Decarburizing Annealing
The intentional or unintentional removal of carbon from the surface layer via an oxidizing or decarburizing furnace atmosphere. For certain soft magnetic materials, this step is necessary to reduce magnetic core losses.
Decarburization annealing is a heat treatment defect where the edge layer of the steel loses carbon. This happens when the furnace atmosphere contains oxygen or CO2, which react with the carbon. As a result, the edge zone becomes softer and ferritic, standing out clearly from the harder core structure. When welding, you notice it immediately: the decarburized zone melts differently and can lead to porosity or poor wetting. If you have to weld such a part, grind off the decarburized layer completely beforehand, otherwise you won't get a clean weld. You can recognize the depth of decarburization in a ground section by the bright, carbon-poor edge zone.