A hollow bar is a bar of metal with a central bore running along its full length. Hollow bars are produced in the same way as seamless tubes, extruded from a forged bar and then cut to shape. This gives excellent, consistent mechanical properties and good impact toughness compared with rolled or forged components, along with very good uniformity of shape and dimensional tolerances.
What is stainless steel?
Stainless steel is an alloy of steel and chromium, with roughly 10.5% of the bar being added chromium. That chromium forms a layer of chromium oxide on the surface on contact with oxygen or water. The layer is protective and can even repair itself if damaged, which is what makes stainless steel such a good construction material.
How hollow bars differ from tube
The only real difference between a stainless steel hollow bar and a seamless tube is the wall thickness. A tube is designed to transport fluids, so any machining is limited to the ends to form couplings or connectors. A hollow bar typically has a much greater wall thickness, allowing various finished components to be machined from it.
Why use hollow bars
Starting from a near-net-shape hollow bar rather than a solid bar achieves direct savings in material and tooling costs, reduces machining time and increases productivity. Less metal is lost as low-value swarf and less tooling is consumed, so there can be immediate input-cost savings from this more sophisticated product form.
More significantly, reducing or eliminating machining steps can dramatically improve manufacturing productivity, whether valued as reduced machining time and cost for a given part, or as higher throughput when machines are fully loaded. Replacing solid bars with hollow bars also eliminates the need for trepanning when producing components with a central bore, a step that work-hardens the material and is detrimental to subsequent machining.
Sizes and specifications
Hollow bars are produced in a wide range of sizes, typically with 3-5 different internal diameters (ID) for each given outside diameter (OD).
We stock more than 80 different sizes of stainless steel hollow bar in Sanmac® 316L (UNS S31603, 1.4404) and Sanmac 2205 (UNS S32205, 1.4462). Sanmac is a proprietary family of steels produced by Alleima with enhanced machinability as standard, which further strengthens the case for using hollow bars in place of solid bars to produce components with a central bore.