CN111996347A - Method for strengthening austenitic stainless steel through low-temperature circulating strain after room-temperature pre-strain - Google Patents
Method for strengthening austenitic stainless steel through low-temperature circulating strain after room-temperature pre-strain Download PDFInfo
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- CN111996347A CN111996347A CN202010743454.3A CN202010743454A CN111996347A CN 111996347 A CN111996347 A CN 111996347A CN 202010743454 A CN202010743454 A CN 202010743454A CN 111996347 A CN111996347 A CN 111996347A
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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Abstract
The invention relates to a method for strengthening austenitic stainless steel by low-temperature circulating strain after room-temperature prestrain, which comprises the following steps: step one, performing tensile pre-strain on a test piece at room temperature: clamping the sample on a testing machine for unidirectional stretching until the preset prestrain amount is reached, then unloading the load to 0, wherein the strain loading rate in the stretching process is 0.0004s‑1(ii) a Step two, carrying out cyclic reinforcement treatment on the test piece in a low-temperature environment: clamping a test piece on a testing machine and placing the test piece into a low-temperature box, carrying out cooling operation after the temperature of the low-temperature box is set, carrying out heat preservation for a set time after the temperature in the low-temperature box reaches a set temperature and is stabilized, then starting a cyclic strengthening treatment test, axially stretching and compressing the test piece in a low-temperature environment for a certain number of cycles, determining the number of cycles according to the stress response rule of the low-temperature cyclic test, unloading the load to 0, taking out the test piece by a dismounting device, and completing strengthening treatment. The method greatly improves the strength of the material and is beneficial to improving the fatigue life of the material.
Description
Technical Field
The invention belongs to the field of strain strengthening of metals, relates to a method for improving fatigue performance of austenitic stainless steel, and particularly relates to a method for strengthening austenitic stainless steel by low-temperature circulating strain after room-temperature pre-strain.
Background
Fatigue failure is a common failure mode for mobile pressure vessels. The material S30408 adopted by the inner cylinder container of the low-temperature tank car isThe metastable austenite stainless steel has good corrosion resistance, low temperature resistance, good processability and toughness. However, the yield ratio is small, so that the allowable stress of the material is small, the wall thickness of the design is increased, the plastic bearing potential of the material is not fully utilized, and the production requirement of light weight cannot be met. At present, the performance of the material is improved mainly by adopting a strain strengthening technology for processing, the strain strengthening technology is used for carrying out external force loading on the material, when the external force loading exceeds the yield limit, the material can generate plastic deformation which cannot be recovered, and the next external force loading needs to exceed the force sigma loaded before1After which plastic deformation occurs again, so σ1Corresponding to the new yield strength of the material, thereby achieving the function of strengthening.
At present, the strain strengthening treatment in engineering is single, and mainly comprises the steps of carrying out hydraulic loading at a specified temperature after the container is manufactured, so that the container generates certain plastic deformation under a specific strengthening pressure, and the aim of improving the yield strength is fulfilled. At present, the research on the mechanical property and the cyclic fatigue property of the material after strain strengthening mainly focuses on two modes of stretching pre-strain and twisting pre-strain at room temperature, and the obtained general rule is that the pre-strain treatment increases the strength of the material to some extent, and the fatigue life is reduced to some extent. After the tensile pre-strain strengthening treatment is carried out at room temperature, the anisotropy of the material is caused, namely the tensile yield strength of the material is improved, and the compressive yield strength is reduced, so that the average stress exists in the subsequent uniaxial fatigue test, and the strain strengthening effect is not uniform. In addition, the material after room temperature strain strengthening has a 'hardening-softening-secondary hardening' process in a subsequent fatigue test, and the strengthening effect is not stable enough.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a method for strengthening austenitic stainless steel by circulating strain at low temperature after room temperature prestrain, which can uniformly and stably improve the strength of materials greatly and is beneficial to improving the fatigue life of the materials.
The above object of the present invention is achieved by the following technical solutions:
a method for strengthening austenitic stainless steel by low-temperature circulating strain after room-temperature prestrain is characterized by comprising the following steps:
step one, performing tensile pre-strain on a test piece at room temperature: clamping the test sample on a testing machine for unidirectional stretching until the preset prestrain amount is reached, then unloading the load to 0, wherein the strain loading rate in the stretching process is 0.0004s-1；
Step two, carrying out cyclic reinforcement treatment on the test piece in a low-temperature environment: clamping a test piece on a testing machine and placing the test piece into a low-temperature box, carrying out cooling operation after the temperature of the low-temperature box is set, carrying out heat preservation for a set time after the temperature in the low-temperature box reaches a set temperature and is stable, then starting a cyclic strengthening treatment test, axially stretching and compressing the test piece in a low-temperature environment for a certain number of cycles, determining the number of cycles according to the stress response rule of the low-temperature cyclic test, then unloading the load to 0, taking out the test piece by a dismounting device, and completing strengthening treatment.
Further: in the second step, the operation is carried out on a self-built low-temperature test platform; the low-temperature test platform comprises a low-temperature box, a liquid nitrogen tank and a control system, wherein the liquid nitrogen tank is used for providing refrigeration source liquid nitrogen, the low-temperature box is connected with the liquid nitrogen tank, and a connecting pipeline of the low-temperature box and the liquid nitrogen tank is provided with a regulating valve; the low-temperature box is provided with a fan for realizing uniform temperature distribution in the box and a temperature sensor for monitoring the temperature in the box in real time.
Further: the heat preservation time in the second step is 20 minutes.
Further: in the second step, the cyclic strain amplitude delta/2 is 0.5%, and the strain rate is 0.005s-1The period is 4s, and the loading waveform is a triangular wave.
The invention has the advantages and positive effects that:
1. the invention combines two driving forces of plastic strain and low temperature for inducing martensite phase transformation in the metastable state austenitic stainless steel to strengthen the material, thereby greatly improving the strength of the material.
2. The invention creatively provides the treatment of cyclic plastic strain strengthening under the condition of low temperature, greatly promotes the martensitic phase change and simultaneously leads the strengthening of the material to be more uniform and stable.
3. The invention mainly utilizes the martensite phase transformation generated by the metastable austenite stainless steel to strengthen the material without changing the chemical composition of the material.
4. The invention greatly improves the stress response and yield strength of the material, enables the material to bear larger external alternating load in the using process, improves the yield ratio of the material, is beneficial to reducing the design wall thickness of the pressure container and meets the production requirement of light weight when being practically applied to the strengthening treatment of the pressure container.
5. The invention strengthens the material more uniformly, improves the yield strength in the stretching and compressing directions, effectively avoids the influence of the anisotropy of the material on the overall performance of the material and reduces the potential safety hazard of the material in the using process.
6. The material strengthened by the invention does not have hardening or softening process in the subsequent fatigue test process, thereby ensuring the performance of the material to be more stable and avoiding considering the influence caused by cyclic hardening or softening in the container design process.
7. The fatigue life of the reinforced material is obviously longer than that of the room-temperature pre-strain reinforcement mode and the original material, and the higher fatigue life is more favorable for the practical engineering application of the material.
Drawings
FIG. 1 is a schematic view of a test piece according to the present invention;
FIG. 2 shows a schematic diagram of the test piece, the testing machine and the low temperature chamber;
FIG. 3 is a graph comparing fatigue life;
fig. 4 is a stress response comparison graph.
Detailed Description
The present invention will be described in further detail with reference to the following embodiments, which are illustrative, not restrictive, and the scope of the invention is not limited thereto.
A method for strengthening austenitic stainless steel by low-temperature circulating strain after room-temperature prestrain is disclosed, please refer to fig. 1-4, the invention point is that the method comprises the following steps:
step one, performing tensile pre-strain on a test piece at room temperature: in the embodiment, a tubular test piece 100 is adopted, the test piece is clamped on a testing machine to be subjected to unidirectional stretching until a preset prestrain amount is reached, then the load is unloaded to 0, and the strain loading rate in the stretching process is 0.0004s-1. Wherein the prestrain amount can adopt 8 percent.
Step two, carrying out cyclic reinforcement treatment on the test piece in a low-temperature environment: specifically, the operation is carried out on a self-built low-temperature test platform. The low-temperature test platform mainly comprises a low-temperature box 201, a liquid nitrogen tank 202 and a control system (not shown in the attached drawing), wherein the liquid nitrogen tank is used for providing a refrigeration source liquid nitrogen, the low-temperature box is connected with the liquid nitrogen tank, and a regulating valve 205 is arranged on a connecting pipeline of the low-temperature box and the liquid nitrogen tank; the low-temperature box is provided with a fan 203 for realizing uniform temperature distribution in the box and a temperature sensor 204 for monitoring the temperature in the box in real time. The low temperature box builds low temperature environment through the business turn over of control liquid nitrogen, adjusts the temperature through the governing valve of control connection liquid nitrogen container. The test piece was clamped on the testing machine and placed in a cold box, see fig. 3. Specifically, two ends of the test piece are respectively and fixedly connected with an upper connecting rod 302 and a lower connecting rod 303, and the two connecting rods respectively penetrate through the upper side wall and the lower side wall of the low-temperature box and are respectively connected with an upper chuck 301 and a lower chuck 304 of the fatigue testing machine. After the temperature of the low-temperature box is set (for example, -163 ℃), a valve of a liquid nitrogen tank is opened to start introducing liquid nitrogen, and after the temperature reaches the set temperature and is stable, the temperature is kept for 20 minutes, and then a circulation strengthening treatment test is started. The test piece is axially stretched and compressed in a low-temperature environment for a certain number of cycles (such as 300), the cyclic strain amplitude delta/2 is 0.5 percent, and the strain rate is 0.005s-1The period is 4s, and the loading waveform is a triangular wave. Wherein the number of cycles is determined according to the stress response law of the low temperature cycle test, for example, at-163 ℃, from the stress response law of the uniaxial fatigue test of S30408 stainless steel, the hardening stage of the material is basically concentrated in the first 300 cycles, at which the stress level isTo the maximum, the subsequent cycle test will maintain this stress level until the specimen fails at fatigue fracture, so 300 cycles are selected for the number of strain cycles in this test. This is because the metastable austenitic stainless steel induces martensitic transformation at low temperature or when plastic strain occurs, and the strength and hardness of the martensite phase are higher than those of the parent phase austenite phase, thus having a certain strengthening effect on the material. Tests prove that the speed and the quantity of martensite phase transformation in the test piece subjected to the cyclic plastic strain treatment at low temperature are far more than those of a pre-strain treatment mode at room temperature, so that the martensite phase transformation in the test piece is saturated after the test piece is subjected to cyclic plastic strain treatment at low temperature for a certain period, namely the best strengthening effect is achieved. And then unloading the load to 0, and taking out the sample by the dismounting device to finish the strengthening treatment.
The comparison result of the performance of the material strengthened by the method in the subsequent fatigue test and the original material and other traditional strengthening methods is shown in figures 3 and 4, wherein PRT is tensile pre-strain strengthening at room temperature; PLT is low temperature draw pre-strain reinforcement; PRT-C300LT is the enhancement of this method. After the method is adopted for strengthening treatment, the fatigue life of the sample in a subsequent fatigue test is obviously longer than that of the sample treated by other methods; as can be seen from FIG. 4, the stress response of the sample in the subsequent fatigue test is significantly higher after the strengthening treatment by the method than after the strengthening treatment by other methods.
In practical application, the periodic change of the pressure can be controlled by adjusting the flow and the flow speed of liquid nitrogen inside and outside the container, and the strain quantity of the material can be quantitatively monitored by using the low-temperature strain gauge, so that the cyclic strain hardening process at low temperature is simulated, and the material is strengthened.
Although the embodiments of the present invention and the accompanying drawings are disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the disclosure of the embodiments and the accompanying drawings.
Claims (4)
1. A method for strengthening austenitic stainless steel by low-temperature circulating strain after room-temperature prestrain is characterized by comprising the following steps:
step one, performing tensile pre-strain on a test piece at room temperature: clamping the sample on a testing machine for unidirectional stretching until the preset prestrain amount is reached, then unloading the load to 0, wherein the strain loading rate in the stretching process is 0.0004s-1；
Step two, carrying out cyclic reinforcement treatment on the test piece in a low-temperature environment: clamping a test piece on a testing machine and placing the test piece into a low-temperature box, carrying out cooling operation after the temperature of the low-temperature box is set, carrying out heat preservation for a set time after the temperature in the low-temperature box reaches a set temperature and is stabilized, then starting a cyclic strengthening treatment test, axially stretching and compressing the test piece in a low-temperature environment for a certain number of cycles, determining the number of cycles according to the stress response rule of the low-temperature cyclic test, unloading the load to 0, taking out the test piece by a dismounting device, and completing strengthening treatment.
2. The method of room temperature prestrained post-low temperature cyclic strain strengthened austenitic stainless steel of claim 1, characterized in that: in the second step, the operation is carried out on a self-built low-temperature test platform; the low-temperature test platform comprises a low-temperature box, a liquid nitrogen tank and a control system, wherein the liquid nitrogen tank is used for providing refrigeration source liquid nitrogen, the low-temperature box is connected with the liquid nitrogen tank, and a regulating valve is arranged on a connecting pipeline of the low-temperature box and the liquid nitrogen tank; the low-temperature box is provided with a fan for realizing uniform temperature distribution in the box and a temperature sensor for monitoring the temperature in the box in real time.
3. The method of room temperature prestrained post-low temperature cyclic strain strengthened austenitic stainless steel of claim 1, characterized in that: the heat preservation time in the second step is 20 minutes.
4. The method of room temperature prestrained post-low temperature cyclic strain strengthened austenitic stainless steel of claim 1, characterized in that: in the second step, the cyclic strain amplitude delta/2 is 0.5 percent, and the strain rate is 0.005s-1The period is 4s, and the loading waveform is a triangular wave.
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024114081A1 (en) * | 2022-11-28 | 2024-06-06 | 中国商用飞机有限责任公司北京民用飞机技术研究中心 | Thermal test apparatus and test method for hybrid connection structure |
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| CN102901672A (en) * | 2012-10-18 | 2013-01-30 | 华东理工大学 | Test method for researching transformation mechanism of martensite in austenitic stainless steel |
| CN204666454U (en) * | 2015-06-08 | 2015-09-23 | 天津大学 | A kind of fretting fatigue testing machine of contact load real-time, tunable |
| CN107739815A (en) * | 2017-09-20 | 2018-02-27 | 浙江大学 | Support method is protected in a kind of austenitic stainless steel deep cooling container strain hardening of optimization |
| CN109777936A (en) * | 2019-02-26 | 2019-05-21 | 东南大学 | A kind of martensitic stain less steel ultralow temperature strain hardening method |
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- 2020-07-29 CN CN202010743454.3A patent/CN111996347A/en active Pending
Patent Citations (4)
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| CN102901672A (en) * | 2012-10-18 | 2013-01-30 | 华东理工大学 | Test method for researching transformation mechanism of martensite in austenitic stainless steel |
| CN204666454U (en) * | 2015-06-08 | 2015-09-23 | 天津大学 | A kind of fretting fatigue testing machine of contact load real-time, tunable |
| CN107739815A (en) * | 2017-09-20 | 2018-02-27 | 浙江大学 | Support method is protected in a kind of austenitic stainless steel deep cooling container strain hardening of optimization |
| CN109777936A (en) * | 2019-02-26 | 2019-05-21 | 东南大学 | A kind of martensitic stain less steel ultralow temperature strain hardening method |
Non-Patent Citations (1)
| Title |
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| 徐淮建: ""亚稳态奥氏体不锈钢(S30408)深冷低周疲劳性能研究"", 《中国优秀硕士学位论文全文数据库（电子期刊）·工程科技Ⅰ辑》 * |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024114081A1 (en) * | 2022-11-28 | 2024-06-06 | 中国商用飞机有限责任公司北京民用飞机技术研究中心 | Thermal test apparatus and test method for hybrid connection structure |
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