Ferrite Content in Austenitic Stainless Steels

Solution annealing will dissolve most of the ferrite retained from ingot solidification. Because cast products are balanced to have a higher amount of ferrite, annealing will not transform all of it into austenite. However, little or no ferrite will remain in an annealed wrought product.
The DeLong diagram also predicts the amount of ferrite likely to be present in a weld deposit made from the wrought product. Plotting the composition of the wrought plate or bar product will indicate how much ferrite might be present if the material were re-melted (i.e., welding). Still, it is not a valid indicator of the ferrite content in the annealed product. Ferrite content in the actual product can be determined through laborious metallographic examination or by using sensitive magnetic instruments in a laboratory setting (magnetic permeability). However, in most cases, comparative calibrated magnets can establish an approximate ferrite content within 0.5 – 1%. Standard annealed 304/304L, 316/316L, 309, and 310 testing tends to yield 2% or less values. Often, ferrite contents are less than 0.5%.

It should also be noted that the matching filler metals used to join these alloys will also have higher ferrite contents. This is a precaution against solidification cracking during welding.

Depending on the alloy, the ferrite contents in the weld deposits will be between 5 – 10% ferrite. If ferrite-free welds are required, a higher nickel alloy filler should be selected, such as Alloy 20 (ER320LR) or Alloy 625 (ERNiCrMo-3).