In heating steel to harden it, there arise many considerations, the principal of which are as follows:-

As the steel becomes heated, it expands; if one part becomes hotter than another, it expands more, and the form of the steel undergoes the change necessary to accommodate this local expansion, and this alteration of shape becomes permanent. In work finished and fitted this is of very great consideration, and in the case of tools it often assumes sufficient importance to entirely destroy their value. If, then, an article has a thin side, it requires to be so manipulated in the fire that such side shall not become heated in advance of the rest of the body of the metal, or it will become locally distorted or warped, because, though there exist but little difference in the temperature of the various parts, the more solid parts are too strong to give way to permit the expansion; hence the latter is accommodated at the expense of form of the weakest part of the article. It does not follow, however, that the part having the smallest sectional area is the weakest when in the fire, unless it is as hot as the rest of the body.

For example, suppose we have an eccentric ring, say 1/2 in. thicker on one side than the other, and heat it midway between the thick and thin sides to a cherry - red; while those sides are barely red - hot, the part heated to cherry - red will be the weakest, and will give way most to accommodate the expansion, because the strength due to its sectional area has been more than compensated for by the reduction of strength due to its increased temperature. The necessity of heating an article according to its shape then becomes apparent, and it follows that the aim should be to heat the article evenly all over, taking care especially that the thin parts shall not get hot first. If, then, the steel is heated in the open fire, it may be necessary to take it from the fire occasionally, and cool it with water, and to so hold it in the fire that the thin part is least exposed to the heat. If the article is large enough, the thin part may be covered, or partially so, during the first of the heating by wet ashes.

If, however, the article is of equal sectional area all over, it is necessary to so turn it in the fire as to heat it uniformly all over; and in either case care should be taken not to heat the steel too quickly, unless, indeed, it is desirable to leave the middle somewhat softer than the outside, so as to have the outside fully hardened and the inside somewhat soft, which will leave the steel stronger than if hardened equally all through. Sometimes the outside of an article is heated more than the inside, so as to modify the tendency to crack from the contraction during the quenching; for to whatever degree the article expands during the heating, it must contract during the cooling. Whether the heating be done in the open fire or in a heating mixture, it must be done uniformly, so that it may often be necessary to hold the article for a time with the thick part only in the melted lead or other heating material; but in this case it should not be held quite still, but raised and lowered gradually and continuously, to ensure even heating.

The size of an article will often be an important element for consideration in heating it, because, by heating steel in the open fire, it becomes decarbonized; and it follows that, the smaller the article in sectional area, the more rapidly this decarbonization takes place. In large bodies of metal, the decarbonization due to a single heating is not sufficient to have much practical significance; but if a tool requires frequent renewal by forging, the constant reheating will seriously impair its value; and in any event it is an advantage to maintain the quality of the steel at its maximum.