Tempering and annealing
State-of-the-art hardening and tempering technology
Hardening is a heat treatment that makes steels hard and wear-resistant.
In particular, tools and components subject to wear are hardened.
Hardening consists of several work steps:
- First, the tool is heated to hardening temperature and held at this temperature if necessary.
- It is then quenched, i.e. immersed in water or oil. This makes the steel very hard, but also brittle and fragile.
- This is why the workpiece is then tempered, i.e. heated to tempering temperature. In general, the aim of tempering is to increase the deformation capacity of hardened components and to reduce the risk of cracking.
- The workpiece is then left to cool in the air.
When heated, the body-centered cubic ferrite lattice transforms into the face-centered cubic austenite lattice. The vacant space in the center of the crystal is occupied by a C atom. If the austenitic steel is cooled (quenched) very quickly, the face-centered cubic austenite lattice suddenly collapses into the body-centered cubic ferrite lattice. The C atom in the middle does not have time to move out of the lattice. There is now a C atom and an additional iron atom in the center of the lattice. This strongly distorts the crystal lattice. The result is a needle-like structure called martensite. Martensite is very hard but brittle and is only formed if the steel contains at least 0.2% carbon. With certain high-alloy steels, cooling in the air already leads to martensite formation.
Precise and efficient annealing with the latest induction technology
Annealing is the heat treatment of a workpiece at a defined temperature, taking into account a specific holding time and subsequent slow cooling.
A distinction is made between the following annealing processes:
- Normalizing is mainly carried out after previous hot forming of components. Normalizing is intended to achieve a fine-grained ferrite-pearlite structure. As a result, coarse-grained and uneven microstructures can be transformed into new, homogeneous and fine structures. Depending on the C content, annealing takes place at 750°C to 950°C.
- Stress relief annealing is used to reduce residual stresses in the workpiece as a result of cold forming, structural transformation, thermal stress or machining. Stress relief annealing is usually carried out between 550°C and 650°C with sufficiently long holding times and subsequent very slow cooling. There are no significant changes to the microstructure or mechanical properties.
- Soft annealing refers to annealing at a temperature of 680°C to 750°C followed by slow cooling in order to achieve the softest possible state. The aim is to produce granular pearlite, a soft microstructure that provides the best workability for chipless forming and machining.
- Recrystallization annealing is used when a microstructure that has been distorted by cold working is to be returned to an undistorted microstructure.
- Diffusion annealing refers to long-term annealing at 1050°C to 1250°C. It is used to reduce concentration differences that occur when casting large iron castings.
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