Investigation of the microstructure in the area of perforations in bent sections

1. For all steels and thicknesses tested, perforation induces strain hardening of the metal in the area of the holes. Considerable deformation of grains is observed at the edges of the holes and increase of the microhardness by as much as 70% as compared with the original metal. The depth of the strain-hardened layer was 0.8–1.2 mm, but substantial strain hardening extended only to a distance of 0.5–0.8 mm from the edges of the holes. 2. The degree of strain hardening depends on the mechanical properties of the steel—the more ductile the steel, the higher the strain hardening. 3. Forming of perforated blanks has no effect on the strain hardening of the metal at the edges of holes or the depth of strain hardening, which makes it possible to manufacture perforated sections by a more progressive method and guarantees reliable operation of structures built form them.