Is ASTM A500 Tubing Seamless
ASTM A500 covers cold-formed welded and seamless carbon steel structural tubing in round, square and rectangular shapes, and it applies to both product forms. The answer to the common question is therefore that A500 tubing may be seamless or welded, and the buyer has to state which is required, because the standard lists them as separate types with separate requirements.
| Type | Product form |
|---|---|
| Type S | Seamless, produced without a weld seam |
| Type H | Welded, round cross section |
| Type F | Welded, square or rectangular cross section |
Welded A500 tubing is normally produced by cold forming strip into shape and joining the edges by electric resistance welding, followed by sizing to the final dimensions. Seamless A500 tubing is made by piercing and rolling a solid billet, which avoids a seam altogether. Both routes are covered by the same standard but not by identical testing requirements.
Grades and Mechanical Properties
| Grade | Yield strength min, round | Tensile strength min, round |
|---|---|---|
| Grade A | 228 MPa (33 ksi) | 400 MPa (58 ksi) |
| Grade B | 290 MPa (42 ksi) | 400 MPa (58 ksi) |
| Grade C | 345 MPa (50 ksi) | 427 MPa (62 ksi) |
Grades A, B and C are the usual structural grades, and the standard also includes Grade D together with metric equivalents in the A500M designation. Values for square and rectangular sections are tabulated separately from those for rounds, and the minimum yield strength of a shaped section is lower than that of the round section of the same grade. Elongation requirements depend on the type, the grade and the wall thickness, so the full table in the standard has to be consulted for a specific size.
Because the tubing is cold formed, the section carries work hardening from the forming operation, and the corners of a square or rectangular section carry more than the flats. This is one reason why A500 delivers higher yield strength than hot rolled structural steel of similar chemistry.
How Welded A500 Tubing Is Made and Tested
The process begins with slit strip, which is uncoiled, levelled, edge trimmed and progressively formed through a series of rolls into a round, square or rectangular shape. The edges are heated and forged together by resistance welding, the external and internal flash is removed, and the tube is sized, straightened and cut to length. Weld quality is monitored in line, and the finished product is inspected for dimensions, straightness and surface condition.
The standard requires mechanical testing of the finished tubing, including tensile testing to confirm grade properties, and flattening tests on welded round tubing to demonstrate that the weld and the tube body can sustain plastic deformation without opening. Product analysis of the finished tube verifies that the carbon, manganese, phosphorus and sulfur levels remain within the limits for the grade.
A500 Compared With Other Carbon Steel Products
A500 compared with A36: A36 covers structural shapes, plates and bars, while A500 covers hollow structural sections. A500 is cold formed, which generally gives it a higher yield strength than hot rolled A36 material.
A500 compared with A53: A53 is a pipe standard for pressure and mechanical applications, in sizes defined by pipe schedules. A500 is a tubing standard for structural hollow sections, where the outside dimensions, corner radii and wall thicknesses follow a different table.
A500 compared with line pipe: line pipe grades such as API 5L serve fluid transmission with a specified minimum yield strength tied to the pipeline design code, while A500 is intended for structural framing and load carrying members.
Selection, Corrosion and Finishing
A500 is a low carbon steel with a carbon content normally not more than about 0.26 %, which places it in the mild steel family. It is weldable, formable and suitable for general structural work. Like all plain carbon steels it will rust when exposed to moisture and oxygen, so outdoor and corrosive environments call for a protective system such as paint, powder coating, hot-dip galvanizing or another barrier coating. For exposed architectural work, galvanizing is often chosen because it protects the inside and outside surfaces and the cut ends receive sacrificial protection from the zinc.
When ordering, specify the standard, the type, the grade, the shape, the outside dimensions, the wall thickness, the length and the finish. Stating the type is essential: Type S, Type H and Type F are distinct products, and a certificate for one does not confirm the other.
FAQ
Q: Is A500 tubing seamless?
A500 tubing is available in both seamless and welded form. The standard covers Type S seamless, Type H welded round and Type F welded square or rectangular sections, so the required type has to be stated when the material is ordered.
Q: Is A500 steel mild steel?
Yes. A500 is a low carbon steel, with carbon normally not more than about 0.26 %, which places it in the mild steel family. That low carbon content is what makes the tubing weldable and formable.
Q: What is the difference between A36 and A500 steel?
A36 covers structural shapes, plates and bars, while A500 covers hollow structural sections in round, square and rectangular form. A500 is cold formed, which generally raises its yield strength above that of hot rolled A36 material.
Q: Does A500 steel rust?
Yes. As a plain carbon steel it oxidises when exposed to moisture and oxygen. Outdoor or corrosive service therefore requires a coating such as paint, powder coating, galvanizing or a similar protective system.
Q: What is the difference between A53 and A500?
A53 is a pipe standard for pressure and mechanical applications sized by pipe schedule, while A500 is a structural tubing standard for hollow sections sized by outside dimension and wall thickness. The two are not interchangeable without a design review.
Q: Which grade of A500 should be selected?
Grade A suits lightly loaded framing, Grade B is the general structural choice, and Grade C is selected where the highest yield strength of the series is needed. Square and rectangular sections carry lower tabulated yield values than round sections of the same grade.
