washers
Washers material is used for carbon steel S45CM(SAE1045), S65CM(SAE1065), S70CM(SAE1070), S75CM(SAE1074). You can see a lot of washers used with bolts and nuts. It's a thin flat one with a hole in the middle. There is a hole in the middle, and the flat one is inserted between the object, the nut and the bolt.