Heat treatment method for improving low-temperature impact performance of maraging stainless steel

Abstract

The invention relates to the field of metal material heat treatment methods, in particular to a heat treatment method for effectively improving the low-temperature impact performance of maraging stainless steel, which comprises maraging stainless steel and comprises the following steps: a, low-temperature solution treatment: heating a vacuum tube type heat treatment furnace at a heating rate of 8-10 DEG C / min, keeping the temperature for 10-15 minutes after heating to 830-870 DEG C, and cooling to room temperature; then the maraging stainless steel is put into a vacuum tube type heat treatment furnace for heat preservation for 0.5-1 h, and after heat preservation is finished, the maraging stainless steel is discharged out of the furnace and water-cooled to the And b, aging treatment is conducted, specifically, the vacuum tube type heat treatment furnace is heated at the heating rate of 8-10 DEG C / min, heat preservation is conducted for 10-15 min after the temperature is increased to 460-485 DEG C, then the maraging stainless steel subjected to low-temperature solution treatment in the step a is put into the vacuum tube type heat treatment furnace to be subjected to heat preservation for 3-4 h, and after heat preservation is completed, the maraging stainless steel is taken out of the furnace and air-cooled to the room temperature. The method is particularly suitable for a process for improving the low-temperature impact performance of the maraging stainless steel.

Description

Technical Field

[0001] The present invention relates to the field of heat treatment methods for metal materials, and particularly to a heat treatment method for improving the low-temperature impact performance of maraging stainless steel. Background Art

[0002] With the continuous progress and development of technology, steel materials are gradually transforming towards high-end, intelligent, and green directions, and the design and development of high-strength steels have become one of the main directions for future development in the steel field. Compared with traditional high-strength steels, maraging stainless steels generally contain a relatively low C content, making their strengthening means no longer rely on carbides, but rather disperse precipitates are precipitated during appropriate aging treatment to achieve strengthening. At the same time, combined with the martensite lath structure obtained after quenching, the strengthening and toughening effect is jointly achieved. Among them, at low temperatures, the impact toughness of maraging stainless steel is particularly important. Impact toughness is an important indicator for measuring the ability of materials to resist fracture under dynamic loads, and it is of great significance for ensuring the safe operation of structural components under low-temperature conditions. Therefore, how to improve the low-temperature impact performance of maraging stainless steel through simple and effective methods is a problem that needs to be solved.

[0003] The solution of patent publication number CN109439870A discloses a method for improving the low-temperature impact energy of 17-4PH maraging stainless steel forgings based on microstructure control. It mainly includes the following steps: First, through chemical composition optimization, the total amount of high-temperature ferrite generated during steel smelting is reduced; second, by controlling the forging heating temperature, deformation amount, and deformation direction, the high-temperature ferrite during the smelting process is broken up, and then by forging along the direction perpendicular to the length direction of the long-strip high-temperature ferrite, the long-strip high-temperature ferrite is upset, effectively eliminating its directionality; finally, secondary solution + aging treatment is carried out to further eliminate the fine high-temperature ferrite during the recrystallization process, and the area content of the long-strip high-temperature ferrite is controlled below 0.6% to improve the low-temperature impact energy. The final relevant mechanical properties of the stainless steel forgings obtained after treatment are KV2(-40°C)≥27J, and the room-temperature tensile strength≥1000MPa, but its low-temperature impact toughness still cannot meet higher application requirements. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a heat treatment method for effectively improving the low-temperature impact performance of maraging stainless steel.

[0005] The technical solution adopted by the present invention to solve its technical problems is: A heat treatment method for improving the low-temperature impact performance of maraging stainless steel, including the following steps:

[0006] a. Low-temperature solution treatment: Heat up the vacuum tube heat treatment furnace at a heating rate of 8 - 10 °C / min. After heating to 830 - 870 °C, hold for 10 - 15 min. Then put the maraging stainless steel into the vacuum tube heat treatment furnace and hold for 0.5 - 1 h. After that, take it out of the furnace and cool it in water to room temperature;

[0007] b. Aging treatment: Heat up the vacuum tube heat treatment furnace at a heating rate of 8 - 10 °C / min. After heating to 460 - 485 °C, hold for 10 - 15 min. Then put the maraging stainless steel that has completed the low-temperature solution treatment in step a into the vacuum tube heat treatment furnace and hold for 3 - 4 h. After that, take it out of the furnace and air-cool it to room temperature.

[0008] Furthermore, the mass percentages of the chemical components of the maraging stainless steel are as follows: C: 0.01% - 0.03%; Cr: 12.00% - 14.00%; Ni: 8.00% - 9.50%; Mo: 2.00% - 2.80%; Al: 1.00% - 1.80%, and the rest is Fe.

[0009] The beneficial effects of the present invention are as follows: First, through low-temperature solution treatment, relatively small original austenite grains can be obtained, and a certain amount of austenite can be introduced into the matrix. The smaller grain size can play a role in fine grain strengthening, and the existence of austenite can effectively improve the interfacial stress concentration. When the impact crack encounters austenite during the propagation process, it will deflect to a certain extent, increasing the energy required for propagation, so that the low-temperature impact work of the material macroscopically increases, and the low-temperature impact performance is significantly improved. Second, through aging treatment, a certain number density and appropriately sized precipitation phases can continue to precipitate in the matrix, further playing a role in precipitation strengthening, so that the final strength of the material is maintained at a relatively high level, achieving the goal of significantly improving the low-temperature impact performance on the premise of ensuring its strength grade. Third, for the maraging stainless steel treated by this solution, its main mechanical properties can reach the following levels: tensile strength at room temperature ≥1400 MPa, KV2(-60 °C) ≥55 J, and the low-temperature impact performance is greatly improved. Fourth, without changing the material composition, through a new heat treatment system, the strength grade is kept stable, and at the same time, the impact work KV2 is greatly increased, effectively broadening the service scenario of the material. The present invention is particularly applicable to the process of improving the low-temperature impact performance of maraging stainless steel. Description of the Drawings

[0010] Figure 1 is a schematic structural diagram of a tensile specimen when the tensile test of the present invention is carried out.

[0011] Figure 2 is a schematic diagram of an impact specimen when the impact test of the present invention is carried out.

[0012] The markings in the figure are: tensile specimen 1, impact specimen 2, and V-notch 21. Specific implementation method

[0013] A heat treatment method for improving the low-temperature impact performance of maraging stainless steel. The mass percentages of the chemical components of the maraging stainless steel to be treated are as follows: C: 0.01% - 0.03%; Cr: 12.00% - 14.00%; Ni: 8.00% - 9.50%; Mo: 2.00% - 2.80%; Al: 1.00% - 1.80%, and the rest is Fe. The microstructure of the maraging stainless steel obtained by the heat treatment system of this solution includes: relatively small original austenite grain size (10.78 ± 0.49 μm), austenite with a volume fraction of 10.5 - 13.2%, and NiAl precipitation phases with a number density of (5.11 ± 0.47)×10 23 m -3 , and the size range is (3 - 5 nm). Through the synergistic effect of a certain content of austenite in the matrix and a certain number density and appropriate size of precipitation phases, the low-temperature impact performance of maraging stainless steel is greatly improved while ensuring its mechanical properties.

[0014] The following three groups of examples and three groups of comparative examples are used to illustrate the related effects of this solution.

[0015] Example 1

[0016] The heat treatment is carried out according to the following steps:

[0017] Step 1: Heat the vacuum tube heat treatment furnace to 850°C at a heating rate of 10°C / min. After pre-insulating for 10 min, quickly put the prepared maraging stainless steel specimen into the furnace. Since opening and closing the furnace door will have a certain impact on the furnace temperature, start timing after the temperature stabilizes at 850°C. After insulating for 1 h, quickly water-cool to room temperature, and the low-temperature solution treatment process is completed.

[0018] Step 2: Heat the vacuum tube heat treatment furnace to 482°C at a heating rate of 10°C / min. After pre-insulating for 10 min, quickly put the maraging stainless steel specimen that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the furnace temperature, start timing after the temperature stabilizes at 482°C. After insulating for 4 h, air-cool to room temperature, and the aging treatment process is completed.

[0019] Step 3: Perform room-temperature tensile (the tensile rate of tensile specimen 1 is 2*10 -4 ) and low-temperature impact (the temperature of impact specimen 2 is -60°C, and impact specimen 2 includes V-notch 21) tests on the maraging stainless steel specimen that has completed the above aging treatment.

[0020] Example 2

[0021] The heat treatment is carried out according to the following steps:

[0022] Step 1: Heat the vacuum tube heat treatment furnace to 830°C at a heating rate of 10°C / min. After pre-insulating for 12 minutes, quickly put the prepared maraging stainless steel specimen into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 830°C. After insulating for 0.75 hours, quickly water-cool to room temperature, thus completing the low-temperature solution treatment process.

[0023] Step 2: Heat the vacuum tube heat treatment furnace to 468°C at a heating rate of 10°C / min. After pre-insulating for 10 minutes, quickly put the maraging stainless steel specimen that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 468°C. After insulating for 3 hours, air-cool to room temperature, thus completing the aging treatment process.

[0024] Step 3: Conduct room-temperature tensile test (the tensile rate of tensile specimen 1 is 2*10 -4 ) and low-temperature impact test (the temperature of impact specimen 2 is -60°C, and impact specimen 2 includes V-notch 21) on the maraging stainless steel specimen that has completed the above aging treatment.

[0025] Example 3

[0026] The heat treatment is carried out according to the following steps:

[0027] Step 1: Heat the vacuum tube heat treatment furnace to 870°C at a heating rate of 10°C / min. After pre-insulating for 15 minutes, quickly put the prepared maraging stainless steel specimen into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 870°C. After insulating for 0.5 hours, quickly water-cool to room temperature, thus completing the low-temperature solution treatment process.

[0028] Step 2: Heat the vacuum tube heat treatment furnace to 473°C at a heating rate of 10°C / min. After pre-insulating for 10 minutes, quickly put the maraging stainless steel specimen that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 473°C. After insulating for 4 hours, air-cool to room temperature, thus completing the aging treatment process.

[0029] Step 3: Conduct room-temperature tensile test (the tensile rate of tensile specimen 1 is 2*10 -4 ) and low-temperature impact test (the temperature of impact specimen 2 is -60°C, and impact specimen 2 includes V-notch 21) on the maraging stainless steel specimen that has completed the above aging treatment.

[0030] Comparative Example 1

[0031] The heat treatment is carried out according to the following steps:

[0032] Step 1: Heat the vacuum tube heat treatment furnace to 925°C at a heating rate of 10°C / min. After pre-insulating for 15 min, quickly put the prepared maraging stainless steel specimen into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 925°C. After insulating for 1 h, quickly water-cool to room temperature, thus completing the low-temperature solution treatment process.

[0033] Step 2: Heat the vacuum tube heat treatment furnace to 480°C at a heating rate of 10°C / min. After pre-insulating for 10 min, quickly put the maraging stainless steel specimen that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 480°C. After insulating for 4 h, air-cool to room temperature, thus completing the aging treatment process.

[0034] Step 3: Conduct room-temperature tensile test (the tensile rate of tensile specimen 1 is 2*10 -4 ) and low-temperature impact test (the temperature of impact specimen 2 is -60°C, and impact specimen 2 includes V-notch 21).

[0035] Comparative Example 2

[0036] The heat treatment is carried out according to the following steps:

[0037] Step 1: Heat the vacuum tube heat treatment furnace to 900°C at a heating rate of 10°C / min. After pre-insulating for 15 min, quickly put the prepared maraging stainless steel specimen into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 925°C. After insulating for 1.5 h, quickly water-cool to room temperature, thus completing the low-temperature solution treatment process.

[0038] Step 2: Heat the vacuum tube heat treatment furnace to 510°C at a heating rate of 10°C / min. After pre-insulating for 10 min, quickly put the maraging stainless steel specimen that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 510°C. After insulating for 4 h, air-cool to room temperature, thus completing the aging treatment process.

[0039] Step 3: Conduct room-temperature tensile test (the tensile rate of tensile specimen 1 is 2*10 -4 ) and low-temperature impact test (the temperature of impact specimen 2 is -60°C, and impact specimen 2 includes V-notch 21).

[0040] Comparative Example 3

[0041] The heat treatment is carried out according to the following steps:

[0042] Step 1: Heat the vacuum tube heat treatment furnace to 925°C at a heating rate of 10°C / min. After pre-insulating for 15 min, quickly put the prepared maraging stainless steel sample into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 925°C. After insulating for 0.5 h, quickly water-cool to room temperature, and thus complete the low-temperature solution treatment process.

[0043] Step 2: Heat the vacuum tube heat treatment furnace to 570°C at a heating rate of 10°C / min. After pre-insulating for 10 min, quickly put the maraging stainless steel sample that has completed the above low-temperature solution treatment into the furnace. Since opening and closing the furnace door will have a certain impact on the temperature in the furnace, start timing after the temperature stabilizes at 570°C. After insulating for 4 h, air-cool to room temperature, and thus complete the aging treatment process.

[0044] Step 3: Perform room temperature tensile test (the tensile rate of tensile sample 1 is 2*10 -4 ) and low-temperature impact test (the temperature of impact sample 2 is -60°C, and impact sample 2 includes V-notch 21) on the maraging stainless steel sample that has completed the above aging treatment.

[0045] For the room temperature tensile and low-temperature impact tests carried out on the above three groups of examples and three groups of comparative examples, the test results are as follows: All tests are carried out in three groups according to the national standard, and the average value among them is selected for display. The obtained results are in good agreement and the fluctuation range is small. Therefore, one of the groups is selected for description, and the results are shown in the following table.

[0046] Tensile and Low-temperature Impact Test Results

[0047] Number Tensile strength (MPa) Low-temperature impact energy (J) Example 1 1440 58 Example 2 1443 55 Example 3 1412 56 Comparative example 1 1520 20 Comparative example 2 1490 17 Comparative example 3 1250 50

[0048] Examples 1, 2, and 3, compared with the mechanical properties of Comparative Examples 1 and 2, the tensile strength is basically maintained at the same level, and the low-temperature impact energy is increased by about one time. Although the newly designed heat treatment system introduces a certain amount of austenite, and austenite has an adverse effect on the strength of the material, due to the use of low-temperature solution treatment, relatively small original austenite grains can be obtained, and the fine grain strengthening compensates for a certain strength loss. At the same time, combined with the precipitation strengthening brought by the subsequent aging treatment, the final strength of this type of maraging stainless steel is relatively high, and the low-temperature impact performance is greatly improved. Examples 1, 2, and 3, compared with Comparative Example 3, although the low-temperature impact performance is basically close, the tensile strength is quite different. The tensile strengths of Examples 1, 2, and 3 are much higher than that of Comparative Example 3, indicating the combined effect of low-temperature solution and subsequent aging treatment, making the maraging stainless steel have almost unchanged strength and about doubled low-temperature impact performance under the newly designed heat treatment system. In addition, through the impact fracture surfaces provided by Example 1 and Comparative Example 1 after the test, it can be seen that there is an obvious necking phenomenon on the impact fracture surface of Example 1, which belongs to a ductile impact fracture surface. While there is no necking phenomenon on the impact fracture surface of Comparative Example 1, which belongs to a brittle impact fracture surface.

Claims

1. A heat treatment method for improving the low-temperature impact performance of maraging stainless steel, characterized in that, It includes the following steps: a. Solution treatment at low temperature: Heat up the vacuum tube heat treatment furnace at a heating rate of 8 - 10 °C / min, heat up to 830 - 870 °C and keep it warm for 10 - 15 min, then put the maraging stainless steel into the vacuum tube heat treatment furnace and keep it warm for 0.5 - 1 h, and after that, take it out of the furnace and cool it in water to room temperature; b. Aging treatment: Heat up the vacuum tube heat treatment furnace at a heating rate of 8 - 10 °C / min, heat up to 460 - 485 °C and keep it warm for 10 - 15 min, then put the maraging stainless steel that has completed the solution treatment at low temperature in step a into the vacuum tube heat treatment furnace and keep it warm for 3 - 4 h, and after that, take it out of the furnace and cool it in air to room temperature.

2. The heat treatment method for improving the low-temperature impact performance of maraging stainless steel according to claim 1, characterized in that, The mass percentages of the chemical components of the maraging stainless steel include: C: 0.01% - 0.03%; Cr: 12.00% - 14.00%; Ni: 8.00% - 9.50%; Mo: 2.00% - 2.80%; Al: 1.00% - 1.80%, and the rest is Fe.

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