Abstract
Phase equilibria among δ-Fe, γ-Fe, and Fe2M phases in the Fe-Cr-M (M: Hf, Ta) ternary systems were determined using bulk alloys heat-treated at high temperatures. The final goal of this study is to develop novel ferritic heat resistant steels strengthened by precipitation of Fe2M phase on the eutectoid type reaction path: δ → γ + Fe2M. The phases present in heat-treated samples were identified by microstructural characterization and X-ray diffraction pattern analysis. The chemical compositions of the phases were analyzed by energy dispersive spectroscopy. A pseudo-eutectoid trough (δ → γ + Fe2M) exists at ~1220 °C at a Hf content of 0.1% and at ~1130 °C at a Ta content of 0.6% on the vertical section at a Cr content of 9.5% in each ternary system, respectively. A thermodynamic calculation with a database that reflects reported binary phase diagrams and the present study indicates that an increase in the Cr content decreases the temperature and the Hf/Ta contents of the pseudo-eutectoid troughs. The determined phase equilibria suggest that the supersaturation of Hf/Ta for the formation of γ phase is higher in the Hf doped system than in the Ta doped system, which is probably an origin of a much slower kinetics of precipitation on the eutectoid path in the latter system.
Highlights
Fossil fuel-fired steam and gas turbine power generations are currently supporting about 80% of the energy supply in the world [1]
High Cr ferritic heat resistant steels are an important class of materals for high temperature components such as pipes and turbines in steam turbine power generation systems due to their low thermal expansion coefficient, high thermal conductivity, and relatively low cost compared with austenitic heat resistant steels and nickel based superalloys
In the 4 Hf alloy heat-treated at 1150 ◦ C for 48 h, three types of micro-constituents are observed (Figure 1a)
Summary
Fossil fuel-fired steam and gas turbine power generations are currently supporting about 80% of the energy supply in the world [1]. Steam and gas turbine power generation systems, have advantages in providing energy security, including an ability to supply energy in case of an emergency and to back up the variable output from the renewable energy, and will be necessary in future as important power sources while using clean fuels such as NH3 and H2 and/or carbon capture technologies. It is, important to continue make efforts to further increase the efficiency and the durability of steam/gas turbine power generation systems, thereby improving the high temperature durability of heat-resistant steels and alloys for the systems. The strengthening precipitates are, prone to particle coarsening and decomposition, which are assumed to be the main cause of the degradation of long-term creep strength of the steels [3,4]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
