Hardening generally is understood to mean the heating of a piece of steel to a certain temperature and plunging it into a bath of some kind for the purpose of cooling it. While this definition holds good for most steels, a few alloy steels now on the market reverse this method; steels known as air-hardened or self-hardening obtain their hardest and toughest state by a slow cooling process rather than by a sudden one.
Two reasons may be mentioned for the desirability of hardening steel: (1) to give the steel a cutting edge such as is required for all cutting tools; and (2) to alter the static strength and dynamic qualities of the metal so it will give the best results for the moving parts of machinery.
To harden steel, therefore, it is requisite that the heating produce* a change in the structure, and that the quenching which follows the heating retain the whole or a part of the elements produced by this change. It is therefore necessary, as in annealing, that the temperature of the steel be raised to a point slightly above the point of transformation.
As the point of transformation varies with the different ingredients which are alloyed with steel, it is necessary to find out where this point is in the steel to be hardened. A steel may be heated to 1300° Fahrenheit, which is above the point of transformation in some steels, and no change in structure may occur; therefore no result in hardness is obtained. If this same piece is heated to 1450° Fahrenheit, which may be considered the point of transformation in this piece, the intermolecular transformation which consists of the passage of the carbon from the combined into the dissolved state takes place, and the steel assumes the hardest state possible, if properly cooled.
Thus the factors which have an influence on the results of hardening are: (1) the nature and composition of the metal; (2) the temperature of the metal when quenched; and (3) the nature, volume, and temperature of the quenching bath.
The constitution of a given steel is not the same in the hardened as in the normal condition, owing to the carbon not being similarly disposed. In the annealed or normal steel it is in the free state, while in a hardened steel it is in a state of solution which we may call martensite, and this contains more or less carbon according to the original carbon content of the steel. The composition, and therefore the mechanical properties, depend principally upon the carbon content, the mechanical properties being changed, slowly and gradually by the increase in carbon.
In regard to mechanical properties, the higher the temperature above the critical point, the lower the tensile strength, and the less the elongation, which means that the steel becomes more brittle with each increase in temperature. Coarsening of the grain, reduction of the tensile strength, and elongation lead to the conclusion that in practice 40° Fahrenheit above the highest point of transformation is the extreme limit to which steel should be raised to obtain the best results in hardening. The same figures hold also for annealing.
The results obtained in hardening steel are: increase of tensile strength and of elastic limit, and reduction of elongation - the effect being greater in proportion to the carbon content; and due to quenching at the proper temperature, greater homogeneity, which aids resistance to shock.
The most important feature to be taken into consideration is the method of heating, which should be such that the tool may be brought slowly and evenly to the hardening point without coming into contact with air or any form of gas which would oxidize the steel. Great care should be used, first, to protect the cutting edges and working parts from heating more rapidly than the bodies of the pieces; next, that the whole part to be hardened be heated through uniformly without any part being visibly hotter than another. A good oil or gas muffle furnace, provided care is taken to assure proper combustion, makes the best heating medium. For small tools, etc., a lead bath is very good.
The liquid baths commonly used for heating steel preparatory to hardening are molten lead, cyanide of potassium, barium chloride, a mixture of barium and potassium chlorides and other metallic salts.
Anyone using any of these different baths cannot be too careful in preventing an accident. If there is ever so little moisture of any kind on the tool to be immersed, it causes the molten liquid to fly in all directions. Before putting any tool into the bath, be sure that it is perfectly dry. The safe method is to pre-heat all tools before immersing them.
Lead at any heat above 1200° Fahrenheit gives off a slight vapor which is poisonous. Cyanide of potassium should be carefully used, as it is a virulent poison. The furnaces for heating these liquid baths should be equipped with hoods to carry away the fumes. It is a very good idea to put powdered charcoal on top of the molten liquid as charcoal acts as a purifier.
As it is necessary to maintain the steel in the state it was at the moment quenching began, the quenching bath is a very important part of the process of hardening. The better the bath, the more nearly perfection is attained.
Various baths are used for cooling steel when hardening, on account of the different rates at which they cool the heated metal. An oil bath is used when the steel is wanted tougher and not excessively hard, as the oil cools the steel more slowly than water. Brine or an acid bath is used when the steel is wanted very hard, as they absorb heat more rapidly than water. For excessively hard work mercury or quicksilver is sometimes used, as it absorbs the heat very rapidly.
In the hardening of steels, the influence of the bath depends upon its temperature, its mass, and its nature, or, to express this in another way, upon its specific heat, its conductivity, its volatility, and its viscosity. With other things equal, the lower the temperature of the bath, the quicker the metal cools and the more pronounced is the hardening effect. Thus water at 60 degrees makes steel harder than does water at 150 degrees, and, when the bath is in constant use, the first piece quenched will be harder than the twentieth, owing to the rise in the temperature of the bath. Therefore, if uniform results are to be obtained, the bath must either be of a very large volume or be kept cool by some mechanical means; in other words, the bath must be kept at a constant temperature.
A bath consisting of a liquid which volatilizes easily at the highest temperature it reaches, from plunging the metal into it, forms a space filled with vapor around the steel which retards the cooling action of the liquid. Motion of the bath throws off this vapor, as it brings the liquid in contact with the metal and tends to equalize the temperature. To agitate the piece to be hardened gives better results than trusting to the volatility of the bath, as it is more energetic in distributing the vapor.