Ferritic stainless steel molten pool columnar grain control method based on high-power laser welding

Abstract

The present application relates to the field of ferritic stainless steel laser welding, and is based on a columnar crystal control method for a molten pool of ferritic stainless steel by high-power laser welding, which comprises the following steps: step one: high-power low-speed welding; step two: low-angle large-flow gas protection; and step three: ultra-low-temperature protective gas welding, which is realized by a protective helium gas cooling device. The present application greatly improves the self-fusion weldability of ferritic stainless steel, greatly improves the strength and toughness of the welded joint, and meets the mechanical performance requirements in most cases. The present application expands the application range of ferritic stainless steel and can replace austenitic stainless steel in some fields, thereby reducing the use cost.

Description

Technical Field

[0001] This invention relates to the field of laser welding of ferritic stainless steel, and more particularly to a method for controlling columnar crystals in the molten pool of ferritic stainless steel based on high-power laser welding. Background Technology

[0002] Ferritic stainless steel is a type of stainless steel with good corrosion resistance, mainly used in applications where corrosion resistance is not a primary concern rather than mechanical properties. However, its poor weldability greatly limits its application range. This is mainly manifested in the fact that during autogenous welding, the weld grains easily grow, forming opposing columnar crystals, resulting in extremely poor weld mechanical properties that cannot withstand large loads, thus restricting the expansion of its application market.

[0003] The laser industry has developed rapidly in recent years. Due to the heat input sensitivity of ferritic stainless steel, laser welding is commonly used in industrial production. Furthermore, given the high energy density of laser welding, it is frequently employed for connections in critical load-bearing structures. However, while laser welding of ferritic stainless steel can significantly reduce grain growth compared to other welding methods, a large number of opposing columnar crystals still appear in the weld. A distinct grain boundary appears at the center of the weld, where compositional segregation and dislocation density are high. In situations with high constraint, cracks are highly likely to form along this centerline, leading to weld deterioration.

[0004] Therefore, there is an urgent need for a method to control the growth of columnar crystals in the weld, allowing them to meet at the center and form a final solidified grain boundary at the center of the weld.

[0005] The purpose of this invention is to propose a method for controlling columnar crystals in the ferritic stainless steel weld pool based on high-power laser welding. This method can form equiaxed crystals at the center of the weld, avoid compositional segregation and dislocation concentration at the center of the weld, and improve the mechanical properties of the weld. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned problems by providing a method for controlling columnar crystal formation in the molten pool of ferritic stainless steel based on high-power laser welding.

[0007] The objective of this invention is achieved as follows: a method for controlling columnar crystal formation in the molten pool of ferritic stainless steel based on high-power laser welding, comprising the following steps: Step 1: High-power low-speed welding: applicable to ferritic stainless steel with a thickness of 3.0-8.0mm, welding speed of 2-5m / min, and matched with a corresponding laser power of 10-12kw; Step 2: Low-angle high-flow-rate gas protection: the angle between the protective gas nozzle and the workpiece being welded is 10°-15°, and the gas flow rate is 70-80L / min; Step 3: Cryogenic protective gas welding: liquid nitrogen is used to cool the protective gas, and helium gas at -50℃ to -80℃ is used to suppress the plasma and cool the upper surface of the molten pool, forming three-dimensional heat flow conditions in the molten pool. The temperature gradient of the molten pool changes from the original 2 gradients to 3 gradients, ultimately forming an elliptical weld pool and a large number of equiaxed crystals at the center of the weld.

[0008] Step 3: Cryogenic shielded gas welding is achieved through a shielding helium cooling device. This device includes a liquid nitrogen cooling tank, a shielding gas nozzle, and a shielding gas cylinder. The liquid nitrogen cooling tank is a sealed container, and a cooling copper tube is placed inside it. The cooling copper tube is fixed to the top of the liquid nitrogen cooling tank by its internal thread fitting into the tank opening. The inlet and outlet of the cooling copper tube are connected to the shielding gas cylinder and the shielding gas nozzle through ventilation hoses A and B, respectively. The shielding gas nozzle is positioned directly below the laser head to weld the molten pool, enabling laser welding under cryogenic protection. The liquid nitrogen cooling tank contains 40%–60% liquid nitrogen. The cooling copper tube does not contact the bottom of the liquid nitrogen cooling tank. The inner diameter of the cooling copper tube is required to be 3mm. With a liquid nitrogen capacity of 40%–60%, a length of 18–25m of cooling copper tube is immersed in the liquid nitrogen to ensure that the shielding gas temperature at the nozzle outlet is between -50℃ and -80℃.

[0009] The beneficial effects of this invention are: 1) The molten pool shape changes from a teardrop shape to an elliptical shape. Figure 1 , Figure 2 As shown. 2) The weld microstructure changes from a single pair of columnar crystals to columnar crystals + central equiaxed crystals, avoiding compositional segregation at the weld center and dislocation concentration, such as... Figure 2 , Figure 3 As shown. 3) It greatly improves the self-fluxing weldability of ferritic stainless steel, significantly increasing the strength and toughness of the welded joint and meeting the mechanical performance requirements in most cases. 4) It expands the application range of ferritic stainless steel, and can replace austenitic stainless steel in some fields, reducing the cost of use. Attached Figure Description

[0010] The present invention will now be further described with reference to the accompanying drawings.

[0011] Figure 1 Schematic diagram of a teardrop-shaped molten pool.

[0012] Figure 2 Schematic diagram of an elliptical molten pool.

[0013] Figure 3 The microstructure of conventional laser-welded welds is mainly composed of columnar crystals (see attached figure).

[0014] Figure 4 The microstructure of the laser weld seam in this invention is columnar crystals + central equiaxed crystals (see attached diagram).

[0015] Figure 5 Schematic diagram of low-angle, high-flow-rate gas protection.

[0016] Figure 6 Schematic diagram of the protective helium cooling device.

[0017] The components are: 1. Protective nozzle, 2. Cooling copper pipe, 3. Hose B, 4. Air outlet, 5. Air inlet, 6. Hose A, 7. Liquid nitrogen cooling tank, and 8. Gas cylinder. Implementation

[0018] Ferritic stainless steel is sensitive to heat input, and grains tend to grow during welding, resulting in a large number of columnar crystals in the weld, which greatly affects the mechanical properties of the weld and cannot meet the requirements of heavy-load applications. This invention uses three methods—high-power low-speed welding, low-angle high-flow-rate gas protection, and ultra-low temperature shielded gas welding—to create three-dimensional heat flow conditions, improve the solidification mode of the molten pool, and ultimately form an elliptical weld pool with a large number of equiaxed crystals in the center of the weld, thereby improving the mechanical properties of the weld and meeting the requirements of some heavy-load applications.

[0019] The macroscopic shape of the weld pool is determined by the combination of the material's physical properties, welding parameters, and heat transfer conditions. Common weld pool shapes include teardrop and elliptical. Currently, ferritic stainless steel has relatively low thermal conductivity. When using high-power laser welding for rapid welding, the weld pool tends to be teardrop-shaped. In situations with high constraint, cracks are easily generated along its centerline, so teardrop-shaped weld pools should be avoided. Elliptical weld pools form a large number of equiaxed crystals at the weld center, resulting in non-jointed nucleation and growth, preventing compositional segregation and dislocation concentration at the weld center, and improving the weld's mechanical properties. To achieve this, the present invention adopts the following technical solutions: 1. High-power, low-speed welding: Adjusting process parameters, appropriately reducing the welding speed and matching the corresponding laser power (≥10kW), slightly delaying the solidification rate of the weld pool. 2. Low-angle, high-flow-rate gas protection: The angle between the protective gas nozzle and the workpiece is approximately 10°-15°, and the gas flow rate is 70-80L / min, reducing porosity while enhancing the protective effect. Figure 5 As shown. 3. Cryogenic shielded gas welding uses liquid nitrogen as the shielding gas and helium gas at -50℃ to -80℃ to suppress the plasma and cool the upper surface of the molten pool, creating three-dimensional heat flow conditions in the molten pool. The temperature gradient of the molten pool changes from the original two to three, ultimately forming an elliptical weld pool and a large number of equiaxed crystals at the center of the weld. 4. The shielding helium cooling device is as follows: Figure 6As shown, the system includes a liquid nitrogen cooling tank, a protective gas nozzle, a protective gas cylinder, and a cooling copper tube section placed inside the liquid nitrogen cooling tank. The cooling copper tube section is fixed to the top of the liquid nitrogen cooling tank by an internal thread fitting into the tank opening. The inlet and outlet of the spiral copper tube are connected to the protective gas cylinder and protective gas nozzle via ventilation hoses A and B, respectively. The protective gas nozzle faces the welding pool below the laser head, enabling laser welding under cryogenic protection. The liquid nitrogen cooling tank contains 40%–60% liquid nitrogen. The spiral copper tube does not contact the bottom of the liquid nitrogen cooling tank. The cooling copper tube has an inner diameter of 3mm, and with a liquid nitrogen capacity of 40%–60%, 18–25m of cooling copper tube is immersed in the liquid nitrogen, ensuring that the protective gas temperature at the nozzle outlet is between -50℃ and -80℃. Figure 6 As shown. Example 1

[0020] Welding of 4.5mm thick 439 stainless steel was performed using a laser with a power of 11kW, a welding speed of 4m / min, a shielding gas flow rate of 60L / min, and an angle of 13° between the shielding gas nozzle and the workpiece. The liquid nitrogen level in the cooling tank was 50%. The temperature at the outlet of the shielding gas nozzle was -68℃ as measured by a temperature gun. There was very little spatter during the welding process. Metallographic analysis of the weld microstructure showed a large number of equiaxed crystals and no obvious opposed columnar crystals. The cup-shaped crack point and crack direction were both on the base material. The tensile test showed that the fracture occurred on the base material, indicating good mechanical properties of the weld. Example 2

[0021] Welding of 8.0mm thick 443 stainless steel was performed using a 12kW laser, a welding speed of 2.0m / min, a shielding gas flow rate of 60L / min, a 14° angle between the shielding gas nozzle and the workpiece, and liquid nitrogen at 42% level in the cooling tank. Temperature readings from the shielding gas nozzle outlet were -53℃. Welding spatter was minimal. Metallographic analysis of the weld microstructure revealed numerous equiaxed crystals and no obvious opposing columnar crystals. Mechanical properties showed that the cup-shaped crack point and crack direction were both on the base material, and the tensile test fracture occurred on the base material, indicating good mechanical properties of the weld. Example 3

[0022] Welding of 3.0mm thick 430 stainless steel, laser power 10kw, welding speed 5m / min, shielding gas flow rate 60L / min, angle between shielding gas nozzle and workpiece 10°, liquid nitrogen level in cooling tank 59%, temperature measured by temperature gun at shielding gas nozzle outlet -78℃, minimal spatter during welding, metallographic analysis of weld microstructure shows abundant equiaxed crystals and no obvious opposed columnar crystals, mechanical properties cup crack point and crack direction are both on the base material, tensile test fracture point is on the base material, weld mechanical properties are good.

[0023] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A method for controlling columnar crystal formation in the molten pool of ferritic stainless steel based on high-power laser welding, characterized in that: Includes the following steps: Step 1: High-power low-speed welding: suitable for ferritic stainless steel with a thickness of 3.0-8.0mm, welding speed of 2-5m / min, and matched with corresponding laser power of 10-12kw; Step 2: Low-angle, high-flow-rate gas protection: The angle between the protective gas nozzle and the workpiece being welded is 10°-15°, and the gas flow rate is 70-80L / min; Step 3: Cryogenic shielding gas welding: Liquid nitrogen is used to cool the shielding gas, and helium gas at -50℃ to -80℃ is used to suppress the plasma and cool the upper surface of the molten pool, forming three-dimensional heat flow conditions in the molten pool. The temperature gradient of the molten pool changes from the original 2 to 3, and finally an elliptical welding molten pool is formed and a large number of equiaxed crystals are formed in the center of the weld. Step 3: Cryogenic shielded gas welding is achieved through a shielding helium cooling device. This device includes a liquid nitrogen cooling tank, a shielding gas nozzle, and a shielding gas cylinder. The liquid nitrogen cooling tank is a sealed container, and a cooling copper tube is placed inside it. The cooling copper tube is fixed to the top of the liquid nitrogen cooling tank by its internal thread fitting into the tank opening. The inlet and outlet of the cooling copper tube are connected to the shielding gas cylinder and the shielding gas nozzle through ventilation hoses A and B, respectively. The shielding gas nozzle is positioned directly below the laser head to weld the molten pool, enabling laser welding under cryogenic protection. The liquid nitrogen cooling tank contains 40%–60% liquid nitrogen. The cooling copper tube does not contact the bottom of the liquid nitrogen cooling tank. The inner diameter of the cooling copper tube is required to be 3mm. With a liquid nitrogen capacity of 40%–60%, a length of 18–25m of cooling copper tube is immersed in the liquid nitrogen to ensure that the shielding gas temperature at the nozzle outlet is between -50℃ and -80℃.

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