How to choose the material of bolts and nuts?

The difference between the mechanical function requirements of bolts and nuts is:
①The mechanical function of the nut mainly depends on the hardness of the nut, the height of the nut, and the wall thickness of the nut. Therefore, the requirements for materials are not high. Of course, if there is a requirement for nut resistance, higher-level materials should also be used, such as chemical industry. Fastener nuts have requirements for durability and require higher-grade materials.
② The plastic target required by the bolt, the higher the functional level, only the higher-level material can be reached. The lower-level material only needs to reduce the tempering temperature to meet the strength requirements, but it cannot meet the requirements of plasticity and resistance. In fastener testing , Brittle fractures were judged as unqualified. Steel structure bolts also stipulated higher tensile strength, which is to ensure the plastic resistance of the bolts.