Soft annealing
Soft annealing is a process of heat treatment of metals in which the machinability and the cold deformability are improved by annealing . The material - for example brass or steel - is heated until it glows (between 650 ° C and 750 ° C), and then kept for some time at an elevated temperature until the structure is completely transformed . Then let it cool down slowly. Soft annealing reduces hardness and strength and thereby facilitates further processing steps such as rolling and wire drawingor punching considerably, precisely because it prevents cracks from forming.
General processes during soft annealing
During soft annealing, defects such as dislocations are initially healed; Stresses in the material are reduced. Then a recrystallization takes place , in which new nuclei form ( nucleation ) and replace highly strained crystallites . Eventually crystal growth occurs .
Soft annealing for steels
With hypoeutectoid steel (below 0.8% C), the soft annealing takes place in the area below the PS line on the iron-carbon diagram . In the case of hypereutectoid steel, the soft annealing takes place with the oscillation around A1 (or the SK line) with subsequent slow cooling. As a result, a state that is sufficiently soft and low in tension is achieved for the respective purpose.
Note: A1 denotes a stop or break point in the cooling curve, i.e. transition temperatures ; In addition, the terms Ac1, Ar1, Ac2, Ar2 etc. are used in the iron-carbon diagram. A stands for “arrêt”, ie stopping point or kink point, r for “refroidissement”, ie cooling and c for “chauffage”, ie warming. Depending on whether it is cooled or heated, the transition temperatures are u. U. different. Thus, a transition temperature can only be referred to as Ac1 or Ar1, but never as Acr1 or Arc1 etc.
With this annealing it is irrelevant in which state the structure is before the soft annealing. The focus is only on changing the elongation at break , strength and hardness. Due to the annealing temperature, the lamellar (strip-shaped) cementite loses its strength and can pursue its quest for a body with the smallest possible surface area (the sphere). Granular cementite is formed, which is why one speaks of annealing on spherical cementite (GKZ annealing) . Thus, the material is readily formable and cuttable . However, sub-eutectoid steels tend to "smear" when they are soft-annealed during machining.
- Soft annealing improves the machinability of steels with a carbon content > 0.4 percent by mass.
- In contrast, steels with a carbon content of <0.4 mass percent do not improve the machinability. In contrast, low carbon steels tend to smear when drilling.
- This method is not suitable for use prior to induction hardening .
Web links
- kupferinstitut.de: Heat treatment of copper materials
- Working group heat treatment + materials technology e. V .: Heat treatment of non-ferrous metals