Welding deformation refers to the phenomenon where the shape and size of the welded workpiece change due to uneven heat input and output during the welding process. Welding deformation not only affects welding quality and appearance but also reduces the strength and stiffness of welded structures, and may even lead to defects such as cracks and delamination. Therefore, understanding the causes and classification of welding deformation, mastering methods to eliminate or reduce welding deformation, is of significant importance for improving welding efficiency and ensuring welding performance.
I. Causes and Classification of Welding Deformation
The generation of welding deformation is mainly due to the following three factors:
Welding Temperature Field: During the welding process, the welded workpiece is subjected to the action of high-temperature heat sources, resulting in temperature gradients and thermal cycles. These temperature changes cause thermal expansion and contraction and phase changes in the welded workpiece, leading to different degrees of shrinkage in different parts, resulting in internal stress and deformation.
Weld Seam Contraction: During the welding process, the molten metal undergoes volume contraction as it solidifies. This contraction causes the molten metal to exert tension on the surrounding base metal, leading to movement of the base metal towards the weld seam and causing deformation.
Structural Stiffness: During the welding process, the welded workpiece is constrained or supported by the surrounding environment or other components, preventing it from freely contracting or expanding. This constraint or support affects the stiffness of the welded workpiece, i.e., its ability to resist deformation. Generally, the greater the structural stiffness, the smaller the deformation; the smaller the structural stiffness, the greater the deformation.
According to whether the deformation occurs within the plane or outside the plane, and the direction and form of the deformation, welding deformation can be classified into the following types:
Longitudinal Shrinkage Deformation: Refers to shrinkage deformation occurring along the length of the weld seam. Longitudinal shrinkage deformation mainly depends on the length and cross-sectional area of the weld seam and structural stiffness. Generally, longitudinal shrinkage increases with the increase of weld seam length and cross-sectional area and decreases with the increase of structural stiffness.
Transverse Shrinkage Deformation: Refers to shrinkage deformation occurring perpendicular to the direction of the weld seam. Transverse shrinkage deformation mainly depends on heat input, plate thickness, and groove angle. Generally, transverse shrinkage increases with the increase of heat input and plate thickness and decreases with the increase of groove angle.
Angular Deformation: Refers to torsional deformation caused by uneven transverse shrinkage along the plate thickness direction or improper assembly or welding sequence, leading to irregular shrinkage of the weld seam. Angular deformation mainly depends on groove type, number of layers, and sequence. Generally, angular deformation increases with the increase of groove depth, number of layers, and sequence.
Bending Deformation: Refers to bending deformation caused by shrinkage of the weld seam as a result of the asymmetric welding seam arrangement or asymmetric cross-sectional shape of the welded workpiece. Bending deformation mainly depends on the position, length, and direction of the weld seam. Generally, bending deformation increases with the increase of distance from the neutral axis, length, and angle of the weld seam.
Torsional Deformation: Refers to the twisting deformation of the component caused by uneven angular deformation of the weld seam along the length direction or improper assembly or welding sequence, leading to irregular shrinkage of the weld seam. Torsional deformation mainly depends on the symmetry, stiffness, and welding sequence of the structure. Generally, torsional deformation increases with the asymmetry of the structure, decrease in stiffness, and improper welding sequence.
Wave Deformation: Refers to the wave-like deformation caused by relatively low structural stiffness, resulting in significant compressive stress under the combined effect of longitudinal and transverse shrinkage of the weld seam. Wave deformation mainly occurs in thin plates or areas with dense weld seams. Generally, wave deformation increases with decreasing plate thickness and increasing weld seam density.
II. Hazards of Welding Deformation
Welding deformation can cause several hazards to welded products:
Reduced Assembly Quality: Welding deformation leads to deviations in the dimensions and shapes of welded workpieces from design requirements, resulting in problems such as excessive or insufficient clearances, misalignment, or overlap during assembly, affecting assembly accuracy and stability.
Impact on Appearance Quality: Welding deformation results in surface defects such as unevenness and distortion, affecting the aesthetics and integrity of products.
Reduced Load-Bearing Capacity: Welding deformation changes the geometric dimensions of welded workpieces, altering their stress states and affecting their ability to withstand loads. Especially for structures subjected to bending or torsion, the decrease in sectional performance due to bending or torsional deformation reduces their ability to resist bending or torsion moments.
Increased Correction Processes: To eliminate or reduce welding deformation, corresponding correction measures such as mechanical correction and thermal correction must be taken. These correction processes not only increase labor and cost but also affect the quality and performance of products.
Increased Manufacturing Costs: Welding deformation reduces product quality, necessitating rework or scrapping, resulting in material and labor waste. Moreover, to prevent or reduce welding deformation, corresponding control measures such as providing allowances, anti-deformation measures, and rigid fixation are required. These measures also increase material and equipment input, raising manufacturing costs.
III. Methods to Eliminate or Reduce Welding Deformation
To eliminate or reduce welding deformation, appropriate control measures can be taken from the following three aspects:
1) Design Measures: Consider the effects of welding deformation during the design stage.
Design measures consider the effects of welding deformation during the design stage, selecting reasonable structural forms, materials, dimensions, weld types, quantities, positions, etc., to minimize the possibility and degree of welding deformation. Specific design measures include:
Increasing Structural Stiffness: Structural stiffness refers to the ability of a structure to resist deformation. Generally, the greater the structural stiffness, the smaller the welding deformation; the smaller the structural stiffness, the greater the welding deformation. Therefore, efforts should be made to increase structural stiffness during design, such as adding stiffeners, strengthening ribs, or using box-type structures.
Reducing Weld Seam Size: Weld seam size refers to the width, depth, and length of the weld seam. Generally, larger weld seam sizes result in more heat input and larger welding deformation, while smaller weld seam sizes result in less heat input and smaller welding deformation. Therefore, weld seam sizes should be minimized while ensuring welding quality and strength, such as using smaller groove angles, clearances, and layers.
Reducing Weld Seam Quantity: Weld seam quantity refers to the number of weld seams that need to be welded in the same structure. Generally, more weld seams result in more heat input and greater thermal stress and deformation, while fewer weld seams result in less heat input and smaller thermal stress and deformation. Therefore, the number of weld seams should be minimized while ensuring structural integrity and load-bearing capacity, such as using thicker plates, larger sections, or simpler shapes.
Optimizing Weld Seam Positions: Weld seam position refers to the distance and direction of the weld seam relative to the structure's central axis or neutral plane. Generally, weld seams closer to the central axis or neutral plane result in better symmetry and balance and smaller welding deformation, while weld seams farther from the central axis or neutral plane result in poorer symmetry and balance and larger welding deformation. Therefore, efforts should be made to position weld seams closer to the central axis or neutral plane while maintaining symmetry and balance.
2) Process Measures: Adopt appropriate methods to control deformation during assembly and welding processes.
Process measures adopt appropriate methods during assembly and welding processes to control deformation, reducing or eliminating shrinkage and distortion caused by uneven temperature fields. Specific process measures include:
Selecting Proper Assembly Sequence: The assembly sequence refers to the order in which various components are assembled into a whole. Generally, the assembly sequence should follow principles such as symmetry before asymmetry, simplicity before complexity, freedom before constraint, and small before large. This can reduce assembly errors and cumulative errors, improving assembly accuracy and quality.
Selecting Proper Welding Sequence: The welding sequence refers to the sequence and direction in which various weld seams are welded. Generally, the welding sequence should follow principles such as butt welding before fillet welding, center welding before perimeter welding, symmetry welding before asymmetry welding, segmented welding before continuous welding, and skip welding before return welding. This can reduce the accumulation of thermal stress and deformation, improving welding quality and efficiency.
Selecting Proper Welding Parameters: Welding parameters refer to various factors affecting the welding process, such as current, voltage, speed, polarity, gas flow rate, etc. Generally, welding parameters should aim to minimize heat input, maximize thermal efficiency, and achieve uniform heat distribution. This can reduce the range and depth of the heat-affected zone and reduce the size of thermal stress and deformation.
Pre-add reverse deformation method: Pre-add reverse deformation method involves giving the welded workpiece a certain amount of deformation in reverse direction before welding based on experience or calculation, so that the deformation after welding is offset or compensated for by the pre-added deformation. This method is suitable for structures with certain stiffness and elasticity, such as box structures or frame structures.
Rigid Fixation Method: The rigid fixation method involves firmly fixing the welded workpiece in a certain position and shape using jigs or other devices before welding, preventing it from freely contracting or expanding during welding. This method is suitable for structures with small stiffness or large degrees of freedom, such as thin plate structures or pipe structures.
Hammering Method: The hammering method involves lightly and densely tapping the weld seam and its surrounding area with a round-headed hammer during or after welding while the weld is still hot, inducing plastic deformation and ductile contraction, thereby reducing or eliminating residual stress and deformation. This method is suitable for materials with good ductility, such as low-carbon steel or cast iron.
3) Correction Measures: Employ necessary methods to restore deformation after welding.
Correction measures involve employing necessary methods to restore deformation after welding, ensuring that the welded workpiece meets design or usage requirements. Specific correction measures include:
Mechanical Correction Method: The mechanical correction method involves using mechanical force to stretch, compress, bend, or twist the welded workpiece, inducing plastic deformation or elastic recovery, thereby eliminating or reducing deformation. This method is suitable for structures with high stiffness or small deformation, such as steel beams or steel columns.
Thermal Correction Method: The thermal correction method involves using localized heating or overall heating to treat the welded workpiece, inducing uneven shrinkage or phase change shrinkage, thereby eliminating or reducing deformation. This method is suitable for structures with high stiffness or large deformation, such as steel bridges, steel towers, or steel box girders. Specific thermal correction methods include:
Localized Heating Method: The localized heating method involves locally heating the concave part or symmetrical surface of the welded workpiece using flames or electric currents, inducing uneven shrinkage to offset or reduce deformation. This method requires control of heating temperature and time, with heating temperature generally not exceeding 600°C and heating time not being too long to avoid affecting material performance and structural stability. After heating, timely cooling should be applied to prevent re-deformation. The advantage of the localized heating method is its simplicity and effectiveness, while its disadvantage is the potential for localized stress concentration and crack formation.
Overall Heating Method: The overall heating method involves uniformly heating the entire welded workpiece or the main stress-bearing part in a heating furnace, inducing phase change shrinkage to eliminate or reduce deformation. This method requires selection of heating temperature and insulation time, with heating temperature generally being lower than the critical temperature, and insulation time being determined based on the material's phase change rate and thickness. After heating, slow cooling should be applied to prevent residual stress and new deformation. The advantage of the overall heating method is its ability to eliminate large deformations and residual stresses, while its disadvantage is the high equipment requirements, cost, and potential impact on material microstructure and properties.
Flame Correction Method: The flame correction method utilizes the linear expansion coefficient of steel at 1.2×10⁵/℃ after heating to extend in all directions. When cooled to the original temperature, in addition to shrinking to the original unheated length, further shrinkage occurs at a rate of 1.48×10⁶/℃, resulting in shorter length after shrinkage compared to the original length. This property is utilized by heating the appropriate parts of the deformed component with a flame, followed by cooling, resulting in significant shrinkage stress after cooling, achieving the purpose of correcting deformation. This method requires control of parameters such as flame movement speed, direction, distance, and temperature, with flame temperature generally being 600-700°C, and movement speed being 3-5 seconds/point. The advantage of the flame correction method is its ability to correct complex shapes and large-sized structures, while its disadvantage is the possibility of local overheating, overcooling, and cracking issues.
In conclusion, welding deformation is an unavoidable phenomenon that can impact the quality, performance, and cost of welded products. Therefore, effective control measures should be taken from three aspects: design, process, and correction, to eliminate or reduce the occurrence of welding deformation. Moreover, suitable methods and parameters should be selected based on different materials, structures, and conditions to ensure welding safety and reliability.
