Chapter VII: Case-Hardening or Surface-Carburizing

Carburizing, commonly called case-hardening, is the art of producing a high-carbon surface, or case, upon a low carbon steel article. Wrenches, locomotive link motions, gun mechanisms, balls and ball races, automobile gears and many other devices are thereby given a high-carbon _case_ capable of assuming extreme hardness, while the interior body of metal, the _core_, remains soft and tough.

The simplest method is to heat the piece to be hardened to a bright red, dip it in cyanide of potassium (or cover it by sprinkling the cyanide over it), keep it hot until the melted cyanide covers it thoroughly, and quench in water. Carbon and nitrogen enter the outer skin of the steel and harden this skin but leave the center soft. The hard surface or "case" varies in thickness according to the size of the piece, the materials used and the length of time which the piece remains at the carburizing temperature. Cyanide case-hardening is used only where a light or thin skin is sufficient. It gives a thickness of about 0.002 in.

In some cases of cyanide carburizing, the piece is heated in cyanide to the desired temperature and then quenched. For a thicker case the steel is packed in carbon materials of various kinds such as burnt leather scraps, charcoal, granulated bone or some of the many carbonizing compounds.

Machined or forged steel parts are packed with case-hardening material in metal boxes and subjected to a red heat. Under such conditions, carbon is absorbed by the steel surfaces, and a carburized case is produced capable of responding to ordinary hardening and tempering operations, the core meanwhile retaining its original softness and toughness.

Such case-hardened parts are stronger, cheaper, and more serviceable than similar parts made of tool steel. The tough core resists breakage by shock. The hardened case resists wear from friction. The low cost of material, the ease of manufacture, and the lessened breakage in quenching all serve to promote cheap production.

For successful carburizing, the following points should be carefully observed:

The utmost care should be used in the selection of pots for carburizing; they should be as free as possible from both scaling and warping. These two requirements eliminate the cast iron pot, although many are used, thus leaving us to select from malleable castings, wrought iron, cast steel, and special alloys, such as nichrome or silchrome. If first cost is not important, it will prove cheaper in the end to use pots of some special alloy.

The pots should be standardized to suit the product. Pots should be made as small as possible in width, and space gained by increasing the height; for it takes about 1-1/2 hr. to heat the average small pot of 4 in. in width, between 3 and 4 hr. to heat to the center of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in. box; and the longer the time required to heat to the center, the more uneven the carburizing.

The work is packed in the box surrounded by materials which will give up carbon when heated. It must be packed so that each piece is separate from the others and does not touch the box, with a sufficient amount of carburizing material surrounding each. Figures 27 to 31 show the kind of boxes used and the way the work should be packed. Figure 31 shows a later type of box in which the edges can be easily luted. Figure 30 shows test wires broken periodically to determine the depth of case. Figure 28 shows the minimum clearance which should be used in packing and Fig. 29 the way in which the outer pieces receive the heat first and likewise take up the carbon before those in the center. This is why a slow, soaking heat is necessary in handling large quantities of work, so as to allow the heat and carbon to soak in equally.

While it has been claimed that iron below its critical temperature will absorb some carbon, Giolitti has shown that this absorption is very slow. In order to produce quick and intense carburization the iron should preferably be above its upper critical temperature or 1,600°F.,--therefore the carbon absorbed immediately goes into austenite, or solid solution. It is also certain that the higher the temperature the quicker will carbon be absorbed, and the deeper it will penetrate into the steel, that is, the deeper the "case." At Sheffield, England, where wrought iron is packed in charcoal and heated for days to convert it into "blister steel," the temperatures are from 1,750 to 1,830°F. Charcoal by itself carburizes slowly, consequently commercial compounds also contain certain "energizers" which give rapid penetration at lower temperatures.

The most important thing in carburizing is the human element. Most careful vigilance should be kept when packing and unpacking, and the operator should be instructed in the necessity for clean compound free from scale, moisture, fire clay, sand, floor sweepings, etc. From just such causes, many a good carburizer has been unjustly condemned. It is essential with most carburizers to use about 25 to 50 per cent of used material, in order to prevent undue shrinking during heating; therefore the necessity of properly screening used material and carefully inspecting it for foreign substances before it is used again. It is right here that the greatest carelessness is generally encountered.

Don't pack the work to be carburized too closely; leave at least 1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the top of pots, and for a 6-hr. run, have the pieces at least 1/2 in. apart. This gives the heat a chance to thoroughly permeate the pot, and the carburizing material a chance to shrink without allowing carburized pieces to touch and cause soft spots.

Good case-hardening pots and annealing tubes can be made from the desired size of wrought iron pipe. The ends are capped or welded, and a slot is cut in the side of the pot, equal to one quarter of its circumference, and about 7/8 of its length. Another piece of the same diameter pipe cut lengthwise into thirds forms a cover for this pot. We then have a cheap, substantial pot, non-warping, with a minimum tendency to scale, but the pot is difficult to seal tightly. This idea is especially adaptable when long, narrow pots are desired.

When pots are packed and the carburizer thoroughly tamped down, the covers of the pot are put on and sealed with fire clay which has a little salt mixed into it. The more perfect the seal the more we can get out of the carburizer. The rates of penetration depend on temperature and the presence of proper gas in the required volume. Any pressure we can cause will, of course, have a tendency to increase the rate of penetration.

If you have a wide furnace, do not load it full at one time. Put one-half your load in first, in the center of the furnace, and heat until pots show a low red, about 1,325 to 1,350°F. Then fill the furnace by putting the cold pots on the outside or, the section nearest the source of heat. This will give the work in the slowest portion of the furnace a chance to come to heat at the same time as the pots that are nearest the sources of heat.

To obtain an even heating of the pots and lessen their tendency to warp and scale, and to cause the contents of the furnace to heat up evenly, we should use a reducing fire and fill the heating chamber with flame. This can be accomplished by partially closing the waste gas vents and reducing slightly the amount of air used by the burners. A short flame will then be noticed issuing from the partially closed vents. Thus, while maintaining the temperature of the heating chamber, we will have a lower temperature in the combustion chamber, which will naturally increase its longevity.

Sometimes it is advisable to cool the work in the pots. This saves compound, and causes a more gradual diffusion of the carbon between the case and the core, and is very desirable condition, inasmuch as abrupt cases are inclined to chip out.

The most satisfactory steel to carburize contains between 0.10 and 0.20 per cent carbon, less than 0.35 per cent manganese, less than 0.04 per cent phosphorus and sulphur, and low silicon. But steel of this composition does not seem to satisfy our progressive engineers, and many alloy steels are now on the market, these, although more or less difficult to machine, give when carburized the various qualities demanded, such as a very hard case, very tough core, or very hard case and tough core. However, the additional elements also have a great effect both on the rate of penetration during the carburizing operation, and on the final treatment, consequently such alloy steels require very careful supervision during the entire heat treating operations.

RATE OF ABSORPTION

According to Guillet, the absorption of carbon is favored by those special elements which exist as double carbides in steel. For example, manganese exists as manganese carbide in combination with the iron carbide. The elements that favor the absorption of carbon are: manganese, tungsten, chromium and molybdenum those opposing it, nickel, silicon, and aluminum. Guillet has worked out the effect of the different elements on the rate of penetration in comparison with steel that absorbed carbon at a given temperature, at an average rate of 0.035 in. per hour.

His tables show that the following elements require an increased time of exposure to the carburizing material in order to obtain the same depth of penetration as with simple steel:

When steel contains Increased time of exposure
2.0 per cent nickel 28 per cent
7.0 per cent nickel 30 per cent
1.0 per cent titanium 12 per cent
2.0 per cent titanium 28 per cent
0.5 per cent silicon 50 per cent
1.0 per cent silicon 80 per cent
2.0 per cent silicon 122 per cent
5.0 per cent silicon No penetration
1.0 per cent aluminum 122 per cent
2.0 per cent aluminum 350 per cent

The following elements seem to assist the rate of penetration of carbon, and the carburizing time may therefore be reduced as follows:

When steel contains Decreased time of exposure
0.5 per cent manganese 18 per cent
1.0 per cent manganese 25 per cent
1.0 per cent chromium 10 per cent
2.0 per cent chromium 18 per cent
0.5 per cent tungsten 0
1.0 per cent tungsten 0
2.0 per cent tungsten 25 per cent
1.0 per cent molybdenum 0
2.0 per cent molybdenum 18 per cent

The temperature at which carburization is accomplished is a very important factor. Hence the necessity for a reliable pyrometer, located so as to give the temperature just below the tops of the pots. It must be remembered, however, that the pyrometer gives the temperature of only one spot, and is therefore only an aid to the operator, who must use his eyes for successful results.

The carbon content of the case generally is governed by the temperature of the carburization. It generally proves advisable to have the case contain between 0.90 per cent and 1.10 carbon; more carbon than this gives rise to excess free cementite or carbide of iron, which is detrimental, causing the case to be brittle and apt to chip.

T. G. Selleck gives a very useful table of temperatures and the relative carbon contents of the case of steels carburized between 4 and 6 hrs. using a good charcoal carburizer. This data is as follows:

TABLE 15.--CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES

At 1,500°F., the surface carbon content will be 0.90 per cent
At 1,600°F., the surface carbon content will be 1.00 per cent
At 1,650°F., the surface carbon content will be 1.10 per cent
At 1,700°F., the surface carbon content will be 1.25 per cent
At 1,750°F., the surface carbon content will be 1.40 per cent
At 1,800°F., the surface carbon content will be 1.75 per cent

To this very valuable table, it seems best to add the following data, which we have used for a number of years. We do not know the name of its author, but it has proved very valuable, and seems to complete the above information. The table is self-explanatory, giving depth of penetration of the carbon of the case at different temperatures for different lengths of time:

---------------------------------------------------------
| Temperature
Penetration |-----------------------------
| 1,550 | 1,650 | 1,800
---------------------------|---------|---------|---------
Penetration after 1/2 hr. | 0.008 | 0.012 | 0.030
Penetration after 1 hr. | 0.018 | 0.026 | 0.045
Penetration after 2 hr. | 0.035 | 0.048 | 0.060
Penetration after 3 hr. | 0.045 | 0.055 | 0.075
Penetration after 4 hr. | 0.052 | 0.061 | 0.092
Penetration after 6 hr. | 0.056 | 0.075 | 0.110
Penetration after 8 hr. | 0.062 | 0.083 | 0.130
---------------------------------------------------------

From the tables given, we may calculate with a fair degree of certainty the amount of carbon in the case, and its penetration. These figures vary widely with different carburizers, and as pointed out immediately above, with different alloy steels.

CARBURIZING MATERIAL

The simplest carburizing substance is charcoal. It is also the slowest, but is often used mixed with something that will evolve large volumes of carbon monoxide or hydrocarbon gas on being heated. A great variety of materials is used, a few of them being charcoal (both wood and bone), charred leather, crushed bone, horn, mixtures of charcoal and barium carbonate, coke and heavy oils, coke treated with alkaline carbonates, peat, charcoal mixed with common salt, saltpeter, resin, flour, potassium bichromate, vegetable fibre, limestone, various seed husks, etc. In general, it is well to avoid complex mixtures.

H. L. Heathcote, on analyzing seventeen different carburizers, found that they contained the following ingredients:

Per cent
Moisture 2.68 to 26.17
Oil 0.17 to 20.76
Carbon (organic) 6.70 to 54.19
Calcium phosphate 0.32 to 74.75
Calcium carbonate 1.20 to 11.57
Barium carbonate nil to 42.00
Zinc oxide nil to 14.50
Silica nil to 8.14
Sulphates (SO3) trace to 3.45
Sodium chloride nil to 7.88
Sodium carbonate nil to 40.00
Sulphides (S) nil to 2.80

Carburizing mixtures, though bought by weight, are used by volume, and the weight per cubic foot is a big factor in making a selection. A good mixture should be porous, so that the evolved gases, which should be generated at the proper temperature, may move freely around the steel objects being carburized; should be a good conductor of heat; should possess minimum shrinkage when used; and should be capable of being tamped down.

Many "secret mixtures" are sold, falsely claimed to be able to convert inferior metal into crucible tool steel grade. They are generally nothing more than mixtures of carbonaceous and cyanogen compounds possessing the well-known carburizing properties of those substances.

QUENCHING

It is considered good practice to quench alloy steels from the pot, especially if the case is of any appreciable depth. The texture of carbon steel will be weakened by the prolonged high heat of carburizing, so that if we need a tough core, we must reheat it above its critical range, which is about 1,600°F. for soft steel, but lower for manganese and nickel steels. Quenching is done in either water, oil, or air, depending upon the results desired. The steel is then very carefully reheated to refine the case, the temperature varying from 1,350 to 1,450°F., depending on whether the material is an alloy or a simple steel, and quenched in either water or oil.

There are many possibilities yet to be developed with the carburizing of alloy steels, which can produce a very tough, tenacious austenitic case which becomes hard on cooling in air, and still retains a soft, pearlitic core. An austenitic case is not necessarily file hard, but has a very great resistance to abrasive wear.

The more carbon a steel has to begin with the more slowly will it absorb carbon and the lower the temperature required. Low-carbon steel of from 15 to 20 points is generally used and the carbon brought up to 80 or 85 points. Tool steels may be carbonized as high as 250 points.

In addition to the carburizing materials given, a mixture of 40 per cent of barium carbonate and 60 per cent charcoal gives much faster penetration than charcoal, bone or leather. The penetration of this mixture on ordinary low-carbon steel is shown in Fig. 32, over a range of from 2 to 12 hr.

EFFECT OF DIFFERENT CARBURIZING MATERIAL

Each of these different packing materials has a different effect upon the work in which it is heated. Charcoal by itself will give a rather light case. Mixed with raw bone it will carburize more rapidly, and still more so if mixed with burnt bone. Raw bone and burnt bone, as may be inferred, are both quicker carbonizers than charcoal, but raw bone must never be used where the breakage of hardened edges is to be avoided, as it contains phosphorus and tends to make the piece brittle. Charred leather mixed with charcoal is a still faster material, and horns and hoofs exceed even this in speed; but these two compounds are restricted by their cost to use with high-grade articles, usually of tool or high-carbon steel, that are to be hardened locally--that is, "pack-hardened." Cyanide of potassium or prussiate of potash are also included in the list of carbonizing materials; but outside of carburizing by dipping into melted baths of this material, their use is largely confined to local hardening of small surfaces, such as holes in dies and the like.

Dr. Federico Giolitti has proven that when carbonizing with charcoal, or charcoal plus barium carbonate, the active agent which introduces carbon into the steel is a gas, carbon monoxide (CO), derived by combustion of the charcoal in the air trapped in the box, or by decomposition of the carbonate. This gas diffuses in and out of the hot steel, transporting carbon from the charcoal to the outer portions of the metal:

If energizers like tar, peat, and vegetable fiber are used, they produce hydrocarbon gases on being heated--gases principally composed of hydrogen and carbon. These gases are unstable in the presence of hot iron: it seems to decompose them and sooty carbon is deposited on the surface of the metal. This diffuses into the metal a little, but it acts principally by being a ready source of carbon, highly active and waiting to be carried into the metal by the carbon monoxide--which as before, is the principal transfer agent.

Animal refuse when used to speed up the action of clean charcoal acts somewhat in the same manner, but in addition the gases given off by the hot substance contain nitrogen compounds. Nitrogen and cyanides (compounds of carbon and nitrogen) have long been known to give a very hard thin case very rapidly. It has been discovered only recently that this is due to the steel absorbing nitrogen as well as carbon, and that nitrogen hardens steel and makes it brittle just like carbon does. In fact it is very difficult to distinguish between these two hardening agents when examining a carburized steel under the microscope.

One of the advantages of hardening by carburizing is the fact that you can arrange to leave part of the work soft and thus retain the toughness and strength of the original material. Figures 33 to 37 show ways of doing this. The inside of the cup in Fig. 34 is locally hardened, as illustrated in Fig. 34, "spent" or used bone being packed around the surfaces that are to be left soft, while cyanide of potassium is put around those which are desired hard. The threads of the nut in Fig. 35 are kept soft by carburizing the nut while upon a stud. The profile gage, Fig. 36, is made of high-carbon steel and is hardened on the inside by packing with charred leather, but kept soft on the outside by surrounding it with fireclay. The rivet stud shown in Fig. 37 is carburized while of its full diameter and then turned down to the size of the rivet end, thus cutting away the carburized surface.

After packing the work carefully in the boxes the lids are sealed or luted with fireclay to keep out any gases from the fire. The size of box should be proportioned to the work. The box should not be too large especially for light work that is run on a short heat. If it can be just large enough to allow the proper amount of material around it, the work is apt to be more satisfactory in every way.

Pieces of this kind are of course not quenched and hardened in the carburizing heat, but are left in the box to cool, just as in box annealing, being reheated and quenched as a second operation. In fact, this is a good scheme to use for the majority of carburizing work of small and moderate size. Material is on the market with which one side of the steel can be treated; or copper-plating one side of it will answer the same purpose and prevent that side becoming carburized.

QUENCHING THE WORK

In some operations case-hardened work is quenched from the box by dumping the whole contents into the quenching tank. It is common practice to leave a sieve or wire basket to catch the work, allowing the carburizing material to fall to the bottom of the tank where it can be recovered later and used again as a part of a new mixture. For best results, however, the steel is allowed to cool down slowly in the box after which it is removed and hardened by heating and quenching the same as carbon steel of the same grade. It has absorbed sufficient carbon so that, in the outer portions at least, it is a high-carbon steel.

THE QUENCHING TANK

The quenching tank is an important feature of apparatus in case-hardening--possibly more so than in ordinary tempering. One reason for this is because of the large quantities of pieces usually dumped into the tank at a time. One cannot take time to separate the articles themselves from the case-hardening mixture, and the whole content of the box is droped into the bath in short order, as exposure to air of the heated work is fatal to results. Unless it is split up, it is likely to go to the bottom as a solid mass, in which case very few of the pieces are properly hardened.

A combination cooling tank is shown in Fig. 38. Water inlet and outlet pipes are shown and also a drain plug that enables the tank to be emptied when it is desired to clean out the spent carburizing material from the bottom. A wire-bottomed tray, framed with angle iron, is arranged to slide into this tank from the top and rests upon angle irons screwed to the tank sides. Its function is to catch the pieces and prevent them from settling to the tank bottom, and it also makes it easy to remove a batch of work. A bottomless box of sheet steel is shown at _C_. This fits into the wire-bottomed tray and has a number of rods or wires running across it, their purpose being to break up the mass of material as it comes from the carbonizing box.

Below the wire-bottomed tray is a perforated cross-pipe that is connected with a compressed-air line. This is used when case-hardening for colors. The shop that has no air compressor may rig up a satisfactory equivalent in the shape of a low-pressure hand-operated air pump and a receiver tank, for it is not necessary to use high-pressure air for this purpose. When colors are desired on case-hardened work, the treatment in quenching is exactly the same as that previously described except that air is pumped through this pipe and keeps the water agitated. The addition of a slight amount of powdered cyanide of potassium to the packing material used for carburizing will produce stronger colors, and where this is the sole object, it is best to maintain the box at a dull-red heat.

The old way of case-hardening was to dump the contents of the box at the end of the carburizing heat. Later study in the structure of steel thus treated has caused a change in this procedure, the use of automobiles and alloy steels probably hastening this result. The diagrams reproduced in Fig. 39 show why the heat treatment of case-hardened work is necessary. Starting at _A_ with a close-grained and tough stock, such as ordinary machinery steel containing from 15 to 20 points of carbon, if such work is quenched on a carbonizing heat the result will be as shown at _B_. This gives a core that is coarse-grained and brittle and an outer case that is fine-grained and hard, but is likely to flake off, owing to the great difference in structure between it and the core. Reheating this work beyond the critical temperature of the core refines this core, closes the grain and makes it tough, but leaves the case very brittle; in fact, more so than it was before.

REFINING THE GRAIN

This is remedied by reheating the piece to a temperature slightly above the critical temperature of the case, this temperature corresponding ordinarily to that of steel having a carbon content of 85 points, When this is again quenched, the temperature, which has not been high enough to disturb the refined core, will have closed the grain of the case and toughened it. So, instead of but one heat and one quenching for this class of work, we have three of each, although it is quite possible and often profitable to omit the quenching after carburizing and allow the piece or pieces and the case-carburizing box to cool together, as in annealing. Sometimes another heat treatment is added to the foregoing, for the purpose of letting down the hardness of the case and giving it additional toughness by heating to a temperature between 300° and 500°. Usually this is done in an oil bath. After this the piece is allowed to cool.

It is possible to harden the surface of tool steel extremely hard and yet leave its inner core soft and tough for strength, by a process similar to case-hardening and known as "pack-hardening." It consists in using tool steel of carbon contents ranging from 60 to 80 points, packing this in a box with charred leather mixed with wood charcoal and heating at a low-red heat for 2 or 3 hr., thus raising the carbon content of the exterior of the piece. The article when quenched in an oil bath will have an extremely hard exterior and tough core. It is a good scheme for tools that must be hard and yet strong enough to stand abuse. Raw bone is never used as a packing for this class of work, as it makes the cutting edges brittle.

CASE-HARDENING TREATMENTS FOR VARIOUS STEELS

Plain water, salt water and linseed oil are the three most common quenching materials for case-hardening. Water is used for ordinary work, salt water for work which must be extremely hard on the surface, and oil for work in which toughness is the main consideration. The higher the carbon of the case, the less sudden need the quenching action take hold of the piece; in fact, experience in case-hardening work gives a great many combinations of quenching baths of these three materials, depending on their temperatures. Thin work, highly carbonized, which would fly to pieces under the slightest blow if quenched in water or brine, is made strong and tough by properly quenching in slightly heated oil. It is impossible to give any rules for the temperature of this work, so much depending on the size and design of the piece; but it is not a difficult matter to try three or four pieces by different methods and determine what is needed for best results.

The alloy steels are all susceptible of case-hardening treatment; in fact, this is one of the most important heat treatments for such steels in the automobile industry. Nickel steel carburizes more slowly than common steel, the nickel seeming to have the effect of slowing down the rate of penetration. There is no cloud without its silver lining, however, and to offset this retardation, a single treatment is often sufficient for nickel steel; for the core is not coarsened as much as low-carbon machinery steel and thus ordinary work may be quenched on the carburizing heat. Steel containing from 3 to 3.5 per cent of nickel is carburized between 1,650 and 1,750°F. Nickel steel containing less than 25 points of carbon, with this same percentage of nickel, may be slightly hardened by cooling in air instead of quenching.

Chrome-nickel steel may be case-hardened similarly to the method just described for nickel steel, but double treatment gives better results and is used for high-grade work. The carburizing temperature is the same, between 1,650 and 1,750°F., the second treatment consisting of reheating to 1,400° and then quenching in boiling salt water, which gives a hard surface and at the same time prevents distortion of the piece. The core of chrome-nickel case-hardened steel, like that of nickel steel, is not coarsened excessively by the first heat treatment, and therefore a single heating and quenching will suffice.

CARBURIZING BY GAS

The process of carburizing by gas, briefly mentioned on page 88, consists of having a slowly revolving, properly heated, cylindrical retort into which illuminating gas (a mixture of various hydrocarbons) is continuously injected under pressure. The spent gases are vented to insure the greatest speed in carbonizing. The work is constantly and uniformly exposed to a clean carbonizing atmosphere instead of partially spent carbonaceous solids which may give off very complex compounds of phosphorus, sulphur, carbon and nitrogen.

Originally this process was thought to require a gas generator but it has been discovered that city gas works all right. The gas consists of vapors derived from petroleum or bituminous coal. Sometimes the gas supply is diluted by air, to reduce the speed of carburization and increase the depth.

PREVENTING CARBURIZING BY COPPER-PLATING

Copper-plating has been found effective and must have a thickness of 0.0005 in. Less than this does not give a continuous coating. The plating bath used has a temperature of 170°F. A voltage of 4.1 is to be maintained across the terminals. Regions which are to be hardened can be kept free from copper by coating them with paraffin before they enter the plating tank. The operation is as follows:

Operation
No. Contents of bath Purpose
1 Gasoline To remove grease
2 Sawdust To dry
3 Warm potassium hydroxide solution To remove grease and dirt
4 Warm water To wash
5 Warm sulphuric acid solution To acid clean
6 Warm water To wash
7 Cold water Additional wash
8 Cold potassium cyanide solution Cleanser
9 Cold water To wash
10 Electric cleaner, warm sodium Cleanser to give good
hydroxide case-iron anode plating surface
11 Copper plating bath of copper Plating bath
sulphate and potassium cyanide
solution warm

There are also other methods of preventing case-hardening, one being to paint the surface with a special compound prepared for this purpose. In some cases a coating of plastic asbestos is used while in others thin sheet asbestos is wired around the part to be kept soft.

PREPARING PARTS FOR LOCAL CASE-HARDENING

At the works of the Dayton Engineering Laboratories Company, Dayton, Ohio, they have a large quantity of small shafts, Fig. 40, that are to be case-hardened at _A_ while the ends _B_ and _C_ are to be left soft. Formerly, the part _A_ was brush-coated with melted paraffin but, as there were many shafts, this was tedious and great care was necessary to avoid getting paraffin where it was not wanted.

To insure uniform coating the device shown in Fig. 41 was made. Melted paraffin is poured in the well _A_ and kept liquid by setting the device on a hot plate, the paraffin being kept high enough to touch the bottoms of the rollers. The shaft to be coated is laid between the rollers with one end against the gage _B_, when a turn or two of the crank _C_ will cause it to be evenly coated.

THE PENETRATION OF CARBON

Carburized mild steel is used to a great extent in the manufacture of automobile and other parts which are likely to be subjected to rough usage. The strength and ability to withstand hard knocks depend to a very considerable degree on the thoroughness with which the carburizing process is conducted.

Many automobile manufacturers have at one time or another passed through a period of unfortunate breakages, or have found that for a certain period the parts turned out of their hardening shops were not sufficiently hard to enable the rubbing surfaces to stand up against the pressure to which they were subjected.

So many factors govern the success of hardening that often this succession of bad work has been actually overcome without those interested realizing what was the weak point in their system of treatment. As the question is one that can create a bad reputation for the product of any firm it is well to study the influential factors minutely.