Hot - rolled round bars are produced by heating the steel billet to a high temperature (usually above 1000°C) and then passing it through rolling mills to reduce its cross - section and form the round shape. The production standard for hot - rolled round bars, such as ASTM A29, focuses on chemical composition, mechanical properties, and dimensional tolerances. The dimensional tolerances for hot - rolled bars are relatively loose compared to cold - drawn ones. For example, the diameter tolerance for hot - rolled round bars can be ±0.5 - 1.0% of the nominal diameter. On the other hand, cold - drawn round bars, as per ASTM A108, are produced by pulling the hot - rolled bars through dies at room temperature. This process allows for much tighter dimensional tolerances, as mentioned earlier, with tolerances as low as ±0.0015 inches for small diameters. Cold - drawn bars also have a better surface finish, which is crucial for applications like precision machining. The mechanical properties of cold - drawn bars are also different; they generally have higher strength and hardness due to the work - hardening effect during the cold - drawing process.