A method for controlling cracks of laser cladding layer of spheroidal graphite cast iron for spent fuel

Abstract

The present application belongs to the technical field of laser cladding of cast iron parts, and particularly relates to a method for controlling cracks of laser cladding layer of nodular cast iron for spent fuel. Iron-based alloy powder is laser cladded on the surface of nodular cast iron, and the iron-based alloy powder is composed of the following components in mass percentage: C≤0.05%, Mn 0.8-1.2%, Si 0.8-1.0%, P≤0.015%, S≤0.015%, Cr 8-12%, Ni 7-9%, Mo 0.3-0.6%, B 0.6-1.0%, Ce 0.1-0.3%, and Fe in the remainder. The iron-based alloy powder of the present application is laser cladded on the surface of QT400-18 nodular cast iron, and the cladding material and the substrate have almost the same linear expansion coefficient, and low-temperature martensitic phase transformation can occur at the same time, so that the surface stress of the cladding layer is changed to compressive stress, and the generation of cracks in the cladding layer is effectively inhibited.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser cladding of cast iron parts, and particularly relates to a method for controlling cracks in a laser cladding layer of nodular cast iron for spent fuel. BACKGROUND

[0002] With the increase in the service time of nuclear power plants in China, a major problem is the rapid increase in the generated spent fuel, which needs to be transported and stored. Compared with stainless steel and carbon steel, nodular cast iron has the advantages of low manufacturing cost, good safety, dual purposes of storage and transportation, etc., and thus becomes the main candidate material for manufacturing the cylinder of the storage and transportation container. However, since the inner wall of the container cylinder will be subjected to corrosion in an acidic water environment during loading and unloading of the spent fuel, and the nodular cast iron has poor corrosion resistance, it is urgent to study the anti-corrosion technology for the inner wall of the cylinder. The processes such as thermal spraying, electroplating and surfacing are complex, low in efficiency and high in energy consumption. The laser cladding technology has the advantages of energy concentration, small deformation of the workpiece and metallurgical bonding between the cladding layer and the substrate, and thus has great advantages in the preparation of the cladding layer for the inner wall of the cylinder of the storage and transportation container. However, after laser cladding of stainless steel powder and nickel-based powder on the surface of the nodular cast iron, it is found that cracks are prone to occur on the surface and the interface region of the cladding layer. Therefore, it is one of the urgent problems to be solved in the manufacturing of the cylinder of the spent fuel storage and transportation container to develop new iron-based alloy powder and solve the problem of cracks in the laser cladding layer of the spent fuel nodular cast iron.

[0003] Generally speaking, the closer the linear expansion coefficient of the cladding material and the substrate, the more conducive to avoiding the generation of cracks in the cladding layer. In addition, under the action of high-energy laser beam, tensile stress is easy to produce, if the tensile stress can be reduced or even changed from "tension" to "compression", it is also conducive to inhibiting the generation of cracks in the cladding layer. There is a difference in the linear expansion coefficient between nickel-based alloy, stainless steel and ductile iron substrate, combined with the high carbon equivalent of ductile iron, under the action of laser thermal tensile stress, the tendency of thermal cracking is high. The iron-based alloy powder and the ductile iron substrate have better physical property matching, and the cost of the iron-based alloy powder is also lower than that of the nickel-based alloy powder and the stainless steel powder. Chinese patent (CN113136532B, an iron-based alloy powder for laser cladding and a preparation method thereof) introduces an iron-based alloy powder composition and a preparation method thereof, which is mainly used for surface coating preparation of steel, tool steel or stainless steel, but not for ductile iron. Chinese patent (CN107245713B, alloy powder for laser cladding repair of surface of ductile iron roller) introduces an alloy powder for repairing the surface of a ductile iron roller, but the powder is a nickel-based alloy, and B4C and graphene are also added, which makes it difficult to ensure the sphericity of the powder and increases the manufacturing cost. Li et al. (Y. Li, S. Dong, P. He, et al. Journal of Materials Processing Tech. 269 (2019) 163-171) developed an iron-based alloy powder for laser cladding remanufacturing of ductile iron, but the Ni content in the powder is as high as 35wt.%, which increases the manufacturing cost. SUMMARY

[0004] The purpose of the present application is to provide a spent fuel ductile iron laser cladding layer crack control method, which designs a new type of iron-based alloy powder with almost the same linear expansion coefficient as the ductile iron QT400-18, and can produce low-temperature martensitic phase transition, reduce the tensile stress, and even change the stress to compressive stress, which can effectively solve the problem of cracks in the laser cladding layer of ductile iron QT400-18.

[0005] The present application adopts the following technical solutions:

[0006] A spent fuel ductile iron laser cladding layer crack control method, an iron-based alloy powder is laser cladded on the surface of a ductile iron, the iron-based alloy powder is composed of the following components in mass percentage: C≤0.05%, Mn 0.8-1.2%, Si 0.8-1.0%, P≤0.015%, S≤0.015%, Cr 8-12%, Ni 7-9%, Mo 0.3-0.6%, B 0.6-1.0%, Ce 0.1-0.3%, and Fe in the remainder.

[0007] The crack control method of the ductile cast iron laser cladding layer for spent fuel, the iron-based alloy powder is prepared by a vacuum air atomization method, and the particle size of the iron-based alloy powder is 45-200 mu m.

[0008] The crack control method of the ductile cast iron laser cladding layer for spent fuel, the laser cladding process parameters of the iron-based alloy powder are as follows: an output power is 1.2-1.8 kW, a scanning speed is 6-10 mm / s, a powder feeding amount is 8-12 g / min, a protective argon flow is 12-20 L / min, an overlapping rate is 35-45%, and a powder feeding mode is synchronous coaxial conveying.

[0009] The crack control method of the ductile cast iron laser cladding layer for spent fuel, before laser cladding, the iron-based alloy powder is dried in a 120 DEG C drying box for 2 h.

[0010] The crack control method of the ductile cast iron laser cladding layer for spent fuel, the iron-based alloy powder is used as the laser cladding material, and the linear expansion coefficient of the iron-based alloy powder is same as that of the base body QT400-18 ductile cast iron.

[0011] The crack control method of the ductile cast iron laser cladding layer for spent fuel, the surface residual stress of the laser cladding layer is compressive stress.

[0012] The design idea of the application is as follows:

[0013] The Ni element is a main alloy element for enabling the base body to obtain stable austenite structure, has the effects of oxidation resistance, corrosion resistance and wear resistance, but if the content of the Ni element is too high, the impact toughness of the cladding material is reduced, and the content of the Ni element can be controlled in the range of 7-9%.

[0014] The Cr element can promote the generation of martensite and has good corrosion resistance, but if the content of the Cr element is too high, the hot brittleness tendency is increased, and the content of the Cr element can be controlled in the range of 8-12%.

[0015] The B and Si elements have the effects of improving the wear resistance and hardness of the cladding material and promoting the self-fluxing of the coating powder, the Si element can be preferentially gathered on the surface to form an O and Si rich layer, which is beneficial to prevent the corrosion liquid from penetrating, therefore, the content of the B element is controlled in the range of 0.6-1.0%, and the content of the Si element is controlled in the range of 0.8-1.0%.

[0016] The C and Mn elements improve the strength of the cladding layer material, but if the contents of the C and Mn elements are too high, the toughness of the cladding material is greatly reduced, and cracks are easily generated, therefore, the content of the C element is not more than 0.05%, and the content of the Mn element is controlled in the range of 0.8-1.2%.

[0017] The Mo element can improve the hot brittleness tendency of the steel, but if the content of the Mo element is too high, the cladding material is embrittled, and the content of the Mo element can be controlled in the range of 0.3-0.6%.

[0018] The rare earth element Ce can refine the grain and improve the ability to inhibit the crack, but too much Ce can generate inclusions and promote the crack.

[0019] The content of each element in the iron-based alloy is determined by considering the important role of C, Mn, Ni, Cr, Mo and other elements in reducing the martensite phase transition starting temperature Ms, so that Ms is about 200 DEG C, and a complete martensite phase transition should occur during the solidification process of the cladding metal.

[0020] The content of each element in the iron-based alloy is determined by considering the important role of each element (especially Ni and Cr) on the linear expansion coefficient of the cladding material, so that it is similar to the linear expansion coefficient of the substrate QT400-18 ductile iron.

[0021] The advantages and beneficial effects of the present application are that:

[0022] The present application provides a crack control method for laser cladding layer of spent fuel ductile iron, develops a new type of iron-based alloy powder, and clads the iron-based alloy powder of the present application on the surface of QT400-18 ductile iron by laser cladding, so that the cladding material and the substrate have almost the same linear expansion coefficient, and low-temperature martensite phase transition can occur, so that the surface stress of the cladding layer is changed to compressive stress, and the generation of cracks in the cladding layer is effectively inhibited. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The linear expansion coefficients of the iron-based alloy powder, Ni625 powder and QT400-18 ductile iron described in the present application are compared.

[0024] Figure 2 The schematic diagram of the test position of the surface residual stress of the laser cladding layer described in the present application is shown.

[0025] Figure 3 The longitudinal residual stress of the surface of the laser cladding layer of the iron-based alloy powder and the Ni625 powder described in the present application is compared.

[0026] Figure 4 The penetration flaw detection result diagram of the laser cladding layer of the Ni625 powder described in the present application is shown.

[0027] Figure 5 The penetration flaw detection result diagram of the laser cladding layer of the iron-based alloy powder described in the present application is shown. DETAILED DESCRIPTION

[0028] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0029] Example 1

[0030] In this embodiment, the iron-based alloy powder is prepared by vacuum gas atomization method and consists of the following components in mass percentage: C 0.03%, Mn 1.0%, Si 0.9%, P 0.01%, S 0.008%, Cr 9%, Ni 9%, Mo 0.5%, B 0.75%, Ce 0.2%, and Fe the balance. The particle size of the iron-based alloy powder is 45-150 μm. As a comparative example, the Ni625 nickel-based alloy powder produced by Tianjin Chugang Science and Technology Development Co., Ltd. is purchased and consists of the following components in mass percentage: C 0.1%, Si 0.63%, Cr 20.35%, Fe 2.0%, Mo 8.62%, Nb 3.31%, and Ni the balance. The particle size of the Ni625 nickel-based alloy powder is 50-200 μm.

[0031] The substrate material is QT400-18 nodular cast iron. The surface oxide is removed by machining, and after grinding, the substrate surface is cleaned with acetone to remove oil stains, cleaned with alcohol, and dried with a blower. The substrate size is 110 mm x 95 mm x 8 mm.

[0032] The α-Fe The laser cladding experiment is carried out by using the α-Fe

[0033] The linear expansion coefficient of the iron-based alloy powder is calculated by using the JMatPro software, the linear expansion coefficient of the Ni625 nickel-based alloy powder is taken from the In625 alloy in the material database of the SYSWELD software, and the linear expansion coefficient of the QT400-18 nodular cast iron is the measured data. The comparison results are shown in Figure 1 As can be seen from the figure, the linear expansion coefficient of the iron-based alloy powder is almost the same as that of the substrate QT400-18 nodular cast iron and is lower than that of the Ni625 nickel-based alloy powder.

[0034] The martensite transformation start temperature Ms of the material can be calculated by formula (1):

[0035] Ms(℃) = 561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] (1)

[0036] In the formula, [C], [Mn], [Ni], [Cr], [Mo] are the mass percentages of C, Mn, Ni, Cr, Mo elements in the powder. Through calculation, the martensite phase transition starting temperature Ms of the iron-based alloy powder is 197℃. The simulation study finds that when the martensite phase transition starting temperature of the cladding material is about 200℃, compressive stress can be generated near the weld.

[0037] The residual stress on the surface of the cladding layer is measured by X-ray method, and the measurement position is shown in Figure 2 . Among them, positions P1 and P7 are located in the heat-affected zone on both sides of the cladding layer, and positions P2-P6 are located on the surface of the cladding layer with a spacing of 10mm. As shown in Figure 3 , the longitudinal residual stress at positions P1-P7 after laser cladding of Ni625 powder and iron-based alloy powder. When the cladding material is Ni625 powder, the longitudinal residual stress at positions P1-P7 is tensile stress, and when the cladding material uses iron-based alloy powder, not only the longitudinal residual stress value at positions P1-P7 can be reduced, but also the longitudinal residual tensile stress can be changed to compressive stress.

[0038] As shown in Figure 4 and Figure 5 , from the results of the penetrant test, compared with the results of the Ni625 powder laser cladding layer Figure 4 , no cracks Figure 5 appear in the iron-based alloy powder laser cladding layer.

[0039] Example 2

[0040] In this embodiment, the iron-based alloy powder is prepared by vacuum air atomization method, which is composed of the following mass percentages of components: C 0.03%, Mn 0.9%, Si 0.9%, P 0.008%, S 0.008%, Cr 10%, Ni 8%, Mo 0.45%, B 0.7%, Ce 0.15%, Fe balance. The particle size of the iron-based alloy powder is 45-150μm. As a comparative example, Ni625 nickel-based alloy powder produced by Tianjin Cast Gold Technology Development Co., Ltd. is purchased, which is composed of the following mass percentages of components: C 0.1%, Si 0.63%, Cr 20.35%, Fe 2.0%, Mo 8.62%, Nb 3.31%, Ni balance. The particle size of the Ni625 nickel-based alloy powder is 50-200μm.

[0041] The substrate material is QT400-18 nodular cast iron. The surface scale is removed by machining, and after grinding, acetone is used to remove the oil stains on the surface of the substrate, alcohol is used for cleaning, and a blower is used for drying. The size of the substrate is 110mmx95mmx8mm.

[0042] The corundum The fiber laser remanufacturing forming system is used to carry out laser cladding experiment. Laser cladding process parameters are as follows: output power 1.5 kW, scanning speed 8 mm / s, powder feeding rate 9 g / min, protective argon flow rate 18 L / min, overlap rate 40%, and powder feeding mode is synchronous coaxial delivery. Single-layer laser cladding is carried out on the middle part of the substrate surface, and the area size is 60 mm*60 mm. Before laser cladding, the alloy powder is dried in a 120°C drying oven for 2 h. After laser cladding, the substrate is subjected to penetration detection.

[0043] The linear expansion coefficient of the iron-based alloy powder is calculated by using the JMatPro software. The linear expansion coefficient of the Ni625 nickel-based alloy powder is taken from the In625 alloy in the material database of the SYSWELD software, and the linear expansion coefficient of the QT400-18 ductile iron is the measured data. It can be seen from comparison that the linear expansion coefficient of the iron-based alloy powder is almost the same as that of the substrate QT400-18 ductile iron and is lower than that of the Ni625 nickel-based alloy powder.

[0044] The martensite transformation start temperature Ms of the material can be calculated by formula (1). After calculation, the martensite transformation start temperature Ms of the iron-based alloy powder is 202°C. The simulation study finds that when the martensite transformation start temperature of the cladding material is about 200°C, compressive stress can be generated near the weld.

[0045] In this embodiment, after using the iron-based alloy powder as the cladding material, not only the longitudinal residual stress value can be reduced, but also the longitudinal residual tensile stress can be changed to compressive stress. It can be seen from the penetration detection result that, compared with the penetration detection result of the Ni625 powder laser cladding layer, no cracks are generated in the iron-based alloy powder laser cladding layer.

[0046] Example 3

[0047] In this embodiment, the iron-based alloy powder is prepared by using the vacuum gas atomization method and is composed of the following components by mass percentage: C 0.04%, Mn 0.85%, Si 1.0%, P 0.005%, S 0.01%, Cr 10.5%, Ni 7.5%, Mo 0.55%, B 0.8%, Ce 0.25%, and Fe the balance. The particle size of the iron-based alloy powder is 45-150 μm. As a comparative example, the Ni625 nickel-based alloy powder produced by Tianjin Chugold Technology Development Co., Ltd. is purchased and is composed of the following components by mass percentage: C 0.1%, Si 0.63%, Cr 20.35%, Fe 2.0%, Mo 8.62%, Nb 3.31%, and Ni the balance. The particle size of the Ni625 nickel-based alloy powder is 50-200 μm.

[0048] The substrate material is QT400-18 ductile iron. After machining to remove surface oxide scale, grinding, acetone is used to remove oil stains from the substrate surface, followed by alcohol cleaning and drying with a hairdryer. The substrate dimensions are 110mm × 95mm × 8mm.

[0049] Using Huirui Laser cladding experiments were conducted using a fiber laser remanufacturing system. Laser cladding process parameters were: output power 1.6 kW, scanning rate 8 mm / s, powder feed rate 10 g / min, protective argon flow rate 16 L / min, overlap rate 40%, and synchronous coaxial powder feeding. A single layer of 25 laser cladding passes was performed on the central area of ​​the substrate surface, covering an area of ​​60 mm × 60 mm. Before laser cladding, the alloy powder was dried in a 120°C oven for 2 hours. After laser cladding, the substrate underwent penetrant testing.

[0050] The linear expansion coefficient of the iron-based alloy powder was calculated using JMatPro software. The linear expansion coefficient of the Ni625 nickel-based alloy powder was taken from the In625 alloy in the SYSWELD software material database, while the linear expansion coefficient of QT400-18 ductile iron was obtained from measured data. The comparison shows that the linear expansion coefficient of the iron-based alloy powder is almost the same as that of the QT400-18 ductile iron matrix, and lower than that of the Ni625 nickel-based alloy powder.

[0051] The martensitic transformation initiation temperature Ms of the material can be calculated using equation (1). According to the calculation, the martensitic transformation initiation temperature Ms of the iron-based alloy powder is 196℃. Simulation studies have found that when the martensitic transformation initiation temperature of the cladding material is around 200℃, compressive stress can be generated near the weld.

[0052] In this embodiment, using iron-based alloy powder as the cladding material not only reduces the longitudinal residual stress value but also transforms the longitudinal residual tensile stress into compressive stress. The penetrant testing results show that, compared to the Ni625 powder laser cladding layer, the iron-based alloy powder laser cladding layer did not exhibit any cracks.

[0053] The results show that the present invention has developed a novel iron-based alloy powder with a lower nickel content compared with the alloy powders reported in the prior art. By adjusting the alloy ratio, its coefficient of linear expansion is matched with that of ductile iron QT400-18, and it can produce a low-temperature martensitic phase transformation. This can change the tensile stress on the surface of the cladding layer to compressive stress, thereby suppressing the generation of cracks.

Claims

1. A method for controlling cracks in the laser cladding layer of ductile iron used for spent fuel, characterized in that, Iron-based alloy powder is laser-clad onto the surface of QT400-18 ductile iron. The iron-based alloy powder is composed of the following components by mass percentage: C≤0.05%, Mn 0.8~1.2%, Si 0.8~1.0%, P≤0.015%, S≤0.015%, Cr 8~12%, Ni 7~9%, Mo 0.3~0.6%, B 0.6~1.0%, Ce 0.1~0.3%, and Fe balance. Through alloy proportioning, the linear expansion coefficient of the iron-based alloy powder is matched with that of ductile iron QT400-18, and a low-temperature martensitic phase transformation is generated. This transforms the longitudinal residual tensile stress on the cladding layer surface into compressive stress, effectively suppressing the generation of cracks in the cladding layer. Laser cladding process parameters for iron-based alloy powder: output power 1.2~1.8kW, scanning rate 6~10mm / s, powder feeding rate 8~12g / min, protective argon flow rate 12~20L / min, overlap rate 35~45%, and powder feeding method is synchronous coaxial conveying.

2. The method for controlling cracks in the laser cladding layer of ductile iron for spent fuel as described in claim 1, characterized in that, The iron-based alloy powder was prepared by vacuum atomization, and the particle size of the iron-based alloy powder was 45-200 μm.

3. The method for controlling cracks in the laser cladding layer of ductile iron for spent fuel as described in claim 1, characterized in that, Before laser cladding, the iron-based alloy powder was dried in a 120°C drying oven for 2 hours.

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