The Effect of Boundary Precipitates on the High-Temperature Fatigue Strength of Austenitic Stainless Steels

Abstract

Austenitic stainless steels of base composition Fe-18 Cr-12 Ni have been tested in low-stress-amplitude fatigue in the range 600–8000 C (873–1073K) (0.5–0.62 T_m ). Metallographic evidence, including transmission electron microscopy, illustrates the extent of boundary sliding and subsequent intergranular crack formation that occurs in the solid-solution 18/12 alloy. Pinning the grain boundaries, by M₂₃C₆ carbide precipitates, in an 18/12/0.05 C alloy improves the high-temperature fatigue properties by inhibiting both grain-boundary sliding and migration, so that the fracture-mode transition temperature is increased from ∼ 600 to 800°C. Nucleation of the triple-point cracks formed at the carbide/matrix interface at 800°C may be explained by the Smith-Barnby analysis of the stresses generated at particles along sliding boundaries. This analysis gives a value of 820 erg/cm² (mJ/m²) for the carbide/matrix fracture surface energy. The experimental observations thus confirm the prediction that the fracture mode in this type of alloy is determined primarily by the applied stress and the spacing of the grain-boundary precipitates.