4130 Square Tubing

Why 4130 Is Not Standard in Square Form

The manufacturing route for seamless alloy steel tube — billet piercing and mandrel drawing — naturally produces circular cross-sections. Square or rectangular hollow sections are typically produced from welded strip, which is problematic for 4130 due to its elevated carbon equivalent (see the welded tubing page). For structural applications requiring square or rectangular steel hollow sections, engineers typically use structural steel hollow sections (e.g., ASTM A500 cold-formed steel) rather than 4130 alloy.

What to Specify Instead

If a design requires the strength-to-weight properties of 4130 alloy in a non-round profile, the following should be evaluated:
1. Whether the design can be modified to use round 4130 tube (often possible with changed connection geometry)
2. Whether a structural hollow section in lower-alloy steel meets the load requirement in the available square section sizes
3. Whether a 4130 round tube machined to a custom shape is a feasible alternative for small quantities

For roll cages, aircraft airframes, and bicycle frames, round section is specified and required.

Practical Ways to Get a Square Section

• Use structural hollow section. Cold-formed square and rectangular steel sections are made in large volumes to structural standards. They are carbon steel, so the section must be sized for their lower strength.
• Fabricate from 4130 sheet. Aircraft practice has long built box sections and fittings by forming and welding 4130 sheet. This gives true 4130 properties in a custom shape at higher labour cost.
• Redesign with round tube. Flat mounting faces can be provided with welded tabs or plates on a round tube.
• Machine from solid or heavy tube. Workable for short parts in small numbers.

Tell us what the square section is for. Often the real need is a flat surface to bolt to, which a round-tube design can provide. Round sizes are listed in the size chart; the related case is covered under rectangular tubing.