They are used for pipe connections or parts which needed to increase the fitting surface with nut mostly.
A3 low carbon steel ,35K,45#steel,40CR,35Crmoa
Color ZP,White ZP,Black ZP(usually cold electricity galvanizing )
It’s one piece of washer and nut ,and there is the anti-slip tooth on the bottom,which increases the bonding between nut and the workpiece,compared to the ordinary nut and washer combination,it’s stronger and more tension.
The size above M20 are flat flange mostly(There is no tooth on its surface.)These nuts are used for some special equipment and special places.Generally,the manufacturers don’t have ready goods.