Hot working involves heating metals above their critical point to make them easier to shape. Cold working can cause stress and eventual cracking. Hot working creates a ductile metal, but may require high temperatures and special equipment. Annealing and tempering can strengthen the metal.
Hot working is a term for heating steel or other metals and then rolling, stamping or hammering them into desired shapes. Heating metals above their critical point, where the metal crystals can flow as very thick liquids, reduces stress and makes them easier to roll or shape. Hot-worked metal may be less brittle, or less prone to cracking, than cold-worked metals.
Metals contain a crystalline structure, a random mix of multifaceted crystals that can be compared to grains of sand. When most metals are at room temperature, the crystals are tightly packed together and the metal is considered stiff. If the metal is heated above its crystallization temperature, referred to as the critical point, the crystals lose their grip on each other and the metal will flow or be easily shaped.
Cold working metal at room temperature creates very strong metal parts, but can cause stresses in the metal that lead to eventual cracking or failure. This occurs because the metallic crystals cannot move around each other and want to return to a lower stress condition. Cold shaping can be compared to stretching a rubber band, with the crystals under stress. The metals can be heated later in a process called annealing, where the metal is heated to a high temperature, held there, then slowly cooled to reduce the stress. Hot working prevents this stress and makes the metal much easier to handle.
When hot metals are rolled, the metallic crystals will slide past each other and change shape from circular shapes to long, thinner shapes. This change can make the metal stronger in the direction of the crystals, but weaker if strength is added in the opposite direction. Rolled metals can be annealed to recover some of the original crystal shape, which strengthens the metal.
Working at higher temperatures creates a ductile metal, which is one that can flex or is less brittle. Ductility can be a disadvantage for parts that require high impact resistance, such as tools or blades. Hot working can be followed by tempering, which is heating followed by rapid cooling in water or oil, to recover some of the toughness of the cold worked metal.
A disadvantage of hot working is the temperature required to reach the critical point of some metals. Some high-strength steels may require heating to very high temperatures to allow them to work without creating metallic stresses. These temperatures may require special machine tools that can maintain their strength and toughness when exposed to the high temperatures of the metal. This equipment may also require high temperature heat loops on rolls or stamped parts to keep the metal hot and avoid cold work.
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