Powder Metallurgy (PM) processing technology covers most metallic and alloy materials and a wide variety of shapes. PM processing was originally developed and is used for manufacturing small-scale parts, but is often neither economical nor reliable for fabricating large-scale product. Typically, ferritic steel and oxide dispersoid powders are mechanically alloyed in a high energy mill over a period of at least twenty hours. The mechanically alloyed powder blend is poured into a steel can, which is evacuated and welded shut, then placed into a hot isostatic press (HIP) for consolidation at high temperature and pressure to near theoretical density. Once the resulting billet is stripped of the outer can, it undergoes a series of plate rolling or tube piercing/cross-rolling and heat treatment steps to form the finished plate or pipe geometry. The oxide dispersion strengthened (ODS) material is prone to picking up impurities during the extensive milling process; pore defects during HIPing; and dispersoid, inclusion, and/or pore alignment (stringering) during rolling, all of which degrade material properties at high and low temperatures.
One approach to enabling the full potential of ferritic ODS materials in advanced fossil energy power plant cycle is to reduce these defects and also reduce production cost by a new processing methodology. PNNL’s recent progress in friction stir welding of ODS alloys suggests that stainless steel and powder blend can be directly mixed and consolidated into full density rod and tube shapes via a one-step friction stir or shear consolidation process (SCP). This project will investigate this new powder metallurgy process, which has the potential to significantly reduce the cost of fabricating ODS products and enable their use in coal and other fossil fuel power plant applications.
To obtain significant increases in the efficiency of coal fired power plants, steam pressure and temperature must be increased beyond current technology to advanced ultra-supercritical (A-USC) conditions -temperatures and pressures up to 760 degrees Celsius (°C) and 35 megapascals (MPa). The upper bounds of operating pressure and temperature are limited by the properties of the current set of materials employed in the boiler components. Key concerns are creep resistance, corrosion resistance, and cost-effectiveness of the materials used for critical pressure-boundary components such as boiler waterwall tubing, steam headers, piping, and superheater/reheater tubes.
Historically, materials selection for these components has focused on ferritic steels, which are less costly than austenitic stainless steels and nickel based alloys. These alloys display greater thermal conductivities and lower coefficients of thermal expansion (CTE) than austenitic stainless steels, making them less susceptible to thermal fatigue cracking.
However, at temperatures higher than 620 °C, steamside oxidation and fireside corrosion of ferritic steels increases. Corrosion rates can be reduced to some extent by increasing the chromium content of the steel, but at chromium levels greater than ten weight percent, the creep strength of ferritic steels is reduced. Such operating conditions typically require the use of austenitic stainless steels, but at about 700 °C the corrosion resistance and creep strength of these materials also degrades. Although Ni-based superalloys meet the creep- and oxidation/corrosion resistance requirements of the various boiler components, they are very expensive in terms of raw material cost and processability (e.g., in casting and welding).
Adding insoluble nanoscale oxide dispersoids to ferritic alloys greatly improves their high-temperature mechanical properties. However, several barriers currently limit the deployment of these advanced materials in fossil fuel power plants. The National Technology Energy Laboratory (NETL) is partnering with Pacific Northwest National Laboratory (PNNL) to develop a process to fabricate these materials at lower cost and thus overcome the barriers to their deployment.
Development of effective fabrication methods to maintain the material performance of high-performance alloys will enable the use of such alloys in high-temperature, high-pressure, corrosive environments including A-USC steam turbines and boilers. This project will contribute to more efficient use of fossil fuels in A-USC power plants, which will simultaneously lead to lower emissions of carbon dioxide and other emissions.
Goal and Objectives
The goal of this project is to develop a low-cost method of producing high-strength, creep resistant ODS ferritic steel mill product for high-temperature applications. Specific objectives include adapting a high shear, friction-based method of consolidating metal powders directly into round billets, rods, and tubes.
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