Effect of Boron on the Thermal Stability of the Precipitate Phase of FC92B and FC92N Steels During Aging and Creep

In this study, precipitation strengthening alloying elements (C, N, V, Nb, Ta) and solution strengthening alloying elements (Cr, W, Mo, B) were appropriately combined to fabricate FC92B and FC92N alloys. Long-term aging heat treatments at 650 °C of up to 20,000 h were conducted to compare the high-temperature properties of each alloy, investigate the effects of microstructure stability on creep performance characteristics, and analyze the precipitate growth behavior in relation to the long-term thermal stability of the precipitate phase. Transmission electron microscopy and nanoscale secondary ion mass spectrometry were carried out to investigate microstructural changes according to the aging treatment time and the microstructure of creep specimens in relation to the addition of trace alloying elements. Although FC92B contains a higher B content than FC92N, FC92B did not exhibit significant changes in hardness or tensile strength regardless of the aging time, which extended to a maximum of 20,000 h. The FC92B alloy with an adequate addition of B showed the highest creep performance and exhibited rupture durations that were more than twice those of FC92N under stress conditions of 100 MPa and 140 MPa at 650 °C. The precipitate size of FC92N was significantly coarser than that of FC92B. In FC92B aged for 20,000 h, B was distributed in the grain boundaries, as shown by Nano-Secondary Ion Mass Spectrometry. B content was found to be high within the M₂₃C₆ precipitate.