Graduation thesis of ship welding-on the causes and preventive measures of common defects in ship welding
Abstract: Ship welding is the key to ensure the tightness and strength of the ship. This paper introduces the causes of several common defects in ship welding in detail, and puts forward preventive measures.
Keywords: preventive measures for welding defects of ships
Ship welding is the key to ensure the tightness and strength of the ship, the key to ensure the quality of the ship and the important condition to ensure the safe navigation and operation of the ship. If there are defects in welding, the structure may break, leak or even sink. According to the investigation of ship brittle fracture accidents, 40% brittle fracture accidents began with weld defects. In township shipbuilding, the welding quality of ships is particularly prominent. In the process of ship inspection, the inspection of weld is particularly important. Therefore, the defects should be found as soon as possible and the welding defects should be limited to a certain range to ensure the safety of navigation.
There are many kinds of welding defects in ships, which can be divided into external defects and internal defects according to different positions. Common defects include blowhole, slag inclusion, welding crack, incomplete penetration, incomplete fusion, unsatisfactory weld shape and size, undercut, flash, arc pit, etc.
I. Stomata
Porosity refers to the void formed by the failure of bubbles in the molten pool to escape during solidification during welding. The main reasons for porosity are: the edge of the groove is not clean, with water, oil and rust; Covered electrode or flux is not baked according to regulations, core corrosion or coating deterioration and peeling. In addition, when welding low hydrogen covered electrode, the arc is too long and the welding speed is too fast; The voltage of submerged arc automatic welding is too high, which is easy to produce blowholes in the welding process. Because of the existence of air holes, the effective cross section of weld is reduced, and too many air holes will reduce the strength of weld and destroy the compactness of weld metal. The way to prevent blowholes is to choose proper welding current and welding speed, and carefully clean up the moisture, oil stain and rust on the edge of the groove. Keep, clean and bake welding materials in strict accordance with regulations. Don't use spoiled covered electrode. When covered electrode coating is found to be deteriorated, peeled off or corroded, the application scope should be strictly controlled. When submerged arc welding, appropriate welding process parameters should be selected, especially the automatic welding of thin plates, and the welding speed should be as low as possible.
Second, slag inclusion
Slag inclusion is the slag remaining in the weld. Slag inclusion will also reduce the strength and compactness of weld. The main reason of slag inclusion is the slag left at the weld edge by oxygen cutting or carbon arc gouging. The groove angle or welding current is too small, or the welding speed is too fast. When acid covered electrode is used, "paste residue" will be formed due to too small current or improper transportation; When using alkaline covered electrode, slag inclusion will also occur due to too long arc or incorrect polarity. When submerged arc welding is used to seal the bottom, the welding wire deviates from the center of the weld, which is also easy to form slag inclusion. The measures to prevent slag inclusion are: correctly selecting the groove size, carefully cleaning the groove edge, selecting the appropriate welding current and welding speed, and swinging the bar appropriately. During multi-layer welding, the melting situation on both sides of the groove should be carefully observed, and the welding slag should be carefully cleaned for each welding layer. Welding slag for back cover should be thoroughly removed, and welding deviation should be prevented during submerged arc welding.
Third, bite the edge
The depression left by the welding edge is called undercut. The causes of undercut are too large welding current, too fast rod feeding speed, too long arc drawing or improper covered electrode angle. The welding speed of submerged arc welding is too fast or the welding trajectory is not equal, which will cause the weldment to melt to a certain depth, and the filler metal can not be filled in time, resulting in undercut. Undercutting reduces the working section of the base metal joint, resulting in stress concentration at the undercut, so it is generally not allowed to exist in important structures or structures under dynamic load, or the undercut depth is limited. The methods to prevent undercut are as follows: selecting appropriate welding current and tape feeding mode, and paying attention to controlling covered electrode angle and arc length at any time; Submerged arc welding process parameters should be appropriate, especially the welding speed should not be too high and the welding trajectory should be flat.
Four. Incomplete infiltration and fusion
When welding, the phenomenon of incomplete penetration at the root of the joint is called incomplete penetration; There is local incomplete fusion between weldment and weld metal or weld layer, which is called incomplete fusion. Incomplete penetration or incomplete fusion is a serious defect. Due to incomplete penetration or fusion, the weld will be discontinuous or abrupt, the weld strength will be greatly reduced, and even cracks will occur. Therefore, incomplete penetration and fusion are not allowed in the important structural parts of the hull. The reasons for incomplete penetration and fusion are that the assembly gap or bevel angle is too small, the blunt edge is too thick, the covered electrode diameter is too large, the current is too small, the speed is too fast and the arc is too long. If the oxide film and oil stain on the weld groove surface are not cleaned up, or the slag flowing into it hinders the fusion between metals during welding, or the arc deviates to the groove side, the edge will not fuse. To prevent incomplete penetration or fusion, it is necessary to correctly select the groove size, reasonably select the welding current and speed, and remove the oxide scale and oil stain on the groove surface; The back cover welding shall be thoroughly cleaned and the rod shall swing properly. Pay close attention to the fusion on both sides of the groove.
Verb (abbreviation of verb) welding crack
Welding crack is a very serious defect. The damage of structures mostly begins with cracks. All necessary measures should be taken to prevent cracks during welding, and various methods should be used to check whether there are cracks after welding. Once cracks are found, they should be thoroughly removed and then repaired.
Welding cracks include hot cracks and cold cracks. The cracks produced in the process of crystallization of weld metal from liquid to solid are called hot cracks, which are characterized by being visible immediately after welding, mostly occurring in the center of weld and distributed along the length of weld. Most of the thermal cracks run through the surface, showing oxidation color, and the crack ends are slightly round. The reason of hot cracking is that there are low melting point impurities (such as FeS) in the welding pool. Because of the low melting point of these impurities, crystallization and solidification are the latest, and the plasticity and strength after solidification are extremely low. Therefore, under the constraint stress of external structure and solidification shrinkage of weld metal, these low melting point impurities in the molten pool are pulled away during solidification or shortly after solidification, resulting in intergranular cracks. When the weldment and covered electrode contain more impurities such as sulfur and copper, it is easy to produce hot cracks. The measures to prevent hot cracks are as follows: first, strictly control the welding process parameters, slow down the cooling rate, appropriately improve the weld shape coefficient, and adopt low-current multi-layer and multi-pass welding as far as possible to avoid cracks in the weld center; Second, seriously implement the process specification, choose a reasonable welding process, and reduce the welding stress.
During or after the cooling of the welding metal, the cracks produced on the fusion line between the base metal or the base metal and the weld are called cold cracks. This kind of crack may appear immediately after welding, or it may appear hours, days or even longer after welding. The main causes of cold cracks are: 1) under the action of welding thermal cycle, hardened structure is formed in hot zone; 2) There is excessive diffusion of hydrogen in the weld, and there are concentrated conditions; 3) The joint bears large restraint stress. The measures to prevent cold cracking are: 1) choose low hydrogen covered electrode to reduce the content of diffused hydrogen in the weld; 2) Strictly abide by the system of storage, baking and use of welding materials (covered electrode, flux) to prevent from being affected with damp; 3) Carefully clean the oil, moisture and rust on the edge of the groove to reduce the source of hydrogen; 4) According to material grade, carbon equivalent, component thickness, welding environment, etc. Choose reasonable welding process parameters and line energy, such as preheating before welding, slow cooling after welding, multi-layer and multi-pass welding, and controlling certain interlayer temperature. 5) emergency post-heat treatment to remove hydrogen, internal stress and temper hardening structure, thus improving the toughness of the joint; 6) Adopt reasonable welding technology and sectional welding withdrawal method to reduce welding stress.
Six, other defects
In welding, some defects such as flash, arc pit and weld shape and size are also common. The main cause of flash is uneven bar conveying, which leads to high molten pool temperature and slow drop of liquid metal solidification, thus forming metal flash on the weld surface. Excessive welding current and arc length are used in vertical welding and overhead welding, and flash may also occur. The reason of arc crater is that the arc extinguishing time is too short, or the welding is suddenly interrupted, or the current is too large when welding the thin plate. The appearance of weld surface is affected by flash, which is easy to cause slag inclusion on the surface; Arc pits are often accompanied by cracks and blowholes, which seriously weaken the welding strength. The main measure to prevent flash is to strictly control the temperature of molten pool. During vertical welding and overhead welding, the welding current should be less than that of flat welding 10- 15%. When alkaline covered electrode is used, short arc welding should be adopted to ensure uniform transportation of steel bars. The main measure to prevent the occurrence of arc pits is that when the arc is closed by manual welding, covered electrode should stay for a short time or make several round bars.
The existence of some defects is very dangerous to the safe navigation of ships, so once they are found, they should be corrected in time. For the correction of blowholes, especially internal blowholes, after the position is determined, all blowhole defects are removed by air shovel or carbon arc gouging, and corresponding grooves are formed, and then welding repair is carried out; For slag inclusion, incomplete penetration and incomplete fusion, it is also necessary to remove the defects in the same way first, and then carry out welding repair according to regulations. For cracks, before eliminating defects, the starting point and ending point of cracks and the depth of cracks should be carefully checked. When eliminating crack defects with air shovel, crack stops should be drilled at both ends of the crack to prevent the crack from spreading. 8 ~ 12 mm bit should be used when drilling, and the depth should be 2 ~ 3 mm greater than the crack depth. When carbon arc gouging is used to eliminate cracks, gouging should be started from both ends of the cracks until the cracks are eliminated, and then the whole cracks should be gouged out. No matter what method is used to eliminate the crack defect, the corresponding groove should be formed and repaired by welding according to the regulations.
Attention should be paid to the correction of weld defects: 1) When repairing welding defects, small current, no swing, multi-layer and multi-pass welding should be adopted, and excessive current repair welding is prohibited; 2) When repairing the rigid structure, except for the first layer and the last layer of weld bead, it can be hammered in the hot state after welding. The arc starting and closing of each weld bead should be staggered as far as possible; 3) For materials that need preheating, when the working environment temperature is lower than 0℃, corresponding preheating measures should be taken; 4) For weldments requiring heat treatment, defects shall be corrected before heat treatment; 5) For the weld defects of D and E grade steel and high strength structural steel, when manual arc welding is used to repair them, the control line energy welding method should be adopted. Each defect should be repaired by welding at one time, and it is not allowed to stop halfway. The preheating temperature and interlayer temperature should be kept above 60℃. 6) Welding repair under pressure and backwater is not allowed to eliminate weld defects; 7) The modified weld shall be reinspected according to the flaw detection requirements of the original weld. If defects exceeding the allowable limit are found again, they should be corrected again until they are qualified. The number of welding repairs shall not exceed the specified number of repairs.
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