Volume 3, Issue 5 (May 2006)
Comparison of the Corrosion Fatigue Characteristics of 23Mn-21Cr-1Mo Low Nickel, 22Cr-13Ni-5Mn, and 18Cr-14Ni-2.5Mo Stainless Steels
The purpose of this study was to evaluate and compare the notched corrosion fatigue (CF) characteristics of a series of stainless steel alloys including 18Cr-14Ni-2.5Mo (ASTM F 138), 22Cr-13Ni-5Mn (ASTM F 1314), and 23Mn-21Cr-1Mo low nickel stainless steel (ASTM F 2229). Evaluation of alloy composition, microstructure, static single cycle mechanical properties, stress corrosion cracking, and smooth sample corrosion fatigue was performed on the same lots of each material in a previous study in our laboratories. Notched CF tensile samples of each alloy were machined using low stress grind techniques to a 2.5 mm notch root diameter (Kt=3.2) and tested at 1 Hz according to ASTM F 1801 in Ringer's solution at 37°C. A minimum of three samples was tested at five tension-tension sinusoidal load levels including a run-out level at 106 cycles. SEM analysis was performed on the fractured surfaces of representative samples of each alloy to characterize and compare the failure mechanisms. Previous evaluation of corrosion fatigue on smooth samples of these alloys found no significant differences in the fracture modes or cycles to failure between the samples run in distilled deionized water and those run in Ringer's solution indicating corrosion fatigue mechanisms were not in effect. However, a significant lowering of fatigue strength was observed for the notched samples of each alloy compared to the smooth samples previously tested. This suggests that even though the notch sensitivity of these alloys under static single cycle conditions has been shown to be acceptable, a notch under dynamic fatigue mechanisms may cause a substantial reduction in strength and implant life. Despite this reduction in fatigue strength, in high strength applications where the implant must be bent during surgery or may be scratched during implantation BioDur 108 proved to be the best alternative. For lower strength applications, 316LS revealed the least fatigue notch sensitivity of any of the alloys tested.