Precipitation correlation between MC carbide and Nb-rich M2P phosphide in a TP347H austenitic stainless steel

The precipitation correlation between Nb-rich MC carbides and Nb-rich M2P phosphides has been investigated in a TP347H austenitic stainless steel during rupture test at 750 °C. Needle-like Nb-rich M2P phosphides as well as round-shaped Nb-rich MC is observed during rupture test. The Nb-rich M2P phosphides form an orthorhombic NbNiP-type structure and show the orientation relationship of [01−0]M2P//[100]γ and (201−)M2P//(01−1−)γ. The fraction of the Nb-rich MC carbides decreases abruptly after a critical rupture time, while that of M2P phosphides is roughly little changed. This means that the dissolution of the pre-formed Nb-rich MC carbides into the matrix for meeting the equilibrium phase fraction of each phase activates the active precipitation and growth of the Nb-rich M2P phosphides. Such a precipitation correlation may delay intergranular cracking occurring in the austenitic stainless steel.