Welding in shipbuilding is one of the crucial processes directly impacting a vessel's structural strength, safety, and durability. However, welding defects inevitably occur during ship welding processes, such as porosity, slag inclusion, lack of fusion, cracks, etc. These defects degrade the quality of welding, potentially leading to welding failures and maritime accidents. Therefore, understanding the mechanisms behind ship welding defects and implementing effective preventive and corrective measures are vital tasks for enhancing welding standards and ensuring maritime safety. This article introduces common types of welding defects in shipbuilding, their causes, consequences, and corresponding solutions, providing reference and guidance for ship welding engineers and managers.
I. Types and Causes of Welding Defects in Ships
Welding defects in ships refer to discontinuities, non-uniformities, or non-compliance phenomena in welded joints or base materials resulting from various factors during or after welding. Based on the nature, location, and morphology of defects, welding defects in ships can be categorized into the following types:
1）Porosity:
Porosity refers to gas cavities remaining in the weld seam, typically appearing as round or elliptical voids of various sizes and uneven distribution. The formation of porosity is mainly due to excessive hydrogen, oxygen, nitrogen, or other gases in welding materials, base metals, or welding environments. Poor gas shielding during welding can trap gases in the molten pool, leading to porosity. Porosity reduces the effective cross-sectional area of the weld, affecting its strength, toughness, and serving as crack initiation sites, leading to brittle fracture of the weld.
2）Slag Inclusion:
Slag inclusion refers to solid or semi-solid non-metallic impurities remaining in the weld seam, typically appearing as linear, planar, or punctate entities with darker color distinctly different from metal. The formation of slag inclusion is mainly due to excessive oxides, sulfides, nitrides, or other impurities in welding materials, base metals, or welding environments. Inadequate slag removal during welding can result in impurities being trapped in the molten pool. Slag inclusion reduces the density and uniformity of the weld, affecting its strength, toughness, and serving as crack initiation sites, leading to brittle fracture of the weld.
3）Lack of Fusion:
Lack of fusion refers to incomplete or insufficient fusion between the weld and the base metal or the previous weld layer, resulting in discontinuous or inadequate connection. Lack of fusion typically appears as linear or planar features with indefinite positions and distinct boundaries with metal. The formation of lack of fusion is mainly due to factors such as insufficient welding current, excessive welding speed, inappropriate welding angle, inadequate assembly clearance, and mismatched welding materials, preventing the molten pool from fully penetrating or diffusing into the base metal or the previous weld layer. Lack of fusion reduces the effective bonding area of the weld, affecting its strength and stiffness, and serving as stress concentration areas, leading to fatigue fracture of the weld.
4）Cracks:
Cracks refer to elongated fissures formed in the weld or base metal, typically appearing as straight or curved lines of varying lengths and indefinite directions. The formation of cracks is mainly due to thermal stresses, structural stresses, or residual stresses exceeding the material's strength limit during or after welding, leading to material fracture. Cracks are the most dangerous welding defects, severely reducing the strength and toughness of the weld, rapidly propagating, leading to brittle fracture or delayed fracture of the weld.
II. Remedies for Welding Defects in Ships
The remedies for welding defects in ships primarily involve preventive and corrective measures. Prevention refers to taking measures before, during, and after welding to avoid or reduce the occurrence of welding defects, while correction refers to taking measures to eliminate or mitigate the effects of welding defects after their discovery. The preventive methods for various welding defects in ships are as follows:
1）Prevention of Porosity:
Choose appropriate welding materials, control the hydrogen content and humidity of welding materials, avoid using moist or contaminated welding materials, and perform drying or deoxidation treatment if necessary.
Clean the surface of the base metal thoroughly, remove oil, rust, water stains, and other impurities, avoid using moist or contaminated base metals, and perform preheating or deoxidation treatment if necessary.
Select suitable welding methods, control welding parameters such as current, voltage, welding speed, welding angle, etc., avoid using excessively large or small current, excessively fast or slow welding speed, excessively large or small welding angle, etc.
Choose appropriate gas shielding, control the flow rate, pressure, purity of the gas, avoid using excessive or insufficient gas, excessive or low pressure, excessive or low purity, etc.
Pay attention to environmental factors such as airflow, temperature, humidity, etc., during welding, avoid welding in harsh environments such as wind, rain, snow, frost, etc., and take measures such as shielding or heating if necessary.
2）Prevention of Slag Inclusion:
Choose appropriate welding materials, control the content of oxides, sulfides, nitrides, and other impurities in welding materials, avoid using materials with excessive impurities, and perform drying or deoxidation treatment if necessary.
Clean the surface of the base metal thoroughly, remove oil, rust, water stains, and other impurities, avoid using base metals with excessive impurities, and perform preheating or deoxidation treatment if necessary.
Select suitable welding methods, control welding parameters such as current, voltage, welding speed, welding angle, etc., avoid using excessively large or small current, excessively fast or slow welding speed, excessively large or small welding angle, etc., so that the molten pool can be fully stirred and impurities removed.
Clean the slag in a timely manner, avoid slag residue in or near the weld, affecting the quality of the next layer of welding.
Pay attention to environmental factors such as airflow, temperature, humidity, etc., during welding, avoid welding in harsh environments such as wind, rain, snow, frost, etc., and take measures such as shielding or heating if necessary.
3）Prevention of Lack of Fusion:
Choose appropriate welding materials, control the melting point, coefficient of thermal expansion, thermal conductivity, and other physical properties of welding materials, avoid using welding materials incompatible with the base metal, causing uneven or unstable molten pools.
Clean the surface of the base metal thoroughly, remove oil, rust, water stains, and other impurities, avoid using base metals with excessive impurities, affecting the penetration or diffusion of the molten pool.
Select suitable welding methods, control welding parameters such as current, voltage, welding speed, welding angle, etc., avoid using excessively large or small current, excessively fast or slow welding speed, excessively large or small welding angle, etc., so that the molten pool can fully penetrate or diffuse into the base metal or the previous weld layer.
Choose appropriate assembly methods, control assembly clearances, misalignment, joint angles, etc., avoid using excessively small or large assembly clearances, excessively large or small misalignment, excessively large or small joint angles, causing incomplete or inadequate molten pools.
Pay attention to welding sequence, direction, posture, etc., during welding, avoid using unreasonable welding sequences, causing the accumulation or release of welding stress, affecting the formation and distribution of the molten pool.
；Avoid using inappropriate welding directions, causing the flow or stagnation of the molten pool, affecting the penetration or diffusion of the molten pool.
Avoid using incorrect welding postures, causing offset or oscillation of the welding gun or electrode, affecting the stability and uniformity of the molten pool.
4）Prevention of Cracks:
Choose appropriate welding materials, control the chemical composition, mechanical properties, metallographic structure, etc., of welding materials, avoid using welding materials containing excessive carbon, sulfur, phosphorus, and other harmful elements, causing embrittlement or hardening of the weld.
Avoid using welding materials with excessive alloying elements, which can cause phase transformation or precipitation in the weld seam, affecting the toughness or ductility of the weld.
Clean the surface of the base material thoroughly, removing oil, rust, water stains, and other impurities. Avoid using base materials with excessive carbon, sulfur, phosphorus, and other harmful elements, which can cause embrittlement or hardening of the base material.
Avoid using base materials with excessive alloying elements, which can cause phase transformation or precipitation in the base material, affecting the toughness or ductility of the base material.
Select appropriate welding methods and control welding parameters such as current, voltage, welding speed, welding angle, etc. Avoid using excessively high or low current, excessively fast or slow welding speed, excessively large or small welding angle, etc., which can cause overheating or chilling of the weld or base material, affecting the thermal stress or structural stress of the weld or base material.
Choose suitable preheating and post-heating temperatures, and control the preheating and post-heating time properly. Avoid using excessively high or low preheating and post-heating temperatures, excessively long or short preheating and post-heating times, which can cause thermal or cold cracking of the weld or base material.
Pay attention to welding sequence, direction, posture, etc., during welding. Avoid using unreasonable welding sequences, causing the accumulation or release of welding stress, affecting the residual stress of the weld or base material.
void using inappropriate welding directions, Acausing uneven contraction of the weld or base material, affecting the deformation or cracking of the weld or base material.
Avoid using incorrect welding postures, causing offset or oscillation of the welding gun or electrode, affecting the melting or solidification of the weld or base material.
Remedies for various welding defects in ships are as follows:
1）Repair of Porosity:
Depending on the size, quantity, and location of porosity, choose appropriate repair methods such as grinding, rewelding, welding filling, etc. Grinding refers to using tools such as grinding wheels or grinding heads to grind off the part of the weld seam where the porosity is located, making the surface of the weld seam smooth. This method is suitable for situations where porosity is small, few, and surface-oriented. Rewelding refers to using tools such as welding guns or electrodes to reweld the part of the weld seam where the porosity is located, making the weld seam full. This method is suitable for situations where porosity is large, many, and deep-seated. Welding filling refers to using tools such as welding wire or welding rods to fill the part of the weld seam where the porosity is located, making the weld seam smooth. This method is suitable for situations where porosity is small, few, and deep-seated.
2）Repair of Slag Inclusion:
Depending on the size, quantity, and location of slag inclusion, choose appropriate repair methods such as grinding, rewelding, welding filling, etc. Grinding refers to using tools such as grinding wheels or grinding heads to grind off the part of the weld seam where the slag inclusion is located, making the surface of the weld seam smooth. This method is suitable for situations where slag inclusion is small, few, and surface-oriented. Rewelding refers to using tools such as welding guns or electrodes to reweld the part of the weld seam where the slag inclusion is located, making the weld seam smooth. This method is suitable for situations where slag inclusion is large, many, and deep-seated. Welding filling refers to using tools such as welding wire or welding rods to fill the part of the weld seam where the slag inclusion is located, making the weld seam smooth. This method is suitable for situations where slag inclusion is small, few, and deep-seated.
3）Repair of Lack of Fusion:
Depending on the size, quantity, and location of lack of fusion, choose appropriate repair methods such as grinding, rewelding, welding filling, etc. Grinding refers to using tools such as grinding wheels or grinding heads to grind off the part of the weld seam where the lack of fusion is located, making the surface of the weld seam smooth. This method is suitable for situations where lack of fusion is small, few, and surface-oriented. Rewelding refers to using tools such as welding guns or electrodes to reweld the part of the weld seam where the lack of fusion is located, making the weld seam full. This method is suitable for situations where lack of fusion is large, many, and deep-seated. Welding filling refers to using tools such as welding wire or welding rods to fill the part of the weld seam where the lack of fusion is located, making the weld seam smooth. This method is suitable for situations where lack of fusion is small, few, and deep-seated.
4）Repair of Cracks:
Depending on the size, quantity, and location of the crack, choose appropriate repair methods such as grinding, rewelding, welding filling, etc. Grinding refers to using tools such as grinding wheels or grinding heads to grind off the part of the weld or base metal where the crack is located, making the surface of the weld or base metal smooth. This method is suitable for situations where cracks are small, few, and surface-oriented. Rewelding refers to using tools such as welding guns or electrodes to reweld the part of the weld or base metal where the crack is located, making the weld or base metal full. This method is suitable for situations where cracks are large, many, and deep-seated. Welding filling refers to using tools such as welding wire or welding rods to fill the part of the weld or base metal where the crack is located, making the weld or base metal smooth. This method is suitable for situations where cracks are small, few, and deep-seated.
III. Conclusion
Welding defects in ships are inevitable problems in shipbuilding, affecting the structural strength, safety, and durability of vessels. Therefore, ship welding engineers and managers should pay attention to the prevention and repair of welding defects in ships, take effective measures to improve the quality and standards of ship welding. This article introduces common types of welding defects in shipbuilding, their causes, and solutions, hoping to be helpful to ship welding engineers and managers.
