CCS and Power Systems
Crosscutting Research - Plant Optimization Technologies
Improving the Performance of Creep Strength-Enhanced Ferritic Steels
Performer: Oak Ridge National Laboratory
Project No: FWP-FEAA107
This project will address two broad technical issues: (1) controlling mechanical properties and microstructure, and their dependence on chemical compositions and heat treatments; and (2) understanding the causes of Type IV failure and developing strategies to minimize or eliminate it in these steels. The overall project approach will rely on fundamental and applied studies of the effects of heat treatment, welding, and process control on microstructural evolution and material properties in CSEF steels.
For microstructure control in CSEF steels in general, two conditions are critical: (1) heat treating temperatures should not exceed the so-called A1 lower critical temperature which can cause hard, brittle, untempered alloys; and (2) the retention of primary ferrite must be avoided because it reduces strength and ductility which can compromise both fabrication processes and performance in service. For this project, researchers will build on exploring the limits for Grade 91 and similar types of steels through an analysis of chemical composition specifications using computational thermodynamics. Microstructure analysis and strength testing will also be conducted to verify thermodynamic predictions and to confirm any unusual effects on mechanical properties. It is anticipated that this information eventually will be used to consider restrictions on ASTM-specified chemical compositions with the development of a more reliable and predictable use of these advanced alloys in power generation equipment. Additionally, this work could establish a basis for identifying alloy compositions with the potential for reaching service temperatures beyond the current limits of commercial alloys. This activity will involve significant interaction with ASME Boiler and Pressure Vessel Code committees and major U.S boiler manufacturers.
Researchers will also study the effects of chemical composition and microstructure on Type IV failure in 9 Cr steels, mainly Grade 91. Past studies suggest that modifying the ASME code-specified post-weld heat treatment of Grade 91 may lead to minimizing the microstructure features associated with Type IV failure. Guided by previous studies, this possibility will be studied in detail by producing tempered plates, welding the plates using a conventional technique, and then further exploring a wider-thanusual range of post-weld heat-treatment temperatures. Weldments will be subjected to characterization of both microstructures and hardness distributions. Conditions leading to microstructures favorable for minimizing the Type IV cracking behavior will be further tested using aging treatments, as well as tensile and creep testing to determine both short- and longterm properties. Post-test microstructural evaluations will be conducted to determine the relationship(s) among alloy/heat affected zones (HAZ) microstructure.