plants are subjected to laser surface melting and hardening treatments to prevent wear during pump rotation and shutdown. Since the sliding surfaces of these thrust disks are in contact with reactor water, there is a concern that stress corrosion cracking (SCC) may occur. Therefore, the surface of the melt-hardened thrust disk is required to have a hardened layer with a distributed compressive residual stress.

[0003] Until the early 2000s, CO2 lasers were used as the light source for laser surface melting and hardening processes because they could emit high-power lasers of over 5kW. However, with CO2 lasers, the output tends to become unstable due to aging of the oscillator, and the oscillation efficiency is also poor, at 10-13% of the input voltage, making it difficult to obtain a stable hardened layer.

[0004] Furthermore, CO2 lasers have low optical absorption in stainless steel substrates, making it impossible to obtain a hardened layer of 0.5 mm or more. Therefore, a blasting process was performed on the stainless steel substrate, projecting particulate abrasives (such as iron oxide) onto the surface to increase the surface roughness Ra (arithmetic mean roughness) to 4 to 7 μm. This improves the optical absorption of the CO2 laser on the surface of the stainless steel substrate, resulting in a hardened layer of 1 mm or more. Furthermore, with CO2 lasers, the efficiency of construction was improved by expanding the beam diameter to a few mm, a process known as "defocusing." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3217400 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the surface of a stainless steel substrate is blasted, abrasives remain on the roughened surface. This residual abrasive leaves iron oxide slag on the surface when the surface is subsequently melted by laser irradiation. Therefore, after laser irradiation, the surface must be polished with a large stock removal amount, which requires a long polishing time to remove the slag from the surface of the stainless steel substrate.

[0007] Laser surface melting is an excellent localized hardening method for hardening thrust disks in internal pumps. However, the far-infrared CO2 laser used has an absorption rate of about 10% on the machined surface of Fe-based materials, and as mentioned above, it was not possible to form a hardened layer more than 1 mm deep unless the laser-irradiated surface was roughened by blasting or other methods.

[0008] Furthermore, the light distribution of a "defocused" CO2 laser is a Gaussian beam mode with a high power density in the center. For this reason, when treating a large area such as a thrust disk, multiple passes (6 mm per pass) are unavoidable. In a hardening specification with a Vickers hardness of 400 or more, with an irradiation beam diameter of 6 mm and an overlap of 3 mm in the treatment area, the hardened layer depth in the overlap area, which should be 2 mm, is significantly reduced to 1.3 to 1.5 mm.

[0009] In this way, when a CO2 laser is "defocused" and irradiated with a Gaussian mode beam, the power density in the center of the laser beam, which is a few millimeters in diameter, is high, so only the center of the surface molten bead melts deeply, while the surrounding area melts shallowly, making it impossible to obtain a uniform hardened layer. Furthermore, when setting an overlap between beads as described above, it is necessary to overlap by about half the diameter of the irradiated beam, which not only increases the construction time with multiple passes, but also causes fluctuations in the penetration depth between the overlap and the center of the bead, making it impossible to obtain a stable hardened layer.

[0010] The embodiments of the present invention have been made in consideration of the above circumstances, and have as their object to provide a laser surface melting and hardening treatment method that can obtain a stable hardened layer on the surface of the workpiece without requiring pretreatment such as blasting, and that allows for high-speed application. [Means for solving the problem]

[0011] In an embodiment, the laser surface melt hardening treatment method includes the steps of irradiating a surface of a workpiece made of martensitic stainless steel with laser light in the near-infrared region without performing pretreatment to roughen the surface of the workpiece, melting the surface of the workpiece by irradiating the laser light, rapidly cooling a region of the molten surface that has reached the quenching temperature of the martensitic stainless steel by self-cooling to form a hardened layer, and removing a surface portion of the hardened layer that exhibits tensile residual stress to expose a surface that exhibits compressive residual stress. [Effects of the Invention]

[0012] According to an embodiment of the present invention, a laser surface melting and hardening treatment method is provided that forms a stable hardened layer on the surface of an object to be treated without requiring pretreatment such as blasting, and that enables high-speed application. [Brief explanation of the drawings]

[0013] [Figure 1] 1A to 1C are explanatory diagrams illustrating a laser surface melting hardening treatment method according to an embodiment of the present invention. [Figure 2] FIG. 1 is a cross-sectional view of a thrust disk of an internal pump that is used as a treatment object in a laser surface melting hardening treatment method according to an embodiment. [Figure 3] Chemical composition of martensitic stainless steel, the material used to make the thrust disk. [Figure 4] (A) A three-dimensional graph showing the power density distribution of laser light in Gaussian mode, (B) a three-dimensional graph showing the power density distribution of laser light in top-hat mode, (C) a longitudinal cross-sectional view showing the hardened layer of a workpiece processed in Gaussian mode, and (D) a longitudinal cross-sectional view showing the hardened layer of a workpiece processed in top-hat mode. [Figure 5] 1 is a graph showing the reflectance of the surface of an Fe-based alloy versus the wavelength of the irradiated laser light. [Figure 6] 1 is a graph showing Vickers hardness versus depth from the surface of a treated object. [Figure 7] 1 is a graph showing residual stresses on the tensile side and compressive side with respect to the depth from the surface of the treated object. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is an explanatory diagram of a laser surface melt hardening treatment method according to an embodiment of the present invention. Fig. 2 is a cross-sectional view of a thrust disk of an internal pump in a nuclear power plant, which is applied as a treatment object 20 in the laser surface melt hardening treatment method according to the embodiment.

[0015] The internal pump here refers to the reactor internal pump (RIP) installed in an advanced boiling water reactor (ABWR). In conventional BWRs, the corresponding reactor coolant recirculation pump was installed outside the reactor pressure vessel. The thrust disk, a component of this internal pump, is made of martensitic stainless steel with the composition shown in Figure 3, and a hardened layer 21 is formed on both main surfaces that serve as sliding surfaces.

[0016] As shown in Figures 1 and 2, the laser surface melt hardening treatment method includes the steps of irradiating a surface of a workpiece 20 made of martensitic stainless steel with laser light 11 in the near-infrared region without performing any pretreatment to roughen the surface, melting the surface of the workpiece 20 by irradiating the laser light 11, rapidly cooling the area of ​​the molten surface that has reached the quenching temperature of the martensitic stainless steel by self-cooling to form a hardened layer 21, and removing a surface portion 22 (Figure 7) of this hardened layer 21 that exhibits tensile residual stress to expose a surface that exhibits compressive residual stress.

[0017] FIG. 3 shows the chemical composition of martensitic stainless steel, which is the material constituting the thrust disk 20. Martensitic stainless steel is a classification of stainless steel by metal structure, and is stainless steel containing only chromium as the main component. This martensitic stainless steel can be hardened by quenching, and has high strength and wear resistance in addition to corrosion resistance. Note that the martensitic stainless steel to which this embodiment is applied is not limited to the composition shown in FIG. 3, as long as it has a chemical composition with a chromium content in the range of 11 to 18 wt % and a carbon content of 1.2 wt % or less.

[0018] Returning to Figure 1, the explanation will continue. In this embodiment, a fiber laser system is adopted as the irradiation device 10 that irradiates laser light 11 in the near-infrared region. The fiber laser system is a type of solid state, and inside the oscillator 12, a double fiber structure is formed, with a Yb-doped fiber at the center, and a semiconductor laser for excitation is incident on the outside of this, and is amplified by resonators installed before and after this, and laser light 11 with a wavelength of 1070 nm is emitted outside the oscillator 12.

[0019] The laser light 11 emitted from the oscillator 12 is transmitted through the optical fiber 15, and the laser light 11a emitted from the output end of the optical fiber 15 spreads at a certain angle, but is collimated by the collimator lens 17 into parallel light.

[0020] The parallel laser beam 11 is then focused by the condenser lens 19, reducing the spot diameter and further increasing the power density of the laser beam 11 irradiated onto the object to be processed 20. The light distribution of the laser beam 11a emitted from the optical fiber 15 is a Gaussian beam mode with a high power density at the center.

[0021] Beam shaper 18 is placed downstream of collimator lens 17 and upstream of condenser lens 19, and converts laser light 11a having a Gaussian beam mode light distribution into laser light 11b having a top-hat beam mode light distribution with uniform power density. There are various types of beam shapers 18, one example of which is a DOE (Diffractive Optical Element), which intentionally causes diffraction and performs beam shaping by forming irregularities on the surface of a quartz optical substrate on the order of the wavelength.

[0022] Another example of the beam shaper 18 is a homogenizer that splits the incident laser beam 11b in Gaussian beam mode into smaller beams and forms the laser beam 11b in top hat beam mode at the back focus position of the lens. The beam shaper 18 is not particularly limited as long as it has the function of changing the light distribution of the laser beam 11 from Gaussian to top hat beam mode, and other types besides those mentioned above can be used as appropriate.

[0023] Fig. 4(A) is a three-dimensional graph showing the power density distribution of laser light in Gaussian beam mode. Fig. 4(C) is a longitudinal cross-sectional view showing a hardened layer 21a (20) of a workpiece 20 in Gaussian beam mode. Fig. 4(B) is a three-dimensional graph showing the power density distribution of laser light in top hat beam mode. Fig. 4(D) is a longitudinal cross-sectional view showing a hardened layer 21b (20) of a workpiece 20 in top hat beam mode.

[0024] Of the collimator lens 17, beam shaper 18, and condenser lens 19 that make up the optical system 16, the surface hardened layer can be obtained without the beam shaper 18, but by adopting the beam shaper 18, the laser light 11 can be switched from Gaussian beam mode to top hat beam mode, which makes it possible to suppress depth variations in the hardened layer 21 and shorten the construction time by reducing overlap between passes.

[0025] Figure 5 is a graph showing the reflectance R of the surface of an Fe-based alloy versus the wavelength λ of the irradiated laser light 11. As shown in Figure 5, with a CO2 laser (λ = 10600 nm), which is far outside the near-infrared region (λ = 900 to 1100 nm), the reflectance R is approximately 90%. In contrast, with a fiber laser (λ = 1070 to 1080 nm), the reflectance is approximately 60%, and by changing from a CO2 laser to a fiber laser, this improves in terms of absorption rate from 10% to 40%, resulting in a fourfold increase in the amount of laser light 11 absorbed.

[0026] In this embodiment, a fiber laser system is used as the irradiation device 10, but the present invention is not limited to this. Any other system capable of irradiating laser light 11 in the near-infrared region, such as a disk laser, a YAG laser, or a semiconductor laser, can ensure sufficient absorption into the metal workpiece 20. [Example]

[0027] An example will be described in which the effect of this embodiment was confirmed for a martensitic stainless steel having the composition shown in Figure 3. The working conditions are as follows. Laser type: Fiber laser Laser wavelength: 1070nm Process fiber core diameter: Φ0.2mm~Φ1.0mm Collimation lens focal length: 100~150mm Focal length of condenser lens: 250~450mm Irradiation beam mode: Top hat mode Irradiation beam shape: circular (Φ5mm~Φ9mm), Rectangle (□5mm~□9mm) Laser output: 4 to 10 kW Processing speed: 300~1000mm / min Shielding gas: 20 to 80 liters / min Gas type: N2 gas, Ar gas, or He gas

[0028] 6 is an example of a graph showing Vickers hardness versus depth from the surface of the workpiece 20. From this graph, it can be seen that the depth at which the Vickers hardness is 400 or more (HV≧400) is approximately 2.7 mm, and the depth of the hardened layer 21 is the required 2 mm or more. By using laser light 11 in the near-infrared region, the absorption rate is improved, and it can be said that a hardened layer 21 of sufficient depth is stably formed even without blasting.

[0029] FIG. 7 is an example of a graph showing the tensile and compressive residual stresses versus depth from the surface of the workpiece 20. The graph shows the results of measuring the residual stress at 50 μm intervals in the depth direction from the surface irradiated with the laser beam 11. From this, it can be said that the compressive residual stress is stably distributed from a depth of 0.1 mm from the surface, and that stress corrosion cracking susceptibility can be significantly reduced by setting the polishing allowance to 0.1 mm or more. If the polishing allowance is less than 0.1 mm, the tensile residual stress remains, and the compressive residual stress may not be exposed. If the polishing allowance is more than 0.3 mm, the thickness (2 mm) of the hardened layer 21 becomes too thin.

[0030] According to at least one embodiment of the laser surface melting and hardening treatment method described above, by irradiating a laser beam in the near-infrared region, a stable hardened layer can be obtained on the surface of the workpiece without requiring pretreatment such as blasting, and further, by using a top hat mode, high-speed processing can be achieved.

[0031] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0032] 10...irradiation device, 11 (11a, 11b)...laser light, 12...oscillator, 15...optical fiber, 16...optical system, 17...collimating lens, 18...beam shaper, 19...condensing lens, 20...object to be treated (thrust disk), 21...hardened layer, 22...surface layer portion

Claims

1. a step of irradiating a surface of a martensitic stainless steel object to be treated with a laser beam in the near-infrared region without performing a pretreatment for roughening the surface of the object; a step of melting the surface of the object to be treated by irradiating the laser light; a step of rapidly cooling a region from the molten surface that has reached the quenching temperature of the martensitic stainless steel by self-cooling to form a hardened layer; and removing a surface portion of the hardened layer that exhibits tensile residual stress to expose a surface that exhibits compressive residual stress.

2. 2. The laser surface melting hardening method according to claim 1, A laser surface melting and hardening treatment method using the laser light having a wavelength in the range of 900 nm to 1100 nm in the near-infrared region.

3. 3. The laser surface melting and hardening treatment method according to claim 1 or 2, The laser surface melting and hardening treatment method uses a top hat beam mode with a uniform power density as the laser light.

4. 3. The laser surface melting and hardening treatment method according to claim 1 or 2, A laser surface melting and hardening treatment method, wherein when removing the surface layer portion from the hardened layer, a polishing amount of the surface of the object to be treated is set to 0.1 mm or more.

5. 3. The laser surface melting and hardening treatment method according to claim 1 or 2, A laser surface melting and hardening treatment method in which the treatment object is a thrust disk of an internal pump in a nuclear power plant.

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