Welding techniques have matured to the point where almost every austenitic, duplex, and full ferritic grade that is made in strip form can be manufactured into a tubular product by welding. Common grades, such as TP 304, TP 316, and their derivatives, are chemistry balanced to form a small amount of ferrite during solidification. This ferrite formation makes these grades particularly tolerant of variations in the welding process because the weld shrinkage during solidification is compensated for by the different densities of the two phases. This also allows for higher welding speeds.
The high quality of continuously cold rolled steel and coiled strip is one of the keys to getting repeatable high-quality welded tube. Currently, stainless steel coils are made as large as 50,000 pounds and 72 inches wide. Because of improvements in the rolling process, thickness tolerances are commonly held to 50% of that in ASTM specifications. Surface finishes are commonly 20 µinch Ra or better on both sides of the strip. (Ra is the surface roughness found as the arithmetic average deviation of the surface valleys and peaks expressed in micro inches.) The combination of high-quality surface and tight tolerance translates into a very concentric product with excellent surface quality on both the outer and inner diameter (OD and ID) of the tube.
Step 1: Slit the coils
The coils are slit in a continuous process to a width needed to make the desired tube hollow size (Figure 1). The slitting process is designed to develop a square edge that's optimized for the square butt weld commonly used in manufacturing welded tubular products.
1. Cut stock to width. Stainless steel coils are split to the required width to begin the manufacturing process. Courtesy: Plymouth Tube
The most commonly ordered tubing specifications are SA 249 or SA 688 for austenitics, SA 268 or SA 803 for ferritics, and SA 789 for duplex grades. Ferritic grades that form martensite upon cooling are more of a challenge. The martensite is brittle and notch sensitive and may crack due to thermal stresses prior to the annealing operation. Some welded mills have developed proprietary processing and heat treatment methods that temper the martensite before it has a chance to crack.
Grades that do not form the compensating second phase during solidification, such as the higher alloyed austenitics and the full ferritics, require more care. This is typically provided by slowing welding speeds and using tooling just behind the weld melt pool that squeezes the pool at the same rate as the shrinkage occurs. This technique provides a high-integrity weld with no solidification shrinkage defects.
Step 2: Uncoil and roll form
An uncoiler (Figure 2) is located to provide precision alignment of the strip into the roll-forming operation. The strip is then roll formed using a series of polished rolls (Figure 3) in a progressive series (Figure 4) to provide the two strip edges to the welding location in the form of a square butt weld.
4. By the numbers. Forming a round tube from flat stock requires several gradual and precision rolling operations. Source: Plymouth Tube
The number of forming stations varies depending upon the design of the mill and the nature of the material being welded. There could be as few as six or as many as 14. Alignment of these stations and the uncoiler is critical to keep the weld seam perfectly centered under the welding head. Any misalignment can result in rolling of the tube and an off-seam condition in which the weld is not centered on the strip edges; in such cases, only partial penetration may result.
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