Austenitic stainless steels with around 10-12% nickel (e.g. grades 304, 316, 321 and 347) are predominantly non-magnetic due to the face centered cubic (fcc) crystal structure of the austenite phase, which imparts so-called ‘paramagnetic’ (i.e. non-magnetic) behavior.
Although a number of second phases, e.g. inclusions or ferrite stringers, may exist in wrought austenitic stainless steel products, the structure is almost exclusively made up of the austenite phase and hence they are essentially non-magnetic. However, weld metals made with filler metal compositions matching the 300 series austenitic steels (e.g. 308, 309, 316 and 347 types) are designed to have a small fraction of delta ferrite phase, typically around 5-15%, to reduce significantly the risk of solidification cracking, which exists for purely austenitic compositions. The delta ferrite phase has a body centered cubic (bcc) structure, which has ‘ferromagnetic’ properties (i.e. typical magnetic properties, as associated with, for example, mild steels, carbon steels and low alloy steels).
Some delta ferrite may form also in autogenous weld metal (i.e. parent metal which is fused without filler metal addition) and the high-temperature heat-affected zone of a weld, where partial transformation to delta ferrite can occur on heating and where cooling is too rapid for full re-transformation. Hence, overall, welds in many of the 300 series austenitic stainless steels, made with no filler or approximately matching filler will typically show mildly magnetic properties.
Some higher alloyed austenitic steels, with significantly more than 12% nickel, such as type 310, do not show such magnetism as they have a stable, fully-austenitic structure as a result of the high level of nickel, which stabilizes the fcc structure in preference to bcc. Resistance to solidification cracking is then achieved by adding manganese to the filler metal and keeping impurity levels to a minimum.
Where low magnetic permeability is a desirable property for welds in common 300 series steels, welding filler metal should be chosen to give very low ferrite levels. Type 310 is an option and ‘nil-ferrite’ 316 types are available.