Alloy steel and processing method and application for surface strengthening of alloy steel

Abstract

The invention relates to the technical field of alloy steel surface strengthening, in particular to alloy steel and a surface strengthening processing method and application thereof, the alloy steel is quenched and tempered to obtain quenched and tempered alloy steel, the quenched and tempered alloy steel is processed into a sample, the sample is subjected to induction quenching, and the sample is put into a water-based quenching liquid to be cooled to the room temperature after induction quenching. A surface hardened layer strengthened sample is obtained; the method comprises the following steps: taking alloy steel as a research object, carrying out surface hardening treatment on the sample, cutting out a surface hardening part of the surface hardening layer strengthening sample, carrying out pipe sealing treatment, and carrying out secondary tempering treatment on the sample subjected to the pipe sealing treatment to obtain the surface-strengthened alloy steel, so that efficient and uniform surface hardening is realized by taking the alloy steel as the research object and carrying out surface hardening treatment on the sample; and the surface hardness, the wear resistance and the fatigue strength are obviously improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of alloy steel surface strengthening, in particular to an alloy steel and a surface strengthening processing method and application thereof. BACKGROUND

[0002] The crankshaft is the core component of the engine, and its main function is to convert the reciprocating motion of the piston into rotary motion to drive the engine. During operation, the crankshaft bears bending, torsional load and friction, and is prone to fatigue failure and wear failure after long-term use, which may cause serious problems such as fracture. Therefore, higher requirements are put forward for the wear resistance and fatigue performance of the crankshaft parts. Especially in high-power-density diesel engines, the wear and fatigue failure of the crankshaft significantly affect its service life, so surface strengthening treatment measures need to be taken to prolong its service life. At present, the material processing of the crankshaft generally adopts surface hardening technologies such as nitriding treatment and induction quenching. However, the traditional induction quenching uses oil-based quenching medium as the cooling liquid, which has high cost and pollutes the environment. SUMMARY

[0003] In view of the above problems of the prior art, the present application provides an alloy steel and a surface strengthening processing method and application thereof. The present application takes the alloy steel as the object, studies the influence factors of different high-frequency induction quenching processes on the microstructure, properties and residual stress of the alloy steel, and uses a specific processing method to improve the properties of the alloy steel and thereby prolong the service life of the crankshaft made of the surface strengthened alloy steel.

[0004] To solve the above technical problems, the present application adopts the following technical scheme.

[0005] A surface strengthening processing method of an alloy steel, comprising the following steps: The alloy steel is subjected to quenching and tempering treatment to obtain a quenched and tempered alloy steel, and the microstructure of the quenched and tempered alloy steel is uniform distribution of tempered sorbite. The quenched and tempered alloy steel is processed into a sample. The quenching and tempering process is quenching at 870-900 DEG C for 45-60 min and first tempering at 610-650 DEG C for 2-2.5 h.

[0006] The sample is subjected to induction quenching, and after induction quenching, it is cooled to room temperature in a water-based quenching liquid to obtain a surface hardened layer strengthened sample. During induction quenching, the current is 400-600 mA, and the heating time is 3-7 s.

[0007] The surface hardened layer strengthened sample is cut to obtain a surface strengthened part, which is subjected to tube sealing treatment. The sample after tube sealing treatment is subjected to second tempering treatment, and the material in the tube is taken out to obtain a surface strengthened alloy steel.

[0008] In the preferred embodiment of the present application, the quenched and tempered alloy steel is processed into a rod-shaped sample with standardization and repeatability.

[0009] In the preferred embodiment of the present application, the rod-shaped sample is cleaned and dried before induction quenching, which can remove surface oil and other impurities and ensure a dry and impurity-free surface.

[0010] In the preferred embodiment of the present application, the equipment used for induction quenching is a high-frequency induction heating power supply.

[0011] In the preferred embodiment of the present application, the water-based quenching liquid is a PAG-based quenching liquid, a polyvinyl alcohol quenching liquid, a polyether quenching liquid, or an inorganic salt aqueous solution.

[0012] The PAG-based quenching liquid is UCON Quenchant E, AQUA-QUENCH 365 C, or AQ251C with a mass fraction of 10%; the polyether quenching liquid is NJ-3600 quenching liquid; and the inorganic salt aqueous solution is a nitrate aqueous solution.

[0013] In the preferred embodiment of the present application, the second tempering treatment temperature is 250°C to 400°C, and the time is 1h to 4h. More preferably, the second tempering treatment temperature is 330°C, and the tempering treatment time is 2h.

[0014] In the preferred embodiment of the present application, the alloy steel is a medium-carbon alloy steel. It is suitable for medium-carbon alloy steels that can obtain high hardness and strength through heat treatment, such as 34CrNiMo6 alloy steel, which is an alloy steel used for crankshaft manufacturing.

[0015] The 34CrNiMo6 alloy steel comprises the following components in terms of mass percentage: C: 0.34% to 0.38%, Si: 0.155 to 0.4%, Mn: 0.45% to 0.7%, Cr: 1.44% to 1.7%, Mo: 0.2% to 0.3%, Ni: 1.4% to 1.7%, P: ≤0.0025%, S: ≤0.015%, Cu: ≤0.2%, and the rest is impurities, totaling 100%.

[0016] Another object of the present application is to provide an alloy steel prepared by the processing method described in any one of the above.

[0017] In the preferred embodiment of the present application, the operation process of the sealed tube treatment is as follows: connecting the test tube containing the surface hardened layer of the surface strengthened part of the sample into a vacuum system, pumping to a vacuum degree of ≤0.1 Pa, filling with inert gas, heating one end of the test tube and rotating, and finally forming a sealed test tube. Further preferably, the test tube is a quartz tube, and the inert gas is argon.

[0018] In a preferred embodiment of the present application, a quartz test tube is used for the tube sealing process, and a high-temperature sealing rubber ring is used at the tube opening; the test tube with the sample is connected to a vacuum system to remove oxygen and water vapor, and then filled with inert gas argon; the steps of "vacuuming and filling with inert gas" are repeated 2-3 times until a vacuum degree of ≤0.1 Pa (10 -3 mbar) is reached; one end of the quartz tube is heated by an elongated flame of a hydrogen-oxygen flame gun, and the quartz tube is rotated to be completely sealed, thereby providing an inert gas environment for the sample.

[0019] A third object of the present application is to provide an application of the above-mentioned surface-strengthened alloy steel in a low-cost high-performance crankshaft. The high-performance crankshaft refers to a hardened layer hardness ≥ 500 HV, and a material hardened layer hardness ≥ 420 HV after induction quenching + second tempering; a matrix hardness ≥ 250 HV.

[0020] Compared with the prior art, the present application has the beneficial effects that: 1. The present application takes alloy steel as the research object, and obtains a quenched and tempered alloy steel after quenching and tempering treatment. The microstructure of the quenched and tempered alloy steel is uniform distribution of fine tempered sorbite. The quenched and tempered alloy steel has high strength and hardness, wherein the tensile strength is 1261 MPa, and the elongation after fracture is 20.4%. Meanwhile, the plasticity and toughness are also good, and the good strength and toughness are exhibited. The sample is subjected to induction quenching to meet the requirements of surface strengthening treatment, high-efficiency and uniform surface hardening are realized, and the surface hardness, wear resistance and fatigue strength are significantly improved. Through accurate control of high-frequency induction quenching parameters (especially the cooperation of extremely short heating time 3s-7s and specific current range 400-600 mA) and matching quenching cooling medium and tempering process, the following are realized on the alloy steel (especially 34CrNiMo6 alloy steel) for the first time: 1) ultra-fine and uniform hardened layer microstructure; 2) more optimal depth and gradient distribution of residual compressive stress field; 3) extremely narrow and performance-controlled heat-affected zone; and 4) significantly higher bending / torsional fatigue strength and service life than the existing induction quenching process due to the synergistic effect of the above factors.

[0021] According to the experimental results of the present application, the fatigue stress limit of the quenched and tempered alloy steel is 385 MPa, which is increased to 440 MPa after quenching at 400 mA for 4s. Compared with the quenched state, the fatigue performance of the sample is improved by 55 MPa. After quenching and tempering at 330 DEG C for 2h, the fatigue stress limit is increased to 460 MPa, which is increased by 20 MPa compared with the quenched state. The fatigue stress limit of the quenched and tempered steel is increased by 75 MPa after tempering, which shows that the high-frequency induction quenching and tempering can significantly improve the fatigue performance of the sample, and the fatigue life is also significantly improved.

[0022] The application adopts a water-based quenching liquid to replace a traditional oil-based quenching medium, which not only reduces the cost, but also reduces environmental pollution, while ensuring the cooling effect and material performance. The second tempering process after the sealed tube treatment effectively eliminates the quenching stress, further improves the toughness and comprehensive mechanical properties of the material, while maintaining the performance advantages of the surface hardened layer, significantly reduces the energy consumption and material loss, realizes the surface strengthening of low cost and high performance, and is suitable for large-scale industrial production.

[0023] In addition, in the induction quenching process, eddy current heating is generated on the surface layer of the workpiece by 400mA-600mA current, so that the surface is rapidly heated to an austenitizing temperature (about 850-900°C), and then the water-based quenching liquid rapidly cools to make the austenite transform into high-hardness martensite (HRC 55-60), and at the same time, residual compressive stress is generated due to the volume expansion of the surface layer; the tempering treatment at 250°C-400°C for 1h-4h promotes the decomposition of martensite into tempered troostite and the precipitation of fine carbides, which improves the toughness while maintaining high hardness (HRC 50-55), and finally realizes the synergistic strengthening effect of high hardness and high wear resistance on the surface layer and good toughness in the core. If the induction quenching current (400mA-600mA) and the tempering parameters (250°C-400°C, 1h-4h) of the application are exceeded, the surface strengthening effect will be significantly reduced. For example, the comparative experimental data show that low current (<400mA) will cause the surface hardness to decrease, and the austenitizing is not complete due to insufficient heat input, and the quenching layer is shallow, for example, when the current is 300mA for 3s, the austenitizing is not complete due to insufficient heat input and low temperature, and the quenching layer thickness is 0mm, and the hardness is 275HV; high current (>600mA) will cause overheating, leading to grain coarsening, high hardness but increased brittleness, insufficient internal toughness, and even micro-cracks.

[0024] 2. The alloy steel surface strengthening processing method of the application, the key lies in suitable quenching process parameters, and accurate control of heating, cooling, and first tempering and second tempering parameters, to ensure high stability and repeatability of the processing process. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The scanning microstructure graph of the 34CrNiMo6 steel surface strengthening treatment prepared for the embodiment 1 of the application.

[0026] Figure 2 The surface strengthening hardness curve graph of the 34CrNiMo6 steel prepared for the embodiment 1 of the application.

[0027] Figure 3 The scanning microstructure graph of the 34CrNiMo6 steel surface strengthening treatment prepared for the embodiment 2 of the application.

[0028] Figure 4A surface strengthening hardness curve of the 34CrNiMo6 steel prepared in Embodiment 2 of the present application is shown in the figure. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present application are clearly and completely described in combination with the preferred embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0030] It should be noted that all the professional terms used in the present application are only for the purpose of describing the specific embodiments, and are not intended to limit the protection scope of the present application. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the following embodiments of the present application can be purchased from the market or prepared by the existing method.

[0031] In the following embodiments, during induction quenching, the test uses ROGP-25 high-frequency induction heating power supply, and the induction quenching coil is Φ20mm. The AQ251C cooling liquid is prepared into a mass fraction of 10% of the good fortune AQ251C water-soluble quenching liquid.

[0032] Embodiment 1 A processing method for surface strengthening of a 34CrNiMo6 alloy steel, comprising the following steps: (1) Selecting 34CrNiMo6 alloy steel as the research object, and the specific chemical composition is as follows in mass percentage: C: 0.34%, Si: 0.25%, Mn: 0.65%, Cr: 1.50%, Mo: 0.25%, Ni: 1.50%, P: 0.025%, S: 0.015%, Cu: 0.2%, and the rest is inevitable impurities, totaling 100%.

[0033] The 34CrNiMo6 alloy steel is subjected to quenching and tempering treatment to obtain an alloy steel after treatment; wherein the quenching and tempering process is 870℃ quenching for 45min, and 610℃ first tempering for 2h.

[0034] The 34CrNiMo6 alloy steel after quenching and tempering treatment is processed into a rod-shaped sample with a length of 80mm and a diameter of Φ15mm.

[0035] (2) The rod-shaped sample is thoroughly cleaned using an ultrasonic cleaner to effectively remove surface-attached impurities and oil stains. After cleaning, the sample is dried and properly stored to ensure that it is not contaminated or affected by other external factors during subsequent processing, ensuring the processing quality and accuracy of the experimental results.

[0036] (3) The rod-shaped sample after cleaning in step (2) is inductively quenched using a high-frequency induction heating power source. When inductively quenching, the current parameter of the ROGP-25 high-frequency induction heating power source is first adjusted to 500 mA, and the heating time is set to 5 s. Then, the rod-shaped sample is accurately placed in the center of the heating coil using a special tweezer to ensure uniform heating. After starting the power source, the rod-shaped sample completes the austenitizing process within the set heating time. After heating, the rod-shaped sample is immediately transferred to a 10% by mass AQ251C cooling liquid for rapid cooling until room temperature is reached, so that a surface hardened layer strengthened sample is obtained, thereby effectively improving the surface hardness, wear resistance and fatigue strength thereof.

[0037] (4) The surface hardened layer strengthened sample after step (3) is cut into a surface strengthened part using a wire cutting machine, and the surface strengthened part is subjected to a tube sealing treatment. The operation process of the tube sealing treatment is as follows: the test tube with the surface hardened layer strengthened sample is connected to a vacuum system, vacuumized to 0.1 Pa, filled with inert gas, and one end of the test tube is heated and rotated to form a sealed test tube.

[0038] After the tube sealing treatment, the tube and the sample inside are placed in a box-type resistance furnace for a second tempering treatment. The second tempering treatment is performed at a holding temperature of 330°C for 2 h. The sample in the tube is taken out, and a surface strengthened 34CrNiMo6 alloy steel is obtained, which is suitable for low-cost high-performance crankshafts.

[0039] Figure 1 The scanning microstructure diagram of the surface strengthened 34CrNiMo6 alloy steel sample after the second tempering treatment for Example 1. It can be seen that the hardened layer of the surface strengthened 34CrNiMo6 alloy steel is essentially uniform and fine martensite, which plays a great role in the surface layer, because the quenching temperature is higher than the austenitizing temperature, causing the sorbite structure to change. Figure 1

[0040] Figure 2 The hardness curve diagram of the surface strengthened 34CrNiMo6 alloy steel prepared in Example 1. It can be seen that the hardness of the hardened layer of the surface strengthened 34CrNiMo6 alloy steel is 456 HV, and the hardness of the matrix is 285 HV. Figure 2

[0041] Example 2 A processing method for surface strengthening of a 34CrNiMo6 alloy steel, comprising the following steps: ​​(1) Select 34CrNiMo6 alloy steel as the research object, the specific chemical composition mass percentage is as follows: C: 0.34%, Si: 0.25%, Mn: 0.65%, Cr: 1.50%, Mo: 0.25%, Ni: 1.50%, P: 0.025%, S: 0.015%, Cu: 0.2%, the rest is inevitable impurities, total 100%.

[0042] Quenching and tempering treatment is carried out on 34CrNiMo6 alloy steel to obtain treated alloy steel; wherein the quenching and tempering process is 870℃ quenching for 45min, 610℃ first tempering for 2h.

[0043] The quenched and tempered 34CrNiMo6 alloy steel is processed into a rod-shaped sample with a length of 80mm*Φ15mm.

[0044] (2) The rod-shaped sample is thoroughly cleaned with ultrasonic cleaner to effectively remove surface-attached impurities and oil stains. After cleaning, the sample is dried and properly stored to ensure that it is not contaminated or affected by other external factors during subsequent processing, ensuring the accuracy of the processing quality and experimental results.

[0045] (3) The rod-shaped sample cleaned in step (2) is inductively hardened using a high-frequency induction heating power source. During inductive hardening, first adjust the current parameter of the ROGP-25 high-frequency induction heating power source to 600mA and set the heating time to 3s. Then, use a special tweezer to accurately place the rod-shaped sample in the center of the heating coil to ensure uniform heating. After starting the power source, the rod-shaped sample completes the austenitizing process within the set heating time. After heating is completed, immediately transfer the rod-shaped sample to an AQ251C cooling liquid with a mass fraction of 10% for rapid cooling until room temperature is reached. The surface hardened layer strengthening sample can be obtained, thereby effectively improving its surface hardness, wear resistance and fatigue strength.

[0046] (4) The surface hardened layer strengthening sample treated in step (3) is cut using a wire cutting machine to obtain a surface strengthening part. The surface strengthening part is subjected to tube sealing treatment, and the operation process is as follows: connect the test tube containing the surface hardened layer strengthening sample to the vacuum system, extract to a vacuum degree of 0.1Pa, fill with inert gas, heat one end of the test tube and rotate, and finally form a sealed test tube.

[0047] After tube sealing treatment, the tube and its internal sample are placed in a box-type resistance furnace for second tempering treatment. The second tempering treatment is carried out at a holding temperature of 330℃ for 2h. The sample in the tube is taken out, and the surface-strengthened 34CrNiMo6 alloy steel is obtained, which is suitable for low-cost high-performance crankshafts.

[0048] Figure 3 The scanning microstructure diagram of the surface strengthened 34CrNiMo6 alloy steel sample after quenching and second tempering prepared in Example 2 is shown in Figure 2. Figure 3 It can be seen that the hardened layer of the surface strengthened 34CrNiMo6 alloy steel is essentially a uniform and fine martensite, which plays a great role in the surface layer, because the quenching temperature is higher than the austenitizing temperature, causing the change of sorbite structure.

[0049] Figure 4 The hardness curve diagram of the surface strengthened 34CrNiMo6 alloy steel prepared in Example 2 is shown in Figure 3. Figure 4 It can be seen that the hardness of the hardened layer of the surface strengthened 34CrNiMo6 alloy steel is 422HV, and the hardness of the matrix is 284HV.

[0050] Example 3 A processing method for surface strengthening of a 34CrNiMo6 alloy steel, comprising the following steps: (1) Selecting 34CrNiMo6 alloy steel as the research object, the specific chemical composition and mass percentage are as follows: C: 0.34%, Si: 0.25%, Mn: 0.65%, Cr: 1.50%, Mo: 0.25%, Ni: 1.50%, P: 0.025%, S: 0.015%, Cu: 0.2%, and the rest is unavoidable impurities, totaling 100%.

[0051] Quenching and tempering treatment is performed on the 34CrNiMo6 alloy steel to obtain the treated alloy steel; wherein the quenching and tempering process is 870℃ quenching for 45min, and 610℃ first tempering for 2h.

[0052] The quenched and tempered 34CrNiMo6 alloy steel is processed into a rod-shaped sample with a length of 80mm x Φ15mm.

[0053] (2) The rod-shaped sample is thoroughly cleaned using ultrasonic cleaning agent to effectively remove surface-attached impurities and oil stains. After cleaning, the sample is dried and properly stored to ensure that it is not contaminated or affected by other external factors during subsequent processing, ensuring processing quality and accuracy of experimental results.

[0054] (3) The rod-shaped sample after cleaning in step (2) is inductively quenched by using a high-frequency induction heating power supply. When inductively quenching, the current parameter of the ROGP-25 high-frequency induction heating power supply is first adjusted to 400 mA, and the heating time is set to 7 s. Then, the rod-shaped sample is accurately placed in the center of the heating coil by using a special tweezer to ensure the uniformity of heating. After starting the power supply, the rod-shaped sample completes the austenitizing process within the set heating time. After heating, the rod-shaped sample is immediately transferred to the 10% by mass AQ251C cooling liquid for rapid cooling until room temperature is reached, so that a surface hardened layer strengthened sample is obtained, thereby effectively improving the surface hardness, wear resistance and fatigue strength of the sample.

[0055] (4) The surface hardened layer strengthened sample after step (3) is cut into a surface strengthened part by using a wire cutting machine, and the surface strengthened part is subjected to a tube sealing treatment. The operation process of the tube sealing treatment is as follows: the test tube with the surface hardened layer strengthened sample is connected to a vacuum system, and is pumped to a vacuum degree of 0.1 Pa. Inert gas is filled, one end of the test tube is heated and rotated, and finally a sealed test tube is formed.

[0056] After the tube sealing treatment, the tube and the sample in the tube are placed in a box-type resistance furnace for a second tempering treatment. The holding temperature of the second tempering treatment is 330℃, and the duration is 2 h. The sample in the tube is taken out, and a surface strengthened 34CrNiMo6 alloy steel is obtained.

[0057] In addition, the residual stress of the example 1 (500 mA, 5 s quenching + second tempering treatment), the example 2 (600 mA, 3 s quenching + second tempering treatment) and the example 3 (400 mA, 7 s quenching + second tempering treatment) is tested respectively, and the results are shown in Table 1.

[0058] Table 1 Residual stress of different processes After the inductive quenching and the second tempering treatment, the residual stress of the sample is compressive stress. After inductive quenching, the sample presents the characteristics of compressive stress, which is due to the difference in cooling rate between the surface and the interior of the sample. Specifically, the surface cools rapidly and preferentially shrinks; on the contrary, the interior cools relatively slowly and the shrinkage lags behind. The difference in cooling rate leads to tensile stress (thermal stress) on the surface layer, and the martensitic transformation forms greater compressive stress (microstructure stress). The final residual stress is mainly compressive stress, which is the result of the joint action of the two. After the combined process of inductive quenching and second tempering treatment, the second tempering treatment significantly reduces the residual compressive stress of the hardened layer, while the matrix region maintains a relatively high compressive stress level, which reflects the regulation effect of the second tempering treatment on the residual stress.

[0059] In addition, the application also provides examples 4, 5 and 6.

[0060] The difference between example 4 and example 1 is that: The quenching process is quenching at 900 DEG C for 60 min, and the first tempering is at 650 DEG C for 2.5 h.

[0061] The difference between example 5 and example 1 is that: The temperature of the second tempering process is 200 DEG C to 400 DEG C, and the time is 4 h.

[0062] The difference between example 6 and example 1 is that: The temperature of the second tempering process is 400 DEG C, and the time is 1 h.

[0063] The results show that the low-cost high-performance crankshaft made of the surface-strengthened 34CrNiMo6 alloy steel prepared by examples 4 to 6 has a hardened layer hardness of greater than or equal to 420 HV and a matrix hardness of greater than or equal to 250 HV.

[0064] It should be noted that when the application involves a numerical range, both endpoints of each numerical range and any number between the two endpoints can be selected. Since the same steps and examples are used, the preferred examples are described in the application to prevent redundancy. Although the preferred examples of the application have been described, those skilled in the art can make additional changes and modifications to the examples once they understand the basic creative concept. Therefore, the appended claims are intended to include the preferred examples and all changes and modifications falling within the scope of the application.

[0065] Obviously, those skilled in the art can make various modifications and variations to the application without departing from the spirit and scope of the application. Thus, if these modifications and variations of the application fall within the scope of the claims of the application and their equivalents, the application also intends to include these modifications and variations.

Claims

1. A method for surface strengthening of alloy steel, characterized in that: The following steps are involved: The alloy steel is subjected to quenching and tempering treatment to obtain quenched and tempered alloy steel, and the quenched and tempered alloy steel is processed into a sample; wherein the quenching and tempering process comprises quenching and holding at 870°C to 900°C for 45 minutes to 60 minutes, and first tempering at 610°C to 650°C for 2 hours to 2.5 hours; The sample is induction hardened and then placed in a water-based quenching liquid to cool to room temperature to obtain a surface hardened layer strengthened sample; wherein, during induction hardening, the current is 400mA~600mA and the heating time is 3s~7s; The surface strengthened portion of the surface hardened layer strengthened sample is cut off and sealed, and the sealed sample is subjected to a second tempering treatment to obtain a surface strengthened alloy steel.

2. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The equipment used for induction hardening is a high-frequency induction heating power supply.

3. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The water-based quenching liquid is a PAG quenching liquid, a polyvinyl alcohol quenching liquid, a polyether quenching liquid or an inorganic salt aqueous solution.

4. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The PAG quenching liquid is UCON Quenchant E, AQUA-QUENCH 365 C or 10% by mass of AQ251C; The polyether quenching liquid is NJ-3600 quenching liquid; The inorganic salt aqueous solution is a nitrate aqueous solution.

5. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The temperature of the second tempering treatment is 250℃~400℃, and the time is 1h~4h.

6. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The alloy steel is medium carbon alloy steel.

7. The method for surface strengthening of alloy steel according to claim 5, characterized in that: The medium carbon alloy steel is 34CrNiMo6 alloy steel.

8. The method for surface strengthening of alloy steel according to claim 1, characterized in that: The operation process of the tube sealing treatment is as follows: connect the test tube with the surface strengthened part of the surface hardened layer strengthened sample to the vacuum system, evacuate to a vacuum degree of ≤0.1Pa, fill with inert gas, heat one end of the test tube and rotate it, and finally form a sealed test tube.

9. A surface-strengthened alloy steel obtained by the processing method according to any one of claims 1 to 8.

10. Use of the alloy steel according to claim 9 in the preparation of a high-performance crankshaft, characterized in that: The hardness of the hardened layer of the high-performance crankshaft is ≥420HV, and the hardness of the matrix is ​​≥250HV.

Images