What are the causes and prevention measures for weld cracking in welded steel pipes

Welding cracks are one of the most common and serious defects in welded components. Under the combined action of welding stress and other embrittlement factors, cracks form at new interfaces due to the breakdown of the atomic bonding forces in localized areas of the weld joint. They are characterized by sharp notches and a large aspect ratio. Cracks affect the safe use of welded components and are a very dangerous process defect. Welding cracks not only occur during the welding process, but some also have a certain latency period, and some occur during post-weld reheating.

First, what are the causes of welding cracks in welded steel pipes?
The following factors can cause cracking in welded steel pipes during welding: stress, restraint force, rigidity, chemical composition, the gap left in the weld seam, current, weld bead, and the cleanliness of the base material. These factors can all contribute to weld cracking in welded steel pipes. Although there are many causes of weld cracking in welded steel pipes, in different situations, multiple factors may be involved, or even two or three factors. However, regardless of the number of factors, there must be one primary factor. There are also cases where various conditions have little impact, and only one factor causes the weld seam of the welded steel pipe to crack. Therefore, when weld seam cracking occurs, it is essential to first correctly analyze the primary and secondary factors causing the cracking, and then take corresponding measures to address them. The weld seam of the welded steel pipe formed during the welding process is created when the welding rod and the base metal are melted at high temperature by an electric current. This process involves the welding rod and the base metal changing from solid to liquid. The high-temperature liquid expands thermally, and the cooling back to solidifies causes contraction. Due to thermal expansion and contraction, stress is naturally generated in the welded structure. Some welded structures inherently possess restraint and rigidity. The welding process involves the transformation from solid to liquid, that is, from solid to liquid (usually referring to molten iron), and then from liquid to solid, thus forming the weld seam of the welded steel pipe. The transformation from liquid to solid (that is, the transformation of molten iron into grains) is the crystallization process. Crystallization begins in the lower-temperature areas of the base metal and gradually extends towards the center of the weld seam, with the center of the weld seam being the last to crystallize. Due to thermal expansion and contraction, welded structures are affected by stress, restraint, or rigidity, causing the base metal grains to fail to bond together. This can result in small cracks in the weld seam of the welded steel pipe or even significant cracks in the weld seam. Even if the base metal and welding rod have good chemical composition, cracks or fissures can still occur due to the restraint, rigidity, and stress generated during the welding process. Poor chemical composition of the base metal and welding rod (high levels of carbon, sulfur, phosphorus, etc.); excessive gaps in the weld seam; excessive impurities in the base metal at the weld seam edge; excessive current; and excessively fast or slow welding speeds, or excessively wide weld bead widths will further exacerbate weld seam cracking.

Second, Types of Welding Cracks in Welded Steel Pipes and Preventive Measures. 
Welding cracks can be classified in different ways depending on their location, size, cause, and mechanism. Based on the conditions of crack formation, they can be divided into four categories: hot cracks, cold cracks, reheat cracks, and lamellar tears. Based on on-site observations of weld cracks in welded steel pipes, most are caused by stress, restraint forces, and rigidity. It can be said that stress, restraint forces, and rigidity are often the main factors contributing to weld cracking in welded steel pipes. A relatively effective method to address weld cracking caused by stress, restraint forces, and rigidity is to employ fixed welding and distributed welding. Fixed welding involves first fixing all weld seams of the welded steel pipes, or those in critical areas, using low current, narrow weld beads, and short distances. This prevents the welded part from generating excessive stress. Even when fixing all parts of the welded part, it is crucial not to weld sequentially from the same position, and even more importantly, to avoid using high current and large-gauge welding rods. Welding positions should be varied to prevent excessive heat generation in localized areas. The same method can be used for structures with restraint forces and rigidity. Distributed welding, on the other hand, is crucial for large structures. Welding should never be done sequentially from the same position; positions should be varied. For large structures, not only must fixed welding be performed first, followed by distributed welding, but the first weld bead should also avoid using high current and large-gauge welding rods. For the overall structure, all welded steel pipe welds must be welded separately from beginning to end; otherwise, although the welded steel pipe welds will not crack, the residual stress will be too great.