Alloy steel and composite nitriding heat treatment method thereof

Abstract

The present invention relates to an alloy steel and a composite nitriding heat treatment method thereof. The composite nitriding heat treatment method for alloy steel comprises the following steps: performing heat preservation treatment on the alloy steel; performing a primary heat treatment on the alloy steel under the condition of a mixed gas of ammonia and cracked ammonia, wherein the nitrogen potential value K of the primary heat treatment is N is 3~4, and the ammonia flow rate is 3~5m 3 / h; Under the mixed gas conditions of ammonia, cracked ammonia and carbon dioxide, the alloy steel is subjected to secondary heat treatment. The nitrogen potential value K N is 5~6, and the ammonia flow rate is 4~6m 3 / h, the carbon dioxide flow rate is 0.5-1 m 3 / h; in an inert gas atmosphere, the temperature of the alloy steel is lowered to below 100°C to obtain a composite nitrocarburized alloy steel. The present invention also provides an alloy steel produced by the heat treatment method of the present invention. The present invention effectively improves the surface hardness of the alloy steel.

Description

Technical Field

[0001] The present invention relates to the technical field of surface heat treatment, in particular to an alloy steel and a composite nitriding heat treatment method thereof. Background Art

[0002] Nitriding and nitrocarburizing are performed on the surface of steel to infiltrate nitrogen atoms and carbon atoms into conventional materials to improve the hardness, mechanical properties, wear resistance, corrosion resistance and other properties of the material surface. It is a commonly used heat treatment process method for steel materials and is widely used in the industrial field.

[0003] 40CrNiMo material is often treated by nitriding heat treatment method for surface strengthening. The conventional nitriding process is generally carried out at a temperature of 500-520℃ and a nitrogen potential K N With a hardness of about 0.8-2, after this type of nitriding process, the surface hardness of the material can only reach about 680-700HV, which is difficult to improve further, thus limiting the application of 40CrNiMo material. Summary of the Invention

[0004] The object of the present invention is to provide an alloy steel and a composite nitriding heat treatment method thereof, so as to improve the surface hardness of the alloy steel and thus broaden the application of the alloy steel.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A composite nitriding heat treatment method for alloy steel comprises the following steps:

[0007] Pretreatment stage: heat preservation treatment of alloy steel;

[0008] The first stage of nitriding: the alloy steel is subjected to a heat treatment under the mixed gas conditions of ammonia and cracked ammonia. The nitrogen potential value K N is 3-4, and the ammonia flow rate is 3-5m 3 / h;

[0009] The second stage of composite nitrocarburizing: the alloy steel is subjected to secondary heat treatment under the mixed gas conditions of ammonia, cracked ammonia and carbon dioxide. The nitrogen potential value K N is 5-6, and the ammonia flow rate is 4-6m 3 / h, carbon dioxide flow rate is 0.5-1m 3 / h;

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

[0011] According to the above technical means, by setting up a pretreatment stage, on the one hand, it plays a role in temperature uniformity, preparing for the subsequent rise to a higher nitriding temperature, and on the other hand, pre-oxidizing the alloy steel, providing a good prerequisite for the penetration of nitrogen atoms into the alloy steel during the nitriding stage. In the nitriding stage, a large flow of ammonia and a high nitrogen potential value are first used, so that high-concentration nitriding is carried out at the beginning of nitriding, so that a large amount of nitrogen atoms are penetrated into the surface of the alloy steel in a short time, thereby effectively increasing the nitrogen content on the surface of the alloy steel. In the second stage, the high concentration of nitrogen potential is continued to be maintained for diffusion, and at the same time, an appropriate amount of cracked ammonia and carbon dioxide gas is added to form active carbon atoms to promote the continued diffusion of nitrogen atoms. This ensures that the infiltrated nitrogen atoms diffuse internally, thereby effectively improving the hardness of the alloy steel surface. After the composite nitrocarburizing is completed, the temperature is lowered to below 100°C through protective gas protection, effectively avoiding the problem of oxidation of the alloy steel and affecting the concentration of nitrogen atoms on the surface.

[0012] 40CrNiMo steel, classified as an alloy structural steel, boasts excellent mechanical properties and machinability, and is widely used. Available primarily in sheet and bar form, its overall performance surpasses that of 40Cr and 42CrMo, earning it industry recognition. 40CrNiMo steel is a highly robust steel, characterized by high strength and toughness, good hardenability, and no temper brittleness. After quenching and tempering, it exhibits a high fatigue limit and resistance to multiple impacts, as well as excellent low-temperature impact toughness.

[0013] 40CrNiMo steel is suitable for manufacturing large and medium-sized plastic molds that require a certain degree of strength and toughness. The composition of 40CrNiMo steel by mass percentage is as follows: carbon: 0.37-0.44%, silicon: 0.17-0.37%, manganese: 0.50-0.80%, sulfur: residual content allowed ≤ 0.025%, phosphorus: residual content allowed ≤ 0.025%, chromium: 0.60-0.90%, nickel: 1.25-1.65%, copper: residual content allowed ≤ 0.25%, molybdenum: 0.15-0.25%, and the remainder is iron.

[0014] After extensive research and analysis, the inventors of this case discovered that the reason why the surface hardness of 40CrNiMo steel workpieces after nitriding disclosed in the prior art is not high is mainly due to the difficulty of high concentrations of nitrogen atoms infiltrating into the surface and subsurface of the 40CrNiMo steel workpiece. The nitrogen absorption capacity of iron increases with increasing temperature, but after the temperature reaches a certain level, the change will become less obvious. When the active nitrogen atoms formed by the decomposition of ammonia accumulate on the surface of the 40CrNiMo steel workpiece, they must be promptly infiltrated into the 40CrNiMo steel workpiece surface; otherwise, the active nitrogen atoms will immediately polymerize into nitrogen molecules, thereby losing their activity and becoming unusable for nitriding. In addition, after a certain concentration of nitrogen atoms has been infiltrated into the subsurface of the 40CrNiMo steel workpiece, it becomes very difficult to infiltrate a higher concentration of nitrogen atoms into the surface. This application adopts a two-stage nitriding process. After a certain concentration of nitrogen atoms is infiltrated in the first stage, a second stage of composite nitrocarburizing is performed to continue to maintain the high concentration of nitrogen and carbon atoms infiltration, so that the 40CrNiMo steel workpiece still has a high nitrogen absorption capacity. Therefore, selecting appropriate process methods and parameters to ensure high concentration of nitrogen and quickly and effectively penetrate the surface of 40CrNiMo steel workpiece is the key technology to obtain high hardness.

[0015] After high-concentration nitrogen atoms have completed the penetration into the surface of the 40CrNiMo steel workpiece, if the concentration of the penetrated nitrogen atoms is high and they cannot diffuse into the interior of the 40CrNiMo steel workpiece in time, they will quickly focus with the alloy elements contained in the 40CrNiMo steel workpiece to form nitrides and precipitate at the grain boundaries, thereby reducing the strengthening effect of the surface of the 40CrNiMo steel workpiece. When nitriding is carried out at a temperature of 500-560°C, three phase regions of ε, γ', and α will be formed on the surface of the 40CrNiMo steel workpiece from the surface to the inside. The diffusion rate of nitrogen atoms in these three phase regions is Vα>Vε>Vγ', and the diffusion rate in the γ' phase is the lowest. Therefore, solving the diffusion rate of nitrogen atoms in the γ' phase is the most urgent problem to be solved. The inventors of this case selected a suitable nitrogen potential K in the second stage of nitriding. N The value is set to determine a reasonable ammonia decomposition rate, while maintaining a high nitriding temperature of 540-560℃. In addition, a suitable carbon dioxide gas is introduced to infiltrate a certain amount of carbon atoms into the surface of the 40CrNiMo steel workpiece to promote the optimal diffusion rate of nitrogen atoms.

[0016] Based on the above analysis, the inventors conducted relevant experiments and carried out corresponding comparative tests by varying the ratio of nitriding temperature, nitriding time, flow rate and nitrogen potential value, and obtained a set of optimal process parameters, which solved the diffusion difficulty of high-concentration nitrogen and obtained the high nitriding hardness of 40CrNiMo material.

[0017] Preferably, the pretreatment stage is to perform homogenization heat treatment on the alloy steel at a temperature of 400-450° C., and the heat preservation treatment time is 0.5-1 hour.

[0018] Experiments have shown that by setting the pretreatment temperature between 400 and 450°C and the pre-oxidation time between 0.5 and 1 hour, the uniform temperature effect is effectively guaranteed, and it is beneficial to the penetration of nitrogen atoms into the alloy steel during the nitriding stage.

[0019] Preferably, in the first stage of nitriding, the flow rate of cracked ammonia is 1.5 to 2.5 m 3 / h.

[0020] Experiments have shown that in the first nitriding stage, by controlling the flow rate of cracked ammonia at 1.5 to 2.5 m 3 / h, ensuring that nitrogen atoms penetrate quickly and effectively into the alloy steel.

[0021] Preferably, in the second stage of composite nitrocarburizing, the flow rate of cracked ammonia is 1 to 2 m 3 / h.

[0022] Experiments have shown that in the second nitriding stage, by controlling the flow rate of cracked ammonia to be smaller than that in the first nitriding stage and introducing a certain amount of carbon dioxide at the same time, the surface nitrogen concentration is effectively increased, and the nitrogen atoms that have penetrated the surface diffuse into the interior.

[0023] Preferably, the temperature of the primary heat treatment is 490°C to 500°C, and the temperature of the secondary heat treatment is 540°C to 560°C.

[0024] Experiments have shown that in the first nitriding stage, controlling the temperature between 490°C and 500°C results in a high nitrogen potential on the workpiece surface, giving the active nitrogen atoms a high potential energy. This creates a high nitrogen potential on the alloy steel surface, enabling the penetration of high-concentration nitrogen. In the combined nitrocarburizing stage, higher nitriding temperatures and the infiltration of a certain amount of carbon atoms further promote the penetration of nitrogen atoms into the workpiece surface, resulting in ultra-high hardness on the alloy steel surface.

[0025] Preferably, the time for the first heat treatment is 5 to 7 hours, and the time for the second heat treatment is 3 to 4 hours.

[0026] Because the heat treatment process has a high nitrogen potential and a large amount of nitrogen atoms accumulate, the primary heat treatment duration is controlled within 5-7 hours to avoid the formation of severe network nitrides. Since the secondary heat treatment process has a high nitriding temperature and a relatively high nitrogen potential, the secondary heat treatment duration is controlled within 3-4 hours to effectively ensure high surface hardness.

[0027] Preferably, the alloy steel is 42CrNiMo steel.

[0028] Preferably, in the cooling stage, the inert gas is nitrogen, and the flow rate of nitrogen is 4 to 6 m 3 / h.

[0029] By controlling the nitrogen concentration during the cooling stage, the surface of the alloy steel is effectively protected from oxidation, thereby ensuring the hardness of the alloy steel surface.

[0030] The present invention also provides an alloy steel, which is manufactured by the heat treatment method of the present invention.

[0031] Preferably, the depth of the composite nitrocarburized layer on the surface of the alloy steel is 0.15 to 0.23 mm, and the hardness of the surface of the alloy steel is above 800 HV.

[0032] Beneficial effects of the present invention:

[0033] The alloy steel composite nitriding heat treatment method of the present invention, by setting a pretreatment stage, on the one hand plays a role in temperature uniformity, preparing for the subsequent increase to a higher nitriding temperature, and on the other hand pre-oxidizes the alloy steel, providing a good prerequisite for the penetration of nitrogen atoms into the interior of the alloy steel in the nitriding stage. In the nitriding stage, a large flow of ammonia and a high nitrogen potential value are first used, so that high-concentration nitriding is performed at the beginning of nitriding, so that a large amount of nitrogen atoms are penetrated 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, nitriding is performed at a higher temperature, and an appropriate amount of carbon dioxide is added to promote the penetration of nitrogen atoms, ensuring that a high-concentration nitrided layer is obtained on the surface, while ensuring that the penetrated nitrogen atoms want to diffuse to the inside, thereby effectively increasing the hardness of the alloy steel surface. After the composite nitrocarburizing is completed, the temperature is lowered to below 100° C. by protective gas protection, effectively avoiding the problem of oxidation of the alloy steel and affecting the concentration of nitrogen atoms on the surface. The method has the advantages of simple operation, short process time, and suitability for industrial application.

[0034] The alloy steel prepared by the method of the present invention has been tested to have a depth of a composite nitrogen-carbon co-diffusion layer on the surface of the alloy steel of 0.15 to 0.23 mm and a surface hardness of the alloy steel of above 800 HV, which effectively improves the surface hardness of the alloy steel, thereby broadening the application of the alloy steel. The method has promotional application value in the field of alloy steel surface heat treatment technology in some application fields requiring high hardness and high wear resistance, such as plastic molds, heavy-loaded gears, high-pressure plunger friction pairs, and rollers, roller pins and other transmission components. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A process roadmap of the heat treatment method for alloy steel of the present invention;

[0036] Figure 2 This is the hardness test position diagram;

[0037] Figure 3 and Figure 4 This is the SEM image of the nitrided 40CrNiMo steel workpiece prepared in Example 1. DETAILED DESCRIPTION

[0038] The following describes the embodiments of the present invention with reference to the accompanying drawings and preferred embodiments. Those skilled in the art will readily appreciate the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the various details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are intended only to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0039] It should be noted that the illustrations provided in the following embodiments are merely schematic illustrations of the basic concept of the present invention. Therefore, the illustrations only show components related to the present invention and are not drawn according to the number, shape, and size of components in actual implementation. In actual implementation, the type, quantity, and proportion of each component may be changed arbitrarily, and the component layout may also be more complex.

[0040] Example 1

[0041] like Figure 1 As shown, a composite nitriding heat treatment method for alloy steel comprises the following steps:

[0042] S1. Pretreatment: First, use a water-based detergent or gasoline-based detergent with strong detergency to thoroughly clean the oil and impurities on the surface of the 42CrNiMo steel workpiece. When using a water-based detergent for cleaning, it is necessary to dry the workpiece after cleaning to completely eliminate the residual moisture on the surface and in the pores of the 42CrNiMo steel workpiece. Secondly, apply a protective layer to the parts of the 42CrNiMo steel workpiece that do not require high hardness and high nitrogen concentration.

[0043] S2, pretreatment stage: heat treatment of the pre-treated 42CrNiMo steel workpiece at a temperature of 450°C for 60 minutes;

[0044] S3. First stage nitriding: Under the mixed gas condition of ammonia and cracked ammonia, the 42CrNiMo steel workpiece that has completed the pretreatment is subjected to a heat treatment. The nitrogen potential value K N is 3.5, and the ammonia flow rate is 3.5m 3 / h, cracking ammonia flow rate is 2.5m 3 / h, temperature is 495℃, time is 6h;

[0045] S4. Second stage composite nitrocarburizing: Under the mixed gas conditions of ammonia, cracked ammonia and carbon dioxide, the 42CrNiMo steel workpiece that has completed the first stage nitriding is subjected to secondary heat treatment. Among them, the nitrogen potential value K N is 5, and the ammonia flow rate is 5.5m 3 / h, cracking ammonia flow rate is 1.5m 3 / h, temperature is 555℃, time is 4h;

[0046] S5, cooling stage: in nitrogen atmosphere, the temperature of the 42CrNiMo steel workpiece that has completed the second stage of composite nitrocarburizing is reduced to below 100 ° C to obtain the nitrided 42CrNiMo steel workpiece, wherein the flow rate of nitrogen is 5m 3 / h.

[0047] Example 2

[0048] like Figure 1 As shown, a composite nitriding heat treatment method for alloy steel comprises the following steps:

[0049] S1. Pretreatment: First, use a water-based detergent or gasoline-based detergent with strong detergency to thoroughly clean the oil and impurities on the surface of the 42CrNiMo steel workpiece. When using a water-based detergent for cleaning, it is necessary to dry the workpiece after cleaning to completely eliminate the residual moisture on the surface and in the pores of the 42CrNiMo steel workpiece. Secondly, apply a protective layer to the parts of the 42CrNiMo steel workpiece that do not require high hardness and high nitrogen concentration.

[0050] S2, pretreatment stage: heat treatment of the pre-treated 42CrNiMo steel workpiece at a temperature of 450°C for 60 minutes;

[0051] S3. First stage nitriding: Under the mixed gas condition of ammonia and cracked ammonia, the 42CrNiMo steel workpiece that has completed the pretreatment is subjected to a heat treatment. The nitrogen potential value K N is 4.0, and the ammonia flow rate is 5.0m 3 / h, cracking ammonia flow rate is 2.5m 3 / h, temperature is 500℃, time is 7h;

[0052] S4. Second stage composite nitrocarburizing: Under the mixed gas conditions of ammonia, cracked ammonia and carbon dioxide, the 42CrNiMo steel workpiece that has completed the first stage nitriding is subjected to secondary heat treatment. Among them, the nitrogen potential value K N is 6, and the ammonia flow rate is 6.0m 3 / h, cracking ammonia flow rate is 2.0m 3 / h, temperature is 560℃, time is 3h;

[0053] S5, cooling stage: in nitrogen atmosphere, the temperature of the 42CrNiMo steel workpiece that has completed the second stage of nitriding is reduced to below 100 ° C to obtain the nitrided 42CrNiMo steel workpiece, wherein the flow rate of nitrogen is 5m 3 / h.

[0054] Comparative Example 1

[0055] A heat treatment method for alloy steel comprises the following steps:

[0056] S1. Pretreatment: First, use a water-based detergent or gasoline-based detergent with strong detergency to thoroughly clean the oil and impurities on the surface of the 42CrNiMo steel workpiece. When using a water-based detergent for cleaning, it is necessary to dry the workpiece after cleaning to completely eliminate the residual moisture on the surface and in the pores of the 42CrMo steel workpiece. Secondly, apply a protective layer to the parts of the 42CrNiMo steel workpiece that do not require high hardness and high nitrogen concentration.

[0057] S2, pretreatment stage: heat treatment of the pre-treated 42CrNiMo steel workpiece at a temperature of 450°C for 60 minutes;

[0058] S3, nitriding stage: under the mixed gas condition of ammonia and cracked ammonia, the 42CrNiMo steel workpiece which has completed the pretreatment is subjected to a heat treatment, wherein the nitrogen potential value K N is 2.5, and the ammonia flow rate is 2.0m 3 / h, cracking ammonia flow rate is 1.0m 3 / h, temperature is 500℃, time is 10h;

[0059] S4, cooling stage: in nitrogen atmosphere, the temperature of the 42CrNiMo steel workpiece that has completed the second stage of nitriding is reduced to below 100 ° C to obtain the nitrided 42CrNiMo steel workpiece, wherein the flow rate of nitrogen is 5m 3 / h.

[0060] Detection and Analysis

[0061] Surface hardness test

[0062] Make as Figure 2 After nitriding treatment according to the process and parameters in Example 1, Example 2 and Comparative Example 1, the surface hardness of the nitrided 42CrNiMo steel workpieces prepared in Example 1, Example 2 and Comparative Example 1 was tested using an LM247AT microhardness tester. The experimental load was 200 g, and the hardness detection position was as follows: Figure 2 The test results are shown in Table 1.

[0063] Table 1 Hardness test results

[0064]

[0065] From the analysis in Table 1, it can be seen that the surface hardness of the nitrided 42CrNiMo steel workpiece treated by the method of the present invention is above 800 HV, showing ultra-high strength.

[0066] Scanning electron microscopy analysis

[0067] The nitrided 40CrNiMo steel workpiece prepared in Example 1 was subjected to scanning electron microscopy analysis, and the results were as follows: Figure 3 and Figure 4 shown.

[0068] from Figure 3 and Figure 4 The analysis shows that the surface of the 40CrNiMo steel workpiece treated by the method of the present invention has a uniform structure after nitrocarburizing, the white nitrogen-carbon compound layer on the surface is uniform and dense, and no harmful network nitride appears in the entire composite nitriding layer, which is a good nitrided structure.

[0069] In summary, the alloy steel composite nitriding heat treatment method of the present invention first pre-treats the alloy steel to slightly oxidize the surface of the alloy steel, and then nitrides the alloy steel in the first stage, using a large flow rate of ammonia and a high nitrogen potential K of 3 to 4. N The composite nitriding heat treatment process of alloy steel of the present invention adopts the composite nitriding treatment method, which breaks the inherent concept that nitriding and nitriding processes of traditional processes cannot be combined. In addition, the higher nitrogen potential value KN parameter setting is adopted to improve the activity of nitrogen atoms during nitriding, ensure that the high concentration nitrogen atoms on the surface of the alloy steel penetrate into the interior of the alloy steel before combining into nitrogen molecules, accelerate the nitriding effect, and improve the surface hardness of the steel. It has promotion and application value in the field of alloy steel surface heat treatment technology.

[0070] The alloy steel produced by the method of the present invention has been tested to have a depth of the composite nitrocarburized layer on the surface of the alloy steel of 0.15 to 0.23 mm and a surface hardness of the alloy steel of above 800 HV, which effectively improves the surface hardness of the alloy steel, thereby broadening the application of the alloy steel in some fields requiring high hardness and high wear resistance, such as plastic molds, heavy-loaded gears, high-pressure plunger friction pairs, and high-load transmission shaft parts.

[0071] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or modification made by those skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. A composite nitriding heat treatment method for alloy steel, characterized in that: The following steps are involved: Pretreatment stage: performing heat preservation treatment on the alloy steel, wherein the alloy steel is 42CrNiMo steel; The first stage of nitriding: the alloy steel is heat treated under the mixed gas conditions of ammonia and cracked ammonia. The nitrogen potential value K N is 3~4, and the ammonia flow rate is 3~5m 3 / h; the temperature of the primary heat treatment is 490°C~500°C, and the time of the primary heat treatment is 5~7h; The second stage of composite nitrocarburizing: the alloy steel is subjected to secondary heat treatment under the mixed gas conditions of ammonia, cracked ammonia and carbon dioxide. The nitrogen potential value K N is 5~6, and the ammonia flow rate is 4~6m 3 / h, carbon dioxide flow rate is 0.5-1m 3 / h; the temperature of the secondary heat treatment is 540°C~560°C, and the time of the secondary heat treatment is 3~4h; Cooling stage: In an inert gas atmosphere, the temperature of the alloy steel is lowered to below 100°C to obtain composite nitrocarburized alloy steel.

2. The alloy steel composite nitriding heat treatment method according to claim 1, characterized in that: The pretreatment stage is to perform homogenization heat treatment on the alloy steel at a temperature of 400-450° C., and the heat preservation time is 0.5-1 hour.

3. The alloy steel composite nitriding heat treatment method according to claim 1, characterized in that: In the first stage of nitriding, the flow rate of cracked ammonia is 1.5~2.5m 3 / h.

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

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

6. An alloy steel, characterized in that: The composite nitriding heat treatment method for alloy steel is used to manufacture the composite nitriding heat treatment method for alloy steel according to any one of claims 1 to 5.

7. The alloy steel according to claim 6, characterized in that The depth of the composite nitrocarburized layer on the surface of the alloy steel is 0.15-0.23 mm, and the hardness of the surface of the alloy steel is above 800 HV.

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