Microstructure-mechanical property variations in HT-9

This research is directed at determining microstructure-property-property correlations for cleavage fracture in HT-9. Such correlations can be used to assess the potential for operating fusion reactor structures in temperature regimes corresponding to lower shelf toughness. Microstructure and mechanical property specimens of the ESR Heat of HT-9 were subjected to a set of heat treatments. These consisted of combinations of five austenitizing temperatures (950-12000C) and five tempering conditions (650-7800C). The prior austenite grain (PAG) size and lath packet size were found to increase with increasing austenitization temperature. Five different carbide types were identified. They appear to be largely M23C6 and perhaps M6C and Mo2C. Large blocky carbides at PAG boundaries appeared only at low austenitization temperatures, and fine needlelike carbides in the matrix were absent at the higher tempering temperatures. Elongated lath and boundary carbides increased slightly in size at higher tempering temperatures. Lower shelf toughness showed a slight minimum at an austenitization temperature of 10500C and increased with increasing tempering temperature and time. Yield strength, ultimate tensile strength and ductility all showed a peak for an austenitization temperature of 10500C. Values of critical microcleavage fracture stress (o*/sub f/) also showed a peak with austenitizing temperature. This may indicate a change in the controlling microstructural feature. Fracture surfaces exhibited largely cleavage facets with ductile tear ridges at PAG and lath packet boundaries. These tear ridges may contribute to the magnitude of o*/sub f/