Heat treatment method for improving toughness of ultrahigh-strength stainless steel

Abstract

The invention relates to the technical field of metal heat treatment, in particular to a heat treatment method for improving toughness of ultrahigh-strength stainless steel. The method comprises the following steps: S1, putting a sample into a heat treatment furnace, heating to 1080 DEG C, and carrying out solution treatment; s2, the sample subjected to solution treatment is discharged out of a furnace and quenched, the thickness of martensite is 0.2-0.8 micrometer, and the thickness of retained austenite is 1.0-3 micrometers; s3, after quenching, the sample is put into a cold treatment box, the temperature is reduced to-86 DEG C to-67 DEG C, and retained austenite is converted into martensite; the thickness of the retained austenite is 0.1-0.7 mu m, and the thickness of the martensite is 0.9-2.7 mu m; discharging the sample out of the furnace and returning to room temperature; s4, the sample subjected to cold treatment is subjected to aging treatment at the temperature of 560-570 DEG C, and then air cooling is conducted to the room temperature; the thickness of retained austenite is 0.3-1 [mu] m, and the thickness of martensite is 1.5-3.5 [ According to the method, the fracture toughness of the sample is ensured by determining the relationship between the thickness of the retained austenite and the thickness of the martensite after quenching, subzero treatment and aging treatment.

Description

Technical Field

[0001] The present invention relates to the technical field of metal heat treatment, and specifically to a heat treatment method for improving the toughness of ultra-high strength stainless steel. Background Art

[0002] Brief introduction to the heat treatment characteristics of ultra-high strength stainless steel:

[0003] Stainless steel containing Cr, Co, Ni, and W has excellent strength, toughness, and corrosion resistance. At room temperature, the tensile strength is ≥1930 MPa and the yield strength is ≥1500 MPa. Compared with precipitation hardening ultra-high strength steel, ultra-high strength stainless steel has excellent corrosion resistance. Currently, as the preferred material for structural components, it is widely used in high-tech fields such as aviation, aerospace, and marine. The characteristics of a certain type of ultra-high strength stainless steel are that after aging treatment in the range of 530 - 550 °C, a large number of fine and dispersed Laves phases precipitate in the steel. Their sizes are about 10 - 15 nm, which improves the strength and toughness of the steel.

[0004] The conventional heat treatment system is: quenching at 885 °C * 1 h, oil cooling; cryogenic treatment (-73) °C ≥ 1 h, warming up in air; aging at 482 °C * (5 - 8) h, air cooling; however, it is found in actual production that the mechanical properties fluctuate greatly, and the fracture toughness is between 70 - 80, which has not yet reached the acceptance requirement of fracture toughness greater than 90. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a heat treatment method for improving the toughness of ultra-high strength stainless steel to ensure the fracture toughness of the stainless steel after heat treatment.

[0006] The technical solution adopted by the present invention to solve its technical problems is a heat treatment method for improving the toughness of ultra-high strength stainless steel, including the following steps:

[0007] S1: Place the specimens in the material frame in an orderly manner, put the material frame and the specimens into the heat treatment furnace, heat the material frame and the specimens to 1080 °C, keep the temperature for 1 - 2 h, and perform solution treatment; completely transform the pearlite structure inside the specimens into austenite structure;

[0008] S2: Take out the material frame and the specimens that have completed the solution treatment for quenching, and the quenching transfer time is 5 s - 10 s, so that a part of the austenite structure inside the specimens is transformed into lath martensite structure, and the retained austenite is arranged in a lamellar shape between the martensite structures. The thickness of the martensite is 0.2 - 0.8 μm, and the thickness of the retained austenite is 1.0 - 3 μm;

[0009] S3: Immediately after quenching, the sample is placed in a cryogenic treatment box, the temperature is lowered to -86°C to -67°C and held for 2 to 3 hours to transform retained austenite into martensite structure, reducing the thickness of retained austenite to 0.1 to 0.7 μm and the thickness of martensite to 0.9 to 2.7 μm; then the sample is taken out of the furnace and left in the air to return to room temperature;

[0010] S4: The cryogenically treated sample is subjected to aging treatment:

[0011] The aging treatment temperature is 560°C to 570°C and held for 4 to 8 hours; then air-cooled to room temperature; the thickness of retained austenite is 0.3 to 1 μm and the thickness of martensite is 1.5 to 3.5 μm.

[0012] Furthermore, the quenching medium used in step S2 is B-244 quenching oil.

[0013] Furthermore, in step S2, the sample is placed in B-244 quenching oil and shaken up, down, left and right, the shaking frequency is 10 to 20 times per minute, the shaking time is 5 - 15 min, the up-and-down shaking amplitude is the height of the sample, and the left-and-right shaking amplitude is the width of the sample.

[0014] Furthermore, in step S1, the solution treatment is carried out using a heating furnace with a stirring fan for heating.

[0015] Furthermore, in step S3, the cooling rate of cryogenic treatment is 5°C / min to 10°C / min, and a low-temperature box furnace is used during the cryogenic treatment.

[0016] Furthermore, the sample is a three-point bending fracture toughness sample after rough machining before heat treatment, its surface roughness Ra ≤ 0.8 μm, and ultrasonic cleaning is carried out to remove the surface oxide layer before step S1.

[0017] The beneficial effects of the present invention are: By determining the relationship between the thickness of retained austenite and martensite after quenching, cryogenic treatment and aging treatment, the fracture toughness of the sample is ensured. The retained austenite exists in the form of lamellae between martensite laths. At the same time, the retained austenite can also hinder the propagation of microcracks in the steel. The lamellar retained austenite between martensite laths can improve the fracture toughness of the material. By controlling the cryogenic treatment time, the amount of austenite transformation is different after different cryogenic treatment heating temperatures and holding times, which can increase the content and distribution of martensite, and the fracture toughness can be significantly improved. Using the improved process for sample heat treatment can ensure that the strength is basically not reduced while increasing the toughness qualification rate, reducing the frequency of repeated heat treatment and repeated tests, and reducing the production cost and production cycle. Description of the Drawings

[0018] Figure 1 is a flow schematic diagram of the present invention;

[0019] Figure 2 is a schematic diagram of the specimen;

[0020] Figure 3 is a micrograph of the specimen after being processed by the technological process of the present invention. Detailed implementation manners

[0021] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are only used to explain the present invention and should not be construed as a limitation to the present invention.

[0022] As Figures 1 - 3 shown, a heat treatment method for improving the toughness of ultra-high strength stainless steel according to the present invention includes the following steps:

[0023] S1: The specimens are orderly placed in the material frame, and the material frame and the specimens are placed in a heat treatment furnace. The material frame and the specimens are heated to 1080 °C and held for 1 - 2 h for solution treatment; to completely transform the pearlite structure inside the specimens into austenite structure; if the temperature is lower than 1080 °C, the precipitated phases cannot be fully redissolved into the matrix, which will lead to a reduction in the number of nanoscale strengthening phases precipitated during tempering and affect the final properties of the material. If the temperature is higher than 1080 °C, the original austenite grains will grow excessively, and the coarse grains will cause the martensite laths obtained by quenching to be thick, and the yield strength will decrease significantly; if the holding time < 1 h, the precipitated phases do not have enough time to redissolve into the matrix, and if the holding time > 2 h, it will also cause the original grains to be coarse.

[0024] S2: The material frame and the specimens after solution treatment are taken out of the furnace for quenching, and the quenching transfer time is 5 s - 10 s, so that part of the austenite structure inside the specimens is transformed into lath-shaped martensite structure, and the retained austenite is arranged in a lamellar shape between the martensite structures. The thickness of the martensite is 0.2 - 0.8 μm, and the thickness of the retained austenite is 1.0 - 3 μm; after solution treatment, the material is cooled to room temperature by oil cooling; this process is to rapidly cool the steel after complete austenitization to promote the transformation of austenite into martensite.

[0025] S3: Immediately after quenching, the specimen is placed in a cryogenic treatment box, the temperature is reduced to -86°C to -67°C and held for 2 to 3 hours to transform retained austenite into martensite structure, reduce the thickness of retained austenite to 0.1 to 0.7 μm, and the thickness of martensite to 0.9 to 2.7 μm; then the specimen is taken out of the furnace and left in the air to return to room temperature; the cooling rate of cryogenic treatment is 5°C / min to 10°C / min. If the cooling rate is too fast, it will cause cracking due to the volume shrinkage of the material caused by rapid cooling. If the cooling rate is too slow, the martensite transformation will be incomplete. The controlled cooling temperature is to provide sufficient supercooling degree to provide sufficient phase transformation driving force for martensite transformation. At the same time, due to the extremely low temperature, the lattice constant of iron has a tendency to shrink, increasing the driving force for the precipitation of carbon atoms, promoting the precipitation of fine carbides during tempering, and playing a role of secondary phase strengthening.

[0026] S4: The specimen after cryogenic treatment is subjected to aging treatment:

[0027] The temperature of aging treatment is 560°C to 570°C and held for 4 to 8 hours; then air-cooled to room temperature; the thickness of retained austenite is 0.3 to 1 μm, and the thickness of martensite is 1.5 to 3.5 μm. After aging treatment, the strength of the steel reaches the maximum value. Thin-film retained austenite precipitates between martensite laths, increasing the thickness of retained austenite. The thin-film retained austenite plays an important role in improving the toughness of the steel, enabling the steel to have good toughness while reaching the peak strength.

[0028] Furthermore, the quenching medium used in step S2 is B-244 quenching oil.

[0029] Too fast cooling rate will lead to an increase in internal stress of the material, which will increase the risk of steel cracking. If the cooling rate is too slow, the martensite transformation will be incomplete, easily resulting in phenomena such as a high content of retained austenite and coarse martensite laths. The retained austenite will undergo martensite phase transformation during long-term use, leading to poor dimensional stability of the material and affecting its service performance, while coarse martensite laths will reduce the material strength. Further, in step S2, the specimen is placed in B-244 quenching oil and shaken. In step S2, the specimen is shaken up, down, left, and right in B-244 quenching oil. The shaking frequency is 10 - 20 times per minute, the shaking time is 5 - 15 minutes, the up-and-down shaking amplitude is the height of the specimen, and the left-and-right shaking amplitude is the width of the specimen. The shaking frequency is 10 - 20 times per minute. The higher the shaking frequency, the shorter the corresponding shaking time. The shaking process can be to shake upward first, then downward, then leftward, and finally rightward. One complete cycle of shaking in the four directions is considered one time. The up-and-down shaking amplitude is the height of the specimen, and the left-and-right shaking amplitude is the width of the specimen. This can ensure sufficient agitation of B-224 quenching oil, enabling the specimen to continuously contact with the cold B-244 quenching oil during the shaking process. A shaking frequency of 10 - 20 times / min can control the frequency of contact between the specimen and the cold B-244 quenching oil, ensuring that the cooling rate is within the range of 1 - 3 °C / second.

[0030] In order to ensure the heating efficiency of the specimen, further, in step S1, the solution treatment is carried out using a heating furnace with a stirring fan for heating.

[0031] Further, the specimen is a three-point bend fracture toughness specimen after pre-heat rough machining, with a surface roughness Ra ≤ 0.8 μm, and ultrasonic cleaning is carried out before step S1 to remove the surface oxide layer.

[0032] Example 1

[0033] Step S1: Solution treatment

[0034] The three-point bend fracture toughness specimens after pre-heat rough machining are orderly placed in the material frame. The surface roughness of the specimens Ra ≤ 0.8 μm, and ultrasonic cleaning is carried out before placing them in the material frame to remove the surface oxide layer. The material frame and the specimens are placed in a heating furnace with a stirring fan and heated to 1080 °C, and held for 1.5 hours for solution treatment. During this process, the pearlite structure inside the specimens is completely transformed into austenite structure.

[0035] Step S2: Quenching treatment

[0036] Quickly take out the material frame and specimens that have completed solution treatment. The quenching transfer time is 7 seconds, and B-244 quenching oil is used as the quenching medium. During quenching, the specimens are shaken at a speed of 15 times per minute, and the single shaking amplitude is 8% of the specimen length. After quenching, part of the austenite structure inside the specimens is transformed into lath martensite structure, and the retained austenite is arranged in lamellar form between the martensite structures. The thickness of the martensite is 0.5 μm, and the thickness of the retained austenite is 2.0 μm.

[0037] Step S3: Cryogenic treatment

[0038] Immediately put the specimens after quenching into the cryogenic treatment box, and conduct cryogenic treatment using a low-temperature box furnace. The cooling rate is 7 °C / min, and the temperature is reduced to -75 °C and held for 2.5 hours. After cryogenic treatment, the retained austenite is transformed into martensite structure, the thickness of the retained austenite is reduced to 0.4 μm, and the thickness of the martensite is increased to 1.8 μm. Take out the specimens and let them return to room temperature in the air.

[0039] Step S4: Aging treatment

[0040] Conduct aging treatment on the specimens after cryogenic treatment. The aging treatment temperature is 565 °C and held for 6 hours. After aging treatment, the specimens are air-cooled to room temperature. The thickness of the retained austenite is 0.6 μm, and the thickness of the martensite is 2.5 μm.

[0041] Example 2

[0042] Step S1: Solution treatment

[0043] Orderly place the three-point bend fracture toughness specimens after pre-heat rough machining into the material frame. The surface roughness Ra of the specimens is ≤ 0.8 μm, and ultrasonic cleaning is carried out before putting them into the material frame to remove the surface oxide layer. Put the material frame and specimens into a heating furnace with a stirring fan, heat to 1080 °C, hold for 2 hours, and conduct solution treatment. During this process, the pearlite structure inside the specimens is completely transformed into austenite structure.

[0044] Step S2: Quenching treatment

[0045] Quickly take out the material frame and specimens that have completed solution treatment. The quenching transfer time is 5 seconds, and B-244 quenching oil is used as the quenching medium. During quenching, the specimens are shaken at a speed of 10 times per minute, and the single shaking amplitude is 5% of the specimen length. After quenching, part of the austenite structure inside the specimens is transformed into lath martensite structure, and the retained austenite is arranged in lamellar form between the martensite structures. The thickness of the martensite is 0.3 μm, and the thickness of the retained austenite is 1.5 μm.

[0046] Step S3: Cryogenic treatment

[0047] The specimens after quenching were immediately placed in a cryogenic treatment box and cryogenically treated using a low-temperature box furnace. The cooling rate was 5 °C / min, and the temperature was lowered to -86 °C and held for 3 hours. After cryogenic treatment, the retained austenite was transformed into martensite structure, the thickness of the retained austenite decreased to 0.2 μm, and the thickness of the martensite increased to 1.2 μm. The specimens were taken out of the furnace and left in the air to return to room temperature.

[0048] Step S4: Aging treatment

[0049] The specimens after cryogenic treatment were subjected to aging treatment at a temperature of 560 °C for 8 hours. After aging treatment, the specimens were air-cooled to room temperature. The thickness of the retained austenite was 0.3 μm, and the thickness of the martensite was 1.5 μm.

[0050] Example 3

[0051] Step S1: Solution treatment

[0052] The three-point bend fracture toughness specimens after rough machining before heat treatment were orderly placed in a material frame. The surface roughness Ra of the specimens was ≤ 0.8 μm, and ultrasonic cleaning was carried out before placing them in the material frame to remove the surface oxide layer. The material frame and the specimens were placed in a heating furnace with a stirring fan and heated to 1080 °C and held for 1 hour for solution treatment. During this process, the pearlite structure inside the specimens was completely transformed into austenite structure.

[0053] Step S2: Quenching treatment

[0054] The material frame and the specimens after solution treatment were quickly taken out of the furnace. The quenching transfer time was 10 seconds, and B-244 quenching oil was used as the quenching medium. During quenching, the specimens were shaken at a speed of 20 times per minute, and the single shaking amplitude was 10% of the specimen length. After quenching, part of the austenite structure inside the specimens was transformed into lath martensite structure, and the retained austenite was arranged in lamellar form between the martensite structures. The thickness of the martensite was 0.8 μm, and the thickness of the retained austenite was 3.0 μm.

[0055] Step S3: Cryogenic treatment

[0056] The specimens after quenching were immediately placed in a cryogenic treatment box and cryogenically treated using a low-temperature box furnace. The cooling rate was 10 °C / min, and the temperature was lowered to -67 °C and held for 2 hours. After cryogenic treatment, the retained austenite was transformed into martensite structure, the thickness of the retained austenite decreased to 0.7 μm, and the thickness of the martensite increased to 2.7 μm. The specimens were taken out of the furnace and left in the air to return to room temperature.

[0057] Step S4: Aging treatment

[0058] The cold-treated specimen was subjected to aging treatment at a temperature of 570 °C for 4 hours. After aging treatment, the specimen was air-cooled to room temperature. The retained austenite thickness was 1.0 μm, and the martensite thickness was 3.5 μm.

[0059] Example 4

[0060] Step S1: Solution treatment

[0061] The three-point bend fracture toughness specimens after rough machining before heat treatment were orderly placed in a material frame. The surface roughness Ra of the specimens was ≤ 0.8 μm, and ultrasonic cleaning was performed before placing them in the material frame to remove the surface oxide layer. The material frame and the specimens were placed in a heating furnace with a stirring fan and heated to 1080 °C and held for 1.5 hours for solution treatment. During this process, the pearlite structure inside the specimens was completely transformed into austenite structure.

[0062] Step S2: Quenching treatment

[0063] The material frame and the specimens after solution treatment were quickly taken out of the furnace. The quenching transfer time was 8 seconds, and B-244 quenching oil was used as the quenching medium. During quenching, the specimens were shaken at a speed of 18 times per minute, and the single shaking amplitude was 7% of the specimen length. After quenching, part of the austenite structure inside the specimens was transformed into lath martensite structure, and the retained austenite was arranged in lamellar form between the martensite structures. The martensite thickness was 0.6 μm, and the retained austenite thickness was 2.5 μm.

[0064] Step S3: Cryogenic treatment

[0065] The specimens after quenching were immediately placed in a cryogenic treatment box, and cryogenic treatment was carried out using a low-temperature box furnace. The cooling rate was 8 °C / min, and the temperature was lowered to -80 °C and held for 2.5 hours. After cryogenic treatment, the retained austenite was transformed into martensite structure, the retained austenite thickness decreased to 0.5 μm, and the martensite thickness increased to 2.0 μm. The specimens were taken out of the furnace and left in the air to return to room temperature.

[0066] Step S4: Aging treatment

[0067] The cold-treated specimens were subjected to aging treatment at a temperature of 565 °C for 5 hours. After aging treatment, the specimens were air-cooled to room temperature. The retained austenite thickness was 0.8 μm, and the martensite thickness was 2.8 μm.

[0068] Comparative Example 1

[0069] The three-point bend fracture toughness specimens after rough machining before heat treatment are orderly placed into the material frame. The surface roughness Ra of the specimens is ≤ 0.8 μm, and ultrasonic cleaning is carried out before placing them into the material frame to remove the surface oxide layer. The material frame and the specimens are placed into a heating furnace with a stirring fan, heated to 885 °C, and held for 1 hour for solution treatment. Step S2: Quenching treatment

[0070] The material frame and the specimens after solution treatment are quickly taken out of the furnace. The quenching transfer time is 8 seconds, and B-244 quenching oil is used as the quenching medium.

[0071] Step S3: Cryogenic treatment

[0072] The specimens after quenching are immediately placed into a cryogenic treatment box, and cryogenic treatment is carried out using a low-temperature box furnace. The temperature is reduced to -73 °C and held for 2 hours.

[0073] Step S4: Aging treatment

[0074] The specimens after cryogenic treatment are subjected to aging treatment. The aging treatment temperature is 482 °C, and it is held for 5 hours. After aging treatment, the specimens are air-cooled to room temperature.

[0075]

[0076] It can be seen from the above table that the fracture toughness of the specimens obtained by the heat treatment process of the present invention is higher than the acceptance requirements.

[0077] The embodiments of the specific implementation manners are all preferred embodiments of the present invention, and do not limit the protection scope of the present invention accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A heat treatment method for improving the toughness of ultra-high strength stainless steel, characterized in that, It includes the following steps: S1: Place the specimens in the material frame in an orderly manner, put the material frame and the specimens into the heat treatment furnace, heat the material frame and the specimens to 1080 °C, hold for 1 - 2 h, and perform solution treatment; completely transform the pearlite structure inside the specimens into austenite structure; S2: Take out the material frame and the specimens that have completed the solution treatment and quench them. The quenching transfer time is 5 s - 10 s, so that part of the austenite structure inside the specimens is transformed into lath martensite structure, and the retained austenite is arranged in a lamellar shape between the martensite structures. The thickness of the martensite is 0.2 - 0.8 μm, and the thickness of the retained austenite is 1.0 - 3 μm; S3: Immediately after quenching, put the specimens into the cryogenic treatment box, lower the temperature to -86 °C - -67 °C and hold for 2 - 3 h, transform the retained austenite into martensite structure, reduce the thickness of the retained austenite to 0.1 - 0.7 μm, and the thickness of the martensite is 0.9 - 2.7 μm; then take the specimens out of the furnace and let them return to room temperature in the air; S4: Perform aging treatment on the specimens after cryogenic treatment: The aging treatment temperature is 560 °C - 570 °C, hold for 4 to 8 hours; then air cool to room temperature; the thickness of the retained austenite is 0.3 - 1 μm, and the thickness of the martensite is 1.5 - 3.5 μm.

2. The heat treatment method for improving the toughness of ultra-high strength stainless steel according to claim 1, characterized in that, The quenching medium used in step S2 is B - 244 quenching oil.

3. A heat treatment method for improving the toughness of ultra-high strength stainless steel according to claim 2, characterized in that, In step S2, the specimens are placed in B - 244 quenching oil and shaken up and down, left and right. The shaking frequency is 10 - 20 times / minute, the shaking time is 5 - 15 min, the up - and - down shaking amplitude is the height of the specimens, and the left - and - right shaking amplitude is the width of the specimens.

4. A heat treatment method for improving the toughness of ultra-high strength stainless steel according to claim 1, characterized in that, In step S1, the solution treatment is carried out by heating with a heating furnace equipped with a stirring fan.

5. A heat treatment method for improving the toughness of ultra-high strength stainless steel according to claim 1, characterized in that, In step S3, the cooling rate of the cryogenic treatment is 5 °C / min - 10 °C / min, and a low - temperature box - type furnace is used during the cryogenic treatment process.

6. The heat treatment method for improving the toughness of ultra-high strength stainless steel according to claim 1, characterized in that, The specimens are three - point bend fracture toughness specimens that have been rough - machined before heating. Their surface roughness Ra ≤ 0.8 μm, and ultrasonic cleaning is carried out before step S1 to remove the surface oxide layer.

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