Bendability of Shot Blasted Ultrahigh-Strength Steels

Abstract

The effect of shot blasting on the bendability of two tempered trial ultrahigh-strength steels has been studied by comparing the bending behaviour of otherwise identical plates with and without shot blasting. The yield strength of the studied 10 mm thick trial steel was 700 MPa and 7.5 mm trial steel 1100 MPa. The local microstructures below the different surfaces were characterized using laser scanning confocal microscopy and FESEM. Microhardness profiles and surface roughness (Raand Rz) were measured and bendability was determined using three-point brake press bending. Shot blasting was found to decrease the bendability of both steels. For the 700 MPa steel shot blasting increased the surface roughness metric Rasubstantially from 7.4 μm with the normal scale surface to 12.6 μm, and for the 1100 MPa steel from 2.8 μm to 6.8 μm. For the 700 MPa steel with the bend axis parallel to rolling direction, the minimum usable punch radius for defect-free bends with normal scale surfaces was 13 mm (1.3 x thickness) and for the 1100 MPa steel, 14 mm (1.9 x thickness), while for shot blasted surfaces corresponding values were respectively 20 mm (2.0 x thickness) and 18 mm (2.4 x thickness). All of these values are excellent considering the strength levels involved. Shot blasting increased the subsurface hardness and introduced work hardening of surface layers. Possible explanations for the observed effects of shot blasting on the bendability are discussed.