Mechanical property changes in steel during steel pipe production
Today, welded steel pipe has been the most widely utilized pipe material for a variety of applications around us. As we all know, raw steel is first cast into a more workable starting form. It is then made into a pipe by stretching the steel out into a seamless tube or forcing the edges together and sealing them with a weld in a mill.During the pipe manufacturing process, the steel undergoes various permanent changes in shape. These changes require the steel to yield, which may result in alterations to the mechanical properties of the original steel coil. The pipe ends may also be expanded past yield point or swaged to shape a bell, while spigot ends may be rolled to accommodate gaskets. All of this processing can have varying effects on the steel’s mechanical properties.
With regard to cold rolled steel pipe, it is well understood that cold working steel will change its mechanical properties. The amount of change that can occur is dependant on the amount of work done to the steel. The actual amount of change that can be expected when forming steel pipe has been explored and studied in the oil and gas steel pipe market, but not much work on the subject has been completed in the water pipe market. However it cannot be denied that cold working steel will increase the tensile-yield strength, and therefore some of the minor changes seen are likely attributed to the cold working.
The changes in yield strength are insignificant in typical water pipe applications, first because the amount of possible change would be considered normal variance in properties are received from the steel mills. Second, the change in yield strength from cold working will be an increase, not a decrease, adding to the design safety factor of the pipeline. Ultimately the amount of variation is negligible in pipe applications. The most commonly used pipe manufacturing method for round steel pipe used for water system is spiral welding coil steel in a continuous process. The process starts with steel coils placed in the pipe mill and the material unrolled off the coil and leveled. The edges of the coil are trimmed or machined for welding and the steel is fed through forming rolls at a set helix angle. The feed angle and the width of the coil determine the diameter of the pipe. The pipe is formed and welded in a continuous operation and typically joined with submerged arc welding, with a weld on the inside of the pipe and one on the outside. The pipe sections are then cut at the desired length.
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