Characterization of a nitrogen-rich austenitic stainless steel used for osteosynthesis devices

Two laboratory melts of the standardized nitrogen-rich austenitic stainless steel specified in ISO 5832-9:1992(E) have been characterized with respect to corrosion properties, mechanical properties, and microstructure. The two melts differ essentially in nitrogen concentration, namely, 0.42 and 0.46 wt %, respectively. Both melts were found to fulfill the requirements in the ISO standard for corrosion and mechanical properties. The resistance to pitting corrosion in a solution of 0.9% NaCl, intended to simulate the conditions in the human body, was demonstrated by a critical pitting temperature of about 70 degrees C for both alloys, which should be compared with 40 degrees C for the biocompatible reference steel AISI 316. While no difference in corrosion resistance was observed between the two alloys, a significantly higher mechanical strength and lower toughness were observed for the nitrogen-rich melt. Using electron diffraction Z-phase was identified in unaged material. These were present as primary precipitates, most likely precipitated in the liquid state owing to the high concentration of nitrogen in combination with the presence of the strong nitride former niobium. However, the influence of Z-phase on pitting corrosion is believed to be of minor importance. The ageing behavior was studied indirectly in terms of toughness as a function of ageing. Formation of the intermetallic phase--chi-phase--was observed, particularly during prolonged ageing at 800 degrees C. The total absence of chi-phase in forged bar condition shows that the cooling rates during production are sufficient to suppress the formation of chi-phase.