Rolled‑steel wheels for railway rolling stock are directly formed from a solid steel billet by forging and rolling. The hub is the thickened cylindrical section at the wheel centre; it cannot be separately cast or forged and then joined by welding. The hub must possess sufficient toughness to withstand alternating impact loads, while its inner bore shall achieve extremely high dimensional accuracy to meet interference press‑fit requirements for mounting onto the axle wheel seat.
Hot‑Forming Process of the Hub
Raw materials are smelted and vacuum‑degassed to produce clean continuously‑cast round steel billets. After cutting‑to‑length, the billets are fed into a ring‑type reheating furnace and heated to 1250‑1300 °C.
First, the billet is upset in a hydraulic press. The cylindrical billet is flattened into a thick round disc, which compacts internal porosity and lays a foundation for hub forming. During pre‑forging indentation, metal flows toward the centre to preliminarily build up the thick blank for the hub. The workpiece is then transferred to a dedicated wheel rolling mill for rolling and diameter expansion, which mainly shapes the rim and web. Metal at the central hub zone does not spread outwards, retaining a thick base material.
After rolling, a hydraulic press performs central punching to remove the excess central core material and produce the rough inner bore of the hub. The web is subsequently bent using dies to finalize the outer shape and end‑face contour of the hub blank. Hot blanks are then slowly cooled out‑of‑line to relieve residual stresses generated during forging and rolling.
Throughout the hot‑forming stage, the hub blank is obtained via plastic metal flow. The metal fibre structure at the hub remains continuous and dense with no lamination permitted.
Heat Treatment of the Hub
Rolled‑steel wheels adopt a local spray quenching process applied only to the wheel tread; the hub receives no quench hardening treatment.
The entire wheel is first heated to the austenitizing temperature and held for soaking. Water‑spray quenching is applied exclusively to the rim tread and flange, whereas the hub together with the web cools gradually with the workpiece. A subsequent full tempering soak is carried out to relieve residual stresses.
This process is designed to maintain relatively high toughness of the hub and prevent it from becoming hard and brittle. Excessively high hub hardness raises the risk of radial cracking under assembly stress and alternating service loads. Hub hardness shall be lower than the hardness of the quenched rim tread.
Precision Machining of the Hub Inner Bore
After heat treatment, scale is removed by rough turning. The most critical operation for the hub is finish boring of its inner bore.
A numerical‑control boring machine performs fine machining of the bore. Bore diameter and cylindricity are strictly controlled, alongside inner‑bore surface roughness. Obvious tool marks or pits are not allowed on the bore surface. Tiny residual tool marks will act as stress‑raisers after press‑fitting and may initiate fatigue cracks during service. Both hub end‑faces on either side and the hub‑web transition fillets are also machined. Fillet profiles are machined per drawing specifications to mitigate stress concentration at these locations.
Non‑Destructive Testing of the Hub
Specialized non‑destructive testing is performed for the hub upon completion of machining.
Ultrasonic testing scans from the hub inner bore and both wheel end‑faces to detect internal cracks and inclusion defects inside the hub as well as in the hub‑web transition zone. Magnetic particle testing inspects surface and near‑surface micro‑cracks on hub end‑faces and transition fillets. Demagnetization is conducted after inspection.
The complete wheel is directly rejected for scrapping once any defect exceeding acceptance limits is detected; repair is not permitted. Identification marking is stamped after satisfactory inspection. At this point the wheel (with its monobloc hub) finishes manufacture and is ready for hydraulic press‑fit assembly into a wheelset.
Conclusion
Hub manufacture is integrated within the full production workflow of monobloc rolled‑steel wheels. Hot forging and rolling yield a dense hub base; differential heat‑treatment ensures high hub toughness; high‑precision boring guarantees press‑fit mating quality; non‑destructive testing screens for various crack‑type defects. Only through rigorous process control at every step can the hub satisfy assembly and long‑term in‑service performance requirements for railway rolling stock.
