Remove by mechanical polishing or chemical cleaning. Residual temperature blackening film is a black protective film formed by thermal polymerization reaction on the surface of fastener by using the residual temperature of fastener tempering, so as to achieve the purpose of corrosion resistance. This protective film can be removed by mechanical polishing or chemical cleaning. The biggest feature of residual temperature blackening film is that it is combined with heat treatment equipment to realize continuous work of heat treatment and surface blackening operation, improve production efficiency and reduce cost.