The Strength and Structure of Alloys.
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It was a happy thought which impelled Col. Crompton, the President of the Institate of Automobile Engineers, to invite Mr. Walter Rosenhain, of the National Physical Laboratory, to read a paper on the strength and structure of alloys. The subject is one to which Mr. Rosenhain has devoted special attention tor a good many years past. In association with Prof. Ewing, he made valuable contributions to our knowledge of the internal structure of metals, and has, during recent years, carried out original researches of considerable importance. Probably few of those present at last week's meeting realised the extent to which the work discussed was original research work, Mr. Rosenhain's method of preparing sections for examination by the microscope is an achievement of no mean order, and is likely to prove of very practical use in arriving at the causes of the mysterious failures of metal that are met in motor engineering.
Scientific Testing of Materials.
Mr. Rosenhain said that probably no branch of industry necessitated more careful and scientific testing of materials than motor engineering, and the close connection between metallurgy and the motor industry was emphasised by the fact that the distinguished French metallurgist, M. Guillet, had been for a considerable time associated with the De Dion-Bouton Company. The point of view from which the engineer was chiefly interested in metals was in regard to their mechanical strength and ductility. The facts governing the strength of metals might be divided into two classes. The first was grouped under the phrase, molecular cohesion. It was not necessary to consider that particular aspect, as little was known concerning it, and in any case the amount of knowledge was too vague and speculative for practical purposes. On the other hand, the modern method of microscopic examination of metals had thrown a great flood of light upon their internal structure, and their behaviour when deformed or broken. For this purpose of microscopic examination the pieces of metal were reduced to a smooth condition, and, finally, polished until no trace of ssratches or other marks remained upon the surface. The specimen to be examined was then attacked by some chemical reagent, which acted differently upon the different constituents, and produced a surface pattern representing the internal structure of the metal. A special microscope was used for examining the section, and the internal structure of a pure metal thus re. vealed showed a number of roughly polygonal areas, each of these areas being the section of a true crystal, although, in con. sequence of their close intermixture, their shapes were not regular or geometrical. The strength of an aggregation of crystals, and, therefore, the strength of a metal, depended upon two factors: the first, the force with which the crystals adhered to each other; and, secondly, the resistance of the individual crystal to deformation or breakage. The crystalline structure, when it commenced in the liquid metal, started at a considerable number of points, and the crystalline structure grew by the production of arms growing out like the branches of a fir tree and meeting branches coming from other centres. Upon the degree and nature of the interlocking of these arms the force of the crystals to resist tearing apart from each other must largely depend. Photographs of steel, iron and alloys, when examined, revealed this structure.
Coming. to a consideration of the behaviour of individual crystals, It was found that they were not capable of undergoing distortion when exposed to force in the way that putty, for example, would undergo distortion, but behaved in a characteristic manner, the crystals adapting themselves to the new shapes imposed upon them under the stress to which they were exposed by a process of sliding which went on among the layers of minute crystalline elements of which each crystal was built up. The crystal remained as truly crystalline after distortion in that way as it was when first cast, which was proved by the fact that, if a specimen of metal was polished and examined under a microscope, it was seen to have the usual polygonal structure, but when the metal had been distorted in any way by tension, compression or bending, the surface of the polygons was no longer blank white as in the untested metal, but showed a number of minute dark lines. Investigation had made it clear that these lines were the surface traces of the slip which had taken place between the crystals.
Study of Fractures in Metal.
The next point to come under consideration was the manner in which the crystals behaved when deformation was pushed to its limit and fracture occurred. It would be interesting to note the mode of fracture of iron under pure tension, and he had devised a special method of studying such fractures in longitueinal section. For this purpose the broken piece was embedded in a mass of electro-deposited copper, a section being afterwards cut through the fracture and the adherent copper. The result was that the ssction showed a clear, sharp outline upon which it was possible to trace tae exact nature of the fracture. The apparently fibrous fracture of every metal broken in static tension was deceptive, for a metal could never be really fibrous, but remained essentially crystalline, the strength of the metal depending, as previously stated, upon the size and arrangement oi the crystals. He would next consider the manner in which the structure of a metal became altered when a second metal was alloyed with it in increasing proportions. Alloys might be divided into two classes: in the first class the addition of the second body did not produce any visible change of structure, but might at the same time proauce very vast changes in the mechanical properties. Thus, the addition of so small a quantity is .02 per cent. of vanadium to steel altered its mechanical properties to a marvellous extent, while no visible alteration of s.ructura took place. The extraordinary influence of such small additions might be attributed to their effect upon the crystalline arrangement of the metal through the alteration of the gliding a ad cleavage planes, it being upon the arrangement ut these platses that the strength of the metal depended.
Influence of Eutectic on Steel.
In the second group of alloys, the addition of a second constituent produced a new structural element known to metallurgists as the eutectic. He would be able to show by a series of photographs the nature and properties of this second class of constituent, and to illustrate by examples the effect of its pre
sence upon the mechanical properties of alloys. The true structure of the eutectic was not well understood. It was not even certain if eutectics were truly crystalline, but, if they were crystalline, they could only be a mass of very minute crystals, and, consequently, any slip or deformation taking place must change its direction very frequently, so that a eutectic was always a stiffer and more resistant body than pure metal. It frequently happened that one of the constituents of the eutectic was a hard, brittle substance, as, for instance, cementite in the case of steel. In other alloys the second constituent was a definite compound, and was always exceedingly brittle. In an alloy of this kind the eutectic first made its appearance when a comparatively small quantity of the second element was intro• duced, and the quantity of eutectic present increased as the perc,entage of the alloy increased. The eutectic structure was at first only interspersed here and there among the crystals of pure metal, but occupied snore and more of the whole volume as the proportion of the alloy increased, and its effect on the mechanical properties became manifest by a stiffening and hardening ot the metal until, finally, with a certain amount of the eutectic present, the metal became non-ductile and brittle. He would iilustrate by photographs the relation between the micro-structure and strength of carbon steel and of copper aluminium alloys. It would be noted that as the proportionate pearlite increased the hardness increased, and the tenacity increased at first, but finally fell off as the proportions of the brittle constituent, pearlite, became predominant. He would show, by a series of highly magnified sections prepared by his special method, the mode of fracture of a duplex alloy. He took, first, as an example mild carbon steel, and would demonstrate the way in which such an alloy failed under tension, shock, alternate bending, alternate stress, and in this way the relative behaviour of ferrite and pearlite would be clearly indicated.
Dangers of Commercial Steel.
Ordinary commercial steels invariably contained enclosures of foreign bodies, ailicide of manganese being a common occurrence. It was held that under the best conditions such substances became separated from the steel while still fluid, but in actual practice the separation was never complete, and numerous minute enclosures of that kind were always found in steel. For some purposes they were harmless, weakening the metal very little in tension, but when they were elongated by compression, and stresses came upon the metal in a direction at right angles to their length, the presence of such impurities was a fruitful source of failure in the metal, the metal showing a strong tendency to split along lines started from the existence of these enclosures. The method of studying fractures, which he had described, was capable of giving valuable information both as to the mechanical proprties of the various constituents of alloys, and also as to the possible causes of failure in parts of machines or structures which had given way during work. The study of mysterious failures, and the causes which led up to them was in the ordinary way exceedingly difficult. If a section of metal were taken somewhere near the fracture and examined, it might still be impossible to detect where it was weak or de. fective. It might happen that the weakness of the metal was entirely local, and only by an actual section taken through the fracture itself was it at all possible to detect the presence of particular sources of weakness. Such sections, through the actual fracture, could not be obtained by any other method save the special method he had illustrated, and he would point out that if specimens were to be examined in that way it was very necessary that the fracture should be kept clean and uninjured. The best method of preserving the fracture from injury was that of wrapping it in tin foil and having it examined as soon as possible after breakage. If that were done, it might be possible to gain knowledge of the actual causes of many mysterious fractures. He hoped he had demonstrated to them the possibilities arising out of the microscopic examination of metals, and the new knowledge that it was thus possible to obtain would prove of importance to all branches of the engineering industry, and in particular the motor industry.
The New Zinc Alloy.
Mr. A. E. Tucker, in opening the discussion, said the suggested presence of slag in steel was of great importance to the motor industry. He could not accept the statement that failures in steel aro%e from that cause, as slag was never present in steel with ordinary care in casting. The effect of aluminium on alloys was somewhat analogous to that of phosphorus and arsenic, and, in brass castings for motor work where high quality was desired, some manufacturers invariably used phosphorus, with the object of obtaining a result similar to that given by aluminium in increasing the mechanical strength, and, owing to the effect upon the crystalline arrangement, the sectional area of the metal for structural work could be reduced. He was particularly interested in vanadium steel, and an interesting point arose in connection with the extraordinary properties of that steel in respect to elongation, when considered in relation to machinery, in which it resembled copper. He had recently had brought before him an alloy of zinc containing something like 9 per cent, of aluminium, which had a tensile strength of '22 tons per square inch, which was a most extraordinary result.
Dr. H. S. Hele-Shaw referred to the very complete manner in which French automobile engineers had studied their materials, and used the correct steel for the different parts of a chassis.
If France was to maintain her place in the front rank of the industry, it would be by specialisation. Mr. Rosenhain had done the English industry good service by showing how necessary it was to study the material used in manufacture. The subject was one of the greatest importance, and it would be well if the institution could initiate original work on the materials used in motor engineering, in order that steel makers could fill their special needs.
The Solidification of Metals.
Prof. Ewing put forward a theory to account for the stratified structure in metals. He said that the process of solidification was always an unstable process. The eutectic solution might be regarded as a saturated solution of the two constituents. If the two constituents were called A and B, a condition was set up in which alternately there was a supersaturated solution of A in B, and then a supersaturated solution of B in A. The whole process of solidification thus became oscillatory, each constituent crystallising out in turn. Assuming a process of this character, the peculiarities of structure were explained. He congratulated Mr. Rosenhain—who was an old pupil of his— on the excellent work he had accomplished.
Case-hardening Problems.
Mr. J. S. Critchley said that ten years ago the structure of steels was not well understood, but the Alloys Research Committee of the Institution of Mechanical Engineers had added greatly to the information available. He himself had had great difficulty in obtaining suitable steel for motorcar construction. At last, a French steel maker recommended nickel-chrome steel, but English manufacturers had said it was not possible to make it, save at very high prices. To-day, that steel was being largely used for gear wheels. Nothing was more interesting in connection with motorcar work than the question of case-hardening, which had been the bugbear of motor engineers for many years. In case-hardening ordinary mild steel, the best results were obtained by double quenching, but nickel steels only required one quenching. The De than Company had lately used a steel with 1,12 per cent. of carbon, and 7 per cent, of nickel, which gave a case-hardened steel without quenching. lie was glad to be able to state that British steel makers now recognised the importance of producing steel for motorcar work, and were lending great assistance to the industry. Mr. P. L. Renouf asked a question about fatigue in metal. it would be interesting, he said, to know if metal, which, in motor engineering, had to be subject to severe stresses, often extending over long periods, had the power of reconstitution.
Working Expenses of Commercial Vehicles.
Col. Crompton said that during recent years great progress had been made in regard to the study of metals. One matter, in which motor engineers were greatly interested, was in obtaining materials of very high elastic limit. There was no doubt
that road shock could be minimised, and the working expenses of commercial vehicles reduced by the use of steel rims of the correct material, and he believed the right material was very near being evolved. Importance lay not only in the material itself, but in the best treatment, and in the power of arresting the segregating process.
Mr. Rosenhain, in replying on the discussion, said that the zinc-aluminium alloy referred to had marvellous tensile strength, but, must not be compared with steel, as it had practically no ductility. With regard to the case-hardening of the carbonnickel steel without quenching, that was one of the achievements which the microscopic study of metals had made possible. On the subject of the impurities in commercial steel, to which he had referred, their danger was very much underrated.