Widmanstätten structure
The Widmanstätten structure (named after Alois Widmanstätten ( 1754 - 1849 ), also known as the overheating structure ) is characteristic of unannealed cast steel . It is a brittle ferritic - pearlitic structure .
It is to be distinguished from the Widmanstätten structure made visible in nickel-iron meteorites by etching .
Emergence
The Widmanstätten structure occurs when the austenite crystals are too large or are cooled too quickly by too high temperatures . The γ → α conversion from austenite to ferrite is abnormal.
It arises due to segregation phenomena with accelerated cooling from a coarse-grained austenite structure when the temperature has dropped below A c3 (see iron-carbon diagram ). Pre-eutectoid ferrite with a needle-like or plate-like structure, which is embedded in a pearlitic matrix, forms in the relatively large austenite grains. With a finer-grain austenite, the ferrite would form at the grain boundaries; With the Widmannstätten structure, the ferrite does not arise on the grain boundaries, but inside the austenite grains as ferrite plates on the crystallographically preferred lattice planes .
A Widmanstatt structure only arises in steels with a carbon content of around <0.3%.
meaning
Steel with a coarse structure can be machined well due to its lower toughness and the tool life increases. For this purpose, a Widmanstätten structure is occasionally deliberately created. However, since this would lead to undesirable properties in use, the Widmanstätten structure is removed by subsequent normalizing and the yield point , elongation and strength are increased again.