Low-temperature steel bar tensile test method and device

Abstract

The invention relates to a tensile test method and device for a low-temperature reinforcing steel bar, which are characterized in that connecting pieces are welded at two ends of a low-temperature reinforcing steel bar sample, and heat treatment including heating, cooling quenching and low-temperature tempering is carried out on welding positions of the low-temperature reinforcing steel bar sample and the connecting pieces, so that quenched and tempered structures including tempered martensite and bainite are formed at the welding positions; then the low-temperature steel bar sample and the connecting pieces are placed in a low-temperature environment, the connecting pieces at the two ends are connected through a tensile testing machine, the low-temperature steel bar sample is subjected to a tensile test, it can be guaranteed that the steel bar sample is in the low-temperature environment, meanwhile, the requirement that the steel bar is not machined is met, breakage can occur in a steel bar base material section, and the test success rate is high; the mechanical property of the low-temperature steel bar at low temperature can be effectively reflected, and a test basis is provided for the application of the low-temperature steel bar in an LNG (Liquefied Natural Gas) project.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material testing, and in particular relates to a low-temperature steel bar tensile testing method and device. Background Art

[0002] Liquefied natural gas (LNG) storage tanks are facilities used to store liquefied natural gas, and their operating temperature is usually around -165 degrees Celsius. Low-temperature steel bars for LNG storage tanks are specially designed steel bars used to maintain their strength and toughness for a long time under extremely low temperature conditions. Therefore, during the development and design of low-temperature steel bars or the application of LNG projects, it is necessary to conduct a tensile test at -165°C on the low-temperature steel bars to reflect the mechanical properties of the mechanical properties material at low temperatures and provide a test data basis for development and application.

[0003] The low-temperature tensile test device or method in the prior art, for example, a low-temperature tensile test system for threaded steel bars disclosed in patent CN203396654U, a test device suitable for ultra-low-temperature steel bar tensile test disclosed in patent CN104020053A, and a low-temperature steel bar specimen for tensile test disclosed in patent CN116448515A, usually processes the low-temperature steel bar test sample into a standard specimen, uses a special hanging or threaded clamp to clamp the standard specimen at room temperature, and then places the clamp and the standard specimen in a low-temperature environmental box, which simulates the low-temperature environment, and stretches the standard specimen by a tensile testing machine connected to the clamp to obtain the test result; but on the one hand, the low-temperature environment will increase the strength of the material itself, for example: at a low temperature of -165°C, the low-temperature steel exposed in the low-temperature environmental box The increase in reinforcement strength will form a large strength difference between the reinforcement and the low-temperature steel bar material clamped in the fixture. Therefore, during the stretching process, the standard specimen will break at the clamping position at room temperature, causing the test failure and failing to reflect the performance of the material at low temperatures. On the other hand, the end of the reinforcement is machined for assembly with the fixture, but the test standard does not allow the processing of the reinforcement matrix. The processed threaded reinforcement cannot reflect the performance under actual working conditions, making the test method and equipment unsuitable for the stretching of low-temperature reinforcement. Although medium-frequency quenching heat treatment and strengthening transition at both ends of the reinforcement matrix can be used to improve the low-temperature hardening problem of the test, it may affect the performance of the reinforcement under actual working conditions. At the same time, the parallel length of the reinforcement in the test standard is required to be more than 500 mm, which will cause the required standard specimen size to be too long due to the heat treatment process, limiting the applicable range of the test. Summary of the invention

[0004] The present invention aims to solve at least one of the above technical problems to a certain extent. The present invention provides a low-temperature steel bar tensile test method and device, which does not require machining of the low-temperature steel bar, can improve the test success rate and effectively reflect the mechanical properties of the low-temperature steel bar at low temperatures.

[0005] The technical solution adopted by the present invention to solve its technical problem is: A low-temperature steel bar tensile test method, the method comprising: welding connectors at both ends of a low-temperature steel bar sample, heat treating the welding position of the low-temperature steel bar sample and the connector, the heat treatment comprising heating, cooling quenching and low-temperature tempering steps performed in sequence, so that the welding position forms a quenched and tempered structure including tempered martensite and bainite; then placing the low-temperature steel bar sample and the connector in a low-temperature environment, connecting the connectors at both ends with a tensile testing machine and performing a tensile test on the low-temperature steel bar sample.

[0006] In a preferred embodiment, the material of the connecting piece is 18CrNiMo7-6, which has the characteristics of high strength, high toughness, good welding performance, and wear resistance. The preferred length is 150mm~250mm.

[0007] In order to quickly and effectively connect the connecting piece with the low-temperature steel bar sample matrix, in a preferred embodiment, the welding adopts friction welding, and the friction welding includes primary pressure friction preheating, secondary pressure friction heating and top forging welding. Friction welding connects the low-temperature steel bar sample and the connecting piece by the heat generated by mechanical friction. During the friction welding process, pressure is applied to cause friction movement on the contact surface. Friction heat raises the temperature of the local area of ​​the contact surface to above the melting point, and then the friction movement is stopped. Pressure is continued to be applied to cause the molten material to form a weld under solid conditions. It has the advantages of fast welding speed, no need for filling materials, and applicability to a variety of sizes.

[0008] In order to further improve the welding effect and weld strength, selecting appropriate friction speed and spindle speed can accelerate the heating rate of the welding area, generate enough heat at the interface, make the material reach the plastic state faster and avoid local overheating and deterioration of the structure, select appropriate friction pressure and time, make the interface material better mixed and diffused, form a good metallurgical bond, select suitable upsetting parameters can make the welding interface in the plastic state further tightly bonded, enhance the welding bonding strength, in a preferred embodiment, the friction welding controls the first-level friction speed to be 1.1mm / s~1.4mm / s, the first-level friction speed pressure to be 2.5~3.5MPa, the first-level friction time to be 3~5s, the second-level friction speed to be 1.1mm / s~1.4mm / s, the second-level friction speed pressure to be 3~5MPa, the second-level friction deformation to be 7~9mm, the upsetting pressure to be 6~9MPa, the upsetting speed to be 4mm / s~6mm / s, the upsetting 3~4s, the upsetting holding time to be 1~2s, and the spindle speed to be 2000~2400r / min.

[0009] During the heating process of the heat treatment, a suitable heating temperature is selected, and the welding position can be heated to above the critical temperature to prepare for cooling and quenching. In a preferred embodiment, during the heating process, the heating temperature is controlled to be 900-950°C, and the welding position can be further preferably heated to 900°C.

[0010] The cooling and quenching process uses appropriate quenching parameters to promote the quenching transformation of the fabric to martensite and bainite, significantly improve the hardness of the welding position, and increase the strength of the bonding part. In a preferred embodiment, during the cooling and quenching process, the cooling medium is controlled to be oil, and the cooling temperature is 20~28°C, and the cooling temperature can be further preferably 25°C.

[0011] The low-temperature tempering selects appropriate tempering parameters, which can form a quenched and tempered structure including tempered martensite and bainite at the welding position, appropriately reduce the quenching stress, maintain high strength at the welding position, so as to improve local strength and toughness, and reduce the influence of the heat affected zone in the welding area during friction welding. In a preferred embodiment, during the low-temperature tempering process, the tempering temperature is controlled to be 250~300℃, and the insulation time is 1.5~2 hours. It can be further preferably controlled to be 250℃, the insulation time is 2h, and the cooling method is air cooling.

[0012] In a preferred embodiment, liquid nitrogen is used to simulate a cryogenic environment.

[0013] A low-temperature steel bar tensile test device, which implements any one of the above-mentioned low-temperature steel bar tensile test methods, comprises two connecting pieces, a tensile test machine and a low-temperature environment box, wherein the tensile test machine is used to drive the two connecting pieces inside the low-temperature environment box to move towards each other.

[0014] In a preferred embodiment, connectors are provided at both ends of the low temperature environment box, one end of the connector is connected to the tensile testing machine, and the other end of the connector is detachably connected to the connecting piece, and the connector can slide relative to the low temperature environment box in the moving direction of the connecting piece.

[0015] In a preferred embodiment, the connector and the connecting piece are detachably connected via threaded engagement.

[0016] In a preferred embodiment, a PLC controller is included. A plurality of temperature sensors are provided in the low-temperature environment box. The PLC controller is used to control the low-temperature environment box to simulate a low-temperature environment according to feedback from the temperature sensors.

[0017] In a preferred embodiment, the low temperature environment box is connected to a liquid nitrogen pipeline, and the PLC controller is used to control the liquid nitrogen pipeline.

[0018] Compared with the prior art, the beneficial effects of the present invention are at least: The present invention innovatively adopts welding combined with heat treatment to form a quenched and tempered structure with higher strength and toughness, including tempered martensite and bainite, at the welding position of the two ends of the low-temperature steel bar and the connecting piece, so as to realize the effective connection between the connecting piece and the steel bar matrix, thereby ensuring that the steel bar samples are all in a low-temperature environment, and at the same time meet the requirement of not machining the steel bar itself. The connecting piece can be stretched by a tensile testing machine and a tensile test can be performed on the low-temperature steel bar. It has been verified that the fracture occurs in the steel bar matrix material section, and there is obvious local plastic deformation at the fracture. The test success rate is high, and the reproducibility and reliability are good. The test data can accurately and effectively reflect the mechanical properties of the low-temperature steel bar at a low temperature of -165°C, and solves the problem that the low-temperature steel bar is machined in the existing tensile test, or cannot be directly stretched due to the strength difference between normal temperature and low temperature, and cannot reflect the performance of the material at low temperature. It is convenient to meet the requirement of the test standard for the parallel length of the steel bar to be above 500mm, so as to reduce the restriction on the low-temperature test of the low-temperature steel bar, and can provide an important test data basis for the application of low-temperature steel bars in LNG projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1 is a schematic structural diagram of Embodiment 1 of the present invention; Figure 2 This is a sample image after welding in Example 2 of the present invention; Figure 3 This is a structural diagram of the core of the welding position of Example 2 of the present invention; Figure 4 This is a sample diagram after the successful test of Example 2 of the present invention; Figure 5 is a displacement-stress curve diagram of Example 2 of the present invention; Figure 6 This is a sample diagram after the failure of the comparative example 1 of the present invention; Figure 7 is a displacement-stress curve diagram of Comparative Example 1 of the present invention; Figure 8 This is the core organization diagram of the welding position of Comparative Example 1 of the present invention; Fig. 9 It is a fractured sample diagram of comparative examples 2 and 3 of the present invention after the test failure.

[0020] Markings in the figure: low-temperature steel bar specimen 1, connecting piece 2, low-temperature environmental box 3, connector 4, push fixture of tensile testing machine 5, temperature sensor 6, welding position 7. DETAILED DESCRIPTION

[0021] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0022] In the description of the present invention, it should be understood that the terms "upper", "middle", "lower", "axial", "inner", "outer", etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In the description of the present invention, "multiple" and "several" mean two or more, unless otherwise clearly and specifically defined.

[0023] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. Embodiment 1:

[0024] In view of the problem that low-temperature steel bars cannot be machined, have strength differences between room temperature and low temperature, and cannot be directly stretched, such as Figure 1 As shown, a preferred embodiment of the low-temperature steel bar tensile test device of the present invention is shown, the low-temperature steel bar tensile test device comprises two connectors 2, a tensile testing machine and a low-temperature environment box 3, the tensile testing machine is used to drive the two connectors 2 inside the low-temperature environment box 3 to move toward each other, the two connectors 2 are used to connect the low-temperature steel bars by welding and heat treatment, and meet the requirement of not machining the low-temperature steel bars themselves, during the test, the connectors 2 can be added to ensure that the low-temperature steel bar samples 1 are all in the low-temperature environment of the low-temperature environment box 3, and then the connectors 2 can be stretched by the tensile testing machine to perform a tensile test on the low-temperature steel bars, so as to avoid the sample breaking at the clamping position at normal temperature and causing the test failure due to the strength difference between normal temperature and low temperature during the stretching process, and at the same time, the heat treatment is only used for the welding position 7, which will not have too much impact on the low-temperature steel bar matrix, and is more conducive to the performance of the reaction material under real working conditions, and then effectively reflects the performance of the material at low temperature, and is convenient to meet the requirement of the test standard for the parallel length of the steel bars to be above 500mm, thereby reducing the restrictions on the low-temperature test of the low-temperature steel bars.

[0025] Furthermore, connectors 4 are provided at both ends of the low temperature environment box 3, one end of the connector 4 is connected to the tensile testing machine, and the other end of the connector 4 is detachably connected to the connecting piece 2, and the connector 4 can slide relative to the low temperature environment box 3 in the moving direction of the connecting piece 2; for example: Figure 1 As shown, the tensile testing machine is arranged outside the low-temperature environment box 3, a hole is opened on the low-temperature environment box 3, and the connector 4 is slidably and sealedly installed on the hole of the low-temperature environment box 3. The sealing installation can adopt a seal and other structures. One end of the connector 4 is clamped and connected by the flat push clamp 5 of the tensile testing machine, and the other end of the connector 4 passes through the low-temperature environment box 3 and is connected to the connector 2 inside the low-temperature environment box 3. In this way, the low-temperature steel bar sample 1 and the connector 2 can both be in the low-temperature environment box 3. During the stretching process of the tensile testing machine, the two connectors 2 are pulled to move in opposite directions through the connectors 4 at both ends, and the tensile force on the low-temperature steel bar sample 1 is transmitted through the connector 4 and the connector 2. This type of clamping can effectively improve the utilization rate of materials. The use of the connector 4 can also effectively increase the airtightness of the low-temperature environment box 3 and maintain the low-temperature environment temperature.

[0026] Furthermore, the connector 4 and the connector 2 are detachably connected through threaded engagement; for example: Figure 1 As shown, threads are turned on the connector 2, and the connector 4 adopts a sleeve structure with threads. The end of the connector 2 can be screwed into the sleeve through threaded cooperation to achieve quick connection. After the test, the connector 2 can be removed through the threads, so that the connector 2 can be quickly replaced, ensuring that the connector 4 can be reused as a connecting part.

[0027] Furthermore, a PLC controller is included, and a plurality of temperature sensors 6 are provided in the low-temperature environment box 3. The PLC controller is used to control the low-temperature environment box 3 to simulate a low-temperature environment according to feedback from the temperature sensor 6; for example, the temperature sensor 6 has three temperature probes, which are respectively arranged at the upper, middle and lower positions inside the low-temperature environment box 3. The low-temperature environment box 3 is connected to a liquid nitrogen pipeline, and a liquid nitrogen inlet valve connected to the PLC controller is provided on the liquid nitrogen pipeline. When the liquid nitrogen pipeline passes liquid nitrogen into the low-temperature environment box 3 to simulate a low-temperature environment, the temperature in the low-temperature environment box 3 can be detected by a plurality of temperature sensors 6 respectively, and fed back to the PLC controller. The PLC controller controls the opening of the liquid nitrogen inlet valve according to the feedback temperature and the target temperature, thereby realizing low-temperature environment control, ensuring the uniformity and stability of the low-temperature environment during the test, and enabling the low-temperature environment and the temperature control method to accurately simulate the low-temperature use conditions. Embodiment 2:

[0028] In view of the problem that low-temperature steel bars cannot be directly stretched due to the strength difference between room temperature and low temperature without machining, a preferred implementation of the low-temperature steel bar tensile test method of the present invention adopts the low-temperature steel bar tensile test device described in Example 1, and the method includes the following steps: S1. Weld connectors 2 at both ends of low-temperature steel bar specimen 1. Specifically: The material of the connecting piece 2 is 18CrNiMo7-6, the outer diameter of the welding part is 27mm after turning, and the pitch is 3mm; the material of the low-temperature steel bar sample 1 is HRB500DW, the specification is φ25mm, and the length is 500mm.

[0029] The friction welding machine is used to perform friction welding on the low-temperature steel bar sample 1 and the connector 2. The friction welding includes primary pressure friction preheating, secondary pressure friction heating and upsetting welding. The primary friction speed of the friction welding machine is controlled to be 1.3 mm / s, the primary friction speed pressure is 3 MPa, the primary friction time is 3 s, the secondary friction speed is 1.3 mm / s, the secondary friction speed pressure is 4 MPa, the secondary friction deformation is 8 mm, the upsetting pressure is 8 MPa, the upsetting speed is 5 mm / s, the upsetting is 3 s, the upsetting holding time is 1 s, and the spindle speed is 2200 r / min. After welding, the sample is as follows Figure 2 shown.

[0030] S2. Considering that the heat affected zone (HAZ) in the welding area is inevitable during welding, the local microstructure will inevitably change, which will affect the overall strength of the weld. In order to increase the strength of the combined part and reduce the cold brittleness, the welding position 7 between the low-temperature steel bar sample 1 and the connector 2 is heat treated to improve the local strength and toughness and reduce the influence of the heat affected zone in the welding area. The heat treatment includes heating, cooling quenching and low-temperature tempering processes performed in sequence. Specifically: During the heating process, the welding position 7 is heated by an induction heating machine, and the heating temperature is controlled to be 950°C; during the cooling and quenching process, the cooling medium is controlled to be oil, and the cooling temperature is 25°C; during the low-temperature tempering process, a blast drying oven is used, the tempering temperature is controlled to be 250°C, and the temperature is kept for 2 hours, and the cooling method is air cooling. This heat treatment process can effectively improve the strength of the weld. The cross section of the sample welding area is subjected to metallographic structure analysis. The metallographic structure is as follows Figure 3 As shown, thanks to the heat treatment of the alloy element nickel in the low-temperature steel bar at welding position 7, a quenched and tempered structure including tempered martensite and bainite is formed at the edge, center and 1 / 2 radius, with higher strength and toughness.

[0031] S3, using liquid nitrogen to simulate a low temperature environment, placing the low temperature steel bar sample 1 and the connector 2 in a low temperature environment, connecting the connectors 2 at both ends with a tensile testing machine and performing a tensile test on the low temperature steel bar sample 1, specifically: Open the low temperature environment box 3, place the low temperature steel bar sample 1 and the connector 2 after step S2 in the low temperature environment box 3, connect the connectors 2 at both ends to the inner ends of the corresponding connectors 4 through threads, seal the low temperature environment box 3, use the tensile testing machine flat push fixture 5 to clamp the outer end of the connector 4, set the low temperature environment box 3 to simulate a -165°C low temperature environment, and start the tensile testing machine after the low temperature environment box 3 reaches the target temperature. The tensile tester is used to transmit the tensile force through the connector 4 and the connector 2, and the low temperature steel bar sample 1 is stretched axially until the low temperature steel bar breaks. The tensile test result is recorded by the tensile testing machine. The fractured sample after the test is successful is as follows: Figure 4 It can be seen that the fracture position occurs in the matrix material segment and is caused by Figure 5 It can be clearly seen from the displacement-stress curve that the sample has undergone plastic fracture, and there is obvious local plastic deformation at the fracture site, that is, the fracture, which shows that the low-temperature steel bars that have not been machined can reflect the performance under real working conditions and are an effective experiment. Embodiment 3:

[0032] Another preferred embodiment of the low-temperature steel bar tensile test method of the present invention is different from Example 2 in that the welding parameters of step S1 and the heat treatment parameters of step S2 are replaced. Specifically: Replace the welding parameters of S1 to control the first-level friction speed of the friction welding machine to 1.1mm / s, the first-level friction speed pressure to 3MPa, the first-level friction time to 5s, the second-level friction speed to 1.4mm / s, the second-level friction speed pressure to 4MPa, the second-level friction deformation to 8mm, the upsetting pressure to 8MPa, the upsetting speed to 4mm / s, the upsetting 4s, the upsetting holding time to 2s, the spindle speed to 2200r / min, and replace the heat treatment parameters of step S2 to control the heating temperature to 900°C; in the cooling and quenching process, the cooling medium is controlled to be oil, and the cooling temperature is 20°C; in the low-temperature tempering process, a blast drying oven is used to control the tempering temperature to 300°C, and the heat preservation is 1.5 hours. The fracture position occurs in the base material section, which is an effective test.

[0033] In order to further illustrate the feasibility and beneficial effects of the low-temperature steel bar tensile test method of the present invention, the present invention also conducted the following tests: (1) Verify the reproducibility and reliability of test results: According to the methods described in Examples 2 and 3, 9 low-temperature steel bar samples were re-prepared respectively, wherein 3 low-temperature steel bar samples had a specification φ of 25 mm, 3 low-temperature steel bar samples had a specification φ of 20 mm, and 3 low-temperature steel bar samples had a specification φ of 16 mm. Tests were carried out under the same conditions, and the test results were as follows: the fracture positions of the low-temperature steel bar samples all occurred in the matrix material section, indicating that the test was effective and the test success rate was greatly improved. At the same time, the matrix part of the low-temperature steel bar was not processed. The test data obtained by the present invention can accurately and effectively reflect the mechanical properties of the low-temperature steel bar, solve the problem of low tensile success rate of the low-temperature steel bar due to poor strength at low temperatures, and provide an important test data basis for the application of low-temperature steel bars in LNG projects.

[0034] (2) Investigate the effect of heat treatment on the tensile test of low-temperature steel bars: Step S2 of Example 2 is omitted, and a low-temperature steel bar tensile test is performed. As Comparative Example 1, multiple tests are performed. The fracture of the low-temperature steel bar in Comparative Example 1 is as follows: Figure 6 As shown, the displacement-stress curve of the low-temperature steel bar in comparative example 1 is as follows Figure 7 The test results of Example 1 and Comparative Example 1 are shown in Table 1 below: Table 1. Tensile test results of low temperature steel bars with and without heat treatment

[0035] Depend on Figure 6 It can be seen that the fracture of the sample of the low-temperature steel bar in Comparative Example 1 occurred in the welding area, and Figure 7 It can be clearly seen from the displacement-stress curve that the sample of the low-temperature steel bar in comparative example 1 fractured brittlely without obvious local plastic deformation at the fracture site, indicating that it was a failed and invalid experiment.

[0036] The microstructure of the sample with low temperature steel bar fractured at the weld position in comparative example 1 was analyzed. Figure 8 As shown, it is found that the metallographic structures at the edge, core and 1 / 2 radius of the low-temperature steel bar are ferrite, bainite and pearlite. The heat-affected zone generated during welding is rapidly cooled at a higher temperature to form bainite. Although the strength and hardness of bainite are between pearlite and martensite and it has good toughness, most of the material structure is still ferrite and pearlite, and the strength and hardness are relatively low, which is the main reason for the fracture of the test weld. From the comparison results of Example 2 and Comparative Example 1, it can be seen that the present invention innovatively uses friction welding combined with heat treatment, which can form a quenched and tempered structure with higher strength and toughness, including tempered martensite and bainite, at the welding position, which can effectively increase the strength of the bonding part, reduce cold brittleness, and thus reduce the influence of the heat-affected zone of the welding part, so that the success rate is greatly improved.

[0037] (3) Investigate the influence of friction welding parameters on low-temperature steel bar tensile test: The friction welding parameters of step S1 in Example 2 were replaced to conduct a low-temperature steel bar tensile test. As a comparative example, multiple tests were conducted. The welding parameters of each example are shown in Table 2 below: Table 2. Welding parameters of different low temperature steel bar tensile tests

[0038] The broken samples after the failure of the comparative examples 2 and 3 are as follows Fig. 9 The test results are shown in Table 3 below: Table 3. Results of low temperature steel bar tensile test with different welding parameters

[0039] From the comparison results of Example 2 with Comparative Examples 2 and 3, it can be seen that the selection of appropriate friction welding parameters can further improve the welding effect and weld strength, so as to improve the success rate of the low-temperature steel bar tensile test.

[0040] (4) Investigate the influence of heat treatment parameters on low-temperature steel bar tensile test: The heat treatment parameters of step S2 in Example 2 were replaced to carry out a low-temperature steel bar tensile test. As a comparative example, multiple tests were carried out. The welding parameters of each example are shown in Table 4 below, and the test results are shown in Table 5 below: Table 4. Heat treatment parameters for tensile tests of different low temperature steel bars

[0041] Table 5. Tensile test results of low temperature steel bars with different heat treatment parameters

[0042] From the comparison results of Example 2 with Comparative Examples 4 and 5, it can be seen that the selection of appropriate heat treatment parameters can further change the structure of the welding part and improve the welding strength, so as to improve the success rate of the low-temperature steel bar tensile test.

[0043] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. All equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low temperature steel bar tensile test method, characterized in that: The methods include: Connectors (2) are welded at both ends of a low-temperature steel bar sample (1), and a heat treatment is performed on a welding position (7) between the low-temperature steel bar sample (1) and the connector (2), wherein the heat treatment comprises heating, cooling quenching and low-temperature tempering steps performed in sequence, so that a quenched and tempered structure including tempered martensite and bainite is formed at the welding position (7); Afterwards, the low-temperature steel bar sample (1) and the connecting piece (2) are placed in a low-temperature environment, and the connecting pieces (2) at both ends are connected by a tensile testing machine to perform a tensile test on the low-temperature steel bar sample (1).

2. The low-temperature steel bar tensile test method according to claim 1, characterized in that: The material of the connecting piece (2) is 18CrNiMo7-6.

3. The low-temperature steel bar tensile test method according to claim 1, characterized in that: The welding adopts friction welding, and the friction welding includes primary pressure friction preheating, secondary pressure friction heating and top forging welding.

4. The low-temperature steel bar tensile test method according to claim 3, characterized in that: The friction welding is controlled such that the first-level friction speed is 1.1 mm / s to 1.4 mm / s, the first-level friction speed pressure is 2.5 to 3.5 MPa, the first-level friction time is 3 to 5 s, the second-level friction speed is 1.1 mm / s to 1.4 mm / s, the second-level friction speed pressure is 3 to 5 MPa, the second-level friction deformation is 7 to 9 mm, the upsetting pressure is 6 to 9 MPa, the upsetting speed is 4 mm / s to 6 mm / s, the upsetting is 3 to 4 s, the upsetting holding time is 1 to 2 s, and the spindle speed is 2000 to 2400 r / min.

5. The low-temperature steel bar tensile test method according to claim 1, characterized in that: During the heating process, the heating temperature is controlled at 900-950°C.

6. The low-temperature steel bar tensile test method according to claim 5, characterized in that: During the cooling and quenching process, the cooling medium is controlled to be oil, and the cooling temperature is 20-28°C.

7. The low-temperature steel bar tensile test method according to claim 6, characterized in that: During the low-temperature tempering process, the tempering temperature is controlled to be 250-300° C., the holding time is 1.5-2 hours, and the cooling method is open furnace air cooling.

8. A low temperature steel bar tensile test device, characterized in that: The device implements the low-temperature steel bar tensile test method according to any one of claims 1 to 7, comprising two connecting pieces (2), a tensile testing machine and a low-temperature environment box (3), wherein the tensile testing machine is used to drive the two connecting pieces (2) inside the low-temperature environment box (3) to move towards each other.

9. The low-temperature steel bar tensile test device according to claim 8, characterized in that: Connectors (4) are provided at both ends of the low-temperature environment box (3); one end of the connector (4) is connected to the tensile testing machine, and the other end of the connector (4) is detachably connected to the connecting piece (2); the connector (4) is capable of sliding relative to the low-temperature environment box (3) in the moving direction of the connecting piece (2).

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