A method of plasma arc welding of low carbon steel plate

Abstract

The present application relates to a kind of medium plate low carbon steel plasma arc welding method, belong to serial type plasma arc welding technical field, for low carbon steel medium plate using plasma arc welding and carbon dioxide gas shielded welding composite welding process, open Y type groove, using plasma arc welding to realize intelligent control in blunt edge, realize stable welding by special current waveform, improve welding quality.Carbon dioxide gas shielded welding is used in groove edge to carry out multi-layer multi-pass welding.Through strict control of each stage process parameter, the mechanical properties of weld at room temperature and low temperature meet the expected target, greatly improve the welding efficiency of low carbon steel medium plate.At the same time in the welding effect, weld is more flat and smooth, welding point is more firm, at the same time will not produce bubble and hydrogen and other impurities.

Description

Technical Field

[0001] This invention relates to a plasma arc welding method for medium-thick low-carbon steel plates, belonging to the field of serial-port plasma arc welding technology. Background Technology

[0002] Low-carbon steel is steel with a carbon content of less than 0.25%, usually represented by the SAE standard. The main characteristics of low-carbon steel are low hardness, good plasticity, good toughness, good weldability, and good machinability. Low-carbon steel, by simultaneously meeting the requirements of strength, toughness, weldability, machinability, and economy, is widely used in various manufacturing industries.

[0003] Plasma arc welding (PAW) is a fusion welding method that uses a high-energy-density plasma arc beam as the welding heat source. PAW uses technical means to compress the electric arc, concentrating its energy for application in welding and cutting processes. In PAW, a water-cooled nozzle is used to confine the arc burning between the tungsten electrode and the workpiece. Because the orifice of the water-cooled nozzle is relatively small, the welding arc is mechanically compressed and its size shrinks as it passes through the orifice. A low-temperature, non-ionized cooling gas film existing on the inner wall of the water-cooled nozzle cools the arc through its surface. According to the law of minimum voltage, the arc size will shrink under good external heat dissipation conditions. This compression due to low-temperature cooling is called thermal compression. Charges moving in the same direction attract each other, reducing the arc cross-section, increasing the current density, and increasing the Lorentz force on the charges from the arc's magnetic field, further compressing the arc. Through these three compression effects, the bell-shaped free arc is compressed into a cylindrical plasma arc with a temperature as high as (10⁻⁶). 4 K), which can be applied to welding various materials such as low carbon steel and stainless steel.

[0004] The welding current for through-hole plasma arc welding is generally between 100 and 300 A. Through the matching of welding current and plasma gas flow, the arc penetration is strong, and during welding, the plasma arc completely melts through the workpiece, forming a small hole that penetrates both the upper and lower surfaces. It is mainly used for welding 6-10 mm thick stainless steel and carbon steel.

[0005] The traditional welding method for medium and heavy plates of low carbon steel is gas shielded welding, which requires multi-layer and multi-pass welding, resulting in low efficiency and high cost. Therefore, it is necessary to upgrade the existing welding process to improve the welding efficiency and welding quality of medium and heavy plates of low carbon steel. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a plasma arc welding method for medium-thick low-carbon steel plates, specifically a composite welding process combining plasma arc welding and carbon dioxide gas shielded welding for medium-thick low-carbon steel plates.

[0007] The technical solution of the present invention is as follows: A plasma arc welding method for medium-thick low-carbon steel plates includes the following steps: (1) Beveling the weld: The weld is beveled in a Y-shape. Preferably, in step (1), the weld bead is cleaned by using 99.5% anhydrous alcohol to remove oil, rust, welding slag and other impurities from the surface of the weld bead before welding, so as to ensure the cleanliness of each layer of weld bead.

[0008] Preferably, in step (1), the thickness of the weldment is 16-20mm, the welding material is low carbon steel, the thickness of the blunt edge of the Y-shaped groove is 6-10mm, the angle between the groove surface and the vertical plane is 45°, the bottom of both ends of the groove is rounded with R3-5mm, and the distance between the bottom of the groove slope and the center line of the Y-shaped groove is 3mm.

[0009] (2) Inspect the welding equipment to confirm that the welding power source, wire feeder, welding torch, etc. are working properly, and adjust the parameters such as welding current, wire feed speed, and gas flow rate to ensure the stability of the electric arc and the transition mode of the molten droplets. Inspect the welding environment to eliminate potential hazards such as fire sources, flammable materials, and power lines. Maintain good ventilation and lighting to ensure welding safety and efficiency.

[0010] (3) Prepare gas protection. During plasma arc welding, use high-purity argon as an inert gas to protect each layer of the weld to prevent oxidation. Carbon dioxide gas shielded welding uses carbon dioxide to protect the weld. Preferably, the carbon dioxide purity should be above 99.5%, the water content should not exceed 0.1%, and the carbon content should not exceed 0.1%. High-purity argon gas should have a purity of 99.999%.

[0011] (4) Spot welding: Before welding begins, a short welding is performed at the starting point of the weld to fix the position and direction of the workpiece, and the weld is preheated to improve the quality and efficiency of welding. (5) Blunt edge welding: rotate the workpiece 180° to invert the Y-shaped groove, and perform plasma arc welding on the blunt edge of the back of the workpiece. Shielding gas is introduced into both the front and back of the workpiece. Preferably, the blunt edge thickness is 6~10 mm, the welding current is 200~300 A, the welding speed is 3~8 mm / s, and high-purity argon is used for the plasma gas, the front shielding gas and the back shielding gas. The welding plasma gas flow rate is 4~6 L / min, the front shielding gas flow rate is 5~10 L / min, and the back shielding gas flow rate is 25~30 L / min.

[0012] Preferably, the plasma arc welding current is in the form of periodic pulses, and the time within one cycle is divided into T phases according to the direction of time occurrence. P T k T B T PThe current remains at I during the time period P T k The current changes from I within a certain time period. P Down to I k In T B The current remains at I during the time period B constant.

[0013] Further preferred, T k The current decreases linearly over the time period, with a slope of K.

[0014] Preferred, I P with I k The difference is a, I k with I B The difference is b, and the ratio of a to b is 3:2.

[0015] Preferred, T P Within the time period, there are times T0 and T... k There is a time T within the time period C At time T0, the pinhole penetrates the workpiece, and subsequently the area of ​​the molten pool penetrated by the pinhole continuously increases, until the pinhole grows to T. P The current is reduced at the end, and then the orifice is at T. k It reaches its maximum value within a certain time period, and finally penetrates the small hole at T. C Always closed.

[0016] The pulsed current is specially designed to incorporate a falling phase with a slope of K at the falling edge of the pulsed current waveform, so as to gradually reduce the welding current from the peak value to the base value.

[0017] During the peak current phase, the control system continuously monitors the pinhole penetration signal, and the pinhole penetrates the workpiece at time T0. When the pinhole status feedback signal increases above the judgment threshold, the welding current initially decreases from the peak value I with a relatively small decreasing slope K. P Slowly decrease to I k Due to the thermal hysteresis effect of the thermal field on the workpiece, the base metal around the pinhole continues to melt at a relatively rapid rate. Although the welding heat input decreases at this stage, the molten pool still tends to grow. Subsequently, the pinhole... k The weld pool reaches its maximum value within a certain time period. Due to the decrease in current, the welding heat input decreases sharply, the weld pool stops expanding and begins to shrink, and the keyhole shrinks accordingly. The penetration point is at T... C The circuit is closed at the specified time. At the base current I... BDuring the action time, the orifice depth and molten pool volume shrink. After the base phase, the peak current applies again, a new pulse cycle begins, and the orifice penetrates the workpiece at time T0 of the next pulse cycle. Thus, the orifice is dynamically controlled by a specially designed pulse current waveform, penetrating the workpiece for a period within one current pulse cycle before closing again, operating in a periodic "one pulse, one penetrating orifice" pattern. Therefore, this pulse current ensures complete melting of the workpiece with the lowest possible heat input while avoiding burn-through defects.

[0018] (6) Turn the workpiece over so that the bevel faces upward and use carbon dioxide gas shielded welding to perform the root pass welding. The root pass welding is one layer, mainly to cover the back weld of the blunt edge weld. The welding current is 100~150 A, the welding speed is 2~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min. (7) On the basis of the root pass welding, carbon dioxide gas shielded welding is used for the filler weld. The filler weld consists of 3 to 5 layers. The welding current is 150 to 200 A, the welding speed is 1 to 5 mm / s, and the carbon dioxide shielding gas flow rate is 10 to 20 L / min. (8) After the filler weld, the cover weld is performed using carbon dioxide gas shielded welding. The welding current is 100~150 A, the welding speed is 1~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min.

[0019] Preferably, in the multi-layer, multi-pass welding process, the control of preheating temperature and interpass temperature is very important. In step (5), the preheating temperature is 100~150℃, and the subsequent interpass temperature is 100~150℃. During the welding process, an infrared thermometer is used to monitor the interpass temperature, and flame heating is used to maintain the interpass temperature.

[0020] The beneficial effects of this invention are as follows: This invention relates to a composite welding process combining plasma arc welding and CO2 gas shielded welding for medium-thick low-carbon steel plates. Plasma arc welding offers advantages such as concentrated energy, high productivity, fast welding speed, low stress deformation, and stable arc, making it particularly suitable for welding enclosed containers and pipelines. For medium-thick plates (6-10 mm), plasma arc welding can achieve single-sided penetration in a single pass without beveling, filler wire, or back-side forced forming protection, resulting in good double-sided forming and significantly improving welding production efficiency. Compared to using CO2 gas shielded welding alone, plasma arc welding can penetrate the blunt edge more quickly, eliminating the need for filler wire and beveling, thus greatly improving welding efficiency.

[0021] Plasma arc welding enables intelligent control, achieving stable welding through a specific current waveform and improving weld quality. By strictly controlling the process parameters at each stage, the mechanical properties of the weld at both room temperature and low temperature meet the expected targets, greatly improving the welding efficiency of medium and heavy-duty low-carbon steel plates. Furthermore, in terms of welding effect, the weld is flatter and smoother, the weld points are stronger, and no impurities such as bubbles or hydrogen are generated. Attached Figure Description

[0022] Figure 1 Schematic diagram of weld bevel design; Figure 2 This is a schematic diagram of plasma arc welding of the blunt edge of medium-thick low-carbon steel plates. Figure 3 A schematic diagram of the root pass, cover pass, and fill pass welding for CO2 shielded welding; Figure 4 This is a waveform diagram of the plasma arc welding current. Among them, 1. Plasma gas, 2. Protective gas, 3. Plasma arc, and 4. Wire feeder. Detailed Implementation

[0023] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0024] Example 1

[0025] A plasma arc welding method for medium-thick low-carbon steel plates includes the following steps: (1) Beveling the weld: The weld is beveled in a Y-shape. Weld cleaning: Before welding, use 99.5% anhydrous alcohol to remove oil, rust, welding slag and other impurities from the weld surface to ensure the cleanliness of each weld layer.

[0026] The weldment thickness is 16-20mm, the welding material is Q235 low carbon steel, the blunt edge thickness of the Y-shaped bevel is 6-10mm, the angle between the bevel surface and the vertical plane is 45°, the bottom of both ends of the bevel has a radius of 3-5mm, and the distance between the bottom of the bevel slope and the center line of the Y-shaped bevel is 3mm. For example... Figure 1 As shown.

[0027] (2) Inspect the welding equipment to confirm that the welding power source, wire feeder, welding torch, etc. are working properly, and adjust the parameters such as welding current, wire feed speed, and gas flow rate to ensure the stability of the electric arc and the transition mode of the molten droplets. Inspect the welding environment to eliminate potential hazards such as fire sources, flammable materials, and power lines. Maintain good ventilation and lighting to ensure welding safety and efficiency.

[0028] (3) Prepare gas protection. During plasma arc welding, use high-purity argon as an inert gas to protect each layer of the weld to prevent oxidation. Carbon dioxide gas shielded welding uses carbon dioxide to protect the weld. Carbon dioxide should have a purity of 99.5% or higher, with a water content not exceeding 0.1% and a carbon content not exceeding 0.1%. High-purity argon should have a purity of 99.999%.

[0029] (4) Spot welding: Before welding begins, a short welding is performed at the starting point of the weld to fix the position and direction of the workpiece, and the weld is preheated to improve the quality and efficiency of welding. (5) Blunt edge welding: Rotate the workpiece 180° to invert the Y-shaped groove, and perform plasma arc welding on the blunt edge of the back side of the workpiece. Shielding gas is introduced into both the front and back sides of the workpiece; Figure 2 As shown. Bevel opening direction ( Figure 2 The bottom side is the front, with a blunt edge. Figure 2 The top (top) is the back.

[0030] The blunt edge thickness is 6~10 mm, the tungsten electrode diameter is Φ4.0 mm, the welding current is 200~300 A, the welding speed is 3~8 mm / s, and high-purity argon is used for the plasma gas, front shielding gas and back shielding gas. The welding plasma gas flow rate is 4~6 L / min, the front shielding gas flow rate is 5~10 L / min, and the back shielding gas flow rate is 25~30 L / min.

[0031] The welding current settings for different blunt edge thicknesses are shown in Table 1.

[0032] Table 1 Plasma Arc Welding Current Parameters 6 200 300 150 160 200 8 240 300 200 180 300 10 280 400 200 220 400 The welding current, peak / base value duration, and blunt edge thickness have a non-linear relationship. However, as the blunt edge thickness increases, a larger heat input is required to penetrate the blunt edge. Therefore, both peak and base value welding currents will increase, and the peak / base value duration will also increase accordingly.

[0033] The plasma arc welding current is in the form of periodic pulses, and the time within one cycle is divided into T_time according to the direction of time occurrence. P T k T B T P The current remains at I during the time period P T k The current changes from I within a certain time period. P Down to I k In T B The current remains at I during the time period B constant.

[0034] T kThe current decreases linearly over the time period, with a slope of K. P with I k The difference is a, I k with I B The difference is b, and the ratio of a to b is 3:2. P Within the time period, there are times T0 and T... k There is a time T within the time period C At time T0, the pinhole penetrates the workpiece, and subsequently the area of ​​the molten pool penetrated by the pinhole continuously increases, until the pinhole grows to T. P The current is reduced at the end, and then the orifice is at T. k It reaches its maximum value within a certain time period, and finally penetrates the small hole at T. C Closed at any time. For example... Figure 4 As shown, Figure 4 The three connected line segments in the upper middle section represent current pulse waveforms. Figure 4 The three connected line segments in the lower middle part represent the perforation signal intensity change curve. The thermal hysteresis and heat accumulation during the welding process is a complex process. After the pinhole is penetrated, the pinhole signal will continue to rise and will not drop immediately due to the decrease in current.

[0035] The pulsed current is specially designed to incorporate a falling phase with a slope of K at the falling edge of the pulsed current waveform, so as to gradually reduce the welding current from the peak value to the base value.

[0036] During the peak current phase, the control system continuously monitors the pinhole penetration signal, and the pinhole penetrates the workpiece at time T0. When the pinhole status feedback signal increases above the judgment threshold, the welding current initially decreases from the peak value I with a relatively small decreasing slope K. P Slowly decrease to I k Due to the thermal hysteresis effect of the thermal field on the workpiece, the base metal around the pinhole continues to melt at a relatively rapid rate. Although the welding heat input decreases at this stage, the molten pool still tends to grow. Subsequently, the pinhole... k The weld pool reaches its maximum value within a certain time period. Due to the decrease in current, the welding heat input decreases sharply, the weld pool stops expanding and begins to shrink, and the keyhole shrinks accordingly. The penetration point is at T... C The circuit is closed at the specified time. At the base current I... B During the action time, the orifice depth and molten pool volume shrink. After the base phase, the peak current applies again, a new pulse cycle begins, and the orifice penetrates the workpiece at time T0 of the next pulse cycle. Thus, the orifice is dynamically controlled by a specially designed pulse current waveform, penetrating the workpiece for a period within one current pulse cycle before closing again, operating in a periodic "one pulse, one penetrating orifice" pattern. Therefore, this pulse current ensures complete melting of the workpiece with the lowest possible heat input while avoiding burn-through defects.

[0037] (6) Turn the weldment so that the bevel faces upwards, as shown. Figure 3 As shown, carbon dioxide gas shielded welding is used for the root pass welding. The root pass welding is a single layer, mainly covering the back weld of the blunt edge weld. The welding current is 100~150 A, the welding speed is 2~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min.

[0038] (7) On the basis of the root pass, the fill pass is performed using carbon dioxide gas shielded welding. The fill pass consists of 3 to 5 layers. The welding current is 150 to 200 A, the welding speed is 1 to 5 mm / s, and the carbon dioxide shielding gas flow rate is 10 to 20 L / min. After each pass is completed, the weld is cleaned to remove spatter, oxide scale, slag and other impurities from the weld surface.

[0039] (8) After the filler weld, the cover weld is performed using carbon dioxide gas shielded welding. The welding current is 100~150 A, the welding speed is 1~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min.

[0040] In steps (6)-(8), the welding wire used for carbon dioxide gas shielded welding is of grade H08Mn2SiA, with a diameter of Φ1.0-1.2 mm and a wire extension length of 10-15 mm. Too long or too short will affect the intensity of the arc and the transition of the molten droplets.

[0041] In the process of multi-layer and multi-pass welding, the control of preheating temperature and interpass temperature is very important. In step (5), the preheating temperature is 100~150℃, and the subsequent interpass temperature is 100~150℃. During the welding process, an infrared thermometer is used to monitor the interpass temperature, and flame heating is used to maintain the interpass temperature.

[0042] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments, and can be applied to various fields suitable for the present invention. Those skilled in the art will readily implement other improvements and modifications without departing from the principles of the present invention; the present invention is not limited to the specific details and examples shown and described herein.

Claims

1. A plasma arc welding method for medium-thick low-carbon steel plates, characterized in that, The steps include the following: (1) Beveling the weld: The weld is beveling in a Y-shape. The thickness of the weldment is 16-20mm, the welding material is low carbon steel, and the thickness of the blunt edge of the Y-shape is 6-10mm. (2) Inspect the welding equipment to confirm that the welding power source, wire feeder and welding torch are working properly, and adjust the welding current, wire feed speed and gas flow parameters. (3) Prepare gas protection. In plasma arc welding, use inert gas high-purity argon to protect each layer of weld. In carbon dioxide gas shielded welding, use carbon dioxide to protect the weld. (4) Spot welding: Before welding begins, a short welding is performed at the starting point of the weld. (5) Blunt edge welding: Rotate the workpiece 180° to invert the Y-shaped groove, and perform plasma arc welding on the blunt edge of the back side of the workpiece. Shielding gas is introduced into both the front and back sides of the workpiece. The welding current is 200~300 A, the welding speed is 3~8 mm / s, and high-purity argon is used for the plasma gas, the front shielding gas and the back shielding gas. The flow rate of the welding plasma gas is 4~6 L / min, the flow rate of the front shielding gas is 5~10 L / min, and the flow rate of the back shielding gas is 25~30 L / min. (6) Turn the workpiece over so that the bevel faces upward and use carbon dioxide gas shielded welding to perform the root pass welding. The root pass welding is one layer. The welding current for the root pass welding is 100~150 A, the welding speed is 2~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min. (7) On the basis of the root pass welding, carbon dioxide gas shielded welding is used for filler welding. The filler welding consists of 3 to 5 layers. The filler welding current is 150 to 200 A, the welding speed is 1 to 5 mm / s, and the carbon dioxide shielding gas flow rate is 10 to 20 L / min. (8) After the filler weld, the cover weld is performed using carbon dioxide gas shielded welding; the cover weld welding current is 100~150 A, the welding speed is 1~5 mm / s, and the carbon dioxide shielding gas flow rate is 10~20 L / min.

2. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 1, characterized in that, In step (1), the weld bead is cleaned by using 99.5% anhydrous alcohol to remove oil and rust from the surface of the weld bead before welding.

3. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 1, characterized in that, In step (3), the purity of carbon dioxide should reach 99.5% or higher, the water content should not exceed 0.1%, and the carbon content should not exceed 0.1%; the purity of high-purity argon gas should be 99.999%.

4. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 1, characterized in that, In step (1), the angle between the bevel surface and the vertical plane is 45°, the bottom of both ends of the bevel is rounded with R3-5mm, and the distance between the bottom of the bevel slope and the midline of the Y-shaped bevel is 3mm.

5. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 1, characterized in that, In step (5), the plasma arc welding current is in the form of periodic pulses, and the time within one cycle is divided into T according to the direction of time occurrence. P T k T B T P The current remains at I during the time period P T k The current changes from I within a certain time period. P Down to I k In T B The current remains at I during the time period B constant.

6. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 5, characterized in that, T k The current decreases linearly over the time period, with a slope of K.

7. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 5, characterized in that, I P with I k The difference is a, I k with I B The difference is b, and the ratio of a to b is 3:

2.

8. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 5, characterized in that, T P Within the time period, there are times T0 and T... k There is a time T within the time period C At time T0, the pinhole penetrates the workpiece, and subsequently the area of ​​the molten pool penetrated by the pinhole continuously increases, until the pinhole grows to T. P The current is reduced at the end, and then the orifice is at T. k It reaches its maximum value within a certain time period, and finally penetrates the small hole at T. C Always closed.

9. The plasma arc welding method for medium-thick low-carbon steel plates according to claim 1, characterized in that, In step (5), the preheating temperature is 100~150℃, and the interpass temperature is 100~150℃. During the welding process, an infrared thermometer is used to monitor the interpass temperature, and flame heating is used to maintain the interpass temperature.

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