CHARACTERIZATION OF A NITROGEN-RICH AUSTENITIC STAINLESS-STEEL USED FOR OSTEOSYNTHESIS DEVICES

Two laboratory melts of the standardized nitrogen-rich austenitic stai nless steel specified in ISO 5832-9:1992(E) have been characterized wi th respect to corrosion properties, mechanical properties, and microst ructure. The two melts differ essentially in nitrogen concentration, n amely, 0.42 and 0.46 wt %, respectively. Both melts were found to fulf ill 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 demon strated by a critical pitting temperature of about 70 degrees C for bo th alloys, which should be compared with 40 degrees C for the biocompa tible reference steel AISI 316. While no difference in corrosion resis tance was observed between the two alloys, a significantly higher mech anical strength and lower toughness were observed for the nitrogen-ric h melt. Using electron diffraction Z-phase was identified in unaged ma terial. These were present as primary precipitates, most likely precip itated 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 t o be of minor importance. The ageing behavior was studied indirectly i n terms of toughness as a function of ageing. Formation of the interme tallic phase-chi-phase-was observed, particularly during prolonged age ing at 800 degrees C. The total absence of chi-phase in forged bar con dition shows that the cooling rates during production are sufficient t o suppress the formation of chi-phase. (C) 1996 John Wiley & Sons, Inc .