Today, the vast majority of engineering steel structures are created by welding. Welding is an essential piece of steel pipeline engineering and is viewed as the essential joining technique utilized in gas and oil pipelines. It has for some time been the utmost well-known and efficient method of metal joining. The oil and gas pipelines are made by utilizing combination welding processes and a consistent pipe shaping interaction. The generally used welding technique is submerged arc welding (SAW). SAW is preferred over different methods for pipe welding, including fuel tanks, due to its intrinsic characteristics like simple process control factors, superior quality, profound infiltration, smooth completion, ability to welder thicker segments, and deterrence of atmospheric pollution weld pool (James & Hudgins, 2016). However, the welded structures are regularly exposed to welding defects. Weld cracking is one of the primary weld defects in oil and gas pipelines.
Weld cracks framed can be recognized into two principal classes, hot and cold cracks. Cracks formed in the cooling cycle at raised temperatures d are called hot cracks. Hot cracking emerges if specific blends of unfavorable conditions happen, for example, low width/profundity proportion of the weld infiltration, contraction stresses present as the metal cools, and high metal’s carbon and Sulfur contents. Submerged arc welding presents a risk of hot cracking because of profound infiltration and substantial melting of the welded material, making elements from the workpiece component result in the weld metal (Hadjoui et al., 2012). These cracks can be clarified by the solidification of a molten zone front pushing before it comprises greater convergences of materials lowering the melting point than in the remainder of the weld material. The weld metal solidifies in a profound, thin joint to leave a debilitated tract caught in the weld, which then cracks to create a longitudinal break influenced by shrinkage stresses.
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Elimination of hot cracking can be achieved by compelling the weld from the base towards the surface to cool, with the goal that the essential crystals are obligated to develop obliquely upwards towards the surface of the weld, for example, by welding contrary to a heat eliminating base. When welding thicker plates, hot cracking can happen if the cooling rate is excessively high. Preheating might be required. On the other hand, cold cracking occurs when the weld material has been cooled to room temperature. It can occur days or hours after the formation of the welding solidification is complete; hence it is referred to as delayed cracking (Hadjoui et al., 2012). Cold cracking is probably going to happen in all martensitic and ferritic steels, for example, carbon steel, low-alloy steel, and high-alloy steel, except if adequate precautions, essentially preheating, are utilized.
Cold cracks are brought about by the joint impacts of low weld ductility, weld diffusible hydrogen, and residual stress. The ductility of a weld may decrease with a high cooling rate after solidification and a high carbon equivalent. A weld’s residual stress can be more critical than anticipated in the event that it contains weld discontinuities like incomplete joint infiltration, incomplete fusion, slag inclusions, undercut, and porosity (Böllinghaus et al., 2016). The diffusible hydrogen source in a weld is primarily moistness in the welding consumables and air. Cold cracking, accordingly, can be forestalled by controlling the three primary variables; (1) residual stress, low ductility, and diffusible hydrogen. That is, preheating the base component to decrease the weld cooling rate. This forestalls the weld embrittlement and eliminates hydrogen dissolved from the weld. (2) Preventing weld breaks to avoid stress consolidation and (3) limiting diffusible hydrogen in the weld using welding consumables low in hydrogen.
After submerged arc welding, as the weld metal cools, shrinkage strains occur, resulting in hot and cold types of weld cracking. If the shrinkage is confined, residual stresses that lead to cracking will be initiated by the welding strains. Two opposing forces exist: The tensions brought about by the metal contraction and the encompassing stiffness of the base material. The contraction stresses increment as the capacity of contracting metal increments. Enormous weld magnitude and profound infiltrating welding methods increment the contraction strains. When base materials and greater strength filler metals are involved, the stresses prompted by welding strains will increment. Greater residual stresses will be available in welded joints with higher residual stresses (Sharma & Maheshwari, 2017).
Weld cracks are usually unsatisfactory discontinuities in any form and are considered generally detrimental to the weld performance. That is the reason product weldability is primarily dependent on its cracking propensity before and after welding. The cracking deformities can go about as stress concentration sites that lead to untimely failure through fatigue and provide ideal hydrogen-aided cracking and stress corrosion cracking if undetected. Such deformities increase the risk of disastrous in-service failures for welded metal items, for example, deep-ocean oil and gas transportation pipes. Such failures can prompt obliterating natural, financial, and social harm. Understanding the essentials behind the occurrence of weld cracks can help welder keep those cracks from happening in any case. However, the best guard against cracking is appropriate pre-weld and post-weld heat treatments alongside practices curtail hydrogen exposure.
References
Böllinghaus, T., Lippold, J., & Cross, C. E. (2016). undefined . Springer.
Hadjoui, F., Benachour, M., & Benguediab, M. (2012). Undefined. Materials Sciences and Applications , 03 (09), 596-599. https://doi.org/10.4236/msa.2012.39085
James, B., & Hudgins, A. (2016). undefined. Handbook of Materials Failure Analysis with Case Studies from the Oil and Gas Industry , 1-38. https://doi.org/10.1016/b978-0-08-100117-2.00001-7
Sharma, S. K., & Maheshwari, S. (2017). A review on welding of high strength oil and gas pipeline steels. Journal of Natural Gas Science and Engineering , 38 , 203-217. https://doi.org/10.1016/j.jngse.2016.12.039
