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Analysis on the weld defects on the low carbon steel pipeline

Carbon steel pipe

Encountered hole problems are very common in the welding process, welding materials drying, corrosion of the base metal and welding consumables, welding process is not stable enough oil and impurities and to protect the poor will be varying degrees of blowholes. Classification of weld porosity, the blowholes fundamental reason is due to the high temperature of the molten metal by dissolving a large amount of gas, the portion of the gas in the weld when the crystallization time to escape, the formation of pores.

In accordance with the generated gas composition, porosity in the weld metal after welding is mainly hydrogen the hole and CO stomatal. For low-carbon steel welding, hydrogen hole in the weld surface in the case of most of the cross-section of the pores screw-shaped, smooth wall loudspeaker mouth-shaped when viewed from the surface of the weld. Due to the high temperature, the hydrogen solubility in the molten bath and the metal droplet is high, rapid solidification of the weld metal in the welding process, hydrogen is too late to escape, is formed in the weld gas. CO generation of holes is mainly due to metallurgical reactions produce CO gas remaining in the crystallization inside the weld. Stomatal distribution along the crystal orientation has a worm-like appearance. Factors affecting gas pores are created metallurgy and workmanship. The major impact of the metallurgical aspects of the metallurgical reaction of the welding material in the welding process, the nature of the protective atmosphere, the material to be welded before welding original condition, for example, the impact of moisture, corrosion of the weld porosity generation. Welding process as a metallurgical process for having characteristics different from the ordinary metallurgical, short reaction time and protection Atmosphere poor city so that the weld metal and the base metal material, organization microscopic difference exists. The oxidation of the slag stomatal sensitivity of the resistance of the weld has a great influence, experiments show that both acidic and basic electrode CO stomatal tendency increases with increasing slag oxidation resistance. But the tendency of the hydrogen hole to the contrary. Than the CO due to the hydrogen in the metal structure of Compliance Compliance so often taken suppressed in order to achieve to maintain slag appropriate oxidizing hydrogen. It is also used in the actual welding production such methods. Added with strong oxidizing acid electrode composition. Hydrogen generation of the hole can be prevented. The alkaline electrode was added fluorite, often contain a certain amount of carbonate. During the welding process, the welding arc heating effect of Fe203 decomposition emits O generated Fe30 Fe | 0 HO reaction at a high temperature to regenerate Fe20, and H, elemental Fe and HO FeO and H is generated at high temperatures, can be seen rust for the formation of two types of pores have a greater impact. Normal welding operation welding specification for the formation of weld porosity have a certain impact, but to take appropriate specifications for operation, welding current, welding voltage have little impact on the formation of pores in the low-carbon steel welding.

In the low-carbon steel welding, the weld fusion zone, inclusions, will cause a decrease in toughness of the weld metal, increasing the tendency to hot cracking and lamellar tearing. Inclusions in the weld mainly oxides, nitrides and sulfides. Oxide inclusions and more generally exist in the form of a silicate, silica; oxide inclusions generated mainly metallurgical reactions due to the welding process, the actual welding process in the welding material and select the appropriate circumstances due to improper welding operation mixed the weld oxide inclusions is a small amount of. Nitride mixed mainly N in the low-carbon steel welding, the Fe4N welding metal precipitates during aging, needle-like form distributed in the grain or through grain boundaries, resulting in the decline in the toughness of the weld metal. Low carbon steel welding, a source of nitrogen for the surrounding atmosphere, i.e. from the surrounding atmosphere penetrated weld puddle while nitrogen gas was caused by inclusions and poor protection of the welding process is mainly due to the formation of nitride inclusions. Sulfide sulfur in mostly comes mainly from welding materials, such as welding rod coating, flux, weld sulfide MnS and FeS. Weld inclusions cause actual welding production generally: the selection of a welding incorrect or unreasonable; welding specifications inappropriate, such as welding current, welding voltage inappropriate resulting in slag is difficult to go up; tack welding and multi- welding, the weld is not clean slag; welding, the angle of the electrode and the transport bar inappropriate molten metal and slag mixed together; the poor weld pool protection operation, the air intrusion welding protective atmosphere.

Technical measures to prevent low-carbon steel pipe weld defects

Selection of welding materials directly affect the performance and the composition of the weld metal, which has a very important significance for the prevention and control of welding defects. Process factors include aspects of the welding specifications, type of current and welding operation. The appropriate choice of welding power supply for welding materials is a prerequisite for the prevention of welding defects. Welding equipment replacement very quickly, due to the rapid development of electronic technology, welding power source is also undergoing a profound transformation. For ordinary low carbon steel pipe welding materials, the ordinary welding equipment to construction. But the good performance of the device operation caused can not be ignored. Must prepare a reasonable premise equipment conditions have targeted the welding process measures. Including welding specifications, type of current, welding operations, groove and layer clean process requirements. Welding specifications include welding current, welding voltage, welding speed and other parameters. Welding operations should be carried out in the normal specification. In the case of the welding voltage. The welding current increases will lead to the increase of the depth of penetration of the molten bath; in the case of welding current, welding voltage increases will lead to the increase in the width of the bath melt; but the current increases. Result in the arc transition molten metal particles decreases. Ratio of the surface area increases, the gas absorbed by the transition of molten metal particles Multiple increased stomatal tendency; arc voltage caused by increased the welding protective atmosphere decreased, outside air easily penetrated; welding speed increases, so that the crystallization speed is increased, but also easy to make the residual gas The increased tendency to cause holes in the weld metal. Therefore, as far as possible to maintain the normal construction norms conducive to the generation of defects.


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