Property of THE CHEMICAL NEWS AND JOURNAL OF PHYSICAL SCIENCE Edited by Established (WITH WHICH IS INCORPORATED THE "CHEMICAL GAZETTE"). in 1859. Published Weekly. Annual Subscription, free by post : Entered at the New York Post Office as Second Class Mail Matter. Transmissible through the Post-United Kingdom, at Newspaper rate; Canada and Newfoundland, at Magazine rate. Vol. 119.-No. 3110 [Copyright Friday, November 21, 1919 ARTICLES reserved. The Hardening of Steel, by H. C. H Carpenter Canadian News ..... PROCEEDINGS OF SOCIETIES PHYSICAL SOCIETY MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY THE TECHNICA INSP CTION ASSOCIATIO◄ NORWEGIAN CHEMICAL SOCIETY, CHRISTIANIA ROYAL SOCIETY NOTICES OF Books NOTES...... MARTINGS FOR THE WENK ........ 240 Registered as PRICE 4d. a Newspaper. LPOST FREE 4§d. ssistant Analyst required to go to Messina, Sicily. One with experience of Essential Oil Analysis preferred-Apply, with particulars of experience, to B., care of G. H. Ogston and Moore, 87-89, Aldgate High S reet, E. 1. 240 Chemist (30) desires Appointment in Works. 241 242 242 243 243 244 244 Thirteen years in Public Analyst's Laboratory. Knowledge of Food and Drugs, Water, Fertilisers, Oils, Paints, Fuels, &c. -Address, "Analyst," CHEMICAL NEWS Office, 16, Newcastle Street, Farringdon Street, London, E C 4. equired, a capable Analytical Chemist as Re Assistant in Laboratory in large Works in South Americe. Age 25 to 35. Good prospects -Write, giving fullest details of experience, qualifications, age, and salary required, to "A. C.." care of Street's, 30, Cornhill, E.C. 3. BEDFORD COLLEGE FOR WOMEN. (UNIVERSITY OF LONDON). The Council invite applications for the Post of DEMONSTRATOR in the Department of Inorganic Che mistry, vacant from January, 1920. Candidates must have taken an Honours Degree or its equivalent in Chemistry. Applications must be sent in by SATURDAY, NOVEMBER 2". Further information can be obtained from THE SECRETARY, Bedford College, Regent's Park, N.W. 1. B FILTER PAPERS Note a few of our later specialities: For use wit No. 42-"ASHLESS." Double Acid-washed and extremely close of texture. No. 50-Hardened by treatment with Nitric Acid. Very tough, it will resist great pressure, and SOLE MANUFACTURERS: W. & R. BALSTON, LTD., MEDWAY MILL, MAIDSTONE, KENT. FIND OUT ALL ABOUT OUR COMPLETE RANGE FROM OUR REVISED BOOKLET AND PRICE LIST. In case of difficulty in obtaining Free Samples, write the Sole Mill Representatives H. REEVE ANGEL & CO., 9, BRIDEWELL PLACE, LONDON, E.C. 4. THE capacity of steel for hardening by being quenched from a bright red beat in water is the most important property possessed by any metallic substance. This property is utilised practically in the arts in a great variety of ways, and is the basis of all modern engineering work. To take two types of application only (1) It is utilised in the great variety of tools which are employed in modern engineering work, for machining metals and alloys to a high degree of accuracy so that they may constitute a given part of one of the thousand and one machines employed in the mechanic arts of modern civilisation, e.g.. the locomotive, turbine, gas engine, electro motor, &c. (2) The razor, the balance spring of the chronometer, the knife, the needle, the pair of scissors, the surgeon's lancet, and the dentist's twist drill are instances of the utilisation of this property in the finished steel as well, and they depend upon the capacity of such material to retain its hardness in the absence of stress indefinitely at the ordinary temperatures. Not only, however, is this property of the greatest practical import, but of the highest scientific interest, inasmuch as the search for the explanation of the capacity of steel for hardening has given rise to a large number of scientific investigations, which have done more than any. thing else to throw light on the constitution of steels and metallic alloys generally, and have helped to establish the modern science of metallography. My object this evening is to trace rapidly the history of some of the salient features of this scientific work, and to bring to your notice the most modern views as to the scientific explanation of this wonderful property of steel. I must preface my remarks, however, with this warning that within the compass of an hour's lecture it is not possible to attempt a complete exposition of the theories relating to the great variety of steel alloys that are now used in the arts, and I shall be obliged to confine my thesis to one of the simplest, albeit the best known of these materials-namely, what is called the pure carbon steel turning tool. Let us begin by considering the manufacture of the tool itself, and afterwards the conditions under which it is put to work and what follows then. some Although even the purest iron containing only the merest trace of carbon can be hardened to extent by quenching from a high temperature, a certain minimum percentage of carbon is necessary before the hardening is sufficiently marked to confer what may be called practical hardening properties on the steel. This minimum is about 07 per cent, and the cutting tools used in the arts range from this figure up to 1.5 per cent. For reasons which will be subsequently apparent, it is simplest to consider the case of a steel tool containing o'9 per cent carbon. The operation of manufacture of such a tool is briefly as follows: In the first place, a charge designed to give the correct composition is melted down in a crucible in a furnace whose temperature ranges from 1500-1600° C. The steel is cast from the correct temperature into a metallic mould, giving what is called an ingot, e.g., a rectangular bar about 2 inches square and 2 feet long. The ingot is subsequently forged down to a bar, say, 1-1 inch in diameter; it is then cut up into lengths suitable for the dimensions of the tool itself, and * A Lecture delivered at the Royal Institution, March 7. 1919. next it passes to the hands of the smith, who forges it by hand and fashions the tool to shape. During this operation it is of the greatest importance that the composition of the steel should be altered as little as possible by oxidation of the carbon, otherwise the tool will not harden properly or evenly. Then follows the actual hardening operation, in which the nose of the tool, as it is called, is carefully heated to a given temperature in the neighbourhood of 800° C., withdrawn either from the smith's fire, or, better still, the hardening bath, and quenched outright in a bath of cold water at the ordinary temperature. This is an operation requiring the utmost skill. If the surface of the metal has been decarbonised in heating and scale has formed on the surface, the rapid transference of heat from the metal to the bath will not take place, imperfect hardening will result, and very likely cracks will be formed. If, however, the tool, as in the best modern practice, is heated up in a bath of fused salts, no loss of carbon takes place, and when it is removed from the bath it is covered with a thin film of fused salt. This dissolves almost instantaneously in the water, and the necessary rapid transference of heat from the metal to the water takes place. The best quenchings always have a peculiar "bite," accompanied by a dull, albeit sharp, sound, as the large bubbles of steam generated by the heat are absorbed in the surrounding water. The steel thus hardened, although possessing the neces sary hardness, is too brittle to be used as a tool. Accordingly the tempering process follows, in which it is heated to a moderate temperature depending on the work to which it is to be put. This temperature varies usually from 200°-300° C. This tempering process, while it withdraws some of the hardness produced by quenching, confers a most valuable property on the tool-namely, toughness-by means of which it stands up to its work, at any rate for a time, without cracking. Then comes the last stage, namely, the grinding of the tool on the grindstone, whereby a clean cutting edge of the required shape is produced. The tool is then ready for use, Let us suppose that it is to be used in taking a cut from a cylindrical bar of an unhardened steel. The latter is fixed in a lathe and rotated at an appropriate speed (Fig. 1). The tool. held in a tool-holder, actuated by suitable mechanism, is gradually brought up to the end of the rotating bar and a given rate of feed maintained. For an instant there is actual contact between the cutting edge of the tool and the bar. The moment, however, that this Bappens a chip is formed and a shearing stress is set up, as a result of which the work falls, not on the actual edge of the tool, but on an area inside. If the metal which is being machined is ductile, it is cut away in long shavings, whose thickness depends on the feed; if, on the other hand, it is brittle, it breaks off into short chips. The effect of this friction between the two metals and the pressure of the shaving is to generate heat and to raise the temperature of the tool to a much greater extent than that of the bar which is being machined, and deterioration of the tool sets in; its hardness gradually diminishes, and pari passu wear of the tool itself by abrasion occurs. Eventually a stage is reached at which the cutting operation has to be discontinued, and the tool requires re-hardening or regrinding. The carbon tool of which I have just spoken cannot be used for taking the heavy cuts of which the modern alloy tools are capable, but they retain their preeminence even to-day in all machinery work where the highest degree of accuracy attainable is desired. A complete theory of the action of such tools must account for both the hardening and toughening of the steel, and its gradual loss of these properties by the operations I have described. Before attempting this, however, it may be interesting to refer briefly to the methods of hardening which chiefly occupied the attention of early workers. The Greek alchemical manuscripts give various recipes, from which it is clear that in the early days the nature of the quenching liquid was considered to be all-important. seconds. As the diameter of the bar increases, the rate of chilling is, of course, diminished, and no doubt in the case of a rectangular turning tool of 1-inch diameter several seconds would be required for the same drop in temperature at the centre. As McCance has pointed out, the perfect theoretical conditions for quenching are "that a specimen beated uniformly has its surface suddenly cooled to a lower temperature, and kept at that lower temperature without alteration until it has once more obtained uniformity, but at the lower temperature. The temperature of the bar changes then in a manner depending on its thermal properties and dimensions, and the rate of change thus obtained cannot be exceeded between similar temperature limits. Practical quenching depends on how far these conditions are satisfied, which really resolves itself into the simple question: How constant can the surface of the specimen be kept at the lower temperature after immersion in the liquid, and what properties must the liquid possess to fulfil this purpose best?" McCance accepts Benedicks' conclusion that vaporisation plays the most important part in the quenching power of a liquid, and the fact that water with its high latent beat of vaporisation is the best quenching liquid known accords with this view. There were certain rivers the waters of which were sup- | until the temperature had dropped to 500° C. was 11 posed to be specially efficacious, and Pliny mentions that the difference between waters of various rivers can be recognised by workers in steel. Many old recipes for hardening and tempering have been lost, but a number of them have come down through the ages, and from them I take the following illustrations:-The first is from a work entitled " Rechtegebranch & Alchimei" (1531), of which an English translation appeared in 1583. "Take snayles and first-drawn water of a red die, of which water, being taken in the first two monthes of harvest when it raynes, boil it with the snayles, then heate your iron red hot and quench it therein, and it shall be as hard as steele." "Ye may do the like with the blood of a man of XXX years of age and of a sanguine complexion, being of a merry nature and pleasant, . . . distilled in the middst of May." It would be interesting to know the circumstances which led to this particular experiment being first tried. These in structions may seem trivial, but a belief in the efficacy of such solutions has continued, for in a work published in 1810 ("The Laboratory or School of Arts") the artist is directed to take the root of blue lilies, infuse it in wine, and quench the steel in it; the steel will be hard. On the other hand, he is told that if he takes the juice or water. f common beans and quenches iron or steel in it, it will be soft as lead. When the practice of an art is purely empirical it is liable to take fantastic forms. Even at the present day, however, there are many workshops where steel is hardened in which some quaint nostrum still holds sway. Occasionally, but not often, the use of these compounded baths was supported by theoretical views. Otto Tachen, for instance, writing of steel about the year 1666, says that steel "when it is quenched in water acquires strength because the light alcaly in the water is a true comforter of the light acid in the iron, and cutlers do strengthen it with alcaly of animals." Hence the use of snails. Though the practice and theories in the periods I have mentioned were frequently fantastic, these early workers were quite right in attaching importance to the liquid in which the steel was to be quenched. To-day much simpler methods are used in the best modern practice. If severe quenching is required, cold water or brine is used; if a lesser degree of hardening is necessary, hot water or oil; while in many cases the processes of hardening and tempering are combined in one operation by plunging the tool in a bath of molten metal such as lead. The rapid changes of temperature which occur when a bar of steel is quenched in a liquid from a high temperature have been studied experimentally by Le Chatelier, Benedicks, and Lejeune, and their results utilised by McCance in calculating the theoretical curve of quenching. Le Chatelier found that water gave a more rapid rate of cooling than any other liquid, and concluded that the most important property in determining the quenching power of a liquid was its specific heat. Benedicks, by means of an automatic quenching and photographically recording apparatus, obtained the most accurate results which are available as to the temperature at the centre of small round bars quenched in water. On the whole his conclusions corroborated those of Le Chatelier, but he showed that the latent heat is the most important property in determining the quenching power of a liquid under practical conditions. The appended cooling curve (Fig. 2) deduced by McCance from Benedicks' results shows what happened when a bar of steel containing 1 per cent carbon, 2 inches long and inch in diameter, was quenched from 850° C. in water at 15° C. This curve gives the temperature at the centre of the bar at any given moment. It will be observed that it does not fall for the first fraction of a second; that the rate of fall starts slowly, then increases rapidly to a maximum, afterwards decreasing more and more slowly until it reaches the temperature of the surrounding water. From our present point of view the important thing to notice is that the time from the moment of immersion What happens when a uniformly heated bar is plunged into water at o° C. is somewhat as follows:-The layers of water in immediate contact with the bar are rapidly heated to their boiling point, and steam is formed. This expands outwards and causes a fresh layer to come in contact with the surface, and so the process goes on. The surface of the bar alternates then between o° C. and 100 C., the steam acting as a carrier of heat to the body of the liquid. This goes on until the supply of heat is sufficient to form steam, when the transference of heat and the cooling of the bar take place by convection only; and at this stage of the process a high conductivity in the liquid is an actual disadvantage, since it retards convection. A low viscosity is wanted to enable the steam to travel outwards with as little resistance as possible, while a high specific heat ensures that the temperature changes of the liquid are small. We have next to consider certain fundamental properties of iron and its alloys with carbon which have been established by modern research, and without a knowledge of which any discussion of theories of hardening would be unintelligible. These properties have been for the most part investigated during the last fifty years by a small band of devoted research workers who have been the pioneers in founding the modern science of metallography. The application of the microscope to the study of the constitution of iron and steel, regarding these materials as igneous rocks, is due to Henry Clifton Sorby, of Sheffield, who died only a few years ago, while the use of the thermo-electric pyrometer as a means of studying the changes of internal energy in the same materials is the work of a Frenchman who is still living-M. Henri le Chatelier. The methods opened up by these pioneers have proved extremely fruitful, and the results achieved by their systematic exploitation have brought certainty into a field where previously only speculation reigned. With these results it is necessary to deal briefly. It will be convenient first to consider the evidence afforded by accurate pyrometry, although historically the microscope led the way. It is now known that the purest iron which can be obtained, containing as it does not more than three parts of impurity per 10,000, can exist in at least three different modifications, and there is every reason to believe that these modifications are a property of the iron, and not in any way due to the small amount of impurities mentioned. If the cooling of a sample of such iron from the melting point (1505° C.) is followed by means of a delicate pyrometer and potentiometer the cooling is found to be normal for about 600°. At about 900° C., however (Fig. 3), there is a sudden and large evolution of heat, which completes itself within a few degrees, and is |