Investigation of application of friction welding to dissimilar metal joints for electric power plants

Summary

Dissimilar joints between high temperature ferritic steel and austenitic stainless steel are employed in boiler tubes of thermoelectric power plants. The friction welding method is under consideration as a new method to manufacture these dissimilar joints and fundamental investigation of this method is in progress. As indicated in the previous report, partial interfacial fracture was noted following the joint impact test; thus, in this study, improvement in impact properties was attempted by examination of the friction welding conditions. In parallel to this, evaluation of creep properties was also carried out. The effects of the friction welding conditions (upsetting pressure, heating pressure, time-lag of revolution) upon impact and creep properties were considered using dissimilar joints of modified 9Cr steel and Inconel 600. The energy absorbed by the impact test varied little, even with increased upsetting pressure, but the ratio of interfacial fracture decreased. During creep testing, the creep rupture time showed roughly similar values, irrespective of the friction welding conditions and, in addition, interfacial fracture was observed in every specimen. After observation of the creep specimen fracture surfaces, numerous minute cracks were observed in the region from the weld interface to several μm on the Inconel 600 side. Observation of element distribution of Fe and Ni near the weld interface indicated concentration peaks in the region extending from the weld interface to several μm on the Inconel 600 side for both the as-welded state and the state after creep testing. This indicates that a secondary phase was formed during friction welding and it is thought that this Fe rich secondary phase was a major factor in interfacial fracture during the creep test. A further investigation is planned into the relationship between composition analysis of the secondary phase and creep test conditions.