The Equipment of Case-hardening Rooms.
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The Construction of Thermo-electric and other Forms of Pyrometers.
The importance of obtaining • and maintaining the correct temperatures for the hardening or tempering of steel cannot be overestimated. Mr. Brayshaw, whose salt-bath furnace was described in our last issue; when speaking at the Institution of Mechanical Engineers on the 2gth January, 1904, during the discussion which followed the presentation of the sixth report of the Alloys Research Committee of that institution, remarked : " He was in a position to say that at the hardening point, one degree Centigrade made a percep. tible difference in the hardened steel. Recent work showed that hardening should be more carefully undertaken than was considered necessary a few years ago. Five degrees Centigrade made the difference between good and bad hardening." The only way in which the best results can be obtained is by the use of an accurately-calibrated thermometer, and of these we now give some particulars. The temperatures at which steel is treated during the processes of hardening or tempering are much too high to be recorded on a mercurial thermometer ; therefore, an instrument which depends on magnetic or thermo-electric action must be employed. Many men in the shops, not a few of whom hold important positions, regard such instruinents with a certain amount of awe, and they are inclined to abuse them, or even wilfully to damage them, whenever an opportunity occurs. The fireman who gives it as his opinion that them there clock things ain't no use " has got. to realise that they are far more reliable than his " eye " which, by the way, is controlled by the state of his general health, and is, therefore, not to be relied upon to give the same judgment on two different occasions.
Magnetic Temperature Indicator.
Instruments of this class depend upon the fact that at the critical temperature from which steel can be hardened by quenching, but below which it cannot be satisfactorily hardened, the steel loses its magnetic property. In order
to snake use of this characteristic for the purpose of giving audible warning to the fireman, the steel to be hardened is placed, in a muffle, between the poles of a magnet, one limb of which is delicately poised and held up by the steel until the critical temperature is reached. At that moment, the arm is set free, and swings, to establish an electrical contact and ring a bell, thus giving warning to the attendant, who then withdraws the steel from the furnace and quenches. Very good results have been obtained by the use of such a device which is made by Taylor, Taylor and Hobson, Limited, of Leicester.
ThermowEIectric Pyrometers.
In thermo-electric pyrometry, the temperatures are measured by the magnitude of the electro-motive forces which are set up when the junction of two dissimilar metals is exposed to heat. This method of heat measurement was proposed by Becquerel, in 1826, but it was first put into service, in a scientific manner, by Pouillet, in 1836. Many improvements in the construction and nature of the couples employed have from time to time been made. Experience has proved that the couples, which are best suited to meet the requirements of the workshop or the laboratory, are those made of pure platinum and platinum-iridium, or pure platinum and platinum-rhodium. It is necessary that the wires of the couple be as pure and homogeneous as possible, in order to obtain absolute accuracy in the readings. There is little trouble at the present time in getting materials of sufficient purity and homogeneity; this being the case, it is only necessary to secure a galvanometer which is sufficiently sensitive to record the minute variations in the strength of the electric current that are produced when the junction of the " couple " is subjected to heat, and, instead of indicating the electro-motive force of the current as so
many " to give the equivalent value in degrees Centigrade or Fahrenheit.
The thermo-couple consists of two wires, which are fused together at one end (by means of the electric arc or by an oxy-hydrogen flame) and are insulated from each other. Where compactness is of great importance, the wires may be enclosed in magnesia tubes one-eighth of an inch in external diameter; or, a more practical and durable method is to enclose them in separate porcelain tubes of small diameter, and then to enclose these tubes within a single porcelain tube of larger diameter, which larger tube, in turn, is protected by its being enclosed in a steel sheath, the sheath having to be removed if the " couple" is to be used for temperatures exceeding 1,300 degrees Fahrenheit. We illustrate a "couple " built up on the principle just outlined, and which is made by the Cambridge Scientific Instrument Company, Limited. Another method of insulating the wires is to use clay tubes which are sufficiently refractory to stand the high temperatures, and one of our illustrations shows such a " couple." This is of American manufacture, and is known as Price's patent pyrometer; it is handled in this country by Messrs. Markt and Company, of 6, City Road, Finsbury Square, London, .E.C.
Either of the above-named pyrometers is suitable for tilt taking of temperatures in such furnaces as have been illustrated and described during the course of the present series.
In commercial work, a high-resistance galvanometer i employed for the purpose of reading the differences Of th electro-motive forces set up in the couples; the coil of sue a galvanometer carries a pointer, which either moves over scale, or is depressed, by means of clockwork mcchanisa on to an inking arrangement which prints a mark upon sheet of paper that is wrapped round a revolving drum, an in this manner all variations in the temperature of th.e fui mice are recorded. Owing to the high resistance of On winding of the galvanometer, the instrument may he set ts at a considerable distance from the source of heat, witlacn the necessity of making any allowance for the resistance ( the conducting wires. The armatures of these galvam meters are very delicately mounted, sometimes on a sing: jewel, or by a fine strand of silk, in order to reduce the is ternal friction of the instrument practically to nothing, an make it possible to obtain indications of temperature withi
a fraction per cent. of absolute accuracy. The Cambridge Scientific Instrument Company, Limited, is one of the largest makers, in this country, of these delicate instruments, and two of that company's galvanometers—the portable and the wall types—are illustrated herewith. This company also makes another form of portable galvanometer, in which all the vital parts are mounted on gimbals, and consequently self-levelling, whilst still another model is a recording instrument which may be fixed in any convenient position in either the works or the manager's office.
Price's recording galvanometer, as sold by Messrs. Markt and Company, differs from most others in that a needle indicates, on a scale, the temperature of the furnace, but, every minute, a tiny hole is perforated through a continuous sheet of paper. The paper is moved by clockwork, and it has marked across the surface the minutes and hours to correspond with the time of day at which it passes through the instrument, and, parallel to its edges, lines representing the temperatures are ruled. The holes are similar to the puncture of a pin, but their edges are browned by the spark.
The thermo-electric system has many advantages over other systems, principally due to its simplicity. It requires no outside battery, no adjustable resistance, and nothing that can be varied by the workman or which depends upon that individual's judgment. Any number of " couples "may be connected to the same instrument, by the use of a multiple-contact switch, as is illustrated in the accompanying diagram of one of Price's instruments, wired up to suit four " couples," permanently fixed in different furnaces.
Amongst other British makers of pyrometer-galvanometers are Messrs. Baird and Tatlock, of Glasgow, and Messrs. J. W. and C. J. Phillips, of College Hill, Cannon Street, E.C.
Radiation Pyrometers.
The chemical activity of the furnace gases when working at very high temperatures has, in a few cases, rendered difficult the adequate protection of the thermo-couple. With a view to the overcoming of these troubles, M. Fery, Professor of Physics at the Ecole de Physique et de Chimie, Paris, has invented an instrument which can be placed at some distance from the furnace, and of which no part is ever raised more than 180 degrees Fahrenheit above the terriperature of the atmosphere.
The heat rays which emanate from a hot body, or which pass out through an observation hole in the wall of a furnace, fall upon a concave mirror, and are thus brought to a focus. In this focus is a thermo-electric couple, whose temperature is raised by the heat rays falling upon it : the hotter the furnace, the greater the rise of temperature of the "couple." The arrangement of the instrument is such that it is uninfluenced, within wide limits, by the size of the hot body or observation hole on the one hand, or by the distance which separates it from the furnace on the other hand. The absorbtion of some small amount of radiant heat in passing through the atmosphere cannot, of course, be strictly without effect, but in practice the error is not appreciable. The Eery device has been placed on the market by the Cambridge Scientific Instrument Company, Limited. Conclusion.
In addition to its proper equipment of furnaces, temperature'indicators, and recorders, a case-hardening room is not complete without quenching tanks, for both oil and water, and proper ventilating fans for the purpose of exhausting the offensive fumes which are given off when a piece of
white-hot, or red-hot, metal is plunged into oil. Such fittings and accessories, however, will not be specially de scribed in this series. The accompanying tables will be found very useful in any hardening shop, and for these we are indebted to the Wolseley and the Cambridge Companies..