Nitrogen bearing austenitic stainless steels for surgical implants

Nitrogen addition promotes substantial improvements on general and localized corrosion performance of stainless steels. In recent times high Nitrogen (up to 0.6 wt%) and Mn bearing super austenitic stainless steel has been studied for medical applications due to its low Ni content, the so called body friendly alloys. 18%Cr, 0.4%N and 15%Mn stainless steels were cast either from electrolytic or commercial master alloys in induction furnace, forged, solubilized at 1423K for 3 hours and water quenched. Delta ferrite and carbide precipitate free structures were observed. Potentiodynamic corrosion tests of all alloys in 0.5M, sulfuric acid solution presented passive current density as low as those observed for AISI: 316L stainless steel. Intergranular corrosion susceptibility testing, according to ASTM A262 - Practice A, were performed and samples made of electrolytic raw material presented step structures with no ditches at grain boundaries, pointing out no sensitization. Mechanical testing presented high yield and tensile strengths with 60% elongation These properties can be attributed to the Nitrogen action on stacking fault energy of austenitic structure of the alloy. These results fulfill orthopedic implant materials application requirements. In vivo biological compatibility tests for two months showed no adverse reactions, no necrosis and no evidence of corrosion products on implanted materials.