Flange Nut:

A flange nut is a nut that has a wide flange at one end that acts as an integrated washer. This serves to distribute the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface. These nuts are mostly hexagonal in shape and are made up of hardened steel and often coated with zinc.

General :

They may be serrated to provide a locking action. Moreover On a serrated flange nut, these are angled such that they keep the nut from rotating in the direction that would loosen the nut. Because of the serrations they cannot be used with a washer or on surfaces that must not be scratched. The serrations help in preventing the vibration of the nut from moving the fastener, thus maintaining the holding power of nut. DIN 6923 are with serration and that’s why they don’t need washers. The serration is keeping that the nut doesn’t unscrew. The flange may be serrated to provide a locking action. The serration are angled such that they keep the nut from rotating in the direction that would loosen the nut. Moreover, the serration help in preventing the vibration of the nut from moving the fastener. Thus maintaining the holding power of nut.