Weichglühen

Weichglühen, or soft annealing, is a heat treatment used to increase the ductility of metals by reducing hardness and relieving internal stresses. The process typically aims for a soft, homogeneous microstructure that improves formability and machinability.

The procedure involves heating the material to a temperature within or just above the metal’s transformation range, holding it there long enough for diffusion and recrystallization, and then cooling it slowly. The exact temperatures and hold times depend on the alloy and composition. For steel, the target temperature is chosen according to the alloy class (for example, in the austenitic range for many steels) and is followed by a slow cool, usually inside a furnace. Protective atmospheres may be used to minimize oxidation and decarburization.

Materials and scope

Weichglühen is most commonly applied to ferrous metals, particularly steels and cast irons, to relieve stresses introduced during casting, rolling, or forming. It can also be used for certain nonferrous alloys, such as copper or aluminum, to restore ductility after significant work hardening. The method is distinct from other heat treatments by its emphasis on achieving maximum softness and uniform ductility rather than hardening or refining the microstructure.

Effects and outcomes

The primary outcomes are reduced hardness and yield strength, increased ductility and toughness, and improved machinability and suitability for subsequent forming operations. A microstructure becomes more uniform, and residual stresses from manufacturing or welding are relieved. Depending on the material, grain growth can occur during the process, which influences mechanical properties.

Relation to other treatments

Weichglühen is contrasted with normalizing, quenching and tempering, or full annealing, each producing different balances of hardness, strength, and ductility. It is typically used when the goal is maximum softness and workability before further processing.