EVOLUTION OF
Symposium
B. D. SAKLATWALLA U. S. Rustless Steel & Iron Corporation, Gulf Building, Pittsburgh, Pa.
on
New Metals
and Alloys Applicable to
the Chemical Industry Presented before the Division of Industrial and Engineerjng Chemistry at the 9Znd Meeting of the Amerioan Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936.
(Pages 1366-14 16 )
T
HE
basis of our mechanistic civilization of today is accurate scientific knowledge and understanding. Egypt, China, India, and other ancient countries may have produced remarkable bronzes, the iron pillar of Delhi, unmatchable sword blades, bewitching pottery and glass, yet the excellence of these products was dependent on pure empiricism. The arts flourished by tradition handed down from father to son. This made progress slow, involving centuries. Our present technical evolution has been a matter of hardly a few decades. Progress was speeded through scientific knowledge, elucidating and clarifying the mechanism by which newer products were born, suitable to the requirements of the new civilization. We are concerned in this symposium with recent progress in the metallurgical art. As we recount the applications and uses of the newer metals, we survey the whole modern progress of chemical technology. The chemical engineer owes the broadening of his horizon to the metallurgist. Before the advent of modern metallurgy he was obliged to carry on his reactions in equipment constructed of materials which had marked limitations. A few years ago materials for handling chemicals were mostly of nonmetallic origin. The available structural metals were not capable of resisting chemical action, nor did they possess any degree of strength a t extreme temperatures, high or low. Metals that did display resistance to chemical action, such as lead, had poor physical properties and lack of strength, hardness, and resistance to abrasion or heat. Today the picture has changed, and the number of metals and alloys, ferrous and nonferrous, that are available to the chemical engineer, fulfilling his specific requirements, is astonishing and bespeaks well for metallurgical progress. At the turn of the present century, through application of the microscope to the investigation of metals, the metallurgist was afforded an entirely new vision. He began to realize that the physico-chemical laws applicable to reactions in general were applicable to processes by which metals were produced. The microscope indicated to him that his problems were somewhat analogous to those of the mineralogist. As nature had produced the minerals from molten magma through certain stages of cooling and crystallization, SO had man produced his crystalline-structured metals, although under controlled conditions. The metallurgist immediately recognized the possibility of producing different properties in one and the same metal or alloy by differences in physical structure brought about by varying the rates of solidification and subsequent cooling. He also found that solid metal, when heated considerably below its melting point and cooled under controlled conditions, produced definite structural results. By such structural differences wide variations in strength, hardness, and other engineering properties can be brought about. Thus was born the science of heat treatment, enlarging the possibilities of achieving physical properties in practically unlimited combinations in metals.
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NEW METALS Within the last quarter of a century the science of metal.. has undoubtedly advanced with great strides. The application of the laws of thermodynamics has given us ti picture of the genesis of the various constituents of metals. The physicist has provided new means of testing, such as x-ray, magnetic, and spectrographic analyses. We must, however, remember that there is a distinction between the science of metals and the art of metallurgy. Compared to the scientific advancement of our knowledge of metals, we have probably not progressed a t an equal rate in the art. With the exception of the advent of the electric furnace, and particularly that of the high-frequency f u r n a c e , the means and methods available to the production metallurgist have remained practically the same. The most m a r k e d advance has been made in the manufacturing p r o c e s s e s of metallurgical products, in the rolling, shaping, and fabricating of metals, and in applying a u t o m a t i c mechanical and electrical devices, curtailing cost and inc r e a s i n g production. The processes of direct-rolling of sheets and even the rolling of m e t a l l i c sheet products directly from the fluid metal, such as by the Hazelet process, come to mind. Also advancement has been made in t h e forming, shaping, a n d joining of metals. The great strides made in the art of welding may be cited as example.
constantly changing that it becomes impossible to simulate such variable conditions in a laboratory test. This is most apparent in the case of corrosion tests. If a metal is subjected to the action of a chemical, the rate of its corrosion is influenced by the prevailing temperature, pressure concentration, flow velocity, impurities, protective colloids, and many other such factors. They would be impossible of exact reproduction in laboratory testing. It is well known that extremely minute traces of foreign substances picked up in a chemical operation often change the corrosion rate materially. Under testing conditions such traces of foreign material may not be picked up. It is also well known that chemical action in the presence of mechanical stresses, such as vibrations, alternate expansions and contractions, materially speed up the attack. These factors, which are the result of equipment design, cannot be reproduced in a laboratory test. It is apparent, therefore, that information gained in the laboratory can, as a rule, be only 8 rough indication of the types of materials to be used for a particular operation. In fact the laboratory is more
Limitations of Metal Testing If the existing limitations to the t e c h n i c of t e s t i n g
metals for chemical service by laboratory methods were not present, the exploitation of metals in the chemical industry would h a v e b e e n much more rapid. In spite of our scientific knowledge of the chemistry, physics, and mechanics of metals, we are unable to predict with any degree of certainty the suitability of a metal for a particular chemical operation. The results obtained under laboratory conditions cannot and do not simulate service. The factors which affect deterioration in chemical operations are 30 manifold and so
Cnurtesv, Aluminum Company nf Bmerica
RECEIVERAND GOOSENECK OF 35 ALUMINUM ALLOYUSED IN 1367
THE
STEARIC ACIDINDUSTRY
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helpful in eliminating than in selecting certain materials as appropriate. The final answer to the adaptability of a metal for any chemical operation is obtained by actual trial. The most'logical procedure is to construct equipment on a pilotplant scale where practically the operating conditions are present and run such equipment for some reasonable length of time. The duration of the test is important, since it is possible that some material which may show a high initial rate of deterioration may reach a sort of equilibrium, and its corrosion rate may then rapidly decrease so that in the
In the development of new metals and alloys, careful consideration must be given the abundance of the elements in the mineral world before undertaking their exploitation. Advance in the science of metals has made the technological strides in the art of metals possible, yet for specific purposes it is still necessary to find the suitability of any new metal by empirical trial. In the development of alloys, especially ferrous, when several alloying elements are used simultaneously, novel effects rather than the superimposed effect of the individual elements are obtained. In the case of pure metals improvement seems to have been achieved by two diametrically opposite paths, the one by obtaining the metals in a superpure state, the other by adding t o pure metals very small traces of impurities. In spite of the advance of the art of alloy-
h a 1 analysis such material may be more economical in use than another which may have shown higher resistance a t the start but may have a much shorter ultimate life. Chemical equipment, like all other structures, depends for its design upon physical properties of the metals from which it is constructed. The designing engineer has laid great worth on certain factors generally used for expressing the engineering characteristics of a metal. These factors are usually represented by the tensile strength, yield point, elongation, and reduction of area as obtained on a test piece. As long as metals were used for static structures, evaluation based on these factors was satisfactory. With the increased use of metals in motive parts, the factor of dynamic strength came into the picture, and values for fatigue and impact were taken into consideration. Then stress began to be laid on grain size, tendency to grain growth, air-hardening propensity, phenomena of aging, freedom from nonmetallic inclusions, and similar factors. Next the importance of resistance to corrosion and creep properties a t high temperatures began to be realized. It is obvious that corrosion-resistant property in a metal without regard to its physical-strength properties is important in engineering construction, since it gives a longer useful life of any equipment constructed of it; however, it becomes all the more important when combined with high strength as in the case of the new high-tensile steels and other high-tensile metals. In designing equipment it is possible to take advantage of economy of high yield-strength material only if
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it is permissible to curtail weight of the structure by the use of much thinner sections. In the case of very high-strength material, if full advantage of the strength is taken, such sections become so thin that, if extent of corrosion penetration remains the same, a larger percentage of the total volume of metal would be corroded and failure would be accelerated. Such high-strength material must necessarily be simultaneously resistant to corrosion. Only then can full advantage of the strength be taken and sections be designed of extreme light weight, thus saving in initial cost of equip-
ing, a strong tendency exists towards composite metal articles where two or more different metals are united by some process of adherence, such as sintering, welding, electroplating, etc. Similarly there are considerable possibilities of composite articles of metals and nonmetals. The importance of corrosion is stressed not only as a matter of economic waste but also as a factor in equipment design, making possible light-weight construction of equipment combined with safety. Consideration is given t o the requirements of extremely high or low temperatures, important for the future of the chemical industry. From the standpoint of the chemical engineer suitable metals are just as necessary for handling waste effluents and for shipping containers as for process equipment.
ment. Stainless steel affords a striking example, and its wide use is due to the combination of highstrength properties with extreme corrosion resistance. Resistance to corrosion, apart from the increased useful life of the equipment, thus actually becomes a factor in engineering design, and its importance should be stressed from this standpoint. For chemical engineering design, corrosion has to be considered from the standpoint of action of materials to be processed as well as from that of the prevailing atmosphere. It may appear that the chemical engineer is concerned only with resistance to corrosion to the particular chemicals with which his equipment comes in contact. Resistance to atmospheric corrosion, however, is of considerable importance to him. Exposure of equipment to the action of chemicals is generally intermittent so that a t intervals it is exposed to the atmosphere only. Also temperature conditions are suddenly variable. This type of alternate action is the most severe, and therefore metals which are resistant to the particular chemical action and also to the prevailing atmospheric conditions are to be preferred. Moreover, for general plant construction, the modern high-tensile steels and alloys which resist atmospheric corrosion are admirably suited for general structural purposes. They help to withstand the effect of the corrosive atmosphere, which is usually severe in the vicinity of chemical plants, and a t the same time to lighten structural dead weight and perhaps thus lessen initial cost of the original investment. Bt this point it may be advisable to review atmospheric
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corrosion tests conducted by exposure of the metals in various natural atmospheres. For materials that are to be used for building and roofing construction, such tests are adequate. However, when a piece of equipment, subject to strains, vibration, abrasion, etc., is to be constructed of such metal, other factors have to be taken into consideration. For instance, in the building of hoppers, tanks, hopper cars, ore and coke chutes, conveyors, piping systems, etc., the question of corrosion fatigue becomes paramount. The resistance to atmospheric corrosion, especially of the newer high-tensile alloy steels, depends upon their ability to acquire an oxide film or adherent scale which persists under ordinary circumstances and excludes the elements of the atmosphere from further action. This initial film or scale formed on such steels is very different in its physical characteristics from the rust on ordinary steel. Such film is more uniform over the entire surface so that corrosion takes place evenly and not in localized spots or pits. When corrosion takes place by pitting, the steel is rendered useless even though the loss in weight of the metal may have been comparatively small. The protective qualities of alloy steel oxidation films are different according to the nature and content of the alloying elements in the steel. Therefore, in the selection of a high-tensile corrosion-resistant steel for such equipment, preference should be given to the one that produces initially or scale. the toughest, most homogeneous, and densest If the film is of appropriate characteristics, it will not be readily destroyed by physical disturbances, such as temperature changes, abrasion, etc., necessitating that it be frequently renewed. It can thus afford protection to the metal against mechanical influences as well as against corrosion from elements of the atmosphere, ensuring increased life of the structure. For this reason considerable care has to be exercised in evaluating the life of finished equipment, such as hopper cars, bins, or chutes, from the results of atmospheric exposure tests of static specimens. Experience in the transportation of coal in railroad hopper cars has more or less established the fact that the factors of abrasion, indentation, etc., play such a part as to change the relative corrosionresistant evaluation obtained from exposure tests. Generally, if a low-alloy steel shows a loss in weight through exposure to the atmosphere only of about a fourth or a sixth that of the loss suffered by ordinary carbon steel, it may be presumed that, although the life of the equipment built from such lowalloy steel will be considerably greater, it need not be in the same relative value as represented by the results of the static tests. Further, if several low-alloy steels show equal loss of weight in atmospheric exposure tests, it does not follow that equipment constructed from them will have equal life, since physical characteristics of their protective Elms may be vastly different.
Low-Alloy High-Tensile Steels In the field of steel metallurgy, the most recent important advancement is the commercial production and application of the so-called low-alloy high-tensile corrosion-resistant steels. These steels, for the same carbon centent, have practically twice the yield strength of ordinary carbon steel. When high yield strength is obtained by increasing the carbon content, a proportionate rise in the tensile strength takes place. In the case of these modern low-alloy high-tensile steels, yield strength is obtained by the balanced presence of various alloying elements which increase the yield strength without correspondingly increasing the tensile strength. These steels, therefore, have a much higher ratio of yield to tensile strength compared to the older steels. The effect of this high ratio is to produce steels of a relatively very high degree of ductility in combination with the high yield strength. Further, the alloys can be so selected as to obtain high fatigue and impact
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values. Since the amount of alloying elements in these steels is low, they do not show any objectionable air-hardening propensity, as do steels containing larger percentages of alloying elements. On this account, such steels can be subjected to hot-forming or welding operations without manifestation of objectionable hardening or brittleness. The most important desideratum in these steels is, however, the ability to resist corrosion from atmospheric elements and other mild corroding media usually encountered by engineering structures. Attention should be drawn to the fact, however, that these steels are not rustless or noncorrosive but have much greater resistance to atmospheric corrosion; some of them, especially those containing chromium, also show remarkable resistance to mild acid solutions, waste liquors, mine waters, hot and cold tap water, and the like. On account of the nature of the initial oxide film formed on such steels, they take paint better, and the paint coat also lasts longer. Equipment made of such steels will, therefore, have less upkeep cost in protective coatings and have a much longer life than can be expected of ordinary carbon-steel structure. It should also be remembered that all the high-tensile low-alloy steels do not display the same degree of corrosion resistance ( J l the same degree of resistance to abrasion and indentation, nor do they display the same film characteristics of toughness, imperviousness, etc., of the initial oxide layer formed on the steel. Care should therefore be exercised in the selection of the appropriate steel for a particular use. The alloying elements used in these new high-tensile steels are well known in steel metallurgy. In the older alloy steels most of these elements, together with carbon, had been used individually, or sometimes in combination, for the purpose of obtaining higher physical properties, mostly tensile strength. The alloys practically augmented and somewhat modified the effect of the carbon, but the requisite properties had to be brought out by heat treatment, with its accompanying disadvantages, such as quenching strains and added cost of heat-treating procedure. The alloys made the steels more amenable to heat treatment and brought out the full effect of the carbon content. Structures made of such steels, if they were welded, had to be subjected to heat treatment after completion of the unit. This property has precluded the use of the older alloy steels in the fabrication of large structures, such as freight cars. The new low-alloy hightensile steels possess their physical properties in the as-rolled condition. This condition is achieved by the proper combination, within definite limits, of the alloying elements with a low carbon content so that the properties are derived from the alloys themselves and not from the carbon content. The principle on which the newer low-alloy steels are founded is one of balance of effects and properties. A survey of the steel-alloying elements shows that each one has certain definite properties from physical as well as corrosion-resistant standpoint. It is, however, only a comparatively recent realization that, if two or more such alloying elements are introduced into low-carbon steel, the resultant properties have no definite relation to the properties that would be obtained if these alloying e l e m e n t s were used individually in a steel. Therefore it becomes apparent t h a t t o produce definite property results by means of two or more alloys, an extended program of experimentat i o n a n d trial is necessary. Knowledge has to
INDIISTRIAL AND ENtiINEERING CHEMISTRY
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be acquired of the properties of a certain alloying element in the presence of a certain definite combination of other elements. I n the presence of such other combinations tlie effect is entirely novel and unexpected, and could even be contrary to the effect produced by the element when present only by itself. It is well recognized now in alloy steel rnctallurgy that in the presence of a low-carbon content the d k c t of a combination of elements depends upon a delicate balance of properties. For instance, if an element is known t i l produce high-tensile strength but to give a low impact ~ a l u e and another low strength but high impact value, tlicir combination will riot necessarily give a combination of the desired properties of high strength plus high impact. T i r e characteristics of the steel may be quite different from expeetations. I n fact, a metal with novel characteristics niay have been produced. The final art of the production of such steels, therefore, suinmarizes itself in achieving a balance whereby the salutary physical properties due ta the elemcnts are brought out and a t the same time the deleterioiis properties are suppressed. The achievementof this balancc becomes all the more difficult when, simultaneously with the pligsiod properties, corrosion-resistant properties are to be considered. Imckilv. ~.~ however. some of the elements which lravc Irei+n used for bringing ahout strength and ductility hnppen also to possess useful corrosion-resistant properties. Tile elements that have beer1 used up to the present for the production of hight e n s i l e corrosion-resista,nt s t e e l s a r e mainly copper, chromium, nickel, silicon, and p h o s p h o r u s . Such steels on the rdarket today contain copper and can be conveniently d i v i d e d i n t o t w o main groups, a chromium-copper group and a nickel-copper group. Both these groups contain other alloying elements hut differ essentially in that they contain either chromium or nickel. Also, the use of phosphorus as an alloying element is a new departure in the development of these steels, since i t is well known that phosphorus had, until very recently, heen eonsidered a deleterious element in steels, especially in alloy steels of hightensile properties. Of these two groups, the c h r o m i u m copper type has been in e n g i n e e r i n g service for a considerably longer time than the nickel-copper group. Chromium-copper steels were developed in this country about fifteen years ago but have enjoyed much larger commnercial exploitation in foreign countrics. I n Great Britain chromium-copper steel under the trade name Chromador, in France under the trade name Durapso, in India under the t,radename Tischrorn, and in Germany under several commercial names has been in actual extensive service for nearly a decade with excellent r e s u l t s . I n the United States the attention of the engineering profession was directed to this type of steel by the advent of a chromium-copper-silicon-phosphorus s t e e 1 This steel has found wide application iri this country within tlie last year or two. It has a test history under service conditions for approximately five or six years and has been i n c o m m e r c i a l u s e i n ~
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railroad operations, such ti? in hopper freight cars, passenger conches, 1)iesel power cars, etc., for about three years. Tlrc chromium-copper group of steels, therefore, seems to iiave the advant.age of having reliable back history with favora1.1lc refiults in actual commercial operation. Chromium is well recognized in the metallurgical world as prohahly tlie element par excellence for corrosion resistance. Silicon fonns a salutary combination with chroniiuin, helping its ~ioncorroiiveproperties; wheii phosphorus is added to tlie coml)inatiou, together with copper, considerably more corrosion resistance is achieved. Act.ually chromium-coppersilicoii-pliosphorus steels are more durable under salt water and brine spray coiiditions than any of the lon-alloy tonnage atecls.
High-Chromium Steels Ainoiig ferrous metals of prime importance to tlie chemical industry are the so-called stainless steels. Although they have been in use for a considerable period, methods for their proper fabrication and welding, knowledge of their exact compo~ition,and effect of various added elements are recent. For these reasons the total toiinage of these steels produced in the I:iiited St,ates, which was approsimatelg about 27,000
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tons in 1932, rose to over 71,000 tons in 1935. Although the major composition of stainless steel has remained the same and it has definitely been established that different chromium percentages from 12 to 25 produce certain definite qualifications for particular uses in the chemical industry, and also that the 18-8 chromium-nickel composition has found a general adoption for all purposes where stainless steel is used, nevertheless it has been recognized only recently that the presence of other elements favorably alters the properties of such steels. For instance, silicon and aluminum have become more or less essential when the steel is exposed to high temperatures. Tungsten also has been added for this purpose. Addition of selenium makes the steel more free-cutting. Copper, alone or with silicon, is used to modify some properties of high-chromium or high-chromium-nickel steels. Addition of 2 to 4 per cent molybdenum makes stainless steel of the 18-8 type practically indispensable in the sulfite pulp and very useful in the phosphoric acid industries. It may be said, then, that the full development of stainless steels suitable for resistance to chemical and temperature attack has taken place recently; therefore such steels find justification in being included in a symposium covering only the newer metals. Of the important advancements that have been made in stainless steel of the 18-8 type in recent years, the following deserve special notice. The addition of nitrogen has brought about remarkable grain refinement. The use of titanium or columbium has solved a baffling problem which confronted the use of stainless steels when they were subjected to high temperatures between 500" and 800" C. At such temperatures, even though the carbon content of stainless steel is low (approximately 0.08 per cent), a carbide precipitation occurs within the grain boundaries. The steel then becomes vulnerable to attack by penetration of the corroding medium in the interstices between the grains. The presence of titanium or columbium apparently stabilizes the carbides and prevents their segregation intergranularly. The 18-8 composition was developed abroad, but its wide application in industry was made possible by the pioneering work and metallurgical developments for the production of low-carbon ferro-chromium carried out in the United States. It is, therefore, remarkable that the difficulty of intergranular corrosion, which would curtail wide application of stainless steel, was also overcome by developments here-namely, by the production of suitable alloys of titanium and columbium which are added to the steel. Although the so-called stainless type steels containing high percentages of chromium have been popularly known, the usefulness, especially in chemical engineering, of steels containing from 2 to 10 per cent of chromium has not been so broadly recognized. The modern oil-cracking industry would not have been possible if such steels had not been available; i t is fitting, therefore, to pay tribute to these steels for the part they have played in the development of the petroleum industry. The products from petroleum have revolutionized our civilization. Without them we would not have seen the enormous strides which have been made in the transportation industry, whether i t be railroads, marine transportation, automobiles, or aeroplanes. Just as in the case of the highchromium stainless steels, these lower percentage chromium steels have also been modified by the addition of other elements for specialized service. In this connection molybdenum has played an important part. Some special alloys containing chromium, cobalt, nickel, and similar elements have been important to the chemical industry. Stellite and Hastelloy deserve mention. Advances in the metallurgy and application of these alloys have continuously been made. Under extremely severe conditions-for instance, where hydrochloric acid is present
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or where chemical corrosion, abrasion, and high temperature are combined-these alloys are the only means available to the chemical engineer to combat such conditions. In the absence of such alloys he would have to resort to nonmetallic substances with their well-know disadvantages.
Nonferrous Metals The alloying elements used in the production of special steels are not confined t o rolled products only. Recently progress has been made in developing alloy cast irons and also in methods of introducing alloying elements into cast iron. The use of alloy briquets with a binder has made possible the commercial introduction of alloys into cast iron. Irons alloyed with nickel, silicon, chromium, and copper have proved their merit in chemical industry, and the use of other elements such as molybdenum and vanadium for special-purpose cast irons is being developed. Since separate papers are to be presented on each of the important nonferrous metals their progress will not be recounted here in detail. It seems remarkable that resistance to corrosion in nonferrous metals is brought about by two diametrically opposite procedures. When metals are purified to a superpure state, they seem to withstand corrosion to a tremendously higher degree than if they were just in the commercially pure state. Zinc, aluminum, and lead of over 99.99 per cent purity are examples. On the other hand, when very small impurities of the magnitude of approximately 0.05 per cent are introduced into a commercially pure metal, corrosion resistance is considerably increased. The addition of 0.05 per cent tellurium to lead is an example of this effect. The use of small percentages of beryllium in copper has imparted to it remarkable properties. Aluminum, nickel, lead, zinc, silver, gold, platinum, and rhodium have all contributed to the modern progress of chemical industries. Tantalum, tungsten, molybdenum, as commercially pure metals, portend useful application in engineering. Besides the use of metals for equipment construction, they are of interest to the chemical engineer from the standpoint of their use as catalysts or anodes.
Composite Metals Although the science of metals has progressed remarkably and the art of alloying various elements into metals has reached a high degree of accomplishment, the ultimate goal seems to be distant. In cases where a certain set of physical properties is required in the body of the metal, together with certain surface properties which the body does not possess, recourse is still had to composite materials. The art of plating a metal with a nobler metal by galvanic current is old. Gold, silver, and nickel have surfaced articles of household use. Recently advances have been made in producing electrolytic coatings of zinc, lead, cadmium, and tin, and in some cases with bright finish. Also advances have been made in plating ferrous and nonferrous metals with chromium and with tungsten. Possibly tantalum will some day perform the same function. Such galvanic deposits for chemical engineering use have certain shortcomings, and there has been a persistent effort to produce so-called duplex materials whereby two metals are rolled together, the one for surface protection and the other for engineering strength. Copper- and nickelclad steels have been successfully made and used in the chemical industry. Autogenous lead coatings on steel have been common. Only recently, however, have attempts been made to produce ordinary carbon steel with a surface of stainless steel or with a surface of aluminum. Aluminum alloys coated with pure aluminum for better resistance to corrosion have been produced. Metals have also been coated by spraying with a molten noncorrosive metal, but such
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coatings suffer from porosity and pin holes as do metals deposited electrolytically. Considerable advancement has been made in coating steels or high-tensile nonferrous alloys with nonmetallic substances, such as glass, enamel, plastics, or rubber. I n such cases the metallurgist’s services are required to produce the parent metal with a surface capable of making a solid bond with the lining material. Development of the technic of producing anodic oxide protective iilms on metals, as in the case of aluminum, is recent. Mention should be made of the recent development of a new field known as powder metallurgy. Powders of various pure metals are produced and mixed in the proportions desired and pressed into a rod or briquet which is sintered, and the sintered product is then rolled or forged. This art may prove of considerable advantage to the chemical industry inasmuch as i t permits the mixing of metals, independent of their inherent alloying solubility in each other. By this technic i t is also possible to obtain a composition of the surface entirely different from the body of the sintered metal. Great advance has recently been made in coating common structural metals with corrosion-resistant alloys by applying a fused layer of the nobler metal by means of the welding torch. The advancement in the technic of welding in the construction of equipment and in attaching linings to containers is due to the advance in the metallurgy not only of the metals of construction but also that of the metal of the welding rod and the coatings used on the welding rods. The advance in welding technic has also made it permissible to construct welded equipment of rolled metal instead of using a casting, thus considerably diminishing the weight of the equipment and obviating the usual deficiencies of cast parts. This is particularly useful where i t is advantageous to construct equipment requiring different properties in different parts. It is thus possible to use two or three different materials, each appropriate for the function. This would be impossible if a casting were used. A new process of welding by high-frequency current has been announced. The metal is welded a t a low temperature without being subjected to temperatures high enough to change the structure of the metal in the parts adjacent to the weld. This process becomes of great importance to the chemical engineer, especially in the welding of extremely thin sections or of low-melting metals.
Metals Resistant to Extremes of Temperature In the chemical industry, while chemical attack plays the major part, the effect of temperature, alone or combined with pressure, is very pronounced. I n some branches of chemical technology, as in the glass industry, this condition is accentuated by action of the molten glass a t the high temperature. The immediate future of metallurgical progress in the chemical industry lies in the development of metals that will withstand high temperatures, as well as abrasion and chemical attack. Besides the direct use of such metals in actual chemical plant equipment, they will have a great future in allied industries, such as power generation. For fire boxes, boiler tubes, and tubes for metal recuperators and heat exchangers, such new metals will have an unlimited field. They will undoubtedly change the aspect of the chemical industry from the standpoint of production cost. The development of materials resistant to low temperatures is also important. For use in refrigeration, it is important that metals should be developed which will function at extreme low temperatures. Processes have been developed for removing wax from paraffin products by refrigeration, and similar other chemical operations of fractional crystallization can be carried out and certain separations achieved a t extremely low temperatures not permissible a t present. This will open up a new and vast low-temperature chemical technic.
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The attention of the metallurgist, enthused with the idea of developing metals with the use of new alloying elements, should be called to the fact that primary consideration should be accorded to the possibility of abundant supply of the particular metal or element in the mineral world. The demands of the engineering profession for any successful new material increase so rapidly that it is futile to undertake the development of an element and then find that the available raw material supplies are not sufficient to justify broad use or mass production a t a reasonable cost. With the restricted availability of a mineral, the price a t which the metal produced from it can be marketed will be high; and, since the prime purpose of the newer metals and alloys should be to effect a better economic picture in industry, a comparatively high price would preclude the use of such element. Fortunately the metallic elements which are important to the chemical engineer-namely, iron, nickel, chromium, copper aluminum, silicon, and molybdenum-are abundantly supplied in nature and capable of being marketed a t a fairly low cost.
Metals for Handling Wastes and for Shipping Containers To the chemical engineer the disposal of effluents and wastes from the manufacture of his product is also important. The waste products generally have corrosion characteristics different from any of the product occurring in the process and therefore may demand entirely different corrosionresistant materials. The disposal of waste acids, condensation of waste fumes, and the like present extreme and acute conditions and require materials more resistant to corrosion than required in the process itself. As a specific instance, cement-lined steel conduits are finding wide use in the mining, oil, and sulfur industries for handling saline waters, spent sulfuric acid wastes, water containing hydrogen sulfide, etc. Another problem is that of containers of the finished product such as tank cars for various acids and cylinders for condensed gases. Shipping containers of lighter weight become essential, and undoubtedly new lighbweight alloys of high strength will help the situation. The tendency seems to be towards containers which are composite in structure. A striking example is found in beer cans which are made of thin-gage tin-coated steel and lined with lacquer. Possibly with the perfection of corrosion-resistant high-strength metals, containers can be made which will resist the corrosive action of the contents, will have the ability not to contaminate the products shipped in them, and also be extremely light in weight.
Metallurgy and Chemical Engineering Advancement in the development of metals and alloys suitable for the chemical industry will be materially accelerated if the chemical industry will give the metallurgist an opportunity to study its problems in detail. As has been pointed out, very small variables and differences in operating practice are sufficient to cause failure or success of the metal used. It is therefore extremely important that the man who is called upon to produce a balanced alloy suitable for a specific purpose should be fully acquainted with the minutest details of the operation in question, The knowledge of superficial generalities imparted to the metallurgist today is not sufficient if a specific tailor-made metal for the job is to be produced by him. On the other hand, the producer of metals should also be alive to the needs of industries and equip himself with such experimental production facilities as will enable him, by extensive trials, to adjust the proper composition of his alloys and to determine their useful characteristics before placing them in the hands of his customer. Just as
DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
chemical industry has found that its processes for production of new products must be tried out not only in the laboratory but on a pilot-plant scale, so also must the producer of metals have facilities to produce new alloys and metals in commercially small quantities. He will then have material in such quant,ity and form as t o enable him to test it practically under service conditions. This will give him knowledge of the fabricating and welding properties of the metal on commercial sizes and sections and suggest the possibilities of the application of the new product to various other industries. Research in metallurgy from the scientific and theoretical standpoint has been and is carried out on a large scale. Methods and means of testing metals have been developed from various angles. Considerable work is being done to improve fabricating plant practice, but there still seems to be room for intensive work in applied metallurgy. New alloys can be created only by knowledge of the specific need of a metal of definite characteristics for a new engineering requirement. The application of existing alloys into newer fields
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usually demands change in design of existing structures. After a new alloy is introduced, its performance must be closely followed because various initial deficiencies of the alloy may have to be remedied even to the extent of changing the composition. I n spite of the wide use of metals in all fields of engineering, there are still applications where metals could be used in place of nonmetallic substances. The competition lies no longer within the metallurgical camp itself but is facing outside adversaries, such as glass, paper, plastics, etc. Not only engineering aspects but also those of daily life are changing, such as our system of housing, house furnishing, and preservation and transportation of food supplies. The metallurgical industry has to be cognizant of these changes and prepare itself to meet the demand of a new era. As in all activities, so in metallurgical industry cooperation between consumer and supplier and coordination by the manufacturer of all factors will undoubtedly lead to success, furthering the cause of metallurgy as well as of chemical engineering. RECEIVED September 9, 1936.
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Discussion CLYDE E. WILLIAMS Battelle Memorial Institute, Columbus,Ohio
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N CONNECTION with Saklatwalla's paper, it is of interest to note that in the development of some of the newer steela the two elements copper and phosphorus have been used, whereas they were previously considered as nuisance elements. Their presence in most steels was merely adventitious, the phosphorus coming largely from ore and the copper either from ores or steel scrap. Copper, in amounts up to about 0.25 per cent, has been used to increase the atmospheric corrosion resistance of steel; phosphorus in amounts up to 0.07 or 0.08 per cent has been used in sheet to be pack-rolled to thin gages. Aluminum is another element that has been in common use by the steel makers whose value in making better steel is becoming increasingly recognized. Saklatwalla's reference to the unsuitability of laboratory appraisal of corrosion resistance of metals shows the need for more fundamental work on the causes of corrosion and on the development of laboratory and accelerated test methods that will give results more closely simulating those obtained when the metals are used commercially. Similarly, in the development of steels for service a t elevated temperatures, more knowledge is needed as to the effect of variables such as melting practice, composition, heat treatment, structure, grain size, and stability, on resistance to deformation under sustained loads a t elevated temperatures. Short-time testa are not generally acceptable, although they are useful in making a preliminary survey of a field of alloys. Now creep tests are ordinarily run for 1000 or 2000 hours, and even longer times have been reported and advocated. As in the case of ordinary corrosion the character of the scale developed at elevated temperatures, whether by products being treated or by the combustion gases, further determines the quality of the metal. In addition, resistance to creep and to embrittlement are essential. The high chromium content in ferrous alloys is not only beneficial in resisting scaling, but it also greatly aids resistance to creep. Thus chromium, usually with nickel, is used in alloys for extreme service conditions. The well-known 18-8 chromiumnickel steel modified to resist intergranular corrosion by titanium or columbium has come into common use and has good creep properties up to 1250" F. (678' C.). For higher temperatures (up to 1800" F. or 983" C,), 25 per cent chromium and 12 per
cent nickel compositions are used and, where corrosion by sulfur dioxide must be combated, 29-9 chromium-nickel is preferred. For still higher temperatures and when resistance to sulfur dioxide is not required, 35-16 nickel-chromium iron alloys are used. Scaling by oxidation is satisfactorily combated in alloys containing chromium contents above 24 per cent. For less severe conditions the cheaper 4 to 6 per cent chromium steel, containing about 0.5 per cent molybdenum or tungsten is used. This steel retains high strength and good creep resistance up to 1100" F. (594" (2,). A still cheaper product, but one with considerably better properties than plain carbon steel, contains 1.25 per cent chromium, 0.75 per cent silicon, and 0.5 per cent molybdenum. This type of steel is generally used where corrosion conditions are less severe than those requiring the 4 to 6 per cent chromium alloy. In these low-alloy steels, molybdenum is the effective agent in enhancing creep resistance, but ita effect is greatly increased by the presence of chromium. Recent research work which has not yet been checked in commercial installations indicates that phosphorus is likely to become a useful alloying element in steels for high temperature service. These tests show that phosphorus in amounts up to 0.20-0.30 per cent increase both short-time tensile properties and resistance to creep at elevated temperatures. Thus, the effect of phosphorus is similar to that imparted by molybdenum and tungsten. Also, the effect of phosphorus is increased by the presence of chromium and other elements which also serve to maintain a satisfactory toughness. Preliminary creep tests indicate that in low-carbon (0.15 per cent) 1 per cent chromium steels, phosphorus may be substituted for a portion of the molybdenum (0.5 per cent) sometimes used, with little change in properties. In recent years much progress has been made in the improvement of the serviceability of cast iron a t elevated temperatures. Cast iron is useful as a structural material because, owing to ita easy castability and machinability, it can be made into varioua and intricate shapes. Its disadvantage of being subject to growth and consequent warping and cracking when held a t elevated temperatures can now be greatly reduced. This improvement is achieved largely by producing a more uniform structure in which the graphite is finely divided and which has higher strength. It can be effected either by holding the silicon content to B point where extreme graphitization does not take place, by melting
