SOME
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OF STEEL CHEMISTRY JOHN JOHNSTON United States Steel Corporation, Kearny, N. J.
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usefulness uf a building depends upon its architecture, the arrangement of its structural units. -4change in structure brings with it some change in properties; a change in properties implies some change in real structure, even if we are unable to detect this change directly. Moreover, just as the mechanical stability of a building may depend largely upon the kind of cement used to tie the building units together, even though the total weight of the cement is only a emall percentage of that of the whole building, so may the properties of a metal depend upon the kind, amount, and distribution of the material a t its grain boundaries, as well as upon the kind, size, and shape of the grains themselves. A metal or alloy, as we use it, is an aggregate of crystal grains, which may be of one kind or of more than one kind, which may be large or small or may differ widely in size; and in manipulating metal, such as steel, in order to obtain the combination of qualities desired for a particular purpose, we are in effect altering the kind, size, and arrangement of our structural units, the crystal grains. Therefore, not the “chemistry” in the restricted sense will be discussed here, but what many would call the “physical chemistry” (a term to be avoided, since i t implies a difference which does not in fact exist) and the writer would call the “chemical philosophy” underlying the properties and behavior of steel. This concept enables us greatly to simplify the whole picture and makes it unnecessary to keep continuously in mind a great deal of scattered and apparently unrelated information.
H E geiieral whject of the chemistry of steel is q o large that a whole course of lecture>, and more than a single lecturer, x-ould be required to discuqs it thoroughly. Such a. discussion mould bring up a number of point? which have been settled only within recent years as a direct outcome of the more systeniatic scientific research directed to the solution of steel problems; it would raise a much larger number of questions, which cannot be answered except by further investigations along many different lines; and it would differ in many respects from what is in the textbooks read by chemists. The subject is so large that one is forced to deal with certain aspects of it only, and to omit inany significant points. Those chosen have been deemed most likely to be novel and interesting to chemists; i t is hoped, however, that they will serve to $ketch some of the high lights of the picture and to outline what may be called a basic general philosophy of the behavior of the various kinds and types of steel. The steel man uses this word “chemiztry” in a special and limited sense-namely, to denote the composition of a steel, a$ determined by the usual analytical methods, with respect to carbon, manganese, qilicon, and any other alloying elements which may have been added by intention. He has paid, in R sense, too niuch attention to thir so-called chemistry, almost as if he believed that this alone determines the properties of the steel. Perhaps Tve should say rather that he has, until recently, paid too little attention to other factors which influence the useful properties of the steel-for iriqtance, to \mall percentages of the noiimetallic elements oxygen, nitrogen, hydrogen which are difficult to determine by analysis, yet do affect the steel in some ways, or to the precise conditions of roiling, finishing, and cooling, which may exert an appreciable influence on properties now of practical significance. In short, recent work has demonstrated again and again that we must take into account not only this limited “chemistry,” but everything which affects the real intimate structure of the metal: for its usefulnew depends upon its real s t r u c t u r e what may he termed it; molecular architecture-juqt a; the
Steel-Making Processes Let u j first review in outline the general methods now commonly in use for making steel. The first step is the reduction, in the blast furnace, of the ore by coke with lime as a flux, to produce pig iron, which contains 3 to 4 per cent carbon, usually about 1 per cent silicon, and such other reducible metals as were present in the ore. As now carried out, this step is much more efficient than is commonly believed; further improvement ic possible, but this is more in the direc141;
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tion of greater uniformity of product day after day than in improved efficiency in utilization of coke or ore. The next step is to convert the pig iron to steel and to refine the steel, either in ( a ) the Bessemer converter, (b) the open-hearth furnace, or ( e ) the electric furnace. In spite of the apparent differences between these three processes, they are all essentially the same; each is a process of controlled oxidation, in which we try to oxidize out of the liquid
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usefulness of its products, though they are still called by the same name and in many cases sell now for less than ever before. The subsequent series of operations can, for the present purpose, be recounted briefly. The liquid steel is transferred from the furnace to a ladle, in which its composition may be finally adjusted by the addition of alloying elements and of deoxidizing agents; from the ladle it is teemed or poured into ingot molds in which it crystallizes (the highest melting solid phases separating earliest) and finally solidifies. The solid, but still hot, ingot is removed from the mold, transferred to a soaking pit to bring it throughout to the proper temperature, and then rolled down to the shape desired. This may be a finished product, such as a rail or beam or plate; or an intermediate product for later conversion to tube or wire or sheet, or any of the multifarious articles made from steel. In all of these operations careful attention must be paid to the temperature, to a nice adjustment of the rolls, to proper heating, annealing, and cooling cycles, and to many other details.
L /QUID Temperature
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[FIGURE 1. THERMAL EXPANSION OF ELEMENT IRON
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metal those elements which we wish to eliminate from the steel and to retain in the right proportion those whose presence is considered desirable. Which of the three methods is used is largely a matter of relative economy and convenience, and of the precise kind of product specified. The greatest tonnage is now being made in the open-hearth furnace, a much smaller tonnage in the Bessemer converter; the more expensive electric furnace is limited t o the making of special steels, particularly of so-called alloy steels, and the process is in many cases more a melting of the constituents than a refining of the product. In any case, the properties of a steel depend much more upon the skill of the man who made it than upon the type of process used; and for a specified real composition (but not necessarily for a specified "chemistry"), steels made by the several processes are indistinguishable. It would lead too far to discuss the methods by which the composition of the liquid steel is controlled and adjusted. It must suffice to state that they involve direct control of (a) the temperature of the liquid steel, which in many cases must be brought up to within about 30" C. of the temperature a t which the silica brick in the furnace roof melt down rapidly, (b) the composition of the slag, the slag being the reagent acting upon the liquid metal to alter its real composition, ( c ) the final rate of change of carbon and manganese content. Adequate control of these factors raises in turn many other problems such as the provision of methods of analysis which are accurate yet rapid enough to be useful in practice, or of refractory substances with the necessary combination of qualities, or even of a proper method of measuring the temperature, and so on. Much of this control has been a matter of the personal skill of the operator, and i t is wonderful what he can achieve on batches of 100 tons or more; but he is finding difficulty in meeting day after day the more and more rigorous specifications now imposed upon him (for instance, for a carbon range of 0.05 per cent or even smaller, and indirectly for an oxygen range still smaller) and is perforce adopting methods of closer control developed by systematic investigations of the various problems. The steel industry has been and is spending a large amount of time and money in studying its processes intensively; and within a decade there has been great improvement in the quality and
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DIAGRAM OF IRON-CHROFIGURE 2. EQUILIBRIUM MIUM SYSTEM
A steel article is thus the end result of a long series of operations, from the selection and transport of the raw materialsore, coal, limestoneand the making of the pig iron to the manufacture of the steel and its subsequent fashioning and finishing; and the steel may not fit its purpose if there has been inadequate care in any one of these many steps. Withal steel is sold for about 2 cents a pound, a considerable fraction of which is for transportation.
Nature of Steel What is steel? And why, apart from its cheapness, should it be so preeminently useful, serving so much wider a variety of purposes than any other single metal or alloy? Steel is an alloy of the elements iron and carbon, the latter in proportion ranging from a few hundredths up to about one per cent; it usually contains manganese, to the extent of
FIGURE 3 (Right). RELATION BETWEEN TIME REQUIRED FOR ONSETAND COMPLETION OF TRANSFORMATION AT CONSTANT TEMPERATURE AND THE TEMPERATURE FOR A EUTECTOID STEEL Micrographs in the shaded band represent the structure when the transformation has gone halfway: those t o the right, t h e final structure a t t h e corresponding temperature.
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martensite. This martensite, like ordinary glass, will, though unstable, persist for centuries a t ordinary temperature: but when heated it changes, with partial or 90( complete separation of its constituents. This is the process known as tempering, and the precise structure of the product depends again on the tempering temperature 85 and on the time interval at that temperature. Figure 3 also illustrates how the actual transformation temperature affects the mode of precipitation of the 80( carbon, and hence the structure and useful properties of the product. A few separate micrographs at full magnification make this clearer. At a constant temperature just under the equilibrium temperature, the carbon pre75 cipitates in lagers or lamellas spaced fairly regularly, yielding the type of structure known as coarse pearlite (Figure 4 9 ) ; at successively lower constant tempera70( tures, the distance betn-een lamellas becomes smaller and smaller until the pearlite is so fine that it is finally not resolvable by the microscope (Figures 4B and C). There 65( is thus a regular series of pearlites, from coarse to extremely fine, with a corresponding regular gradation in properties of the steel, which becomes continuously 60( harder and stronger as the temperature of its transformation is lower. I n other words, the architecture and hence the properties of a steel depend upon the temperature range within which the crystal transformation 551 and consequent carbon precipitation actually occurred. This temperature may be fixed by quenching the steel, T I M E O F C O O L / Y G (M/NUTES-SECONDS) while still austenitic, into a bath of molten lead or salt 50 loo 15 0 maintained a t the desired temperature, and holding it FIGURE5 . EFFECTO F TI50 R4TES O F PRIOR COOLING UPOX there until the transformation is complete, as was T R 4 N S F O R M A T I O N TEMPERITI-RE I Y D FISkL STRUCTCRE O F A STEEL done in preparing Figures 3 and 4. When this is done within the range 250' to 500" C., the product has remarkable properties; a piece brought by this method to a can obtain the wide modification of properties only because, given hardness has much greater ductility and resistance as the steel is cooled, the transformation from gamma to to fatigue than a companion piece brought to the same hardalpha sets in, not instantaneously as the real equilibrium ness by the ordinary quench and temper method, corretemperature is passed, but only after a definite interval; sponding to a marked difference in structure between the two and it requires another definite interval to go to completion. pieces. For a given steel these definite intervals depend upon the The temperature a t which transformation begins may also temperature a t which the transformation actually goes, as be altered by changing the rate of cooling prior to its onset is illustrated by the shaded band in Figure 3. (Figure 5 ) . When this rate of cooling is slow, the actual Influence of Transformation Temperature on transformation temperature is higher and the product Final Structure softer; when it is rapid, the temperature is lower and the Figure 3 shows the typical relation, for a single steel, besteel harder. This behavior, which is merely another example tween actual transformation temperature and the time reof the familiar phenomenon of undercooling, is illustrated in Figure 6, which reproduces schematically part of the timequired a t that temperature for the onset and completion of the reaction, austenite +.ferrite carbide. The upper portion temperature S-shaped curve, previously referred to, along of this curve is entirely analogous to the similar curve showing with some curves representing different rates of prior cooling the time required for a glass to devitrify (that is, to crystallize) of the austenitic steel. a t various constant temperatures. Its form is due to the fact Effect of Austenite Grain Size that two main factors are involved in the reaction: (1) the If we examine the rate of transformation of two steels departure from equilibrium or the "driving force," which which are identical in composition, as ordinarily determined, increases steadily as the temperature is lower; (2) a factor but differ in austenite grain size, we find that a t all temperaanalogous t o viscosity-a resistance to diffusion and precipitatures the coarse grained steel takes more time to transform tion of the carbon-which, as the temperature is lowered, than the fine (Figure 7). This difference in speed is readily likewise increases but a t a rate greater than the driving force. explained by the observation that the transformation starts Thus the viscosity factor, so to speak, overtakes the driving a t the grain boundary; obviously, therefore, it would take force and begins to delay, and finally in effect to stop, the longer to sweep across a large grain than a small grain of the reaction. same material. The consequence of this difference is that such The form of the lower part of the curve is due to the fact a pair of steels, though cooled identically, will not have that, if vie cool the metal so fast that it does not stay long enough in the region of fastest transformation to permit identical structure; in fact they harden differently. This is an important matter in practice, particularly when the final the tramformation to start (the process knon-n as quenching), heat treatment of parts is more or less an automatic process n e obtain a different reaction and consequently a different and has brought about the recent developments in the control product. V i t h fast cooling the carbon, instead of precipitatof the grain size of steels. ing, reiliains in superiaturated solution just as happens in The fact that the transformation qtarts at points in the many TT d l - k n o ~ ~cases, n this is the typical constituent of grain boundary is another example of a phenomenon well Iiririlrirril *tecl a i protliicetl by quenching and i 4 known aq I
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nhat finer laiiiellar -tiuc.turc n hich is hartlcr and stronger than the plain $tee1 n hen similarly handled. If, however, me take siiiiilar m a l l piecec: of a plain and of an alloy ?teel, both of the same carbon content, and force them to tranqforni a t the w n e temperature (as we can readily do by immersing them in a lead bath held a t that temperature), n e find that the final structure cf both pieces is substantialh identical, and so therefore arc their properties. This slowing of the transformation proresis indeed the predominant mode in m-hich an alloying element, properly so-called (that is, a11 element present in solution in the iron rather than one which is chiefly a chemical reagent in the steel-making process), influences the, mechanical properties of steel. By this meanwe can obtain structure.;-hence, propertieqwhich could hardly be expected with plain steels. Nevertheleqs in parts of small cross section it is possible by appropriate heat treatment t o develop in a carbon steel mechanical properties which are not excelled by those of a n alloy steel; yet the latter may be the ea-ier. and more economical, to use. This merely emphasizes the point that alloy steels are primarily steels, and that their final mechanical propeities and usefulness depend upon how n e treat them, especially with regard to the temperature a t which the crystal transformation actiiallj takes place. In general, therefore, any iteel (alloy or carbon) must be heat-treated if n cx are to develop to the fullest extent the most (le&LO fU fm r, 000 quuu able combination of properties inherent in nic.tal fime m $!!on& /Iogmfim/c $cde] of that particular real composition. RATEUPON TRASSFORMATION FIQURE6. EFFECTOF PRIORCOOLING This lead? to the .tatement that most ordiTEMPERATURE nary steels are in effect heat-treated, even when we do not d o it consciouslv. For the steel is rolled and fashioned in the gamma stkte but, as known in solid catalytic agents-namely, that catalytic acit cools, transforms and is used in the alpha state. It is tivity is greatest a t a n interface or boundary, the so-called therefore necessary to pay attention to the mode of this promoters increasing the number of effective interfaces. The cooling and not to leave it largely uncontrolled, as has been grain boundaries comprise material rejected by the growing usual, with the result that the final product was not always crystals as the liquid metal freezes; the effectiveness of this grain boundary material in inducing the transformation depends upon its nature and upon its degree of dispersion. A solid particle precipitated Tithin a n austenite grain can also serve as a nucleus; this happens in some alloy steels-for instance, steels containing in the matrix vanadium whose carbide is very sparingly soluble.
Effect of Alloying Elements upon Rate of Transformation The time required for the onset of transformation at a given temperature, and the rate at which it then progresses, both depend upon the composition of the steel. The presence of any one of the usual alloying elements (including carbon) in solution in the austenite delays its transformation by an amount which depends upon the element and upon the proportion of it present in the matrix. This effect is illustrated by a few experimental curves for several additions of nickel (Figure 8) or of manganese (Figure 9); comparison of these curves shows that manganese in the higher concentrations is about fifty times as effective as nickel in hindering the transformation. In other words, alloy additions displace the S-shaped curve towards the right-that is, towards longer time intervals. This implies that, for a given mode of cooling, the alloy steel transforms a t a lower temperature, therefore t o a some-
FIGURE7. EFFECTOF GRAINSIZEUPON CURVEFOE HALF-WAYTRANSFORMATION OF A EUTECTOID STEEL,AT CONSTANT TEMPERATURE
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For we may have two steels of identical “chemistry,” which yet differ in some of their significant properties, notably in response t o heat treatment and in aging characteristics. These differences depend upon the grain size, which in turn seems to depend upon the presence of very small proportions of certain nonmetallic elements (notably, oxygen), and upon whether these elements are present in solid solution in the matrix or as solid particles a t grain boundaries or within the grains. Incidentally, a really pure steel would not be desirable, because it would be too coarse-grained and remain so in spite of heat treatments; what is desired is the right , i 2 5 1 0 2 0 I 5 10 20 1 5 IO kind of particle, properly dispersed, and this usually imSECONDS MINUTES HOURS plies that in the finished liquid steel there must be some small proportion (measured in hundredths per cent) of oxygen, and that this effective oxygen content must be kept within quite narrow limits if the product is to be as uniform from heat to heat as is now demanded. The assumption that the “chemistry” of a steel is all important had led many consumers to analyze a steel which exactly suited their purpose, and then to demand steel of precisely this composition with respect t o the 5 1 0 I 2 S I 0 4 i I O 20 I elements commonly determined by analysis. But this ! > 3 ”10 70 I MjMurcs HOURS o i l s wcrx‘ SCCONDS has not solved the consumer’s problem and has had the FIGURE8 (.T O ., D). EFFECTOF NICKELUPON RATE O F TR.4NSFORdisadvantage of leading to a multiplicity of nominal steel MATION AT 315’ C. AND AT 480’ C , compositions, a number which is certainly five times, FIGURE 9 (Bottom). EFFECT OF MANGANESEUPON RATE OF possibly ten times, as great as it need be to cover TRANSFORMATIOS AT 315’ c. .4ND A T 523’ c. every purpose satisfactorily. It would be to the aduniform because of differences in the rate of cooling prior to vantage of everyone to lessen the total number of nominal the onset of the transformation. The importance of better compositions; many of them are now so similar that they control of final cooling is being realized more widely every can be distinguished definitely only by well-made chemiday, and is responsible for developments such as the sciencal analyses. Another point which illustrates the changing tifically regulated heat treatment of rails and railroad car attitude with respect to this “chemistry” is that phosphorus wheels. The same is true with respect to welding, for some and sulfur have commonly been regarded as nuisance elements, portion of the metal a t and near the weld must have been in and in most specifications are limited to 0.05 per cent or less, the gamma state, from which it transformed as the weld although there is little real evidence that, for many purposes cooled; some control of this cooling is thus necessary if a t least, such low limits are required. There is no doubt uniform results are to be secwred. In short, to obtain the that sulfur or phosphorus may be a nuisance if it is not best performance from any steel, plain or alloy, we must properly taken into account; yet within recent years comqee to it that the final heat treatment (cooling is, of course, paratively large quantities of sulfur or of phosphorus are a mode of heat treatment) is chosen and carried out so as being deliberately added to certain grades of steel because to develop the structure which will best fulfill the particuthey confer some useful property on the steel. It is clear lar purpose. In practice the matter is complex because of therefore that the rather naive ideas hitherto prevalent on the number of possible variables (size, shape, composition, this matter will be modified, as we learn more as to the etc.) ; but the time-temperature curve, whose fundamental precise effect of each element, alone and with others, on the qignificance we have been discussing, enables us t o disproperties of a steel properly made and treated. The real ‘ entangle these apparent complexities and furnishes a rational effects of the several elements are not, in general, additive; picture of the whole matter o f the heat treatment of steel. and so there are still large numbers of permutations of compositions and of heat treatments awaiting the careful inConclusion vestigator, From the considerations just outlined it is clear that the RECEIVED September 12, 1936. Presented before the General Meeting a t ”chemistry” of a steel, in the steelman’s use of the word, is the 92nd Meeting of the American Chemical Society, Pittsburgh, P a . , Sepfar from telling the whole story of the behavior of that steel. tember 7 to 11, 1936.
