Abstract
The microstructure of SA508 Gr3 steel heavy forgings features, in some zones coming from the original ingot where solidification took place with equiaxial grains, local segregations between the solidification grains. These segregations were studied in two different forgings, where the mean local composition is offset from the original heat analysis, at two different levels because of macro-segregation. The local chemical compositions show significant peaks in alloying elements associated with a rather high level of impurity (phosphorus). The resulting structures after forging feature “bands” of segregated material elongated in directions normal to the short direction (perpendicular to the surfaces) of the forging. These structures are analyzed, by metallographical and micro-analytical means. Mechanical testing and fractographic analysis showed that these segregations contribute to brittle fracture initiation in the ductile-brittle transition because of their harder and more brittle character than the surrounding matrix. Because of the relatively large distance between them (several mm) and depending on their random position with respect to the specimen notch tip, significant scatter is observed in the Charpy impact energy. An analysis of the scatter of Charpy impact energy in relation with the position of segregated bands in the ligament and their distance from the specimen notch tip is presented.
