Soft annealing

Soft annealing is a high temperature heat treatment process performed around A1. As the name suggests the aim of the process is to make a material as soft as possible. After soft annealing the material will have a soft and easy to machine structure.

Benefits

Steels with higher carbon content, and most high-alloy steels, which are allowed to air cool after hot working, such as forging or hot rolling, are usually hard to machine. Soft annealing reduces the hardness and makes the material easier to machine. Soft annealing of low carbon steels < 0,35% C will normally result in a structure too soft and sticky for cutting operations.

The risk of hardening cracks during re-hardening of quenched and tempered steel can be reduced by soft annealing prior to the hardening and tempering process.

Application & materials

Soft annealing can be performed on steel as well as on copper and brass alloys.

Soft annealing is normally performed on steels with a high carbon content and on tool steels to provide a softer and easier to machine structure.

During cold forming of copper and brass components, the material will become harder. The greater the cold forming is, the harder the material will become. The effect of cold forming can be eliminated by soft annealing.

The hardness obtained after soft annealing is dependent on steel type or the type of alloy.

Materials typically soft annealed:

• Steel 170-300 HB
• Copper 40-70 HV
• Brass 50-100 HV

Process details

During soft annealing of a pearlitic steel structure the cementite lamellae will spheroidise creating small round cementite particles in a soft ferritic matrix. This is a rather time consuming diffusion process and the soaking time is normally >4h. After the spheroidising process the component is cooled down slowly (free).

The process temperature should be chosen dependent on the steel alloy, normally between 700°C and 900°C.

Soft annealing is usually performed in a furnace with protective gas or in a vacuum furnace to avoid oxidation or de-carburisation.

For copper and brass alloy the physical process is different and the soft annealing temperature is between 300°C and 650°C for copper alloys and between 425°C and 650°C for brass alloys.