The Working of Steel / Annealing, Heat Treating and Hardening of Carbon and Alloy Steel

A current from the dry cell Ba is constantly flowing through the main, or so-called potentiometer circuit, ABCDGEF. The section DGE of this circuit is a slide wire, uniform in resistance throughout its length. The scale is fixed on this slide wire. The current from the cell Ba as it flows through DGE, undergoes a fall in potential, setting up a difference in voltage, that is, an electromotive force, between D and E. There will also be electromotive force between D and all other points on the slide wire. The polarity of this is in opposition to the polarity of the thermo-couple which connects into the potentiometer at D and at G. By moving G along the slide wire a point is found where the voltage between D and G in the slide wire is just equal to the voltage between D and G generated by the thermo-couple. A galvanometer in the thermo-couple circuit indicates when the balance point is reached, since at this point the galvanometer needle will stand motionless when its circuit is opened and closed.

The voltage in the slide wire will vary with the current flowing through it from the cell Ba and a means of standardizing this is provided. SC, Fig. 111, is a cadmium cell whose voltage is constant. It is connected at two points C and D to the potentiometer circuit whenever the potentiometer current is to be standardized. At this time the galvanometer is thrown in series with SC. The variable rheostat R is then adjusted until the current flowing is such that as it flows through the standard resistance CD, the fall in potential between C and D is just equal to the voltage of the standard cell SC. At this time the galvanometer will indicate a balance in the same way as when it was used with a thermo-couple. By this operation the current in the slide wire DGE has been standardized.

Development of the Wiring Scheme of the Cold-end Compensator.—The net voltage generated by a thermo-couple depends upon the temperature of the hot end and the temperature of the cold end. Therefore, any method adopted for reading temperature by means of thermo-couples must in some way provide a means of correcting for the temperature of the cold end. The potentiometer may have either of two very simple devices for this purpose. In one form the operator is required to set a small index to a point on a scale corresponding to the known cold junction temperature. In the other form an even more simple automatic compensator is employed. The principle of each is described in the succeeding paragraphs, in which the assumption is made that the reader already understands the potentiometer principle as described above.

As previously explained the voltage of the thermo-couple is measured by balancing it against the voltage drop DG in the potentiometer.

As shown in Fig. 111, the magnitude of the balancing voltage is controlled by the position of G. Make D movable as shown in Fig. 112 and the magnitude of the voltage DG may be varied either from the point D or the point G. This gives a means of compensating for cold end changes by setting the slider D. As the cold end temperature rises the net voltage generated by the couple decreases, assuming the hot end temperature to be constant. To balance this decreased voltage the slider D is moved along its scale to a new point nearer G. In other words, the slider D is moved along its scale until it corresponds to the known temperature of the cold end and then the potentiometer is balanced by moving the slider G. The readings of G will then be direct.

The same results will be obtained if a slide wire upon which D bears is in parallel with the slide wire of G, as shown in Fig. 113.

Automatic Compensator.—It should be noted that the effect of moving the contact D, Fig. 113, is to vary the ratio of the resistances on the two sides of the point D in the secondary slide wire. In the recording pyrometers, an automatic compensator is employed. This automatic compensator varies the ratio on the two sides of the point D in the following manner:

The point D, Fig. 114, is mechanically fixed; on one side of D is the constant resistance coil M, on the other the nickel coil N. N is placed at or near the cold end of the thermo-couple (or couples). Nickel has a high temperature coefficient and the electrical proportions of M and N are such that the resistance change of N, as it varies with the temperature of the cold end, has the same effect upon the balancing voltage between D and G that the movement of the point D, Fig. 114, has in the hand-operated compensator.

Instruments embodying these principles are shown in Figs. 115 to 117. The captions making their uses clear.

PLACING THE THERMO-COUPLES

The following illustrations from the Taylor Instrument Company show different applications of the thermo-couples to furnaces of various kinds. Figure 118 shows an oil-fired furnace with a simple vertical installation. Figure 119 shows a method of imbedding the thermo-couple in the floor of a furnace so as to require no space in the heating chamber.

Various methods of applying a pyrometer to common heat-treatment furnaces are shown in Figs. 120 to 122.

LEEDS AND NORTHRUP OPTICAL PYROMETER

The principles of this very popular method of measuring temperature are sketched in Fig. 123.

The instrument is light and portable, and can be sighted as easily as an opera glass. The telescope, which is held in the hand, weighs only 25 oz.; and the case containing the battery, rheostat and milliammeter, which is slung from the shoulder, only 10 lb.

A large surface to sight at is not required. So long as the image formed by the objective is broader than the lamp filament, the temperature can be measured accurately.

Distance does not matter, as the brightness of the image formed by the lens is practically constant, regardless of the distance of the instrument from the hot object.

The manipulation is simple and rapid, consisting merely in the turning of a knurled knob. The setting is made with great precision, due to the rapid change in light intensity with change in temperature and to the sensitiveness of the eye to differences of light intensity. In the region of temperatures used for hardening steel, for example, different observers using the instrument will agree within 3°C.

Only brightness, not color, of light is matched, as light of only one color reaches the eye. Color blindness, therefore, is no hindrance to the use of this method. The use of the instrument is shown in Fig. 127.

Optical System and Electrical Circuit of the Leeds & Northrup Optical Pyrometer.—For extremely high temperature, the optical pyrometer is largely used. This is a comparative method. By means of the rheostat the current through the lamp is adjusted until the brightness of the filament is just equal to the brightness of the image produced by the lens L, Fig. 123, whereupon the filament blends with or becomes indistinguishable in the background formed by the image of the hot object. This adjustment can be made with great accuracy and certainty, as the effect of radiation upon the eye varies some twenty times faster than does the temperature at 1,600°F., and some fourteen times faster at 3,400°F. When a balance has been obtained, the observer notes the reading of the milliammeter. The temperature corresponding to the current is then read from a calibration curve supplied with the instrument.

As the intensity of the light emitted at the higher temperatures becomes dazzling, it is found desirable to introduce a piece of red glass in the eye piece at R. This also eliminates any question of matching colors, or of the observer's ability to distinguish colors. It is further of value in dealing with bodies which do not radiate light of the same composition as that emitted by a black body, since nevertheless the intensity of radiation of any one color from such bodies increases progressively in a definite manner as the temperature rises. The intensity of this one color can therefore be used as a measure of temperature for the body in question. Figures 124 to 126 show the way it is read.

CORRECTION FOR COLD-JUNCTION ERRORS

The voltage generated by a thermo-couple of an electric pyrometer is dependent on the difference in temperature between its hot junction, inside the furnace, and the cold junction, or opposite end of the thermo-couple to which the copper wires are connected. If the temperature or this cold junction rises and falls, the indications of the instrument will vary, although the hot junction in the furnace may be at a constant temperature.

A cold-junction temperature of 75°F., or 25°C., is usually adopted in commercial pyrometers, and the pointer on the pyrometer should stand at this point on the scale when the hot junction is not heated. If the cold-junction temperature rises about 75°F., where base metal thermo-couples are used, the pyrometer will read approximately 1° low for every 1° rise in temperature above 75°F. For example, if the instrument is adjusted for a cold-junction temperature of 75°, and the actual cold-junction temperature is 90°F., the pyrometer will read 15° low. If, however, the cold-junction temperature falls below 75°F., the pyrometer will read high instead of low, approximately 1° for every 1° drop in temperature below 75°F.

With platinum thermo-couples, the error is approximately 1/2° for 1° change in temperature.

Correction by Zero Adjustment.—Many pyrometers are supplied with a zero adjuster, by means of which the pointer can be set to any actual cold-junction temperature. If the cold junction of the thermo-couple is in a temperature of 100°F., the pointer can be set to this point on the scale, and the readings of the instrument will be correct.

Compensating Leads.—By the use of compensating leads, formed of the same material as the thermo-couple, the cold junction can be removed from the head of the thermo-couple to a point 10, 20 or 50 ft. distant from the furnace, where the temperature is reasonably constant. Where greater accuracy is desired, a common method is to drive a 2-in. pipe, with a pointed closed end, some 10 to 20 ft. into the ground, as shown in Fig. 128. The compensating leads are joined to the copper leads, and the junction forced down to the bottom of the pipe. The cold junction is now in the ground, beneath the building, at a depth at which the temperature is very constant, about 70°F., throughout the year. This method will usually control the cold-junction temperature within 5°F.

Where the greatest accuracy is desired a compensating box will overcome cold-junction errors entirely. It consists of a case enclosing a lamp and thermostat, which can be adjusted to maintain any desired temperature, from 50 to 150°F. The compensating leads enter the box and copper leads run from the compensating box to the instrument, so that the cold junction is within the box. Figure 129 shows a Brown compensating box.

If it is desired to maintain the cold junction at 100°: the thermostat is set at this point, and the lamp, being wired to the 110- or 220-volt lighting circuit, will light and heat the box until 100° is reached, when the thermostat will open the circuit and the light is extinguished. The box will now cool down to 98°, when the circuit is again closed, the lamp lights, the box heats up, and the operation is repeated.

BROWN AUTOMATIC SIGNALING PYROMETER

In large heat-treating plants it has been customary to maintain an operator at a central pyrometer, and by colored electric lights at the furnaces, signal whether the temperatures are correct or not. It is common practice to locate three lights above each furnace-red, white and green. The red light burns when the temperature is too low, the white light when the temperature is within certain limits—for example, 20°F. of the correct temperature—and the green light when the temperature is too high.

Instruments to operate the lights automatically have been devised and one made by Brown is shown in Fig. 130. The same form of instrument is used for this purpose to automatically control furnace temperatures, and the pointer is depressed at intervals of every 10 sec. on contacts corresponding to the red, white and green lights.

AN AUTOMATIC TEMPERATURE CONTROL PYROMETER

Automatic temperature control instruments are similar to the Brown indicating high resistance pyrometer with the exception that the pointer is depressed at intervals of every 10 sec. upon contact-making devices. No current passes through the pointer which simply depresses the upper contact device tipped with platinum, which in turn comes in contact with the lower contact device, platinum-tipped, and the circuit is completed through these two contacts. The current is very small, about 1/10 amp., as it is only necessary to operate the relay which in turn operates the switch or valve. A small motor is used to depress the pointer at regular intervals. The contact-making device is adjustable throughout the scale range of the instrument, and an index pointer indicates the point on the instrument at which the temperature is being controlled. The space between the two contacts on the high and low side, separated by insulating material, is equivalent to 1 per cent of the scale range. A control of temperature is therefore possible within 1 per cent of the total scale range. Figure 131 shows this attached to a small furnace.

PYROMETERS FOR MOLTEN METAL

Pyrometers for molten metal are connected to portable thermocouples as in Fig. 132. Usually the pyrometer is portable, as shown in this case, which is a Brown. Other methods of mounting for this kind of work arc shown in Figs. 133 and 134. The bent mountings are designed for molten metal, such as brass or copper and are supplied with either clay, graphite or carborundum tubes. Fifteen feet of connecting wire is usually supplied.

The angle mountings, Fig. 134, are recommended for baths such as lead or cyanide. The horizontal arm is usually about 14 in. long, and the whole mounting is easily taken apart making replacements very easy. Details of the thermo-couple shown in Fig. 132 are given in Fig. 135. This is a straight rod with a protector for the hand of the operator. The lag in such couples is less than one minute. These are Englehard mountings.

PROTECTORS FOR THERMO-COUPLES

Thermo-couples must be protected from the danger of mechanical injury. For this purpose tubes of various refractory materials are made to act as protectors. These in turn are usually protected by outside metal tubes. Pure wrought iron is largely used for this purpose as it scales and oxidizes very slowly. These tubes are usually made from 2 to 4 in. shorter than the inner tubes. In lead baths the iron tubes often have one end welded closed and are used in connection with an angle form of mounting.

Where it is necessary for protecting tubes to project a considerable distance into the furnace a tube made of nichrome is frequently used. This is a comparatively new alloy which stands high temperatures without bending. It is more costly than iron but also much more durable.

When used in portable work and for high temperatures, pure nickel tubes are sometimes used. There is also a special metal tube made for use in cyanide. This metal withstands the intense penetrating characteristics of cyanide. It lasts from six to ten months as against a few days for the iron tube.

The inner tubes of refractory materials, also vary according to the purposes for which they are to be used. They are as follows:

Marquardt mass tubes for temperatures up to 3,000°F., but they will not stand sudden changes in temperature, such as in contact with intermittent flames, without an extra outer covering of chamotte, fireclay or carborundum.

Fused silica tubes for continuous temperatures up to 1,800°F. and intermittently up to 2,400°F. The expansion at various temperatures is very small, which makes them of value for portable work. They also resist most acids.

Chamotte tubes are useful up to 2,800°F. and are mechanically strong. They have a small expansion and resist temperature changes well, which makes them good as outside protectors for more fragile tubes. They cannot be used in molten metals, or baths of any kind nor in gases of an alkaline nature. They are used mainly to protect a Marquardt mass or silica tube.

Carborundum tubes are also used as outside protection to other tubes. They stand sudden changes of temperature well and resist all gases except chlorine, above 1,750°F. Especially useful in protecting other tubes against molten aluminum, brass, copper and similar metals.

Clay tubes are sometimes used in large annealing furnaces where they are cemented into place, forming a sort of well for the insertion of the thermo-couple. They are also used with portable thermo-couples for obtaining the temperatures of molten iron and steel in ladles. Used in this way they are naturally short-lived, but seem the best for this purpose.

Corundite tubes are used as an outer protection for both the Marquardt mass and the silica tubes for kilns and for glass furnaces. Graphite tubes are also used in some cases for outer protections.

Calorized tubes are wrought-iron pipe treated with aluminum vapor which often doubles or even triples the life of the tube at high temperature.

These tubes come in different sizes and lengths depending on the uses for which they are intended. Heavy protecting outer tubes may be only 1 in. in inside diameter and as much as 3 in. outside diameter, while the inner tubes, such as the Marquardt mass and silica tubes are usually about ¾ in. outside and 3/8 in. inside diameter. The length varies from 12 to 48 in. in most cases.

Special terminal heads are provided, with brass binding posts for electrical connections, and with provisions for water cooling when necessary.

APPENDIX

TABLE 32.—Temperature Conversion Tables.

TABLE 33.—Comparison Between Degrees Centigrade and Degrees Fahrenheit.

TABLE 34.—Weight of Round, Octagon and Square Carbon Tool Steel per Foot.

TABLE 35.—Weight of Round Carbon Tool Steel 12 In. in Diameter and Larger, per Foot.

TABLE 36.—Decimal Equivalents of a foot.

TEMPERATURE CONVERSION TABLES

By ALBERT SAUVEUR

NOTE.—The numbers in bold face type refer to the temperature either in degrees Centigrade or Fahrenheit which it is desired to convert into the other scale. If converting from Fahrenheit degrees to Centigrade degrees the equivalent temperature will be found in the left column, while if converting from degrees Centigrade to degrees Fahrenheit, the answer will be found in the column on the right. These tables are a revision of those by Sauveur & Boylston, metallurgical engineers, Cambridge, Mass. Copyright, 1920.

Those using pyrometers will find this and the preceding conversion table of great convenience: