The precipitation strengthening behavior of Cu-rich phase in Nb contained advanced Fe-Cr-Ni type austenitic heat resistant steel for USC power plant application

Copper has been used as a strengthening element in newly developed FeCrNi type austenitic heat resistant steel for inducing Cu-rich phase precipitation to meet high temperature strength requirement for 6 degrees C Ultra Super-Critical (USC) coal fired power plants for many years. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in these advanced austenitic heat resistant steels is still unclear. In order to understand the precipitation strengthening behavior of Cu-rich phase and to promote high strength austenitic heat resistant steel development, 18 Cr9 NiCuNb steel which is a Cu-added Nb contained advanced FeCrNi type austenitic heat resistant steel has been selected for this study to be aged at 65 degrees C till to 1,h. Micro-hardness and room temperature tensile test were conducted after long-time aging. SEM, TEM, HRTEM and three dimensional atom probe (3DAP) technology accompanying with thermodynamic calculation have been used to investigate the Cu-rich phase precipitation behavior during 65 degrees C aging. The experimental results showed that Cu atoms can quickly concentrate in clusters at very early precipitation stage to form the fine nano-size Cu-rich segregation areas within less than 1h at 65 degrees C. With increasing aging time at 65 degrees C Cu atoms continuously concentrate to Cu-rich segregation areas (clusters) and simultaneously other kinds of atoms such as Fe, Cr and Ni diffuse away from Cu-rich segregation areas to austenitic matrix, and finally to complete the transformation from Cu-rich segregation areas to Cu-rich phase. However, there is only Cu atoms concentration but not crystallographic transformation from early stage of Cu-rich clusters forming to the final Cu-rich phase formation. Even the Cu atom becomes the main composed element after 5h aging at 65 degrees C the Cu-rich phase still keeps coherent relationship with austenitic matrix. According the experimental results in this study, Cu-rich phase precipitation sequence which starts from the Cu atom segregation followed by the Cu diffusing from matrix to segregation areas and Fe, Cr and Ni atoms diffuse out from Cu-rich areas to matrix without crystallographic transformation is proposed. The Cu-rich phase is the most dispersed phase and contributes the most important strengthening effect among all precipitated phases (M23C6, MX and Cu-rich phase). It has been found that Cu-rich phase is very stable and still keeps in nano-size even for 1,h aging at 65 degrees C. The unique precipitation strengthening of Cu-rich phase in combination with nano-size Nb-rich MX phase and grain-boundary M23C6 carbide contributes excellent strengthening effect to 18 Cr9 NiCuNb austenitic heat resistant steel.