Analysis of Heat Resistant Steel State by Changes of Lattices Parameters of Carbides Phases

Abstract

Stability of heat resistant steel crystal structure and carbides lattices parameters during exploitation of power plant is one of the most important factor of reliability and durability of its components. The changes of carbides lattices parameters of pearlite heat resistant 12X1MФ steel, during long-term operation thermal power plant facilities at 550 °C temperature and 14 MPa pressure and isothermally aged 600 h at 700 °C under laboratory conditions, were analysed. X-ray diffraction method was used for identification of carbides M23C6 and M7C3 phases (where M indicates a mixture of metals atoms) and for measurement of accurate lattice parameters of carbides in steel during ageing. At long service times of steel or heating in laboratory conditions, alloying elements Cr, Mo, V replace iron within M23C6 and M7C3 carbides, the lattices parameters change and more thermodynamically stable carbides precipitate. The changes of carbides lattice parameters were calculated by structure refinement program Powder Cell and confirmed by GSAS program. Lattice parameter of M23C6 carbide increases with ageing duration of steel and reaches 1.058 nm in steel exploited 227000 h at 550 °C temperature and 14 MPa pressure. The changes of lattice parameters of orthorhombic M7C3 are more complicated.