Reheat cracks usually manifest themselves as macroscopic defects, which are centimeters long and deep, and are detectable by the usual nondestructive examination (NDE) procedures or as microscopic grain boundary decohesidris (GBD) that an beyond the limit of detection by commercial NDE procedures. This report has concentrated on the significance of the microscopic cracks that may go undetected;The probability that GBD exist in the heat-affected zones (HAZ) of weldments of pressure vessel steels is high;particularly in SA 508 Class 2 weldments.nThe GBD reside in the coarse-grained region of the HAZ:The microstructure of this region tends to be a tempered martensite or lower bainite, a structure whose fracture toughness is superior to that of higher temperature transformation product upper bainite and pearlite ferrite aggregate). Further, this region should be less sensitive to irradiation embrittlement. Toughness data for this region in cither the unirradiated or irradiated condition are sparse;However those data that are available indicate that this area is superior in toughness to that of the base metal.
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