This has been moved a few times over the years. Notice that 825 and AL-6XN have "N" UNS numbers. At one time an alloy that was not 50% Fe was not stainless.
Today the rule is based on the dominant element. So if the alloy is 45% Fe and less of any other specific element then it is a steel.
Maybe the best example is alloy 33 (R20033). It is considered a Cr based alloy because it is 33% Cr and only 31% Ni, and then after a few other elements bal Fe (works out to be about 32%).
@EdStainless, but for Alloy 825 the Fe content is around 22% and the Ni can be as high as 46%, but it is considered a higher austenitic stainless steel?!
What is the actual problem with the definition? Is it trying to work out the tables of ISO 15156-3, because everything with >= 10.5% Cr is a "stainless steel" for the purposes of that standard?
@SJones, I was trying to explain to a colleague that Alloy 825 is a higher austenitic stainless steel not a nickel alloy and the UNS number does not always reflect that - a clear example is the 904L (in addition to the examples given by @EdStainless) - then he asked the question I had posted - How can we decide from the compositional point of view if an alloy is a stainless steel or a nickel alloy?
Special Metals does it for you, Incoloy is Fe based and Inconel is Ni based.
Seriously though, 825 is an attempt at pushing enough Ni in a SS to get beyond the CSCC range, and still keep the alloy stable. There are plenty of other SS grades with better pitting corrosion resistance than 825 as it has modest Cr and Mo content (but this does help phase stability).
Most of the true Ni alloys have little Fe, usually <10%. There are some odd balls such as X or 718, but you have to think of corrosion resistant alloys separately from high temperature grades.
If you are in oil & gas, just stick to the ISO 15156-3 classification of N08825: solid solution nickel based alloy. It saves a lot of mental wrangling.
