Effects of Orthogonal Heat Treatment on Microstructure and Mechanical Properties of GN9 Ferritic/Martensitic Steel

Microstructure and mechanical properties of GN9 Ferritic/Martensitic steel for sodium- cooled fast reactors have been investigated through orthogonal design and analysis. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimeter (DSC), tensile and impact tests were used to evaluate the heat treatment parameters on yield strength, elongation and ductile-to-brittle transition temperature (DBTT). The results indicate that the microstructures of GN9 steel after orthogonal heat treatments consist of tempered martensite, M23C6, MX carbides and MX carbonitrides. The average prior austenite grains increase and the lath width decreases with the austenitizing temperature increasing from 1000 °C to 1080 °C. Tempering temperature is the most important factor that influences the dislocation evolution, yield strength and elongation compared with austenitizing temperature and cooling methods. Austenitizing temperature, tempering temperature and cooling methods show interactive effects on DBTT. Carbide morphology and distribution, which is influenced by austenitizing and tempering temperatures, is the critical microstructural factor that influences the Charpy impact energy and DBTT. Based on the orthogonal design and microstructural analysis, the optimal heat treatment of GN9 steel is austenitizing at 1000 °C for 0.5 h followed by air cooling and tempering at 760 °C for 1.5 h.