Heat Treatment of Steel

From the 1924 edition of Machinery's Handbook This is section 2 of 7

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Pyrometers

Pyrometers are of great value in connection with the heat-treatment of steel, as they make if possible to determine high temperature accurately; moreover, the temperature, when heating for hardening, can be regulated to conform with the temperature that has given the best results in practice. There are several different types of pyrometers commonly used in industrial service, which may be classified according to the principle upon which they operate.

Thermo-electric Pyrometer. -- In this type of pyrometer, temperature variations are determined by the measurement of an electric current generated by the action of heat on the junction of two dissimilar metals; that is, when one junction of the thermo-couple has a temperature different from the other, a current is developed and a meter indicates the temperature, the relation between the strength of the current and the temperature being constant. The thermo-couple and the meter form the essential parts. The two dissimilar metals composing the thermo-couple are connected at one end, which is called the "hot end", and placed in the furnace or heated place the temperature of which is required. Except at the hot end, the two wires or elements do not touch. The free ends, called the "cold end", are kept away from the heat. When the hot end is heated, the intensity of the current generated depends upon the difference between the temperature of the hot and cold ends. The meter is connected to the cold end and shows the value of the current in degrees Fahrenheit or Centigrade. Some pyrometers of this type may be used, intermittently, for temperatures up to 3000 degrees F.

Resistance Pyrometer. -- The variation in electric conductivity due to changes in temperature is the principle upon which the resistance pyrometer is based. This type is very accurate for temperatures below 1600 degrees F., but should not be used continuously for higher temperatures. The maximum temperature is about 2200 degrees F. The thermo-electric type is preferable for indicating high-speed steel hardening temperatures, etc., because the resistance type will not stand exposure to intense heats, except for short periods.

Radiation Pyrometer. -- This type measures radiated heat and is adapted for very high temperatures. The FÈry radiation pyrometer is practically a reflecting telescope having a concave mirror which focuses the radiant heat of the object upon the "hot" junction of a small thermo-couple. There is a diaphragm for reducing the aperture when the instrument is pointed at a very hot object, in order to prevent over-heating the thermo-couple. With the Brown radiation pyrometer, the rays of heat from the furnace or molten metal which enter the pyrometer tube are reflected from concave mirror onto a sensitive thermo-couple, and the temperature is indicated on a milli-voltmeter, graduated in temperature degrees, the same as a thermo-electric pyrometer. No part of the instrument is inserted in the high heat to be measured. If the temperature of a furnace is being measured, the tube is either held on a tripod or in the hand, and is pointed toward the furnace. The temperature can then be read off on the indicator.

Optical Pyrometers. -- There are several classes of optical pyrometers. The Morse thermo-gage indicates the temperature by heating the filament of an electric lamp to the same color as that of the incandescent body, the temperature of which is required. The small low-voltage lamp is placed inside a tube through which the heated object is observed. To determine the temperature, the current for the lamp is so regulated (by means of a rheostat) that the color of the lamp filament corresponds to that of the heated object which is observed through the instrument. The current then being consumed is indicated by a small milli-ammeter, and the corresponding temperature is determined. This instrument is accurate to within 2 or 3 degrees C. When absorbent glasses are used to reduce the brilliancy of the heated part, the highest temperatures required for industrial work can be gaged. The Mesure and Nouel optical pyrometer is a very simple type, which, by means of prisms and reflectors, enables temperatures to be determined by utilizing the colored field produced by the polarization and refraction of light from the heated part. The accuracy of a reading depends upon the observer's judgment of relative colors and may vary 50 degrees C. (90 degrees F.). This type is adapted to the taking of frequent readings. With the photometric type (including the Wanner and Le Chatelier optical and FÈry absorption pyrometers) there is an illuminated field, one-half of which received light from the heated object, and the other half, from a standard source of light forming part of the instrument. With the Le Chatelier instrument, the amount of light admitted from the heated part is regulated by an adjustable diaphragm. When both halves are of the same intensity or brightness, the temperature is indicated by a scale on the diaphragm.

Judging Temperatures by Color. -- The U. S. Bureau of Standards states that skilled observers may vary as much as 100 degrees F. in their estimation of relatively low temperatures by color; beyond 2200 degrees F. it is practically impossible to make estimations with any certainty.

Seger Temperature Cones. -- The "sentinel" pyrometer or Seger temperature cones are in the form of triangular pyramids (about 3 inches high), composed of metallic and mineral substances which fuse at certain temperatures. They are made in series, each successive cone having a fusing temperature that differs slightly from the one above or below in the scale; that is, if the series were placed placed in a furnace and the temperature gradually raised, one cone after another would melt as its melting point was reached. These cones are sometimes used in pairs to determine the minimum and maximum temperatures for a given process, one cone being selected for the lowest and another for the highest temperature required. Tests have shown that this method for determining temperatures is very trustworthy within 35 degrees F.

Melting Temperatures of Seger Cones
No. of ConeMelting Temp., Deg. F.No. of ConeMelting Temp., Deg. F.No. of ConeMelting Temp., Deg. F.No. of ConeMelting Temp., Deg. F.No. of ConeMelting Temp., Deg. F.
010174301206692390 182714273038
09177812102102426 192750283074
08181422138112462 202786293110
07185032174122498 212822303146
06188642210132534 222858313182
05192252246142570 232894323218
04195862282152606 242930333254
03199472318162642 252966
02203082354172678 263002

Calibration of Pyrometers. -- Pyrometers should occasionally be compared with a standard pyrometer or be calibrated in some other way. The following general instructions are given by the Hoskins Mfg. Co. The accuracy of both meter and thermo-couple should be checked. When checking the meter see that all connections are tight and that the protection tubes are sound. Set the zero of the check meter and the service meter to the same temperature and then take readings of both meters when they are alternately connected to any couple. If the instruments are both calibrated for the same external resistance, then only one set of leads is necessary, this set being connected first to one and then the other meter. If the meters are calibrated for different external resistances, then the individual leads of proper resistance must be used with each. In this method of checking, the check meter and the one being tested must be of the same kind; that is, both must be high or low resistance. When checking the thermo-couple, if only one thermo-couple is being used with the meter, see that the zero setting of the meter corresponds to the "cold end" of the couple. If several couples are used with one meter, the zero setting should be in agreement with the average temperature of the cold ends of the several couples. Set the zero of the check meter in agreement with the cold-end temperature of the check couple; place check couple and service couple in same protection tube and compare the readings of the two meters.
If the meter operates with only one couple, then the indicated error is the actual error of the thermo-couple, assuming that the zero settings of both the check and the service meter are correct. If the service meter proved to be accurate, and it is serving more than one couple, then the difference between the readings of the two meters in this test is the combined error of the thermo-couple and the error due to the cold-end setting of the meter. To determine the portion of this due to the thermo-couple, note the difference in temperature between the zero setting of the service meter and the actual cold-end temperature of the particular couple being tested. Subtract this difference from the indicated error, as shown by the meter readings and the result is the error in the thermo-couple.

Calibrating by the Melting Point of Copper. -- For calibrating pyrometers for temperatures above a red heat, the welded or "hot end" of the thermo-couple should be covered with a tight winding of No. 14 or 16 B. & S. gage, standard melting-point wire. The couple should then be inserted in a tube furnace with the welded end approximately in the center. The furnace should be of the required heat before inserting the couple, and should be kept at a temperature approximately 100 degrees F. higher than the melting point of the calibrating wire. The pointer of the meter will then move up the scale with a gradually decreasing speed until the calibrating wire begins to melt, when the pointer will come to rest. After the wire has melted, the pointer will again move upward. Pure copper wire, under oxidizing conditions, melts at 1083 degrees C. (1981 degrees F.), and pure zinc wire, at 419 degrees C. (786 degrees F.). In order to have a strictly oxidizing atmosphere, an open-end electric furnace should be used for calibrating. With this method of calibrating, care should be taken not to have the furnace temperature too far above the melting point of the calibrating wire, because the pointer will move so rapidly and the melting will be of such short duration that the temporary pause of the pointer may not be observed.

Calibrating by the Freezing Point of Melted Salts. -- A very satisfactory way of calibrating pyrometers is by using the "freezing points" of melted salts. Pure common salt (NaCl) is melted in a pure graphite crucible. When the salt has been raised to a temperature of 100 degrees to 200 degrees F. above its melting point, the bare welded end of the thermo-couple is inserted to a depth of 2 to 3 inches. The crucible is then removed from the furnace and allowed to cool. The pointer on the meter will drop gradually until the salt begins to freeze or solidify; then the pointer will stop until the salt is frozen. The freezing point of pure salt is taken at 800 degrees C. (1472 degrees F.). After calibrating and before being further used, the couple end should be washed in hot water to remove all traces of the salt, as otherwise the couple will deteriorate rapidly, especially when heated considerably above the melting point of salt in an open furnace. When calibrating pyrometers, care should be taken that the zero setting of the meter agrees with the cold end of the couple, which is always kept away from the heat and generally at the temperature of the outside air. The following table gives the latest available data by the Bureau of Standards on certain substances which may be used for calibrating pyrometers.

   Water boils at . . . . . . . . . . . . .  100   deg. C  (212   deg. F.)
   Tin freezes at . . . . . . . . . . . . .  231.9 deg. C  (449.4 deg. F.)
   Zinc freezes at  . . . . . . . . . . . .  419.4 deg. C  (786.9 deg. F.)
   Common salt freezes at . . . . . . . . .  800   deg. C (1472   deg. F.)
   Copper freezes at  . . . . . . . . . . . 1083   deg. C (1981.4 deg. F.)

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