The most common types of steel pipe are seamless and welded. The names refer to the method used to manufacture the pipe. Each type is better suited for particular uses.
Seamless Steel Pipe
Just as the name implies, seamless steel pipe is seamless. In other words, the pipe is smooth all the way around. This is established by stretching a single piece of steel pipe to the desired length. Seamless tubes and pipes are generally pretty light weight. They are also thinner due to the stretching. Some of the most common uses for seamless steel pipe are small parts in things such as bicycles, although it is also used for some automobile parts. This type of pipe is also often used to transport liquids.
Welded Steel Pipe
This type of pipe is not seamless due to the welding. It is created by rolling a sheet of steel and then welding along the long edge where the ends of the roll meet. This leaves a long, rigid seam along the whole pipe or tube that is often manipulated to be flush against the roll. Seamed steel pipe is heavier and typically more rigid but also straighter. Some of the best uses for welded steel pipe are an electrical conduit and plumbing applications. This type of pipe is great for low-pressure situations but not ideal when a lot of stress may be applied to the tube.
Controllable Features
Whether seamless or welded, there are some characteristics of steel pipe that are determined by the manufacturing process. For example, the length of the pipe is decided up front and then the steel is manipulated to meet those specifications. The overall wall thickness is also controlled during manufacturing. X-ray gauges are used to test the steel thickness after a pipe is manufactured to ensure that the mandatory requirements are met.
Many steel pipes are coated with a finish or paint before use as well. The coating requirements are different for each application, so that is also controlled through manufacturing. The last thing that is determined while the pipe is being built is the amount of pressure it can hold. Each pipe is tested before being released for use to ensure that it can handle the necessary amount of pressure.