Low-temperature application precipitation hardening martensitic stainless steel forge piece and heat treatment process thereof

Abstract

The invention relates to a heat treatment process for a low-temperature application precipitation hardening martensitic stainless steel forge piece. The heat treatment process comprises the following steps: S1, blanking; s2, upsetting and punching are carried out; s3, ring rolling is conducted, specifically, the blank is heated to 1100-1150 DEG C before rolling, heat preservation is conducted, and a forge piece is cooled after rolling is completed; s4, solid solution is conducted, specifically, the forge piece is heated to 830-850 DEG C for heat preservation, and then the forge piece is cooled; and S5, aging is conducted, specifically, the forge piece is subjected to first-step aging firstly, the forge piece is heated and heated to 480-700 DEG C for heat preservation and then cooled to the room temperature, and the forge piece is heated and heated to 480-700 DEG C for heat preservation in the second-step aging. The method has the advantages that the content of inverted austenite is increased through first-step aging, precipitation of NiAl and Ni3Ti is enhanced through second-step aging, and the fracture performance of the material is improved.

Description

Technical Field

[0001] The invention relates to the field of stainless steel preparation, and in particular to a low-temperature application precipitation hardening martensitic stainless steel forging and a heat treatment process thereof. Background Art

[0002] Martensitic stainless steel is stainless steel with martensite as the main strengthening phase. It has high yield strength and tensile strength, much higher than ordinary stainless steel. This makes it perform well under high load and strong impact environments. It has excellent comprehensive mechanical properties and is therefore widely used in aerospace, nuclear reactors, petrochemical and other fields.

[0003] The main reason for the high strength of martensitic stainless steel is the strengthening effect caused by the precipitation of ultrafine nanophases such as Cu-rich phase, NiAl, Ni3Ti, etc. during the aging process. The degree of strengthening depends on the size, density, spatial distribution of the precipitated phase and the interaction between the precipitated phase and dislocation. However, in the aging process of martensitic stainless steel, there is reversed austenite. The formation mechanism of reversed austenite is diffusion-type phase transformation. The precipitation hardening phase and the production of reversed austenite occur during the aging process, which have a huge impact on its performance, resulting in poor low-temperature performance of traditional martensitic stainless steel.

[0004] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0005] In view of the deficiencies of the prior art, the present invention discloses a low-temperature application precipitation hardening martensitic stainless steel forging and a heat treatment process thereof.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A heat treatment process for low temperature application precipitation hardening martensitic stainless steel forgings, characterized by comprising the following steps:

[0008] Step S1, blanking;

[0009] Step S2, upsetting and punching;

[0010] Step S3, ring rolling, the blank is heated to 1100-1150°C before rolling and kept warm, and the forging is cooled after rolling is completed;

[0011] Step S4, solid solution, heating the forging to 830-850°C and keeping the temperature, and then cooling the forging;

[0012] Step S5, aging, the forging is first subjected to the first step of aging, the forging is heated to 480-700°C and kept warm, then cooled to room temperature, and the second step of aging, the forging is heated to 480-700°C and kept warm.

[0013] Furthermore, in step S3, the insulation time ranges from 50 to 100 minutes.

[0014] Furthermore, in step S3, the deformation amount per firing is controlled to be 30-50%.

[0015] Furthermore, in step S4, the insulation time ranges from 60 to 90 minutes.

[0016] Furthermore, in step S4, the forging is cooled by air cooling or oil cooling.

[0017] Furthermore, in step S5, the first aging heat preservation time is 3 to 6 hours.

[0018] Furthermore, in step S5, the second aging holding time is 4 hours.

[0019] Furthermore, in step S5, the cooling liquid method of the forging is air cooling.

[0020] Furthermore, in step S5, after the solid solution of the forging is completed, it is transferred to the aging process, and the transfer time is controlled within 90 seconds.

[0021] A martensitic stainless steel comprises the following elements in percentage by mass: C 0.01-0.20%, Cr 11.0-12.0%, Ni 8.5-9.5%, Mo 0-0.5%, Ti: 0.9-1.4%, Al: 0.05-1.5%, Si: 0.1-0.5%, and the balance is Fe.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. Through solid solution and two-stage aging, the first step of aging increases the content of reversed austenite, and the second step of aging strengthens the precipitation of NiAl and Ni3Ti. These precipitated phases are evenly distributed on the matrix, which improves the overall strength of the material. This heat treatment process can improve the low-temperature impact performance of the material while maintaining a certain strength. The reason why reversed austenite can improve the impact performance of the material is that reversed austenite is a face-centered cubic structure with many slip systems, easy dislocations to start, and more able to coordinate deformation, which alleviates the stress concentration caused by dense lath martensite. The process of crack extension between martensite is hindered. At the same time, due to the existence of phase transformation induced plasticity effect (TRIP effect), during the deformation process, reversed austenite will undergo martensitic phase transformation and transform into martensite. This process will absorb additional deformation work, thereby significantly improving the fracture performance of the material.

[0024] 2. Strictly control the aging temperature and do not let it be too high. Increasing the aging temperature will increase the content of reverse austenite, coarsen the precipitation phase, and improve the plasticity of the material, but the strength will decrease significantly. Therefore, the most appropriate aging temperature range should be selected to take into account both the strength and plasticity of the material.

[0025] 3. Strictly control the chemical composition of stainless steel, the C content is controlled at 0.01-0.20%, the Cr content is controlled at 11-12%, to ensure its good hardenability and corrosion resistance, the Ni content is controlled at 8.5-9.5%, to avoid the formation of delta ferrite and residual austenite, and at the same time, an appropriate amount of aging strengthening elements is required, which is conducive to the precipitation of Cu-rich phase, NiAl, and Ni3Ti. These fine precipitates are densely distributed on the matrix, hindering the movement of dislocations. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the steps of the present invention.

[0027] Figure 2 This is the metallographic diagram of the forging after solid solution.

[0028] Figure 3 This is the metallographic diagram of the forging after the first step of aging.

[0029] Figure 4 This is the metallographic diagram of the forging after the second step of aging. DETAILED DESCRIPTION

[0030] The specific implementation of the present invention will be described below in conjunction with the accompanying drawings.

[0031] In order to make the purpose, technical solution and advantages of the present invention more clear, the device proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. According to the following description, the advantages and features of the present invention will be more clear.

[0032] Embodiment 1:

[0033] A heat treatment process for a low temperature application precipitation hardening martensitic stainless steel forging, comprising the following steps:

[0034] Step S1, blanking;

[0035] Step S2: upsetting and punching to obtain a ring forging.

[0036] Step S3, ring rolling, firstly, the forging is heated to 1100-1150°C by a gas furnace, and the holding time is controlled at 50-100 minutes. Then, the forging is rolled on a rolling mill for multiple times, and the deformation amount of each time is controlled at 30%-50%. After the ring is rolled, it is cooled by furnace cooling.

[0037] Step S4, solid solution, the forging is heated to 850°C and kept warm for 60 to 90 minutes. After the end of the warming, it is cooled by air or oil, and then transferred to the next process. The transfer time is controlled within 90 seconds.

[0038] Step S5, aging, the first step of aging forgings is heating up to 600°C and keeping warm, the first step of aging keeping warm time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 480°C and keeping warm, the second step of aging keeping warm time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0039] Martensitic stainless steel includes the following elements in mass percentage: C 0.01-0.20%, Cr 11.0-12.0%, Ni 8.5-9.5%, Mo 0-0.5%, Ti: 0.9-1.4%, Al: 0.05-1.5%, Si: 0.1-0.5%, and the balance is Fe.

[0040] Embodiment 2:

[0041] The difference from Example 1 is:

[0042] Step S5, aging, the first step of aging forgings is heating up to 480°C and keeping it warm, the first step of aging keeping time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 600°C and keeping it warm, the second step of aging keeping time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0043] Embodiment 3:

[0044] The difference from Example 1 is:

[0045] Step S5, aging, the first step of aging forgings is heating up to 600°C and keeping warm, the first step of aging keeping warm time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 540°C and keeping warm, the second step of aging keeping warm time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0046] Embodiment 4:

[0047] The difference from Example 1 is:

[0048] Step S5, aging, the first step of aging forgings is heating up to 540°C and keeping warm, the first step of aging keeping warm time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 600°C and keeping warm, the second step of aging keeping warm time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0049] Embodiment 5:

[0050] The difference from Example 1 is:

[0051] Step S5, aging, the first step of aging forgings is heating up to 700°C and keeping it warm, the first step of aging keeping time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 480°C and keeping it warm, the second step of aging keeping time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0052] Embodiment 6:

[0053] The difference from Example 1 is:

[0054] Step S5, aging, the first step of aging forgings is heating up to 480°C and keeping it warm, the first step of aging keeping time is 3-6h, then cooling to room temperature, the second step of aging forgings is heating up to 700°C and keeping it warm, the second step of aging keeping time is 4h, then the forgings are air-cooled to room temperature, and the forgings are sent for inspection.

[0055] Comparative Example 1:

[0056] The same forgings as in Example 1 are used, except that:

[0057] Solution treatment: Heat the forging to 850°C and keep it warm for 60 to 90 minutes.

[0058] Aging: Heat the forgings to 480°C and keep warm for 3 to 6 hours.

[0059] Comparative Example 2:

[0060] The same forgings as in Example 1 are used, except that:

[0061] Solution treatment: Heat the forging to 850°C and keep it warm for 60 to 90 minutes.

[0062] Aging: Heat the forgings to 480°C and keep warm for 3 to 6 hours.

[0063] Comprehensive mechanical testing of forgings:

[0064] Test group: samples of Examples 1 to 6 and samples of Comparative Examples 1 to 2.

[0065] The specific test results are shown in Table 1:

[0066]

[0067] Table 1

[0068] Analysis of test results:

[0069] Comparing the forgings of the embodiment with the forgings of the comparison example, the hardness performance of the forgings of the embodiment is reduced, the toughness performance of the forgings of the embodiment is increased, and the low-temperature performance of the forgings of the embodiment is significantly improved, which proves from a macroscopic perspective that the alloy structure of the forgings increases the reverse austenite content after the second-step aging.

[0070] Metallographic inspection of forgings:

[0071] Magnification: 50μm.

[0072] After the forging is solid-solutionized, samples are taken to test the metallographic structure, such as Figure 2 As shown in the figure, it can be seen that there are a large number of precipitation phases in the forging structure, and the grain size reaches level 9. Due to the fine grain strengthening effect, its strength is very high, but the plasticity is poor, and there is almost no reverse austenite in the structure.

[0073] After the first step of aging, the forgings are sampled and tested for metallographic structure, such as Figure 3 As shown in the figure, some reversed austenite is produced in the forging structure, with a volume fraction of about 15%. With the increase of aging temperature, the content of reversed austenite increases, the precipitation phase coarsens, and the precipitated phase is reversed austenite, which has many slip systems, easy dislocation, and better coordination of deformation, which relieves the stress concentration caused by dense lath martensite, and the process of crack extension between martensite is hindered.

[0074] After the second step of aging, the forgings are sampled and tested for metallographic structure, such as Figure 4 As shown in the figure, after the second step of aging, a large amount of precipitated phases NiAl and Ni3Ti are precipitated, and no delta ferrite and residual austenite are found in the final metallographic structure, and an ideal metallographic structure is obtained.

[0075] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0076] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A heat treatment process for low temperature application precipitation hardening martensitic stainless steel forgings, characterized in that: The following steps are involved: Step S1, blanking; Step S2, upsetting and punching; Step S3, ring rolling, the blank is heated to 1100-1150°C before rolling and kept warm, and the forging is cooled after rolling is completed; Step S4, solid solution, heating the forging to 830-850°C and keeping the temperature, and then cooling the forging; Step S5, aging, the forging is first subjected to the first step of aging, the forging is heated to 480-700°C and kept warm, then cooled to room temperature, and the second step of aging, the forging is heated to 480-700°C and kept warm.

2. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 1, characterized in that: In step S3, the temperature keeping time ranges from 50 to 100 minutes.

3. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 2, characterized in that: In step S3, the deformation amount of each firing is controlled to be 30-50%.

4. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 1, characterized in that: In step S4, the temperature keeping time ranges from 60 to 90 minutes.

5. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 1, characterized in that: In step S4, the forging is cooled by air or oil.

6. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 1, characterized in that: In step S5, the first step of aging and heat preservation time is 3 to 6 hours.

7. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 6, characterized in that: In step S5, the second aging heat preservation time is 4 hours.

8. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 7, characterized in that: In step S5, the cooling liquid method of the forging is air cooling.

9. The heat treatment process for low temperature applied precipitation hardening martensitic stainless steel forgings according to claim 1, characterized in that: In step S5, after the forging is solid-solutionized, it is transferred to the aging process, and the transfer time is controlled within 90 seconds.

10. A martensitic stainless steel prepared by the heat treatment process of low temperature applied precipitation hardening martensitic stainless steel forgings according to any one of claims 1 to 9, characterized in that: The elements include, in mass percentage, C 0.01-0.20%, Cr 11.0-12.0%, Ni 8.5-9.5%, Mo 0-0.5%, Ti: 0.9-1.4%, Al: 0.05-1.5%, Si: 0.1-0.5%, and the balance is Fe.

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