One of the most serious losses common to our tool and implement manufactories is that of the cracking and splitting of steel during the hardening process. Not only is the article or piece lost after having incurred the cost of its manufacture, but in many cases the completion of the machine of which it forms a part is arrested until the lost piece is replaced. In many cases this is done at increased expense, because the piece has to be made singly instead of with a number of others, involving as much setting of machine and adjustment of tools as would be required for a large number of pieces. Successful hardening and tempering is indeed, even under ordinary and unvarying conditions, considered and kept as a trade secret. Visitors are excluded from the hardening and tempering room. In some cases the method of heating, in other cases the material used for heating, in yet others the cooling mixtures, form the supposed secret. As a matter of fact, however, some of the very best tool manufacturers employ the simple open fire or furnace and water, and it is probable that with these two simple agents good cast - steel can be as successfully and properly hardened for any purpose as it can be under any other process, and the advantage gained by heating in fluxes consists in increased expedition and the necessity for a less expert manipulation.
The splitting or cracking of steel occurs during the cooling part of the hardening process, and is to be easily avoided even with the most unfavourable of steels, if the conditions of cooling are made to conform to the form and size of the article. The cooling is, in a majority of cases, performed by dipping the heated steel in water; and the manner in which the dipping is performed may be made at will to crack, warp, or straighten the article.
The instant the surface of a piece of red - hot steel enters the water, a rapid contraction of the submerged portion takes place, and unless this contraction is kept equalized to suit the shape of the article, the side or part most contracted will bend hollow, causing the diametrically opposite metal to bend to accommodate the inner carve. Suppose, for example, we heat a piece of steel, 1 in. square and 12 in, long, to a red - heat, and dip it slowly in water, so that one side of the square will strike the surface flat and evenly, then that surface will contract while the diametrically opposite or upper surface will remain expanded; the lower face will curve to a concave, the upper one to a convex. If, then, such a bar were curved during the heating process, we may help to straighten it by dipping it slowly in the water with its convex side downward. If it was bent at one end only, we may dip at that end, first diagonally, and with the convex side downward. If, however, we dip it with the length lying either diagonally or horizontally, we are apt to warp it, no matter how quickly it may be dipped, and the reason is, in addition to the above, as follows: - Experiments have demonstrated that the greater part of the hardness of steel depends upon the quickness with which its temperature is reduced from about 500° F. (260° C.) to a few degrees below 500° F., and metal heated to 500° F. must be surrounded by a temperature which renders the existence of water under atmospheric pressure impossible; hence, so long as this temperature exists, the steel cannot be in contact with the water, or, in other words, the heat from the steel vaporizes the immediately surrounding water.
The vapour thus formed penetrates the surrounding water and is condensed, and from this action there is surrounding the steel a film of vapour separating the water from the steel, which continues so long as the heat from the steel is sufficiently great to maintain that film against the pressure of the water and the power of the water which rushes towards the steel to fill the spaces left vacant by the condensation of the vapour as it meets a cooler temperature and condenses. The thickness of the vapour film depends mainly upon the temperature of the steel, but here another consideration claims attention. As the heated steel enters the water the under side is constantly meeting water at its normal temperature, while the upper side is surrounded by water that the steel has passed by, and, to a certain extent, raised the temperature of. Hence the vapour on the under side is the thinnest, because it is attacked with colder water and with greater force, because of the motion of the steel in dipping. Suppose, now, we were to plunge a piece of heated steel into water, and then slowly move it laterally, the side meeting the water would become the hardest, and would be apt to become concave in its length.
From these considerations we may perceive how important a matter the dipping is, especially when it is remembered that the expansion which accompanies the heating is a slow process compared to the contraction which accompanies the cooling (although their amounts are of course precisely equal), and that while unequal expansion can only warp the article, unequal contraction will in a great many, or indeed in most cases, cause it to crack or split.
After an article is dipped to the required depth, it should, if straightness is of importance, be held quite still until reduced to the temperature of the water, because if taken out before so reduced in temperature, it is especially apt to crack; and it is better to have a deep tank of water if the body of metal is great, so that the steel may be dipped slowly downwards, and become cooled sufficiently rapidly to harden without any lateral movement, except it be after the steel has lost its redness.
When a piece of steel requires to be hardened at one end only, the dipping must be performed with a view to make the graduation from the soft to the hard metal extend over a broad section of metal, for if the junction of the hardened with the soft metal is abrupt, the hardened end is apt to break short off. The method of dipping, therefore, is in this case to plunge the end of the steel vertically into the water to a depth a little more than equal to the depth it requires hardening, and after holding it still there until it is black hot (that is, as soon as its redness is gone), dip it slowly a little deeper, and then raise it up to the amount of the increased dip-ping, and slowly immerse again.
When a piece of metal requires hardening and tempering at one part only, we may heat the steel back of the part to be tempered to redness, and dip the article so as to harden the required part, and leave sufficient heat in the contiguous metal to raise the temperature of the hardened part enough to temper it. This plan is always followed in the tempering of lathe and planer tools, flat drills, etc. If, however, the method of dipping is to hold the steel in the water at an even depth after the immersion, the temper colour will be very narrow, while if the steel is raised and lowered in the water, the colour - band will be broad. Burning - - The whole value of the temper will be destroyed if the steel is made too hot and becomes what is known as "burned." As a general rule, unsatisfactory results will be found to have arisen from overheating the steel, for steel may have its quality impaired without giving evidence of being what is known as burned. If a piece of hardened tool steel shows brightness and crystalline formation under fracture, it has probably been burned; but if the fracture appears dull and even, it has not been burned.