Method for testing mechanical property of heat affected zone of low-alloy ultrahigh-strength steel welded joint

Abstract

The invention discloses a method for testing the mechanical property of a heat affected zone of a low-alloy ultrahigh-strength steel welded joint. The method comprises the following steps: preparing a low-alloy ultrahigh-strength steel induction heating sample; when the temperature of the middle position of the induction heating sample reaches the expected peak temperature, heating is stopped, the welding cooling process is simulated in an air cooling or air cooling mode, and the sample is cooled to the room temperature; preparing a tensile sample and a hardness sample from the induction heating sample; and respectively measuring the tensile strength of the tensile sample and the hardness of the hardness sample by adopting a universal testing machine and a microhardness tester, so as to obtain the tensile strength and the hardness of the low-alloy ultrahigh-strength steel under the normal-temperature condition after being influenced by the welding heat cycle. According to the method, the welding heat process is accurately simulated, and the simulated welding heat cycle is applied to the low-alloy ultrahigh-strength steel sample, so that the mechanical behavior of the low-alloy ultrahigh-strength steel sample is consistent with the mechanical behavior of a local microcell of a welding joint. The problem that the mechanical properties of different micro-areas of the heat affected zone of the low-alloy ultrahigh-strength steel welding joint are difficult to evaluate is solved.

Description

Technical Field

[0001] The invention belongs to a metal material welding joint mechanical property detection technology, and specifically relates to a simple and practical testing method for the mechanical properties of a heat-affected zone of a low-alloy ultra-high-strength steel welding joint. Background Art

[0002] Low alloy ultra-high strength steel is a martensitic strengthened steel. It is treated by direct quenching or quenching + low temperature tempering to obtain quenched martensite or tempered martensite structure, which makes it have high strength and good wear resistance. Low alloy ultra-high strength steel is currently mainly connected by melting welding, and the heat affected zone of the welded joint has the problem of uneven material and performance.

[0003] The peak temperature of the heat affected zone is C3 The above areas are all completely quenched areas. The higher the peak temperature, the more serious the grain growth. The heat input and cooling rate are different. Not only martensite but also bainite is generated. Therefore, the organizational characteristics of the completely quenched area are a mixed organization of martensite of different coarseness and fineness and a small amount of bainite. C1 ~A C3 The area between is the incomplete quenching zone, and its structure is a mixed structure composed of multiple phases and intermediates. In addition to the complete quenching zone and the incomplete quenching zone, there is also a tempering zone. The width of the tempering zone is related to the heat treatment process of low-alloy ultra-high strength steel. The lower the tempering temperature, the larger the tempering zone of the heat-affected zone, and the greater the change in structure and performance. The closer the peak temperature is to A C1 The higher the tempering degree of the tempering zone, the higher the softening degree. In the case of multiple thermal cycles, not only the tempering zone softens, but also the heat-affected zone generated by the first weld bead except the last weld bead softens.

[0004] It can be seen that compared with the materials in the unaffected areas, the microstructure and mechanical properties of the heat affected zone of low alloy ultra-high strength steel have undergone significant changes. Traditionally, the mechanical properties of the heat affected zone of the joint are evaluated by taking tensile specimens from the welded joint for tensile testing. However, due to the large gradient of changes in the microstructure and mechanical properties of the heat affected zone and the narrow width, there is a problem of uneven microstructure in the parallel section of the tensile specimen. Therefore, it is inappropriate to use the tensile results obtained by the traditional method to evaluate the mechanical properties of the heat affected zone. The use of a thermal simulation test machine can more accurately simulate the welding heating and cooling process, but the experimental cost of the thermal simulation test machine is too high, and due to the large size of the specified specimen, a more uniform microstructure and mechanical properties cannot be obtained.

[0005] Therefore, in order to more accurately and conveniently obtain the mechanical properties of the heat-affected zone of low-alloy ultra-high-strength steel welded joints, it is necessary to develop a simple and practical mechanical properties testing method to address the above problems. Summary of the invention

[0006] In view of the above problems existing in the prior art, the purpose of the invention is to provide a simple and practical test method for accurately obtaining the mechanical properties of the heat-affected zone of low-alloy ultra-high-strength steel welded joints. The mechanical properties of the heat-affected zone are linked to the peak temperature experienced, which improves the test accuracy of the mechanical properties of the heat-affected zone and saves measurement costs. At the same time, it also provides accurate material parameters for mechanical analysis of welded joints or structures (such as numerical simulation of welding residual stress).

[0007] The present invention solves the above technical problems by the following technical means: a simple and practical test method for the mechanical properties of the heat-affected zone of a low-alloy ultra-high-strength steel welded joint, comprising the following steps:

[0008] S1: Preparation of low alloy ultra-high strength steel induction heating specimens by wire EDM;

[0009] S2: Use electromagnetic induction equipment to simulate the welding thermal process, and heat the induction heating sample to the expected peak temperature. Use a thermocouple to monitor the temperature at the middle of the sample to control the peak temperature, and adjust the power of the electromagnetic induction equipment to control the heating speed.

[0010] S3: When the temperature at the middle position of the induction heated sample reaches the expected peak temperature, stop heating and control the cooling speed of the sample by wind cooling or air cooling to cool the sample temperature to room temperature;

[0011] S4: Prepare tensile specimens and hardness specimens from induction heated specimens using wire EDM;

[0012] S5: The tensile strength and hardness of the low alloy ultra-high strength steel tensile specimens and hardness specimens after heating were measured using a universal testing machine and a microhardness tester, respectively.

[0013] Preferably, the size of the S1 medium-low alloy ultra-high strength steel induction heating sample is 80 mm×10 mm×6 mm, which can ensure that the temperature of the outer surface and the core of the induction heating sample is consistent during the heating process.

[0014] Preferably, in S2, the maximum power of the electromagnetic induction equipment is 15KW, the number of coil turns is 5, the inner diameter of the coil is 30mm, and the length of the coil is 60mm; the output power is adjusted to ensure that the temperature of the induction heated sample can reach the expected peak temperature in a very short time, the peak temperature range is 300℃~1300℃, and the heating rate range is 4℃ / s~40℃ / s.

[0015] As a preference, in S3, different cooling methods are used according to different peak temperatures to quickly cool the induction heated sample to room temperature, so that the cooling speed is close to the welding condition, especially when the peak temperature is A. C1When the fan is above 1000W, the fan power has two gears (15W and 9W), and the fan volume is 220mm×205mm×205mm. The peak temperature is A C1 When the temperature is above 20°C, the cooling rate range of t8 / 5 is 5°C / s to 20°C / s.

[0016] Preferably, after steps S2 and S3, there is a 20 mm region in the middle of the induction heating sample with relatively uniform structure and hardness; in S4, the uniform structure region is placed in the parallel section of the tensile sample (also 20 mm in length), and the hardness sample is also 20 mm in length.

[0017] Compared with the prior art, the present invention has at least the following advantages:

[0018] 1. The traditional technology is to directly cut the tensile specimen from the weld joint and test its tensile properties. Since the width of the heat-affected zone of the molten weld joint of low-alloy ultra-high-strength steel is relatively narrow (2-4 mm), the tensile specimen obtained by the traditional method cannot guarantee that the structure of the parallel section is completely uniform. Therefore, the mechanical properties obtained by the final stretching cannot truly reflect the mechanical properties of the heat-affected zone after experiencing different peak temperatures. The present invention directly applies a thermal cycle similar to the actual welding situation to the induction heating specimen, so that the structure of the middle area of ​​the induction heating specimen is uniform and the length is sufficient (20 mm) to make a tensile specimen.

[0019] 2. Since the peak temperature of the induction heated sample can be accurately controlled by adjusting the heating power of the induction heating equipment in the present invention, the mechanical properties (yield strength and hardness) of any micro-area in the heat affected zone of low-alloy ultra-high strength steel can be measured very accurately.

[0020] 3. The present invention uses electromagnetic induction heating to quickly heat the sample. The designed sample size can ensure that the tensile sample with uniform structure and mechanical properties is obtained, while greatly reducing the experimental test cost. It costs about 1,000 yuan to do an experiment using a thermal simulation test machine, and it costs about 100 yuan to do an experiment using the method of the present invention, which is 1 / 10 of the traditional method. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the thermal simulation process of low alloy ultra-high strength steel welding. Figure 1 a is the electromagnetic induction heating equipment and coil, Figure 1 b is a schematic diagram of the processing of tensile specimens and hardness specimens.

[0022] Figure 2 The thermal cycle curves of induction heating simulation and actual welding conditions are shown in Figure 2. Figure 2 a is the thermal cycle simulated by induction heating, Figure 2 b is the thermal cycle under real welding conditions.

[0023] Figure 3 Schematic diagram of tensile specimen dimensions.

[0024] Figure 4 The graphs of yield strength and hardness as a function of peak temperature are shown in Figure 2. DETAILED DESCRIPTION

[0025] The specific implementation modes of the present invention are further described in detail below with reference to the accompanying drawings.

[0026] The heat affected zone in the present invention refers to the area where the structure or performance is affected by heat under welding conditions. For low-alloy ultra-high strength steel welded joints, the heat affected zone includes a complete quenching zone, an incomplete quenching zone and a tempering zone. The present invention provides a practical and simple test method for the mechanical properties of the heat affected zone of metal material welding. Specifically, it includes the following steps:

[0027] (1) Welding thermal simulation process

[0028] Electromagnetic induction heating equipment is used to simulate the rapid heating process of welding. In order to ensure that the sample can be heated to the expected peak temperature in a relatively short time, the maximum peak temperature is 1300°C (the peak temperature of the coarse grain zone), and the recommended size of the induction heating sample is 80mm×10mm×6mm. For low-alloy ultra-high-strength steel, the peak temperature range of the heat affected zone (including the complete quenching zone, the incomplete quenching zone and the tempering zone) is very wide, from 300°C to 1300°C. Different peak temperatures are heated with different powers, and the heating rate range is 4°C / s to 40°C / s, which is in line with the heating rate of the heat affected zone heating stage of the actual welded joint. A thermocouple is used to monitor the temperature at the center of the induction heated sample, and heating is stopped when the temperature reaches the expected peak temperature. Different cooling methods are then used to conform to the actual welding cooling process, and the peak temperature is at A. C1 When the temperature is above 0.05°C, the cooling rate range is 5°C / s to 20°C / s. For low alloy ultra-high strength steel, the heating power, heating time and cooling method corresponding to different peak temperatures were determined through repeated tests, as shown in Table 1. The thermal cycle diagram simulated by the induction heating equipment is different from the thermal cycle diagram under the actual welding condition. Figure 2 As shown. The thermal cycle under real welding conditions is simulated by finite element software, which is consistent with the experimental measurement results. The induction heating equipment mentioned in the present invention can achieve a heating rate of 40°C / s at the maximum power, i.e. 15KW, which is in line with the heating rate of the real welding heat affected zone. Different peak temperature thermal cycles have different heating rates and cooling rates. The induction heating equipment provided by the present invention can meet the heating rate and cooling rate at any peak temperature by adjusting the heating power.

[0029] Table 1 Induction heating parameters of low alloy ultra-high strength steel at different peak temperatures

[0030] Peak temperature(℃) Heating power(KW) Heating speed (℃ / s) Cooling method Cooling rate (℃ / s) 1300 15 40 Air cooling (15W) 20 1000 15 38 Air cooling (15W) 18 800 14.5 35 Air cooling (15W) 16 750 9.5 31 Air cooling (15W) 15 700 6.5 29 Air cooling (9W) 13 600 4.5 23 Air cooling (9W) 11 500 4.5 15 air cooling 9 400 4.5 9 air cooling 7 300 4.5 4 air cooling 5

[0031] (2) Preparation and performance testing of tensile specimens and hardness specimens

[0032] The induction heating method mentioned in the present invention can make the structure and hardness of the sample in the range of about 20mm in the middle very uniform. The 20mm wide area is used as the parallel section of the tensile sample. The size of the tensile sample is as follows: Figure 3 As shown in the figure, the yield strength obtained by the tensile test is the mechanical property of the material in the uniform area. The hardness specimen is also taken from the 20mm wide area, as shown in the figure. Figure 1 As shown in b.

[0033] pass Figure 3 It can be seen that the total length of the tensile specimen in the middle is 80mm, and the length of the parallel section of the tensile specimen is 20mm, which just makes the area with uniform structure and hardness located in the parallel section. The specimen size meets the national standard for metal room temperature stretching.

[0034] The yield strength of the tensile specimens was measured using a universal testing machine, and the microhardness of the hardness specimens was measured using a microhardness tester. The relationship between the yield strength and hardness and the peak temperature is shown in Figure 4 shown.

[0035] pass Figure 4 It can be seen that the method of the present invention can obtain more detailed mechanical properties (hardness and yield strength) of different areas of the heat affected zone. It can be seen from the figure that for low-alloy ultra-high strength steel, the hardness and yield strength of the fully quenched zone are equivalent to those of the parent material. When the thermal cycle temperature is 600°C, the hardness and yield strength values ​​are reduced to the minimum.

Claims

1. A test method for the mechanical properties of the heat-affected zone of a low-alloy ultra-high-strength steel welded joint. It is characterized in that The steps include: S1: Preparation of low alloy ultra-high strength steel induction heating specimens; S2: Use electromagnetic induction equipment to simulate the welding heat process, and heat the induction heating sample to the expected peak temperature. Use a thermocouple to monitor the temperature at the middle position of the induction heating sample to control the peak temperature. Adjust the power of the electromagnetic induction equipment to control the heating speed. The peak temperature is 300℃~1300℃, and the heating speed is 4℃ / s~40℃ / s S3: When the temperature at the middle position of the induction heated sample reaches the expected peak temperature, stop heating and control the cooling speed of the sample by wind cooling or air cooling. When the peak temperature is above 800℃, t 8 / 5 The cooling rate is 5℃ / s~20℃ / s, so that the sample temperature is cooled to room temperature; S4: Prepare tensile specimens and hardness specimens from induction heated specimens using wire EDM; S5: The tensile strength and hardness of the tensile specimens and hardness specimens of low alloy ultra-high strength steel after heat influence were measured by a universal testing machine and a microhardness tester, respectively.

2. The method for testing the mechanical properties of the heat affected zone of a low alloy ultra-high strength steel welded joint according to claim 1, Features: The size of the S1 medium-low alloy ultra-high strength steel induction heating sample is 80 mm×10 mm×6 mm.

3. The method for testing the mechanical properties of the heat affected zone of a low alloy ultra-high strength steel welded joint according to claim 1, Features: In S2, the maximum power of the electromagnetic induction device is 15KW, the number of coil turns is 5, the inner diameter of the coil is 30mm, and the length of the coil is 60mm.

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