A stainless is defined as 12% minimum Chromium, which can encompass many stainless grades. There are sub categories of this Austenitic type stainless though. Depending on the composition, some materials are more Austenitic than others; primarily due to their Nickel (Ni), Manganese (Mn), Carbon (C) and Nitrogen (N) contents. To consider this further, you need to understand the corollary to Austenite; Ferrite and Martensite are what many people are looking for when they require magnetism, or magnetic permeability. When you mix Austenite and Ferrite it is a Duplex (contains both, nearly equal amounts). A low Ferrite number generally indicates a lower magnetic permeability. Stainless that is high in Ni, Mn, C, or N or a combination thereof can be an indicator that it will have a lower Ferrite content. Some stainless steels with generous amount of these attain a negative ferrite calculation, using Schaefflers diagram you can predict these on a graph or us a Ferrite Number (FN) formula to arrive at the estimate.
Why is the Austenitic structure so important in many applications? Austenite is a Face Centered Cubic (FCC) crystalline structure, the most resistant to corrosion. Many Nickel based alloys are very well known for their austenitic structure to aid in the slowing of corrosion in extreme environments. Here is where it becomes a bit unclear. Not all Austenitic stainless grades will remain austenitic – they may partially change structure to Body Centered Cubic (BCC) Ferrite with deformation and become more magnetic. This phase change also can create problems with corrosion that were unexpected. These are two things that Austenitic stainless steels normally would be detrimental. This phase change is known as Transformation Induced Plasticity (TRIP). 304 and 316 are affected by this phenomenon.
If you want your austenitic stainless to remain austenitic, you need to make sure that it is a fully austenitic stainless – one that is stable at temperature, and also after forming (deformation). Two grades that are excellent replacements for 300 series stainless steels are: Nitronic 50 for replacing 316, and Nitronic 60 for replacing 304. Both are fully austenitic with low magnetic permeability, with higher Nickel and Chromium to stabilize the grades. As an added bonus, the Nitrogen austenitizing additions help to elevate the yield strength of the Nitronic grades. High Performance Alloys calls these Super Austenitic Stainless steels.