January, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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White Metals By Wm. A. Cowan NATIONAL LEADCo.,NEWYORK,N. Y.
N THE CLASSIFICATION of alloys, as established by ordinary usage, the term “white metals” is understood to designate alloys of lead, tin, and antimony. I n some cases copper also is one of the lesser constituents, and alloys containing zinc may be included, but aluminium alloys are classified separately. Fusible alloys containing bismuth and cadmium are included as white metals. The nomenclature of most alloys of this class is based on the purpose for which they are used, the principal ones beihg bearing metals, solder, type metals, and casting metals. The names of the first three describe specifically the purposes for which they are used. They consist essentially of white metals, although the term “bearing metals” also covers bearing brasses or plastic bronzes. Casting metals, composed of white metals, are used for toys, plated articles, storage battery grids, bullets, etc., as well as in the manufacture of what are commonly termed “die castings.” The development of standardized metal parts by die-casting to accurate size has resulted in large-quantity production by this process, bearing linings of white metal being frequently so made and in some cases die-cast directly on a brass back. The development of the die-casting industry has been very rapid, and a great deal of work has been done on the investigation of the alloys most suitable for the various purposes for which die castings are now manufactured. Rapid advances have been made in the art, and the alloys utilized have covered a wide range of composition, aluminium and zinc base alloys being used to a large extent. Since the subject covers a broader field than white metals as defined above, alloys for die castings will not be considered here.’ Them are two additional kinds of white metals, the names of which are not derived from their fields of service. These are pewter and Britannia metal, the former being now very little used, while the latter is employed to some extent for silver-plated ware. Since they are of comparatively small commercial importance, and no noteworthy developments have been brought out in connection with them, these alloys will not be discussed. BEARINQMETALS The use of white metal alloys as lining metal for bearings has developed greatly since the introduction by Isaac Babbitt of a tin base alloy now commonly called “Genuine Babbitt.” There is need of a comparatively soft metal as a lining for bearings to provide a bearing surface that is softer than the metal of the shaft, and that is sufficiently plastic to conform to any irregularities in alignment or running. Such a metal prevents wear of the shaft, and provides for the economical replacing of the lining when it is itself excessively worn. Metals of homogeneous structure such as possessed by pure metallic elements might have the requisite qualities of softness and plasticity, and yet they have been found unsuitable for bearings. Metals which have proved satisfactory are alloys of heterogeneous structure having hard components imbedded in a softer ground mass. When properly cast so that the hard components are more or less uniformly distributed and interlaced throughout the softer matrix, a structure is provided which is sufficiently resistant to pressure and yet has requisite plasticity. In actual service, such an
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See Tour, “Die Castings;’ p . 26 of this issue.
alloy has strength to withstand the load and sufficient hard ness of the rubbing surface, while the softer plastic matrix imparts to the metal the property of flowing under pressure, thus enabling it to conform to any irregularities in the running of the shaft. These requirements in the structure of alloys have long been established as constituting satisfactory bearing metals. It has more recently been recognized that under the ideal condition of running, which requires an unbroken film of lubricating oil, the question of friction and wear on the bearing linings is of very little moment, and it is only when the film is broken that the peculibr properties of the bearing metal come into play. An important requirement, therefore, in the design of bearings is to insure as far as possible the maintenance of a perfect oil film. It is now believed that in the characteristic structure of alloys previously found best for bearings, where the interlaced crystals of the hard component are brought out by wear and the softer metal of the ground mass is depressed, the resulting formation of grooves and hollows, although almost infinitesimal in size, tends to retain the oil film, somewhat as by capillary action. For many years the practical development of bearing metals simply followed rule-of-thumb methods, alloys being made up to certain formulas according to hearsay or because of customary practice, without real study of the relation of actual requirements to the structure of the alloys employed. As careful investigations have been made of the properties of alloys required to render them serviceable for bearings, it has been found that many long-established ideas were based simply on prejudice or lack of full knowledge, and that a change in grade of metal should be made in order to give best and most economical service. This has resulted in finding many cases where drastic changes have been desirable, such as substitution of a tin base alloy for a lead base alloy, or vice versa, and also where slighter changes in composition could be made to produce greater uniformity. However, this situation has frequently been taken advantage of by unscrupulous dealers to persuade the nontechnical consumer that there was some peculiar quality in the particular brand of alloy of his manufacture, whereas the demonstrated difference has in fact been due only to change in grade or composition of the alloy. Following this, many manufacturers have made extravagant claims of exceptional quality in alloys of their make based on secret processes, addition of substances said to be impossible to be found by analysis, or presence of small amounts of elements, which may appear to give improvement in one direction but may be actually detrimental on the whole. These claims are often presented in a pseudoscientific manner, and have been a frequent cause of misleading the user having little technical knowledge. It is now well established that such methods are both unwise and unnecessary, since the best alloys can be made by the use of good practice in the art based on skilful experience and well-known metallurgical principles. Alloys of wide variations in composition have been suggested and put into practical service as bearing metals. In many cases these have varied from each other by only small percentages of the constituent elements, so that the difference in actual properties has been slight. It is now the tendency to concentrate on a fewer number of formulas, selected a s
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INDUSTRIAL A N D ENGINEERING CHEMIXTRY
giving the best characteristics in each grade, as based on the properties conferred on the alloy by its structural components. This has been recommended by committees of the War Service Association of Manufacturers of Solder and Bearing Metals and of the American Society for Testing Materials. The grades specified can be classed very roughly as lead base, tin base, and intermediate grades. The latter, containing considerable amounts of both lead and tin, have as a structural component the eutectic mixture of these two elements. This has a low melting point, while the other components have a higher one, thus giving a wide range of temperature through which the metal continues to freeze in cooling. Partly on account of this and the presence of the eutectic mixture, these alloys have certain undesirable characteristics as bearing metals and are being largely eliminated in favor of either tin base or lead base alloys. They are, however, suitable for a special purpose, where very thin linings are desired, requiring a high degree of fluidity in pouring and the quality of adhering well to the metal in connection with which they are used as a lining. A new alloy has been introduced as a bearing metal which has given excellent results in service. It is known as Frary metal and consists of an alloy of lead with only small amounts of barium and calcium. Although it contains approximately 98 per cent of lead, the added elements impart to it a hardness similar to that of other bearing metals, and also similar ‘structural characteristics. It is used in the same way as other white metals for bearing linings, and has besides been found to give extraordinary service when cast in form of the entire journal for bearings of railway cars, particularly on interurban electric railways. The study of the application of alloys to the needs of their service as bearings is based on the proportions and methods of distribution of their several structural componentsnamely, solid solution, eutectic mixture, chemical compound, or pure metal. Difference in hardness of the separate crystals of microscopic size comprising these components can now be investigated by means of an apparatus called the “Microcharacter,” recently developed by a subcommittee of the American Society of Mechanical Engineers, C. H. Bierbaum, Chairman. This apparatus is to be applied to the general study of alloys by the Subcommittee on Microhardness of the American Society for Testing Materials. There is more attention now being given to the question of proper pouring temperatures and rate of cooling in casting, as determining the distribution of structural components without undesirable segregation in bearings when put into service. The properties of each separate casting of an alloy as influenced in this way depend upon its particular treatment since last remelting. The need is now recognized of giving consideration to changes in properties of the alloys which take place a t temperatures attained by bearings while running in actual service. Some work has been done on determining the properties of certain bearing metals a t elevated temperatures. SOLDER
There has been little change in the composition of alloys used for solder. For ordinary soft solder, lead tin alloys have been consistently used. They have the following qualities requisite for the purpose: low melting point, high degree of fluidity at temperature only little above the melting point, and property of adhering well to surfaces of most other metals. Although the alloy having the composition of the eutectic mixture-63 per cent tin and 37 per cent lead-is actually best in these respects, there is seldom need of using it, since the 50/50 alloy is about as serviceable. The latter contains some excess of lead, which, however, does not
Vol. 15, No. 1
solidify a t a temperature sufficiently above the melting point of the eutectic to affect the practical working qualities. The advance which has been made in the practice of soldering has been in adapting the composition of the alloy to requirement of the work in hand as determined by the proportions of eutectic and excess lead. The alloys of this series within a short range of composition contain such proportions of these two components that they acquire the property which is af value in making a so-called wiping solder. The metal becomes plastic on cooling when passing through a comparatively long range of temperature wherein it is partly fluid and partly solid. This property is valuable in wiping pipe joints, the solder being applied in this plastic condition and held in place until entirely solidified. It was formerly a frequent practice to allow the individual workman to use his personal skill in mixing wiping solder to the proper composition by the addition of tin or lead. With better knowledge of the properties of the alloys, the workman is now usually provided with solder ingots of the correct composition and not allowed to make any change. During the war, when there was a scarcity of tin, an alloy made by replacing some of the tin with a less amount of cadmium was suggested for solder. The composition recommended, and on which much work was done by the U. S. Bureau of Standards, was 80 per cent lead, 10 per cent tin, and 10 per cent cadmium. However, since the war this cadmium solder has not been commercially used. The addition of a small amount of phosphor tin to solder has also been frequently suggested for the purpose of replacing to some extent the full amount of tin otherwise required in certain grades. Such an alloy, however, has not been found fully satisfactory. I n the manufacture of electric lamps with tungsten filaments, which attain a higher temperature than the old incandescent lights, a high melting-point solder is required, for which purpose an alloy containing a comparatively low amount of tin is used. Progress has been made in applying solder by means of automatic machinery and by dipping in connection with large production of standard parts or hished articles. An improvement in commercial practice of selling solder by brands was long ago made by many manufacturers indoing away with an old custom of misbranding. It was formerly the universal custom to sell most grades of solder as “half and half,” whether or not they were actually 50/50. Some qualification was made by branding the different grades as “warranted,” “strictly” or “commercial half and half,” but the latter grade contained as little as 37 per cent of tin. Such misbranding has now been stopped, as the result of a United States Supreme Court decision.
TYPEMETALS The introduction of type-setting machines in the printing industry has brought about necessary variations in the percentage composition of type metals. Change has resulted also on account of the general practice of using the type form first set simply as a die in preparing papier-m%chd matrices for casting the plates which are actually used for printing. The latter are called “stereotype plates,” and this method of printing is carried out in general where a great many impressions are to be made, as in newspaper, magazine, and book work. The harder alloys formerly required for small type are now less needed, and for some kinds of type metal, as linotype, alloys consisting of eutectoid mixtures of lead, tin, and antimony are most suitable. I n this case sufficient strength is assured by the fact that the face of the type is cast as a continuous slug of metal the length of one line of type. For the
January, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
various kinds of type-setting machines alloys are required having in general the following properties: low melting point, with solidification taking place in cooling either at one sharp point or else within a very narrow range of temperature; perfect fluidity above this point; slight amount of shrinkage; and property of taking sharp impressions when cast. Some differences in characteristics are required, however, in the particular alloys most suitable for stereotype and for the separate kinds of type-setting machines. according to whether the latter cast single letters, or a whole line. It has been possible to adapt the alloys to these different requirements from a thorough knowledge of the metallography of the ternary series, lead, tin, and antimony. This has been gained from the complete investigations of the thermal equilibrium of the series by Campbel1,z Loebe,3 and from the work of Heyn and Bauer.4 I n the use of stereotype many separate plates cast in the same matrix may be needed to complete an edition, and for the very large issues of weekly magazines, etc., improvement has been made in the number of impressions possible to be taken from each plate by nickel plating the type face. 2 “Le ad-Tin- Antimony and Tin-Antimony-Copper Alloys,” Proc. A m SOC.Testing Materials, 13 (1915),630. 3 “The Constitution of the Ternary Alloy of Lead, Tin, and Antimony,” Metallurgic. 8, 7, 35. 4 Investigation of Bearing Metals, Antimony-Lead-Tin Alloys, conducted by K Materialprufung Berlin-Lichterfelde.
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OTHERRESEARCH ON ALLOYS Valuable articles have lately appeared on the general subject of alloys by W. Guertler; on white metals by Mundy, Bissett, and Cartland, published as a preprint for the September 1922 meeting of the British Institute of Metals; and on bearing metals by J. Ceochralski and others.6 Interest in the study of white metals has greatly increased through the work which is being done in connection with nonferrous metals by the U. s. Bureau of Standards and Bureau of Mines, committees of the American Society for Testing Materials, the Institute of Metals Division of the American Institute of Mining and Metallurgical Engineers, and other organizations. General problems that are being investigated relate to the subject of corrosion, the effect of impurities on physical properties, standardization of methods of physical testing, and effect of shrinkage on the density and form of solid castingsthis having to do both with the contraction taking place from the instant of complete solidification down to room temperature, and also with the change in volume accompanying the difference in density between the liquid and solid phases of the metal. 6 Czochralski, 2. Metallkunde, 12 (1920),371; Guertler, I b i d . , 13 (1921), 257; Czochralski-Welter, “Bearing Metals and Their Technical Value,” Springer, Berlin, 1920.
T h e Wrought Nonferrous Alloys in 1922 By W.H. Bassett THE AMERICAN BRASSCO., WATERBURY, CONN.
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HE PAST YEAR has brought to attention nothing novel in the way of alloys or the treatment of alloys in the wrought nonferrous industry. COPPERALLOYS The outstanding feature has been a wider use of copper and copper alloys following the post-war depression. This has been brought about through a better understanding of the properties of the materials and their adaptation to industrial requirements. Much more attention is being given to the study of the properties of the metals and alloys and to methods of treatment than ever before. The advent of the chemist and metallurgist in the industry dates back hardly more than twenty years, and up to the beginning of the World War only the larger and more progressive manufacturers maintained laboratories or attempted metallurgical control. The engineering specifications from the military establishments forced manufacturers who had been delinquent to establish testing laboratories a t least. Within the industry itself notable improvements of the present time are laboratory control of the quality of the raw materials used, of the composition of the alloys, and of the physical and electrical properties of the products, metallographic control of heat treatment and hardness, electric melting, and the better understanding of the properties of metals and alloys. The statement is frequently made that the quality of copper is not as good as it was twenty or thirty years ago. Such statements are indicative of ignorance of the real facts, for a t no time in the history of the industry has the quality of copper in the form of wire bars, cakes, and ingots been equal to that of the present delivery. The purity minimum of 99.900 per cent is now maintained by the important American pro-
ducers. I n wire bars and cakes the conductivity is regularly held a t 100 per cent of the “Annealed Copper Standard.” All this has been gained through systematic control testing on the part of both producers and consumers. IMPROVEMENTS I N QUALITY Standard specifications for copper, zinc, and nickel have been prepared by the American Society for Testing Materials. Considerable progress has been made by the same society on similar standards for lead and aluminium. Such specifications have marked the progress made in improving the quality of raw materials and rendered the production of alloys of the highest grade a much more exact and simple matter. The analytical laboratory is as essential a part in the production of nonferrous alloys as it has become in the steel works. In every modern metal works the composition and quality of the alloys is strictly a matter of laboratory control. While this condition is not a sudden development, it cannot be said to have become general until within the last few years. It certainly marks a notable feature in the progress of the industry. The metallographic study of nonferrous metals and alloys has gradually grown in importance as a means of control and investigation, and 1922 has seen the publication by a national society’ of the first specification in which grain size has been used as a definite measure of the physical properties of the material. The study of the structure of the nonferrous metals at magnifications of 500 and 1000 diameters in the works laboratories, where nothing much beyond 75 to 100 diameters has heretofore been attempted, is another feature marking progress in applied metallography. * American Society for Testing Materials.
