Evolution of Cr-Mo-V Weld Metal Microstructure During Creep Testing — Part 1: P91 Material

Abstract

Weld metal samples, manufactured in the recently concluded EU project “SmartWeld”, were subjected to various creep tests followed by thorough metallographic and micro-analytical investigations, in order to explain their properties and to predict their long-term behaviour. The microstructural study concentrated on evolution of microstructure and transformations of carbides, which were assisting nucleation of voids and cracks in the creep tests. Results of the conventional constant load short-term/creep rupture test were compared with results of the newly developed accelerated creep test carried out on Gleeble physical simulator. The microstructures generated in these tests were compared with the initial microstructures of the weld metals as well as with microstructures of exploited / crept materials. Most of the effort was dedicated to identification of precipitated phases, mainly carbides, as well as to their transformations. The metallographic study, mainly carried out by means of transmission electron microscopy on carbon extraction replicas and also by scanning electron microscopy on fractures with implementation of EDS x-ray microanalysis, was supplemented by Thermocalc calculations for the appearance of phases at equilibrium conditions. Then, using thin foil technique in TEM, the recovery and recrystallisation of the initial martensitic-bainitic microstructure of the weld metal matrix was studied and the formation of subgrains and grains observed. Finally, the changes in the substructure were correlated with nucleation of fractures/failure modes of various creep tests. In general, the acceleration of creep-like transformation of microstructure appeared dependent on accumulated or localised strain exerted by the testing method. Nevertheless, compatibility of the used tests was confirmed with indication of several nuances affecting variations of the final micro-and sub-structures of the investigated weld metals after exposure to the creep test conditions.