For round bars of medium or smaller diameters (typically less than 100 mm), hot rolling is the mainstream process, ensuring good dimensional tolerances and surface quality. However, when customers specify grain size requirements (such as fine grains or uniform distribution), the forging process cannot be replaced by hot rolling. This is because forging, through repeated hammering, can more effectively break down coarse grains to achieve an isotropic, fine-grained microstructure, thereby improving the material’s fatigue and impact resistance.

Bright bars have minimal variations in outer diameter, with roundness typically controlled within 0.02 mm. Black bars are in their as-rolled or as-forged condition, with scale on the surface, larger dimensional deviations, and roundness generally exceeding 0.1 mm. Since customers of black bars typically perform turning or grinding operations, the final roundness is determined by subsequent processes. Therefore, we do not consider roundness as a factory control specification for black bars.

Polishing is a surface finishing process in which a high-hardness grinding wheel rotating at high speed comes into contact with the surface of the round bar, producing grinding and polishing effects that reduce surface roughness and enhance gloss. Its primary functions in round bar production include: removing scale and surface defects left by the hot-rolling process to provide a clean substrate for subsequent processing; it also improves the dimensional consistency of the round bars.

Because metals exhibit good fluidity at high temperatures. Temperature fluctuations during rolling, elastic deformation of the rollers, and uneven cooling can all lead to dimensional variations. Furthermore, shrinkage deformation occurs during the cooling process after hot rolling, further affecting the final dimensions. However, these tolerances are generally within the normal range and are sufficient for structural materials that do not require precise fits. If the round bars are to be machined into shaft-type parts later, customers can correct the dimensions through turning or grinding.