Intergranular corrosion behaviour of powder metallurgy duplex stainless steels sintered in nitrogen

The degree of sensitization of duplex stainless steels, steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L were examined by exposure methods (ASTM A 763-93, Practice W and Practice Z) and electrochemical reactivation methods (EPR and EPRDL). Three austenite/ferrite ratios of 50/50, 65/75 and 85/25 and two sintering atmospheres, nitrogen-hydrogen (95 %/ 5%) and vacuum, were chosen. The former sintering atmosphere was the focus of the work and the latter was used on comparative basis. The aim of this work is to study the influence of austenite-ferrite powders and the sintering atmosphere on the degree of sensitization. The intergranular corrosion resistance increased when the amount of austenitic powder increased. It was also seen how the nitrogen played a relevant role in the degree of sensitization. The high degree of sensitization registered for samples sintered in nitrogen prevent its use in as-sintered condition and suggests the need of post-sintered heat treatments. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of duplex stainless steels sintered in vacuum. The results were correlated with the microstructural features. Furthermore, it was found that the electrochemical reactivation methods applied to PM duplex stainless steels were suitable to evaluate the degree of sensitization. (authors)