Abstract
The characterization of reheat cracks in FCAW weld metal of a fluid catalysis cracking (FCC) unit has been performed to clarify the effect of bismuth on the reheat cracking sensitivity, and y-groove self-restraint cracking tests have been carried out with FCAW weld metal containing various amounts of bismuth.
As for the weld metal of practical construction, the fractured surfaces of reheat cracks showed a dendritic appearance and cracks mainly propagated along the columnar grain boundary. No evidence of liquation was observed on the fractured surface.
During the y-groove self-restraint cracking test, reheat cracking occurred only in the bismuth-containing weld metal at temperaturs above 823K during heating or holding. The reheat cracking sensitivity was increased with increasing bismuth content in the weld metal. The results of the cross sectional observation showed that the crack initiated at the root of the test bead and mainly propagated along the columnar grain boundary. The fractured surface showed a dendritic appearance. Based on these observations, the cracks that occurred in the y-groove self-restraint cracking test are similar to those that occurred in the weld metal of the FCC unit. Therefore, the y-groove self-restraint cracking test reproduces the reheat cracking in weld metal and bismuth is considered to be the main factor that causes the reheat cracking in the FCAW weld metal during operation.
