Hex socket set screws and cup point ISO 4029-stainless A4 M20x90 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.

Compared with a Phillips-head screw of the same size, a hex socket screw can withstand around ten times the torque. Hex socket screws are well suited to bolted joints where the screw heads are difficult to access due to the design of the structure. Hex socket screws can be countersunk to save space if protruding screw heads are a problem.

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.