Prof. Kich, of Prague, after several experiments with nitric, sulphuric, and hydrochloric acids, and their combinations, with mordants composed of the salts of copper, etc, has arrived at the conclusion that a mixture of equal parts of hydrochloric acid and water, to which is added a trace of solution of antimony, constitutes a mordant especially applicable for the purpose of testing iron and steel. The last ingredient, which was recommended to him by Prof. Gintl, renders the surface attacked more capable of resisting oxidation, and has the effect, after well washing with hot water and the application of a coat of protecting varnish composed of damar resin, of preserving the surface attacked sufficiently pure. The method of proceeding is always to surround the surfaces, previously pre. pared by means of a file or hone, with a wall of wax fully f in. high, the acid, heated to a temperature of 53° to 86° F. (12° to 30° C), is poured on to the surfaces, and soon begins to act, as will become manifest by the disengagement of gas. In winter, owing to the low temperature, the operation cannot be performed so favourably. Its duration is usually 1 to 2 hours, and it should be continued, as a general rule, until the texture of the iron is exposed.
The progress of the action may be easily ascertained by pouring out the acid every 1/2 hour without breaking the wax border, removing by means of a brush or piece of rag the carbon (graphite) deposited on the surface, washing, and again pouring on more acid if the action appears insufficient. If the antimony chloride has been added to the acid in proper proportion, but little time will elapse, after the action has commenced, before it will begin to throw down a black precipitate. This is easy to distinguish from the graphite, inasmuch as the latter is not very appreciable, when for about 1 3/4 pint is added only a single drop of the concentrated solution of antimony chloride, which is sufficient. When the action of the acid has been continued long enough, the wax wall is destroyed, and the surface of the iron is washed by means of a brush with several waters, the first of which is rendered slightly alkaline by the addition of a little lye; it is then carefully dried, and a coat of varnish is applied. If, at the end of a few hours, there are any signs of oxidation, the varnish must be dissolved with spirit of turpentine, the oxide removed, and the varnish again applied.
The indications given by the different kinds of iron are as follows: -
Soft or fibrous iron, when - of very good quality, is attacked by the acid, even when the action is continued for several hours, in a manner so uniform, and with an elimination of the carbon so limited, that the surface acted upon retains a dull lustre - a few incised specks and cinder - like holes being only observable.
Fine - grained iron gives exactly the same indications; the surface generally remains uniform, but it is not quite so bright.
Coarse - grained iron and hot - short iron are attacked by the acid with much greater energy than the 2 kinds above mentioned. Even at the end of about 10 minutes the surface, especially that of the latter kind, becomes quite black. If the acid be allowed to act for nearly 1/2 hour, a black muddy deposit (schlamm) may be removed by washing, and no amount of washing will prevent the surface from remaining black; there will also be a considerable number of small holes distributed over the surface. Some portions of the iron are generally attacked more deeply in this way; others, although they may have become black and a little porous, are better preserved. This appearance will be the more manifest if, after about an hour's action, repeated washing and drying, a fine file be passed over the surface.
Malleable iron or annealed iron becomes rusty, as is well known, more readily than wrought iron; but an interesting fact is that the action of the acid is very violent and irregular.
The appearance exhibited is very like that of puddled steel, and the weldings are also but slightly apparent.
The surfaces of these steels are uniformly grey - the non - homogeneous parts are rare, and but little apparent. The softer the steel the more approaching to grey is the colour. The action of the acid produces very fine fissures. In a sample of Mushet steel the prepared surface was perfectly uniform, but after the treatment with acid narrow transverse fissures were observed over the whole extent. It is probable that the proportion of titanium in this steel was the cause that the surface attacked presented the dark grey colour.
The old blacksmiths' method of calling that steel which can be hardened, is one recognized method. To harden, however, is one thing; to - harden when heated to a definite degree, is another; and to possess a definite degree of elasticity when tempered to a particular point of temperature after hardening, is yet another. So that, in the absence of great uniformity in the grade of the metal, the blacksmith or the temperer must rely upon his judgment and perception. If under a given process the work is not considered hard enough, he may heat the metal to a greater temperature, providing its shape and size will admit of that without injury, or causing it to crack in the quenching. In this case he will try to make up for the deficiency in the metal to temper, by chemical additions to the quenching liquids.
Supposing that steel as operated upon be of uniform grade, the operation of the hardener would not always make it uniform, because steel decarbonizes somewhat by being heated; hence a small tool deteriorates by being heated in the open fire, and one often heated to sharpen or repair suffers in proportion. From all these and other considerations, hardening and tempering processes of steel differ according to the size and nature of the work, the amount of uniformity required, and the duty to which the work is to be put. The only information of value to the practical man is such as will instruct him in the practice of the workshops, giving the conditions and the processes in connection with each other.
If we heat a piece of cast - steel to redness, and plunge it into clean water until its temperature is reduced to that of the water, the result will be that the steel will be hardened. The degree of the hardness will depend upon the quality of the steel, the temperature to which it was heated, and to a small degree upon the temperature of the water in which it was cooled. In any event the operation will be termed hardening. If we reheat the - steel, a softening process will accompany the increasing temperature, until upon becoming again red - hot it will assume its normal softness, and if allowed to tool in the atmosphere the effects of the first hardening will be entirely removed. If, however, after the steel is hardened, we polish one of its surfaces and slowly reheat it, that surface will - assume various colours, beginning with a pale yellow and ending in a blue with a green tinge, each colour appearing when the steel has attained a definite degree of temperature; hence by the appearance of the colours we are informed of the temperature of the steel, or, in other words, how far or to what extent the resoftening has progressed.
This fact is taken advantage of by the machinist to obtain in steel any required degree of hardness less than that of the absolute hardness obtained by hardening, and is termed tempering. The temperature at which these respective colours will appear are as follows:-
Very pale yellow .
Straw yellow . .
Brown yellow .
Dark purple .
Blue tinted with green
To say, then, that a piece of steel has been tempered to a straw colour, implies that it was first hardened and then reheated until the straw yellow appeared upon it, the temperature having arrived at 460° F. (238° C), and that the reheating process was then discontinued. The presence of the straw colour, however, while evidence of temperature to which the heating took place, is no indication of the actual degree of hardness of the steel, because that depends upon the degree to which the steel was hardened before the colour test tempering was resorted to. And since the degree of the first hardening depended upon the quality of the steel, the degree to which it was heated, and the temperature of the water in which it was cooled, it follows that the above quality, heating, and temperature must be uniform in all cases if uniform results are to be reached. The higher the grade of steel, the lower the temperature at which it will harden, and the harder it will be if cooled in water from a given temperature; but any degree of hardness obtained from a temperature equal to or less than the highest at which a colour would appear - that is, 430° F. (221° C.) - will obviously be represent - able under the colour process by a colour, providing, of course, that the steel was first thoroughly hardened.
The colour - scale forming the frontispiece to this volume represents the tint assumed by ordinary steel when, after hardening, it is " let down" to the temperatures recorded on the left margin; while the names of the tools are arranged on the right, opposite the shade of colour to which they are tempered. The scale has been prepared in accordance with the practice of some of the best authorities, and is based upon a somewhat similar but less complete one published by the well - known metallurgist, Joshua Rose, to elucidate some articles on tempering in the Scientific American.