Heat Treatment

The subject of heat treatment could fill a book in itself if explained by an expert on the subject, but since I am not an expert, it won't take many pages to put down what little I do know about the subject. Even though there is probably little point in doing so here, I will attempt first of all to give a brief description of what takes place during the heat treatment of carbon steel.

In carbon steel that has been fully annealed, we would normally find two components apart from Impurities such as phosphorous, sulpher, and others. These components are a chemical compound, iron carbide, in a form metallurgically known as cementite, and the element iron in a form metallurgically known as ferrite. Cementite is made up of 6.67 percent carbon and 93.33 percent iron. A certain proportion of these two components will be present as a mechanical mixture. This mixture, the amount depending on the car bon content of the steel, consists of alternate layers or bands of ferrite and cementite. When examined under a microscope, it frequently resembles mother of pearl and, therefore, has been named pearlite. Pearlite contains some 0.85 percent carbon and 99.15 percent iron, not counting impurities. A fully annealed steel containing at least 0.85 percent carbon would consist entirely of pearlite. Such a steel is known as eutectoid steel.

Steel having a carbon content above 0.85 percent (called hypereutectoid steel) has a greater amount of cementite than is required to mix with the ferrite to form pearlite, so both cementite and pearlite are present in the fully annealed state.

When annealed carbon steel is heated above a lower critical point, a temperature in the range of 1335 to 1355 degrees Fahrenheit depending on the carbon content, the alternate layers or bands of ferrite and cemen-

tite which make up the pearlite will begin to flow into each other. This process continues until the pearlite is thoroughly dissolved, forming what is known as austenite.

If the temperature of the steel continues to rise, any excess ferrite or cementite present in addition to the pearlite will begin to dissolve into the austentite until only austenite is present. The temperature at which the excess ferrite or cementite is completely dissolved in the austenite is called the upper critical point. This temperature has a far wider range, depending on the carbon content, than the lower critical point.

If the carbon steel, which has been heated to a point where it consists entirely of austenite, is cooled slowly, the transformation process which took place during the heating will be reversed. The upper and lower critical points will occur at somewhat lower temperatures than they did during the heating.

Assuming the steel was originally fully annealed, its structure upon returning to atmospheric temperature after slow cooling will be the same. By structure I'm referring to the proportions of ferrite or cementite and pearlite present with no austenite remaining. However, as the steel's cooling rate from an austenetic state is increased, the temperature (at which the austenite begins to change into pearlite) drops more and more below the slow cooling transformation temperature of approximately 1300 degrees Fahrenheit. As the cooling rate is increased, the laminations of the pearlite, formed by the transformation of the austenite, become finer and finer until they can no longer be detected even under a high-power microscope, while the steel itself increases in hardness and tensile strength.

As the cooling rate is further increased, this transformation suddenly drops to around 500 degrees Fahrenheit or lower, depending upon the carbon content. The cooling rate of this sudden drop in transformation temperature is referred to as the critical cooling rate. When a piece of carbon steel is cooled at this rate or faster, a new structure is formed. The austenite is transformed into martensite which is characterized by an angular needlelike structure and an extreme hardness.

If the steel is subjected to a severe quench or to extremely rapid cooling, a small percentage of the austenite may remain instead of being transformed into martensite. Over a period of time, this remaining austenite will be gradually transformed into martensite even if the steel is not subjected to further heating and cooling. Since martensite has a lower density than austenite, such a change or "aging," as it is called, often results in an appreciable increase in volume and the setting up of new internal stresses in the steel.

The process of hardening steel consists fundamentally of two steps. The first step is to heat the steel to a temperature usually at least 100 degrees Fahrenheit above its transformation point so that it becomes entirely austenitic in structure. The second step is to quench the steel at a rate faster than the critical rate to produce a martensitic structure.

The critical or transformation point at which pearlite is heated into austenite is also called the decalescence point. If the temperature of the steel was observed as it passed through the decalescence point, you would notice that the steel continues to absorb heat without appreciably rising in temperature, although the immediate surroundings become hotter than the steel.

Similarly during cooling, the transformation, or critical point at which austenite is transformed back into pearlite, is called the recalescence point. When this point is reached, the steel will give off heat so that its temperature will momentarily increase instead of continuing to fall.

The recalescence point is lower than the decalescence point by anywhere from eighty to 210 degrees Fahrenheit. The lower of these points does not manifest Itself unless the higher one has first been complete ly passed. These critical points have a direct relation to the hardening of steel. Unless a temperature sufficient to reach the decalescence point is obtained, so that the pearlite is changed into austenite, no hardening action can take place. And unless the steel is cooled suddenly before it reaches the recalescence point, thus preventing the changing back again from austenite to pearlite, no hardening can take place. The critical points vary for different kinds of steel and must be determined by testing each case. It is this variation in critical points that makes it necessary to heat different steels to different temperatures when hardening.

After the hardening process, most, if not all, steel parts will require tempering or drawing. The purpose of this is to reduce the brittleness in the hardened steel and to remove any internal strains caused by the sudden cooling in the quenching bath. The tempering process consists of heating the hardened steel to a certain temperature and then cooling. With the steel in a fully hardened state, its structure is made up mostly of martensite. However, when it is reheated to a temperature of about 300 to 750 degrees Fahrenheit, a tougher and softer structure known as troosite is formed.

If the hardened steel is instead reheated to a temperature between 750 and 1285 degrees Fahrenheit, a structure known as sorbite is formed. This has some what less strength than troosite, but it also has considerably greater ductibillty.

Actually, all this boils down to simply this; many of the parts that you have made or will make will require hardening. In certain instances this is required only to prevent undue wear, and in others, both to increase strength and to prevent battering or other malformation.

So it will be necessary for you to heat the part to be hardened to a temperature above the upper critical stage (forming austenite), then rapidly cool it by pumping it into a quenching bath which may be oil, water, brine, etc. (forming martenite). The hardened steel is then heated once more to a temperature somewhat between 300 and 1290 degrees Fahrenheit and cooled (forming either troosite or sorbite). The exact temperature required for this tempering or drawing operation varies considerably, depending on both the carbon content of the steel and the strength and hardness requirements.

A gas or electric furnace is almost a necessity for this type of heat treatment, and if you anticipate treating many parts, I suggest that you either try to buy a commercial furnace or build one. A usable gas furnace may be built by simply lining a steel or iron shell with firebrick, and then by adding a vacuum cleaner motor for power and a fan for a blower. A pyrometer is also necessary to measure and regulate the temperature.

It is also possible to harden and temper parts by using the flame of an oxy-acetylene torch, a forge, or by hot bath. The latter method may be either a chemical solution or molten metal. This method is especially well suited to irregularly shaped parts, parts with holes, and parts varying in thickness or mass. All these parts will heat uniformly to the desired temperature in a bath.

There are times, however, when the only available method will be the torch. While this method may be far from foolproof, satisfactory results may be obtained if sufficient care is taken.

In many cases you will not know the exact composition of your steel, so a bit of experimenting with a scrap of the same material is in order before beginning. Since most of the medium and high carbon steels must be heated to between 1400 and 1650 degrees Fahrenheit for hardening, try heating the scrap to a bright, clear, glowing red, devoid of any yellowish tinge. This is the "cherry-red" so often mentioned in connection with heat treating activities. Then promptly plunge it into a quenching bath of water, at approximately seventy-five degrees Fahrenheit, or a bath consisting of SAE 10 motor oil. It should now be so hard a file won't touch it. If it is not, try another scrap with a little hotter temperature and when the proper combination is found, apply it to the part to be hardened.

Nearly all carbon steels change color in the same way and at almost the same temperatures, so the hardening and tempering colors which appear while heating will indicate the approximate temperature of the metal. The chart at the end of the chapter gives a fairly broad color range and may be used as a guide.

There is currently a product on the market called "Temilag" that will take much of the guesswork out of the temperature control. It is available from gunsmith supply houses, such as Brownell's. A thin coat is applied to the surface to be heat treated. Actually, only a thin smear is required. After it drys to a dull finish, begin heating the metal. When the proper temperature is reached, the Temilag will melt sharply and should be quenched immediately. Temilag is available to indicate temperatures from 350 to 1550 degrees Fahrenheit and is the most foolproof temperature indicator I have found besides the expensive pyrometers.

Regardless of the temperature indicator used, the hardened steel must be drawn or tempered after quenching. So, either wipe on a smear of Tempilag or heat the metal to the color indicating the temperature desired, then allow to cool. It would be wise to again experiment with a hardened scrap of the same material before attempting to temper the actual part, and test it again after tempering with a file and a punch.

Another method which may prove useful for drawing at temperatures up to 500 degrees Fahrenheit is the use of the kitchen oven. Simply place the parts in the oven and set to the desired temperature and let it heat for thirty minutes to an hour.

Still another method which works well on firing pins, sears, pins, and other small parts, is the use of a hardening compound such as "Kasenit." By heating the part to be hardened to a cherry red, coating with the hardening compound, and then reheating to the same cherry red and quenching in water, a hard surface will result with a softer inner core. This is similar to the case-hardening process, which I will not attempt to explain here, since the Kasenit process will give similar results with less equipment.

It might be helpful to include a brief breakdown of the SAE numbers used in drawings and specifications to indicate a certain kind of steel. We read about 2340, 4320,1035, etc., and to the average man these numbers mean little or nothing. The first figure indicates the class to which the steel belongs. Thus, "1" indicates a carbon steel; "2"—nickel steel; "3"—nickel-chromium steel; "4"—molybdenum steel; "5"—chromium steel; "6"—chrome-vanadium steel, etc.

In the case of the alloy steels, the second figure generally indicates the approximate percentage of the predominant alloying element. Usually, the last two or three figures indicate the average carbon content in hundredths of one percent or "points." Thus, 2340 means a nickel steel of approximately three percent nickel and 0.40 (forty-hundredths) percent carbon.

The following color chart may come in handy when tempering by the color method. Brightly polish the part to be tempered so that the color will show and place it on a red hot steel plate until it reaches the desired color, then remove and cool the part.

It should be remembered that the methods and descriptions in this chapter apply to carbon steel only. Certain alloy steels may require entirely different methods of heat-treatment.

Hardening and

Degrees

Tempilag

Tempering Colors

Fahrenheit

Available

Pale Yellow

425

400-413-425

Pale Straw

450

438-450

Pale Straw

455

463

Yellowish Brown

500

475-488-500

Light Purple

525

525

Purple

530

Blue

550

550

Dark Blue

600

575-600

Bluish Green

625

650

Barely visible Red

900

Blood Red

1200

Cherry Red

1400

1350-1400-1425

Light Red

1500

1480-1500

Orange

1650

Yellow

1800

Light Yellow

2000

White

2200

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