Phenomenological Aspects of Fatigue Crack Initiation and Propagation in Type 403 Stainless Steel in Simulated Steam Cycle Environments

The fatigue crack initiation and propagation behavior of type 403 (UNS S40300) stainless steel (SS) was studied in a large number of systems simulating those encountered in steam cycle environments in an attempt to establish relationships between the modes of cracking and the characteristics of the environments. Crack initiation was found to be controlled by mechanical effects during fatigue testing in air, high-pH hydroxide (OH^–) solutions, concentrated phosphate (PO₄^3–) solutions, and silicate (SiO₄^4–) solutions. In these environments, a large number of cracks initiated by an intrusion-extrusion mechanism. These cracks sometimes were associated with mechanical separation of nonmetallic inclusions and debonded zones. Environmentally controlled crack initiation occurred during fatigue testing in chloride (Cl^–) and sulfate (SO₄^2–) solutions. In these aggressive environments, pitting contributed to the crack initiation stage, in addition to dissolved nonmetallic inclusions and emergent boundary etching. Under these conditions, a significant reduction occurred in the number of cracks and the number of cycles to failure, reflecting an increased stress concentration at fewer cracks. When a combination of environmental and mechanical effects controlled the behavior, the characteristics of the initiated cracks became quite variable, being a function of material properties and the properties of the mildly aggressive environments, such as distilled water, low-pH OH^– solutions, and dilute PO₄^3–solutions. The observed fracture mode was found to be a function of potential. At potentials < –350 mV_NHE, ductile fracture was the dominant mode of failure. Intergranular cracking was the dominant mode of failure in the potential range from –350 mV_NHE to –150 mV_NHE, while transgranular cracking took place at potentials > –150 mV_NHE.