Longitudinal Weld

What is Longitudinal Weld

Longitudinal welding is a special type of welding technique used in pipe production application wherein the finished products need an exceptionally high wall thickness because of the high internal or external pressures. That said, the main applications of longitudinal weld pipes include oil, gas, natural gas, central heating, water supply, sewage treatment and other related uses. 

Welded pipes are very prevalent in the oil and gas transporting market due to its broad selection and relatively low cost. Based on the different welding seams, welded pipes can be classified into two categories: the longitudinal welded pipes and helical welded pipes, both of which are utilized extensively in the relative field and quite different due to the distinct manufacturing techniques. We’ll touch base on this in the following section.

Difference in Manufacturing

Compared with helical welding, longitudinal welding is relatively simpler in terms of the manufacturing and welding techniques. As the name implies, longitudinal welding involves welding of line pipes that have straight welding on the pipe surface. Helical welded pipes, on the other hand, are rolled and welded based on certain helical angles, whereas longitudinal pipes are welded vertically according to the axial line of the pipe.

Therefore, due to the complicated welding method, helical welding for line pipe manufacturing is generally more time-consuming. As for longitudinal welding of line pipes, the length of line pipes needs to be taken into account for the related industry projects. For instance, shorter line pipes are always required to be joined and welded into a longer pipe, which can significantly increase the likelihood of welding defect because the appearance of T-shaped welding seams are inevitable. Topping this, T-shaped welding joints can lead to welding residual stress, which can potentially cause fatigue crack at a later time. 

In conclusion, longitudinal welding is less reliable than helical welding in the case of line pipe manufacturing, despite that longitudinal welding is simpler and less expensive. In addition, longitudinal welding is also more useful when it comes to producing pipes with relatively smaller in diameter, whilst helical welding is more ideal for larger line pipes. With the above said, unfortunately, longitudinal welding is the less preferred method of line pipe manufacturing in the market. But that is not to say that helical welding is flawless because it is more complex and expensive.

Longitudinal Weld Process

Steel line pipes are basically long, hollow tubes that are primarily used to transport products from one location to another. The steep pipes are produced mainly using two different methods which result in either seamless line pipes or welded line pipes. For longitudinal welding of line pipes, the first thing to do is to cast raw steel into an initial and more workable form (e.g., hot billet, slabs or flat strip). The pipes are then formed by stretching the hot steel slabs, billets or strips out into a seamless pipe or connecting the end of a flat steel strip, and then sealing them with a weld. 

When high wall thickness due to high internal or external pressures are required as the operational requirements, longitudinal welded pipes are generally the preferred option because they are the most economical in such context. The diameter of a longitudinal welded pipe is typically larger than that of electric resistance welding (ERW), which usually ranges from 16 to 60 inches (i.e., 406mm to 1500mm). The longitudinal welded pipes yield positive performance on high pressure resistance, as well as corrosion resistance on low temperature.

In terms of longitudinal welded pipes milling, the process is performed and achieved by cold forming of heavy carbon steel plates, and the pipe raw materials that translate to the properties of many pipes. The material used for longitudinal welding of line pipes is essentially a high or medium thickness sheet, coiled in the forming or molding machine, which are then welded to expand the diameter. They have a wide specification scope, with superb ductility, toughness, uniformity, compactness and so on.

Simply put, the manufacturing process of longitudinal welded pipes is basically the bending and welding of wide steel plates so that pipes can be formed from leveled hot rolled coils. The coils are shaped during the forming process closely resemble the English alphabets, such as J, C, U, O. 


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