鋭敏化した304ステンレス鋼の腐食疲労過程におけるAE発生におよぼす環境，力学，材料条件の破面解析による検討

Abstract

Corrosion fatigue (CF) tests were conducted for sensitized 304 austenitic stainless steel to investigate effects of environments, mechanical conditions, and sensitization on crack growth and AE (Acoustic Emission) behavior. The same experimental method as in the previous work conducted for solution treated specimens was employed. The crack growth rates of sensitized specimens were found to be accelerated in air as compared with those in the solution treated case. Those obtained in transpassive region (+1.1 V) and at corrosion potential (Ecorr) for sensitized specimens were more accelerated, due to superposition of the acceleration by sensitization and environmental effects such as intergranular corrosion, metal dissolution, or hydrogen evolved from cathodic reaction. The AE activity during crack growth of sensitized specimens was much higher than that of solution treated specimens through all the ΔK values in this study. This indicates that AE techniques are very useful to monitor crack growth processes under sensitized condition. SEM observations of the sensitized fracture surfaces revealed that many microcracks were nucleated at grain boundaries perpendicularly to the main crack surfaces at higher ΔK than ∼34 MPa·m1⁄2. These microcracks (separations) were concluded to be the main AE source during the crack growth in air under sensitized condition, since the numbers of the separations were proportional to the detected AE event counts. In addition to separations, intergranular-like cracks were observed on the sensitized fracture surfaces tested in transpassive region. The high AE activity (event count rate) at relatively low ΔK level was ascribed to the nucleation of these intergranular-like cracks which arose from intergranular corrosion and triaxial stress in the crack tip plastic zone. Intergranular-like cracks due to hydrogen and triaxial stress as well as the separations were observed on the sensitized fracture surfaces tested at Ecorr. The AE sources which provided the high AE activity during CF crack growth under sensitized condition were shown to be such microcracking processes as separations and intergranular-like cracks caused by the co-operation of environments (intergranular corrosion or hydrogen), triaxial stress (mechanical conditions), and sensitization.