Robert W. Swindeman

Advances in processing of Ni3Al-based intermetallics and applications

Abstract The intermetallic-based alloys for structural applications have been an active field of research around the world for the last 20 years. Several major breakthroughs have occurred in this field during this time period. These breakthroughs include: (1) the dramatic effects of boron on ductility improvement for Ni 3 Al at ambient and high temperatures, (2) effect of chromium addition for intermediate temperature ductility improvement of Ni 3 Al, and (3) identification of an environmental effect from hydrogen generated by the reduction of moisture in air by aluminum in the aluminides. The knowledge of the compositional effects has led to the development of Ni 3 Al-based alloys, which allowed them to be taken from laboratory-size melts to commercial applications. This paper will describe the advances in melting practice, casting practices, solidification modeling as it applies to static and centrifugal castings and weld repairs, and welding of castings. This paper will also describe various applications of Ni 3 Al-based alloys and their current status of commercialization.

Structure and phase stability in a cast modified-HP austenite after long-term ageing

Abstract Phase transformations in a cast HP series alloy after long-term ageing were determined by analytical electron microscopy (AEM). Beyond the chromium and niobium carbides normally expected for the as-cast material, an eta phase enriched in Nb and Si is present, indicating phase instability at high temperatures (∼1000 °C).

Selecting and developing advanced alloys for creep-resistance for microturbine recuperator applications

Recuperators are considered essential hardware to achieve the efficiencies desired for advanced microturbines. Compact recuperator technologies, including primary surface, plate and fin, and spiral, all require thin section materials that have high-temperature strength and corrosion resistance up to 750°C or above, and yet remain as low cost as possible. The effects of processing and microstructure on creep-rupture resistance at 750°C and 100 MPa were determined for a range of austenitic stainless alloys made into 0.1-mm foils. Two groups of alloys were identified with regard to improved creep resistance relative to type 347 stainless steel. Alloys with better creep-rupture resistance included alloys 120, 230, modified 803 arid alloy 740 (formerly thermic-alloy), while alloy 214 and 625 exhibited much better creep strength. Alloys 120 and modified 803 appeared to have the most cost-effective improvements in creep strength relative to type 347 stainless steel, and should be attractive for advanced microturbine recuperator applications.

Residual and trace element effects on the high-temperature creep strength of austenitic stainless steels

The heat-to-heat variation in the creep strength and ductility of austenitic stainless steels was reviewed from the viewpoint of residual and trace element effects. Based on data reported in the literature, the creep strength of unstabilized alloys such as types 304 and 316 stainless steel increased with residual element and trace element content. Niobium appeared to be the most potent strengthener. There was no direct evidence that trace elements such as sulfur and phosphorus had a deleterious effect on either strength and ductility. It was assumed that the creep strength and ductility of the unstabilized grades of austenitic stainless steels are controlled by the precipitate characteristics. It follows from this that thermomechanical treatment or residual element additions that affect the precipitate characteristics influence subsequent time dependent mechanical properties. This view is consistant with most of the information in the literature. It was concluded that more systematic studies of trace and residual element effects would be beneficial to the improvement of steels. Incorporated into the studies should be quantitative characterization of evolving precipitate morphology and composition as they are influenced by residual elements. This information should be incorporated into modeling studies of non-equilibrium segregation. Ultimately, optimum elevated-temperature strength could be developed based on a materials science approach.

A Review on Current Status of Alloys 617 and 230 for Gen IV Nuclear Reactor Internals and Heat Exchangers

Alloys 617 and 230 are currently identified as two leading candidate metallic materials in the down selection for applications at temperatures above 760 C in the Gen IV Nuclear Reactor Systems. Qualifying the materials requires significant information related to Codification, mechanical behavior modeling, metallurgical stability, environmental resistance, and many other aspects. In the present paper, material requirements for the Gen IV Nuclear Reactor Systems are discussed; certain available information regarding the two alloys under consideration for the intended applications are reviewed and analyzed. Suggestions are presented for further R&D activities for the materials selection.

The microstructure and mechanical properties of a modified 2.25Cr-lMo steel

Tensile and creep properties were determined on a V-Ti-B-modified 2.25Cr-lMo steel. The modified 2.25Cr-lMo steel had about 0.2 pct V added for improved elevated-temperature strength. Boron was added to improve the hardenability, thus allowing thicker sections to be quenched or normalized to completely bainitic microstructures. Lower carbon and silicon concentrations were used (~0.1 pct C and 0.02 pct Si) than in standard 2.25Cr-lMo steel. The modified steel had substantially better stress-rupture properties than did a standard 2.25Cr-lMo steel (both with bainitic microstructures) with equivalent tensile properties — especially at the lowest stresses and highest temperatures. Comparative transmission electron microscopy studies of the standard and modified 2.25Cr- lMo steels indicated that the differences involved the carbide precipitates and the dislocation substructures present in the steels.

Materials Selection for High Temperature (750°–1000°C) Metallic Recuperators for Improved Efficiency Microturbines

The incorporation of a primary surface recuperator is one method for significantly improving the energy efficiency of a microturbine. The goal of this work is to employ laboratory testing of foil material (≈ 75-125μm or 3-5 mil thickness) to select alloys for higher temperature (750°−1000°C, 1400-1800°F) recuperator performance based on creep strength and corrosion resistance. Alloys of interest are high-Cr, Ni-base superalloys such as alloy 625 and aluminum-containing alloys such as Haynes alloy 214 and Plansee alloy PM2000, which is an oxide-dispersed FeCrAl. The interest in the latter two alloys is based on their corrosion resistance. Particularly in exhaust gas environments, chromia surface oxides are not sufficiently protective at high temperatures to achieve desired lifetimes. The formation of a slow-growing external alumina scale confers exceptional corrosion resistance for this application.Copyright

A Review of Aging Effects in Alloy 617 for Gen IV Nuclear Reactor Applications

The literature was reviewed of aging and aging effects in Alloy 617 to determine the supplementary data needed to understand the response of the alloy to long-time exposure conditions being considered for structural components in Gen IV nuclear reactors. Most of the data were produced in connection with the international research effort supporting High Temperature Gas-Cooled Reactor (HTGR) projects in the 1970s and 1980s. Topics considered included microstructural changes, hardness, tensile properties, toughness, creep-rupture, fatigue, and crack growth. It became clear that, for the long-time, very high temperature conditions of the Gen IV reactors, a significant effort would be needed to fully understand and characterize property changes. Several topics for further research were recommended.Copyright

Composite tube cracking in kraft recovery boilers: A state-of-the-art review

Beginning in the mid-1960s, increasing energy costs in Finland and Sweden made energy recovery more critical to the cost-effective operation of a kraft pulp mill. Boiler designers responded to this need by raising the steam operating pressure, but almost immediately the wall tubes in these new boilers began to corrode rapidly. Test panels installed in the walls of the most severely corroding boiler identified austenitic stainless steel as sufficiently resistant to the new corrosive conditions, and discussions with Sandvik AB, a Swedish tube manufacturer, led to the suggestion that coextruded tubes be used for water wall service in kraft recovery boilers. Replacement of carbon steel by coextruded tubes has solved most of the corrosion problems experienced by carbon steel wall tubes, however, these tubes have not been problem-free. Beginning in early 1995, a multidisciplinary research program funded by the US Department of Energy was established to investigate the cause of cracking in coextruded tubes and to develop improved materials for use in water walls and floors of kraft recovery boilers. One portion of that program, a state-of-the-art review of public- and private-domain documents related to coextruded tube cracking in kraft recovery boilers is reported here. Sources of information that were consulted for this review include the following: tube manufacturers, boiler manufacturers, public-domain literature, companies operating kraft recovery boilers, consultants and failure analysis laboratories, and failure analyses conducted specifically for this project. Much of the information contained in this report involves cracking problems experienced in recovery boiler floors and those aspects of spout and air-port-opening cracking not readily attributable to thermal fatigue. 61 refs.

Selection, Development and Testing of Stainless Steels and Alloys for High-Temperature Recuperator Applications

Compact recuperators/heat-exchangers are essential hardware that increases the efficiency of microturbines and smaller industrial gas turbines. There are several different kinds of recuperator technology (primary surface, plate and fin, spiral, and others), but they all have several common materials needs. Most commercial recuperators today are made from 347 stainless steel sheet or foil. Increased engine size, higher exhaust temperatures and alternate fuels all require greater performance (strength, corrosion resistance) and reliability than 347 steel, especially as temperatures approach or exceed 750°C. To meet these needs, the Department of Energy (DOE) has sponsored programs at the Oak Ridge National Laboratory (ORNL) to measure properties of commercial sheet and foil materials, to analyze recuperator components, and to identify or develop materials with improved performance and reliability, but which also are cost-effective. This paper summarizes high-temperature creep and corrosion testing of commercial 347 used for current recuperators, testing of HR 120 and modified 803 alloys, and development of modified 347 stainless steels.Copyright