Soft Annealing – Process, Advantages and More

Weichglühen bezeichnet ein Verfahren zur Wärmebehandlung von Metallen und  Stählen zur Verbesserung der Spanbarkeit und Umformbarkeit durch Umwandlung im Gefüge und Verringerung von Spannungen im Material. Die Härte des Werkstoffs wird dauerhaft verringert, während die Bearbeitbarkeit gesteigert wird.

Soft annealing phase transition

The temperature range for soft annealing runs along the PSK line. Sub-eutectoid steels (<0.8% carbon content) are heated just below the PS line. This ensures that the lamellar cementite does not disintegrate and transforms into a round shape. This so-called spherical cementite is formed from the strip cementite of the pearlite. Hypereutectoid steels, on the other hand, are pendulum annealed along or just above the PSK line. In addition, hypereutectoid steels should be soft annealed before hardening/tempering, as this allows the carbon to dissolve better when heated.

The material is held at an elevated temperature for a long time until the microstructure has sufficiently transformed (approx. 100 hours). The steel is then cooled slowly to ensure good formability.

The treatment of steels with a carbon content of <0.3% is not suitable for soft annealing. Steels with such a low carbon level are usually already soft anyway.

How the process influences mechanical properties

Soft annealing profoundly influences the mechanical properties of metals in several key ways:

  • Reduction in hardness: The process lowers the material’s hardness, making it less resistant to deformation and easier to machine.
  • Increased ductility: Soft annealing enhances the material’s ability to deform without breaking, which is crucial for manufacturing processes like deep-drawing and rolling.
  • Improved machinability: The transformation of lamellar cementite into a rounded form improves the metal’s ease of cutting, thereby boosting its machinability.
  • Enhanced workability: The overall malleability of the metal is increased, making it more amenable to various mechanical processes such as bending and shaping.

By altering the microstructure through controlled heating and slow cooling, the procedure makes the material more versatile and easier to work with, without compromising its structural integrity.

Applications and industries

Soft annealing is commonly used prior to manufacturing processes like deep-drawing, pressing, and rolling. It is a chosen treatment to improve workability and is particularly useful in the automotive and aerospace industries.

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