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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 metals crystals can flow like very thick liquids, reduces stress and makes them easier to roll or shape. Metal subjected to hot working can be less brittle, or less likely to crack, than metals that are processed cold.
Metals contain a crystal structure, a random mix of many-sided 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 rigid. 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 can be shaped easily.
Cold working of metals at room temperature creates very tough metal parts, but can cause stresses in the metal leading to eventual cracks or failure. This occurs because the metal crystals cannot move around each other, and they want to return to a lower stress condition. Cold shaping can be compared to stretching a rubber band, with the crystals under stress. Metals can be heated later in a process called annealing, where the metal is heated to a high temperature, held there, then cooled slowly to reduce stress. Hot working prevents this stress and makes the metal much easier to handle.
When hot metals are rolled, the metal crystals will slide along each other, and change shape from circular to long and thinner shapes. This change can make the metal stronger in the direction of the crystals, but weaker if force is added opposite to the 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 a metal that can flex or is less brittle. Ductility can be a disadvantage for parts that need 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 regain some of the toughness of cold worked metal.
One disadvantage of hot working is the temperature required to reach the critical point of some metals. Some high-strength steels can require heating to very high temperatures to permit working without creating metal stresses. These temperatures can require special machine tools capable of maintaining their own strength and toughness when exposed to the high metal temperatures. This equipment may also require high-temperature heating circuits on rollers or stamping parts to keep the metal hot and prevent cold working.