Design and exploration of high-temperature steels for electric power generation applications

Project: Research project

Project Details

Description

Overview
Ferritic steels continue to be the most economical structural material in use for a broad spectrum of applications. Conventional steels, however, rapidly loose their strength and creep begins, when exposed to temperatures higher than about 550-600oC. Our goal is to design steels with enhanced high-temperature mechanical properties for use in inexpensive fire-resistant civil infrastructure (buildings, highways and bridges) and components in power generation that operate at high temperatures with moderate corrosion resistance requirements. We propose to extend and refine the use of precipitation hardening by nanometer-scale transition-metal carbide and carbonitride precipitates that are thermodynamically stable at elevated temperatures (up to 760°C), in combination with solid-solution hardening by addition of slow-diffusing refractory elements (Mo, W), to develop a new class of inexpensive low-alloy ferritic steels with improved high-temperature strength and creep resistance. We will use scientific principles to design such steels with small additions of carbide-forming elements (Ti, V, Nb, Cr, Mo, and W). Computational thermodynamics will be used to optimize the alloying element concentrations to maximize the thermal stability of these precipitates. The carbon concentration will be kept low enough (<0.1 wt.%) to facilitate weldability without complicated pre-and post-heating. Alloying additions will be constrained to produce precipitates that are coherent or semi-coherent with the ferritic matrix, with small residual coherency strains. Small coherency strain reduces the interfacial energy and slows detrimental precipitate coarsening. In addition, such coherency strains create drag on nearby dislocations, thereby improving strength. We will study precipitation kinetics in order to control size and volume fraction of precipitates, which will be characterized by various micro- and nanostructural techniques (SEM, TEM and APT, atom-probe tomography). Mechanical properties will be studied at temperatures up to 760°C.

Intellectual Merits
The proposed research aims to develop the intellectual basis for designing ferritic steels that retain a good fraction of room-temperature strength at temperatures up to 760°C, achieved by thermodynamically stable and semi-coherent precipitates. We accomplish this goal by integrating several modeling and experimental tools in this endeavor: computational thermodynamics to determine the alloying element additions required to obtain these precipitates, precipitation kinetic modeling to bracket the heat treatment conditions needed to control precipitate size and volume fraction, micro- and nanostructural analytical tools to characterize the morphology, structure and quantitative composition of such precipitates, and evaluation of these steels in terms of their high-temperature mechanical properties.

Broader Impact
The proposed research will result in high-temperature steels suitable for civil infrastructure and power generation applications. Since 80% of the world’s electricity generation is via steam turbines, higher temperature operations afforded by these steels will increase energy conversion efficiency and literally can change the world. Just as important, this research will provide multi-dimensional training to the doctoral student involved in this research, from metallurgy and phase diagrams, to computational modeling, various micro- and nanostructural characterization tools, and mechanical evaluation. The research will train the student to think more broadly about nanotech
StatusFinished
Effective start/end date9/1/152/29/20

Funding

  • National Science Foundation (CMMI-1462850)

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