Manufacturing method of bainite-austenite dissimilar steel welded joint

Abstract

The invention discloses a manufacturing method of a bainite-austenite dissimilar steel welded joint, which comprises the following steps: step 1, setting the end surface, facing the austenite steel, of bainite steel as a first slope surface, and setting the end surface, facing the bainite steel, of the austenite steel as a second slope surface, the included angle a between the first slope surface and the central axis between the bainite steel and the austenite steel is 60 degrees, and the included angle b between the second slope surface and the central axis between the bainite steel and the austenite steel is 30 degrees; 2, nickel-based alloy powder is adopted for conducting spray welding on the middle position of the first slope face and the middle position of the second slope face, and a welding part is formed; 3, postweld heat treatment is conducted on the welded part; and 4, the welding part is subjected to ultrasonic impact treatment. According to the technical scheme, the problem that an existing bainite-austenite dissimilar steel welded joint has the cracking risk is solved.

Description

Technical Field

[0001] This invention relates to the field of remanufacturing technology, and specifically to a method for manufacturing a bainitic-austenitic dissimilar steel welded joint. Background Technology

[0002] In subcritical power plant boilers, numerous bainitic-austenitic dissimilar steel welded joints exist on the heating surfaces of reheaters and superheaters. These joints are characterized by complex chemical composition, non-uniform properties, macroscopic segregation, complex microstructure, high joint stress, and stress concentration. Due to the combined effects of oxidation, creep, and fatigue, they are prone to early failure. The failure type of dissimilar steel joints is mostly brittle fracture, often only discovered after a leak occurs. By this time, the surrounding dissimilar steel joints are also damaged by the high-temperature, high-pressure steam, necessitating replacement with new dissimilar steel joint sections.

[0003] The manufacturing processes for dissimilar steel welded joints are mostly argon arc welding and shielded metal arc welding. However, the current manufacturing processes do not take into account the characteristics and failure mechanisms of bainitic-austenitic dissimilar steel welded joints and do not take corresponding measures. Therefore, there is still a risk of cracking in a short period of time after welding repair, and the manufacturing process needs to be optimized.

[0004] Conventional manual TIG welding and shielded metal arc welding processes involve high heat input, resulting in a wide heat-affected zone and significant residual stress. They also fail to consider the characteristics of dissimilar steel welded joints, such as macroscopic segregation, complex joint stress, complex microstructure, the characteristics of bainitic steel side fusion line cracking, and failure mechanisms. Furthermore, they lack optimization of the weld bevel and treatment of stress concentration at the weld toe. Additionally, due to the strong reheat cracking sensitivity of bainitic heat-resistant steel, reheat cracking is prone to occur during post-weld heat treatment, and there is a lack of targeted process design to address this characteristic. Dissimilar steel welded joints manufactured using conventional processes still carry the risk of cracking within a short period. Summary of the Invention

[0005] To address the risk of cracking in existing bainitic-austenitic dissimilar steel welded joints, this invention discloses a method for manufacturing bainitic-austenitic dissimilar steel welded joints.

[0006] This invention discloses a method for manufacturing a bainitic-austenitic dissimilar steel welded joint, comprising the following steps: Step 1: Butt-joint bainitic steel and austenitic steel along a straight line at intervals. The end face of the bainitic steel facing the austenitic steel is set as a first slope, and the end face of the austenitic steel facing the bainitic steel is set as a second slope. The first slope and the second slope are arranged in a V-shape. The angle α between the first slope and the central axis between the bainitic steel and the austenitic steel is 60°, and the angle b between the second slope and the central axis between the bainitic steel and the austenitic steel is 30°. Step 2: Use nickel-based alloy powder to spray weld the middle position of the first slope surface and the second slope surface to form a welded part; Step 3: Perform post-weld heat treatment on the welded part; Step 4: Perform ultrasonic impact treatment on the welded part.

[0007] Furthermore, in step 1, the first slope surface and the second slope surface are prepared by machining.

[0008] Furthermore, in step 1, the bottom distance between the first slope and the second slope is 1-2 mm.

[0009] Furthermore, in step 2, before performing spray welding, the spray welding position is preheated, and the pre-welding temperature is controlled at 150℃-180℃.

[0010] Furthermore, in step 2, the spray welding method adopts plasma arc welding, with an arc current of 15-20A, a plasma arc current of 70-90A, an ion gas flow rate of 1.5-3.0L / min, a shielding gas flow rate of 15-20L / min, a powder feeding gas flow rate of 1.5-3.0L / min, and a powder feeding amount of 20-40g / min. The nickel-based alloy powder used is Alloy 182 type nickel-based alloy powder.

[0011] Furthermore, in step 3, the post-weld heat treatment includes: performing a medium-temperature tempering treatment at 550℃ for 0.5 hours, followed by a high-temperature tempering treatment at 730-750℃ for 0.5 hours, and then cooling down, wherein the heating and cooling rate is 300℃ / hour.

[0012] Furthermore, in step 4, an impactor with a diameter of 4mm is used to impact the weld toe on the bainitic steel side of the new weld to induce plastic deformation in the weld toe. The impact amplitude is controlled at 30-45μm, the impact current is controlled at 2-3A, the impact frequency is controlled at 18-20kHz, and the impact speed is controlled at 80-120mm / min.

[0013] Furthermore, after step 4 is completed, phased array ultrasonic testing and surface penetration testing are performed on the dissimilar steel welded joint.

[0014] Early failure of bainitic-austenitic dissimilar steel welded joints is the result of the coupled effects of three mechanisms: oxidation, creep, and fatigue. It is closely related to factors such as the stress state, microstructure, and chemical composition of the welded joint. Under the premise that the chemical composition of dissimilar steel welded joints cannot be changed, a new manufacturing method is proposed based on the principles of reducing joint stress level, improving microstructure, reducing stress concentration, and reducing the coupling effects of oxidation, creep, and fatigue. An asymmetric weld bevel is adopted, increasing the bevel angle on the bainitic steel side. This design can separate the weak zones of creep and fatigue performance from the weak zones of oxidation performance, reducing the coupling effects of oxidation, creep, and fatigue. Plasma spraying of nickel-based powder can eliminate macroscopic segregation, reduce the level of residual welding stress, and reduce the width of the heat-affected zone. A medium-temperature tempering process is added to the post-weld heat treatment; this stepped post-weld heat treatment process can reduce the probability of reheat cracking. After post-weld heat treatment, ultrasonic impact treatment is applied to the weld toe, causing slight plastic deformation, optimizing the geometry, reducing stress concentration, and generating beneficial compressive stress, thus improving fatigue performance. These technical measures can significantly improve the service life of bainitic-austenitic dissimilar steel welded joints. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the structure of bainitic steel and austenitic steel according to the method of the present invention; Figure 2 This is a flowchart of the post-weld heat treatment process of the method of the present invention. Figure 3 This is a schematic diagram of the ultrasonic shock treatment method of the present invention. Detailed Implementation

[0017] This invention discloses a method for manufacturing a bainitic-austenitic dissimilar steel welded joint.

[0018] The technical solutions of this invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a portion of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0019] See Figures 1-3 As shown, this invention discloses a method for manufacturing a bainitic-austenitic dissimilar steel welded joint, comprising the following steps: Step 1: Butt-joint bainitic steel and austenitic steel along a straight line at intervals. The end face of the bainitic steel facing the austenitic steel is set as a first slope, and the end face of the austenitic steel facing the bainitic steel is set as a second slope. The first slope and the second slope are arranged in a V-shape. The angle α between the first slope and the central axis between the bainitic steel and the austenitic steel is 60°, and the angle b between the second slope and the central axis between the bainitic steel and the austenitic steel is 30°. Step 2: Use nickel-based alloy powder to spray weld the middle position of the first slope surface and the second slope surface to form a welded part; Step 3: Perform post-weld heat treatment on the welded part; Step 4: Perform ultrasonic impact treatment on the welded part.

[0020] Early failure of bainitic-austenitic dissimilar steel welded joints is the result of the coupled effects of three mechanisms: oxidation, creep, and fatigue. It is closely related to factors such as the stress state, microstructure, and chemical composition of the welded joint. Under the premise that the chemical composition of dissimilar steel welded joints cannot be changed, a new manufacturing method is proposed based on the principles of reducing joint stress level, improving microstructure, reducing stress concentration, and reducing the coupling effects of oxidation, creep, and fatigue. An asymmetric weld bevel is adopted, increasing the bevel angle on the bainitic steel side. This design can separate the weak zones of creep and fatigue performance from the weak zones of oxidation performance, reducing the coupling effects of oxidation, creep, and fatigue. Plasma spraying of nickel-based powder can eliminate macroscopic segregation, reduce the level of residual welding stress, and reduce the width of the heat-affected zone. A medium-temperature tempering process is added to the post-weld heat treatment; this stepped post-weld heat treatment process can reduce the probability of reheat cracking. After post-weld heat treatment, ultrasonic impact treatment is applied to the weld toe, causing slight plastic deformation, optimizing the geometry, reducing stress concentration, and generating beneficial compressive stress, thus improving fatigue performance. These technical measures can significantly improve the service life of bainitic-austenitic dissimilar steel welded joints.

[0021] In step 1, the first slope surface and the second slope surface are prepared by mechanical processing.

[0022] By machining the first and second slopes, the angle, surface roughness, and flatness of the two slopes can be precisely controlled, ensuring the consistency of the bevel geometry and avoiding defects such as poor welding fusion and poor weld formation caused by insufficient bevel machining precision, thereby improving the manufacturing stability and reliability of the welded joint.

[0023] In step 1, the bottom distance between the first slope and the second slope is 1-2 mm.

[0024] The bottom spacing between the first and second slope surfaces should be controlled at 1-2 mm. Too small a gap will lead to difficulty in welding penetration and a higher risk of incomplete penetration defects; too large a gap will increase the amount of filler material, resulting in excessive heat input and increased weld shrinkage stress. This appropriate gap ensures that the nickel-based alloy powder fully fills the groove, while controlling the welding heat input within a suitable range, reducing the width of the heat-affected zone, lowering residual welding stress, and avoiding weld defects caused by improper gap.

[0025] In step 2, before spray welding, the spray welding position is preheated, and the pre-welding temperature is controlled at 150℃-180℃.

[0026] Preheating the temperature to 150℃-180℃ before welding has several advantages. First, it reduces the temperature difference between bainitic and austenitic steels, thus decreasing thermal stress and the tendency for cold cracking during welding. Second, it improves the plasticity of the metal in the weld zone, preventing joint embrittlement caused by rapid cooling. Third, it optimizes the melting and spreading of nickel-based alloy powder, enhancing the bond strength between the weld and the base metal, and solving the problems of joint cracking and poor bonding caused by improper preheating in existing processes.

[0027] In step 2, the spray welding method adopts plasma arc welding, with an arc current of 15-20A, a plasma arc current of 70-90A, an ion gas flow rate of 1.5-3.0L / min, a shielding gas flow rate of 15-20L / min, a powder feeding gas flow rate of 1.5-3.0L / min, and a powder feeding amount of 20-40g / min. The nickel-based alloy powder used is Alloy 182 type nickel-based alloy powder.

[0028] Plasma arc welding with specific parameters for spraying Alloy 182 nickel-based alloy powder offers several advantages over existing argon arc welding and shielded metal arc welding processes: First, the high temperature and concentrated energy of the plasma arc enable a "keyhole effect," thoroughly stirring the molten pool and eliminating macroscopic segregation common in existing processes. Second, the low dilution rate ensures the stability of the nickel-based alloy composition in the weld, improving the joint's oxidation and creep resistance. Third, precise control of current, gas flow rate, and powder feed parameters reduces the width of the heat-affected zone and lowers residual welding stress, solving the problems of embrittlement and cracking in the heat-affected zone of joints in conventional welding processes.

[0029] In step 3, the post-weld heat treatment includes: performing a medium-temperature tempering treatment at 550℃ for 0.5 hours, followed by a high-temperature tempering treatment at 730-750℃ for 0.5 hours, and then cooling down, wherein the heating and cooling rate is 300℃ / hour.

[0030] The stepped heat treatment method of "550℃ medium-temperature tempering + 730-750℃ high-temperature tempering" has the following advantages compared to existing technologies that use single-temperature tempering or no medium-temperature tempering: First, the 550℃ medium-temperature tempering can eliminate some residual welding stress before the bainitic steel enters the reheat crack sensitive temperature range, significantly reducing the probability of reheat cracking; second, the 730-750℃ high-temperature tempering can further improve the microstructure of the weld and heat-affected zone, refine the grains, and eliminate internal stress; third, the heating and cooling rate of 300℃ / hour can avoid thermal shock damage caused by sudden temperature changes, solving the problems of reheat cracking and uneven microstructure that are prone to occur in existing post-weld heat treatment.

[0031] In step 4, an impactor with a diameter of 4mm is used to impact the weld toe on the bainitic steel side of the new weld to induce plastic deformation in the weld toe. The impact amplitude is controlled at 30-45μm, the impact current is controlled at 2-3A, the impact frequency is controlled at 18-20kHz, and the impact speed is controlled at 80-120mm / min.

[0032] A 4mm diameter impact pin, combined with precise amplitude, current, frequency, and speed parameters, can induce controllable plastic deformation in the weld toe, optimize the weld toe geometry, and eliminate severe stress concentration in existing processes. The beneficial compressive stress generated after impact can offset some of the tensile residual stress, significantly improving the fatigue performance of the joint. Thirdly, it acts specifically on the bainitic steel side (area with a high risk of cracking), which can precisely strengthen weak parts and solve the problem that the weld toe of existing joints is prone to becoming the failure starting point.

[0033] After step 4 is completed, phased array ultrasonic testing and surface penetration testing are performed on the dissimilar steel welded joint.

[0034] Phased array ultrasonic testing can comprehensively detect volumetric defects such as incomplete penetration, slag inclusions, and porosity inside the joint; surface penetrant testing can effectively identify surface defects such as weld toe and cracks on the weld surface; the combination of the two can achieve all-round defect inspection of the welded joint "internal + surface", ensuring that the joint has no defects exceeding the standard, and avoiding the problem of defective joints being put into use and causing early failure due to incomplete inspection in the existing process.

[0035] The present invention will be further described below through specific embodiments: Example 1

[0036] This embodiment illustrates the manufacturing method of the bainitic-austenitic dissimilar steel welded joint provided by the present invention, including the following steps: Step 1: Butt-joint bainitic steel and austenitic steel along a straight line at intervals. The end face of the bainitic steel facing the austenitic steel is set as a first slope, and the end face of the austenitic steel facing the bainitic steel is set as a second slope. The first slope and the second slope are arranged in a V-shape. The bottom distance between the first slope and the second slope is 1mm, and the angle α between the first slope and the central axis between the bainitic steel and the austenitic steel is 60°, and the angle b between the second slope and the central axis between the bainitic steel and the austenitic steel is 30°. Step 2: Preheat the spray welding position, controlling the pre-weld temperature at 170℃. Use Alloy 182 type nickel-based alloy powder to perform plasma arc welding on the middle position of the first and second slope surfaces. The arc current is 20A, the plasma arc current is 80A, the ion gas flow rate is 2.0L / min, the shielding gas is 15L / min, the powder feeding gas is 2.0L / min, and the powder feeding amount is 30g / min to form the welded part. Step 3: Perform post-weld heat treatment on the welded part, including medium-temperature tempering at 550℃ for 0.5 hours, followed by high-temperature tempering at 740℃ for 0.5 hours, and then cooling to room temperature at a rate of 300℃ / hour. Step 4: Use a 4mm diameter impactor to impact the weld toe on the bainitic steel side of the new weld to induce plastic deformation in the weld toe. The impact amplitude is controlled at 40μm, the impact current is controlled at 2A, the impact frequency is controlled at 18-20kHz, and the impact speed is controlled at 100mm / min.

[0037] Comparative Example 1 This comparative example is used to illustrate the manufacturing method of the bainitic-austenitic dissimilar steel welded joint provided by the present invention, including most of the steps in Example 1, the difference being: The angle α between the first ramp surface and the central axis between the bainitic steel and the austenitic steel is 30°, and the angle b between the second ramp surface and the central axis between the bainitic steel and the austenitic steel is 30°.

[0038] Comparative Example 2 This comparative example is used to illustrate the manufacturing method of the bainitic-austenitic dissimilar steel welded joint provided by the present invention, including most of the steps in Example 1, the difference being: The angle α between the first ramp surface and the central axis between the bainitic steel and the austenitic steel is 30°, and the angle b between the second ramp surface and the central axis between the bainitic steel and the austenitic steel is 60°.

[0039] The welded joints obtained above were tested in terms of strength, high-temperature creep aging performance, and high-temperature accelerated thermal aging. The test results are recorded in Table 1: Table 1

[0040] As can be seen from the test results in Table 1, the manufacturing method of the bainitic-austenitic dissimilar steel welded joint provided by the present invention can separate the weak areas of creep, fatigue performance and oxidation performance, reduce the peak value of welding residual tensile stress on the bainitic steel side, reduce the fusion ratio of bainitic steel, optimize the microstructure of the bonding interface, and reduce the possibility of fracture at the weld.

[0041] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a bainitic-austenitic dissimilar steel welded joint, characterized in that, Includes the following steps: Step 1: Butt-joint bainitic steel and austenitic steel along a straight line at intervals. The end face of the bainitic steel facing the austenitic steel is set as a first slope, and the end face of the austenitic steel facing the bainitic steel is set as a second slope. The first slope and the second slope are arranged in a V-shape. The angle α between the first slope and the central axis between the bainitic steel and the austenitic steel is 60°, and the angle b between the second slope and the central axis between the bainitic steel and the austenitic steel is 30°. Step 2: Use nickel-based alloy powder to spray weld the middle position of the first slope surface and the second slope surface to form a welded part; Step 3: Perform post-weld heat treatment on the welded part; Step 4: Perform ultrasonic impact treatment on the welded part.

2. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 1, the first slope surface and the second slope surface are prepared by mechanical processing.

3. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 1, the bottom distance between the first slope and the second slope is 1-2 mm.

4. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 2, before spray welding, the spray welding position is preheated, and the pre-welding temperature is controlled at 150℃-180℃.

5. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 2, the spray welding method adopts plasma arc welding, with an arc current of 15-20A, a plasma arc current of 70-90A, an ion gas flow rate of 1.5-3.0L / min, a shielding gas flow rate of 15-20L / min, a powder feeding gas flow rate of 1.5-3.0L / min, and a powder feeding amount of 20-40g / min. The nickel-based alloy powder used is Alloy 182 type nickel-based alloy powder.

6. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 3, the post-weld heat treatment includes: performing a medium-temperature tempering treatment at 550℃ for 0.5 hours, followed by a high-temperature tempering treatment at 730-750℃ for 0.5 hours, and then cooling down, wherein the heating and cooling rate is 300℃ / hour.

7. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, In step 4, an impactor with a diameter of 4mm is used to impact the weld toe on the bainitic steel side of the new weld to induce plastic deformation in the weld toe. The impact amplitude is controlled at 30-45μm, the impact current is controlled at 2-3A, the impact frequency is controlled at 18-20kHz, and the impact speed is controlled at 80-120mm / min.

8. The method for manufacturing a bainitic-austenitic dissimilar steel welded joint according to claim 1, characterized in that, After step 4 is completed, phased array ultrasonic testing and surface penetration testing are performed on the dissimilar steel welded joint.

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