Method for synergistically strengthening strength and plasticity of martensitic stainless steel

Abstract

The invention provides a method for synergistically strengthening the strength and plasticity of martensitic stainless steel, and belongs to the technical field of metal heat treatment. The method comprises the following steps that S1, martensitic stainless steel is quenched and cooled, and quenched stainless steel is obtained; s2, after the surface roughness of the quenched stainless steel is adjusted, cleaning and drying are conducted, and pretreated stainless steel is obtained; s3, the pretreated stainless steel is heated in the nitrogen atmosphere, after the target temperature is reached, activation gas is introduced for activation treatment, and activated stainless steel is obtained; s4, the activated stainless steel is heated in the nitrogen atmosphere, carburizing gas is introduced for carburizing treatment after the target temperature is reached, and carburized stainless steel is obtained; and S5, the carburized stainless steel is cleaned and dried, and the martensitic stainless steel with the strength and plasticity cooperatively enhanced is obtained. The martensitic stainless steel is strengthened through the method, and the problem that in a traditional strengthening mode, the strength and plasticity cannot be improved at the same time is solved. After the martensitic stainless steel is strengthened through the method, the tensile strength of the martensitic stainless steel is improved, meanwhile, the elongation is also increased, and the effect that the strength and plasticity are synergistically increased is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of metal heat treatment, and in particular to a method for synergistically strengthening the strength and plasticity of martensitic stainless steel. Background Art

[0002] Martensitic stainless steel is a type of stainless steel that obtains high strength and hardness through phase transformation strengthening mechanism. Its performance characteristics mainly depend on its specific chemical composition and heat treatment process. In terms of chemical composition, martensitic stainless steel uses chromium as the main alloying element (usually between 12% and 18%) to ensure its basic corrosion resistance; at the same time, its carbon content is moderate (usually 0.1% to 1.2%), which is an important factor in determining the driving force of martensitic phase transformation and has a significant impact on hardness and strength. In terms of microstructure, the performance of martensitic stainless steel depends on the martensitic matrix structure formed by quenching during heat treatment. Through subsequent tempering treatment, carbides or other dispersed strengthening phases are precipitated inside the material, thereby achieving an optimized balance between strength and toughness. This phase transformation and precipitation mechanism constitutes the theoretical basis for the excellent mechanical properties of martensitic stainless steel.

[0003] The core performance improvement of martensitic stainless steel comes from its special heat treatment process. Usually, its heat treatment process includes quenching and tempering. The quenching process heats the steel to between 950℃ and 1050℃ and cools it rapidly to obtain a high-hardness martensitic structure; the tempering treatment is carried out in the range of 150℃ to 800℃, and its comprehensive mechanical properties and corrosion resistance can be further optimized according to the temperature adjustment. In addition, by controlling the grain size and tissue distribution, its impact resistance and fatigue performance can be significantly improved, making it suitable for more complex service environments.

[0004] Martensitic stainless steel is widely used in industrial fields with high requirements for strength and corrosion resistance, such as cutting tools, bearings, pump shafts, turbine blades, aircraft engine parts, oil and gas equipment, etc., because of its high strength and good corrosion resistance. However, this type of material is often under the combined effects of high loads, friction, impact and corrosive media during service, and its surface performance becomes one of the key factors for failure. In order to meet these stringent working conditions, in addition to conventional heat treatment processes, strengthening treatment is usually required to significantly improve the surface hardness, wear resistance and corrosion resistance of the material, thereby extending its service life and enhancing the reliability of the overall equipment.

[0005] Therefore, there is an urgent need for a method that can synergistically improve the strength and plasticity of martensitic stainless steel. Summary of the invention

[0006] The purpose of the present invention is to provide a method for synergistically strengthening the strength and plasticity of martensitic stainless steel. The present invention uses the present invention to strengthen martensitic stainless steel, getting rid of the problem that the strength and plasticity cannot be improved at the same time in the traditional strengthening method. After strengthening by the present invention, the tensile strength of martensitic stainless steel is improved, and the elongation is also increased, showing the effect of synergistic growth of strength and plasticity.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for synergistically strengthening the strength and plasticity of martensitic stainless steel, comprising the following steps:

[0009] S1. Quenching and cooling the martensitic stainless steel to obtain quenched stainless steel;

[0010] S2. The surface roughness of the quenched stainless steel is adjusted and then cleaned and dried to obtain pretreated stainless steel;

[0011] S3. The pretreated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, an activated gas is introduced for activation treatment to obtain activated stainless steel;

[0012] S4. The activated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, a carburizing gas is introduced for carburizing to obtain carburized stainless steel;

[0013] S5. The carburized stainless steel is cleaned and dried to obtain martensitic stainless steel with synergistically enhanced strength and plasticity.

[0014] Preferably, the quenching temperature in S1 is 950-1050° C., the heating rate is 5-20° C. / min, and the holding time is 0.5-2 h.

[0015] Preferably, the surface roughness of S2 is Ra≤3.2.

[0016] Preferably, the heating temperature in S3 is 250-450° C., and the heating rate is 5-20° C. / min.

[0017] Preferably, the activation gas in S3 is a mixed gas of hydrogen chloride and nitrogen; the volume ratio of hydrogen chloride to nitrogen is (25-40): (60-75).

[0018] Preferably, the activation treatment time in S3 is 2 to 5 hours.

[0019] Preferably, the heating temperature in S4 is 350-450°C, and the heating rate is 5-20°C / min.

[0020] Preferably, the carburizing gas in S4 is a mixed gas of carbon monoxide, hydrogen and nitrogen, and the volume ratio of the carbon monoxide, hydrogen and nitrogen is (20-30): (20-40): (30-60).

[0021] Preferably, the carburizing treatment time in S4 is 10 to 30 hours.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) By adopting the present invention to strengthen the martensitic stainless steel, there is no need to perform tempering after the quenching treatment. The tempering treatment can be performed at the same time as carburizing, thereby eliminating the tempering step and saving the tempering heat treatment time.

[0024] (2) By using the present invention to strengthen martensitic stainless steel, a uniform strengthening layer with a thickness of not less than 50 microns can be obtained for complex porous parts, inner and outer wall surfaces, etc. After strengthening, the surface hardness is greatly improved (increase> 200%), the friction and wear performance is enhanced, and the volume wear rate is reduced by more than one order of magnitude.

[0025] (3) The present invention is used to strengthen martensitic stainless steel, which gets rid of the problem that strength and plasticity cannot be improved at the same time in traditional strengthening methods. After strengthening by the present invention, the tensile strength of martensitic stainless steel is improved while the elongation is also increased, showing the effect of synergistic growth of strength and plasticity. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of the enhanced process of the present invention.

[0027] Figure 2 It is a schematic diagram of the temperature and time flow of the present invention.

[0028] Figure 3 This is the carbon concentration distribution along the depth after strengthening in Example 1 of the present invention.

[0029] Figure 4 This is a comparison chart of the surface hardness before and after strengthening of Example 1 of the present invention.

[0030] Figure 5 This is a comparison diagram of the samples before and after strengthening in Example 1 of the present invention.

[0031] Figure 6 This is a tensile curve diagram before and after strengthening of Example 1 of the present invention.

[0032] Figure 7 This is a volume wear rate diagram of Example 1 of the present invention before and after strengthening under 5N and 20N friction loads. DETAILED DESCRIPTION

[0033] The present invention provides a method for synergistically strengthening the strength and plasticity of martensitic stainless steel, comprising the following steps:

[0034] S1. Quenching and cooling the martensitic stainless steel to obtain quenched stainless steel;

[0035] S2. The surface roughness of the quenched stainless steel is adjusted and then cleaned and dried to obtain pretreated stainless steel;

[0036] S3. The pretreated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, an activated gas is introduced for activation treatment to obtain activated stainless steel;

[0037] S4. The activated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, a carburizing gas is introduced for carburizing to obtain carburized stainless steel;

[0038] S5. The carburized stainless steel is cleaned and dried to obtain martensitic stainless steel with synergistically enhanced strength and plasticity.

[0039] The invention obtains quenched stainless steel by quenching and cooling martensitic stainless steel.

[0040] In the present invention, the heat treatment may be performed in a muffle furnace.

[0041] In the present invention, the quenching temperature is 950-1050°C, the heating rate is 5-20°C / min, and the holding time is 0.5-2h. This ensures that the temperature difference between the inside and outside of the material is small, prevents cracking during the heating process, and ensures that the stainless steel structure is completely austenitized.

[0042] In the present invention, the cooling is carried out by naturally cooling to room temperature in air.

[0043] The invention obtains the pretreated stainless steel by adjusting the surface roughness of the quenched stainless steel and then cleaning and drying it.

[0044] In the present invention, the quenched stainless steel can be finely processed into the required component style before subsequent operations.

[0045] In the present invention, the surface roughness is Ra≤3.2, which ensures that a better strengthening effect is obtained during subsequent processing.

[0046] In the present invention, the cleaning and drying are specifically cleaning with 20-30° C. anhydrous ethanol to remove surface oil stains, and drying with cold air.

[0047] The invention heats the pretreated stainless steel in a nitrogen atmosphere, and introduces an activated gas to perform activation treatment after reaching the target temperature to obtain the activated stainless steel.

[0048] In the present invention, the nitrogen atmosphere is constructed by introducing nitrogen into the muffle furnace to exhaust the air. The present invention prevents surface oxidation by introducing nitrogen.

[0049] In the present invention, the heating temperature is 250-450° C., and the heating rate is 5-20° C. / min.

[0050] In the present invention, the activation gas is a mixed gas of hydrogen chloride and nitrogen; the volume ratio of hydrogen chloride to nitrogen is (25-40): (60-75).

[0051] In the present invention, the activation treatment time is 2 to 5 hours.

[0052] The present invention adopts the above technical solution and uses the activation effect of the activated gas to form a dense chromium oxide film on the surface of the part, thereby opening a channel for the entry of active carbon atoms during the carburizing process.

[0053] The invention heats activated stainless steel in a nitrogen atmosphere, introduces carburizing gas into the activated stainless steel to perform carburizing treatment after reaching the target temperature, and obtains carburized stainless steel.

[0054] In the present invention, the nitrogen atmosphere is exhausted by introducing nitrogen into the muffle furnace to completely exhaust the activated gas to prevent the oxide film from being regenerated; at the same time, the activated gas in the furnace is completely exhausted to avoid corrosion of the stainless steel surface due to the activated gas contacting the parts for too long.

[0055] In the present invention, the heating temperature is 350-450° C., and the heating rate is 5-20° C. / min.

[0056] In the present invention, the carburizing gas in S4 is a mixed gas of carbon monoxide, hydrogen and nitrogen, and the volume ratio of the carbon monoxide, hydrogen and nitrogen is (20-30): (20-40): (30-60).

[0057] In the present invention, the carburizing treatment time in S4 is 10 to 30 hours.

[0058] The present invention performs carburizing after activation treatment. This method does not need to perform tempering treatment independently. Tempering treatment is performed during carburizing, thereby eliminating the tempering step and saving tempering heat treatment time. At the same time, a uniform strengthening layer with a thickness of not less than 50 microns is formed.

[0059] The main chemical reaction formula of carburizing treatment is as follows:

[0060] CO+H2=[C]+H2O

[0061] 2CO=[C]+CO2

[0062] CO+3H2=CH4+H2O

[0063] CH4=[C]+2H2

[0064] The present invention cleans and dries carburized stainless steel to obtain martensitic stainless steel with synergistically enhanced strength and plasticity. The method of the present invention is used to strengthen martensitic stainless steel, which gets rid of the problem that strength and plasticity cannot be improved at the same time in traditional strengthening methods. After strengthening by the present invention, the tensile strength of martensitic stainless steel is improved, and the elongation is also increased, showing the effect of synergistic growth of strength and plasticity.

[0065] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0066] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0067] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0068] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0069] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0070] The "room temperature" and "normal temperature" mentioned in the present invention are all calculated as 25±2°C unless otherwise specified.

[0071] The raw materials used in the following examples of the present invention are all commercially available.

[0072] Example 1

[0073] In this embodiment, commercial 416 stainless steel is used, and the supply state is hot-rolled. The matrix formula is: by mass fraction, C0.133%, Si 0.53%, Mn 0.61%, P 0.027%, S 0.226%, Cr 13.20%, Ni 0.168%, Mo0.029%, and the balance is Fe.

[0074] This embodiment provides a method for synergistically strengthening the strength and plasticity of martensitic stainless steel, the method steps are:

[0075] S1. Take 416 martensitic stainless steel and quench it in a muffle furnace in an air atmosphere at a heating rate of 10°C / min and a temperature of 1050°C, and then air-cool it to room temperature to obtain quenched stainless steel.

[0076] S2. For the quenched stainless steel, finish machining to the surface roughness Ra≤3.2, clean with 25℃ anhydrous ethanol, and dry with cold air to obtain the pretreated stainless steel.

[0077] S3. Put the pretreated stainless steel into the furnace, introduce nitrogen into the furnace to 0.1MPa, and use a vacuum pump to evacuate the furnace. After the vacuum degree in the furnace is ≤1000Pa, introduce nitrogen again. Repeat the vacuuming and filling with nitrogen 3 times to expel the air in the furnace as much as possible; the heating parameters for introducing nitrogen are: introduce nitrogen at 50mL / min, heat to 450℃ in a nitrogen atmosphere, and the heating rate is 10℃ / min.

[0078] After the heating is completed, the temperature is kept constant at 450°C, and activation gas is introduced for surface activation. The components of the activation gas are hydrogen chloride and nitrogen (volume ratio is 30:65). Nitrogen is introduced at 90 mL / min and hydrogen chloride is introduced at 30 mL / min. The temperature is kept for 3 hours to fully remove the passive film on the surface of 416 stainless steel to obtain activated stainless steel.

[0079] S4. After the activation stage, the temperature was maintained and nitrogen was introduced at 90 mL / min for 2 h to completely discharge the activated gas in the furnace;

[0080] Then, carburizing gas was introduced, and then carburizing treatment was carried out at a temperature of 450° C. Carbon monoxide was introduced at 30 mL / min, hydrogen was introduced at 60 mL / min, and nitrogen was introduced at 60 mL / min. The treatment time was 30 hours to obtain carburized stainless steel; the volume ratio of the carbon monoxide, hydrogen and nitrogen was 25:30:45.

[0081] S5. After carburizing, the parts are cooled to room temperature in the furnace, taken out from the furnace, and then ultrasonically cleaned in 20°C deionized water for 20 minutes to obtain strengthened 416 martensitic stainless steel parts.

[0082] Example 2

[0083] In this embodiment, commercial 17-4PH martensitic stainless steel is used, and the supply state is hot-rolled. The matrix formula is: in mass fraction, C 0.043%, Si 0.67%, Mn 0.61%, P 0.031%, S 0.0058%, Cr 16.48%, Ni3.95%, Cu 3.2%, Mo 0.218%, Nb 0.318%, and the balance is Fe.

[0084] This embodiment provides a method for synergistically strengthening the strength and plasticity of martensitic stainless steel, the method steps are:

[0085] S1. 17-4PH martensitic stainless steel is quenched in a muffle furnace in an air atmosphere at a heating rate of 5°C / min and a temperature of 1050°C, and then air-cooled to room temperature to obtain quenched stainless steel.

[0086] S2. For the quenched stainless steel, finish machining to the surface roughness Ra≤3.2, clean with 20℃ anhydrous ethanol, and dry with cold air to obtain the pretreated stainless steel.

[0087] S3. Put the pretreated stainless steel into the furnace, introduce nitrogen into the furnace to 0.1MPa, and use a vacuum pump to evacuate the furnace. After the vacuum degree in the furnace is ≤1000Pa, introduce nitrogen again. Repeat the vacuuming and filling with nitrogen 3 times to expel the air in the furnace as much as possible; the heating parameters for introducing nitrogen are: introduce nitrogen at 50mL / min, heat to 250℃ in a nitrogen atmosphere, and the heating rate is 5℃ / min.

[0088] After the heating is completed, the temperature is kept constant at 250°C, and activation gas is introduced for surface activation. The components of the activation gas are hydrogen chloride and nitrogen (volume ratio is 25:60). Nitrogen is introduced at 90 mL / min and hydrogen chloride is introduced at 30 mL / min. The temperature is kept for 3 hours to fully remove the passive film on the surface of 17-4PH stainless steel to obtain activated stainless steel.

[0089] S4. After the activation stage, the temperature was maintained and nitrogen was introduced at 90 mL / min for 2 h to completely discharge the activated gas in the furnace;

[0090] Then, carburizing gas was introduced, and then carburizing treatment was carried out at a temperature of 350° C. Carbon monoxide was introduced at 30 mL / min, hydrogen was introduced at 60 mL / min, and nitrogen was introduced at 60 mL / min. The treatment time was 10 hours to obtain carburized stainless steel; the volume ratio of the carbon monoxide, hydrogen and nitrogen was 20:20:30.

[0091] S5. After carburizing, the parts are cooled to room temperature in the furnace, taken out from the furnace, and then ultrasonically cleaned in 20°C deionized water for 20 minutes to obtain strengthened 17-4PH martensitic stainless steel parts.

[0092] Comparative Example 1

[0093] The preparation method of this comparative example is the same as that of Example 1, except that the pretreated stainless steel is not activated.

[0094] Comparative Example 2

[0095] This comparative example adopts traditional tempering treatment:

[0096] S1. The 416 martensitic stainless steel in Example 1 is quenched in a muffle furnace in an air atmosphere at a heating rate of 10°C / min and a temperature of 1050°C, and then air-cooled to room temperature to obtain quenched stainless steel.

[0097] S2. The quenched stainless steel was tempered in a muffle furnace in an air atmosphere at a heating rate of 10°C / min and a temperature of 450°C, and then air-cooled to room temperature to obtain conventional tempered 416 stainless steel.

[0098] Test Example 1

[0099] The performance of the strengthened 416 martensitic stainless steel parts obtained in the examples and comparative examples was tested.

[0100] Depend on Figure 3 The results show that after the surface strengthening method of the present invention is used for the 416 stainless steel obtained in Example 1, a strengthening layer of nearly 50 microns is formed on the surface, and the maximum carbon concentration on the surface is nearly 1.4%, which is increased by 1050% compared with the substrate.

[0101] Figure 4 The comparison of the surface hardness of 416 stainless steel before and after strengthening in Example 1 is shown. The surface hardness of 416 stainless steel after strengthening is as high as 881 HV, which is 210% higher than that of untreated stainless steel.

[0102] Figure 5 and Figure 6 A comparison of tensile specimens before and after surface strengthening and a stress-strain curve of the strengthened 416 stainless steel obtained in Example 1 are given. The yield strength and tensile strength of the strengthened 416 stainless steel are both improved, and its tensile strength reaches 1417 MPa, which is an increase of 50 MPa compared to that without carburizing. What is more rare is that the elongation is also increased.

[0103] Figure 7The volume wear rate of the 416 stainless steel surface strengthened in Example 1 before and after strengthening under different friction loads is given. It can be seen from the figure that no matter what load is under, the volume wear rate of the strengthened sample is greatly reduced by more than an order of magnitude, effectively improving the wear resistance of the parts. It can be seen that the present invention effectively reduces the heat treatment steps, greatly improves the surface hardness, friction and wear performance and mechanical properties, and achieves the effect of simultaneous enhancement of strength and plasticity.

[0104] Table 1 Performance comparison table of various examples

[0105] Test items Example 1 Example 2 Comparative Example 1 Comparative Example 2 Surface hardness 881HV 1481HV 252HV 397HV Yield Strength 966MPa 1280MPa 631MPa 873MPa tensile strength 1417MPa 1454MPa 769MPa 1246MPa Elongation 14％ 10％ 13％ 12％

[0106] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for synergistically strengthening the strength and plasticity of martensitic stainless steel, characterized in that: The following steps are involved: S1. Quenching and cooling the martensitic stainless steel to obtain quenched stainless steel; S2. The surface roughness of the quenched stainless steel is adjusted and then cleaned and dried to obtain pretreated stainless steel; S3. The pretreated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, an activated gas is introduced for activation treatment to obtain activated stainless steel; S4. The activated stainless steel is heated in a nitrogen atmosphere, and after reaching the target temperature, a carburizing gas is introduced for carburizing to obtain carburized stainless steel; S5. The carburized stainless steel is cleaned and dried to obtain martensitic stainless steel with synergistically enhanced strength and plasticity.

2. The method according to claim 1, characterized in that In the S1, the quenching temperature is 950-1050° C., the heating rate is 5-20° C. / min, and the holding time is 0.5-2h.

3. The method according to claim 1, characterized in that The surface roughness of S2 is Ra≤3.

2.

4. The method according to claim 1, characterized in that: The heating temperature in S3 is 250-450° C., and the heating rate is 5-20° C. / min.

5. The method according to claim 1, characterized in that The activation gas in S3 is a mixed gas of hydrogen chloride and nitrogen; the volume ratio of hydrogen chloride to nitrogen is (25-40): (60-75).

6. The method according to claim 1, characterized in that The activation treatment time in S3 is 2 to 5 hours.

7. The method according to claim 1, characterized in that In S4, the heating temperature is 350-450°C, and the heating rate is 5-20°C / min.

8. The method according to claim 1, characterized in that The carburizing gas in S4 is a mixed gas of carbon monoxide, hydrogen and nitrogen, and the volume ratio of the carbon monoxide, hydrogen and nitrogen is (20-30): (20-40): (30-60).

9. The method according to claim 1, characterized in that: The carburizing treatment time in S4 is 10 to 30 hours.

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