Microstructure anomaly upon high temperature exposure and its influence on the mechanical properties of a modified 9Cr-1Mo steel weld

Abstract

The present work evaluates the microstructure stability of modified 9Cr-1Mo steel welds when subjected to high temperature exposure (HTE) at 775 °C for different times (5, 100, 500, and 1000 h). Prior to HTE, a post weld heat treatment (PWHT) at 760 °C for 2 h was performed. Upon HTE, the individual weld zones viz., weld metal (WM), coarse-grain heat affected zone (CGHAZ) and fine-grain heat affected zone (FGHAZ) demonstrate distinct microstructural variations in terms of precipitate size, grain size and substructure (lath/block/packet boundaries) fractions. The least- and the highest-microstructure stability are respectively seen in FGHAZ and WM after HTE. A detailed investigation involving electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HR-TEM) confirm that a pronounced recovery of the substructure in FGHAZ happens due to enhanced diffusion along the higher fraction of prior-austenite grain boundaries (PAGBs), leading to M23C6 precipitate coarsening and substantial grain growth. The microstructure degradation upon HTE for 1000 h had eventually resulted in deterioration of mechanical properties, where the hardness reduced the most for FGHAZ (~34%), while the least for WM (~7%). Similarly, after HTE, the tensile strength and toughness had deteriorated by ~67% and ~57%, respectively, from the PWHT condition. Fractography further confirms the pronounced degradation after HTE as they result in failure from FGHAZ.