Method for inhibiting black stripe structure of nitrogen layer below tempering temperature of heat-resistant steel

Abstract

The present application relates to a kind of hot strength steel lower than tempering temperature nitrogen layer black strip organization inhibiting method, comprising: the workpiece is placed in air furnace or vacuum oil quenching furnace and is preheated, then heated to (1000~1020) ℃ and is quenched, after the workpiece after quenching is cleaned to remove oil dirt and is tempered, and the tempering temperature is 550 ℃~570 ℃;After tempering, the workpiece surface is cleaned;After the workpiece after cleaning is placed in ion nitriding furnace at room temperature, hydrogen is filled, and the furnace temperature is raised to 520 ℃~540 ℃, starts to keep warm, and in the keeping warm process, nitrogen and hydrogen ratio is adjusted in two stages;After keeping warm is completed, the workpiece is cooled with furnace, and vacuum pump is kept in working condition during cooling process;When furnace temperature reaches 80 ℃, it is taken out, and there is no black stripe organization in nitriding layer organization.The present application adjusts nitriding parameter, so that black stripe phenomenon in nitriding layer organization can be inhibited.

Description

Technical Field

[0001] This invention relates to a method for inhibiting the black stripe-like structure of nitrogen layer in heat-resistant steel below the tempering temperature, belonging to the field of chemical heat treatment technology. Background Technology

[0002] 1Cr11Ni2W2MoV is a martensitic heat-resistant stainless steel with high room temperature strength and creep strength, as well as good toughness and oxidation resistance. The heat treatment process specification for 1Cr11Ni2W2MoV steel is: preheating at 850℃ + oil quenching at (1010±10)℃, followed by tempering. Tempering significantly affects its final mechanical properties. This steel exhibits two tempering brittleness zones: (350~530)℃ and (600~670)℃. The suitable tempering temperature range is very narrow; even slight deviations can reduce the steel's impact toughness. A tempering temperature of 550℃~570℃ in the material's heat treatment standard yields the best comprehensive mechanical properties. In the application of 1Cr11Ni2W2MoV, some parts require extremely high wear resistance in addition to meeting strength requirements. Nitriding is an effective process for improving surface hardness and wear resistance.

[0003] Nitriding is a process that uses a rarefied nitrogen-containing gas to generate a glow discharge, bombarding and heating the surface of a metallic material to form nitrides for strengthening. To ensure that the material's performance is not degraded after tempering, the conventional nitriding process temperature is generally 20°C to 30°C lower than the tempering temperature. For 1Cr11Ni2W2MoV material, nitriding surface strengthening presents significant challenges: after nitriding at conventional temperatures, black streaks easily appear within the nitrogen layer. Figure 1 The formation of black streaks has many negative effects, not only altering the uniformity of material cross-sectional hardness and wear resistance, but also creating numerous safety hazards during component use. The aggregation of a large number of black streaks in the same area, under external stress, is likely to lead to crack formation and ultimately cause the nitrogen layer to peel off. Figure 2 ).

[0004] The microstructure of stainless steel after nitriding below the eutectoid temperature, from the surface to the core, is ε→ε+γ'→γ'+α→α. N→The matrix contains only the γ'+α metallographic phase, which appears black. According to the Fe-N phase diagram, nitriding below 590℃ will first form the γ' phase when the nitrogen content of the α phase exceeds the limiting saturation solid solubility of 0.1%, providing the necessary conditions for the formation of black streaks. The usual process involves high-temperature nitriding above the tempering temperature, typically between 630℃ and 650℃, nitriding in the γ phase region to limit γ' phase formation and reduce the likelihood of black streaks. The problem with high-temperature nitriding is that when the nitriding temperature is too high, the dispersed nitrides will aggregate, causing a decrease in the hardness of the nitrided layer. Simultaneously, this temperature range falls within the temper brittleness zone, and the hardness, strength, impact toughness, and other mechanical properties of the matrix significantly decrease after nitriding.

[0005] The phenomenon of black streaks in the nitrogen layer when using conventional nitriding temperatures lower than the tempering temperature is common in martensitic heat-resistant stainless steels such as 1Cr11Ni2W2MoV, and the presence of black streaks affects the use of this type of material. Summary of the Invention

[0006] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a method for suppressing the black stripe structure of the nitrided layer below the tempering temperature of heat-resistant steel. By adjusting the nitriding parameters, the black stripe phenomenon in the nitrided layer structure can be suppressed, thereby obtaining a uniform nitrided layer and properties that meet the technical requirements.

[0007] The solution of the present invention is:

[0008] A method for inhibiting the black stripe-like structure of nitrogen layer in heat-resistant steel below the tempering temperature includes: preheating the workpiece in an air furnace or vacuum oil quenching furnace, then quenching it at (1000~1020)℃; cleaning the quenched workpiece to remove oil stains and then tempering it at a tempering temperature of 550℃~570℃; and cleaning the surface of the workpiece after tempering.

[0009] After cleaning, the workpiece is placed in an ion nitriding furnace at room temperature and charged with hydrogen. The furnace is then heated to 520℃~540℃ and held at that temperature. During the holding process, the nitrogen to hydrogen ratio is adjusted in two stages. In the first stage, nitrogen and hydrogen are charged at a ratio of 1:3 to match the volume of the equipment, and the holding time is 3h~4h. Then, in the second stage, the nitrogen to hydrogen ratio is adjusted to 1:10, with the total gas intake the same as in the first stage, and the holding time continues for 8h~12h. Throughout the entire holding process, the voltage of the ion nitriding furnace is controlled between 650V and 750V, and the current is between 20A and 30A, until the holding time is completed.

[0010] After the heat preservation is completed, the workpiece is cooled with the furnace, and the vacuum pump is kept in working condition during the cooling process; when the furnace temperature reaches 80℃, the workpiece is taken out of the furnace, and there are no black striped structures in the nitrided layer.

[0011] Furthermore, when using an air furnace, the workpiece is placed into the furnace at room temperature; when using a vacuum oil quenching furnace, the workpiece is placed into the furnace at room temperature and then oil-quenched after holding at room temperature.

[0012] Furthermore, for workpieces that have undergone vacuum quenching and tempering, clean the surface stains; for workpieces that have undergone air furnace quenching and tempering, clean the surface oxide and decarburization layer.

[0013] Furthermore, the workpiece placed in an air furnace or vacuum oil quenching furnace is preheated at (840-860) °C.

[0014] Furthermore, the surface hardness of the nitrided area is HV≥820, and the nitriding depth D N ≥0.15mm.

[0015] A system for suppressing the black stripe-like structure of nitrogen layer below the tempering temperature in heat-resistant steel, comprising:

[0016] Quenching and tempering module: The workpiece is preheated in an air furnace or vacuum oil quenching furnace, and then heated to (1000~1020)℃ for quenching. After quenching, the workpiece is cleaned to remove oil stains and then tempered at a tempering temperature of 550℃~570℃. After tempering, the surface of the workpiece is cleaned.

[0017] Ion nitriding module: After cleaning, the workpiece is placed in the ion nitriding furnace at room temperature and charged with hydrogen. The furnace is heated to 520℃~540℃ and held at that temperature. During the holding process, the nitrogen and hydrogen ratio is adjusted in two stages. In the first stage of holding, nitrogen and hydrogen are charged at a ratio of 1:3 to match the volume of the equipment, and the holding time is 3h~4h. Then, in the second stage of holding, the nitrogen and hydrogen ratio is adjusted to 1:10, and the total gas intake is the same as in the first stage. The holding time continues for 8h~12h. Throughout the holding process, the voltage of the ion nitriding furnace is controlled between 650V and 750V, and the current is between 20A and 30A, until the holding time is completed.

[0018] Furnace exit inspection module: After heat preservation, the workpiece is cooled with the furnace, and the vacuum pump is kept in working state during the cooling process; when the furnace temperature reaches 80℃, the workpiece is taken out of the furnace, and there is no black stripe structure in the nitrided layer.

[0019] The advantages of this invention compared to the prior art are:

[0020] (1) This invention addresses the phenomenon of black striped structure of nitrogen layer in martensitic heat-resistant stainless steel 1Cr11 Ni2W2MoV after nitriding strengthening. By adjusting the nitriding parameters, it can ensure that the structure of the nitrided layer is normal and that the properties of the matrix do not decrease after nitriding.

[0021] (2) The present invention uses a conventional nitriding temperature lower than the tempering temperature, which effectively avoids the problem of the decline in the properties of the 1Cr11Ni2W2MoV matrix after high-temperature nitriding.

[0022] (3) By adjusting the process and controlling the atmosphere ratio in stages, this invention effectively avoids the problem of black striped structure of nitrogen layer in 1Cr11 Ni2W2MoV nitriding strengthening, and avoids the problem of nitrogen layer peeling during subsequent use.

[0023] (4) By precisely controlling the process time, this invention ensures that nitrogen atoms diffuse into the nitriding process, avoids the aggregation of nitride grain boundaries, and reduces internal stress.

[0024] (5) The present invention uses ion nitriding, which adopts the method of plasma bombarding the stainless steel surface, which can effectively shorten the nitriding time and greatly improve its working efficiency compared with gas nitriding. Attached Figure Description

[0025] Figure 1 The morphology is characterized by black stripes in the nitrided layer.

[0026] Figure 2 The morphology of nitrogen layer exfoliation;

[0027] Figure 3 The nitrogen layer morphology is shown after 8 hours of staged diffusion treatment. There are no black streaks in the nitrogen layer.

[0028] Figure 4 The nitrogen layer morphology is shown after 12 hours of staged diffusion treatment. There are no black striped structures in the nitrogen layer.

[0029] Figure 5 Brittle indentation of the nitrided layer after 8 hours of diffusion treatment;

[0030] Figure 6 Brittle indentation of the nitrided layer after 12 hours of diffusion treatment;

[0031] Figure 7 This is a flowchart of the present invention. Detailed Implementation

[0032] The present invention will be further described below with reference to the embodiments.

[0033] A method for inhibiting the formation of black stripe-like structures in nitrogen layer below the tempering temperature in heat-resistant steel, wherein the heat-resistant steel is 1Cr11 Ni2W2MoV stainless steel, comprising:

[0034] Preheat the workpiece in an air furnace or vacuum oil quenching furnace, then heat it to (1000~1020)℃ for quenching. After quenching, clean the workpiece to remove oil stains and then temper it at a temperature of 550℃~570℃. After tempering, clean the surface of the workpiece.

[0035] After cleaning, the workpiece is placed in an ion nitriding furnace at room temperature and charged with hydrogen. The furnace is then heated to 520℃~540℃ and held at that temperature. During the holding process, the nitrogen to hydrogen ratio is adjusted in two stages. In the first stage, nitrogen and hydrogen are charged at a ratio of 1:3 to match the volume of the equipment, and the holding time is 3h~4h. Then, in the second stage, the nitrogen to hydrogen ratio is adjusted to 1:10, with the total gas intake the same as in the first stage, and the holding time continues for 8h~12h. Throughout the entire holding process, the voltage of the ion nitriding furnace is controlled between 650V and 750V, and the current is between 20A and 30A, until the holding time is completed.

[0036] After the heat preservation is completed, the workpiece is cooled with the furnace, and the vacuum pump is kept in working condition during the cooling process; when the furnace temperature reaches 80℃, the workpiece is taken out of the furnace, and there are no black striped structures in the nitrided layer.

[0037] Example 1

[0038] like Figure 7 As shown,

[0039] Step 1: Before nitriding, 1Cr11Ni2W2MoV is quenched and tempered. An air furnace is used for quenching, with a preheating temperature of 850℃. The workpiece is placed into the furnace at the preheated temperature, held at that temperature, and then heated to 1010℃. After holding at that temperature, it is oil-quenched. After cleaning to remove oil, the quenched workpiece is tempered in an air furnace at 560℃. After tempering, it is air-cooled. The surface of the quenched and tempered workpiece is then cleaned to remove the oxide and decarburized layer.

[0040] Step 2: Place the cleaned workpiece into the ion nitriding furnace at room temperature. Operate the furnace according to its operating requirements, introducing a small amount of hydrogen as per the furnace's instructions. Heat the furnace to 520°C and begin holding the workpiece at this temperature. During this holding period, the nitrogen to hydrogen ratio is adjusted in two stages. In the first stage, introduce nitrogen and hydrogen in a ratio matching the furnace volume at 1.0:3.0 and hold for 3 hours. Then, in the second stage, adjust the nitrogen to hydrogen ratio to 1.0:10.0, maintaining the same total gas flow as in the first stage, and continue holding for 8 hours. Throughout the holding process, maintain the voltage at 700V and the current at 20A until the holding is complete.

[0041] Step 3, Furnace Removal and Inspection. After heat treatment, the workpiece is cooled in the furnace, with the vacuum pump maintained during the cooling process. The workpiece is removed from the furnace when the temperature drops below 80℃. Subsequent performance tests are then performed after removal. The hardness of the nitrided layer is tested using a Vickers hardness tester, and the nitrided layer is observed metallographically. Figure 3 As shown, the nitrided layer exhibits no black streaks, is uniform and defect-free, and meets the required nitriding depth. The surface indentation of the nitrided layer is intact, meeting the requirements for brittleness level 1. Figure 5 As shown.

[0042] Example 2

[0043] like Figure 7 As shown,

[0044] Step 1: Before nitriding, 1Cr11Ni2W2MoV is quenched and tempered. An air furnace is used for quenching, with a preheating temperature of 850℃. The workpiece is placed into the furnace at the preheated temperature, held at that temperature, and then heated to 1010℃. After holding at that temperature, it is oil-quenched. After cleaning to remove oil, the quenched workpiece is tempered in an air furnace at 560℃. After tempering, it is air-cooled. The surface of the quenched and tempered workpiece is then cleaned to remove the oxide and decarburized layer.

[0045] Step 2: Place the cleaned workpiece into the ion nitriding furnace at room temperature. Operate the furnace according to its operating requirements, introducing a small amount of hydrogen as per the furnace's instructions. Heat the furnace to 520°C and begin holding the workpiece at this temperature. During this holding period, the nitrogen to hydrogen ratio is adjusted in two stages. In the first stage, introduce nitrogen and hydrogen in a ratio matching the furnace volume at 1.0:3.0 and hold for 3 hours. Then, in the second stage, adjust the nitrogen to hydrogen ratio to 1.0:10.0, maintaining the same total gas intake as in the first stage, and continue holding for 12 hours. Throughout the holding process, maintain the voltage at 700V and the current at 20A until the holding is complete.

[0046] Step 3, Furnace Removal and Inspection. After heat treatment, the workpiece is cooled in the furnace, with the vacuum pump maintained during the cooling process. The workpiece is removed from the furnace when the temperature drops below 80℃. Subsequent performance tests are then performed after removal. The hardness of the nitrided layer is tested using a Vickers hardness tester, and the nitrided layer is observed metallographically. Figure 4 As shown, the nitrided layer exhibits no black streaks, is uniform and defect-free, and meets the required nitriding depth. The indentation on the nitrided layer surface is intact, as shown... Figure 6 As shown, it meets the requirements for brittleness level 1.

[0047] This invention addresses the phenomenon of black striped nitrogen layer structure appearing during nitriding strengthening of martensitic heat-resistant stainless steel 1Cr11 Ni2W2MoV. By adjusting the nitriding parameters, it is possible to ensure that the nitrided layer structure is normal and that the matrix properties do not decrease after nitriding.

[0048] This invention uses a conventional nitriding temperature lower than the tempering temperature, which effectively avoids the problem of performance degradation of the 1Cr11Ni2W2MoV matrix after high-temperature nitriding.

[0049] This invention effectively avoids the problem of black striped nitrogen layer structure in 1Cr11Ni2W2MoV nitriding strengthening by adjusting the process and controlling the atmosphere ratio in stages, and also avoids the problem of nitrogen layer peeling off during subsequent use.

[0050] This invention ensures that nitrogen atoms diffuse into the nitriding process through precise control of the process time, avoiding the aggregation of nitride grain boundaries and reducing internal stress.

[0051] This invention employs ion nitriding, which uses plasma bombardment of the stainless steel surface, effectively shortening the nitriding time and significantly improving its efficiency compared to gas nitriding.

[0052] This invention enables precise control of the nitriding layer. Surface Vickers hardness testing, metallographic and scanning electron microscopy observations confirm that the nitriding layer hardness and nitriding performance meet requirements; the nitriding layer is uniform and defect-free, satisfying the following: surface hardness HV ≥ 820 at the nitrided area; nitriding depth D... N ≥0.15mm; metallographic observation shows no black streaks in the nitrided layer; the nitrided layer is brittle to grade 1.

[0053] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for inhibiting the formation of black strip-like structures in nitrogen layer below the tempering temperature in heat-resistant steel, characterized in that, The heat-resistant steel is 1Cr11Ni2W2MoV stainless steel; the method includes: placing the workpiece in an air furnace or a vacuum oil quenching furnace for preheating, then heating it to (1000~1020)℃ for quenching treatment, cleaning the quenched workpiece to remove oil stains and then tempering it at a tempering temperature of 550℃~570℃; after tempering and removing it from the furnace, cleaning the surface of the workpiece. After cleaning, the workpiece is placed in an ion nitriding furnace at room temperature and filled with hydrogen. The furnace is then heated to 520℃~540℃ and held at that temperature. During the holding process, the nitrogen to hydrogen ratio is adjusted in two stages. In the first stage, nitrogen and hydrogen are filled in at a ratio of 1:3 to match the volume of the equipment, and the holding time is 3h~4h. Then, in the second stage, the nitrogen to hydrogen ratio is adjusted to 1:10, with the total gas intake the same as in the first stage, and the holding time continues for 8h~12h. Throughout the entire holding process, the voltage of the ion nitriding furnace is controlled between 650V and 750V, and the current is between 20A and 30A, until the holding time is completed. After the heat preservation is completed, the workpiece is cooled with the furnace, and the vacuum pump is kept in working condition during the cooling process; when the furnace temperature reaches 80℃, the workpiece is taken out of the furnace, and there are no black striped structures in the nitrided layer.

2. The method for inhibiting the black stripe-like structure of nitrogen layer in heat-resistant steel below the tempering temperature according to claim 1, characterized in that, When using an air furnace, the workpiece is placed into the furnace at room temperature; when using a vacuum oil quenching furnace, the workpiece is placed into the furnace at room temperature and then oil-quenched after holding at room temperature.

3. The method for inhibiting the black stripe-like structure of nitrogen layer below the tempering temperature in heat-resistant steel according to claim 1, characterized in that, For workpieces that have undergone vacuum quenching and tempering, clean the surface of any dirt; for workpieces that have undergone air furnace quenching and tempering, clean the surface of any oxide and decarburized layer.

4. The method for inhibiting the black stripe-like structure of nitrogen layer below the tempering temperature in heat-resistant steel according to claim 1, characterized in that, The workpiece is placed in an air furnace or a vacuum oil quenching furnace and preheated at (840-860)℃.

5. The method for inhibiting the black stripe-like structure of nitrogen layer below the tempering temperature in heat-resistant steel according to claim 1, characterized in that, The surface hardness of the nitrided area is HV≥820, and the nitriding depth is D. N ≥0.15mm.

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