Chromium and Vanadium
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EARL T. HAYES Bureau of Mines, Department of the Interior, Washington, D. C.
These metals are similar, but the closeness of their relationship is not comparable to that of Ti and Zr or Ta and Ni
F R O M A METALLURGIST'S point of view it is relatively easy to write a critical review on chromium and vanadium, because they have many points of similarity, in their behavior and also in their present stage of development. Both have melting points of about 1800" C., high enough to class them as refractory metals. Both have violent allergic reactions to small amounts of oxygen, nitrogen, and carbon. As bodycentered cubic metals, both have the common failing that bedevils all bvorking operations-i.e., unless kept free of certain contaminating elements, they are subject to ductile-brittle transition above room temperature. Alone, each metal is a n important alloying ingredient in steel manufacture, and Lvhen combined, they have complete solid solubility with one another. Both of these metals are available in ductile form today because of modern metallurg: 's desire to know more about the fundamental properties of pure metals. Also: now that the outer limits of the atomic table have been reached, many researchers are yielding to a sensible inclination to go back and make use of some common materials, rather than to attempt to use newer and more expensive ones.
Seither metal is a shelf item nor are they available from stock, even as simple shapes. Limited amounts of tube, wire, or rod are made for development testing on a negotiation basis. However, chromium and vanadium have no titanium-zirconium or tantalumniobium (columbium) relationship. Therefore. from this point on the) must be discussed separately.
Chromium -4lthough electrolytic plating and stainless-steel production have utilized the desirable characteristics of chromium. there has been considerable discussion in the metallurgical trade about the use of ductile chromium, either as a claddingor forging-stock material or as a basic constituent of chromium-rich alloys. T h e metal's modulus of about 30,000,000 p.s.i. and its excellent resistance to high temperature oxidation have suggested a potential demand by the missile and aircraft industries for thin sheet and foil. In addition, the statement has been made that chromium would have much wider use in the chemical industr). if it were available in ductile form for cladding pressure and reaction vessels
Like chromium, interest in vanadium as a refractory metal is rather academic, but unlike chromium, its future seems to lie in some alloyed form the metal itself has poor oxidation and corrosion resistance. In alloyed form or not, for some time to come, vanadium will be linked with specific uses depending on specific properties such as low neutron absorption of the metal or corrosion resistance of its alloys.
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or in wrought form for valves and meter parts. Actually, most of this is speculation and the area of potential applications not already covered by a more readily available chromium-nickel alloy or a stainless steel is difficult to define. I n the past decade. research work on high purity chromium has been carried out primarily a t five laboratories throughout the world : General Electric Central Research Laboratory, Schenectady, N. Y . ; Battelle Memorial Institute, Columbus, Ohio; Bureau of Mines, Albany. Ore.; Fulmer Institute, Stoke-Poges, England ; and the Australian Aeronautical Institute, Melbourne: Australia. These and ot!iers have tried to produce pure chromium by metallic reductions of the chloride using magnesium, calcium, sodium, zinc or hydrogen. None has been successful in consistently producing high purity chromium for extensive physical-metallurgy studies. Electrolvsis, the preferred production method, can be done either in an aqueous solution or in a fused salt bath. Greenaway took advantage of Brenner's observations that the oxygen content of chromium deposits is reduced markedly a t high plating temperatures. Plating a t temperatures of about 80' to 90' C. produces chromium containing 0.2 to o,l 17c ox)-gen, to a conrent of 0.8 to 1% a t room temperature. strict can be kept to extremely By metallic pa)-ing impurities low. High purity chromium plate produced this rrav is available commercially today. Kroll, the Bureau of Mines, and Horizons, Inc,, among others, Lvere fairly successful in producing experimentally high purity chromium VOL. 53, NO. 2
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by electrolyzing its chlorides in what was essentially a NaCI-KCI carrier bath. There is good reason to believe that at least one large industrial firm in the United States could produce considerable quantities of high-purity chromium by this process if the demand Ivarranted. This electrolj tic product can be further refined, primarily of oxygen, by the van Arkel-de Boer iodide process; or its oxygen content can be lowered by hydrogen treatment a t elevated teniperatures. The hydrogen must be kept extremely pure, because the equilibrium partial pressure of water vapor is 2 mm. a t 1300' C. and 6 mm. a t 1500'C. Reduction proceeds slowly a t temperatures under 1250' C. Iodide or hot-ivire refining as practiced by Battelle has produced the world's purest chromium a substance containing only a few parts per million of oxygen, hydrogen. and metallic impurities. Chromium. having been born a bodycentered cubic, inherited a congenital defect of its family in that it has a ductilebrittle transition somewhat above room temperature tvhich confounds all efforts to work it. There are, in essence, three ways of dealing with this problem: purify the material; alloy it; or, like baldness, make the best of it. This last is not entirely facetious. It is happening to beryllium metal today. Enough work has been done in the last few years to show that attainment of purity is not the final answer in production of ductile chromium. Unlike titanium, zirconium and columbium, which now can be handled with fair facility once they have been obtained in pure form, chromium displays distressing habits which make the production of ductile wire and sheet a real metallurgical challenge. The ductile-brittle transition temperature demands that practically all working be done at approximately room temperature. However, ccmplete recrystallization, which occurs a t about 800' C., destroys all ductility. Ductile chromium wire has extremely poor impact resistance and its impact transition temperature is raised with addition of oxygen, nitrogen, carbon, or silicon. As if that were not enough, the metal is extremely notch-sensitive and any surface defects are transmitted through the metal in a devastating manner. The preparation of sheet, rod, and Xvire has been described by several investigators. Arc melting of high-purity chromium, on a water-cooled copper hearth or in a crucible, avoids contamination of the material during melting. 'The resulting ingot can be forged a t about llOOo C. to break up its cast structure. The forging is then recrystallized by heating it to 1200' C. in hydrogen, sheathed in mild steel, and rolled or swaged at 800" C. to inter-
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mediate sizes of plate and rod, with intermediate recrystallizations for grain refinement after 50 to 70YGreduction. Following this treatment, the chromium is desheathed and rolled in air a t 800' to 400' C., progressively lowering the temperature as the sheet becomes thinner. Contamination of this air-rolled sheet must be remedied by pickling the final product, which may amount to taking 0.003 to 0.004 inch from each side of a sheet. 0.014 inch thick, to achieve bendability. This is best acconiplished by electropolishing in a n ordinary chromium-plating solution in lvhich the sheet is the anode. Several investigators, notably those at Fulmer, have established that ox) gen in concentrations ranging from 0.002 to almost 1% has little effect on the temperature a t which the ductile-brittle transition of chromium occurs, a phenomenon not encountered with many other metals. However, \\ire containing more than 0.2% oxygen loses ductility a t higher temperatures and the impacttransition temperature is raised from 90' to above 200' C. by increasing the oxygen content from 0.004 to 0.228%. T o ensure tensile ductility, then, the oxygen content of pure chromium must be kept below 0.2% The effect of nitrogen is more complex than that of oxygen, but apparently the nitrogen content also must be kept below O.Ol%, if the worked material is to retain tensile ductility. Total nitrogen content, which might be loiv, can also be misleading because ductile chromium can be embrittled by a nitrogen-contaminated layer only 0.001 inch thick. This particular fact, combined with the low notch-sensitivity, represents the greatest deterrent to the development of ductile chromium. Smail amounts of nitrogen in the outer layer of hot-work material can set up stresses that are finally propagated as cracks a t amazing speed through this extremely notch-sensitive material, and result in failure of the entire specimen. Additions of 0.01% nitrogen will raise the impact transition temperature by 35" C. Therefore, from the standpoint of tensile ductility and notch-sensitivity, as well as impact resistance, it is imperative that the nitrogen content of ductile chromium be kept below 0.01%. Carbon additions have shown that this element has a potent influence on the same properties that are affected by nitrogen. and that carbon content of chromium must be kept below O.Olyo. Hydrogen ranging from 3 to 30 p.p.m. apparently has little effect on ductility of chromium. The effect of metallic impurities on the bend-transition temperature of iodide chromium sheet was determined by Battelle; iron to 0.257,, silicon to O.l5%, and tungsten to 0.27; have little
INDUSTRIAL AND ENGINEERING CHEMISTRY
effect on chromium's bend-transition temperature. About 0.1 % nickel raised the temperature by 50' C., and in amounts greater than 0.2%. nickel prevented fabrication of sound sheet. Other sheet, with carbon and sulfur above 0.01 and 0.015%, respectively, could not be bent to room temperature. .4 particular use to which ductile chromium might be suited conceivably could help to determine impurity limits of the metal, because some investigators. employing strain rates of 0.05 inch per inch per minute found that the nitrogen tolerance Mas higher than when test strains of 0.25 inch ivere applied. This is not unusual. because bodycentered cubic metals as a class are strain-rate sensitive. The unsuppressed urge of all metallurgists is to alloy something else with a metal to cure its ills. There are two ways that this can he done. The first is to add small amounts of a more active metal. which will tie up or distribute the deleterious elements in a less obnoxious manner. This is the function performed by the small amounts of manganese that are added to minimize the harmful effect of sulfur in steel, and by the 0.25% titanium bvhich is added to molybdenum to make that metal more workable and ductile. \'irtually every thing in the periodic table has been added to chromium. in amounts up to a few per cent: to accomplish this scavenging action. Thus far, nothing has succeeded in lowering the ductile-brittle transition temperature or improving the ductility. In fact. a few tenths of a per cent of many elements is enough to destroy the ductility of chromium a t anything approaching room temperature. A second method is to alloy chromium with metals that will impart the desired properties. ;May-kuth and Jaffee investigated many common alloys o l chromium and concluded that the highest chromium-content alloys rrould contain iron: nickel, or cobalt additions of about 59%. At points approaching the maximum chromium content, such alloys lost most of their ductility. Recent efforts to alloy chromium with other refractory metals have employed tungsten, molybdenum, and vanadium, ail of which have complete solid solubility in chromium. T o date, hoivever, there are no reports that chromium-base alloys made with any of these elements have room-temperature ductility. Chromium bears a remarkable resemblance to beryllium in that adding almost an)-thing to the metal worsens its already poor ductility. Chromium, however, is vastly superior to beryllium in ductility at room temperature and if only a small fraction of the research effort devoted to the beryllium-ductility problem were applied to chromium: many
LESS COMMON E L E M E N T S of the problems perplexing us today might be solved. Something still may be said for the third approach to chromium's problems-i.e., simply making the best of the metal's properties. One of the most profound changes in engineering materials in recent years has been the acceptance by aircraft and missile designers that beryllium's brittleness need not be a deterrent to its use. They are doing their best to design around the drawbacks of beryllium and are attempting to utilize its better properties as fully as possible. The same psychology applied to chromium might result in more use of the metal. Actually. the big problem today is that most of the people working on ductile chrdmium in various parts of the world have run out of ideas. What we need are new approaches by new people. For example: a couple of years ago the Bureau of Mines sent a piece of 2-mil chromium to the Argonne National Laboratory, where some enterprising investigator. despite all the limitations imposed on the working of this metal, cold-rolled the specimen to foil, 0.0002 inch thick.
Vanadium In contrast to the difficulties encountered with chromium, producing ductile vanadium sheet is a much simpler job. \\'hen purified sufficiently, vanadium has excellent room temperature ductility and fabricability. The ductilebrittle transition temperature of the pure metal is considerably lower than that of chromium. It has good liquid-metal corrosion resistance and is fairly low in absorption of thermal neutrons. However, militating against these traits is the fact that vanadium has probably the poorest resistance to either oxidizing or strongly reducing chemicals of any refractory metal. Oxidation becomes catastrophic above 660" C., when the oxide melts instead of fuming. spalling, or forming a tight coat. We are not blessed with a Ltealth of information on the effect of specific impurities on vaEadium. as we are in the case of chromium. However, it is well known that small amounts of the interstitial elements. oxygen, nitrogen, and carbon, and even many metallic elements have a profound influence on both the ductility and the transition temperature. Consequently, the first order of business in any vanadium program. as it was in research on chromium, is to obtain high purity material as a basis for all future work. Calcium reduction of vanadium chloride in a closed bomb is the preferred commercial method for producing ductile vanadium. Sucli metal has sold for about $40 a pound recently under
Atomic Energy Commission contracts. It contains approximately 0.170 oxygen, 0.05% carbon and 0.04y0 nitrogen. So far, attempts to use a cheaper reductant metal such as magnesium have not been successful. Electrolysis of fused-salt chlorides systcms has yielded metal of excellent quality, but up to now only on a laboratory scale. L'anadium metal can be further refined like chromium. either by the van Arkel-de Boer hot-\vire process or in a fused salt bath. to produce metal of exceptionally low hardness, possessing an extremely low work-hardening coefficient. Research at the Institute for Atomic Studies, i\mes, Iowa, and at the Armour Research Foundation, Chicago, 111.. has pin-pointed the limits of interstitial impurities so that generalizations can be made. Oxygen plus nitrogen should not exceed 0.20% and the carbon content should be under 0.10%. In all probability, carbon is the least harmful of the three, and nitrogen doubtless is more deleterious than oxygen. The impact strength of vanadium a t room temperature falls off rapidly as the sum total of these three elements increases. L'nlike chromium. in which varying amounts of impurities result in differences in impact strength of between and 5 foot-pounds. impact strength of vanadium drops from 100 foot-pounds down to something less than 10. The ductile-brittle transition temperature of calcium-reduced vanadium metal isabout -65' C.. while that of oxide-refined material is about -110' C. The addition of 0.270 oxygen is sufficient to change this to - 100' C. Srnall additions of titanium and zirconium exhibit a strong scavenging action on the interstitial elements; adding only a few tenths of a per cent of these metals ties u p the harmful elements and gives a second dispersed phase of oxides, nitrides, or carbides. In this first stage of deoxidation, vanadium loses ductility, but it is restored as the additions rise above 3%. Rostoker in "The Metallurgy of Vanadium" attributes this to titanium's activity as a deoxidizer, in the sense that it removes soluble oxygen and nitrogen as dispersed sub-oxides and nitrides. The embrittlement trend, then, is the balance between dispersed phase hardening and the solid-solution depletion softening. This same type of mechanism probably can be ascribed to the influence that '12% titanium has on the ductility of molybdenum, and to the efficiency of lanthanum additions in improving the workability of yttrium. Adding some of the more common metals, such as iron, chromium, silicon, aluminum, molybdenum, and nickel to vanadium results in ultimate tensile
strengths of from 60,000 to 90,000 p.s.i. Ivith little elongation or reduction in area. The addition of 2l/2% titanium or zirconium generally retains the ultimate strength and produces eiongations of from 15 to 23% with a reduction in area of approximately 30%. Again unlike chromium, vanadium has several interesting alloying possibilities, because it is completely isomorphous Jvith molybdenum. niobium, and tungsten, and shows high (but not complete) solubility for tantalum and rhenium. Just recently, Union Carbide Metals and the Armour Research Foundation have released reports on the tensile propcrties of vanadium-niobium and vanadium-columbium-titanium alloys which show some amazing properties. A news release from Union Carbide Metals states: "In the \varm-ivork condition, vanadium-columbium alloys have ultimate tensile strengths of 120,000 to 35>000 p.s.i. over the temperature range of 700' to 1000" C. and stress structure properties at 700' C., corresponding to 100-hour life stresses in excess of 100,000 p.s.i. O n samples which had been ivarm-worked and stress-relieved, the tensile strength was 70>000 p.s.i. at 1000' C., and 40,000 p.s.i. at 1200' C. Strain-rate sensitivity can be improved at the expense of strength by titanium additions." Parallel studies at .4rmour showed the 5-vanadium-20-titanium. balance niobium alloy had the phenomenal strength of 34,000 p.s.i. at 2,200' F. The densitycorrected tensile strength of this recrystallized alloy is similar to that for the molybdenum-'j2-titanium and F48 niobium alloys. In addition, this same vanadium alloy had 19% elongation at room temperature and could be cold-rolled as much as 90% after breaking down the cast structure. It \vas completely weldable. .4dditions of silicon to this same alloy produced an age-hardening effect that raised the ultimate strength by as much as 50%. However, an unfortunate property of this alloy is that, like all its constituents, it has no oxidation resistance at elevated temperatures, and preliminary investigations have not been encouraging. Nevertheless, from the standpoint of chemical-corrosion resistance, the high niobium-vanadium alloys appear to be exceptional over a wide range of oxidizing and reducing conditions. This characteristic \vas fully described by Wlodek a t the 1960 spring meeting of the Electrochemical Society. RECEIVED for review November 30. 1960 A C C E P T E D Xovember 30, 1960 Division of Industrial and Engineering Chemistry, 138th Meeting, ACS: New York, N. Y . , September 1960. VOL. 53, NO. 2
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