18-8 is a SUS304 austenite-based stainless steel, generally used as a press washer type fasteners. Because of the extent of deformation of the punched portions of the material varies between about 15% to 40%, so the extent of work hardening of the material are different.
After SUS304 stainless steel sheet cold, microscopic generate a lot of dislocations and grain boundaries will slip surface, resulting lattice distortion. The larger the amount of distortion when the dislocation density is higher, the more severe stress and lattice distortion of the metal deformation resistance and strength, hardness increased with the degree of deformation, plasticity index elongation, reduction of area decreased.
When hardening up to a certain extent, if they continue to deformation, cracking or there's the risk of brittle fracture, after forming the residual stress can easily cause the workpiece blew rupture. Under the effect of the ambient atmosphere, place some time after the workpiece is automatically intergranular cracks (generally called "season cracking"). Therefore, in SUS304 stainless steel stamping process, generally between annealing process must be softened, that intermediate annealing to remove residual stress, reduce the hardness, ductility recovery materials in order to make the next one processing.
Physical properties of type hardness tensile yield strength elongation