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Stress-corrosion cracking (SCC) studies were conducted in the following commonly used pressure vessel steels: ASTM A533 Grade B Class 1 (A533-B-1) plate and a fabricated forging that is equivalent in chemistry and thermomechanical processing to American-made ASTM A508 Class 2 (A508-2) forgings. The purpose of these studies was to determine the response of the materials in a simulated pressurized-water reactor environment. Round tensile specimens were bolt-loaded to 75 to 80 percent of yield and left in the water at 93°C for 2000 h. At the end of this time period, the specimens were taken out of the water, unloaded, and examined by scanning electron microscopy (SEM) and by energy-dispersive X-ray analysis. The specimen cut from A533-B-1 plate did not have any cracks as a result of the SCC tests. Several cracks, some of which were visible with the unaided eye, had developed in the specimen that was cut from the A508-2 forging. The polished and etched sections of the specimen that showed SCC, when examined with the SEM, revealed the presence of many microcracks starting from the outside surface and propagating along either inclusion-matrix or carbide-matrix interfaces; the corresponding microstructure was mostly pearlitic. The microstructure of the specimen that did not crack was bainitic; it had fewer inclusions and carbide particles, and no microcracks were observed. When the fracture surfaces of the larger cracks were examined by SEM, they revealed a cleavage-like failure propagating along inclusion bands. X-rayimage scansand energy spectra of these hands showed thatthey were silicate and manganese-sulfide inclusions. A hydrogen-assisted cracking model has been proposed to explain the experimental results on A508-2 forging. Produced by a cathodic reaction and aided by the stress fields, hydrogen diffuses ahead of the crack tip to the inclusion sites. This causes a preferential decohesion at the inclusion-matrix interfaces and subsequent cracking along inclusion bands. The absence of stress-corrosion cracking in A533-B-1 plate tested under identical experimental conditions is due mainly to fewer inclusion and carbide particles and to the more refined bainitic microstructure of this steel. This type of microstructure is less susceptible than the mainly pearlitic microstructure found in A508-2 forging to hydrogen-assisted cracking. |
