Low-nickel austenitic stainless steel welding method capable of increasing nitrogen content of welding seam

Abstract

The invention discloses a low-nickel austenitic stainless steel welding method for increasing the nitrogen content of a welding seam, and belongs to the technical field of welding. The technical problem that when low-nickel high-nitrogen austenitic stainless steel is welded, the nitrogen content of a weld joint is low, so that the mechanical property and corrosion resistance of a joint are remarkably reduced is solved. A powder feeding device, a welding gun and a liquid nitrogen feeding device are sequentially arranged in the welding direction and are located on the same horizontal line, the surface of the welding gun and the surface of a to-be-welded welding seam inclines backwards by 0-15 degrees, the powder feeding device is arranged at the front end of the welding gun, the liquid nitrogen feeding device is arranged at the rear end of the welding gun, a magnetic field is arranged on the back of the welding seam, and the distance between the magnetic field and the back of to-be-welded base metal ranges from 5 mm to 10 mm. The external magnetic field and the welding gun are distributed in a mirror image mode relative to the surface of the welding test plate, the welding track is controlled, and welding is conducted till filling of the whole welding seam is completed. The welding seam nitrogen content is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a low-nickel austenitic stainless steel welding method for improving the nitrogen content of the weld. Background Art

[0002] High-nitrogen austenitic stainless steel is a high-performance stainless steel that uses cheap nitrogen to replace the expensive and scarce metal nickel. It has high strength and toughness, good corrosion resistance and wear resistance, is non-ferromagnetic and has good biocompatibility. It is widely used in marine engineering, aerospace, petrochemical, biomedicine and other fields, and has become a new generation of resource-saving, high-performance, green advanced materials.

[0003] The alloying element nitrogen in high-nitrogen austenitic stainless steel primarily improves the material's performance through strengthening mechanisms such as solid solution strengthening and grain refinement. Nitrogen's solid solution strengthening effect occurs when nitrogen atoms in high-nitrogen austenitic stainless steel occupy the interstitial spaces of the face-centered cubic lattice, causing lattice distortion and significantly increasing the material's strength. Solution treatment of high-nitrogen stainless steel involves heating the sample to a certain temperature, holding it for a certain period, and then water quenching it. This allows the precipitated nitride particles to dissolve back into the austenite, thereby improving the mechanical properties of the high-nitrogen stainless steel. During the recrystallization process of high nitrogen stainless steel, the nitride second phase particles will inhibit the growth of austenite grains, thereby effectively reducing the grain size and increasing the grain boundary area, thereby achieving the purpose of enhancing the strength and improving the toughness of high nitrogen stainless steel; nitrogen can form stable nitrides CrN and Cr2N in the high nitrogen stainless steel matrix. At the same time, carbon can also precipitate in the stainless steel matrix in the form of carbides. If these nitrides and carbides are dispersed in the stainless steel matrix as fine second phase particles, they can hinder dislocation movement and inhibit grain growth, thereby significantly improving the strength of the steel.

[0004] In summary, nitrogen plays an important role in high nitrogen austenitic stainless steel. However, during the welding process of high nitrogen austenitic stainless steel, nitrogen easily accumulates and escapes in the molten pool, resulting in a significant decrease in the nitrogen content of the welded joint. At the same time, some nitrogen will remain in the molten pool as the molten pool temperature decreases, eventually forming pore defects, causing a sharp drop in the bearing capacity of the welded structure. At the same time, the loss of nitrogen will also cause corrosion, which seriously affects the safety and stability of the welded structure in service. The present invention addresses the technical problem that the mechanical properties and corrosion resistance of the joint are significantly reduced due to the low nitrogen content in the weld during welding of low-nickel, high-nitrogen austenitic stainless steel. The "nitrogen increase" effect is achieved by adding an appropriate amount of nitrogen to the shielding gas and delivering high-nitrogen content metal powder to the front end of the welding gun. The magnetic field is increased at the back of the weld to quickly stir the molten pool during the welding process to accelerate the nitrogen saturation of the liquid metal in the weld. At the same time, liquid nitrogen is added to the rear end of the molten pool to accelerate the solidification of the molten pool, thereby reducing the overflow of nitrogen in the weld to achieve the "nitrogen fixation" effect. The present invention ultimately achieves the purpose of increasing the nitrogen content of the weld through the coordinated regulation of "nitrogen increase" + "nitrogen fixation" during the welding process, and proposes a low-nickel austenitic stainless steel welding method for increasing the nitrogen content of the weld.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions: The object of the present invention is to provide a low nickel austenitic stainless steel welding method for increasing the nitrogen content of the weld, comprising the following steps: Step 1: According to the actual working conditions, a narrow gap groove is formed on the end face of the base material to be welded (low nickel high nitrogen austenitic stainless steel); mechanical grinding is performed to remove oil and oxides on the groove, and then the groove is placed on the platform for clamping and clamping (placed in the welding fixture for assembly and preparation for welding), and the butt gap is controlled at 0mm~1mm; Step 2. Then install the welding wire into the wire feeder, install the high nitrogen austenitic stainless steel powder into the powder feeder, and arrange the powder feeding device, welding gun and liquid nitrogen feeding device in sequence along the welding direction and on the same horizontal line. The welding gun and the surface of the weld to be welded are tilted back by 0°~15°. The powder feeding device is at the front end of the welding gun, and the liquid nitrogen feeding device is at the rear end of the welding gun. The magnetic field is set at the back of the weld, and the distance from the back of the base material to be welded is 5mm~10mm. The external magnetic field and the welding gun are mirror-imaged relative to the surface of the welding test plate. Control the welding trajectory and perform welding until the entire weld is filled.

[0006] It is further defined that the base material to be welded in step 1 is a low-nickel, high-nitrogen austenitic stainless steel plate with a thickness of 1 mm to 6 mm; It is further defined that the narrow gap groove shape described in step 1 is "Y-shaped" or "U-shaped", and the groove angle is ≤160°.

[0007] Further defined, the diameter of the welding wire in step 2 is 0.8 mm to 1.6 mm, and the welding wire is a high nitrogen stainless steel solid wire; It is further defined that the high nitrogen austenitic stainless steel powder described in step 2 has a mesh size of 60 mesh to 80 mesh and a nitrogen content of 0.7% to 0.8% by mass; Further defining, the welding parameters described in step 2 include welding current, welding voltage, dry stickout, wire feed speed, welding speed, shielding gas type, shielding gas flow rate, liquid nitrogen flow rate, powder feed flow rate, magnetic field direction, and magnetic field strength; Further restrictions include: welding current: 150A~220A, welding voltage: 18V~22V, wire stickout: 16mm~20mm, wire feeding speed: 8m / min~12m / min, welding speed: 400mm / min~600mm / min; Further defined, the shielding gas type described in step 2 is N2+Ar mixed gas for welding, N2 gas accounts for 6% to 8% of the total volume of the shielding gas, and the shielding gas flow rate is 18 L / min to 25 L / min; Further defined, the liquid nitrogen flow rate in step 2 is: 5 L / min ~ 10 L / min, and the powder feeding flow rate is: 10 g / min ~ 15 g / min; It is further defined that the direction of the magnetic field described in step 2 is a transverse magnetic field, that is, the magnetic lines of force pass vertically through the arc axis, and the magnetic field strength is: 20 mT ~ 30 mT.

[0008] It is further defined that the relative position of the welding gun and the groove described in step 2 is that the welding gun and the surface of the weld to be welded are tilted back by 0°~15°; the relative position of the welding gun and the powder feeding device is that the welding gun and the powder feeding device are on the same horizontal line, and the powder feeding device is 3mm~4mm from the front end of the welding gun; the relative position of the welding gun and the liquid nitrogen feeding device is that the liquid nitrogen feeding device is on the same horizontal line, and the liquid nitrogen feeding device is 10mm~15mm from the rear end of the welding gun; the relative position of the welding gun and the magnetic field is that the magnetic field is on the back of the test plate to be welded, and the distance from the test plate to be welded is 5mm~10mm, and the external magnetic field and the welding gun are mirror-imaged with respect to the surface of the test plate to be welded.

[0009] Compared with the prior art, the present invention has the following beneficial effects: The present invention achieves the purpose of increasing the nitrogen content of the weld by synergistically regulating "nitrogen addition" and "nitrogen fixation" during the welding process.

[0010] The present invention achieves the "nitrogen increase" effect by adding an appropriate amount of nitrogen to the shielding gas and adopting N2+Ar mixed gas for welding, wherein the proportion of N2 gas is strictly controlled at 6%~8%. When the proportion of N2 gas is less than 6%, the "nitrogen increase" effect cannot be achieved. When the proportion of N2 gas is greater than 8%, the arc stability during the welding process deteriorates, resulting in unstable welding process and poor welding quality.

[0011] The present invention achieves a "nitrogen-enhancing" effect by feeding high-nitrogen metal powder to the front end of the welding gun. The high-nitrogen austenitic stainless steel powder is strictly controlled to 60-80 mesh, and the powder feeding rate is 10g / min-15g / min. When the powder is smaller than 60 mesh, the thrust of the arc will push it out, preventing the powder from entering the molten pool and achieving the "nitrogen-enhancing" effect. When the powder is larger than 80 mesh, the powder cannot be melted smoothly after entering the molten pool, resulting in welding slag defects that affect the welding quality. When the powder feeding rate is less than 10g / min, the "nitrogen-enhancing" effect is not obvious. When the powder feeding rate is greater than 15g / min, the powder cannot be melted smoothly after entering the molten pool, resulting in welding slag defects that affect the welding quality. The powder feeding device is 3mm-4mm from the front end of the welding gun. When the front end of the welding gun is less than 3mm, the powder is lost by the arc energy before entering the molten pool. When the front end of the welding gun is greater than 4mm, the powder cannot be delivered to the arc molten pool and the "nitrogen-enhancing" effect cannot be achieved.

[0012] The present invention achieves the "nitrogen fixation" effect by rapidly stirring the molten pool during the welding process by adding a magnetic field at the back of the weld, accelerating the nitrogen saturation of the liquid metal in the weld. The external magnetic field needs to strictly control the direction of the magnetic field to be a transverse magnetic field, that is, the magnetic lines of force pass vertically through the arc axis, the magnetic field strength is: 20 mT ~ 30 mT, and the distance between the magnetic field and the test plate to be welded at the back is 5mm ~ 10mm; only by strictly limiting the direction of the magnetic field to the transverse magnetic field can the molten pool be stirred during the welding process to accelerate the nitrogen saturation of the liquid metal in the weld. When the magnetic field strength is less than 20 mT, the molten pool cannot be fully stirred during the welding process, and the "nitrogen fixation" effect is not obvious. When the magnetic field strength is greater than 30 When mT, the external magnetic field is too strong, resulting in violent oscillation of the molten pool during the welding process, and the liquid metal in the molten pool separates from the molten pool, resulting in discontinuous weld surface formation and poor weld surface formation quality; when the magnetic field is less than 5 mm away from the back of the test plate to be welded, the external magnetic field is too strong, resulting in violent oscillation of the molten pool during the welding process, and the liquid metal in the molten pool separates from the molten pool, resulting in discontinuous weld surface formation and poor weld surface formation quality; when the magnetic field is greater than 10 mm away from the back of the test plate to be welded, the welding process cannot fully stir the molten pool and the "nitrogen fixation" effect is not obvious.

[0013] The present invention achieves a "nitrogen fixation" effect by adding liquid nitrogen gas to the rear end of the molten pool to accelerate the solidification of the molten pool, thereby reducing the overflow of nitrogen elements in the weld. The liquid nitrogen flow rate is strictly controlled: 5 L / min~10 L / min, and the liquid nitrogen supply device is 10mm~15mm from the rear end of the welding gun; when the liquid nitrogen flow rate is less than 5 L / min, the effect of accelerating the solidification of the molten pool is not obvious; when the liquid nitrogen flow rate is greater than 5 L / min, the molten pool solidification is accelerated too quickly, the weld is prone to cracks, and the welding quality is affected; when the liquid nitrogen supply device is less than 10 mm from the rear end of the welding gun, the molten pool solidification is accelerated too quickly, the weld is prone to cracks, and the welding quality is affected; when the liquid nitrogen supply device is more than 15 mm from the rear end of the welding gun, the effect of accelerating the solidification of the molten pool is not obvious.

[0014] In order to further understand the features and technical contents of the present invention, please refer to the following detailed description of the present invention and the accompanying drawings. However, the accompanying drawings are provided for reference and illustration only and are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a welding system diagram of the present invention; Figure 2 It is the cross-sectional morphology of the weld joint; Figure 3 EBSD analysis diagram of welded joint. (a) IPF diagram of weld area; (b) grain diagram of weld area; (c) Schmidt factor diagram of weld area; Figure 4 It is the tensile DIC test of welded joints; Figure 5 It is the corrosion test result of weld joint. DETAILED DESCRIPTION

[0016] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art further understand the present invention and are not intended to limit the present invention in any way. It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention. These modifications and improvements are all within the scope of protection of the present invention.

[0017] Example 1: The low-nickel austenitic stainless steel welding method for increasing the nitrogen content of the weld in this embodiment is carried out by the following steps: Two 08Cr19Mn6Ni3Cu2N low-nickel, high-nitrogen austenitic stainless steel plates with a size of 300 mm × 200 mm × 5 mm were selected as the base materials. The base materials to be welded were processed into a Y-shaped groove with a groove blunt edge thickness of 1 mm and a single-side groove angle of 60°. The filler metal was a high-nitrogen stainless steel solid welding wire with a diameter of 1.2 mm. The chemical compositions of the base materials and welding wire are shown in Table 1.

[0018] Table 1 Chemical composition of base metal and welding wire (mass fraction, %)

[0019] The two processed test plates (parent metal) were mechanically polished to remove oil and oxides from the processed grooves. The pre-treated test plates (parent metal) were then placed in a welding fixture for alignment and preparation for welding. The polished test plates were then placed on the specimen platform for clamping and tightening, with a butt gap of 0 mm. The welding equipment used was a Cloos StarT 502 Premium welder. The welding power supply, shielding gas cylinder, and other welding equipment were turned on. The welding wire was installed in the wire feeder and the wire feed was ensured to be stable during the welding process. The welding equipment system diagram is shown below. Figure 1 shown.

[0020] The welding parameters are shown in Table 2. Turn on the welding power supply, protective gas cylinder, liquid nitrogen cylinder, powder feeder, electromagnetic device and welding robot and other welding equipment. Install the welding wire into the wire feeder and ensure the stability of wire feeding during the welding process. Install the high nitrogen austenitic stainless steel powder used for welding into the powder feeder and ensure the stability of powder feeding during the welding process. Set the welding parameters, the relative position of the welding gun and the groove, the relative position of the welding gun and the powder feeding device, the relative position of the welding gun and the magnetic field, and the relative position of the welding gun and the liquid nitrogen feeding device. The welding system is as shown in Table 2. Figure 1 As shown, the high-nitrogen austenitic stainless steel powder has a mesh size of 60 and a nitrogen content of 0.8% by mass. The shielding gas used is a mixture of N₂ and Ar, with an N₂ content of 8% by volume and a shielding gas flow rate of 20 L / min. The liquid nitrogen flow rate is 7 L / min, and the powder feed rate is 10 g / min. The magnetic field is transverse, with the magnetic lines of force perpendicular to the arc axis, and the magnetic field strength is 25 mT. The welding gun is tilted 5° back from the weld surface. The welding gun and powder feeder are aligned horizontally, with the powder feeder located 3 mm from the front of the welding gun. The welding gun and liquid nitrogen feeder are aligned horizontally, with the liquid nitrogen feeder located 15 mm from the rear of the welding gun. The welding gun is positioned relative to the magnetic field, with the magnetic field at the back of the test plate, 10 mm from the test plate. The applied magnetic field and the welding gun are mirror images of the test plate surface.

[0021] Table 2 Welding process parameters

[0022] A KUKA robot controlled the welding trajectory and continued welding until the entire test plate was filled. After welding, verification and comparative tests were conducted using an OLYMPUS GX71 optical microscope (OM), a FEI Quanta-200 scanning electron microscope, and an electron backscatter diffractometer (EBSD) to observe the weld microstructure and phases. An oxygen, nitrogen, and hydrogen analyzer was used to measure the nitrogen content in the weld.

[0023] The macroscopic morphology of the cross section of the welded joint is shown in Figure 3No defects such as pores, cracks, or lack of fusion were found in the welds, indicating good metallurgical quality. The transition between the filler metal and the base metal was smooth, and the filler metal molten pool had good fluidity, allowing the liquid metal to wet and spread easily.

[0024] EBSD analysis of weld zone of welded joint Figure 3 , Figure 3 (a) is the IPF diagram of the weld area, Figure 3 (a) It can be seen that the weld area is mainly composed of equiaxed crystals, and the weld grains are finer and more uniform due to the external magnetic field. Figure 3 (b) is the grain map of the weld area. The red lines in the figure represent small-angle grain boundaries of 2°~10°, and the black lines represent large-angle grain boundaries greater than 10°. Figure 3 (b) It can be seen that the low-angle grain boundaries in the test weld area of ​​the present invention account for 6.69%, the high-angle grain boundaries account for 93.3%, the average size of the grains in the weld area is 20.90 μm, the maximum grain size is 140.05 μm, and the minimum grain size is 3.21 μm. Figure 3 (c) is the Schmidt factor diagram of the weld area, Figure 3 (c) It can be seen that the average Schmidt factor of the test weld area of ​​the present invention is 0.45, the minimum value is 0.27, and the maximum value is 0.5.

[0025] The nitrogen content in the weld was tested using an oxygen, nitrogen and hydrogen analyzer and the results are shown in Table 3. The nitrogen content in the weld of the present invention is 0.96%.

[0026] Table 3 Test results of N content in welds

[0027] The room temperature tensile properties test results of the welded joint are shown in Table 4. The room temperature tensile properties of the welded joint are an average tensile strength of 714 MPa, an average yield strength of 415 MPa, and an elongation of 60%, and the fracture location is the base material area.

[0028] Table 4 Room temperature tensile properties test results of welded joints

[0029] The room temperature tensile strength of welded joints was tested by digital image correlation (DIC) technology. The strain field distribution of welded joints is shown in Figure 4, the full-field longitudinal strain distribution of the welded joint under different fracture time percentages, the strain distribution of the entire weld shows that the strain gradient transition from the weld centerline to the base material is smooth and uniform. The maximum local strain of the welded joint during the tensile process occurs in the base material area above the weld center, while the minimum local strain value occurs in the base material near the edge of the specimen. Necking occurs in the base material, and then the strain change is mainly concentrated in the necking area. The weld area of ​​the welded joint is mainly composed of large-angle grain boundaries. The large-angle inter-granular orientation distribution can effectively prevent the expansion of microcracks in the intragranular structure. For small-angle grain boundaries, the crack only needs to deflect a little angle when it propagates to expand from the next grain boundary. At the same time, the Schmidt factor value of the weld area is higher, so the weld area and the heat-affected zone have better resistance to deformation, and the fracture position is the base material area.

[0030] The results of electrochemical corrosion test of weld zone of welded joint in 3.5% NaCl solution are shown in Figure 5 The self-corrosion potential of the weld zone of the welded joint is -7.8mV, and the corrosion current density is -27.69A·cm -2 As the potential gradually shifted positively, the current density of the sample first decreased and then entered the anodic dissolution zone; as the potential continued to shift positively, an obvious passivation zone appeared in the weld, that is, the current density of the sample was maintained within a certain range, the corrosion rate was slow and stable, and the sample showed a "passivation-activation" short-term oscillation behavior in the passivation zone; when the potential reached -7.8mV, the passivation film on the surface of the sample was destroyed, showing an over-passivation phenomenon, and the anodic dissolution current density of the metal continued to increase with the increase of potential E, basically obeying the Tafel law.

[0031] The above describes the specific embodiments of the present invention. It should be noted that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for welding low-nickel austenitic stainless steel to increase the nitrogen content of the weld, characterized in that: The following steps are involved: Step 1: Process the end face of the base material to be welded to form a narrow groove; perform mechanical grinding to remove oil and oxides on the groove, then place it on the platform for clamping and tightening, and control the butt gap to 0mm~1mm; Step 2. Then install the welding wire into the wire feeder, install the high nitrogen austenitic stainless steel powder into the powder feeder, and arrange the powder feeding device, welding gun and liquid nitrogen feeding device in sequence along the welding direction and on the same horizontal line. The welding gun and the surface of the weld to be welded are tilted back by 0°~15°. The powder feeding device is at the front end of the welding gun, and the liquid nitrogen feeding device is at the rear end of the welding gun. The magnetic field is set at the back of the weld, and the distance from the back of the base material to be welded is 5mm~10mm. The external magnetic field and the welding gun are mirror-imaged relative to the surface of the welding test plate. Control the welding trajectory and perform welding until the entire weld is filled.

2. The method according to claim 1, characterized in that The shape of the groove is "Y-type" or "U-type" groove, and the angle is ≤160°.

3. The method according to claim 1, characterized in that The mass content of nitrogen element in the high nitrogen austenitic stainless steel powder is 0.7%~0.8%, and the mesh size is 60 mesh~80 mesh.

4. The method according to claim 1, characterized in that The direction of the magnetic field is transverse magnetic field, and the magnetic field strength is 20 mT ~30mT.

5. The method according to claim 1, characterized in that The direction of the magnetic field is transverse magnetic field, and the magnetic field strength is 20 mT ~30mT.

6. The method according to claim 1, characterized in that The shielding gas is a mixture of N2+Ar, with N2 accounting for 6%~8% of the total shielding gas volume and the shielding gas flow rate being 18L / min~25L / min.

7. The method according to claim 1, characterized in that: Welding current: 150A~220A, welding voltage: 18V~22V, wire stick-out: 16mm~20mm, wire feeding speed: 8m / min~12m / min, welding speed: 400mm / min~600mm / min.

8. The method according to claim 1, characterized in that: Powder feeding flow rate: 10 g / min ~15 g / min.

9. The method according to claim 1, characterized in that: The powder feeding device is 3mm~4mm from the front end of the welding gun.

10. The method according to claim 1, characterized in that: The liquid nitrogen delivery device is located 10mm~15mm from the rear end of the welding gun.

Images