From the Book of War, Appendix A:
3.6. Metal Armor must be made of period metals and alloys such as iron, steel, bronze, brass, or copper. Modern steel alloys are also allowed.
A few people have said in this thread that stainless steel is not allowed. The BoW disagrees with you.
I support the period materials requirement. However, it should be noted that non-period materials are sometimes needed to make armor safe. I'll give a case in point. I bought a set of stainless plate spaulders and bracers and used them in combat for 8 months. [Source was the eBay store Wyndsyngyr's War Gear, found at
http://www.stores.ebay.com/id=31025215 . I recommend them highly.] I had no problems getting them passed by heralds at Spring War.
Then at a summer event someone's sword got damaged by one of the rivets, and they were given extra heavy scrutiny by the heralds. Even though the edges are penny-round and smooth, it was strongly recommended to me that I pad all edges with some thin rubber tubing, just for extra safety. I got 25' of car window molding at an auto parts store and went to town. The result is stunning, and you can't tell its rubber unless you look closely. But it IS a non-period material, and it is visible.