Lock washers type SK forme M stainless steel A4 12,4/27,25/1,8

Stainless steel refers to alloyed or unalloyed steels that contain at least 10.5% chromium and less than 1.2% carbon. Colloquially, stainless steel is also referred to as stainless steel (NiRo), although not all stainless steel is the same as "stainless".

Stainless steels are defined in DIN EN 10088-1 as iron alloys containing at least 10.5 per cent chromium and no more than 1.2 per cent carbon. The multi-part DIN EN ISO 3506 series of standards applies to mechanical fasteners made from stainless steels. Grade A4 is classified as acid-resistant due to its molybdenum content of 2 per cent to 3 per cent. A4 cannot be heat-treated and achieves its various strength classes through cold working to different degrees.