High-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel

Abstract

The invention relates to the technical field of bearing manufacturing, and provides a high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel, which comprises the following steps: a steel ingot of the high-nitrogen bearing steel is subjected to cold rolling after being forged and annealed, the rolling speed is less than 10m / min, and the rolling deformation is 30-40%; after a rolled workpiece is austenitized, the workpiece is sequentially subjected to first-time deep cooling (-196 DEG C to-80 DEG C), pre-strengthening tempering (160-180 DEG C), second-time deep cooling (-196 DEG C to-80 DEG C) and final strengthening tempering (450-500 DEG C), and the high-strength and high-toughness aero-engine high-nitrogen bearing steel is obtained. According to the high-strength, high-toughness and high-corrosion-resistance forming and manufacturing method, the high-strength, high-toughness and high-corrosion-resistance requirements of a bearing under the extreme working condition of an aero-engine are met, the tempering technology of'double-peak tempering 'formed by cold rolling forming in advance, pre-strengthening tempering and final strengthening tempering is organically combined, a double-strength structure of solution strengthening and precipitation strengthening is obtained, and the high-strength, high-toughness and high-corrosion-resistance forming and manufacturing method is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of bearing manufacturing, and particularly to a high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel. Background Technique

[0002] As the third-generation aero-engine bearing steel material, aero-engine high-nitrogen bearing steel significantly improves the corrosion resistance of bearings due to the addition of nitrogen element, and is more suitable for extreme service conditions such as offshore salt spray. It is expected to be applied in the bearings of the new generation of aero-engines. However, due to the compositional differences between high-nitrogen bearing steel and traditional bearing steel, the quenching and high-temperature tempering processes of traditional bearing steel cannot be used. At present, the heat treatment process for high-nitrogen bearing steel in the world is still immature. For example, the processes of multiple tempering and multiple high-temperature tempering are used to promote the precipitation of carbonitrides as much as possible, but the obtained carbonitrides are coarse and the content of retained austenite is extremely low, resulting in the strength and toughness still not reaching the performance indicators of the second-generation bearing steel, and the performance fluctuates greatly, which limits the research and application of aero-engine high-nitrogen bearing steel. Therefore, it is urgent to develop a high-strength and high-toughness heat treatment method for aero-engine high-nitrogen bearing steel. Summary of the Invention

[0003] In view of this, the present invention proposes a heat treatment method for aero-engine high-nitrogen bearing steel with high strength and high toughness. High-nitrogen bearing steel has two hardness peaks in the low-temperature and high-temperature ranges ( Figure 2 ). Through the "low-temperature - high-temperature" double-peak tempering process design, combined with the deep cooling process connection during tempering, the precipitation of fine carbonitrides during tempering is promoted, and the retained austenite in the matrix is stabilized; on this basis, by using the synergistic effect of room-temperature rolling and "double-peak tempering", a microstructure of ultrafine grains, ultrafine carbonitrides, and super-stable retained austenite is obtained, so as to achieve the coordinated improvement of strength and toughness.

[0004] The technical solution of the present invention is realized as follows: On the one hand, the present invention provides a high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel, including the following steps:

[0005] S1, cold rolling (room-temperature rolling): Cold-roll the forging of high-nitrogen bearing steel, the rolling speed < 10 m / min, and the rolling deformation amount is 30% - 40%;

[0006] Specifically: Use a general two-high rolling mill (N-100T) to conduct cold-rolled plate experiments, make the plate specimen bite between the two rolls, at room temperature, adopt a multi-pass method to avoid the occurrence of rolling defects, and control the cold-rolled deformation amount to be 30% - 40% by controlling the reduction ratio,

[0007]

[0008] Among them, h0 is the initial rolling thickness, and h1 is the final rolling thickness. Control the rolling feed speed to be less than 10 m / min, and calculate and control the roll speed through the following formula:

[0009] v = πDn(1 - μ)

[0010] Among them, v is the rolling feed speed, D is the roll diameter, n is the roll speed, and μ is the forward slip value, taking 0.05.

[0011] S2, Heat treatment

[0012] S21, Austenitize the rolled workpiece and then quench it after completion;

[0013] Specifically: Make the material fully austenitized and fully dissolve the primary coarse carbonitrides M 23 (C,N)6 in the form of high-temperature insulation. The insulation temperature is determined according to the empirical formula:

[0014] T A = T AC3 + K × T AC1

[0015] Among them, T AC1 is the starting transformation temperature of austenitization (830 - 850 °C), T AC3 is the ending temperature of austenite transformation (860 - 880 °C). According to the phase diagram, the value range of the correction parameter K is 0.18 - 0.21. The austenite insulation temperature is 1010 - 1050 °C. Select the insulation time according to the workpiece size, and the insulation time is 30 - 60 min. After insulation, use quenching oil for cooling.

[0016] S22, Conduct the first cryogenic treatment on the quenched workpiece at -196 to -80 °C. After completion, wait for the workpiece to return to room temperature, then conduct pre-strengthening tempering treatment at 160 - 180 °C, and cool to room temperature after completion;

[0017] Specifically, for the first cryogenic treatment: After cleaning the oil-quenched workpiece, put it into the cryogenic treatment cabinet to further promote austenite transformation through cryogenic treatment. Control the cryogenic temperature to be -196 to -80 °C, and select the cryogenic time to be 60 - 120 min according to the workpiece size. After cryogenic treatment, place the workpiece in a room temperature environment, and wait for the surface crystallization of the workpiece to completely volatilize before proceeding to the next heat treatment.

[0018] Pre-strengthening tempering: Load the workpiece into a high-temperature tempering furnace, and ensure uniform temperature distribution in the tempering furnace during this process. Pre-strengthening tempering needs to control the precipitation of carbonitrides and ensure the carbon / nitrogen content in martensite. Therefore, the pre-strengthening tempering temperature is selected to be 160 - 180 °C, and the tempering time is 2 h. After tempering, place the workpiece at room temperature for cooling, and wait for the workpiece to cool before proceeding to the next heat treatment.

[0019] S23. After the workpiece is cooled, it is subjected to a second cryogenic treatment at -196 to -80 °C. After completion, after the workpiece returns to room temperature, it is subjected to a final strengthening tempering treatment at 450 to 500 °C. The tempering time is 100 - 150 min, and then it is cooled to room temperature.

[0020] Specifically, for the second cryogenic treatment: The tempered workpiece is placed in a cryogenic treatment cabinet to further stabilize austenite through cryogenic treatment. The cryogenic temperature is controlled at -196 to -80 °C, and the cryogenic time is selected as 60 - 120 min according to the workpiece size. After cryogenic treatment, the workpiece is placed in a room temperature environment, and the next heat treatment is carried out after the surface crystallization of the workpiece completely volatilizes.

[0021] Final strengthening tempering: The workpiece is loaded into a high-temperature tempering furnace, and it is necessary to ensure uniform temperature distribution in the tempering furnace during this process. The final strengthening tempering needs to ensure that the precipitation of carbonitrides is dispersed and uniform, and it is necessary to provide a temperature to reduce the internal stress in martensite. Therefore, the final strengthening tempering temperature is selected as 450 to 500 °C, and the tempering time is 100 - 150 min.

[0022] Based on the above technical solutions, preferably, in step S31, the quenching temperature is 40 - 60 °C.

[0023] Based on the above technical solutions, preferably, the forging is obtained by forging and annealing an ingot of high-nitrogen bearing steel.

[0024] Based on the above technical solutions, preferably, the forging temperature is 1100 - 1220 °C, and forging starts after holding for no less than 2 h; the final forging temperature is 950 - 1000 °C. After forging is completed, it is water-cooled to 180 - 220 °C, and then air-cooled to room temperature.

[0025] Based on the above technical solutions, preferably, the annealing temperature is 850 - 900 °C, the time is 2 - 4 h, then it is cooled to 700 - 750 °C, held for 3 - 4 h and then cooled to 550 - 600 °C, and finally cooled to room temperature.

[0026] Based on the above technical solutions, preferably, by weight percentage, the high-nitrogen bearing steel includes: C 0.25% - 0.35%, Cr 14% - 16%, Si < 1.0%, Mo 0.85% - 1.10%, Mn < 1.0%, N 0.3% - 0.5%, Ni < 0.50%, and the balance is Fe and other inevitable impurities.

[0027] On the other hand, the present invention also provides aero-engine high-nitrogen bearing steel, and the bearing steel is prepared by the above method.

[0028] The high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel of the present invention has the following beneficial effects compared with the prior art:

[0029] (1) Aiming at the requirements of high strength, high toughness and high corrosion resistance of bearings under extreme working conditions of aero-engines, the present invention organically combines the tempering process of "double-peak tempering" composed of pre-cold rolling forming, pre-strengthening tempering and final strengthening tempering to obtain a dual-strengthened structure of solid solution strengthening and precipitation strengthening, and realizes a forming manufacturing method with high strength, high toughness and strong corrosion resistance.

[0030] (2) By cold rolling (cold rolling) treatment at room temperature, the present invention strictly controls the deformation amount to reduce deformation damage while introducing appropriate dislocations, refines grains while increasing diffusion and precipitation channels, and promotes solid solution strengthening and precipitation strengthening during the heat treatment process.

[0031] (3) Through cryogenic treatment, the present invention accelerates the further transformation of unstable retained austenite to obtain high-strength supersaturated martensite structure and stable retained austenite.

[0032] (4) By pre-strengthening tempering, the present invention controls the diffusion process of solid solution elements. Part of the carbon / nitrogen in supersaturated martensite will transfer to adjacent retained austenite to stabilize the retained austenite, and the strain-induced martensite transformation is maximally inhibited, avoiding the dimensional instability caused by the volume expansion of martensite transformation. At the same time, element segregation clusters can be formed during the pre-strengthening tempering process, providing nucleation sites for the precipitation of subsequent carbides and nitrides and reducing the precipitation driving force of the final strengthening tempering.

[0033] (5) By final strengthening tempering, the present invention reduces internal stress and obtains a dispersion distribution of carbides and nitrides. It promotes the precipitation of carbides and nitrides in the high-stress martensite generated during the secondary cryogenic process to improve toughness. While reducing the internal stress in martensite, fine carbides and nitrides are formed at the element segregation clusters to improve strength.

[0034] Finally, through the coordination of the above steps, a structure of ultrafine grains, ultrafine carbides and nitrides, and super-stable retained austenite is obtained, thereby realizing the coordinated improvement of strength and toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0036] Figure 1 It is a diagram of the double-peak tempering heat treatment method for high-nitrogen bearing steel in Embodiment 1 of the present invention;

[0037] Figure 2 This is the microstructure diagram of the high-nitrogen bearing steel in Embodiment 1 of the present invention. The granular substances in the figure are ultra-fine carbonitrides;

[0038] Figure 3 This is the hardness peak of the double-peak tempering of the high-nitrogen bearing steel of the present invention. Specific Embodiments

[0039] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0040] In the previous research on the heat treatment process of high-nitrogen steel by the inventors, the research on the tempering strengthening law of high-nitrogen bearing steel at different tempering temperatures was carried out. It was found that there is a strengthening effect in high-nitrogen bearing steel at two temperature points. The microstructure characteristics in the low-temperature section are tempered martensite, a small amount of fine carbonitrides and some retained austenite, and the strengthening characteristic is that the solid solution elements strengthen the martensite. The microstructure characteristics in the high-temperature section are tempered sorbite, uniformly dispersed spherical carbonitrides and some retained austenite, and the strengthening characteristic is that the precipitated phases hinder the movement of dislocations to play a strengthening role. The inventors believe that there is a solid solution strengthening effect in the low-temperature section and a precipitation strengthening effect in the high-temperature section, and based on this, a heat treatment process method of "double-peak tempering" for high-strength and high-toughness aero-engine high-nitrogen bearing steel is developed. The goal is to utilize the synergistic effect of room-temperature rolling and "double-peak tempering" to obtain a microstructure of ultra-fine grains, ultra-fine carbonitrides, and super-stable retained austenite, so as to achieve the synergistic improvement of strength and toughness.

[0041] The "room temperature" mentioned in the present invention refers to the ambient temperature in the laboratory or workshop, generally 20-30 °C.

[0042] Next, the present invention will be further described in conjunction with the drawings and embodiments.

[0043] Embodiment 1

[0044] By weight percentage, the chemical element components of the high-nitrogen bearing steel in this embodiment include: 0.32% C, 15.21% Cr, 0.95% Si, 0.93% Mo, 0.42% Mn, 0.40% N, 0.15% Ni, and the balance is Fe and other inevitable impurities.

[0045] In the embodiment of the present invention, the smelting equipment used is an electroslag remelting furnace. The steel ingot is processed by wire cutting to prepare a as-cast steel ingot sample with a size of , and then heat treatment is prepared.

[0046] The specific method is realized according to the following steps:

[0047] S1, Forging: Place the ingot in the furnace and heat it at a rate of 80 °C / h to 1200 °C, then hold for a minimum of 2 hours before starting forging. The forging method is the eight-sided four-method, and the final forging temperature is 980 °C. After forging, first spray water to cool to 200 °C, and then air-cool to room temperature of 20 °C to obtain the forgings.

[0048] S2, Annealing: Heat the forgings to 880 °C and hold for 3 hours, cool at a rate of 30 °C / h to 730 °C, hold for 3.5 hours, then cool in the furnace to 600 °C, and subsequently air-cool to room temperature of 20 °C.

[0049] S3, Cold rolling (room temperature rolling): Prepare a specimen with dimensions of 120×40×15 mm from the obtained steel sample. After cleaning and degreasing, place it on the rolling mill and roll it in multiple passes at room temperature of 20 °C to avoid the occurrence of rolling defects. The rolling linear speed is 8 m / min, and the rolling deformation is 30%. When the diameter and height of the workpiece reach the predetermined dimensions, the rolling mill stops moving and the rolling process ends. Inspect whether the diameter and height of the workpiece meet the finished product requirements.

[0050] S4, Double-peak tempering heat treatment:

[0051] Austenitization: Place the steel sample in a vacuum heat treatment furnace for austenitization. The austenitization temperature is 1030 °C, and the austenitization time is 50 minutes. After austenitization is completed, quench the steel sample into a large oil tank for rapid cooling. The quenching medium is U8141 vacuum quenching oil, and the oil temperature is controlled at 50 °C.

[0052] First cryogenic treatment: Use a fully controlled cryogenic treatment equipment to perform ultra-low temperature curing on the quenched steel sample. The process parameters are set to hold at a constant temperature of -90 °C for 90 minutes. The cooling medium is liquid nitrogen, and the cooling rate is 5 °C / min. After cryogenic treatment, place the workpiece in an environment of room temperature of 20 °C. After the surface crystallization of the workpiece completely volatilizes, proceed to the next heat treatment.

[0053] Pre-strengthening tempering: Place the cryogenically treated steel sample in a controlled atmosphere tempering furnace, heat it to 170 °C and hold for 120 minutes. High-purity nitrogen is introduced into the furnace for protection, and then air-cool to room temperature of 20 °C.

[0054] Second cryogenic treatment: Then perform secondary cryogenic treatment on the steel sample. The steps and parameters are the same as those of the first cryogenic treatment.

[0055] Final strengthening tempering: Finally, place the steel sample in a high-temperature tempering furnace for final strengthening tempering treatment. The process parameters are set to hold at 475 °C for 120 minutes, and then air-cool to room temperature of 20 °C.

[0056] Example 2

[0057] By weight percentage, the chemical element composition of the high-nitrogen bearing steel in this embodiment includes: 0.25% C, 14% Cr, 0.8% Si, 0.85% Mo, 0.35% Mn, 0.3% N, 0.1% Ni, and the balance is Fe and other inevitable impurities.

[0058] In the embodiment of the present invention, the smelting equipment used is an electroslag remelting furnace. The steel ingot is subjected to wire cutting to prepare a as-cast steel ingot sample with dimensions of , and then heat treatment is prepared.

[0059] The specific method is implemented according to the following steps:

[0060] S1, forging: Place the steel ingot in the furnace and heat it at a rate of 80 °C / h to 1100 °C, then hold for at least 2 h before starting forging. The forging method is the octagon-four method, and the final forging temperature is 950 °C; after forging, first spray water to cool to 180 °C, and then air-cool to room temperature of 25 °C to obtain a forging.

[0061] S2, annealing: Heat the forging to 850 °C and hold for 2 h, cool it at a rate of 30 °C / h to 700 °C, hold for 3 h, then cool it in the furnace to 550 °C, and then air-cool to room temperature of 25 °C.

[0062] S3, cold rolling (room temperature rolling): Prepare a sample with dimensions of 120×40×15 mm from the obtained steel sample. After cleaning and degreasing it, place it on the rolling mill and roll it in multiple passes at room temperature of 25 °C to avoid the occurrence of rolling defects. The rolling linear speed is 9 m / min, and the rolling deformation is 35%. When the diameter and height of the workpiece reach the predetermined dimensions, the rolling mill stops moving and the rolling process ends. Check whether the diameter and height of the workpiece meet the finished product requirements.

[0063] S4, double-peak tempering heat treatment:

[0064] Austenitization: Put the steel sample into a vacuum heat treatment furnace for austenitization. The austenitization temperature is 1010 °C, and the austenitization time is 60 min. After austenitization is completed, quench the steel sample into a large oil tank for rapid cooling. The quenching medium is U8141 vacuum quenching oil, and the oil temperature is controlled at 40 °C.

[0065] The first cryogenic treatment: Use a full-process controlled cryogenic treatment equipment to perform ultra-low temperature solidification on the quenched steel sample. The process parameters are set to hold at -196 °C for 60 min. The cooling medium is liquid nitrogen, and the cooling rate is 5 °C / min. After cryogenic treatment, place the workpiece in an environment of room temperature of 25 °C, and wait until the surface crystallization of the workpiece completely volatilizes before proceeding to the next heat treatment.

[0066] Pre-strengthening tempering: The steel sample after cryogenic treatment was placed in a controlled atmosphere tempering furnace, heated to 160 °C and held for 100 min. High-purity nitrogen was introduced into the furnace for protection, and then it was air-cooled to room temperature of 25 °C.

[0067] Second cryogenic treatment: Then, the steel sample was subjected to secondary cryogenic treatment, and the steps and parameters were the same as those of the first cryogenic treatment.

[0068] Final strengthening tempering: Finally, the steel sample was put into a high-temperature tempering furnace for final strengthening tempering treatment. The process parameters were set to hold at 450 °C for 150 min, and then air-cooled to room temperature of 25 °C.

[0069] Example 3

[0070] By weight percentage, the chemical element composition of the high-nitrogen bearing steel in this example includes: 0.35% C, 16% Cr, 0.9% Si, 1.1% Mo, 0.5% Mn, 0.5% N, 0.2% Ni, and the balance is Fe and other inevitable impurities.

[0071] In the embodiment of the present invention, the smelting equipment used is an electroslag remelting furnace. The steel ingot was subjected to wire cutting to prepare a as-cast steel ingot sample with dimensions of and then ready for heat treatment.

[0072] The specific method is realized according to the following steps:

[0073] S1, forging: The steel ingot was placed in a furnace and heated to 1220 °C at a rate of 80 °C / h and then held. After holding for no less than 2 h, forging was started. The forging was carried out in the way of the octagonal four-method, and the final forging temperature was 1000 °C; after forging, it was first spray-cooled to 220 °C and then air-cooled to room temperature of 30 °C to obtain a forging.

[0074] S2, annealing: The forging was heated to 900 °C and held for 2 h, cooled at a rate of 30 °C / h to 750 °C, held for 3 h and then furnace-cooled to 580 °C, and then air-cooled to room temperature of 30 °C.

[0075] S3, cold rolling (room temperature rolling): The obtained steel sample was prepared into a specimen with dimensions of 120×40×15 mm. After cleaning and degreasing, it was placed on a rolling mill and rolled in multiple passes at room temperature of 30 °C to avoid the appearance of rolling defects. The rolling linear speed was 9.5 m / min, and the rolling deformation was 40%. When the diameter and height of the workpiece reached the predetermined dimensions, the rolling rolls stopped moving and the rolling process ended. Check whether the diameter and height of the workpiece meet the requirements of the finished product.

[0076] S4, bimodal tempering heat treatment:

[0077] Austenitization: The steel sample was placed in a vacuum heat treatment furnace for austenitization. The austenitization temperature was 1050 °C and the austenitization time was 30 min. After austenitization was completed, the steel sample was quenched into a large oil tank for rapid cooling. The quenching medium was U8141 vacuum quenching oil, and the oil temperature was controlled at 60 °C.

[0078] First cryogenic treatment: A fully controlled cryogenic treatment equipment was used to perform ultra-low temperature solidification on the quenched steel sample. The process parameters were set to maintain a constant temperature at -80 °C for 120 min. The cooling medium was liquid nitrogen, and the cooling rate was 5 °C / min. After cryogenic treatment, the workpiece was placed in an environment at room temperature of 30 °C, and the next heat treatment was carried out after the surface crystallization of the workpiece had completely volatilized.

[0079] Pre-strengthening tempering: The cryogenically treated steel sample was placed in a controlled atmosphere tempering furnace, heated to 180 °C and held for 60 min. High-purity nitrogen was introduced into the furnace for protection, and then it was air-cooled to room temperature of 30 °C.

[0080] Second cryogenic treatment: Then, the steel sample was subjected to secondary cryogenic treatment, and the steps and parameters were the same as those of the first cryogenic treatment.

[0081] Final strengthening tempering: Finally, the steel sample was placed in a high-temperature tempering furnace for final strengthening tempering treatment. The process parameters were set to hold at 500 °C for 100 min, and then it was air-cooled to room temperature of 30 °C.

[0082] Example 4

[0083] The difference between Example 4 and Example 1 is that the final strengthening tempering temperature is 450 °C, and the rest is the same.

[0084] Example 5

[0085] The difference between Example 5 and Example 1 is that the final strengthening tempering temperature is 465 °C, and the rest is the same.

[0086] Example 6

[0087] The difference between Example 6 and Example 1 is that the final strengthening tempering temperature is 485 °C, and the rest is the same.

[0088] Example 7

[0089] The difference between Example 7 and Example 1 is that the final strengthening tempering temperature is 500 °C, and the rest is the same.

[0090] Comparative Example 1

[0091] Compared with Example 1, Comparative Example 1 lacks room temperature rolling treatment, and the rest is the same as Example 1.

[0092] Comparative Example 2

[0093] Comparative Example 2 had a rolling deformation of 45% compared with Example 1, and the rest was the same as in Example 1.

[0094] Comparative Example 3

[0095] Comparative Example 3 had the same temperature and time for pre-strengthening tempering and final strengthening tempering as in Example 1, both being held at 170 °C for 120 min, and the rest was the same as in Example 1.

[0096] Comparative Example 4

[0097] Comparative Example 4 had the same temperature and time for pre-strengthening tempering and final strengthening tempering as in Example 1, both being held at 475 °C for 120 min, and the rest was the same as in Example 1.

[0098] Comparative Example 5

[0099] Comparative Example 5 lacked the final strengthening tempering compared with Example 1, and the rest was the same as in Example 1

[0100] Comparative Example 6

[0101] Comparative Example 6 lacked the second cryogenic treatment and the final strengthening tempering compared with Example 1, and the rest was the same as in Example 1.

[0102] The mechanical properties of the high-nitrogen bearing steel prepared in the above examples and comparative examples were tested. The tensile strength was tested using a universal testing machine (SUNS WDW-50). The Charpy impact toughness tester (Wance

[0103] 452D-4) was used to conduct a room temperature impact test on a standard Charpy U-notch specimen (GB / T 229-2007) with dimensions of 10×10×55 mm. The hardness of the specimen was detected using an HRS-150D Rockwell hardness tester, and the results are shown in the following table.

[0104] Table 1 Mechanical properties of high-nitrogen bearing steel

[0105] Tensile strength (MPa) Impact toughness (J) Hardness (HRC) Example 1 2347 10.41 60 Example 2 2340 10.32 59.2 Example 3 2415 10.58 59.3 Example 4 2279 10.28 59.5 Example 5 2296 10.37 59.7 Example 6 2284 10.36 58.3 Example 7 2213 10.21 57.1 Comparative Example 1 2031 6.32 58.1 Comparative Example 2 2188 6.12 59.7 Comparative Example 3 2093 3.78 58.5 Comparative Example 4 2158 6.89 58.4 Comparative Example 5 1875 5.25 58.8 Comparative Example 6 1754 5.64 57.3

[0106] As shown in Table 1, for the high-nitrogen stainless bearing steel specimens of the examples of the present invention, the tensile strength reached 2213 - 2415 Mpa, the impact energy absorption was 10.21 - 10.58 J, and the hardness reached 57.1 - 60, showing more excellent strength and toughness compared with the comparative examples.

[0107] From Example 1 and Examples 4 - 7, it can be seen that as the temperature of the final strengthening tempering increases, the strength and toughness properties of the high-nitrogen stainless bearing steel show a trend of first increasing and then decreasing, and the properties at 475 °C (Example 1) are the best.

[0108] In Comparative Example 1, rolling was not carried out, and appropriate dislocations were not introduced into the material. The dislocation density inside the material was low, which could not provide sufficient dispersion strengthening effect and channels for subsequent precipitation strengthening. In addition, the austenite grains were coarse, and the phase boundary reaction and element diffusion promoted by subsequent process steps were insufficient, ultimately resulting in a significant decrease in both strength and toughness.

[0109] In Comparative Example 2, excessive rolling led to severe deformation damage of the material, excessive accumulation of defects such as grain boundaries and dislocations, which was prone to induce microcracks or abnormal microstructure. In addition, the residual stress introduced by deformation was too large, making it difficult for subsequent heat treatment to fully and evenly recover, resulting in irregular microstructure evolution, and even prone to grain growth or new adverse phase transformations. These factors led to a decrease in both the strength and toughness of the material.

[0110] One of the keys to double-peak tempering is that "high-temperature tempering" can promote the precipitation of carbonitrides and enhance the secondary strengthening effect. If both temperings are at low temperature (Comparative Example 3), the diffusion of elements such as carbon and nitrogen is restricted, and it is difficult for a large amount of strengthening phases to precipitate in the martensite structure, and dispersed nano-carbonitrides cannot be formed, resulting in a significant reduction in the strengthening effect. Moreover, the low temperature is not sufficient to further stabilize the retained austenite or promote an appropriate amount of martensite transformation, leading to a decrease in tissue uniformity, toughness, and dimensional stability.

[0111] High-temperature tempering is beneficial to the extensive diffusion of carbon / nitrogen and the precipitation of carbonitrides. However, if high-temperature tempering is used twice (Comparative Example 4), it will lead to: excessive precipitation and agglomeration of carbonitrides, losing the nano-scale and dispersed distribution characteristics, coarsening of the precipitates, and reduction of the strengthening effect; poor synergistic effect between martensite and austenite in the tissue, prone to performance deterioration; damaged toughness, and at the same time losing the strengthening effects of element segregation, clusters, and metastable tissues formed in low-temperature tempering; high-temperature tempering may also cause excessive grain growth, further affecting the matrix strength. As a result, the material loses both the due high strength and high toughness.

[0112] Final strengthening tempering can promote the further precipitation of carbon and nitrogen, form nano-compounds and achieve dispersed distribution. In Comparative Example 5, this process is lacking, and the strength and toughness of the steel cannot be fully improved; high quenching stress still remains in the martensite, promoting the tissue to become brittle and significantly reducing the impact toughness.

[0113] The second cryogenic treatment can promote the continued transformation of retained austenite into martensite to obtain more fully supersaturated martensite. In Comparative Example 6, the lack of this process means that some retained austenite fails to be effectively transformed, which will affect the final mechanical properties and hardness. Similar to Comparative Example 5, the lack of final strengthening tempering will lead to insufficient precipitation of carbonitrides, undegraded internal stress, and it is difficult to obtain nano-refined tissue, affecting both strength and toughness.

[0114] Through oil quenching and cryogenic treatment, the present invention obtains a hardened martensite matrix and a stable austenite phase; after the first low-temperature tempering and the second high-temperature tempering, ultrafine carbonitrides are formed in the steel (see Figure 2 ). The examples show that the method of the present invention can significantly improve the strength-ductility performance matching of high-nitrogen stainless bearing steel.

[0115] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high-strength and high-toughness double-peak tempering heat treatment method for high-nitrogen bearing steel for aviation engines, characterized in that: The following steps are involved: S1, cold rolling: cold rolling the high nitrogen bearing steel forgings, with a rolling speed of <10m / min and a rolling deformation of 30% to 40%; S2, heat treatment: S21, austenitizing the rolled workpiece, and then quenching it; S22, subjecting the quenched workpiece to a first deep cryogenic treatment at -196 to -80°C, and after the workpiece returns to room temperature, subjecting the workpiece to a pre-strengthening tempering treatment at 160 to 180°C, and then cooling the workpiece to room temperature; S23, subjecting the cooled workpiece to a second deep cryogenic treatment at -196 to -80°C, and after the workpiece returns to room temperature, subjecting the workpiece to a final strengthening and tempering treatment at 450 to 500°C, and then cooling to room temperature.

2. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: In step S21, the austenitizing temperature is 1010-1050°C, and the time is 30-60 minutes.

3. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: In step S21, the quenching temperature is 40 to 60°C.

4. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: In steps S22 and S23, the time for the first cryogenic treatment and the second cryogenic treatment is 60 to 120 minutes.

5. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: In steps S22 and S23, the time for the pre-strengthening annealing treatment and the final strengthening annealing treatment is 100-150 minutes.

6. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: The forging is obtained by forging and annealing a steel ingot of high nitrogen bearing steel.

7. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 6, characterized in that: The forging temperature is 1100-1220°C, and the forging starts after the holding time is not less than 2h; the final forging temperature is 950-1000°C, and after forging is completed, it is water-cooled to 180-220°C, and then air-cooled to room temperature.

8. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 6, characterized in that: The annealing temperature is 850-900°C for 2-4 hours, then reduced to 700-750°C, kept at this temperature for 3-4 hours, cooled to 550-600°C, and finally cooled to room temperature.

9. A high-strength and high-toughness double-peak tempering heat treatment method for aero-engine high-nitrogen bearing steel as claimed in claim 1, characterized in that: Measured by weight percentage, the high nitrogen bearing steel includes: C 0.25% to 0.35%, Cr 14% to 16%, Si <1.0%, Mo 0.85% to 1.10%, Mn <1.0%, N 0.3% to 0.5%, Ni <0.50%, and the balance is Fe and other inevitable impurities.

10. An aviation engine high nitrogen bearing steel, characterized by: The bearing steel is produced by the method described in any one of claims 1 to 9.

Images