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Most steel tubing is made by cutting rolled steel into thin strips that are cold formed length-wise into a tube shape which is then welded together. Further processing creates the desired mechanical properties, dimensions, and finish. Seamless tubing is also available.
Chromoloy tubing is made from a family of low-alloy steels that contain chromium and molybdenum (SAE 4130 or 4140), along with the iron, carbon and other elements. The chromium adds strength, hardenability and a level of corrosion resistance to mild carbon steel, though Chromoly is not as corrosion resistant as stainless steel.
Chromoloy is heavier than aluminum alloys, but its high strength-to-weight ratio makes it desirable for aerospace components and race car parts. It is also used in automotive gears and crankshafts, gas delivery tubing, and machine shafts.
Electric resistance welding (ERW) is a type of welding process that uses the heat generated by passing a high-frequency electrical current through the metal, along with pressure to hold the parts to be welded together for a specific length of time. ERW can be used for spot welding and also for welding tube seams.
Drawn-over-mandrel (DOM) tubing is not made from any specific alloy – it can be used with mild steel, chromoly or another alloy, such as SAE 1020 or 1026 steel.
DOM tubing is often incorrectly referred to as “seamless tubing” because the seam is almost invisible. DOM is a process that takes the rough cold-formed steel tube and continues to process it further to smooth out the internal surface of the weld seam while improving its mechanical characteristics. This is accomplished by annealing (heating) the tube to soften it so that it can be pulled over a tapered steel shaft (i.e. “drawing the tube over the mandrel).
The mandrel is a little thicker than the inner diameter of the tube, and as it moves through the length of the tube, it smooths the inner surface while stretching it wider. The tube is also drawn through dies that shape and size the outer surface. The combination of mandrel and dies achieves the required wall thickness, inner and outer diameters.
The DOM process creates a more concentric, uniform product with dimensions more closely toleranced to a customer’s exact specifications. DOM tubes also have superior mechanical properties, including increased hardness and tensile strength and a sound welding seam. This makes them ideal for use in mechanical parts, such a hydraulic cylinders and automotive components, without requiring further machining.
True seamless tubing is made from a heated cylindrical steel billet (or blank) which is hollowed out with a rotary piercing process. The solid billet is rolled between two rollers toward a tapered mandrel pointing at the end of the billet. Forces from the rollers create an opening at the center of the billet’s cross section. The opening grows, forming a tube as the billet continues to travel over the mandrel and through the rollers. Once this rough tube cools, it is further processed to achieve the desired thickness, diameter and finish with either a cold or hot forming method.
Cold drawn steel (CDS) is 1018/1026 steel tubing with uniform microstructure, tight tolerances, high strength-to-weight ratio, high tensile strength, thinner tube walls, and a superior surface finish compared with HSF tubing of the same steel grade. It generally requires no additional machining and is used in race cars, truck and auto parts, and hydraulic cylinders.
Hot finish seamless (HFS) tubing is less costly than CDS tubing and can be easily machined to exact specifications. It is used for rollers, sleeves and hydraulic cylinders.