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 attemp t to establish relationships between the modes of cracking and me char acteristics of the environments. Crack initiation was found to be cont rolled by mechanical effects during fatigue testing in air, high-pH hy droxide (OH-) solutions, concentrated phosphate (PO43-) solutions, and silicate (SiO44-) solutions. In these environments, a large number of cracks initiated by an intrusion-extrusion mechanism These cracks som etimes were associated with mechanical separation of nonmetallic inclu sions and debonded zones. Environmentally controlled crack initiation occurred during fatigue testing in chloride (Cl-) and sulfate (SO42-) solutions. In these aggressive environments, pitting contributed to th e crack initiation stage, in addition to dissolved nonmetallic inclusi ons and emergent boundary etching. Under these conditions, a significa nt reduction occurred in the number of cracks and the number of cycles to failure, reflecting an increased stress concentration at fewer cra cks. When a combination of environmental and mechanical effects contro lled the behavior the characteristics of the initiated cracks became q uite variable, being a function of material properties and the propert ies of the mildly aggressive environments, such as distilled water low -pH OH- solutions, and dilute PO43- solutions. The observed fracture m ode was found to be a function of potential. At potentials < -350 m(NH E), ductile fracture was the dominant mode of failure. Intergranular c racking teas 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 mVM(NHE).