Method for improving fatigue performance of ferritic martensitic steel by arc additive manufacturing and heat treatment

Abstract

The present invention relates to a method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment. Dynamic mechanical fatigue tests were carried out on arc additive manufactured modified 9Cr-1Mo ferritic-martensitic steel after different heat treatments and forged modified 9Cr-1Mo ferritic-martensitic steel, and electron backscatter diffraction was used to statistically analyze the original austenite grain size. The data shows that additive manufacturing and heat treatment refine the austenite grains, improve the fatigue life and strength of the modified 9Cr-1Mo ferritic-martensitic steel. The fatigue life is similar to that of the forged modified 9Cr-1Mo ferritic-martensitic steel under the same strain amplitude, while the strength is higher, allowing it to withstand greater loads. On the basis of taking into account the advantages of using arc additive manufacturing to prepare modified 9Cr-1Mo ferritic-martensitic steel, such as shortening the production cycle and being able to manufacture complex parts without a mold, the same fatigue life level as the forged modified 9Cr-1Mo ferritic-martensitic steel is achieved, and the fatigue strength is better than that of forging, improving the fatigue life of the material compared with the original heat treatment method.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment for metal additive manufacturing, and particularly to a method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment. Background Art

[0002] With the development of the economy, the society's demand for energy is increasing day by day. Nuclear energy is currently the only new clean energy proven to be able to provide a large amount of electricity in a base-load manner. In order to ensure the long-term sustainable development of nuclear energy and make nuclear energy truly a safe and clean energy, it is necessary to accelerate the research and development of a new generation of nuclear energy systems. The fourth-generation nuclear reactor has become the focus of research around the world due to the economy, safety, reliability, and sustainability of its fission nuclear energy system. For the fourth-generation nuclear reactor, the materials will face harsh working conditions: high temperature, high neutron irradiation, and material embrittlement caused by contact with liquid metal. Therefore, it is necessary to find new alloys with good mechanical properties, especially high-temperature creep resistance, corrosion resistance, and radiation resistance. P91 ferritic-martensitic steel is obtained by adding a small amount of vanadium (V) and niobium (Nb) to conventional 9Cr-1Mo steel. Due to its good stress corrosion resistance, excellent high-temperature creep strength, high thermal conductivity, and excellent processing performance, P91 ferritic-martensitic steel is the most promising candidate material for fourth-generation nuclear reactor components (such as steam generators, pipelines, or spallation targets of liquid metal-cooled fast breeder reactors). However, in the nuclear power field, for large nuclear components, traditional methods such as forging and casting are limited by long manufacturing cycles and high costs, and the fatigue strength of forged 9Cr-1Mo ferritic-martensitic steel is relatively low. These problems can be solved by using additive manufacturing technology. Additive manufacturing can manufacture complex-shaped parts, reduce time costs, and existing research has shown that additively manufactured 9Cr-1Mo ferritic-martensitic steel has high strength.

[0003] At present, it is relatively mature to prepare modified 9Cr-1Mo ferritic-martensitic steel using arc additive manufacturing technology, which has the advantages of low cost, high material utilization rate, high forming efficiency, high deposition rate, etc. However, the service temperature of nuclear power plant components is 350°C. During the change of operating conditions of nuclear components operating at this temperature, during the startup and shutdown of nuclear reactors, due to the temperature gradient during heating and cooling, they are often subjected to repeated thermal stresses. The cyclic load caused by thermal stress generates severe strain in the material, resulting in low-cycle fatigue failure of engineering components. Arc additive manufacturing materials inevitably have various defects during the manufacturing process, including surface defects such as surface roughness and forming scale accuracy, and internal defects such as pores, cracks, inclusions, and incomplete fusion. The former can be adjusted by subsequent machining, while the latter often cannot be completely eliminated, so it will have a greater impact on the fatigue performance of the material. Moreover, due to the high temperature gradient between the molten pool and other regions during the manufacturing process, large residual stresses are easily generated in the material, leading to deformation and even cracks. A suitable heat treatment process will make the material have higher strength and good toughness to solve the above problems. However, there is no publicly published heat treatment method for the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing to improve its fatigue performance.

[0004] To solve the above problems, this invention patent proposes a method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment. Through dynamic mechanical fatigue tests and microscopic characterization, the relevant dynamic mechanical fatigue test data of the materials processed by the method provided in this patent, the materials processed by the original method, and the forged materials are compared, demonstrating the feasibility and practicality of the method of this invention patent. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies of the poor fatigue life caused by the non-uniformity of the microstructure and the anisotropy of the properties of arc additive manufacturing (WAAM) ferritic-martensitic steel compared with forged ferritic-martensitic steel in the prior art. Utilizing the advantage that the strength of arc additive manufacturing ferritic-martensitic steel is higher than that of forged ferritic-martensitic steel, a method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment is provided. The heat-treated arc additive manufacturing modified 9Cr-1Mo ferritic-martensitic steel refines the austenite grains and improves the fatigue life of the material without changing the cyclic softening characteristics of the material and with a strength higher than that of forged ferritic-martensitic steel.

[0006] The present invention solves its technical problems through the following technical solutions:

[0007] A method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment, comprising the following steps:

[0008] Step 1: Prepare modified 9Cr-1Mo ferritic-martensitic steel using arc additive manufacturing;

[0009] Step 2: Perform Heat Treatment 1 on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 1:

[0010] 2.1. Perform solution treatment at 1200°C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 1, then hold for 2 h and rapidly cool with argon gas;

[0011] 2.2. Perform artificial aging treatment at 760°C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after treatment in 2.1, and then air cool;

[0012] Step 3: Perform Heat Treatment 2 on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 1:

[0013] 3.1. Perform solution treatment at 1200°C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 1, then hold for 2 h and rapidly cool with argon gas;

[0014] 3.2. Perform artificial aging treatment at 760°C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after treatment in 3.1, and then air cool;

[0015] 3.3. Perform solution treatment at 1200°C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after treatment in 3.2, then hold for 2 h and water quench;

[0016] 3.4. Perform artificial aging treatment at 760°C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after treatment in 3.3, and then air cool;

[0017] Step 4: Machine the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and heat treated in Step 2 and Step 3 into rod-shaped specimens respectively, and perform mechanical polishing and electro-polishing on the rod-shaped specimens;

[0018] Step 5: Perform dynamic mechanical fatigue tests on the rod-shaped specimens obtained in Step 4 with a strain amplitude of 0.3% - 0.7%;

[0019] Step 6: Record the dynamic mechanical fatigue test data in Step 5 and analyze the improvement of the fatigue performance of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment methods;

[0020] Step 7. Use the electron backscatter diffraction method to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing obtained in Step 2 after heat treatment;

[0021] Step 8. Use the electron backscatter diffraction method to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing obtained in Step 3 after heat treatment.

[0022] Furthermore, in Step 1, the modified 9Cr-1Mo ferritic-martensitic steel is prepared by arc additive manufacturing. A forming substrate for arc additive manufacturing is prepared using a 10-mm-thick steel plate, which is polished with a wire brush and degreased with a metal cleaning agent. The modified 9Cr-1Mo ferritic-martensitic steel is formed on the treated substrate through arc additive manufacturing technology. The preheating temperature of the substrate is 150°C. A wire with a diameter of 0.9 mm is used as the feedstock for preparation. The wire feeding speed is set to 0.095 m / min, the welding torch travel speed is 10 mm / s, the interlayer temperature is 115°C, the current is 174 A, and the voltage is 21.5 V. A reciprocating additive manufacturing method is adopted. A mixture of 3% CO2 and 97% Ar is used as the shielding gas during the additive manufacturing process, and the shielding gas flow rate is 20 L / min. The modified 9Cr-1Mo ferritic-martensitic steel obtained by arc additive manufacturing is plate-shaped, contains 105 deposition layers, the average height of each layer is about 2.6 mm, and the overall dimensions are 300 mm × 273 mm × 29 mm.

[0023] Furthermore, in Step 2, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing is placed in a box-type electric furnace for solution treatment. The heating rate in the box-type electric furnace is set to 13°C / min, heated to 1200°C, held for 2 h, and then rapidly cooled with argon; subsequently, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing is subjected to artificial aging treatment in the box-type electric furnace. The heating rate in the box-type electric furnace is set to 13°C / min, heated to 760°C, held for 2 h, and air-cooled at room temperature.

[0024] Furthermore, in Step 3, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing treated in Step 2 is placed in a box-type electric furnace for solution treatment. The heating rate in the box-type electric furnace is set to 13°C / min, heated to 1200°C, held for 2 h, and then quenched in water at room temperature; subsequently, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing is subjected to artificial aging treatment in the box-type electric furnace. The heating rate in the box-type electric furnace is set to 13°C / min, heated to 760°C, held for 2 h, and air-cooled at room temperature.

[0025] Furthermore, the modified 9Cr-1Mo ferritic-martensitic steel rod specimens prepared by arc additive manufacturing in step 4 are sampled along the deposition direction. The modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the heat treatment 1 and the heat treatment 2 is machined into rod specimens for dynamic mechanical fatigue tests. For the rod specimens, the gauge section has a length of 13.5 mm and a diameter of 4.5 mm. The roughness requirement for the gauge section is Ra = 0.2 μm. The clamping section has a diameter of 10 mm and a length of 38 mm. The transition fillet from the clamping section to the gauge section is taken as R20. Before performing the dynamic mechanical fatigue test in step 5, the modified 9Cr-1Mo ferritic-martensitic steel rod specimens prepared by arc additive manufacturing after heat treatment 1 and heat treatment 2 and the forged modified 9Cr-1Mo ferritic-martensitic steel rod specimens are mechanically polished, and then electro-polished for 20 s in an electrolyte at room temperature with a voltage of 20 V and a current of 5 A.

[0026] Furthermore, in step 5, a hydraulic servo fatigue testing machine with an axial force bearing capacity of ±25 kN is used to perform dynamic mechanical fatigue tests on the rod specimens obtained in step 4. The dynamic mechanical fatigue test is controlled by a fully reversed triangular waveform R = -1 at 350 °C, with a constant strain rate of 1x10 -3 / s, a strain amplitude of 0.3% - 0.7%. An induction heating generator with a frequency of 15 - 35 kHz is used to heat the rod specimens. A k-type thermocouple with a diameter of 0.25 mm is spot-welded at the center of the cross-section of the rod specimen. The rod specimen is held at a constant temperature for 20 minutes using an induction coil before testing. A high-temperature extensometer with a measuring length of 12.5 mm is used to apply appropriate mechanical strain. For the hysteresis loop, 800 data points are recorded for each cycle. The criterion for fatigue failure is a 20% reduction in the stress range.

[0027] Furthermore, in step 7, the electron backscatter diffraction method is used to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and heat-treated in step 2. The preparation process of the electron backscatter diffraction sample is as follows: Use silicon carbide sandpaper on a polishing machine to polish from coarse to fine in sequence. When changing the sandpaper model, the sample needs to be rotated by 90°. Then, use a solution of 10% HClO4 + 90% C2H5OH by volume ratio to perform electrolytic polishing for about 20 s at a voltage of 20 V and a current of 5 A to remove the scratches formed in the last process. For post-processing, use commercial software TSL OIM Analysis 7.0 to reconstruct the original austenite grain boundaries in the original material IPF.

[0028] Further, in Step 8, the electron backscatter diffraction method is used to statistically analyze the original austenite grain size of the heat-treated modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 3. The preparation process of the electron backscatter diffraction sample is as follows: Use silicon carbide sandpaper on the polishing machine to polish from coarse to fine in sequence. When changing the sandpaper model, the sample needs to be rotated by 90°. Then use a solution with a volume ratio of 10% HClO4 + 90% C2H5OH to perform electrolytic polishing for about 20 s under the conditions of a voltage of 20 V and a current of 5 A to remove the scratches formed in the last process. For post-treatment, use the commercial software TSL OIM Analysis 7.0 to reconstruct the original austenite grain boundaries in the IPF of the original material.

[0029] The advantages and positive effects of the present invention are:

[0030] The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment in the present invention. For the modified 9Cr-1Mo ferritic-martensitic steel prepared by heat-treated arc additive manufacturing, without changing the cyclic softening characteristics of the material and with a strength higher than that of forged ferritic-martensitic steel, the austenite grains are refined, the fatigue life of the material is improved, and the material strength is higher than that of the traditional forged modified 9Cr-1Mo ferritic-martensitic steel under the same strain amplitude, allowing it to withstand a greater load. While taking advantage of the high strength of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing, it overcomes the deficiency that the fatigue life of the modified 9Cr-1Mo ferritic-martensitic steel by arc additive manufacturing is lower than that of the forged 9Cr-1Mo ferritic-martensitic steel. On the basis of taking into account the advantages of using arc additive manufacturing to prepare the modified 9Cr-1Mo ferritic-martensitic steel, such as shortening the production cycle and being able to manufacture more complex parts without a mold, it reaches the same fatigue life level as the forged modified 9Cr-1Mo ferritic-martensitic steel, and the fatigue strength is better than that of forging, improving the fatigue life of the additive manufacturing material compared with the original heat treatment method, which has strong practical value for engineering applications. Description of the Drawings

[0031] Figure 1 It is a physical drawing of the formed modified 9Cr-1Mo ferritic-martensitic steel plate prepared by arc additive manufacturing;

[0032] Figure 2 It is a schematic diagram of the heat treatment method;

[0033] Figure 3 It is a schematic diagram of the selection of rod-shaped specimens;

[0034] Figure 4 It is a schematic diagram of the machining specifications of rod-shaped specimens;

[0035] Figure 5(a) shows the relationship between stress amplitude and fatigue life for a strain amplitude of 0.3%, with heat treatment 1, heat treatment 2, and forging.

[0036] Figure 5 (b) shows the relationship between stress amplitude and fatigue life for a strain amplitude of 0.4%, with heat treatment 1, heat treatment 2, and forging.

[0037] Figure 5 (c) shows the relationship between stress amplitude and fatigue life for a strain amplitude of 0.5%, with heat treatment 1, heat treatment 2, and forging.

[0038] Figure 5 (d) shows the relationship between stress amplitude and fatigue life for a strain amplitude of 0.7%, with heat treatment 1, heat treatment 2, and forging.

[0039] Figure 6 is a comparison chart of the fatigue life of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing with heat treatment 1 and heat treatment 2 and the forged modified 9Cr-1Mo ferritic-martensitic steel at strain amplitudes of 0.3% - 0.7%.

[0040] Figure 7 (a) is the inverse pole figure of the original austenite grains of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing with heat treatment 1.

[0041] Figure 7 (b) is the inverse pole figure of the original austenite grains of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing with heat treatment 2.

[0042] Figure 7 (c) is the inverse pole figure of the original austenite grains of the forged modified 9Cr-1Mo ferritic-martensitic steel.

[0043] In the figure:

[0044] 5.1 - rod-shaped specimen; 5.2 - substrate. Specific implementation manner

[0045] The present invention will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0046] A method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment, comprising the following steps:

[0047] Step 1: Use arc additive manufacturing to prepare a modified 9Cr-1Mo ferritic-martensitic steel.

[0048] Step 2: Perform heat treatment 1 on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in step 1:

[0049] 2.1. Solution treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in step 1 is carried out at 1200 °C, followed by holding for 2 h and then rapid air cooling with argon;

[0050] 2.2. Artificial aging treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the treatment in 2.1 is carried out at 760 °C for 2 h, and then air cooling;

[0051] Step 3. Heat treatment 2 is carried out on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in step 1:

[0052] 3.1. Solution treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in the said step 1 is carried out at 1200 °C, followed by holding for 2 h and then rapid air cooling with argon;

[0053] 3.2. Artificial aging treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the treatment in 3.1 is carried out at 760 °C for 2 h, and then air cooling;

[0054] 3.3. Solution treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the treatment in 3.2 is carried out at 1200 °C, followed by holding for 2 h and then water quenching;

[0055] 3.4. Artificial aging treatment of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the treatment in 3.3 is carried out at 760 °C for 2 h, and then air cooling;

[0056] Step 4. The modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and heat treated in step 2 and step 3 is machined into rod-shaped specimens respectively, and the rod-shaped specimens are subjected to mechanical polishing and electro-polishing treatments;

[0057] Step 5. A dynamic mechanical fatigue test with a strain amplitude of 0.3% - 0.7% is carried out on the rod-shaped specimens obtained in step 4;

[0058] Step 6. Record the dynamic mechanical fatigue test data in step 5 and analyze the improvement of the fatigue performance of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after the heat treatment method;

[0059] Step 7. Use the electron backscatter diffraction (EBSD) method to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and heat treated in step 2;

[0060] Step 8. Use the electron backscatter diffraction (EBSD) method to statistically analyze the original austenite grain size of the heat-treated modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in Step 3.

[0061] In Step 1, a Q345 steel plate with a thickness of 10 mm was used to prepare a formed substrate for arc additive manufacturing. It was polished with a wire brush and degreased with a metal cleaning agent. The modified 9Cr-1Mo ferritic-martensitic steel was formed on the treated Q345 substrate by arc additive manufacturing technology. The preheating temperature of the Q345 substrate was 150 °C. An ER90S-B91 (P91) welding wire with a diameter of 0.9 mm was used as the feedstock for preparation. The wire feeding speed was set at 0.095 m / min, the welding torch travel speed was 10 mm / s, the interlayer temperature was 115 °C, the current was 174 A, and the voltage was 21.5 V. A reciprocating additive manufacturing method was adopted. A mixture of 3% CO2 and 97% Ar was used as the shielding gas during the additive manufacturing process, and the shielding gas flow rate was 20 L / min to avoid oxidation and other contamination of the molten pool during solidification and improve the stability of the arc and the viscosity of the molten pool. The obtained modified 9Cr-1Mo ferritic-martensitic steel by arc additive manufacturing was plate-shaped, which contained 105 deposition layers, with an average height of about 2.6 mm for each layer, and the overall dimensions were 300 mm × 273 mm × 29 mm. The physical object of the formed modified 9Cr-1Mo ferritic-martensitic steel plate prepared by arc additive manufacturing is as Figure 1 shown. In this embodiment, the chemical composition weight percentages of the formed modified 9Cr-1Mo ferritic-martensitic steel by arc additive manufacturing are: carbon accounts for 0.095 wt%, silicon accounts for 0.21 wt%, manganese accounts for 0.49%, phosphorus accounts for 0.006%, sulfur accounts for 0.004%, chromium accounts for 8.98 wt%, molybdenum accounts for 0.99%, vanadium accounts for 0.21%, and niobium accounts for 0.05%.

[0062] In Step 2, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing was placed in a box-type electric furnace for solution treatment. The heating rate in the box-type electric furnace was set at 13 °C / min, and it was heated to 1200 °C. After holding for 2 h, it was rapidly cooled with argon. Subsequently, the modified 9Cr-1Mo ferritic-martensitic steel by arc additive manufacturing was subjected to artificial aging treatment in the box-type electric furnace. The heating rate in the box-type electric furnace was set at 13 °C / min, and it was heated to 760 °C, held for 2 h, and air-cooled at room temperature, as Figure 2 shown.

[0063] In Step 3, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after being processed in Step 2 is put into a box-type electric furnace for solution treatment. The heating rate in the box-type electric furnace is set at 13 °C / min, heated to 1200 °C, held for 2 h, and then water quenched at room temperature. Subsequently, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing is subjected to artificial aging treatment in the box-type electric furnace. The heating rate in the box-type electric furnace is set at 13 °C / min, heated to 760 °C, held for 2 h, and air cooled at room temperature, as Figure 2 shown. The modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment eliminates some of the residual stresses and anisotropy generated in the material due to the high temperature gradient between the molten pool and other regions during the arc additive manufacturing process, making the microstructure of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing have uniform properties.

[0064] In Step 4, in order to maintain the consistency of the selected materials, the modified 9Cr-1Mo ferritic-martensitic steel rod specimens prepared by arc additive manufacturing are sampled along the deposition direction, as Figure 3 shown. For the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing that has undergone the above Heat Treatment 1 and Heat Treatment 2, according to the national standard, rod specimens are designed and machined for dynamic mechanical fatigue tests. The geometric shape of the rod specimens is as Figure 4 shown, with a gauge length of 13.5 mm, a diameter of 4.5 mm, a surface roughness requirement of Ra = 0.2 μm for the gauge section, a diameter of 10 mm and a length of 38 mm for the clamping section, and a transition fillet of R20 from the clamping section to the gauge section. In order to reduce the influence of external environmental factors such as corrosion and scratches on the dynamic mechanical fatigue test, before conducting the dynamic mechanical fatigue test described in Step 5, the rod specimens of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing that have undergone Heat Treatment 1 and Heat Treatment 2 and the rod specimens of the forged modified 9Cr-1Mo ferritic-martensitic steel are mechanically polished to achieve a mirror finish, and then electro-polished for 20 s in an electrolyte (HClO4∶C2H5OH = 1∶9) at room temperature with a voltage of 20 V and a current of 5 A.

[0065] In Step 5, a hydraulic servo fatigue testing machine (MTS370.02) with an axial force bearing capacity of ±25 kN is used to conduct dynamic mechanical fatigue tests on the rod specimens obtained in Step 4. The dynamic mechanical fatigue test is controlled by a fully reversed triangular waveform R = -1 at 350 °C, with a constant strain rate of 1x10 -3 / s, the strain amplitude is 0.3%-0.7%. The temperature of 350 °C is selected because the service temperature of the modified 9Cr-1Mo ferritic martensitic steel used in nuclear power plants is 350 °C. To reach the required test temperature, an induction heating generator with a frequency of 15-35 kHz is used to heat the rod-shaped specimen. To control and measure the temperature, a k-type thermocouple with a diameter of 0.25 mm is spot-welded at the center of the cross-section of the rod-shaped specimen. To obtain a uniform temperature field within the measurement cross-section of the rod-shaped specimen, a self-made induction coil is used to keep the sample at a constant temperature for 20 minutes before the test to ensure that the temperature of the whole sample is consistent. A high-temperature extensometer (Epsilon 7650) with a measurement length of 12.5 mm is used to apply appropriate mechanical strain. For the hysteresis loop, 800 data points are recorded for each cycle to ensure accurate results. The standard definition of fatigue failure is a 20% reduction in the stress range.

[0066] In step 6, the dynamic mechanical fatigue test data, such as Figure 5 shown, for the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing after heat treatment 1 and heat treatment 2, and the forged modified 9Cr-1Mo ferritic martensitic steel, the curves of the response stress peak vs. the number of cycles under the strain amplitude conditions of 3%, 40%, 50%, and 70% at 350 °C can be seen. The response stress values under the same strain decrease with the increase in the number of cycles, and all show cyclic softening characteristics during the strain-controlled cyclic loading process, indicating that the heat treatment method in the present invention has no effect on the change trend of the cyclic stress response. In addition, through comparison, it can be seen that the peak stress of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing after heat treatment 2 is basically the same as that of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing after heat treatment 1, and both are higher than that of the forged modified 9Cr-1Mo ferritic martensitic steel, while the fatigue life of the former is significantly improved compared with the latter, indicating that the heat treatment method of the present invention has a significant effect on improving the fatigue life of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing. It shows that the method for improving the fatigue performance of ferritic martensitic steel by arc additive manufacturing and heat treatment in the present invention, on the one hand, utilizes the advantage of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing having higher strength, and at the same time overcomes the deficiency that the fatigue life of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing is lower than that of the forged 9Cr-1Mo ferritic martensitic steel.

[0067] Comparison of the fatigue lives of the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing after heat treatment 1 and heat treatment 2 and the forged modified 9Cr-1Mo ferritic martensitic steel under a strain amplitude of 0.3%-0.7%, as Figure 6As shown, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment 1 has a fatigue life far lower than that of the forged modified 9Cr-1Mo ferritic-martensitic steel at all strain amplitudes. For strain amplitudes of 0.4%, 0.5%, and 0.7%, the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment 2 has basically the same fatigue strength as the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment 1, while the fatigue life has been significantly improved and is comparable to that of the forged modified 9Cr-1Mo ferritic-martensitic steel. At a strain amplitude of 0.4%, the fatigue life increases from 3442 to 3782; at a strain amplitude of 0.5%, the fatigue life increases from 1560 to 2642; at a strain amplitude of 0.7%, the fatigue life increases from 705 to 834. This shows that the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment 2 of the present invention has significantly improved fatigue mechanical properties compared with the modified 9Cr-1Mo ferritic-martensitic steel prepared only by arc additive manufacturing after heat treatment 1, and has higher material strength than the traditional forged modified 9Cr-1Mo ferritic-martensitic steel under the same strain amplitude and can withstand greater loads. On the basis of taking into account the advantages of using the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing, such as shortening the production cycle and being able to manufacture more complex parts without a mold, it reaches the same fatigue life level as the forged modified 9Cr-1Mo ferritic-martensitic steel, and its fatigue strength is better than that of forging, improving the fatigue life of the material compared with the original heat treatment method. On the one hand, it utilizes the advantage of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing having higher strength, and at the same time overcomes the deficiency that the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing has a lower fatigue life than the forged 9Cr-1Mo ferritic-martensitic steel, which has strong practical value for engineering applications.

[0068] In step 7, the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment obtained in step 2 is statistically analyzed using the electron backscatter diffraction (EBSD) method. The preparation process of the electron backscatter diffraction sample is as follows: Use silicon carbide sandpapers with different mesh numbers on the polishing machine from coarse to fine, successively pass through 400#, 800#, 1000#, 1500#, and 2000# polishing. When changing the sandpaper model, the sample needs to be rotated 90° to eliminate the vertical scratches generated in the previous process. Then, use a solution of 10% HClO4 + 90% C2H5OH by volume ratio, and perform electrolytic polishing for about 20 s under the conditions of a voltage of 20 V and a current of 5 A to remove the scratches formed in the last process. For post-treatment, use the commercial software TSL OIM Analysis 7.0 to reconstruct the original austenite grain boundaries in the original material IPF using the OIM software.

[0069] In step 8, the electron backscatter diffraction (EBSD) method is used to count the original austenite grain size of the modified 9Cr-1Mo ferrite martensitic steel prepared by arc additive manufacturing after heat treatment obtained in step 3. The preparation process of the electron backscatter diffraction sample is: using silicon oxide sandpaper of different meshes on a grinder from coarse to fine, successively through 400#, 800#, 1000#, 1500#, and 2000# grinding and polishing, when changing the sandpaper model, the sample needs to be rotated 90° to eliminate the vertical scratches generated in the previous process, and then use a solution of 10% HClO4+90% C2H5OH by volume, under the conditions of voltage 20V and current 5A, for about 20s of electrolytic polishing to remove the scratches formed in the last process, and the commercial software TSL OIM Analysis 7.0 is used for post-processing, and the original austenite grain boundaries in the original material IPF are reconstructed using the OIM software.

[0070] Generally speaking, as the austenitizing temperature increases, the austenite grains will gradually grow, and the higher the temperature, the more obvious the grain growth. Austenite grain growth is a spontaneous process, because the atoms at the grain boundaries are irregularly arranged, so their energy is relatively high. At higher temperatures, atomic diffusion is easier, so the grains swallow each other to reduce the surface area of ​​the grain boundaries and reduce the surface energy of the grains, thereby causing the austenite grains to grow. The result of austenite grain growth will reduce the mechanical properties of the steel after cooling, especially plasticity and toughness, resulting in performance deterioration. However, the heat treatment method described in the patent of this invention enables the modified 9Cr-1Mo ferritic martensitic steel prepared by arc additive manufacturing to obtain more refined original austenite grains. Figure 7 The reverse pole figures of the original austenite grains of the modified 9Cr-1Mo ferrite martensitic steel prepared by arc additive manufacturing after heat treatment 1 and heat treatment 2 and the forged modified 9Cr-1Mo ferrite martensitic steel are shown. Figure 7 (a) shows that the original austenite grain size of the modified 9Cr-1Mo ferrite martensitic steel prepared by arc additive manufacturing after heat treatment 1 is about 75 μm, which is relatively large; Figure 7 (b) shows that the original austenite grains of the modified 9Cr-1Mo ferrite martensitic steel prepared by arc additive manufacturing after heat treatment 2 of the heat treatment method described in the present invention are significantly refined, about 18μm. In order to compare the forged modified 9Cr-1Mo ferrite martensitic steel with the modified 9Cr-1Mo ferrite martensitic steel prepared by arc additive manufacturing after heat treatment 2 described in the present invention, Figure 7(c) also shows the inverse pole figure of the original austenite grains of the forged modified 9Cr-1Mo ferritic-martensitic steel. The calculated original austenite grain size of the forged modified 9Cr-1Mo ferritic-martensitic steel is 20 μm, which is comparable to the grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing after heat treatment 2, indicating that the modified 9Cr-1Mo ferritic-martensitic steel obtained based on additive manufacturing and heat treatment has refined grains, microscopically verifying the mechanism of enhanced macroscopic fatigue mechanical properties.

[0071] Generally speaking, the heat treatment 2 described in the present invention can reduce the austenite grains of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing by more than three times. When the fatigue life is comparable to that of the forged modified 9Cr-1Mo ferritic-martensitic steel, the strength of the former is significantly higher than that of the latter. While taking advantage of the high strength of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing, the deficiency of the lower fatigue life of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing compared with the forged 9Cr-1Mo ferritic-martensitic steel is overcome.

[0072] The embodiments and drawings of the present invention are disclosed for illustrative purposes. Those of ordinary skill in the art can understand that various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the content disclosed in the embodiments and drawings.

Claims

1. A method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment, characterized in that: It includes the following steps: Step 1: Prepare the modified 9Cr-1Mo ferritic-martensitic steel by arc additive manufacturing; Use a steel plate to prepare a forming substrate for arc additive manufacturing. The preheating temperature of the substrate is 150°C. Use a wire with a diameter of 0.9 mm as the feedstock for preparation. Set the wire feeding speed to 0.095 m / min, the welding torch traveling speed to 10 mm / s, the interlayer temperature to 115°C, the current to 174 A, and the voltage to 21.5 V; Step 2: Perform Heat Treatment 1 on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing in Step 1; 2.

1. Carry out solution treatment at 1200 °C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing in step 1, then keep it warm for 2 h and then rapidly cool it with argon; 2.

2. Carry out artificial aging treatment at 760 °C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing after the treatment in 2.1, and then air cool it; Step 3. Carry out heat treatment 2 on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing in step 1: 3.

1. Carry out solution treatment at 1200 °C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing in step 1, then keep it warm for 2 h and then rapidly cool it with argon; 3.

2. Carry out artificial aging treatment at 760 °C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing after the treatment in 3.1, and then air cool it; 3.

3. Carry out solution treatment at 1200 °C on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing after the treatment in 3.2, then keep it warm for 2 h and then water quench it; 3.

4. Carry out artificial aging treatment at 760 °C for 2 h on the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing after the treatment in 3.3, and then air cool it; Step 4. Machine-process the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing heat-treated in step 2 and step 3 into rod-shaped specimens respectively, and carry out mechanical polishing and electro-polishing treatment on the rod-shaped specimens; Step 5. Carry out dynamic mechanical fatigue tests on the rod-shaped specimens obtained in step 4 with a strain amplitude of 0.3% - 0.7%; Step 6. Record the dynamic mechanical fatigue test data in step 5 and analyze the improvement of the fatigue performance of the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing after the heat treatment method; Step 7. Use the electron backscatter diffraction method to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing heat-treated in step 2; Step 8. Use the electron backscatter diffraction method to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing obtained in Step 3 after heat treatment.

2. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In Step 2, put the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing into a box-type electric furnace for solution treatment. In the box-type electric furnace, set the heating rate to 13°C / min, heat to 1200°C, hold for 2 h, and then rapidly cool with argon. Subsequently, perform artificial aging treatment on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in the box-type electric furnace. In the box-type electric furnace, set the heating rate to 13°C / min, heat to 760°C, hold for 2 h, and air cool at room temperature.

3. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In Step 3, put the modified 9Cr-1Mo ferritic-martensitic steel prepared by the arc additive manufacturing processed in Step 2 into a box-type electric furnace for solution treatment. In the box-type electric furnace, set the heating rate to 13°C / min, heat to 1200°C, hold for 2 h, and then water quench at room temperature. Subsequently, perform artificial aging treatment on the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing in the box-type electric furnace. In the box-type electric furnace, set the heating rate to 13°C / min, heat to 760°C, hold for 2 h, and air cool at room temperature.

4. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In Step 4, sample the modified 9Cr-1Mo ferritic-martensitic steel bar-shaped specimen prepared by arc additive manufacturing along the deposition direction. Machine the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing that has undergone Heat Treatment 1 and Heat Treatment 2 into bar-shaped specimens for dynamic mechanical fatigue tests. For the bar-shaped specimens, the gauge length is 13.5 mm, the diameter is 4.5 mm, the surface roughness requirement of the gauge section is Ra = 0.2 μm, the diameter of the clamping section is 10 mm, and the length is 38 mm. The transition fillet from the clamping section to the gauge section is taken as R20. Before performing the dynamic mechanical fatigue test in Step 5, mechanically polish the bar-shaped specimens of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing that have undergone Heat Treatment 1 and Heat Treatment 2 and the forged modified 9Cr-1Mo ferritic-martensitic steel bar-shaped specimens, and then perform electro-polishing for 20 s in an electrolyte at room temperature with a voltage of 20 V and a current of 5 A.

5. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In step 5, a hydraulic servo fatigue testing machine with an axial force bearing capacity of ±25 kN is used to conduct a dynamic mechanical fatigue test on the rod-shaped specimen obtained in step 4. The dynamic mechanical fatigue test is controlled by a fully reversed triangular waveform with R = -1 at 350 °C, and the constant strain rate is 1x10 -3 / s, the strain amplitude is 0.3% - 0.7%. An induction heating generator with a frequency of 15 - 35 kHz is used to heat the rod-shaped specimen. A k-type thermocouple with a diameter of 0.25 mm is spot-welded at the center of the cross-section of the rod-shaped specimen. The rod-shaped specimen is kept at a constant temperature for 20 minutes using an induction coil before testing. A high-temperature extensometer with a measuring length of 12.5 mm is used to apply appropriate mechanical strain. For the hysteresis loop, 800 data points are recorded for each cycle. The criterion for fatigue failure is a 20% reduction in the stress range.

6. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In step 7, the electron backscatter diffraction method is used to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and subjected to heat treatment in step 2. The preparation process of the electron backscatter diffraction sample is as follows: Use silicon carbide sandpaper on a polishing machine to polish successively from coarse to fine. When changing the sandpaper model, the sample needs to be rotated by 90°. Then, use a solution with a volume ratio of 10% HClO4 + 90% C2H5OH to perform electrolytic polishing for 20 s under the conditions of a voltage of 20 V and a current of 5 A to remove the scratches formed in the last process. For post-treatment, use the commercial software TSL OIM Analysis 7.0, and use the OIM software to reconstruct the original austenite grain boundaries in the IPF of the original material.

7. The method for improving the fatigue performance of ferritic-martensitic steel by arc additive manufacturing and heat treatment according to claim 1, characterized in that: In step 8, the electron backscatter diffraction method is used to statistically analyze the original austenite grain size of the modified 9Cr-1Mo ferritic-martensitic steel prepared by arc additive manufacturing and subjected to heat treatment in step 3. The preparation process of the electron backscatter diffraction sample is as follows: Use silicon carbide sandpaper on a polishing machine to polish successively from coarse to fine. When changing the sandpaper model, the sample needs to be rotated by 90°. Then, use a solution with a volume ratio of 10% HClO4 + 90% C2H5OH to perform electrolytic polishing for 20 s under the conditions of a voltage of 20 V and a current of 5 A to remove the scratches formed in the last process. For post-treatment, use the commercial software TSL OIM Analysis 7.0, and use the OIM software to reconstruct the original austenite grain boundaries in the IPF of the original material.

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