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OF CONSTRUCTION
CHEMICAL E N G I N E E R I N G R E V I E W S
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I l e s s Common Metals I I
T H E current review of recent information concerning the less common metals covers zirconium, hafnium, molybdenum, tantalum: niobium, rhenium, and a newcomer, “ductile” chromium. Although chromium metal of relatively high purity has been available for many years, the commercial grade is not ductile a t room temperature, so that ductile chromium is truly a “less common” metal. T h e year‘s achievements in this field have been largely in improved refining methods, in protective coatings and processing techniques, and in alloy development. T h e release of much hitherto “classified” information, particularly in the cases of zirconium and hafnium, was brought about, in part. by the International Conference on the Peaceful Uses of Atomic Energy which was held in Europe in the summer of 1955. Molybdenum and zirconium received the most attention and, while few publications appeared relating to tantalum and niobium, the usual technical “grapevine” sources more than hinted that a considerable renewed interest in these corrosion-resistant metals was developing. Rhenium, as a scarce and expensive metal, received less attention.
Zirconium and Hafnium Perhaps the most important (and certainly the most voluminous) publication was a book on the metallurgy of zirconium ( 7 9 A ) which included all aspects of occurrence, extraction, refining: fabrication, properties, and applications for this useful metal. Some idea of the wealth of material presented can be gained from the fact that a n 80-page chapter was devoted to the iodide process for the preparation of high-purity metal. T h e present and future of zirconium ivere discussed a t the Atomic Industrial Forum ( 2 4 A ) . Pointing out that the present AEC demand is for 1.200~000 pounds of reactor-grade zirconium annually, commercial producers and their products were listed. Current reduction processes and fabrication methods werc reviewed. T h e Zircaloys, standard commercial materials, were described. Zircaloy 1 contains 2.5 wt. yo tin; Zircaloy
2 contains 0.15Ivt. yo tin, 0.12 \ v t . 5 iron, 0.05 \vt. yc nickel: and 0.10 \vt. % chromium; and Zircaloy 3 contains 0.25 \vt. Tc each of tin and iron. T o date Zircaloy 2 has the best combination of strength and corrosion resistance. Zircaloy 3. designed for higher service temprrature than Zircalov 2. is not yer completely evaluated. The future demand for zirconium depends on government. industrial. and foreign programs. .A less expensive base material may become available chiefly as a result of the development of less costly and more efficient zirconium-hafnium separation methods. T h e preparation of purified halide compounds suitable as sources for ductile zirconium was described. .Australian zircon sand (22.4) \vas mixed Tvith 18 to 20:;; of 20-mesh carbon, heatcd in a carbide tube furnace at 1950’ C.. and chlorinated in a lfonel metal vessel a t 500’ to 700’ C. The 7irconium trtrachloride formed \vas condensed at 350’ C. Small Kroll-process pilot plants producing pound-scale quantities of zirconium can make use of such a system. S o detcctable mass transfer of carbide particl-s !vas encountered at a chlorine floiv rate of 1 gram per minute. \\-hen this floiv rate was doubled, carbide fines were carrit-d oirer ivith the zirconium tetrachloride. In a n investigation of the conversion of zirconium sulfates to anhydroas zirconium tetrafluoride (2-4). particular emphasis was placed on the preparation of pure zirconium sulfate and pentazirconium disulfate. Zirconium \\-as prepared from the fluoride by bomb reduction with calcium.
Recent developments in the Kroll process for zirconium \\’err described by the C . S. Bureau of hlines. Albany, Ore. (709) where studies rvere made of the mechanism of reaction of magnesium and zirconium tetrachloride on an operational scale. It should be recalled that zirconium is desirable as a material of construction for nuclear reactors. not only bccause of its corrosion resistance and mechanical properties, but alsci because it has a loiv cross section for the absorption of thermal neutrons. Since the hafnium “impurity.“ \vliich accompanies zirconium in most of its orcs. has a thrrmal neutron absorption cross section about 600 times that of zirconium? low-hafnium zirconium is desirable. Conversrly: hafnium itself, by virtue of its high absorption, is a n excrllent moderator or damper for. nuc.1-ar reactors. Thus, thr plant of the Carborundum Metals Co.. .Akron. N. Ti.. produces three products: ordinary hafnium-contaminated zirconium, hafnium-free or lo\\.-hafnium zirconium. and hafnium loiv in zirconium (3:l). Carborundutn‘s plant capacity for zirconium is stated to be more t?ian 130.000 pounds prr year. Rased on a n efficient srparation scheme and the Kroll process: rcactor-grade zirconium containin: lrss than 0 . 0 1 ~ ~ hafnium. is prod:ic:d. Hafnium sponge is produced a t the rate of 3000 to 4000 pounds per year. I t should be noted that 500 pounds of reaclor-grade zirconium is valued at 5-000. Sponge is double-arcmeltcd into 6-incli-diameter ingots for qualification tests. For sliipment, steel
E. M. SHERWOOD Battelle Memorial Institute, Columbus, Ohio E. M. S H E R W O O D i s assistant chief in the Division of Inorganic Chemistry a n d Chemical Engineering, Battelle Memorial Institute. Sherwood received his B.S. in engineering physics ( 1 9341, M.S. (19351, a n d Ph.D. ( 1 940) in physics from the Ohio State University, where he was successively university scholar a n d Battelle fellow. After a y e a r as instructor in physics a t O b e r l i n College, he was employed b y the Sperry Gyroscope Co. as a project engineer until 1950, when h e joined the staff a t Battelle. Sherwood i s a member o f the American Society for Metals (chairman of the N e w York Chapter, 1949-50) a n d the Institute of Radio Engineers; currently he i s also secretary-treasurer of the Electrothermics a n d Metallurgy Division of the Electrochemical Society.
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SEPTEMBER 1956
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MATERIALS OF CONSTRUCTION pails, each containing 75 poucds of sponge, are evacnzted, filled \vith dry argon, a n d provided xcith rubbergasketed lids Tvhich zrc scaled in place. Because of its intrinsic value, zirconiniii scrap must bc rcuwd ivlierever possihlp. T h c remelting of c!iips and turnings cut from zirconium innot; not adequately cleaned producm ingots \chose hardncss is above that of t!ir original material (209).T h e ofTendinq contarninants are found to be iron, aliuniinuin, silicon, carn . Techniques ivere developed for rcmoiring cutting oils (centrifugiug, Xvashing in carbon tetrachloride and in alcohol), mechanically adhering contaminants (iron removed by leaching in hydrochloric acid and Mashing), and oxygen (by pickling in a n aqueous mixture of nitric acid and ammonium fluoride), The pickling operation results in 1 2 to 1 5yGweight loss of zirconium. Better methods of consolidating long chips and turnings are still sought. Powder-metallurgy techniques (73A) offer still another method of producing massive zirconium metal. T h e three most important methods of powder preparation include hydrogen embrittlement of massive zirconium, acid leaching of sponge, and fused-salt electrolysis of potassium fluozirconate. I t is predicted that future alloy studies may benefit by using the methods of powder metallurgy as a preparative scheme. Furnaces for zirconium reduction in the Kroll process Icere made more efficient (3A) by use of automatic control, by changes in design: and by improved distillation apparatus. Lots of zirconium, of normal quality, weighing 350 pounds each (twice the amount usually resulting in conventional furnaces) could be made in 70% of the previous working time. A very complete review of the work done a t the U. S. Bureau of Mines was presented a t the International Conference o n the Peaceful Uses of Atomic Energy (27.4). M a n y tables, illustrative figures, and a comprehensive bibliography were included. Laboratory-scale in\,estigation of the Van Arkel--de Borr iodide process for preparing ductile zirconium \cas carried out ( 6 A ) . Rrsults Icere fairly ice11 in accord Lvith previoiis Ivork. Cornmcrcially :ivaiIablt: ultrasonic testing equipment \vas used in determining the soundness of doitblc-arc-melted zirconium ingots 4 to 10 inches in diameter and u p to 48 inches long (33A). I n clusions, voids: and discontinuities could be located by techniques which were found equally applicabie to zirconium with small alloy additions. Forming and fabrication of zirconium were reviewed (784, X 4 j , indications being that, in many respects, zirconium behaves like the well-known stainless steels.
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Arc-melted zirconium, \cliich has been subjected to various degrees of cold \vork was irradiated by neutrons and the suhsequent changes i n tensile propertics and hardness were assessed (77.4). Increases in hardness and Tield strength as a result of irradiation !veri= srr.Ttrst for annealrd material and .\vcrL'progrcssively less for iiicitcrials cold lvorlicd a t increasing Iwels. Th: increases in ulti. matc trnsiie strcngth and dccrrascs in ductility \\-ere ciorr iii7iforin. .Innraling for 160 hours a t 350' C:. rcnioved 707; of the radiation daiiiagc. liccovcry \vas greater in annealed. irradiated material than was recovery of strain !iardening. in 50Yccold-\vorked, unirradiated material subjected to the samr ainealing treatment. T h e internal friction characteristics and temperatiire drpendence of the torsion modulus of iodide zirconium lvith 2.4 \ct. yo hafnium ivere investigated using a low-frequency pendulum technique (5A). The internal friction was a maximum a t the allotropic transformation temperature (862" C . ) . T h e variation of the torsion modulus a t 20' C. was found to be 3450 kg. per square mm. Rotating beam fatigue tests on Zircaloy 2 gave a n endurance limit of 40,000 pounds/square inch, while flexure-fatigue tests on iodide-process zirconium sheet indicated a safe working stress of 25!000 pounds/square inch (31'4). Recent measurement of the total absorption cross section of zirconium for neutrons xcith energies betivecn 0.7 and 1.2 1n.e.v. gave values decreasing from 9 to 6 barns, with increasing neutron energy (72.4). Special techniques of considerable adaptability were developed for study of very thin oxide filrns on zirconium (7.4). Estimates of film thickness as a function of time were made and the results were correlated with hlott's theory of oxidation. Evidence was presented to substantiate the rvorking assumption of unit current efficiency for the film-forming process belo\v those potentials a t Lvhicli oxygen \vas evolved. Adherent, craze-free cnamcls, using a frit composition bascd on lead oxidcsilica giasscs arid matured either bcloiv or above the transformation point o f thr metal (862" (2.1. \ v ~ dcvclopcd c ivhich \cere suitable for use on eithcr sponge or iodide zirconium ( 3 2 1 ) . Sornc of' these protectivc ia)-ers -,vere 5 to 15 inils in thickness and ~ v e r capplicd in trco coats. each followed by heating in gcttered argon for 1 ' 2 hour at 950' to 1100' C. ,4lthoug!i protection was afforded from air oxidation at 600' C. for a period of 1000 hours. the coatings were not protective against high-temperaturr \cater. hIuch progress !vas made in alloy development and evaluation. A heattreatcd alloy of zirconium lcith 4 Lvt. % tin and 1.6 \vt. ';C molyhdeniim !vas readily roll(,d ;it 8 0 0 c C. and. i n vac'uum-
INDUSTRIAL AND ENGINEERING CHEMISTRY
creep-test equipment. showed four times the creep strmgth of unalloyed zirconium a t 500' C. (S'Aj. Ezhihiting a n annealed tensile strength of 917,000pounds/square inch, it could be hrat treatcd to a strength of niore than 140.000 pounds/sqiiare inch. Cold rediictioii of anncalrd matc;+;as possilile Ivithoiit ng the molybdcnum coxtent jvas found LO qivc greater ductility to thc hcat-trcatcd alloy. Alinor variations in coinpiiriii:: !lad only modcratc effects on the mechanical propcrtics of this alloy. Zircaloy 2 !vas cmbrittlcd when multipass icrldrd (2.3.1) due to the precipitation of intcrnictallic compounds in the weld zone brouqht about by the reheatingancl sloiv cooling induced b y the second pass. Highcsr wrld strengths werc obtained by \vatcr-rjucnchinq weldments from temperaturrs of 879' to 980' C. I n a study of thc correlation of composition and structure of a number of zirconium alloys with their corrosion resistance in hot water and steam, the following observations lcrrc made (4-4). An increase of the carbon content of spongeand iodide-zirconium melting stocks above the original 0.02 wt. % value lowered the corrosion resistance. An increase of nitrogen. at a given carbon level, increased the rate of attack, but not above that normally associated with highnitrogen levels. Oxygen! in the range of 0.07 to 0.35 \vt. yc3had no significant effect. Small iron and nickel additions improved the corrosion resistance most effectively, followed by chromium and tin. Optimum corrosion resistance in the nickel-iron-chromium alloys appeared to be associated rcith the presence of critical amounts of the intermetallic compounds characteristic of these elcinents. T h e influence of as much as 1 ivt. 7 0 of chromium. iron. and nickel on the tensile proprrties of zirconium is relatively minor from room temperature LIP to 316' C. (8.4). :It room temperature the 0 . 2 5 offset yield strength of zirconium was raised from 15,000 to 25,000 pounds,'square inch by 1 tvt. ci~ of these elements ivhile the elongation under maximum load \cas drcreased from 24 to 1 9 5 , Chromium appeared to be the most rffcctive hardener ivhilr all three elernrnts loicrrcd the impact strength, iron brill2 the most deleterious h this rcSptTT. The stxngthcning of zirconium by cliroiniiini. lvhich has no solid solubiliq- in tile low-temperature (alpha) phase: iq obtained by "particle hardening" for materials annealed below the alpha ranKe (835' C.) ( 7 B A ) . At 500' C . : the highest yield strength (26,000 pounds 'square inch) is obtained for a n 18 wt. % chromium alloy. T h e strtmgthening effect apparently follows t!ie law of mixtures. Ternary alloys of zirconium with tin and molvbdenuni or aliiminum and niobium
LESS COMMON METALS are relatively easy to fabricate ( 7 A ) . They exhibit good strength and ductility at room temperature and show promise for the development of satisfactory creep resistance a t 500" C. Not all binary and ternary alloys of iodide zirconium with titanium, aluminum, tin, tantalum, or chromium \ v e x suitable for use a t elevated temperatures (30:l). .Alloys with aluminum and/or tin appeared superior, exhibiting high-strcngth properties when tested in a helium atmosphere a t 650 O and 800' C. The strength level of alloyed zirconium a t 650" C. is equivalent to that of Type 347 stainless steel. I n an investigation made to determine whether or not Zircaloy 2 ; Containing more than 75 p.p.m. of hydrogen? would absorb hydrogen more readily than low-hydrogen Zircaloy 2, when irradiated, no detectable effect was noted (32'4). T h e impact strength of loiv-hydrogen Zircaloy 2 was not changed by irradiation. T h e production of hafnium was increased matexially during the past year although separation of this metal from zirconium is difficult. A Russian process for the separation of hafnium from zirconiuni by multiple fractional crystallization of potassium fluozirconate, for example, required 16 to 18 cycles for reducing the hafnium content of the zirconium to brlow O.O1yG (25'4). I t was shown that, if nictliods other than the Kroll process \yere to be used for metal extraction and consolidation, several separation schemes might be considered ( 75A). These included fractional distillation of the complexes formed by the tetrachloridrs xvith phosphorus oxychloride, fractional distillation of alkoxides, paper chromatography, cation exchange processes. and liquid-liquid extraction, using tributyl phosphate. A possible wet process for the preparation of low-hafnium zirconium metal was described. The P. S . Bureau of Mines (74A) prepared tonnage lots of hafnium in Kroll-process equipment designed for zirconium production. .\lthough some slight process modifications were necessary, the over-all operation was the same as for zirconium. Hafnium oxide was chlorinated in the presence of carbon and the resulting hafnium tetrachloride was reduced with magnesium. By-product magnesium chloride was removed in a high-vacuum distillation step and the hafnium sponge was crushed. Arcmelted metal could be forged and rolled into a sheet, but the best material produced was not cold ductile, presumably due to the oxygen content. T h e greater sensitivity of hafnium tetrachloride to atmospheric moisture and the greater stability of hafnyl chloride required additional care and effort in chloride purification. T h e sponge is somewhat reactive to air. The iodide process, so successful in the case of zirconium, was used to pre-
pare ductile hafnium ( 7 7 A ) Lvhich could be double-arc-melted, fabricated, and welded. Hafnium has a higher recrystallization temperature (700' to 750' C.) than does zirconium (500' to 550' (2.). T h e tensile properties of ductile hafnium a t room temperature include a n ultimate tensile strength of 59,400 poundsjsquare inch, a 0 . 2 2 offset yield strength of 22,400 poundsl'square inch, and a 3 5 7 , elongation in 3 inches, accompanied by a 38y0reduction in area. As was the case with zirconium, interest developed in the growth of anodic films on hafnium. I n 707, nitric acid a t room temperature, a n anodic film develops uniformly over hafnium single crystals with a thickness dependent on substrate orientation (27,4). Both highand low-resistance films were found, again, dependent on the orientation of the underlying metal.
Molybdenum hfuch of the interest in this metal centered on the properties of arc-cast material which became available in large ingot sizes. Intensive studies of the influence of impurities on the various mechanical properties, of fabrication methods. and of protection against oxidation a t elevated temperatures were carried out. ,411oy development work also was pursued actively. Research effort was devoted to means other than hydrogen reduction of the oxide for metal extraction. Xfolybdenum trioxide, reduced in a sealed steel bomb (lined with electrically fused magnesia) using a 20% excess of calcium metal over stoichiometric in the presence of iodine, yielded massive molybdenum reguli with 97% efficiency (8B). -4ddition of thorium, aluminum, or carbon to the charge improved the forgeability of the metal produced. I t was necessary to consumable-electrode-arc-melt the 20pound reguli prior to working before satisfactory breakdown characteristics could be secured. Proper location of the refractory lining in the bomb proved difficult. T h e U. S. Bureau of Mines (2B) concluded, after a n extensive and critical review of the literature, that to date only Brenner and Senderoff (7B) have been able to secure satisfactory deposits of molybdenum by electrodeposition and, further, that they were the only investigators who obtained massive deposits of the metal. T h e economics of this electrolytic process have not yet been established. If additional effort is to be directed along these lines, a process involving the complete exclusion of oxygen will probably be desirable. Cast molybdenum of ordinary commerical purity has been noted for its lack of cold ductility. I n one investigation,
high-purity ingots were prepared? each containing a controlled amount of a single impurity (738). T h e influence of oxygen, nitrogrn, and carbon on ductility was determined a t various temperaturcs using bend-test specimens. I t was found that the presence of only 1 to 2 p.p.ni. of oxygen induced room-temperature brittleness. For nitrogen and carbon. the impurity levels producing brittlcness are 30 and 140 p.p.m., respectivcly. The effects of oxygen and nitrogen are not additive; in fact, it appears that nitrogen may even offset some of thc oxygcn-induced brittleness. T h e arc-casting, fabrication, and a p plication of molybdenum in large conimerical ingots of 1000 pounds \ v u e discussed (4B-7B). Hydrogen-rtduced molybdenum powdcr (from ammonium molybdate) of the correct particle size and shape is used in a pelletizing process to form a continuous consumable electrode for arc-melting. The molybdenum powder must also exhibit an 0.(045; maximum weight loss after annealing in dry hydrogen a t about 1050O C. for l,'? hour and have a 4500 pound/square inch minimum compressive strength \vlien pressed into a pellrt) 1 inch in diameter by 1 inch high under a load of 12,000 pounds/square inch. Ingots, 9 inches in diameter and 4 fert long, arc produced a t a furnace melting pressure of 2 microns. T h e present furnace design is capable of scaling upward in size so as to produce ingots of 18 inches in diameter and 5 feet long. T h e high density and low^ gas content of the present largc-size ingots give rise to superior welding and machining characteristics in wrought material. Vacuum fusion analyses indicate the following ranges for gaseous impurities in arc-cast molybdenum: oxygen, 0.0002 to 0.0022%; nitrogen, less than 0.001 to 0.0027,; and hydrogen, 0.0001 to 0.00027,. Forging temperaturesof 1200' to 1300' C. are required in order to break down machined ingots. However, extrusion proved to be a more satisfactory means of modifying the as-cast structure. Billets 65,'s inches in diameter and 20 inches long, coated with glass as a lubricant, are extruded into 4-inch-diameter bars a t 1400' C. After cleaning, cropping, recrystaIlizing, straightening, and machining to a diameter of j 3 / 4 inches, the bars are hot rolled to a 3-inch size (temperature range 1150' to l j O O o C.). Sizes of 6 1 8 inch and under are processed by swaging and drawing in the same manner as powder-metallurgy molybdenum. Fabrication of parts and applications received detailed treatment. The elevated temperature mechanical applications for molybdenum in atmospheric gases should be relatively unlimited a t temperatures below the recrystallization range. if suitable protection can b r provided.
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MATERIALS
OF CONSTRUCTION
T h e welding of inolybdcnum is a n important joining process. LVeldments of good room-temperature ducrilit\- were secured for arc-cast molybdenum containing 0.7 wt. % titanium and carefull>-deoxidized Lvith carbon (less than 0.025 wt. yc carbon in the finished condition) (78B). T h e use of double-arc-mclting \vas required in the preparation of this material in order to avoid subsequent weld porosity. By welding in an atrriosphere of high-purity helium and grinding afterward, excellent results were obtained. In studies of the flash welding of commercial molybdenum (73B), acceptable bend ducrility \vas secured for %veldsmade in air. N o improvement was noted ivhen protective atmospheres of tank argon or 11-lium Tvere used. Poor b m d ductility of flash-welded molybdcxnurn may result from entrapment of oxides at the \veld surface, crearion of a transversc fiber structiire during the LIPsetting operation. and carbide precipitation in the heat-affectid zone. Figure 1 illustrates one step in a process for fabricating molybdenum tubing from thin sheet. The sheet \vas drawn through a bell-shaped die at 100O0 C. using a hydraulic drawbench. The tube. thus formed. then was cleaned and the seam was tungsten arc welded in an inert atmosphrre. Molybdenum hear exchangers were fabricated by weldinc (72B). Rfolybdenuni tubes 0.156 inch in diameter bvith a 0.015-inch \Val1 were fusion Jvelded into headers machined
Figure 1.
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from le-inch-thick sheet. T h e welding conditions were: arc current, 200 amperes at 18 volts, a.c. or d.c. ; arc gap 0.040 inch; time, second. Zirtung clectrodes. ’9 inch in diameter, Lvith a long taper to a ‘16-inch-diameter tip, were used in a helium atmosphere. Parts lvere preheated to 200’ C. T h e production of uniform circumferential welds appeared to result primarily from uniform conditions of heat transfer. Fusion ivelding. thus. \vas considered to be a sarisfacrory means of fabrication in spitc of the fact that ids \vert‘ brittle. LVith molybdenum of better quality: ductile \\.elds would be possible. T h e tensile dcformation of rnolybdrnum \vas eva1uart.d in constant strainrate tensile tests performed on polycn.stalline 1virc.s of hiTh purity ar temps’ratures in tlie range oi‘ -106’ to 1540’ C. ( 3 B ) . I’hrre groups of specimens !\‘ere tcstyd which had average Train diamercrs of 0.018. 0.034. and O.06 inni.: respccrivc.ly.. Results indicated that mo!>-bdenum undergoes discontinLIO~.IS>-i:lding i n tlie temperature ranqr or -55’ to 100° C. and suffers complex changes in !.ield strcngrh, tensile strength. strain hardenin?. and ductility Ivith tcmperaturc. These complvsitics are thought to be related to the presence of interstitial impurities. For the 0.018n 1 m . grain-size material. t h e ductilr-tobrittle transition temperature Tvas found to be 0’ e 2 j 3 6. B:ttt.r insight into r h e tensile properties of moly bdenurn ar
Hot-forming operation in fabrication of molybdenum tubing from sheet
INDUSTRIAL AND ENGINEERING CHEMISTRY
elevated temperatures \vas secured (76B). Investigation carried out under conditions of constant strain rate for short times and constant load for long times, was used in characterizing the remarkable high-temperature strength of molybdenum. A strain phenomenon, such as strain aging, may be responsible for this very attractive property a n d also is considered likely to occur in molybdenumbase alloys. .A properly adjusted combination of purity, deformation, and heat treatment Jvould be of great value in rnhancing the properties of such alloys. T h e mechanism of the initiation of yielding in ductile molybdenum is believed to be substantially the same as the wellknown “delayed” action observed in lowcarbon steel ( 9 B ) . Also, it is thought that nitrogen may be more effective than carbon in strengthening grain boundaries to prevent intergranular fracture. I n the protective coating firld, highnickel alloy cladding \vas found to provide adequate protection of molybdenum against oxidation a t 1000’ C. for long periods of time? including thermal cycling (77B). Roll cladding \vas carried out in hydrogen a t 1200’ C. using an alloy of 80 wt. 76 nickel and 20 Xvt. yc chromium. T h e protective la)-er was about 3 mils in thickness on finished sheets. Protection of the sheared edges of douhle-clad molybdenum sheets \vas secured by electrolytically dissolving thr exposed molybdenum in sodium hydroxide to form, by undercutting, a groove in which the proper filler wire could be inserted and fused using a n oxyacetylene torch. T h e general status of the a r t of prorecting molybdenum from oxidation a t high temperatures was reviewed (70B). Alloying is still in the basic research stage. Metal cladding is satisfactory for simple shapes a t temperatures up to 1100O C. where surface “toughness” is essential. Complex shapes can be electroplated with nickel and chromium where cladding becomes difficult. Selfhealing sprayed coatings, less brittle than ceramics and less ductile than clad or plated coatings, have been moderately effective up to 1300’ C. These also are useful for large and complex shapes. .Above 1300” C., ceramic and molybdenum disilicide coatings can be used only in cases where severe thermal shock, mechanical impact, or high stresses are not involved. Although numerous binary and ternary alloys of molybdenum with aluminum, chromium, cobalt, iron, nickel, silicon, titanium, tungsten, vanadium, and zirconium were screened, none was found to be entirely satisfactory (77B). I t appears unlikely that any molybdenum-base alloy can be developed to combine high oxidation resistance a t 1000O to l l 0 0 O C. with the good physical properties of unalloyed molybdenum.
LESS C O M M O N METALS
yc nickel or 23 tvt. yc chromium to reduce the oxidation rate of molybdenum by a factor of 100 which: in itself, hvould be a far from satisfactory opcrating figure. Presumably, only coated molybdenum will have the proper combination of properties. I n a series of alloys of molybdenum made by poivder-metallurgy methods (15B): it \vas observed that the mean recrystallization temperature of molybdenum (1080' C.) could be raised to 1280' C . \vith zirconium as a n alloying elcmcnt. .A 37; chromium alloy had the best hot hardness. For example, it requires 15 wt.
Tantalum and Niobium An increasing demand for metals with high corrosion resistance spurred efforts to secure better refining techniques for tantalum and niobium ( 3 C ) . Although for many years there has been only one major producer of tantalum, at least four others are now entcring the field. To date, the commercial process used in extracting tantalum and niobium has involved fractional crystallization as a means of separation. HoLvever, thc liquid-liquid extraction scheme, developed by the Bureau of ?clines and outlined in last year's review (K'),is noiv being considered as a replacement for the fractional cr)-stallization step and should lead to increased production and, incidentally, metal of higher purity. I n working metals into useful shapes, such as sheet, it is observed that "textures!' develop-that is, there may be one (or more) crystallographic direction in the grains of the finished sheet which bears a geometrical relationship to the rolling direction. I n general, metals with the same type of crystal structure develop similar textures under the same conditions of mechanical !corking. T h e cold-rolled texture of tantalum is similar to those publishcd for other bodycentered-cubic metals ( E ) . T h e principal results of a recent investigation indicate that, on the basis of present theories, slip and flow in tantalum take place on 110 planes in the 111 direction and that oriented growth is the most important factor during recrystallization. Anion exchange was used in the preparation of tantalum-free niobium pentoxide ( 2 C ) on a small laboratory scale. Starting with a fairly high-purity sample of niobium pentoxide, containing 0.4 to 1.4% tantalum pentoxide, a treatment was developed which yielded gram quantities of niobium pentoxide containing less than 1 p.p.m. of tantalum pentoxide. T h e factors influencing the fabrication of arc-melted niobium a n d niobiumbase alloys were studied (5C). Both shape and purity of the ingots were found
to determine the workability. Small amounts of the impurities carbon, oxygen, and nitrogen appeared to be particularly deleterious. T h e addition of titanium, as a scavenger, improved the workability. .Alloys with chromium, molybdenum, tantalum, titanium! vanadium, and zirconium \rere investigated. Only niobium-titanium alloys, in the form of arc-melted buttons, could be rolled directly, even a t temperatures as high as 12003 C. Il'ith care, alloys Lvith molybdenum, tantalum, titanium, vanadium? and zirconium could be cold Forged. -4dditions of titanium (optimum, 10 Lvt. yo)improved the strength of arcmelted niobium a t 1200' C. At 1000' C.. the 100-hour rupture strength of cold-rolled niobium was 16:jOO pounds/ square inch. Sletals with high corrosion resistance usually possess this characteristic as a result of their ability to form thin but tmacious oxide films. I n order to apply a n adherent coating o n such metals by electrodeposition, this surface film must be removed. A method of preparing niobium for electroplating was developed ( I C ) in lvhich electrolytic etching in hydrofluoric acid \vas employed. Subsequentl\-, '/z mil of either iron or nickel \vas electrodeposited. By heating plated niobium for 1 hour a t 700' C . after baking it a t 200' C., good bonds Were obtained. Iron proved to be more adherent than nickel after this treatment.
Rhenium Further exploration of the fundamental properties of rhenium \vas continued, largely under the auspices of the United States Air Force. although some interest in the metal was evinced by a number of industrial organizations. Determination of the vapor pressure of rhenium was made over a temperature range of 2200' to 2800' C. and a n estimate of its boiling point !vas given (5630' C.) (20). At 2500' C . ) the vapor pressure of rhenium is approximately one and one-half times that of tantalum. Other findings relative to the properties of high-purity rhenium can be summarized somewhat as follows (30). T h e modulus of elasticity of rhenium decreases linearly with increasing temperature u p to 900" C. Mechanical properties include excellent stress-rupture characteristics, these being a 100-hour value of 20,000 pounds/square inch a t 1000" C. and 800 pounds/square inch a t 2000' C. .\nnealed sheet has a O.lyooffset yield strength of 131,000 pounds'square inch, a n ultimate tensile strength of 168,000 poundslsquare inch: and a n elongation of 28Y0 in 1 inch. Cold-worked sheet, reduced 30%, has a 0.1% offset yield strength of 282,000 pounds/square inch, a n ultimate tensile strength of 322.000
poundsjsquare inch, and a n elongation of 2% in 1 inch. Rolling does not work harden rhenium as rapidly as does cold svaging or drawing. T h e presence of thoria reduces the ultimate tensile strength and ductility, and lowers the recrystallization temperature. T h e thermal electromotive-force relationship of the platinum-rhenium thermocouple has been established. T h e couple is more sensitive a t higher temperatures than it is a t lower temperatures. Rhenium is resistant to attack by molten tin, zinc, silver, and copper, is attacked slowly by aluminum, and is dissolved readily by nickel and iron. An investigation of the thermoelectric characteristics of couples of the rheilium-iridium system ( 7 0 ) a t temperatures u p to 2450' C. indicated that the niost satisfactory combination contained 70 wt. 70 rhenium. A recent review of information concerning rhenium predicts that rhenium should appear as a commercial metal in the near future 1(-f0).However, its scarcity (not more than 10 tons per year is available) will limit its use to those areas Ivhere special performance is required, and where the small volumes of metal consumed make the cost competitive.
Chromium Since 1940, a dernand has grown for high-purity chromium suitable for incorporation in high-temperature and corrosion-resistant alloys, carbides, and cermets. I n order to meet this demand. a ne\c process was evolved ( I E ) , which combined standard electric furnace refining and electrolytic deposition into a single integrated procedure capable of producing high-purity chromium from ore. Selling a t a price of approximately $1.20 per pound in large quantities, the metal contains the following impurities : iron, 0.14Yc; carbon. O.OIL;'c; sulfur, 0.0257c; copper O.OO1~o; and lead 0.002yG. O n a metallic basis! the mat:rial, thus, is 99.80% pure. This material, however, is not cold ductile, so that, for the present, it must be used as a starting or feed material for further refining processes capable of yielding a ductile mettil. Electrolytic chromium containing 0.01 to 0.02 wt. yo oxygen: and 0.002 wt. YG nitrogen, as major impurities, has been prepared (LE). Hydrogen reduction is used to lower the oxygen content to 0.005wt. yc and the nitrogen to 0.001 wt. yo. Room-temperature ductility \vas attained in vacuum-treated electrolytic chromium. Further studies ( S E , 9 E ) were carried out. Chromium was prepared having a room-temperature elongation of 1570. It was suggested that small amounts of nitrogen, apparently present in solid solution, produce room-temperature brit-
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MATERIALS OF CONSTRUCTION tleness. O n heating ductile metal in air or nitrogen a t 650’ C., embrittlement occurs. Heating in pure oxygen at this temperature did not embrittle the metal. Chromium is notch sensitive, and surface scratches produced by filing or coarse grinding can seriously reduce its ductility. Light etching or fine polishing eliminates this effect. Recrystallized chromium can be made a room-temperature ductile metal but is more susceptible to embrittlement than is a cold-worked metal. Kitrogen a t 0.02 Wt. yo produces brittleness in coldworked chromium, while less than 0.001 wt. 70is effective in embrittling recrystallized chromium. Ductility is obtained in low-nitrogen recrystallized chromium by a n electropolisliing treatment. T h e results of parallel studies by the Bureau of Mines ( 3 E ) were somewhat in agreement with those reported above? but differed in conclusions regarding the roles of oxygen and iron in the embrittling phenomenon as well as in explanation of the general effect. I n fact, no explanation for the brittle behavior of Bureau of Mines chromium could be advanced on the basis of this work. However, important observations w r e made. For example, a cold-ductile chromium wire could be embrittled by heating in gettered hydrogen at 600” C. By removing as little as 0.002 inch rron1 the surface, cold ductility could be restored. Impurities, such as oxygen, nitrogen, and nickel were considered to have an important influence on ductility and the degree of cold work was thought to be a factor, although a completely cold-worked structure was not absolutely necessary for maximum cold ductility. Cold-ductile wire, heatrd above the recrystallization temperature, was completely and permanently embrittled. To this point, the work discussed may be considered primarily as a background for consideration of more recent work on ductile chromium. T h e purpose of some of this effort ( 6 E ) , supported by the United States Air Force, has been to provide identification of compositional factors affecting the ductility of chromium since strong evidence has been found that pure chromium actually is a ductile metal. Chromium of the highest purity, prepared by an iodide-decomposition process, was used in a study of the effect of deliberate additions of various impurity elements on the properties of this chromium. Both the purity and physical condition of chromium are important in determining its degree of ductility near room temperature. Arcmelted iodide chromium sheet with 0.006 Lvt. yo oxygen, less than 0.002 wt. yo nitrogen, and approximately 0.02 wt. % of metallic elements, had a minimum
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ductile-to-brittle bend transition temperature of -25’ C . Small quantities of oxygen, nitrogen, iron, molvbdenum, tungsten, and silicon had little effect on bend ductilitv Lvhile nickel. carbon, or sulfur adversely affected both hot and cold ductility. TVrought chromium. as noted earlier. is vrrv sensitive to notch
effects. Minute surfacr irregularities, produced by normal machine grinding or milling the edges of bend-test samples in directions parallel or transverse to the rolling direction, increase the ductilcto-brittle transition temperature by 50’ C. Chromium metal, having less than 10 p.p.m. of any single iinpurity, can be made by the thermal decomposition of chromium iodide ( 7 E ) . The real problem Lvhich must be solved is the avoidance of impurity pickup on subsequcJnt processing. Although arc-melting may cause pickup of oxygen by 35 to i0 p.p.m., metal so melted has been fahricated into rod and sheet ductile at room tcmperature. Table I indicates the composition of as-deposited iodide chromium.
Table I.
Impurity Content of Iodide Chromium
Element
Oxygen Hydrogen Kitrogen Carbon Sulfur Phosphorus Silicon Iron Copper
.Antimony
Courtesy B a t t e i l e Memorial I n s t i t u t e
Figure 2. Enlarged photograph of reduced section of miniature tensile b a r of unalloyed iodide chromium (4.5 X )
INDUSTRIAL AND ENGINEERING CHEMISTRY
P.P. .If. 4-6
1 3
10-40
3 1.4 S o t detected Not detected
1-2 Not detected
In general, it was noted that iodide chromium could be fabricated under conditions that hydrogen-reduced electrolytic chromium could not. Some fabricable alloys can be made using larger amounts of iodide chroiiiiuni than with the electrolytic variety. Higher chromium content renders these allo)-s more corrosion resistant. The practicality of the iodide process indicates that metal of this kind may become available in the future a t prices as low as 55.00pcr pound. I n work: unpublished a t the time of this review (JE?5 E ) , further consideration has been given to the propcrties of alloys based on iodide chromium. Binary alloys of iodide chromium (99.99+5% pure) \\.it11 high-purity iron, nickel, and cobalt were studird. Sound rolled sheet was obtained for alloys containing 24 Tvt. yc iron: 37 Lvt. (% nickel, and 55 let. (5cobalt. respectively. The strength of the alloys decreased in proportion to the chromium content. Chromium-nickel alloys showed ihe best combination of strenyth and ductility. Duplicate ingots made \\-ith less pure chromium metal indicated that oxygen, sulfur, and/or nitrogen, in the amounts contained in commercial chromium, impart hot shortness to chromium-rich alloys with iron and nickel. Swaged iodide chromium, with a fibrous structure, tested as a round bar, had excellent tensile ductilitv which was lost after re-
LESS COMMON METALS crystallization. T h e miniature tensile specimen, shown in Figure 2, contained 0.006 wt. % carbon, 0.001 wt. % nitrogen, 0.005 wt. yo oxygen, a n d 0.001 wt. 70sulfur, with the level of total metallic impurities at 0.001 Ivt. 70. Its Lield strength, a t 0.2% offset, was 52,500 pounds/square inch, ultimate tensile strength 60,000 pounds/square inch, elongation 447, in 3/4 inch, a n d reduction of area 787c. T h e modulus of elasticity of iodide chromium decreases from 42,000.000 poundsjsquare inch a t room temperature to 36,000,000 pounds/ square inch a t 800’ C. Swaged a n d recrystallized alloys, containing 50 L v t . of either iron or nickel, had excellent tensile ductility. This became low a t 407, iron a n d was lost a t 40% nickel.
;,‘
BIBLIOGRAPHY Zirconium and Hafnium (1A) Adams, G. B.; Jr.. Rysselberghe, P. van, Xlaraghini. hi., J . Electrocheni. Soc. 102, No. 9, 502-11 (1955). (2.4) Beyer. G. H.. Koerner, E. L., Olson. E. H.. hmes Laboratorv Kept. ’ ISC-634 (unclassified], ; \ u p s t 18, 1955. (3A) Block? F. E.: Abraham, A . D., J . Electrochem. Sac. 102, No. 6, 311-15 (1955). (4.4) Boyd, \V. K., hlaykuth, D. J., Peoples, R. S.: Jaffee, R . I.: Battelle lfemorial Institute Rept. BMI-1056 (unclassified). Sovember 18, 1955. (SA) Bratina, L$’. J., LVinegard, W. J., J . Metals 8, 186-89 (1956). (6.4) Chi, iX. T., Le Vide IO, KO.60, 152-64 (1953). (7A) Chubb, W.,Trans. A m . Soc. Metals, 48, Preprint 27 (1955). ( 8 A ) Chubb, W.,Muehlenkamp, G. T., Battelle Memorial Institute Rept. BMI-938 (unclassified), August 11, 1954. (9A) Chubb, W.,hfuehlenkamp, G. T., Manning, G. K.>Ibid., BMI-987 (unclassified), hlarch 18, 1955. (10‘4) Gilbert, H. L., Alorrison, C. Q., Chem. E n g . Progr. 51, No. 7 > 320-25 (1955). (11A) Goodwin, J. G.. Hurford, I\’. J., J . M e t a l s 7, 1162-68 (1955). (12.1) Guernsey, J. B., Goodman, C., Phys. Reo. 101, Ser. 2, 294-7 (1956). (13.A) Hirsch, H. H . ? .iietal Progr. 68, h-0. 6, 81-5, 160. 162 (19%). (14.\) Holmes, H. P., Barr, hl. M., Gilbert, H. L., C . S. Bur. iMines, Rept. Invest. 5169, h-ovember 1955. (15.1) Hudswell, F.: Hutcheon, J. hl.? “Xlethods of Separating Zirconium from Hafnium and Their Technological Implications,” Intern. Conf. on the Peaceful Uses of -4tomic Energy (Geneva),A/Conf. 8/P/409,UK, July 11, 1935. (16A) Keeler, J. H.: T r a n s . A m . Sac. Metals 48, Preprint 26 (1955). (17A) Kemper, R . S.: Kelly, IV. S., Hanford Atomics Products Operation, Rept. No. HW-38079 (unclassified), July 29, 1955. (18A) Lacy, C. E., Keeler, J. R., Mech. E n g . 77, No. 10, 875-8 (1955).
(19.4) Lustman, B., Kerze, F., Jr., editors‘ “The Metallurgy of Zirconium,” National Nuclear Energy Series, Div. VII, vol. 4, SlcGraw-Hill, New York, 1955. (20A) Metallurzia 51, No. 306, 179-80 (1955). (21A) Misch, R . D.. Fisher, E. S., J . Electrochem. Soc. 103, No. 3, 153-56 (1956). (22‘4) Newnham, I. E., Rutherford, E., Turnbull, A. G., Australian J . Appl. Sci. 6, 218-23 (1955). (23.4) Niemann. J. T., Sopher, R . P., Battelle Memorial Institute Rept. BMI-1006 (unclassified), July 14, 1955. (24A) iVucleonics 14, 45-9, (1956). (25.4) Sajin, N. P., Pepelyaeva, E. A., “Separation of Hafnium from Zirconium and Production of Pure Zirconium Dioxide,” Intern. Conf. on the Peaceful Uses of Atomic Energy (Geneva), A/Conf. 8/P/634, U.S.S.R.?June30, 1955. (26.A) Schultz? J., Tripp, H. P., others, Am. Znst. Chem. Engrs.. Preprint 128. Presented at Nuclear Engineering and Science Congress, Cleveland, Ohio, December 1216, 1955. (27A) Shelton, S. hl., Dilling, E. D., hfcClain, J. H., “Zirconium Metal Production,” Intern. Conf. on the Peaceful Uses of Atomic Energy (Geneva): A/Conf,8/P/533, U.S.A., July 5, 1955. (28A) Steel 137, KO. 24, 113-14 (1955). (29.4) Stephens, \V. L\‘., Morrison, C. Q., J . .Vet& 8, S o . 3. 334-33 (1956). (30.4) Thyne, R . J., McPherson, D. J., Trans. .4m. Sac. Metals 48, Preprint 46 (1955). (31.4) Wallace? I V . P., \Vallate, R . H., Light .Ideta1 Age 14, 24-5, (1956). (32.4) IVheeler, R . G., Kelly, IV. S., Hanford -4tomics Products Operation Rept. No. 39805, (unclassified), November 2, 1955. (33.4) FVood, F. W.j Borg, J. O., U. S. Bur. M i n e s , Rept. Inoest. 5126, March 1955.
(14B) Olds, L. E., Ilengstorff, G. LV. P.. J. Metals 8, 150-55 (1956). (l5B) Pipitz, E., Kieffer, R., Powder M e t . Bull. 7. No. 2. 53-9 (1955). Pugh, J. h’., Trans. Am. So;. Metals 47, 984-1001 (1955). Rengstorff, G. W. P., J . M e t a l s 8, 171-76 (1956). LVeare, N. E , , Monroe, R . E., Rengstorff, G. W. P., Battelle Memorial Institute Rept. BMI1037 (unclassified), September 6, 1955.
Tantalum and Niobium Beach, J. G., Faust, C. L., Battelle Memorial Institute Rept. BMI1004 (unclassified), May 24, 1955. Cabell, M. J., Xlilner. I., J . Appl. Chem. 5, 482-3 (1955). Chem. Tl’eek 78, No. 17, 62, 64, 66, Ami1 28. 1956. (4C) Pugh, J . W.,Ilibberd, \tr. R., Jr., Trans. Am. sue. .Ldetals 48, Preprint 79 ( 1 9 -i 5- 1 39(1955). \ -
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(5C) Sailer, Saller, H . A , . Stacy, J. T., Porembka, S. \V,, Battelle hiemorial Institute Rept. BMI-1003, May 23, .. 1955. (6C) Sherwood, E. hf., IND.EKG.CHEbf. 47, KO.9, 2048-9, (1 955).
Rhenium (1D) Haase, G., Schneider, G., Z . Physik 144 NO. 1-3, 256-62 (1956). (2D) Sherwood, E. hf., Rosenbaum, D. hl.. others, J . Electrochem. SOC. 102, No: 11 (1955). (3D) Sims, C. T., Jaffee, R . I., “Further Studies of the Properties of Rhenium Metal,” J . Metals’8 (1956). (4D) Sims,C. T., Jaffee, R. l.,“The Properties and Applications of Rhenium,” (Presented at Conference on Reactive Metals, Buffalo, N. Y . , March 1956).
Chromium Molybdenum (1B) Brenner, A,, Senderoff, S., J . Electrochem. SOC.101, 16-38 (1954). (2B) Campbell, T. T., Jones: .4., U. S . Bur. Mines, Inform. Circ. 7723, July 1955. (3B) Carreker, R . P., Jr., Guard, R . \V., J . Metals 8, 178-84 (1956). (4B) Deuble. N. L.. hfetal Prom-.. 67. No. 4:87-90‘(1955). ” ’ (5B) Zbid., S o . 5, 89-92. (6B) Zbid., No. 6, 101-105, (7B) Ibid., 68, No. 2 , 77-89 (1955). (8B) Gilbert, H. L., Block, F. E., J . Electrochem. SOC. 102, T\-0. 7, 394-98 (1955). (9B) Hendrickson, J. A., LVood, D. S., Clark, D. S., Trans. Am. Soc. Metals 48, Preprint 36 (1955). (10B) Herzig, .4. J., Blanchard, J. R., Metal Progr., 68, No. 4, 109-16 (1955). (11B) La Chance, h l . H., Jaffee, R . I., Trans. A m . Sod. Metals 48, Preprint 43 (1955). (12B) Alonroe, R. E., Martin, D. C., Battelle lfemorial Institute Rept. BMI-1019 (unclassified), July 20, 1955. (13B) Sippes, E. F., Chang, W. H., Part I, IVeIding J . 34, KO. 3, 132s-140s (1955); Part 11. Ibid.. No. 5 , pp. 251s-64s. ~I
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(1E) Carosella, M. C., hfettler, J. D., M e t a l Progr. 69, No. 6, 51-6 (1956). (2E) Greenaway, H . T., “Production of Ductile Electrolytic Chromium,” Aeronautical Research Laboratories, Research and Development Branch, Department of Supply, Commonwealth of Australia, Rept. A.R.L./Met. 6, December 1954. (3E) Johansen, H., .4sai, G., J . Electrochem. Soc. 101, No. 12, 604-12 (1954). (4E) Maykuth, D. J., Jaffee, R . I., “Influence of Chromium Metal Purity on the Properties of Chromium Metal Alloys,” A m . Sac. Metals, Conf. on Ductile Chromium and Its High Alloys, Philadelphia, Pa., October 17-18, 1955. (5E) Zbid., “The hfechanical Properties Swaged Iodide-Base Chromium and Chromium Alloys.” (6E) Maykuth, D. J., Klopp, I\‘. D., others, J . Electrochem. SOC.102, 3-0.6, 316-31 (1955). (7E) Runck, R . J., Fearnside, T . E., others, see ref. ( 4 E ) . (8E) LVain, H. L., .\letal P r o p . , 49, No. 1, 91-6 (1956). (9E) Wain, H. L., Henderson, F., Johnstone, S. T. hf., “Study of RoomTemperature Ductility of Chromium,” see ref. (2E).
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