Hex Head screws with flange DIN 6921-stainless A4 M6x25 mm

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".

The term plain indicates a completely untreated, merely oiled, smooth surface for all DIN and ISO screws such as wood screws, hexagon head screws, threaded rods, nuts and many types of washers.

The wrench sizes for hexagon bolts and nuts are specified in ISO 272: 1982-01. The nominal wrench size on the workpiece (e.g. bolt, nut) is the same as that on the tool. However, there is some play between the two to allow the tool to be fitted without difficulty. The relationship between the nominal diameter d of the thread and the wrench size s is standardised in EN ISO 4014 and EN ISO 4017 for hexagon bolts, and in EN ISO 4032 for hexagon nuts (dimensions in mm).

Austenitic stainless steels derive their different strength grades from cold working to varying degrees. The number 80 following the grade designation indicates the tensile strength as one-tenth for bolts. A bolt in strength class 70 therefore has a tensile strength of 700 N/mm². For nuts, the number 70 indicates a stress under test load of 700 N/mm².

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.