According to Phil Maziasz, a researcher in ORNL’s Metals and Ceramics (M&C) Division, “ORNL is one of the few facilities in the world that are showing the dramatic effects of composition and processing changes on the properties of ferritic and austenitic steels.” Maziasz has long promoted the advantages of modified steels for energy production facilities. As a result of recent M&C discoveries, industry is noticing ORNL’s work and providing funding through cooperative research and development agreements (CRADAs).
ENGINEERS OF MICROSTRUCTURE
Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) techniques developed in the 1970s and 1980s, as well as the three-dimensional atom probe field ion microscope (3D atom probe tomography) refined and used at ORNL since the 1990s, have provided ORNL researchers with a thorough understanding of microstructure and the compositional nature of ultrafine microscopic precipitates in steels. By heating, cooling, stretching, compressing, and hammering steel samples; measuring their strength, ductility, and fracture resistance; examining their microstructures (including residual stresses and precipitate formation resulting from steel manufacture); and studying the microstructures of steels with the best properties, ORNL researchers have learned how to make better alloys through carefully engineered microstructures.
“We can engineer the microstructure of the steel first by making small or large changes in its composition,” Maziasz says. “Then these cause strategic microstructure changes.”
Much of the steel research in the M&C Division has been supported in past years by the Department of Energy’s Fusion Materials Program and also by the Fossil Energy Materials Program. The modified ferritic and austenitic steels resulting from this earlier research continue to show promise as structural materials for advanced fossil and fission power plants, as well as combined-cycle (gas turbine and steam turbine) power plants, microturbines for distributed power generation, and advanced diesel engines for heavy trucks.
Ferritic steels (which include the common steels worked by blacksmiths) have a body-centered cubic (BCC) structure, consisting mostly of iron atoms at the corners of each cube, with an atom located in the cube’s center. Austenitic steels have a face-centered cubic (FCC) structure: an iron or another atom at every corner of each cube, with an atom in the center of each of the six cube faces.
In both types of steel, carbon has been added during their manufacture, so carbon atoms can be found in the interstices between larger iron and other atoms. Carbon is an important ingredient in steel because it reacts with other alloying elements in the steel, during heating and cooling as part of the alloy preparation, to form metal carbide precipitates (e.g., iron or chromium carbides). These metal carbides can improve the steel’s properties. The key is to design the right alloy recipeto add a small amount of alloying elements, such as chromium (Cr), molybdenum (Mo), nickel (Ni), niobium (Nb), silicon (Si), tungsten (W), and titanium (Ti), in the right proportions.
Unlike the case for steels blacksmiths used to work, today’s ferritic steels can have a wide range of properties, with the proper choice of alloying elements and heat treatment. “When you heat a ferritic steel above about 800°C (the precise temperature depends on the composition), the BCC structure transforms to the FCC, or austenite, structure,” says M&C’s Ron Klueh. “When the steel is then cooled, it transforms to the polygonal ferrite, bainite, or martensite structure, depending on the composition and the cooling rate.”
Most steels are 85% to 93% iron by weight. Commonly used stainless steels, which were discovered at the turn of the 20th century, contain around 12% chromium by weight, but they are austenitic steels that do not transform during heating or cooling. Chromium is added to prevent oxidation or corrosion (or rust stains, hence the term “stainless”), enabling excellent performance at elevated temperatures.
“SUPER BAINITIC” STEEL
Klueh and Maziasz developed an iron-chromium-tungsten-vanadium (Fe-3Cr-3WV) steel for DOE’s Fusion Materials Program that has recently won industrial attention. By weight, it is 3% chromium and 3% tungsten (and the researchers call it 3-chrome steel). This “super-bainitic” steel is being considered by several large industrial firms as a replacement for iron-chromium-molybdenum (Fe-2.25Cr-1Mo) steels in a wide range of applications.
Klueh and his colleagues had been working to produce a “low-activation alloy” from 3-chrome steel to meet the DOE fusion program goals. “We replaced molybdenum with tungsten to make a low-activation three-chrome steel,” he says.
When steel in the first wall of a fusion reactor is bombarded with 14-MeV neutrons from the plasma fuel, some of its alloying elements, such as molybdenum, would be transformed into highly radioactive materials that do not lose their hazardous radioactivity through decay for thousands of years. Such high-activation steels would have to be isolated in geological depositsan expensive proposition. DOE plans to use steels in fusion reactors that do not become dangerously radioactive and that lose most of their radioactivity in about 100 years. These low-activation materials could be disposed of by using shallow land burial or possibly recycled.
After replacing the Mo with W and adding a little V, Klueh and his colleagues characterized the new 3-chrome steel using analytical electron microscopes. They observed that it has a desirable acicular structure, with needlelike metal carbides distributed throughout the microstructure. “The needlelike carbides are probably vanadium carbide precipitates formed during cooling,” Klueh says. “They hinder the motion of dislocations, or line defects, in the steel, increasing its strength at elevated temperatures.”
The ORNL researchers tested the new 3-chrome steel for impact toughness using a Charpy impact test. In a Charpy test, a notched bar is broken by the impact of a heavy pendulum hammer, and the energy absorbed to fracture the specimen is measured. Compared with brittle materials, tough materials absorb considerably more energy when fractured, indicating that the structures they form will better survive the harsh conditions of service.
“We found that our new 3-chrome, 3-tungsten steel has both high-temperature strength and low-temperature toughness because of its unique microstructure,” Klueh says. “Normally, an alloy with high-temperature strength lacks low-temperature toughness, or vice versa. Our alloy is strong at temperatures of 600°C or higher and is also tough at low temperatures. We also might be able to put this alloy into service without temperingheating it up a second time after first heating it up and then cooling itand without heat-treating structures made from it after they are welded together. If that’s possible, this new alloy would be very economical and attractive to industry.”
In 2002, under a CRADA partly funded by DOE’s New Industrial Materials for the Future Program, M&C researchers Vinod Sikka, Mike Santella, Suresh Babu, Klueh, and Maziasz have been working to improve the 3-chrome alloy with Nooter Corporation of St. Louis, which is interested in using the super bainitic steel to build pressure vessels for the chemical industry and make steam drums and tubing for boilers for fossil power plants. “We need to establish that the properties are as good in large, commercial-size heatsup to 50 tonsas they were for small laboratory heats,” Klueh says.
Klueh, Maziasz, and M&C researchers John Vitek and Bob Swindeman are working on a CRADA project with General Electric (GE). They plan to develop ORNL’s “super bainitic” steel and other steels for use in the cast condition, as differentiated from wrought products (e.g., bars and plates) examined in the Nooter CRADA. GE is considering using this steel for casings for steam turbines in natural-gas combined-cycle plants (which include gas turbines) for power production.
What led to the development of this super steel from ORNL? In the mid-1980s Klueh and Swindeman tested steels for toughness, strength, and ductility for pressure vessels that would be used by the chemical and fossil power plant industries. “We compared different steels by studying their microstructure and strength and toughness,” Klueh says. “We would heat treat steel plates by heating them to 900°C and cooling them, causing their microstructure to be transformed from FCC to BCC.
“We noticed that if we cooled steel by water quenching, we would get different properties than if we cooled the steel in air. With water quenching we could cool the steel more rapidly and get a stronger and tougher material. Our TEM studies showed that the air-cooled steel had a granular microstructure, formed during cooling by the concentration of carbon in small regions of the microstructure. We also found that these high-carbon regions were brittle. The tougher, water-quenched steel had an acicular microstructure with needlelike, elongated subgrains distributed throughout. We found that if we cooled the steel fast enough, we obtained a strong, tough acicular product. We also figured that we could get the acicular structure by proper alloying. Thus, we would not have to cool the steel so rapidly, and we would be able to cool thicker sections and get the desired product.”
By using this knowledge and experimentally altering the composition, characterizing, and testing ferritic steels, ORNL researchers came up with the recipe for the “super bainitic” steel that is attracting considerable industrial interest.
ODS STEEL FOR SUPERHOT ENVIRONMENTS
Advanced central power stations designed to produce more electricity while using less fuel and reducing pollutant emissions require higher fuel combustion and steam temperatures. That means the structural materials selected for these plants must withstand the stresses accompanying higher temperatures.
Oxide-dispersion-strengthened (ODS) ferritic alloys are one type of advanced high-temperature material that can withstand very high temperatures (>800°C). ODS alloys were developed over 30 years ago, but they are being con-sidered for high-temperature uses such as boiler and superheating tubing, heat-exchanger piping, and structural components for advanced fossil energy plants, as well as for fission and fusion energy power plants that require additional resistance to radiation effects. The problems are that ODS alloys are both expensive and difficult to form into tubes and other complex shapes.
ODS alloys are made by mechanically alloyingrapidly and strenuously mixingsteel powders together with fine ceramic yttrium-oxide powder, to create a matrix in which very tiny oxide particles on a submicron scale are dispersed. These very fine particles make ODS alloys much stronger at high temperatures than similar alloys made a different way. Such extreme processing is necessary because yttrium oxide (Y2O3, also called yttria) is insoluble in molten steel.
In 1999, in a small project for DOE’s Fusion Materials Program, Maziasz and Klueh studied the mechanical properties and microstructures of several different ODS ferritic alloys made by Kobe Specialty Tube Company of Kobe, Japan. They collaborated with three Japanese partnersT. Okuda from the Kobe firm, Professor K. Miyahara from Nagoya University, and his student I-S. Kim, who visited and conducted research with Maziasz at ORNL.
That year, using AEM, Maziasz found much finer dispersions of oxide particles in a sample of one of Kobe’s ODS alloys. This particular alloy (Fe-12Cr-3W-0.4Ti + Y2O3) also exhibited outstanding high-temperature strength. Maziasz then sought the help of M&C’s Dave Larson (now employed at Seagate Technology), who worked with M&C’s Mike Miller in characterizing materials using ORNL’s unique 3D atom probe. Using this instrument, Larson characterized the sample Maziasz had studied. He discovered that the nanoclusters uniformly dispersed throughout the material were rich in Ti, O, and Y atoms (as well as iron atoms) and, unlike the other ODS alloys (Fe-12Cr + Y2O3 and Fe-17Cr + Y2O3) studied at ORNL, had no traces of yttrium oxides.
“This startling, new finding suggested that the original yttrium-oxide particles had somehow dissolved during the prior processing,” Maziasz says. “This processing included mechanical alloying to make ODS powders, hot extrusion to turn the powders into solid metal, and hot- and cold-rolling to make metal plate. After these particles dissolved, they precipitated out as new nanoclusters of individual atoms.
“We wanted to know why the original yttrium oxide particles dissolved, when during the prior processing the new nanoclusters formed, and how stable these new nanoclusters were,” Maziasz says. M&C’s David Hoelzer is leading a project supported by ORNL’s Laboratory Directed Research and Development Program to answer these and other questions.
To test this ODS alloy’s resistance to creep, or deformation, Maziasz, Klueh, and M&C’s Lee Heatherly hung weights from alloy samples and placed them in furnaces at temperatures of either 800 or 850°C. After 14,000 hours, some alloy samples had stretched only 2% of their original length. “This ODS alloy,” says Maziasz, “has outstanding creep resistance that far exceeds that of other commercial ODS ferritic alloys.”
Atom probe studies by Miller and M&C’s Ed Kenik found the alloy’s nanoclusters to be unbelievably stable, even after the material was annealed at 1300°C. Miller also found evidence that the O-Ti bond plays a crucial role in the formation of the oxygen-rich nanoclusters. Modeling by M&C theorist Chong Long Fu suggests the nanoclusters have the same crystalline structure as the host material.
“Characterization and modeling will help us understand how the nanoclusters form and how they make the ODS alloy so strong at high temperatures,” Maziasz says. That information could lead to the design of a new class of alloys that could perform well at over 1000°C. The new alloys could include ferritic steels, stainless steels, and nickel-based and copper-based superalloys. Just as metals today are strengthened by carbides or intermetallic particles, future alloys may be based on controlled precipitation of oxide particles.”
In the meantime, DOE is interested in the potential of this ODS alloy as a structural material for advanced fossil fuel plants and advanced nuclear power reactors.
ADVANCES IN AUSTENITIC STEEL RESEARCH
Between 1965 and 1985 ORNL researchers obtained a thorough scientific and mechanism-based understanding of how the microstructure of stainless steel changes during the manufacturing process and during use in the environments expected for fast breeder reactors and fusion reactors. In the mid-1980s, Maziasz and M&C’s Bob Swindeman used this knowledge to develop austenitic stainless-steel alloys that resist radiation damage from highly energetic neutrons in fusion reactors, as well as resist creep, for use as boiler tubing for fossil power plants. Using AEM, Maziasz discovered that ultrafine, stable metal carbide precipitates could be produced and dispersed throughout 316 stainless steel, if it were manufactured by a certain process. In 1990 Maziasz and Swindeman received an R&D 100 Award for creating a modified 316 stainless steel that could be used to make extremely creep-resistant and reliable boiler tubing for superheaters in fossil power plants.
Microturbines that burn natural gas are being used to generate electricity and to heat or air condition buildings. To increase their efficiency, recuperators are used to extract heat from the hot exhaust gas and preheat the incoming air. Current recuperators consist of thin sheets or foils made from 347 stainless steel (Fe-18Cr-10Ni-Nb) that cannot withstand temperatures above 700°C. To increase microturbine efficiency by allowing operation at a higher temperature, Maziasz and Swindeman, together with M&C’s Bruce Pint and Karren More, who study oxidation and scale behavior of such steels, have modified several 347 stainless steels so that they have much better creep resistance and corrosion resistance at 750°C than the standard steel. DOE’s Distributed Energy Resources Program funded this work. Capstone Turbines, Ingersoll-Rand, and others are interested in making use of this improved steel and other advanced alloys in their advanced microturbines.
To run more cleanly and efficiently than today’s diesel engines used in heavy trucks, advanced diesel engines will be operated at temperatures higher than 650°C. Thus, the silicon-molybdenum cast iron used in exhaust manifolds and turbocharger casings in today’s engines must be replaced for advanced diesel engines. Under a CRADA project sponsored by DOE’s Office of Transportation Technologies, ORNL researchers led by Maziasz, along with personnel from Caterpillar, Inc., have engineered the microstructure of a commercial cast-austenitic-stainless steel (CF8C) to make a new, modified CF8C alloy that has outstanding strength and creep resistance at 850°C. The knowledge was taken from previous ORNL programs and used to develop the new alloy in about a year. Creep tests on the new steel at 850°C have been running for about two years. Commercial scaleup of the new modified-cast-stainless steel is under way at Caterpillar, together with a new CRADA project to continue this work.
HELPING THE STEEL INDUSTRY
“ORNL is helping the U.S. steel industry make steels that are better and less costly than foreign steels,” says M&C’s Vinod Sikka, an ORNL corporate fellow and leader of the Materials Processing Group. “Our Alloy 4 steel may replace Alloy 803 for ethylene crackers, furnaces for breaking up gaseous fuel into ethylene for making nylon stockings and rubber tires. Our preoxidized iron-chromium-aluminum-titanium alloy is being considered by some as the best material for making roll bearings and roll surfaces in galvanizing-pot hardware. This hardware is used to hold a bath of zinc in which steel sheet is continuously dipped to make galvanized steel for office furniture and the bodies of cars. Also, we are working to develop a better alloy to extend the life of basic open-hearth furnace hoods used in steel mills.”
ORNL researchers’ successes in modifying steels and nickel aluminides have the potential to increase the U.S. steel industry’s competiveness in the world marketplace. ORNL is showing the way as its researchers consistently transform ordinary alloys into extraordinary materials.
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