Precipitation kinetics and strengthening of M23C6 in novel G115 martensitic steel: Utilizing secondary normalizing

G115 martensitic steel is anticipated to be one of the preferred candidate materials in ultra-super critical (USC) power plants with steam temperatures above 650°C. Microstructure evolutions and mechanical properties of G115 martensitic steel after applying various heat treatment processes were investigated. The results demonstrate that the main precipitate in G115 martensitic steel after applying various heat treatment processes is M23C6 phase with Cr enrichment. The time required for M23C6 phase precipitation decreases with increasing secondary normalizing temperature and the extension in holding time according to its precipitation-temperature-time (PTT) curves. Volume fraction of M23C6 phase increases with increasing secondary normalizing temperature and holding time, which strengthens the inhibitory effect of precipitates on dislocations recovery and laths growth. Therefore, G115 martensitic steel can obtain the best mechanical properties after applying the highest secondary normalizing temperature and the longest holding time. In the current work, the excellent strength of G115 martensitic steel mainly derives from precipitates strengthening and laths strengthening.