The latest shields of this type are all made double, as ill.u.s.trated, with an inner sheet of metal an inch or two inside of the front.
In the ill.u.s.tration, _A_, Fig. 102, this inner sheet is smaller, but some are now built the same size as the front and bolted to it with pipe s.p.a.cers between. The advantage of the double sheet is that the inner one bears the brunt of the flame, and, if needs be, burns up before the outer; while, if due to a heavy fire it should be heated red at any point, the outer sheet will still be much cooler and act as an additional shield to the furnace man.
HEAVY FORGING PRACTICE.--In heavy forging practice where the metal is being worked at a welding heat, the amount of flame that will issue from an open-front furnace is so great that a plain, sheet-steel front will neither afford sufficient protection nor stand up in service. For such a place a water-cooled front is often used. The general type of this front is ill.u.s.trated in Fig. 103, and appears to have found considerable favor, for numbers of its kind are scattered throughout the country.
In this case the shield is placed at a slight angle from the vertical, and along the top edge is a water pipe with a row of small holes through which sprays of water are thrown against it. This water runs down in a thin sheet over the shield, cooling it, and is collected in a trough connected with a run-off pipe at the bottom. The lower blast-pipe arrangement is similar to the one first described.
There are several serious objections to this form of shield that should lead to its replacement by a better type; the first is that with a very hot fire, portions in the center may become so rapidly heated that the steam generated will part the sheet of water and cause it to flow from that point in an inverted V, and that section will then quickly become red hot. Another feature is that after the water and fire are shut down for the night the heat of the furnace can be great enough to cause serious warping of the surface of the shield so that the water will no longer cover it in a thin, uniform sheet.
After rigging up a big furnace with a shield of this type several years ago, its most serious object was found in the increase of the water bill of the plant. This was already of large proportions, but it had suddenly jumped to the extent of several hundred dollars.
Investigation soon disclosed the fact that this water shield was one of the main causes of the added cost of water. A little estimating of the amount of water that can flow through a 1/2-in. pipe under 30-lb. pressure, in the course of a day, will show that this amount at 10 cts. per 1,000 gal., can count up rather rapidly.
Figure 103 is a section through a portion of the furnace front and shield showing all of the princ.i.p.al parts. This shield consists essentially of a very thin tank, about 2-1/2 in. between walls, and filled with water. Like other shields it is fitted with an adjustment, that it may be raised and lowered as the work demands.
The tank having an open top, the water as it absorbs heat from the flame will simply boil away in steam; and only a small amount will have to be added to make up for that which has evaporated. The water-feed pipe shown at _F_ ends a short distance above the top of the tank so that just how much water is running in may readily be seen.
An overflow pipe is provided at _O_ which aids in maintaining the water at the proper height, as a sufficient quant.i.ty can always be permitted to run in, to avoid any possibility of the shield ever boiling dry; at the same time the small excess can run off without danger of an overflow. The shield ill.u.s.trated in Fig. 104 has been in constant use for over two years, giving greater satisfaction than any other of which the writer has known. It might also be noted that this shield was made with riveted joints, the shop not having a gas-welding outfit. To f.l.a.n.g.e over the edges and then weld them with an acetylene torch would be a far more economical procedure, and would also insure a tight and permanent joint.
The water-cooled front shown in Fig. 105 is an absurd effort to accomplish the design of a furnace that will provide cool working conditions. This front was on a bolt-heating furnace using hard coal for fuel; and it may be seen that it takes the place of all of the brickwork that should be on that side. Had this been nothing more than a very narrow water-cooled frame, with brickwork below and supporting bricks above, put in like the tuyeres in a foundry cupola, the case would have been somewhat different, for then it would have absorbed a smaller proportion of the heat.
A blacksmith who knows how a piece of cold iron laid in a small welding furnace momentarily lowers the temperature, will appreciate the enormous amount of extra heat that must be maintained in the central portion of this furnace to make up for the constant chilling effect of the cold wall. Moreover, since there would have been serious trouble had steam generated in this front, a steady stream of water had to be run through it constantly to insure against an approach to the boiling point. This is ill.u.s.trated because of its absurdity, and as a warning of something to avoid.
Water-cooled, tuyere openings, as mentioned above, which support brick side-walls of the furnace, have proved successful for coal furnaces used for forging machine and drop-hammer heating, since they permit a great amount of work to be handled through their openings without wearing away as would a brick arch. Great care should be exercised properly to design them so that a minimum amount of the cold tuyere will be in contact with the interior of the furnace, and all interior portions possible should be covered by the bricks. However, a discussion of these points will hardly come in the flame-shield cla.s.s, although they can be made to do a great deal toward relieving the excessive heat to be borne by the furnace worker.
f.l.a.n.g.e SHIELDS FOR FURNACES.--Such portable flame shields as the one ill.u.s.trated in Fig. 106 may prove serviceable before furnaces required for plate work, where the doors are often only opened for a moment at a time. This shield can be placed far enough in front of the furnace, that it will be possible to work under it or around it, in removing bulky work from the furnace, and yet it will afford the furnace tender some relief from the excessive glare that will come out the wide-opened door. To have this shield of light weight so that it may be readily pushed aside when not wanted, the frame may be made up of pipe and fittings, and a piece of thin sheet steel fastened in the panel by rings about the frame.
About the most disagreeable task in a heat-treating shop is the removal of the pots from the case-hardening furnaces; these must be handled at a bright red heat in order that their contents may be dumped into the quenching tank with a minimum-time contact with the air, and before they have cooled sufficiently to require reheating.
Facing the heat before the large open doors of the majority of these furnaces, in a man-killing task even when the weather is moderately cool. The boxes soon become more or less distorted, and then even the best of lifting devices will not remove a hot pot without several minutes labor in front of the doors.
In Fig. 107 is shown a method of arranging a shield on one type of charging and removing truck. This shield cannot afford more than a partial protection to the body of the furnace tender, because he must be able to see around it, and in some cases even push it partly through the door of the furnace, but even small as it is it may still afford some welcome protection. The great advantage in this case of having the shield on the truck instead of stationary in front of the furnace, is that it still affords protection as long as the hot pot is being handled through the shop on its way to the quenching tank.
It might be interesting to many engaged in the heat-treating or case hardening of steel parts, to make a special note of the design of the truck that is ill.u.s.trated in connection with the shield; the general form is shown although the actual details for the construction of such a truck are lacking; these being simple, may be readily worked out by anyone wishing to build one. This is considered to be one of the quickest and easiest operated devices for the removal of this cla.s.s of work from the furnace. To be sure it may only be used where the floor of the furnace has been built level with the floor of the room, but many of the modern furnaces of this cla.s.s are so designed.
The pack-hardening pots are cast with legs, from two to three inches high, to permit the circulation of the hot gases, and so heat more quickly. Between these legs and under the body of the pot, the two forward p.r.o.ngs of the truck are pushed, tilting the outer handle to make these p.r.o.ngs as low as possible. The handle is then lowered and, as it has a good leverage, the pot is easily raised from the floor, and the truck and its load rolled out.
HEATING OF MANGANESE STEEL.--Another form of heat-treating furnace is that which is used for the heating of manganese and other alloy steels, which after having been brought to the proper heat are drawn from the furnace into an immediate quenching tank. With manganese steel in particular, the parts are so fragile and easily damaged while hot that it is frequent practice to have a sloping platform immediately in front of the furnace door down which the castings may slide into a tank below the floor level. Such a furnace with a quenching tank in front of its door is shown in Fig. 108.
These tanks are covered with plates while charging the furnace and the cold castings are placed in a moderately cool furnace.
Since some of these steels must not be charged into a furnace where the heat is extreme but should be brought up to their final heat gradually, there is little discomfort during the charging process.
When quenching, however, from a temperature of 1,800 to 1,900, it is extremely unpleasant in front of the doors. The swinging shield is here adapted to give protection for this work. As will be noted it is hung a sufficient distance in front of the doors, that it may not interfere with the castings as they come from the furnace, and slide down into the tank.
To facilitate the work, and avoid the necessity of working with the bars outside the edges of the shield, the slot-like hole is cut in the center of the shield, and through this the bars or rakes for dragging out the castings are easily inserted and manipulated.
The advantage of such a swinging shield is that it may be readily moved from side to side, or forward and back as occasion requires.
FURNACE DATA
In order to give definite information concerning furnaces, fuels etc., the following data is quoted from a paper by Seth A. Moulton and W. H. Lyman before the Steel Heat Treaters Society in September, 1920.
This considers a factory producing 30,000 lb. of automobile gears per 24 hr. The transmission gears will be of high-carbon steel and the differential of low-carbon steel, carburized. The heat-treating equipment required is:
1. Annealing furnaces 1,400 to 1,600F.
2. Carburizing furnaces 1,700 to 1,800F.
3. Hardening furnaces 1,450 to 1,550F.
4. Drawing furnaces 350 to 950F.
All of the forging blanks are annealed before machining, about three-quarters of the machined gears and parts are carburized, all the carburized gears are given a double treatment for core and case, all gears and parts are hardened and all parts are drawn.
The possible sources of heat supply and their values are as follows:--
1. Oil 140,000 B.t.u. per gallon 2. Natural gas 1,100 B.t.u. per cubic foot 3. City gas 650 B.t.u. per cubic foot 4. Water gas 300 B.t.u. per cubic foot 5. Producer gas 170 B.t.u. per cubic foot 6. Coal 12,000 B.t.u. per pound 7. Electric current 3,412 B.t.u. per kilowatt-hour
For the heat treatment specified only comparatively low temperatures are required. No difficulty will be experienced in attaining the desired maximum temperature of 1,800F. with any of the heating medium above enumerated; but it should be noted that the producer gas with a B.t.u. content of 170 per cubic foot and the electric current would require _specially_ designed furnaces to obtain higher temperatures than 1800F.
TABLE 28.--COMPARATTVE OPERATING COSTS
a.s.suming Cost of oil- and gas-fired furnaces installed as $100.00 per square foot of hearth Cost of coal-fired furnace installed as 150.00 per square foot of hearth Cost of electric furnace 100 kw.
capacity installed as 90.00 per kilowatt Cost of electric furnace 150 kw.
capacity installed as 70.00 per kilowatt
Output 3,000 lb. charge, 8 hr. heat carburizing, 2 hr. heating only. Annual service 7,200 hr. Fixed charges including interest, depreciation, taxes, insurance and maintenance 15 per cent. Extra operating labor for coal-fired furnace 60 cts. per hour, one man four furnaces.
COST OF VARIOUS TYPES OF FURNACES ------------------------------------------------------------------------------- | Cla.s.s fuel | Fuel per | Unit fuel|Installation|Efficiency| Fixed |Cost per | | charge | cost | cost | per cent |charges| charge -|------------|------------|----------|------------|----------|-------|-------- | 1 | 2 | 3 | 4 | 5 | 6 | 7 -|------------|------------|----------|------------|----------|-------|-------- Carburizing -|------------|------------|----------|------------|----------|-------|-------- 1|Oil | 52.0 gal. |$0.15 gal.| $2,400.00 | 12.6 | $.40 | $8.20 2|Natural gas | 4.4 M | 0.50 M | 2,400.00 | 18.8 | 0.40 | 2.60 3|City gas | 8.3 M | 0.80 M | 2,400.00 | 17.0 | 0.40 | 7.04 4|Water gas | 18.7 M | 0.40 | 2,400.00 | 16.4 | 0.40 | 7.88 5|Producer gas| 37.3 M | 0.10 M | 2,400.00 | 14.5 | 0.40 | 4.13 6|Coal |814.0 lb. | 6.00 ton | 3,600.00 | 9.4 | 0.60 | 3.98 7|Electricity |500.0 kw-hr.| 0.015 kw.| 9,000.00 | 53.0 | 1.50 | 9.00 -|------------|------------|----------|------------|----------|-------|-------- Heating -|------------|------------|----------|------------|----------|-------|-------- 1|Oil | 30.8 gal. | 0.15 gal.| 2,400.00 | 21.4 | 0.10 | 4.72 2|Natural gas | 2.61 M | 0.50 M | 2,400.00 | 32.0 | 0.10 | 1.40 3|City gas | 4.9 M | 0.80 M | 2,400.00 | 28.8 | 0.10 | 4.02 4|Water gas | 11.1 M | 0.40 M | 2,400.00 | 27.6 | 0.10 | 4.54 5|Producer gas| 22.1 M | 0.10 M | 2,400.00 | 24.6 | 0.10 | 2.31 6|Coal |348.0 lb. | 6.00 ton | 3,600.00 | 22.0 | 0.15 | 1.38 7|Electricity |329.0 kw-hr.| 0.015 kw.| 10,500.00 | 81.75 | 0.44 | 5.38 -------------------------------------------------------------------------------
This shows but two of the operations and for a single furnace.
The total costs for all operations on the 30,000 lb. of gears per 24 hr. is shown in Table 29.
TABLE 29.--COMPARATIVE ANNUAL PRODUCTION COSTS FOR 30,000 POUNDS OUTPUT IN 24 HOURS
---------------------------------------------------------- No. | Equipment | Installation | | cost -----|-------------------------------------|-------------- 1 | 2 | 3 | | I | Oil | $179,000.00 II | Oil and electric | 213,000.00 III | Natural gas | 117,000.00 IV | (A) Natural gas containing furnaces | 120,000.00 V | Natural gas and electric | 181,000.00 VI | City gas | 122,000.00 VII | City gas and electric | 182,000.00 VIII| Water gas | 214,000.00 IX | Water gas and electric | 238,000.00 X | Producer gas | 246,000.00 XI | Producer gas and electric | 255,000.00 XII | Coal and electric | 194,000.00 XIII| Electric | 257,000.00 ----------------------------------------------------------
--------------------------------------------------------------------- | Annual operating expenses | | Cost No. |----------------------------------------| Total | per lb.
| Fixed | Heat | Labor | | metal, | charges | | | | cents -----|------------|-------------|-------------|-------------|-------- 1 | 4 | 5 | 6 | 7 | 8 | | | | | I | $26,850.00 | $156,000.00 | $105,000.00 | $287,850.00 | $3.19 II | 31,950.00 | 142,770.00 | 97,000.00 | 271,720.00 | 3.02 III | 17,550.00 | 44,250.00 | 97,000.00 | 158,800.00 | 1.78 IV | 18,000.00 | 41,000.00 | 94,000.00 | 153,000.00 | 1.70 V | 27,150.00 | 73,820.00 | 90,000.00 | 190,970.00 | 2.13 VI | 18,300.00 | 123,200.00 | 94,000.00 | 235,500.00 | 2.62 VII | 27,300.00 | 128,820.00 | 90,000.00 | 246,020.00 | 2.74 VIII| 18,600.00 | 104,000.00 | 94,000.00 | 216,600.00 | 2.41 IX | 27,450.00 | 117,420.00 | 90,000.00 | 234,870.00 | 2.62 X | 18,900.00 | 69,300.00 | 90,000.00 | 178,200.00 | 1.98 XI | 27,750.00 | 92,520.00 | 90,000.00 | 210,270.00 | 2.34 XII | 29,100.00 | 87,220.00 | 90,000.00 | 206,320.00 | 2.30 XIII| 38,550.00 | 135,000.00 | 84,000.00 | 257,550.00 | 2.86 ---------------------------------------------------------------------
NOTE.--Producer plant fixed charges are included in the cost of gas and are charged as "heat" in column 5, so they are omitted from column 4.
CHAPTER XII
PYROMETRY AND PYROMETERS
A knowledge of the fundamental principles of pyrometry, or the measurement of temperatures, is quite necessary for one engaged in the heat treatment of steel. It is only by careful measurement and control of the heating of steel that the full benefit of a heat-treating operation is secured.
Before the advent of the thermo-couple, methods of temperature measurement were very crude. The blacksmith depended on his eyes to tell him when the proper temperature was reached, and of course the "color" appeared different on light or dark days. "Cherry"
to one man was "orange" to another, and it was therefore almost impossible to formulate any treatment which could be applied by several men to secure the same results.
One of the early methods of measuring temperatures was the "iron ball" method. In this method, an iron ball, to which a wire was attached, was placed in the furnace and when it had reached the temperature of the furnace, it was quickly removed by means of the wire, and suspended in a can containing a known quant.i.ty of water; the volume of water being such that the heat would not cause it to boil. The rise in temperature of the water was measured by a thermometer, and, knowing the heat capacity of the iron ball and that of the water, the temperature of the ball, and therefore the furnace, could be calculated. Usually a set of tables was prepared to simplify the calculations. The iron ball, however, scaled, and changed in weight with repeated use, making the determinations less and less accurate. A copper ball was often used to decrease this change, but even that was subject to error. This method is still sometimes used, but for uniform results, a platinum ball, which will not scale or change in weight, is necessary, and the cost of this ball, together with the slowness of the method, have rendered the practice obsolete, especially in view of modern developments in accurate pyrometry.
PYROMETERS