Superplastic HSLA steels: Microstructure and failure

Certain materials can show superplasticity when traction tested at temperatures higher than 50% of their melting point and with low strain rates ($$ dot{varepsilon } $$ ε · < 10-2 s-1), showing very high elongations (>100%) without localized necking and mainly intergranular fractures. This behavior requires that the starting grain size is small (<10 μm) so the flow of matter can be non-homogeneous (sliding and rotating of the grain boundaries, accommodated by diffusion). This work presents the superplastic characteristic of shipbuilding steel deformed at 800 C and a strain rate slower than 10 -3 s-1. The fine grain size (5 μm) is obtained when using Nb as a microalloying element and manufactured by controlled rolling processes (three stages). After the superplastic deformation, the steel presents mixed fractures: by decohesion of the hard (pearlite and carbides) and ductile (ferrite) phases and by intergranular sliding of ferrite/ferrite and ferrite/pearlite, just as it happens in stage III of the creep behavior. This is confirmed through the Ashby-Verrall model, according to which the dislocation creep (power-law creep) and diffusion creep (linear-viscous creep) occur simultaneously. © 2013 ASM International.