TECHNOLOGY
New Technique Yields High Strength Metals Du Pont's colloidal chemical process upgrades metal strength at high temperatures; TD Nickel is first commercial product A technique that combines chemistry with metallurgy is being used by Du Pont to upgrade the high temperature strength of certain metals and alloys (C&EN, June 25, page 3 7 ) . Du Pont says the technique involves a patented colloidal chemical process to achieve extremely fine, uniform dispersion of refractory oxides in a metal matrix (U.S. Patents 2,949,358; 2,972,529; 3,019,103). According to Du Pont, the technique is exceptionally versatile and can be applied to materials such as copper, aluminum, cobalt, iron, tungsten, nickel, molybdenum, and chromenickel alloys. Also, a number of dispersing agents can be used including thorium oxide, aluminum oxide, zirconium oxide, titanium oxide, and lanthanum oxide. For its first dispersion-modified metal commercially available, Du Pont has chosen a 9 8 % nickel, 2% thorium oxide (thoria) alloy. Named T D Nickel, it has an ultimate tensile strength of 10,000 p.s.i. at 2400° F . about 90% of the melting point of nickel. Unlike other high temperature materials, it doesn't depend on an exact thermal cycle to produce dispersedphase strengthening properties. And it doesn't rely on mechanical blending
of powders to achieve its very uniform thoria distribution, Du Pont says. According to Du Pont, TD Nickel has mechanical properties superior to many nickel or cobalt base superalloys in the 1800° to 2400° F. temperature range. A conventional superalloy begins to lose its tensile strength at about 1700° to 1800° F., whereas TD Nickel remains stable and becomes the stronger material over 1850° F., Du Pont claims. For example, T D Nickel at 2100° F. under a load of 6000 p.s.i. does not fail. The best superalloys now used in high temperature service failed under similar test conditions, Du Pont says. Another yardstick for measuring the Strength of materials is the stress rupture test in which a specimen is held at a stress well below its tensile strength until it breaks. At 1800° F., T D Nickel will withstand greater stresses than the superalloys for times exceeding 300 hours, Du Pont says. Examined Metallurgy. To explain the differing properties of dispersionmodified metals and conventional superalloys, the basic metallurgy of both must be examined, Du Pont says. Superalloys are strengthened by precipitation hardening. In this method, the hardening agent is first dissolved in
HIGH TEMPERATURE STRENGTH. The flame of an acetylene torch is shot through a tube of Du Pont's TD Nickel to demonstrate the metal's stability at high temperatures. TD Nickel, a dispersion-modified metal containing 98% nickel and 2 % thorium oxide, is produced at the Du Pont Metals Center, Baltimore, Md. 40
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the base metal. The alloy must then be aged at lower temperature for precipitation of small, hard particles to occur in the grains. These particles block intergrain motion and make the metal stronger. However, precipitation hardening has limitations, Du Pont says. First, soluble hardening agents must be used. This eliminates a number of stable materials with limited solubility. Second, at extreme temperatures the hardening agent redissolves and strength of the material is lost. Third, overheating precipitation hardened materials reduces their strength permanently due to overaging or grain growth. According to Du Pont, these limitations do not exist with dispersionmodified metals such as T D Nickel. The alloying agent, thoria, is stable, inert, and essentially insoluble. Through complex chemical technology, tiny particles of thoria (about one millionth of an inch in diameter) are uniformly dispersed through the base metal, Du Pont says. Once locked inside the grain structure of the metal, the thoria acts as a roadblock to the migration of atoms and the movement of grains which normally take place when metal is subjected to extreme heat. Stable Dispersoid. Because thoria is insoluble, T D Nickel does not lose its strength at extreme temperatures as do superalloys in which the hardening agent dissolves, Du Pont says. Also, when T D Nickel is heated to within 50° F. of its melting point, held for a few hours, and then retested, there is no change in its strength. This indicates dispersoid stability without grain growth, the company adds. In addition to its high tensile and stress rupture strength, T D Nickel has other useful properties. It can be fabricated at room temperature, has good corrosion resistance, and excellent thermal and electrical conductivity, Du Pont claims. T D Nickel also has good oxidation resistance at high temperatures. For example, when ex-
posed to air at 1800° F., it will oxidize at a rate of about two ten thousandths of an inch in 100 hr. Du Pont expects gas turbines to be a major market for dispersion-modified metals such as T D Nickel. According to Henry F. Peters, marketing manager for Du Pont's Metals Products Pig ments Department, the aircraft jet en gine is now designed to operate at temperatures no higher than 1750° F. in the turbine. The designers would like to operate the engine 400° F. hotter to allow more power per pound of engine. Dispersion-strengthened metals could withstand these higher temperatures, Mr. Peters says. Du Pont foresees other markets for the metals including high temperature process equipment, such as heat ex changers; electronic components, such as cathodes; aerospace, such as the liquid hydrogen rocket nozzle; and high temperature instrumentation. Powder Fabrication. T D Nickel bar is produced by powder fabrication at the Du Pont Metals Center, Balti more, Md. Powder fabrication is not equivalent to powder metallurgy, Du Pont points out. The powder, or raw material, is made by Du Pont's pat ented colloidal chemical process in a pilot unit at Belle, W.Va. A larger scale plant is under construction at Newport, Del., and is expected to be completed by the end of this year. At the Baltimore plant, the prepared TD Nickel powder is loaded into rubber boots and hydrostatically com pacted at a pressure of 60,000 p.s.i. The compacted billet is then heated or sintered in high purity hydrogen which raises the density and imparts sufficient strength to the billet to permit han dling. The billet is next heated and extruded to a nominal 2-in. diameter bar. The bar is drawn at room tem perature to 1 / 2 - , V-r, or 1-in. diameter. The final step is to heat treat the bar at 1800° F. to restore its room tempera ture ductility which is reduced during cold working. Although thoria is mildly radioactive, the radiation levels experienced during handling and fab rication of T D Nickel are well below the AEC established tolerances, Du Pont says. The 1 / 2 -, 3 Λ - and 1-in. diameter bars are available up to 20 ft. in length at $20 per lb. Sheet and tube blanks will be available in the near future, Du Pont says. Du Pont expects T D Nickel to eventually be competitive with superalloys which today sell for less than $10 per lb.
X-Rays Applied to On-Stream Analysis Norelco's continuous analyzer determines up to six elements in as many as 15 sample streams A new on-stream analyzer based on x-ray fluorescence has been demon strated for the first time by Philips Electronic Instruments, Mount Vernon, N.Y. The Norelco continuous ana lyzer is designed to determine up to six elements simultaneously in as many as 15 sample streams. It can be used for slurries and liquids and powders on conveyor belts. All elements from titanium, atomic number 22, and above can be monitored and analyzed by onstream x-ray fluorescence, Philips says. In a recent plant-scale experiment at New Jersey Zinc's Friedensville, Pa., mill, the incoming feed to a zinc flota tion mill was continuously analyzed by x-ray fluorescence spectrometry. Philips provided the Norelco x-ray equipment in the experiment and co operated in the preliminary develop ment of the sampling system in its laboratories. Philips also helped to conduct the plant-scale trial. In a zinc flotation mill, the optimum quantities of copper sulfate and col lector are closely related to the quan tity of zinc in the feed. Milling prac tice is to use more reagents than needed, thus ensuring adequate amounts to cope with periods of high zinc content. However, by continu ously measuring zinc content of the feed, using x-rays, the collector and copper sulfate feeding rates can be varied with variations in feed grade. The experiment using x-ray control showed that the flotation mill could be operated with satisfactory metallur gical results at a saving of 2 1 % of the reagent costs. With x-ray control, only as much copper sulfate and collector as required by the zinc content in the mill feed was used, thus excess re agents were avoided at all times. As a result, the flotation environment was definitely more selective, New Jersey Zinc says. For example, the x-raycontrolled side of the mill averaged concentrate grades with 4% higher zinc than did the side of the mill not x-ray-controlled. As a result of the plant-scale experiment, New Jersey Zinc decided to purchase from Nor elco a four-stream continuous system to monitor zinc in heads, middlings, concentrates, and tailings.
Flow Cell. The heart of the Norelco on-stream system is a vertical flow cell with a horizontal self-washing Mylar window (patent pending). The sam ple stream actually folds over itself as it passes the cell window, thus the sensing head measures every sus pended particle. Sedimentation, grav ity, and bubble errors are eliminated, Philips says. Sample streams flow to agitated head tanks at the rate of 10 to 15 gal. per min. Samples are taken at atmos pheric pressure with a head pressure of 24 to 36 in. of slurry at a flow rate of 2 gal. per min. Because of this low pressure system, there is no need to use beryllium windows which are ex pensive and are difficult to replace, Philips says. The components of the Norelco onstream analyzer include the following: • A basic constant potential gen erator containing an FA 60 or FA 100 x-ray tube, line stabilizer, and x-ray tube current stabilizer. • A six-element capacity sensor head with built-in density monitor and cor rection, and a traversing mechanism which can continuously scan many sample streams. • A sample holder bed wJiich handles liquids, slurries, powders, and solids. • A console containing programer, data processor, power supplies, and control circuits and cabling. • A recorder panel containing a number of potentiometric strip chart recorders, depending on the num ber of elements and sample streams to be analyzed. The recorder panel also has a maximum/minimum audio alarm for any channel. The Norelco analyzer provides fully automated analysis of one to 15 con tinuously flowing sample streams. Each stream is separately analyzed by an automatic traversing sensor head. Stoppage in any stream doesn't re strict analysis of other streams, the company says. The on-stream analyzer has solid state design in the programer, data processor, and power supplies. Ε 30 JULY
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