hot working
Hot working means shaping metal while it is hot enough that it heals as fast as you damage it. When you deform a metal you pack it with dislocations that tangle and make it harder, but above a certain temperature the metal continually grows fresh, strain-free grains during the deformation, wiping out the tangles as they form. So the metal stays soft no matter how much you squeeze it, like kneading warm dough that never stiffens. That is why a blacksmith heats the bar before hammering.
The threshold that matters is the recrystallization temperature, roughly 0.4 to 0.5 of the melting temperature on the absolute (kelvin) scale. Work the metal above it and dynamic recrystallization keeps replacing the deformed grains with new equiaxed ones, so you can achieve huge shape changes with modest force and never build up work hardening. Hot rolling of a steel slab into sheet, and hot forging and extrusion, all rely on this.
The trade-offs are honest and practical. Working hot needs much less force and lets you break a coarse cast ingot down into a refined, uniform grain structure with the porosity closed up, and it leaves essentially no residual stress. But at high temperature the surface oxidizes into scale, the finish is rough, dimensions are hard to hold as the part cools and shrinks, and you cannot use hot working to make a metal stronger, because whatever hardening you put in immediately recrystallizes away.
Steel that melts near 1500 degrees C is hot rolled at around 1100 degrees C: it deforms with little force, its cast grains recrystallize into fine equiaxed grains, and it emerges soft, ductile, and free of residual stress but with an oxide scale.
Above the recrystallization temperature the metal reforms fresh grains as fast as you deform it, so it never work hardens.
Hot working refines the grain and costs little force, but it cannot strengthen a metal by work hardening because any dislocation tangle recrystallizes away as fast as it forms.