Microstructure and Creep Rupture of P92-grade weld metal

Abstract

The search for new more creep-resistant materials for supercritical components of modern power plants stimulates also welding research to provide joints adequate to withstand highly demanding operation conditions. Seeking for improvement of existing welding consumables, experimental Ti-doped P92 grade weld metals were developed of exceptionally long life in creep rupture tests. In response to this request SEM and TEM investigations were carried out to identify the features of microstructure, which could be responsible for so different behaviour in the creep rupture tests. In this research the microstructures of the Ti-doped FCAW P92 weld metals were also compared with these of a standard P92 pipe material as well as with the submerged arc weld metal deposit W0 manufactured by flux-cored consumable containing no addition of Ti. Transformation of microstructures of this “reference” P92 pipe material and of the Ti-free P92 weld metal was accomplished in the accelerated creep tests on Gleeble physical simulator. An attempt has been made to correlate the fracture appearance of the creep-ruptured samples with the fine structure in their head/grip and gauge portions. In the microstructure of non-affected by the creep strain head portions of the creep-ruptured samples appeared numerous delta-ferrite grains with no carbide precipitates. Evidences were found that creep voids and cracks formed fast in the initially precipitate-free delta-ferrite grains of the sample W1, while in sample W2 during the creep test a peculiar strain-induced precipitation hardening of the delta-ferrite appeared, however no Ti was identified there in the ultra-fine precipitates.