An analytical electron microscope investigation of the phase transformations in a simulated heat-affected zone in alloy 800

Abstract

The heat-affected zone (HAZ) hot-cracking behavior of Alloy 800 was investigated with hot ductility (Gleeble) testing during a simulated HAZ thermal cycle. Microstructural analyses were performed by optical metallography, fractography, electron microprobe analysis, and analytical electron microscopy on specimens that were water quenched from selected temperatures during this thermal cycle. Analysis of analytical electron microscopy (AEM) and Auger electron spectroscopy (AES) data suggests that incipient melting of the grain boundaries occurs at temperatures of 1300 °C and above. The HAZ hot-cracking mechanism was consistent with aspects of a constitutional liquation phenomenon involving a nonequilibrium eutectic-type reaction between grain boundary Ti(C, N) and the austenitic matrix. The extent of expected HAZ cracking would be low as the liquation of Ti(C, N) was localized and no lower melting intermetallic solidification products(e.g., Laves) were observed. The mechanistic observations were consistent with classical thermodynamic and solid state diffusion models.