Alloy steel and method of heat treating the same

Abstract

The present application relates to a kind of alloy steel and its heat treatment method.The heat treatment method of alloy steel includes the following steps: pre-oxidation stage: alloy steel is pre-oxidized treatment;First stage nitriding: under the condition of ammonia and cracked ammonia mixed gas, alloy steel is once heat treatment, the nitrogen potential value K N Of once heat treatment is 4~6, ammonia flow is 3~5m 3 / h;Second stage nitriding: under the condition of ammonia and cracked ammonia mixed gas, alloy steel is twice heat treatment, the nitrogen potential value K N Of twice heat treatment is 1~3, ammonia flow is 2~3m 3 / h;Cooling stage: in inert gas atmosphere, the temperature of alloy steel is reduced to 100 ℃ below, and nitriding alloy steel is obtained.The present application also provides a kind of alloy steel, which is made by the heat treatment method described in the present application.The present application effectively improves the surface hardness of alloy steel material.

Description

Technical Field

[0001] This invention relates to the field of surface heat treatment technology, specifically to an alloy steel and its heat treatment method. Background Technology

[0002] Nitriding is a common heat treatment process for steel materials, incorporating nitrogen atoms into the surface of the material to improve its surface hardness, mechanical properties, wear resistance, corrosion resistance, and other characteristics. It is widely used in the industrial field.

[0003] 42CrMo materials are often surface strengthened by nitriding heat treatment. The conventional nitriding process typically uses a temperature of 500-520℃ and a nitrogen potential of K0. N Using a nitriding process with a viscosity of around 0.8-2, the surface hardness of the material can only reach about 650 HV, which is difficult to improve further, thus limiting the application of 42CrMo material. Summary of the Invention

[0004] The purpose of this invention is to provide an alloy steel and its heat treatment method to improve the surface hardness of the alloy steel, thereby broadening the application of the alloy steel.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A heat treatment method for alloy steel includes the following steps:

[0007] Pre-oxidation stage: The alloy steel undergoes pre-oxidation treatment;

[0008] The first stage of nitriding involves a heat treatment of the alloy steel under a mixed gas environment of ammonia and cracked ammonia. The nitrogen potential value of this heat treatment is K. N The ammonia flow rate is 3-5 m³ / h. 3 / h;

[0009] The second stage of nitriding involves a secondary heat treatment of the alloy steel under a mixed gas environment of ammonia and cracked ammonia. The nitrogen potential value K during this secondary heat treatment is... N The values ​​are 1 to 3, and the ammonia flow rate is 2 to 3 m³ / h. 3 / h;

[0010] Cooling stage: In an inert gas atmosphere, the temperature of the alloy steel is reduced to below 100°C to obtain nitrided alloy steel.

[0011] Based on the aforementioned technical methods, by setting up a pre-oxidation stage, it serves two purposes: firstly, it achieves temperature uniformity, preparing for the subsequent increase to a higher nitriding temperature; secondly, it pre-oxidizes the alloy steel, providing favorable preconditions for nitrogen atoms to penetrate into the alloy steel during the nitriding stage. During the nitriding stage, a large flow rate of ammonia and a high nitrogen potential value are used initially to achieve high-concentration nitriding, allowing a large number of nitrogen atoms to penetrate the alloy steel surface in a short time, effectively increasing the nitrogen content on the alloy steel surface. Then, diffusion is carried out with a slightly lower nitrogen potential, while an appropriate amount of cracked ammonia is added to ensure that the penetrated nitrogen atoms diffuse inward, thereby effectively increasing the surface hardness of the alloy steel. After nitriding, the temperature is lowered to below 100℃ under protective gas, effectively preventing oxidation of the alloy steel and avoiding the problem of affecting the surface nitrogen atom concentration. This method also has the advantages of simple operation, easy process time, and suitability for industrial applications.

[0012] 42CrMo steel, classified as an alloy structural steel, possesses excellent mechanical properties and machinability, making it widely applicable. It is primarily available in plate and bar forms, and its overall performance surpasses that of 40Cr, earning industry recognition. 42CrMo steel is an ultra-high-strength steel, exhibiting high strength and toughness, good hardenability, and no significant temper brittleness. After tempering, it demonstrates a high fatigue limit and resistance to repeated impacts, exhibiting good low-temperature impact toughness. 42CrMo steel is suitable for manufacturing large and medium-sized plastic molds requiring specific strength and toughness. The composition of 42CrMo steel by mass percentage is as follows: carbon: 0.38-0.45%, silicon: 0.17-0.37%, manganese: 0.50-0.80%, sulfur: residual content ≤0.035%, phosphorus: residual content ≤0.035%, chromium: 0.90-1.20%, nickel: residual content ≤0.30%, copper: residual content ≤0.30%, molybdenum: 0.15-0.25%, with the remainder being iron.

[0013] Through long-term research and analysis, the inventors of this case discovered that the reason why the surface hardness of 42CrMo steel workpieces after nitriding in existing technologies is not high is mainly because high concentrations of nitrogen atoms are difficult to penetrate into the surface and subsurface of the 42CrMo steel workpiece. Iron's nitrogen absorption capacity increases with temperature, but this change becomes less significant after a certain temperature. When active nitrogen atoms formed by ammonia decomposition accumulate on the surface of the 42CrMo steel workpiece, they must be promptly absorbed into the surface; otherwise, the active nitrogen atoms will immediately polymerize into nitrogen molecules, losing their activity and becoming unusable for nitriding. In this application, when nitriding at 550-560℃, the ammonia decomposition rate remains around 40-60%, and the 42CrMo steel workpiece still has a high nitrogen absorption capacity. Therefore, selecting appropriate process parameters to ensure a high concentration of nitrogen and its rapid and effective penetration into the surface of the 42CrMo steel workpiece is the key technology for obtaining high hardness.

[0014] After high-concentration nitrogen atoms have diffused into the surface of a 42CrMo steel workpiece, if the high concentration of diffused nitrogen atoms cannot diffuse into the interior of the 42CrMo steel workpiece in time, they will quickly combine with the alloying elements contained in the 42CrMo steel workpiece to form nitrides, which will precipitate at the grain boundaries, thereby reducing the strengthening effect on the surface of the 42CrMo steel workpiece. When nitriding is carried out at a temperature of 550-560℃, three phase regions—ε, γ', and α—will be formed sequentially from the surface to the interior of the 42CrMo steel workpiece. The diffusion rate of nitrogen atoms in these three phase regions is Vα>Vε>Vγ', with the lowest diffusion rate in the γ' phase. Therefore, solving the diffusion rate of nitrogen atoms in the γ' phase is the most important problem to be solved. The inventors of this invention, through selecting a suitable nitrogen potential K during the second-stage nitriding process... N The value is used to determine a reasonable ammonia decomposition rate. In addition, an appropriate amount of ammonia gas and the flow rate of cracked ammonia are introduced to promote the nitrogen atoms in the γ' phase to reach the optimal diffusion rate.

[0015] Based on the above analysis, the inventors conducted relevant experiments. By using different nitriding temperatures, nitriding times, flow rates, and nitrogen potential ratios, they conducted corresponding comparative experiments and obtained a set of optimal process parameters. This solved the problem of diffusion difficulties with high-concentration nitrogen and resulted in high nitriding hardness of 42CrMo material.

[0016] Preferably, the pre-oxidation treatment is to heat-treat the alloy steel at a temperature of 400-450°C for a time of 0.5-1 hour.

[0017] Experiments have shown that setting the pre-oxidation temperature between 400 and 450°C and the pre-oxidation time between 0.5 and 1 hour effectively ensures uniform temperature and facilitates the penetration of nitrogen atoms into the alloy steel during the nitriding stage.

[0018] Preferably, in the first stage of nitriding, the flow rate of cracked ammonia is 2-3 m³ / h. 3 / h.

[0019] Experiments have shown that, in the first nitriding stage, controlling the flow rate of cracked ammonia at 2–3 m³ / h is effective. 3 / h ensures that nitrogen atoms can quickly and effectively penetrate into the alloy steel.

[0020] Preferably, the flow rate for ammonia decomposition during the second stage of nitriding is 1–2 m³ / s. 3 / h.

[0021] Experiments have shown that in the second nitriding stage, controlling the flow rate of cracked ammonia to be lower than that in the first nitriding stage effectively promotes the diffusion of already diffused nitrogen atoms into the interior.

[0022] Preferably, the temperatures for the primary and secondary heat treatments are 550°C to 560°C.

[0023] Experiments have shown that controlling the temperature between 550℃ and 560℃ in both nitriding stages ensures that the active nitrogen atoms have high potential energy, thereby creating a high nitrogen potential on the surface of the aluminum alloy steel and achieving high-concentration nitrogen penetration. In the second nitriding stage, the nitrogen concentration gradient of the aluminum alloy steel is mainly adjusted through nitrogen diffusion, enabling the surface of the aluminum alloy steel to achieve ultra-high hardness.

[0024] Preferably, the first heat treatment lasts for 2 to 4 hours, and the second heat treatment lasts for 15 to 25 hours.

[0025] Because the nitrogen potential is high and nitrogen atoms accumulate in large quantities during the first heat treatment, the heat treatment time is controlled within 2–4 hours to avoid the formation of severe network nitrides. Since the nitrogen potential is relatively low during the second heat treatment, the heat treatment time is extended to effectively ensure nitrogen diffusion.

[0026] Preferably, the alloy steel is 42CrMo steel.

[0027] Preferably, during the cooling stage, the inert gas is nitrogen, and the flow rate of nitrogen is 4-6 m³ / h. 3 / h.

[0028] By controlling the nitrogen concentration during the cooling phase, the surface of the alloy steel is effectively protected from oxidation, thereby ensuring the hardness of the alloy steel surface.

[0029] The present invention also provides an alloy steel, which is manufactured using the heat treatment method described in the present invention.

[0030] Preferably, the depth of the nitrided layer on the alloy steel surface is 0.28 to 0.32 mm, and the hardness of the alloy steel surface is above 800 HV.

[0031] The beneficial effects of this invention are:

[0032] The heat treatment method for alloy steel of the present invention, by setting a pre-oxidation stage, serves two purposes: firstly, it homogenizes the temperature, preparing for the subsequent increase to a higher nitriding temperature; secondly, it pre-oxidizes the alloy steel, providing favorable preconditions for the penetration of nitrogen atoms into the interior of the alloy steel during the nitriding stage. In the nitriding stage, a large flow rate of ammonia gas and a high nitrogen potential value are used to achieve high-concentration nitriding at the beginning of the nitriding stage, allowing a large number of nitrogen atoms to penetrate into the surface of the alloy steel in a short time, thereby effectively increasing the nitrogen content on the surface of the alloy steel. Then, diffusion is carried out with a slightly lower concentration of nitrogen potential, while an appropriate amount of cracked ammonia is added to ensure that the penetrated nitrogen atoms diffuse into the interior, thereby effectively increasing the surface hardness of the alloy steel. After nitriding, the temperature is cooled to below 100°C under protective gas protection, effectively avoiding the problem of oxidation of the alloy steel affecting the surface nitrogen atom concentration. It also has the advantages of simple operation, easy process time, and suitability for industrial application.

[0033] The alloy steel prepared by the method of this invention has a nitriding layer depth of 0.28-0.32 mm and a surface hardness of over 800 HV, which effectively improves the surface hardness of the alloy steel and thus broadens its application. It has promotional and application value in the field of alloy steel surface heat treatment technology. Attached Figure Description

[0034] Figure 1 This is a process flow diagram of the heat treatment method for alloy steel according to the present invention;

[0035] Figure 2 This is a diagram showing the locations for hardness testing. Detailed Implementation

[0036] The embodiments of the present invention will be described below with reference to the accompanying drawings and preferred embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention and not for limiting the scope of protection of the present invention.

[0037] It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0038] Example 1

[0039] A heat treatment method for alloy steel includes the following steps:

[0040] S1. Pretreatment: First, thoroughly clean the oil and impurities on the surface of the 42CrMo steel workpiece using a strong water-based cleaning agent or gasoline-based cleaning agent. When using a water-based cleaning agent, drying is required after cleaning to completely remove any residual moisture from the surface and pores of the 42CrMo steel workpiece. Second, apply a protective layer to areas of the 42CrMo steel workpiece surface that do not require high hardness or high nitrogen concentration.

[0041] S2, Pre-oxidation stage: The 42CrMo steel workpiece that has completed the pretreatment is heat-treated at a temperature of 450℃ for 45 minutes.

[0042] S3. First-stage nitriding: Under mixed gas conditions of ammonia and cracked ammonia, the pre-oxidized 42CrMo steel workpiece undergoes a primary heat treatment. The nitrogen potential value K of this primary heat treatment is... N The value is 5.5, and the ammonia flow rate is 4.5 m³ / h. 3 / h, cracked ammonia flow rate is 2.5m³ / h. 3 / h, temperature 555℃, time 2.5h;

[0043] S4. Second-stage nitriding: Under mixed gas conditions of ammonia and cracked ammonia, the 42CrMo steel workpiece that has completed the first-stage nitriding undergoes a second heat treatment. The nitrogen potential value K of the second heat treatment is... N The concentration is 2.5, and the ammonia flow rate is 2.5 m³ / h. 3 / h, cracked ammonia flow rate is 1.5m³ / h. 3 / h, temperature 555℃, time 18h;

[0044] S5. Cooling Stage: In a nitrogen atmosphere, the temperature of the 42CrMo steel workpiece that has completed the second stage of nitriding is reduced to below 100℃ to obtain the nitrided 42CrMo steel workpiece. The nitrogen flow rate is 5m³ / h. 3 / h.

[0045] Example 2

[0046] A heat treatment method for alloy steel includes the following steps:

[0047] S1. Pretreatment: First, thoroughly clean the oil and impurities on the surface of the 42CrMo steel workpiece using a strong water-based cleaning agent or gasoline-based cleaning agent. When using a water-based cleaning agent, drying is required after cleaning to completely remove any residual moisture from the surface and pores of the 42CrMo steel workpiece. Second, apply a protective layer to areas of the 42CrMo steel workpiece surface that do not require high hardness or high nitrogen concentration.

[0048] S2, Pre-oxidation stage: The 42CrMo steel workpiece that has completed the pretreatment is heat-treated at a temperature of 450℃ for 45 minutes.

[0049] S3. First-stage nitriding: Under mixed gas conditions of ammonia and cracked ammonia, the pre-oxidized 42CrMo steel workpiece undergoes a primary heat treatment. The nitrogen potential value K of this primary heat treatment is... N The value is 5.0, and the ammonia flow rate is 4.0 m³ / h. 3 / h, cracked ammonia flow rate is 2.5m³ / h. 3 / h, temperature 550℃, time 3h;

[0050] S4. Second-stage nitriding: Under mixed gas conditions of ammonia and cracked ammonia, the 42CrMo steel workpiece that has completed the first-stage nitriding undergoes a second heat treatment. The nitrogen potential value K of the second heat treatment is... N The concentration is 2.5, and the ammonia flow rate is 2.5 m³ / h. 3 / h, cracked ammonia flow rate is 1.5m³ / h. 3 / h, temperature 550℃, time 20h;

[0051] S5. Cooling Stage: In a nitrogen atmosphere, the temperature of the 42CrMo steel workpiece that has completed the second stage of nitriding is reduced to below 100℃ to obtain the nitrided 42CrMo steel workpiece. The nitrogen flow rate is 5m³ / h. 3 / h.

[0052] Comparative Example 1

[0053] A heat treatment method for alloy steel includes the following steps:

[0054] S1. Pretreatment: First, thoroughly clean the oil and impurities on the surface of the 42CrMo steel workpiece using a strong water-based cleaning agent or gasoline-based cleaning agent. When using a water-based cleaning agent, drying is required after cleaning to completely remove any residual moisture from the surface and pores of the 42CrMo steel workpiece. Second, apply a protective layer to areas of the 42CrMo steel workpiece surface that do not require high hardness or high nitrogen concentration.

[0055] S2, Pre-oxidation stage: The 42CrMo steel workpiece that has completed the pretreatment is heat-treated at a temperature of 450℃ for 45 minutes.

[0056] S3. Nitriding Stage: Under mixed gas conditions of ammonia and cracked ammonia, the pre-oxidized 42CrMo steel workpiece undergoes a primary heat treatment. The nitrogen potential value K of this primary heat treatment is... N The concentration is 2.5, and the ammonia flow rate is 2.0 m³ / h. 3 / h, cracked ammonia flow rate is 1.0 m³ / h. 3 / h, temperature 555℃, time 23h;

[0057] S4. Cooling Stage: In a nitrogen atmosphere, the temperature of the 42CrMo steel workpiece that has completed the second stage of nitriding is reduced to below 100℃ to obtain the nitrided 42CrMo steel workpiece. The nitrogen flow rate is 5m³ / h. 3 / h.

[0058] Detection and Analysis

[0059] Surface hardness test

[0060] Production Figure 1 Standard workpieces were subjected to nitriding treatment according to the processes and parameters in Examples 1, 2, and Control Example 1. The surface hardness of the nitrided 42CrMo steel workpieces prepared in Examples 1, 2, and Control Example 1 was then tested using an LM247AT microhardness tester. The experimental load was 100g, and the hardness testing locations were as follows: Figure 2 As shown in Table 1, the test results are as follows.

[0061] Table 1. Hardness test results

[0062]

[0063] As can be seen from Table 1, the surface hardness of the nitrided 42CrMo steel workpiece treated by the method of the present invention is above 800HV, exhibiting ultra-high strength.

[0064] In summary, the heat treatment method for alloy steel of the present invention firstly pre-oxidizes the alloy steel to achieve slight surface oxidation, and then performs a first-stage nitriding process using a high flow rate of ammonia and a high nitrogen potential K of 4-6. N The high nitrogen potential value nitriding process of this invention focuses high-concentration active nitrogen atoms on the surface of alloy steel, enabling a high-concentration nitrided layer to be formed on the surface of the alloy steel in a short time. A second stage of nitriding then diffuses the nitrogen atoms from the surface of the alloy steel inward to achieve the required layer depth, while simultaneously adjusting the nitride distribution within the nitrided layer. After nitriding, a protective atmosphere is used to cool the alloy steel to below 100°C, effectively preventing the nitrided layer from being oxidized. This invention's heat treatment process for alloy steel, employing a high nitrogen potential value nitriding method, breaks through the limitations of traditional nitriding processes where the nitrogen potential value K is [value missing]. N The selection of the nitriding temperature must fall within the critical nitrogen potential range at the corresponding temperature. By using a higher nitriding temperature instead of the traditional 500-520℃ nitriding temperature parameter, the activity of nitrogen atoms during nitriding is improved. This ensures that the high concentration of nitrogen atoms on the alloy steel surface penetrates into the alloy steel before combining into nitrogen molecules, thus accelerating the nitriding effect. This method has application value in the field of alloy steel surface heat treatment technology.

[0065] The alloy steel prepared by the method of this invention has a nitriding layer depth of 0.28-0.32 mm and a surface hardness of over 800 HV, which effectively improves the surface hardness of the alloy steel. This expands the application of alloy steel in fields requiring high hardness and high wear resistance, such as plastic molds, heavy-duty gears, high-pressure plunger friction pairs, and high-load transmission shaft parts.

[0066] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A heat treatment method for alloy steel, characterized in that, Includes the following steps: Pre-oxidation stage: The alloy steel, which is 42CrMo steel, undergoes pre-oxidation treatment. The first stage of nitriding involves a heat treatment of the alloy steel under a mixed gas environment of ammonia and cracked ammonia. The nitrogen potential value of this heat treatment is K. N The ammonia flow rate is 3-5 m³ / h. 3 / h; The temperature of one heat treatment is 550℃~560℃, and the time of one heat treatment is 2~4h; The second stage of nitriding involves a secondary heat treatment of the alloy steel under a mixed gas environment of ammonia and cracked ammonia. The nitrogen potential value K during this secondary heat treatment is... N The value is 1~3, and the ammonia flow rate is 2~3m³. 3 / h; the temperature of the secondary heat treatment is 550℃~560℃, and the time of the secondary heat treatment is 15~25h; Cooling stage: In an inert gas atmosphere, the temperature of the alloy steel is reduced to below 100°C to obtain nitrided alloy steel.

2. The heat treatment method for alloy steel according to claim 1, characterized in that, The pre-oxidation treatment involves heat-treating the alloy steel at a temperature of 400~450℃ for 0.5~1h.

3. The heat treatment method for alloy steel according to claim 1, characterized in that, In the first stage of nitriding, the flow rate of cracked ammonia is 2~3 m³ / h. 3 / h.

4. The heat treatment method for alloy steel according to claim 1, characterized in that, In the second stage of nitriding, the flow rate of cracked ammonia is 1~2 m³ / s. 3 / h.

5. The heat treatment method for alloy steel according to claim 1, characterized in that, During the cooling stage, the inert gas is nitrogen, and the flow rate of nitrogen is 4-6 m³ / h. 3 / h.

6. An alloy steel, characterized in that, It is manufactured using the heat treatment method described in any one of claims 1 to 5.

7. The alloy steel according to claim 6, characterized in that, The depth of the nitrided layer on the surface of the alloy steel is 0.28~0.32mm, and the hardness of the surface of the alloy steel is above 800HV.

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