Bending properties of functionally graded 300M steels

The bending behavior of compositionally-graded martensitic steel was investigated in partially decarburized sheets of alloy 300M. The best combination of strength and bendability was obtained when the thickness of the inner hard core was reduced to one third of the sheet thickness. The improved bending behavior of the compositionally-graded sheet is attributed to (a) the suppression of crack propagation by introducing a more ductile phase at the surface and (b) the spreading of plasticity along the circumferential direction as a result of the presence of a yield stress gradient in the thickness direction. The bendability of the graded material was limited by the initiation and rapid propagation of fracture within the hard core of the sheet. Unlike traditional materials in which the springback angle increases remarkably with increasing yield strength, the compositionally graded material had a small springback angle which is comparable to that of steels having a low yield stress similar to that of the soft decarburized surface layer.