A preparation method of 12Cr steel alloy based on vacuum melting and ion implantation technology

Abstract

The present invention mainly proposes a method for preparing a 12Cr steel alloy based on vacuum melting and ion implantation techniques, comprising the following steps: Step 1: Pretreat the raw materials of Fe, Cr and multiple alloying elements; Step 2: Configure the pretreated raw materials into a master alloy, then carry out vacuum melting and casting to form an ingot; Step 3: Carry out hot working treatment on the ingot; Step 4: Carry out quenching and tempering treatment on the hot-rolled slab; Step 5: Use a Cr target to carry out surface modification on the quenched and tempered workpiece by means of ion implantation; Step 6: Carry out post-treatment on the implanted workpiece. The technical solution of the present invention can solve the problem that common high-Cr steel alloys are prone to failure due to large quenching / normalizing stress during use. At the same time, it can solve the problem of poor comprehensive mechanical properties of F / M steel. The elongation of the 12Cr steel prepared by the present invention is increased by about 30%-50% compared with the existing materials, while the strength still meets the use requirements. It enhances the application possibility of high-Cr F / M alloy steel under engineering conditions and improves the service stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloy preparation and surface treatment, and specifically relates to a method for preparing 12Cr steel alloy based on vacuum melting and ion implantation technology. Background Art

[0002] 9% - 12% Cr ferritic / martensitic steel (F / M steel) is listed as a priority candidate system for the cladding material of the fourth-generation nuclear reactor - lead-cooled fast reactor due to its excellent lead-bismuth corrosion resistance and neutron irradiation swelling resistance. In traditional technologies, by adding solid solution strengthening elements such as Mo and W and optimizing the normalizing-tempering process, the yield strength of F / M steel can still be maintained at ≥500 MPa at 600 °C. However, the existing technologies have the following bottlenecks:

[0003] Element distribution defects: Conventional vacuum melting processes still cause dendritic segregation of key elements such as Cr and C (segregation coefficient ≥1.5), especially forming a Cr-rich carbide band (width > 50 μm) in the core of the ingot, which becomes the preferential initiation area of stress corrosion cracks;

[0004] Insufficient stress regulation: After traditional quenching and tempering treatment (quenching + high-temperature tempering), the residual tensile stress inside the material is as high as 200 - 300 MPa, inducing stress corrosion cracking along the original austenite grain boundaries under long-term thermal cyclic loading;

[0005] Surface modification limitations: Existing surface strengthening technologies (such as carburizing and nitriding) can improve hardness, but will exacerbate surface brittleness (when the nano-indentation hardness > 8 GPa, the fracture toughness decreases by 40%), and cannot simultaneously improve the anti-irradiation embrittlement performance. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a method for preparing 12Cr steel alloy based on vacuum melting and ion implantation technology, including the following steps: Step 1: Raw material pretreatment, successively performing ultrasonic pickling, ultrasonic alcohol cleaning, ultrasonic deionized water rinsing, and dehydration drying on raw materials of Fe, Cr, and multiple alloying element raw materials; Step 2: Vacuum melting, preparing a master alloy by mixing the pretreated raw materials according to the Fe-12Cr matrix ratio, melting in a vacuum electromagnetic induction levitation melting furnace and casting into an ingot; Step 3: Hot working treatment, performing hot forging and hot rolling on the ingot; Step 4: Quenching and tempering treatment, successively performing quenching, normalizing, and tempering on the hot-rolled slab; Step 5: Ion implantation, placing the quenched and tempered workpiece in an ion implantation device and using a Cr target for surface modification; Step 6: Post-treatment, cleaning, drying, and encapsulating the implanted workpiece.

[0007] Furthermore, in Step 1, acetic acid solution is used for ultrasonic pickling, and the ultrasonic pickling time ≥ 15 min; the ultrasonic alcohol cleaning time ≥ 25 min; the ultrasonic deionized water rinsing time ≥ 25 min.

[0008] Further, in Step 2, the master alloy composition includes 83% - 85% Fe and 12% Cr by mass percentage.

[0009] Further, the master alloy composition by mass percentage is: Cr 12%, C 0.22%, Ni 0.8%, Mo 0.5%, Mn 1.0%, V 0.2%, Si 0 - 1.6%, W 0.5%, Ta 0.1%, Ti 0.03%, and the balance is Fe and unavoidable impurities.

[0010] Further, in Step 2, the number of melting times is 2 - 4 times, and the melting temperature is 1600 - 1800 °C.

[0011] Further, in Step 3, it is heat-insulated at 1100 - 1200 °C for 30 - 60 min before hot forging; the hot rolling speed is 100 - 200 m / s, and the total deformation amount > 60%; the thickness of the rolled slab after rolling is 5 - 20 mm.

[0012] Further, in Step 4, the quenching medium is water or oil, and the cooling rate is 50 - 100 °C / s; normalizing is carried out by cooling at room temperature for a time ≥ 1 h; the tempering temperature is 600 - 720 °C, and it is heat-insulated for 2 - 4 h, and the furnace cooling rate is 5 - 10 °C / min.

[0013] Further, in Step 5, the background vacuum degree is 1.0 - 1.5 Pa; the injection energy is 1 - 3 MeV, and the beam current intensity is 1 - 100 mA; the injection dose is 1×10 16 ~1×10 18 atoms / cm²; the DC power supply of the Cr target is 40 - 60 A, and the DC power supply of the anode is 20 - 30 A.

[0014] Therefore, the beneficial effects of the present invention are as follows:

[0015] First, it solves the problem that common high-Cr steel alloys are prone to failure due to large quenching / normalizing stress during use. By ion-implanting Cr ions into the material surface, it can excellently improve the stress environment and enhance the service life of the high-Cr steel alloy.

[0016] Second, it solves the problem of poor comprehensive mechanical properties of F / M steel. By quenching and tempering treatment and changing the content of each trace element inside the workpiece, an F / M high-Cr steel alloy with excellent comprehensive mechanical properties is successfully prepared. Its elongation is increased by about 30% - 50% compared with the existing materials, and at the same time, the strength still meets the use requirements. It enhances the application possibility of high-Cr F / M alloy steel under engineering conditions and improves the service stability. Description of the Drawings

[0017] Figure 1 is the flow chart of the steps of the present invention.

[0018] Figure 2 It is a partial optical corrosion photo of the 12Cr steel alloy prepared by the present invention.

[0019] Figure 3 It is an optical corrosion photo of the 12Cr steel alloy prepared by the present invention after tempering treatment.

[0020] Figures 4a - 4d They are scanning electron microscope (SEM) images of different ratios of the 12Cr steel alloy prepared by the present invention after tempering treatment.

[0021] Figure 5 It is an X-ray diffraction (XRD) pattern of the 12Cr steel alloy prepared by the present invention.

[0022] Figure 6 It is a graph of the simulation results of ion implantation SRIM of the 12Cr steel alloy prepared by the present invention. Detailed implementation manners

[0023] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer, reference may be made to the accompanying drawings and embodiments for further description of the technical solutions of the present invention. It should be understood that the embodiments described herein are only used to explain the technical solutions or principles of the present invention, and are not used to limit the protection scope of the present invention. Embodiment

[0024] The following Figures 1 to 6 is a detailed description of Embodiment 1, where Figures 4a - 4d are scanning electron microscope (SEM) images of different ratios of the 12Cr steel alloy after tempering treatment, Figure 4c and Figure 4d the positions of the red frames in indicate the positions where precipitation phases exist.

[0025] This embodiment provides a method for preparing a 12Cr steel alloy based on vacuum melting and ion implantation technology, including the following steps:

[0026] Step 1: Use massive high-purity Fe (99.999%), granular high-purity Cr (99.999%), granular high-purity C (99.999%), granular high-purity Ni (99.999%), granular high-purity Mo (99.999%), granular high-purity Mn (99.999%), granular high-purity V (99.999%), granular high-purity Si (99.999%), granular high-purity W (99.999%), granular high-purity Ta (99.999%), and granular high-purity Ti (99.999%) as raw materials, and successively carry out ultrasonic pickling, ultrasonic alcohol cleaning, ultrasonic deionized water rinsing, and dehydration drying. Among them, the washing times of ultrasonic pickling, ultrasonic alcohol cleaning, and ultrasonic deionized water rinsing are not less than 15 min, 25 min, and 25 min respectively.

[0027] Step 2: Prepare a master alloy by mixing the pretreated raw materials according to the Fe-12Cr matrix ratio. The master alloy has the following mass percentages: Cr 12%, C 0.22%, Ni 0.8%, Mo 0.5%, Mn 1.0%, V 0.2%, Si 0 - 1.6%, W 0.5%, Ta 0.1%, Ti 0.03%, and the balance is Fe and unavoidable impurities. Melt and cast it into an ingot in a vacuum electromagnetic induction levitation melting furnace; in this embodiment, the cast ingot is a 12Cr steel ingot with a height of 20 mm and a width of 80 mm.

[0028] Step 3: Thermal processing. Put the cast ingot after pouring into a high-temperature box furnace for homogenization and heat preservation. The heat preservation temperature is 1100°C - 1200°C, and the time is 30 min - 60 min. Then take out the sample from the heat treatment furnace to complete hot rolling, with a rolling speed of 100 m / s - 200 m / s and a total deformation amount > 60%. Step 4: Quenching and tempering treatment. Carry out quenching, normalizing, and tempering treatments on the hot-rolled slab in sequence; cut the 12Cr steel massive alloy into five pieces of uniform size and put them into a box-type heating furnace for heating respectively. The final heating temperatures are 950°C, 1000°C, 1100°C, and 1200°C respectively, with a heating rate of 5°C / min. After reaching the specified temperature, keep it warm for 1 h, then take out the sample from the high-temperature furnace, and quench the sample in a water / oil cooling quenching box for cooling, with a cooling rate of 50°C - 100°C / s; for the samples subjected to normalizing treatment, place the samples in the air for room temperature cooling, with a cooling time > 60 min, and then carry out tempering treatment on the samples. Reduce the temperature of the box-type heating furnace from 950°C - 1200°C to 600°C - 720°C at a cooling rate of 5°C / min, and the cooling time should be no less than 60 min. Then open the furnace door, and successively put the samples into the heating furnace for heat preservation and tempering treatment, with a heat preservation time of 180 min. After completion, leave the samples in the box-type heating furnace to cool naturally with the furnace body, with a cooling time of not less than 12 h, and then take out the samples.

[0029] Step 5: Ion implantation. Place the quenched and tempered workpiece in an ion implantation device and use a Cr target for surface modification. The 12Cr steel after quenching and tempering treatment in Step 4 is subjected to surface cleaning treatment. Immerse the sample in a 98% alcohol solution and ultrasonically treat it in an ultrasonic machine for 1 hour to remove the impurities attached to the surface. After the ultrasonic treatment, wash it in deionized water and then dry it. Subsequently, place the sample in the vacuum chamber of the ion implantation device, install the Cr target, adjust the background vacuum degree of the vacuum chamber of the ion implantation device and heat it. Turn on the ion implantation source, adjust the ion implantation energy to 1 - 3 MeV, adjust the ion beam current intensity to 1 - 100 mA, and the ion implantation dosage is 1.0×10 16 ~1.0×10 18 atoms / cm 2 , the Cr arc target power supply uses direct current 40 - 60 A, the anode power supply uses direct current 20 - 30 A, the working pressure is 1.0 - 1.5 Pa, the energization time is not less than 30 min, open the baffle. When the ion implantation dosage reaches the preset value, close the baffle, turn off the ion source. Wait until the temperature of the workpiece drops below 100 °C, and then turn off the vacuum system.

[0030] Step 6: Post-treatment. After the implantation, ultrasonically clean the workpiece with alcohol, dry it after cleaning and put it into a vacuum bag for packaging.

[0031] The technical solution of the present invention has the following beneficial effects:

[0032] 1. Optimization of element distribution and coordinated regulation of stress

[0033] Through the synergistic effect of vacuum induction levitation melting (2 - 4 times of remelting) and gradient hot rolling process (1100 - 1200 °C / >60% deformation), the segregation coefficient of Cr element is reduced from more than 1.5 in the traditional process to less than 1.1, and the carbide aggregation zone with a size greater than 50 μm is eliminated. Combining multi-stage quenching and tempering treatment (homogenization at 950 - 1200 °C + stepwise tempering), the internal residual tensile stress of the material is reduced from 200 - 300 MPa in the conventional process to below 80 MPa, significantly inhibiting the tendency of stress corrosion cracking.

[0034] 2. Innovation of surface composite strengthening mechanism

[0035] Adopt high-energy Cr ion implantation technology (1 - 3 MeV / 1×10¹ 6 -1×10¹ 8atoms / cm²), a gradient Cr enrichment layer with a thickness of 10 - 50 μm is formed on the surface layer (the Cr content in the surface layer ≥ 18 wt.%). This structure has both a surface compressive stress layer to offset the tensile stress of the matrix; a dense oxide film (the thickness of the Cr2O3 layer is 200 - 500 nm) reduces the corrosion rate of lead-bismuth to < 0.1 mm / year; high-dose Cr implantation induces nano-scale precipitation phases (5 - 20 nm Cr3C2), improving the anti-irradiation swelling performance (the swelling rate < 0.5% after 3 MeV Au⁺ ion irradiation).

[0036] 3. Synergistic improvement of strength and toughness

[0037] Through the process of "high-temperature homogenization + oil quenching + two-stage tempering" (600 - 720 °C / 2 - 4 h), a fine-grained tempered martensite matrix (grain size 5 - 8 μm) and uniformly distributed M 23 C6 carbides (average size 0.8 μm) are obtained. The elongation is increased by 30 - 50% (the room-temperature elongation ≥ 20% vs. 14 - 15% of traditional materials); the strength retention is excellent (the high-temperature tensile strength at 600 °C ≥ 550 MPa, and the decline rate < 10%); the anti-cyclic creep performance is improved (the steady-state creep rate ≤ 1×10⁻ 8 s⁻¹) under the conditions of 550 °C / 200 MPa.

[0038] 4. Breakthrough in engineering applicability

[0039] The finished size of the prepared 12Cr steel is controllable (100 - 150 mm × 50 - 100 mm × 5 - 20 mm), meeting the processing requirements of fast reactor cladding tubes. After accelerated life testing (in a 550 °C lead-bismuth environment / 10 7 n / cm² irradiation), the service life is increased from 3000 hours of traditional materials to more than 8000 hours, and the cladding breakage rate is reduced to < 5×10⁻ 6 / cycle.

[0040] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a 12Cr steel alloy based on vacuum melting and ion implantation techniques, characterized in that, It includes the following steps: Step 1: Pretreatment of raw materials. The raw materials of Fe, Cr and multiple alloying elements are successively subjected to ultrasonic pickling, ultrasonic alcohol cleaning, ultrasonic deionized water rinsing and dehydration drying; Step 2: Vacuum melting. The pretreated raw materials are made into a master alloy according to the Fe-12Cr matrix ratio, melted in a vacuum electromagnetic induction levitation melting furnace and cast into a steel ingot; The composition of the master alloy includes 83% - 85% Fe and 12% Cr by mass percentage; The number of melting times is 2 - 4 times, and the melting temperature is 1600 - 1800 °C; Step 3: Hot working treatment. The steel ingot is hot forged and hot rolled; Step 4: Quenching and tempering treatment. The hot rolled slab is successively subjected to quenching, normalizing and tempering treatments; The quenching medium is water or oil, and the cooling rate is 50 - 100 °C / s; Normalizing is carried out at room temperature for a time ≥ 1 h; The tempering temperature is 600 - 720 °C, and the holding time is 2 - 4 h, and the furnace cooling rate is 5 - 10 °C / min; Step 5: Ion implantation. The workpiece after quenching and tempering is placed in an ion implantation equipment, and surface modification is carried out using a Cr target; The background vacuum degree is 1.0 - 1.5 Pa; The implantation energy is 1 - 3 MeV, and the beam current intensity is 1 - 100 mA; The implantation dose is 1×10¹ 6 ~1×10¹ 8 atoms / cm²; The DC power supply of the Cr target is 40 - 60 A, and the DC power supply of the anode is 20 - 30 A; Step 6: Post-treatment. The workpiece after implantation is cleaned, dried and encapsulated.

2. The preparation method of a 12Cr steel alloy based on vacuum melting and ion implantation technology according to claim 1, characterized in that In Step 1: Acetic acid solution is used for ultrasonic pickling, and the ultrasonic pickling time is ≥15 min; the ultrasonic alcohol cleaning time is ≥25 min; the ultrasonic deionized water rinsing time is ≥25 min.

3. A method for preparing a 12Cr steel alloy based on vacuum melting and ion implantation technology according to claim 1, characterized in that, The composition of the master alloy is by mass percentage: Cr 12%, C 0.22%, Ni 0.8%, Mo 0.5%, Mn 1.0%, V 0.2%, Si 0 - 1.6%, W 0.5%, Ta 0.1%, Ti 0.03%, and the balance is Fe and unavoidable impurities.

4. A method for preparing a 12Cr steel alloy based on vacuum melting and ion implantation technology according to claim 1, characterized in that, In Step 3, before hot forging, it is held at 1100 - 1200°C for 30 - 60 min; the hot rolling speed is 100 - 200 m / s, and the total reduction is >60%; the thickness of the rolled slab is 5 - 20 mm.

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