Hot rolling is the primary process for shaping AISI 4140 into round bars. The steel is heated above its recrystallization temperature (typically above 1100°C) and passed through a series of rolls to achieve the desired diameter.
Effects and Characteristics:
Microstructure & Properties: The high-temperature deformation breaks down the as-cast structure, refines the grain size, and closes internal voids, improving density and mechanical properties (especially toughness and ductility in the longitudinal direction). However, properties can be anisotropic (direction-dependent).
Surface Finish: The surface has a characteristic bluish-black mill scale (an oxide layer). This scale must be removed (by pickling or shot blasting) for many applications to prevent tool wear during machining or to allow for accurate inspection.
Dimensional Tolerances: Tolerances are wider compared to cold finishing. For example, a 50mm hot-rolled bar might have a diameter tolerance of ±0.8mm. Straightness is also less precise.
Residual Stresses: The non-uniform cooling after rolling can introduce residual stresses near the surface. For precision machining, stress relieving may be required.
Advantages: It is a cost-effective, high-throughput process suitable for large diameters and for producing stock that will be fully machined or subsequently heat-treated.