Chemical composition of stainless austenitic steel welds is selected so that a small quantity of delta ferrite is present in the structure after solidification. Its function is to maintain high resistance of the weld to hot cracking (contaminants dissolve better in ferrite than in austenite).
For steels requiring particularly high resistance to corrosion, the filler material should be selected so as to obtain a completely austenitic structure after curing. Delta ferrite content can only be measured directly with metallography, yet because of the complicated preparation, this method is not preferred for routine delta ferrite testing in corrosion-resistant steels. Because ferrite, unlike austenite, is a ferromagnetic phase, certain methods have been developed based on magnetic induction measurement, ferrite content is computed on the basis of magnetic permeability of the examined material.
We measure ferrite content with a Fisher MP30 Feritscope, which measures ferrite as percentage and as ferrite number (FN).
