Análise estrutural e evolução térmica do aço austenítico-ferrítico astm 2507 submetido à implantação iônica de nitrogênio

Abstract

The microstructure of the super duplex stainelss steels is featured by grains presenting one of two single phases, austenite  (FCC) or ferrite  (BCC). Consequently, plasma-based surface treatments produce different changes in each of them. The prevailing modification in austenite is the S-phase (N); however, no consensus exist regarding ferrite, which can be converted even into N. The metastable N phase decays under high temperatures, a phenomenon that can compromise the performance of such coatings in service. The ASTM 2507 was nitrided by plasma immersion ion implantation under different temperatures (300-400 ºC), times (2-8 h), ion fluences () and energy densities (Epulse), but the same 10 kV voltage. Afterwards, the samples were annealed up to 470 ºC in inert atmosphere, with in situ structure analysis by X-ray diffraction using synchrotron radiation. These experiments allowed the understanding of the thermal stability of phases on the modified layers, as well as the inference of their pristine composition in the austenitic-ferritic alloy, as follows. The S-phase predominated in grains of both  and  structures in all the modified surfaces. By raising the nitriding conditions, -Fe2-3N, `-Fe4N, CrN and Cr2N were also produced. Such phases precipitated mainly in ferrite grains due to the lower nitrogen solubility and higher chromium content. Likewise, the layers on ferrite were up to 59% thinner than those on austenite. Surfaces with precipitates presented the highest hardness values, 12 GPa. The 300 ºC – 4 h nitriding, produced under the lowest  and Epulse, contained only N in both  and  grains, whose layers presented equivalent thicknesses (~0.8 μm), nitrogen contents (9 wt%) and hardness (7 GPa). During the annealing, the S-phase decayed rapidly and partially by diffusion from 400 ºC, resulting in N with lower nitrogen concentration and  with small quantities of retained solute atoms. The layer expanded most in  grains by heating, increasing up to three times in thickness. The  intergranular nitrogen diffusion through grain boundaries was also evidenced. The decay process was observed to depend on the nitrogen amount originally retained in the layers: the chromium depletion and formation of iron nitrides were more significant in the layers produced under the highest  and Epulse values (350 ºC – 8 h and 400 ºC – 4 h). Such kind of decay was not observed in the 300 ºC – 4 h. The thermal expansion coefficient of the S-phase for this sample was 2.2x10-5 K-1, 20 % higher than the austenite one.