Effect of grain size and intergranular stresses on the cyclic behaviour of a ferritic steel

The effect of the grain size and intergranular stresses on the cyclic stress-strain behaviour of a low carbon steel has been investigated at different strain amplitudes. Annealed and prestrained specimens were submitted to fatigue tests. Optical and scanning electron microscopes were used to analyse the grain size and grain boundary effects on the cyclic behaviour and crack nucleation process. An increase in the grain size reduces the strain amplitude for cyclic hardening. However, a reduction in the grain size increases the resistance to macroyielding, decreases the grain boundary affected zone and changes the crack nucleation mechanism from intergranular to transgranular. An original cyclic model, based on geometrically necessary dislocations theory, is proposed to explain the cyclic stress-strain behaviour observed. The improvement of fatigue life, resulting from grain size reduction and previous cold work, is attributed to a more uniform stress distribution within the grains and to the change in the crack nucleation mechanism. © 1993.SCOPUS: ar.j