What A500 Grade B Is
ASTM A500 is the standard for cold-formed welded and seamless carbon steel structural tubing in round, square, and rectangular shapes, known as hollow structural sections (HSS). Grade B is the default grade for general structural work, with a specified minimum yield strength of 46,000 psi (315 MPa) and a specified minimum tensile strength of 58,000 psi (400 MPa). It is used for columns, beams, frames, trusses, and equipment supports in buildings and industrial structures, and it is the grade most designers specify when the project does not demand the higher Grade C.
Mechanical Properties
The guaranteed properties of Grade B are a minimum yield strength of 46,000 psi (315 MPa) and a minimum tensile strength of 58,000 psi (400 MPa). The minimum elongation is set by the wall thickness in the standard's table, with values around 23% in 2 inches for the common wall range, and it is the elongation that guarantees the ductility needed for connections and bending. These values are the contractual minimums; typical mill results are higher. Note that property figures such as 345 MPa yield or 450 MPa tensile belong to other grades, such as the Chinese Q355 class of GB/T 6728, and should not be used for A500 Grade B design.
Chemical Composition
ASTM A500 does not publish a fixed composition table for the grades; the chemistry is selected by the producer to meet the mechanical and weldability requirements, and it is reported on the mill test certificate. In practice, Grade B tubing is made from a low-to-medium carbon steel with manganese strengthening and micro-alloying as needed, and the steel is produced to a fine-grain killed practice. Buyers who need a maximum carbon or a carbon-equivalent limit for welding should state the requirement in the order, because it is not guaranteed by the standard itself.
Performance Characteristics
Grade B tube is selected for its combination of strength, ductility, and formability. It supports cutting, cold bending, and the forming of complex components, and it welds reliably with submerged arc, shielded metal arc, and other standard processes. The cold-formed section has a smooth, uniform surface and tight dimensional tolerances, with the weld flash controlled so that the protruding height of the internal or external seam stays small. The section properties used in design are taken from the AISC manual for the exact size and wall.
Surface and Corrosion Protection
The bare tube has the typical mill surface of cold-formed carbon steel and will rust in outdoor service. For exterior and marine environments, hot-dip galvanizing or a paint system is applied, and the tube can be supplied surface-treated or primed to the project specification. The coating choice should follow the corrosivity category of the site, and the surface preparation before coating follows the coating specification.
Applications
Grade B HSS is used in building frames, roof trusses, mezzanines, industrial plant structures, highway signs and barriers, agricultural and material-handling equipment, and architectural steelwork. The closed section gives an efficient strength-to-weight ratio and a clean appearance, and the flat faces of square and rectangular sections simplify connections. For heavy loads where a lighter section is wanted, Grade C is the alternative; for light, cost-driven duty, Grade A may be sufficient.
Frequently Asked Questions
Q: What are the mechanical properties of A500 Grade B?
The minimum yield strength is 46,000 psi (315 MPa), the minimum tensile strength is 58,000 psi (400 MPa), and the minimum elongation depends on the wall thickness, around 23% in 2 inches for the common range.
Q: What is the chemical composition of A500 Grade B?
A500 does not fix the composition; the mill selects the chemistry to meet the mechanical and weldability requirements, and the actual values are on the certificate. Typical chemistry resembles a low-to-medium carbon, manganese-strengthened steel.
Q: Is A500 Grade B the same as Q355?
No. Q355 is the Chinese structural steel of GB/T 1591 with a 355 MPa minimum yield. A500 Grade B guarantees 315 MPa yield and 400 MPa tensile. The standards and property tables differ.
Q: Can A500 Grade B be welded?
Yes, with standard structural welding procedures for carbon steel.
Q: What is the difference between Grade B and Grade C?
Grade C has higher minimum strengths: 50 ksi yield and 62 ksi tensile against 46 ksi and 58 ksi for Grade B, allowing lighter sections for heavy loads.
Q: How is the tube protected from corrosion?
By hot-dip galvanizing, painting, or other coatings for outdoor service; the bare tube rusts when exposed to moisture.
