Ionically hardened layer and method for its production

Abstract

The application belongs to the technical field of alloy surface modification, and particularly relates to an ion hardening layer and a preparation method thereof. The preparation method comprises the following steps: after the furnace body is vacuumized, arc striking, temperature rising, atmosphere passing, temperature keeping and temperature falling are carried out on the base body. The hardness of the hardening layer prepared by the application is high, and the hardening layer is metallurgically combined with the base body, so that the hardening layer has high bonding strength and good impact resistance, and defects such as low bonding strength of a thermal spraying coating, high cost and high brittleness of stellite alloy can be avoided. The hardening layer obtained by the method is a supersaturated solid solution of nitrogen, and no chromium nitride is precipitated, so that the oxidation performance of the alloy base body cannot be reduced in a water vapor environment at 650-750 DEG C, the hardening layer has high corrosion resistance, and the hardness of the alloy is 700-1100 HV.

Description

Technical Field

[0001] This invention belongs to the field of alloy surface modification technology, specifically relating to an ion-hardened layer and its preparation method. Background Technology

[0002] With the rapid development of my country's national economy, the demand for electricity will continue to grow, while the requirements for environmental protection and pollution control will also become increasingly stringent. Therefore, the development of high-efficiency, energy-saving, and environmentally friendly high-parameter ultra-supercritical thermal power generating units is imperative. In recent years, the structure of my country's thermal power units has been continuously optimized, with a significant increase in the proportion of supercritical and ultra-supercritical units. One of the key technologies and bottlenecks in realizing A-USC unit power generation systems is the development of low-cost, high-performance high-temperature alloys for key components.

[0003] Compared to critical hot-end components such as large pipelines and superheaters / reheaters, high-temperature valves, because they are assembled with the main steam chamber, are constantly subjected to the scouring and disturbance of high-temperature, high-pressure steam on their sealing surfaces. The harsh working environment, strong corrosion, intense scouring, and abrasion easily lead to wear on the sealing surfaces, causing leaks and preventing the unit from operating safely and reliably, resulting in incalculable losses to production. To meet the harsh operating conditions and possess good overall performance, the materials used for valve components have been upgraded to internationally developed high-temperature alloys with superior high-temperature performance. However, the complexity of the operating environment has challenged the wear resistance of these new high-temperature alloys, which have excellent oxidation and mechanical properties.

[0004] To further improve the performance of high-temperature alloys in harsh environments, surface treatment is generally performed. Domestically and internationally, for operating conditions exceeding 600℃, commonly used surface treatment technologies include thermal spraying of NiCr-Cr3C2 wear-resistant coatings, plasma spraying, or laser cladding of Stellite alloys. Thermal spraying of NiCr-Cr3C2 coatings can maintain high hardness and oxidation resistance at 800℃, and has high production efficiency; however, the bond with the substrate is mainly mechanical, with relatively low bond strength and poor impact resistance. Plasma spraying or laser cladding of Stellite 6# or Stellite 21# alloys can maintain high hardness and good oxidation resistance at high temperatures, meeting performance requirements. The bond between the alloy layer and the substrate is metallurgical, with high bond strength and the ability to withstand certain impact loads. However, Stellite alloys are expensive, have high hardness, and are brittle; under high welding stress, the Stellite sprayed layer is prone to cracking. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is that when high-temperature alloys are used as materials for valve components, the sealing surface wears down in actual applications, resulting in poor sealing. The invention overcomes the defects of existing technologies, such as low bonding strength between thermal spray coatings and substrates, poor impact resistance and corrosion resistance, as well as the high cost and easy cracking under high welding stress of Stellite alloy, and provides an ion-hardened layer and its preparation method.

[0006] To this end, the present invention provides the following technical solution.

[0007] This invention provides a method for preparing an ion-cured layer, comprising: after the furnace body is evacuated, arcing, heating, atmosphere introduction, heat preservation, and cooling of the substrate;

[0008] The specific steps of arc striking, heating, and atmosphere ventilation include:

[0009] (1) Adjust the voltage to 650-850V, wait for the pressure to rise to 70-90Pa, and increase the duty cycle until arcing begins in the furnace;

[0010] (2) When the arc weakens, adjust the duty cycle to 60-80% and continue to arc;

[0011] (3) When the arc weakens, introduce the atmosphere, adjust the duty cycle to 15-25%, the pressure to 70-90Pa, and the atmosphere flow rate to 350-450ml / min, and continue to arc;

[0012] (4) When the arc weakens, adjust the duty cycle to 20-32%, the pressure to 300-400Pa, and the air flow rate to 700-900ml / min until the temperature is 420-550℃.

[0013] Among them, arc weakening refers to the phenomenon of reduced discharge glow.

[0014] The atmosphere includes at least one of (1)-(2):

[0015] (1) Ammonia;

[0016] (2) A mixture of hydrogen and nitrogen;

[0017] Preferably, the flow rate ratio of hydrogen to nitrogen is 3:1.

[0018] The matrix is ​​at least one of austenitic stainless steel, nickel-based superalloys, and nickel-iron alloys.

[0019] In this invention, the nickel-iron-based alloy contains 15-18% Cr, 40-45% Fe, 3-5% total aluminum and titanium, and the balance is nickel.

[0020] The heat preservation time is 4-12 hours.

[0021] Furthermore, the power supply used in the method for preparing the ion-hardening layer is a pulsed power supply.

[0022] Furthermore, before performing the arc striking, the vacuum level of the furnace body after evacuation should not exceed 13 Pa;

[0023] Preferably, the vacuum level after evacuation of the furnace body is 6-13 Pa.

[0024] Furthermore, before performing the arc striking, the furnace body is pressurized to 35-45 Pa.

[0025] In addition, the present invention also provides an ion-hardening layer, which is prepared by the above-described method, wherein the ion-hardening layer is metallurgically bonded to the substrate.

[0026] The thickness of the ion-hardened layer is 10-30 μm.

[0027] The technical solution of this invention has the following advantages:

[0028] 1. The present invention provides a method for preparing an ion-hardened layer, which includes evacuating the furnace body, followed by arc striking, heating, atmosphere introduction, heat preservation, and cooling of the substrate. The hardened layer obtained by the present invention has high hardness, a metallurgical bond with the substrate, high bonding strength, and good impact resistance, avoiding the defects of low bonding strength of thermal spray coatings and high cost and brittleness of stellite alloys. The hardened layer obtained by this method is a nitrogen supersaturated solid solution without chromium nitride precipitation. It does not reduce the oxidation performance of the alloy substrate in a water vapor environment of 650-750℃, exhibits high corrosion resistance, and has an alloy hardness of 700-1100 HV.

[0029] By controlling the vacuuming of the furnace body, air can be expelled, an ionization environment can be provided, and the influence of other factors such as the exhaust environment can be eliminated. Temperature and insulation affect whether nitriding is possible, the thickness of the nitrided layer, and the nitriding form. If the temperature is too low, nitriding will not occur, and if the temperature is too high, nitrides will form. By controlling the temperature at 420-550℃, the formation of the nitrided layer can be guaranteed.

[0030] Duty cycle and voltage are the sources of heating energy. Under a certain vacuum, a trace amount of nitrogen-containing gas is used to generate electrons and ions through glow discharge. The ions directly bombard the parts, transferring heat energy and bringing the parts to the processing temperature. This temperature allows N ions to diffuse into the matrix and combine with electrons to form N atoms. Under the same temperature conditions, the N atoms diffuse and dissolve into the matrix without affecting the matrix properties. Controlling the atmosphere flow rate mainly controls the source of nitrogen. During the preparation of the hardening layer, some hydrogen and nitrogen will generate ammonia. The synthesis of ammonia from hydrogen and nitrogen is a reversible reaction. If the atmosphere flow rate is too low, the ammonia cannot decompose to produce hydrogen and nitrogen. If the atmosphere flow rate is too high, there will be excess N for the diffusion layer, resulting in waste. The decomposition rate depends on the flow rate and temperature. The higher the flow rate, the lower the ammonia decomposition rate, and the lower the flow rate, the higher the ammonia decomposition rate. The higher the temperature, the higher the ammonia decomposition rate, and the lower the temperature, the lower the ammonia decomposition rate. Therefore, this invention, carried out at specific atmosphere flow rates and temperatures, can improve the utilization rate of the atmosphere and meet the N requirements of the diffusion layer.

[0031] This invention can ensure the formation of the hardened layer and the bonding performance between the hardened layer and the substrate by controlling various parameters in the ion-hardened layer preparation method.

[0032] This invention improves the hardness and wear resistance of the substrate by controlling parameters such as duty cycle, pressure, atmosphere flow rate, temperature, and vacuum degree during the arc heating process, through nitriding, without changing the properties of the substrate itself. Attached Figure Description

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1 This is an electron microscope schematic diagram of the hardened layer in Embodiment 1 of the present invention;

[0035] Figure 2 This is the curve showing the relationship between the hardness and depth of the hardened layer in Embodiment 1 of the present invention;

[0036] Figure 3 This is an electron microscope schematic diagram of the hardened layer in Embodiment 2 of the present invention;

[0037] Figure 4 This is the curve showing the relationship between the hardness and depth of the hardened layer in Embodiment 2 of the present invention. Detailed Implementation

[0038] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0039] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0040] Example 1

[0041] This embodiment provides a method for preparing an ion-cured layer, including:

[0042] The sample was prepared for installation. Cooling water was introduced to pre-evacuate the furnace. After pre-evacuation, the vacuum level inside the furnace was 13 Pa, and the pressure inside the furnace was set to 40 Pa. The sample was a nickel-iron alloy, in which the mass content of Cr was 16%, the mass content of Fe was 42%, the total mass content of aluminum and titanium was 3%, and the balance was nickel.

[0043] Start the high voltage and adjust the voltage value to 650V. Once the pressure rises to 80Pa, increase the duty cycle until arcing begins.

[0044] When the arcing inside the furnace weakens, adjust the duty cycle to 70% and continue arcing.

[0045] When the arcing inside the furnace weakens again, open the main valves for hydrogen and nitrogen to introduce the atmosphere. The total flow rate of the atmosphere is 400 ml / min, the flow ratio of hydrogen to nitrogen is 3:1, the duty cycle is adjusted to 20%, and the pressure is set to 80 Pa. Then, start the gas injection and arcing process.

[0046] As the arc weakens again, the pressure is increased to 300Pa, the atmosphere flow rate is adjusted to 800ml / min, the hydrogen to nitrogen flow ratio is 3:1, and the duty cycle is adjusted to 21% until the temperature rises to 430℃. Then, the temperature is maintained for 8 hours. After the temperature maintenance is completed, the gas flow rate, duty cycle, voltage and pressure are adjusted to zero. Then, the vacuum valve is turned off and water cooling is continued.

[0047] Figure 1 This is an electron microscope schematic diagram of the hardened layer in this embodiment. Figure 1 As can be seen from the figure, the thickness of the hardened layer obtained in this embodiment is 10 μm; Figure 2 This is the curve showing the relationship between the hardness and depth of the hardened layer in this embodiment, obtained through testing with a microhardness tester. Figure 2As can be seen, the hardness of the outer surface of the nickel-iron alloy of the present invention is in the range of 1080-1134 HV. The hardened layer is a nitrogen supersaturated solid solution, and no chromium nitrides are precipitated. The alloy still has excellent oxidation resistance. When the hardened layer is placed in water vapor at 700℃ for oxidation, an oxide film will be formed on its surface. The oxide film is mainly composed of Cr2O3 and internally oxidized Al2O3.

[0048] Example 2

[0049] This embodiment provides a method for preparing an ion-cured layer, including:

[0050] The sample was prepared for installation. Cooling water was introduced to pre-evacuate the furnace. After pre-evacuation, the vacuum level inside the furnace was 10 Pa, and the pressure inside the furnace was set to 40 Pa. The sample was a nickel-iron-based alloy with a Cr content of 16%, an Fe content of 42%, a total aluminum and titanium content of 3%, and the balance being nickel.

[0051] Start the high voltage and adjust the voltage value to 780V. Once the voltage rises to 80Pa, increase the duty cycle until arcing and heating begin.

[0052] When the arcing inside the furnace weakens, adjust the duty cycle to 70% and continue arcing.

[0053] When the arcing inside the furnace weakens again, open the main valves for hydrogen and nitrogen to introduce the atmosphere. The total flow rate of the atmosphere is 400 ml / min, the flow ratio of hydrogen to nitrogen is 3:1, the duty cycle is adjusted to 20%, and the pressure is set to 80 Pa. Then, start the gas injection and arcing process.

[0054] As the arc weakens again, the pressure is increased to 400Pa, the atmosphere flow rate is adjusted to 800ml / min, the hydrogen to nitrogen flow ratio is 3:1, and the duty cycle is adjusted to 27% until the temperature rises to 470℃. Then, the temperature is maintained for 8 hours. After the temperature maintenance is completed, the gas flow rate, duty cycle, voltage and pressure are adjusted to zero. Then, the vacuum valve is turned off and water cooling is continued.

[0055] Figure 3 This is a schematic diagram of the hardened layer in the embodiment using an electron microscope. Figure 3 As can be seen from the figure, the thickness of the hardened layer obtained in this embodiment is 30 μm; Figure 4 This is the curve showing the relationship between the hardness and depth of the hardened layer in this embodiment. Figure 4 As can be seen from the above, the hardness of the outer surface of the nickel-iron alloy provided by the present invention is in the range of 800-900 HV. The hardened layer is a nitrogen supersaturated solid solution, and no chromium nitrides are precipitated. The alloy still has excellent oxidation resistance. When the hardened layer is placed in water vapor at 700℃ for oxidation, an oxide film will be formed on its surface. The oxide film is mainly composed of Cr2O3 and internal Al2O3 oxide.

[0056] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing an ion-cured layer, characterized in that, After the furnace body is evacuated, the substrate is subjected to arc striking, heating, atmosphere introduction, heat preservation, and cooling; the substrate is a nickel-iron alloy; before the arc striking, the vacuum value of the furnace body after evacuation is not higher than 13Pa, and the furnace body is pressurized to 35-45Pa; the heat preservation time is 4-12h. The specific steps of arc striking, heating, and atmosphere ventilation include: (1) Adjust the voltage to 650-850V, wait for the pressure to rise to 70-90Pa, and increase the duty cycle until arcing begins in the furnace; (2) When the arc weakens, adjust the duty cycle to 60-80% and continue arcing; (3) When the arc weakens, introduce the atmosphere, adjust the duty cycle to 15-25%, the pressure to 70-90Pa, and the atmosphere flow rate to 350-450ml / min, and continue to arc; (4) When the arc weakens, adjust the duty cycle to 20-27%, the pressure to 300-400Pa, and the air flow rate to 700-800ml / min until the temperature is 420-470℃; The atmosphere comprises a mixture of hydrogen and nitrogen; the flow rate ratio of hydrogen to nitrogen is 3:

1. The thickness of the ion-hardened layer is 10-30 μm; The nickel-iron alloy contains 15-18% Cr by mass, 40-45% Fe by mass, 3-5% aluminum and titanium by mass, and the balance is nickel.

2. The preparation method according to claim 1, characterized in that, The power supply used in the method for preparing the ion-hardened layer is a pulsed power supply.

3. The preparation method according to claim 1, characterized in that, Before the arc striking, the vacuum level of the furnace body after evacuation is 6-13 Pa.

4. An ion-cured layer, characterized in that, The ion-hardened layer is prepared by any one of the preparation methods described in claims 1-3, wherein the ion-hardened layer is metallurgically bonded to the substrate.

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