Ferrite transformation on annealing at 1050 and 1150°C in surface melted dupex stainless steel

Three different microstructure morphologies: lathy ferrite (LF), vermicular ferrite (VF) and intercellular austenite (IA) were achieved in duplex stainless steel by surface melting using the GTA technique. The cooling rates were of the order of 10(1)-10(3) degrees C/s. All three structures are exceptionally fine. On annealing at 1050 and 1150 degrees C, the structures proved to be highly unstable. The process kinetics depend only on the attained degree of the thermodynamic equilibrium. The structure morphology, via the delta/gamma boundary characteristics considerably influence the type of secondary austenite, gamma(2), which can be formed in the interior of ferrite, gamma(2i), and on existing delta/gamma boundaries, gamma(2b). The precipitation of highly dispersed gamma(2i) pal-rides causes a very large increase in the parameters Sv delta-gamma and S-v delta gamma/(V-g(delta), With increasing time of annealing intensive coarsening of the structure by coalescence and rounding of the boundaries decreases the effect of the change of the phase composition. Ar 1050 degrees C the sigma-phase is also observed, bur at 1150 degrees C only the delta-->gamma transformation and a coarsening processes occur.