High-Purity Chromium Metal: Supply Issues for Gas-Turbine Superalloys (1995)

This report focuses on the availability of high-purity chromium metal for use in superalloys for aircraft gas-turbine engines. Aircraft gas-turbine engines are used to power military, commercial, and private airplanes and helicopters. The objectives of the study were to determine (1) the health of the domestic chromium-metal industry, (2) the capability and reliability of foreign chromium-metal suppliers, (3) projections of material needs for the future, (4) economic and security benefits that derive from having a domestic supplier base, (5) alternative methodologies (and research and development opportunities) for producing high-purity chromium metal, and (6) suggestions regarding strategies to maintain a core capability.

The operating parameters of an aircraft engine are extremely demanding on materials. Metal temperatures within an engine can reach as high as 1200 °C. Materials are subject to rapid oxidation and creep, high centrifugal stresses, high pressures (up to 4 megapascals), and high torque. The alloys used in engines are referred to as superalloys because of their superior combination of low- and high-temperature mechanical properties, as well as their ability to withstand harsh environments. Many of the superalloys contain high levels of chromium metal. Chromium metal limits the coarsening rate of the intermetallic (and usually coherent) precipitates; forms Cr₂₃ C₆, which strengthens grain boundaries; plays a critical role in the formation of protective scales; and improves hot-corrosion resistance. Thus a major portion of the gross engine weight consists of chromium-bearing alloys, for which there are currently no substitutes.

High-purity materials must be used in aircraft gas-turbine engines because even low concentrations of oxygen, nitrogen, sulfur, iron, and silicon can cause inclusions, areas of hot cracking, and locations of incipient melting. These flaws can cause failure during aggressive forming operations and limit strength, ductility, fatigue life, and creep-rupture resistance during engine operation. To control the purity of the superalloys, the primary metal manufacturer and alloy

producer must control the quality of the incoming material. The original equipment manufacturers and the primary-alloy producers have determined specifications for each major additive, with the minimum and maximum values allowed for each intentional alloying element and a maximum allowed for each undesirable impurity. Unfortunately, each original equipment manufacturer and primary-alloy producer devises its own specifications, forcing chromium-metal producers to qualify their material for each customer.

The production of chromium metal begins with the mining of chromite ore. Chromite ore is then converted either into ferrochromium by smelting or into sodium dichromate by roasting and leaching. Ferrochromium is converted into chromium metal by the electrolytic method. Chromic oxide (made from sodium dichromate) is converted into chromium metal by the aluminothermic process. The metal is then further refined using variations of the vacuum-degassing method. These production methods currently appear to be mature. Although incremental improvements in the efficiencies and economics of the processes are still obtainable and should be pursued, no obvious quantum-leap increase in the purity levels appears likely at this time.

Historically, electrolytic chromium metal was used for those aerospace superalloys that required high-purity specifications, and aluminothermic chromium metal was used for those superalloys that allowed lower-purity applications. The quality of high-grade aluminothermic chromium metal has improved over the past decade, however, and aerospace engine manufacturers have certified this material for some higher-grade superalloys. While aluminothermic material of equivalent quality to the electrolytic material may now be available, changes in such critical applications as rotor-grade materials must be mutually acceptable to producer and user and will have to be subject to extensive qualification studies involving production and testing of alloys, test specimens, final cast and forged products, and possibly engines containing them. Similar constraints are associated with any process changes.

The major chromium-metal producing countries in the world are France, Russia, China, and the United Kingdom for aluminothermic chromium metal and the United States and Russia for electrolytic chromium metal. World capacity

is estimated at 38,300 metric tons per year, with aluminothermic metal accounting for all but 5,100 metric tons of this total. World production is substantially less than capacity, averaging only somewhat over 21,000 metric tons in recent years.

World consumption of chromium metal approximates world production. The United States is the largest user, with reported consumption at roughly 4,000 metric tons per year. This is less than actual usage because not all U.S. consumers elect to report their consumption. With net chromium-metal imports into the United States fluctuating between 2,000 and 6,000 metric tons during the 1982-1992 time period and with domestic production averaging about 2,000 metric tons, actual U.S. consumption is more likely to be 6,000 metric tons per year, 2,000-2,500 metric tons of which is aerospace quality. Roughly half of this total is secondarily treated for further refinement.

Four aluminothermic chromium-metal producers and one electrolytic chromium-metal producer have closed down in Western Europe, North America, and Japan over the past six years. The dismantling of the Tosoh facility in Japan as part of an urban renewal program has left Elkem in the United States as the only significant electrolytic producer outside of Russia.

Although closures have reduced the quantity of high-purity chromium metal available internationally, shortages have not occurred for two reasons. First, worldwide demand for high-purity chromium metal has declined due to reductions in military spending, the recession in the commercial airline industry, and the pending changes in U.S. government stockpiling policy. Second, the end of the Cold War has led to an increase of chromium-metal exports from Russia and China. As noted earlier, the recent annual production over the past several years of about 21,000 metric tons requires only 55 percent of the 38,300 metric tons of available world capacity. The 3,000 metric tons of electrolytic chromium metal produced at a domestic plant could cover the U.S. aerospace requirements of 2,000-2,500 metric tons per year by itself. Thus the international chromium-metal supply appears to be stable and sufficient to satisfy the needs of the domestic aerospace industry.

Based on the information presented above, the committee developed three general conclusions and recommendations.

• Current Status of the High-Purity Chromium-Metal Industry—There is currently not a crisis within the international and domestic chromium-metal industries. Sufficient high-purity chromium metal is being produced domestically and internationally to satisfy the needs of the aerospace industry. The closure of the Tosoh plant in Japan has also made the domestic electrolytic chromium-metal industry even more economically secure. Thus the committee recommends that the government not take any special action to develop additional domestic suppliers of high-purity chromium metal.

• The National Defense Stockpile—The National Defense Stockpile is maintained by the Defense Stockpile Center of the Defense Logistics Agency. The purpose of the stockpile has been to provide the United States with a supply of strategic and critical raw material in the event of a national emergency. These materials can only be used for national security purposes. The National Defense Stockpile currently retains 7,700 metric tons of chromium metal, 3,500-4,500 metric tons of which is of aerospace quality. In the case of a national emergency, the 1994 stockpile inventory is sufficient to provide an approximate two-year supply of metal and sustain the aerospace industry until new aluminothermic and degassing facilities are brought into operation if (1) the metal is totally devoted to the aerospace industry, (2) the demand for chromium metal does not suddenly increase, and (3) the domestic source of metal is no longer in production. The recent collapse of the Soviet Union has caused the U.S. Department of Defense to reassess the quantities of materials required in the stockpile and to liquidate certain items. All indications suggest that the Department of Defense will most likely find little or no continuing need for any of the materials in the National Defense Stockpile, chromium metal included. The committee recommends that the National Defense Stockpile maintain and continually upgrade to industry standards a sufficient quantity of high-purity chromium metal to meet the industry's needs in the event of an emergency.

• Domestic capability—International production capacity of high-purity chromium metal is currently sufficient to supply the needs of the domestic aerospace industry in the event that the domestic chromium-metal industry should fail. Thus the commit-

tee concludes that no special action to subsidize the domestic supplier industry of high-purity chromium metal would be required by the government should the industry falter economically. If the foreign supplier market should also collapse, the 1994 stockpile appears sufficient to supply the domestic aerospace industry until new aluminothermic and degassing facilities for high-purity chromium metal are developed.

• High-Purity Chromium-Metal Qualification—Qualification of high-purity chromium metal has historically been linked to the process used to produce the metal. Improvements in the quality of vacuum-degassed aluminothermic chromium metal over the past 10 years have led to its increasing suitability and certification for high-purity aerospace applications. The committee recommends that chromium-metal specifications be disconnected from production methodology so that any material that meets the required end-product specifications is permissible.

This report is divided into six chapters. Chapter 1 presents an overview of the chromium-metal marketplace, including the nature of the market, the recent and pending supply and demand trends, and the prospects of shortages. Chapter 2 reviews the uses and specifications of high-purity chromium metal. Chapter 3 and Chapter 4 discuss the strengths and weaknesses of the main techniques for producing chromium metal. Chapter 5 presents the criteria for judging the reliability of a supplier company and the assessment of the four major international chromium-metal producers. Chapter 6 presents a number of potential economic scenarios that could result in shortages of high-purity chromium metal and the possible methods for resolving such scenarios.