A MECHANISM OF FERRITE SOFTENING IN A DUPLEX STAINLESS-STEEL DEFORMEDIN HOT TORSION

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni -3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s(-1) at 1200 degrees C. Transmission electron mi croscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/fer rite boundaries. No evidence of discontinuous dynamic recrystallisatio n involving nucleation and growth of new grains was found within ferri te contrary to some suggestions made in the literature for similar exp erimental conditions. The softening mechanism has been classified as e xtended dynamic recovery characterised by a gradual increase in misori entations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by m isorientation angles not exceeding 20 degrees at a maximum obtained st rain of 1.3. (C) 1997 Elsevier Science S.A.