Microstructural evolution and precipitation strengthening in a new 20Cr ferritic trial steel

Microstructural evolution and precipitation strengthening of the newly-developed 20Cr steel were investigated in present work. Three types of precipitates, including G-phase (Ni₁₆Ti₆Si₇, intermetallic silicide), Laves phase (Fe₂Ti, intermetallic compound) and carbide (TiC), were observed and their crystal structures were resolved with combined electrochemical phase extraction, X-ray diffraction and transmission electron microscopy analysis. G-phase particles were observed in the grains and Laves phase occurred at grain boundaries while carbides were present at both of the two sites. A temperature-dependency sketch map for these precipitates was also plotted to illustrate the characteristics of G-particles below 750 °C, Laves phase at 850 °C~1050 °C and carbide below 1150 °C. In particular, G-phase precipitated and had the cubic-cubic orientation with the ferritic matrix, thus showing a strong aging hardening characteristic. Nanodispersion of these G-particles greatly enhanced the yield strength, which was estimated to be up to ~1700 MPa. Finally, theoretical calculations for phase equilibria, critical radius and precipitation strengthening helped to understand the precipitation thermodynamics and strengthening mechanisms for developing high-performance steel by making use of G-phase.