Abstract
Several old generation reactors with shell thickness of approximately six inches (150 mm) were investigated for material property damage and evaluation of defects. These 26 year old reactors were made of annealed 2¼Cr-1Mo steel with double layer stainless steel weld overlay. The UT indications in the weld joints and PT indications on the surface of stainless steel weld overlay were detected during routine inspections.
After retirement of the reactors, several test blocks which included the welded joints and weld overlays were taken from the locations with internal flaws and overlay cracks. Each block was subjected to detailed ultrasonic testing for actual sizing of the flaws; metallographic examination to confirm the accuracy of UT, and metallurgical evaluation to determine the causes of the flaws and other tests for possible in-service material property degradation.
The size of internal weld flaws detected by UT inspection was confirmed in each test block by metallographic examination. Optical and electron microscope examination revealed that the crack-like flaws in the weld deposits increased in size by hydrogen assisted crack growth. Growth may have occurred at the ambient temperature due to shutdown cycles. The weld overlay cracks detected by PT and UT were confirmed to be caused by hot cracking during fabrication due to insufficient ferrite phase. All of the weld overlay cracks stopped at the interface between the weld overlay and base metal.
In-service toughness degradation was also evaluated by Charpy impact testing. Reduced low temperature toughness values were observed only in the weld deposit metal with full bainitic structure. The lower susceptibility to temper embrittlement observed in the base plate is attributed to the annealed structure. Based on the results obtained, serviceability of these reactors is discussed.
