I picked up a surplus power transformer shell at the scap/recycle center last week. It is painted stainless steel, 47 gallons and never used. The scrapyard attendant thought it was carbon steel and sold it to me for $20. It is going to make an awesome boiler!
Transformer steel is often called Silicon Steel - up to about 4% Silicon and 93-96 Iron with minor (< 1%) additions of Nickel, Chromium, Aluminum, Titanium. If it was painted it is very likely not Austenitic (300 Series) Stainless.