Recrystallization annealing

Recrystallization annealing process consists of heating a steel component below A₁ temperature i.e. at temperature between 625⁰C and 675⁰C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi-step cold working operations. In certain case, recrystallization annealing may also be applied as final heat treatment. The cold worked ferrite recrystallizes and cementite tries to spheroidise during this annealing process. Recrystallization annealing relieves the internal stresses in the cold worked steels and weldments, and improves the ductility and softness of the steel. Refinement in grain size is also possible by the control of degree of cold work prior to annealing or by control of annealing temperature and time.

Stress relief annealing

Stress relief annealing process consists of three steps. The first step is heating the cold worked steel to a temperature between 500⁰C and 550⁰C i.e. below its recrystallization temperature. The second step involves holding the steel component at this temperature for 1-2 hours. The final step is to cool the steel component to room temperature in air.

The stress relief annealing partly relieves the internal stress in cold worked steels without loss of strength and hardness i.e. without change in the microstructure. It reduces the risk of distortion while machining, and increases corrosion resistance. Since only low carbon steels can be cold worked, the process is applicable to hypoeutectoid steels containing less than 0.4% carbon. This annealing process is also used on components to relieve internal stresses developed from rapid cooling and phase changes.

Normalizing

Normalizing process consists of three steps. The first step involves heating the steel component above the A₃ temperature for hypoeutectoid steels and above A_cm (upper critical temperature for cementite) temperature for hypereutectoid steels by 30-50⁰C (Figure 4.7.5). The second step involves holding the steel component long enough at this temperature for homogeneous austenization. The final step involves cooling the hot steel component to room temperature in still air. Due to air cooling, normalized components show slightly different structure and properties than annealed components. The same are explained in table 4.7.1.

The properties of normalized components are not much different from those of annealed components. However, normalizing takes less time and is more convenient and economical than annealing and hence is a more common heat treatment in industries. Normalizing is used for high-carbon (hypereutectoid) steels to eliminate the cementite network that may develop upon slow cooling in the temperature range from point Acm to point A₁. Normalizing is also used to relieve internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Normalizing also improves the ductility without reducing the hardness and strength.