As the name suggests, a hollow bar is a bar with a central bore running along its length. It is produced in the same way as seamless tubes, being extruded from a forged bar. This gives it excellent mechanical properties, often with an improvement in consistency and impact toughness compared with rolled or forged components. In addition, the uniformity of shape and dimensional tolerances are very good. The only real difference compared with a seamless tube is the wall thickness. Whereas a tube is produced for the carrying of fluids, and so any machining is limited to the end only (to form couplings or connectors), a hollow bar will typically have a much greater wall thickness to allow the machining of various finished components.
Using hollow bars instead of solid bars can to achieve direct savings in material and tooling costs, reduced machining time and increased productivity. By starting with a near-net shape hollow bar, less metal is lost as low-value swarf and obviously less tooling is consumed too. Therefore, there can be immediate input cost savings when using this more sophisticated product form.
More significantly, reducing machining time or totally eliminating machining steps can dramatically improve productivity. This can either be valued as reduced machining time or cost for a given part, or also as higher throughput when machines are fully-loaded. In addition, replacing solid bars with stainless steel hollow bars eliminates the need for trepanning when producing components with a central bore, which causes material to work harden and is detrimental to subsequent machining operations.
Hollow bars are produced in a wide range of sizes, typically with 3-5 different internal diameters (ID) for each given outside diameter (OD). Langley Alloys stocks 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 Sandvik with ‘enhanced machinability as standard’. Their improved machinability strengthens the argument for using hollow bars in place of solid bars to quickly and efficiently produce components with a central bore.