It is used for rifieproof shutters, guns, etc.
This process of hardening, however, makes the steel very brittle, and in order to make it tough enough for most purposes it has to be tempered.
In order then to produce steel of a certain degree of toughness (without the extreme hardness which causes brittleness), it is gradually reheated, and then cooled when it arrives at that temperature which experience has shown will produce the limited degree of hardness required.
Heated steel becomes covered with a thin film of oxidation, which becomes thicker and changes in colour as the temperature rises. The colour of this film is therefore an indication of the temperature of the steel upon which it appears.
Advantage is taken of this change of colour in the process of tempering, which for ordinary masons' tools is conducted as follows : -
The workman places the point or cutting-end of the tool in the fire till it is of a bright red heat, then hardens it by dipping the end of the tool suddenly into cold water. He then immediately withdraws the tool and cleans off the scale from the point by rubbing it on the stone hearth. He watches it while the heat in the body of the tool returns, by conduction, to the point. The point thus becomes gradually reheated, and at last he sees that colour appear which he knows by experience to be an indication that the steel has arrived at the temperature at which it should again be dipped. He then plunges the tool suddenly and entirely into cold water, and moves it about till the heat has all been extracted by the water.
It is important that considerable motion should be given to the surface of the water while the tool is plunged in, after tempering, otherwise there will be a sharp straight line of demarcation between the hardened part and the remainder of the tool, and the metal will be liable to snap at this point.
In very small tools there is not sufficient bulk to retain the heat necessary for conduction to the point after it has been dipped. Such tools, therefore, are heated, quenched, rubbed bright, and laid upon a hot plate to bring them to the required temperature and colour before being finally quenched.
In some cases the articles so heated are allowed to cool slowly in the air, or still more gradually in sand, ashes, or powdered charcoal. The effect of cooling slowly is to produce a softer degree of temper.
The following Table shows the temperature at which the steel should be suddenly cooled in order to produce the hardness required for different descriptions of tools. It also shows the colours which indicate that the required temperature has been reached.
1 Mushet on Iron and Steel.
Colour of Film.
Temp. Deg. Fahr
Nature of Tool
Very pale straw yellow ...
Lancets and tools for metal.
A shade of darker yellow...
Razors and do.
Darker straw colour..
Still darker straw yellow ...
Cold chisels for cutting iron, tools for wood.
Hatchets, plane irons, pocket knives, chipping chisels, saws, etc.
Yellow tinged with purple..
Light purple ...
Swords, watch springs, tools for cutting sand-
Dark purple ...
Large saws, pit and hand saws.
Paler blue with tinge of green
Too soft for steel instruments.
The tempering colour is sometimes allowed to remain, as in watch springs, but is generally removed by the subsequent processes of grinding and polishing.
A blue colour is sometimes produced on the surface of steel articles by exposing them to the air on hot sand. By this operation a thin film of oxide of iron is formed over the surface, which gives the colour required.
Steel articles are often varnished in such a way as to give them an appearance of having retained the tempering colours.
The exact tempering heat required to produce the same degree of hardness varies with different kinds of steel, and is arrived at by experience. It would be impossible to go very fully into the subject in these Notes. The above remarks will give some idea of the process, and the effects produced upon the strength and ductility of steel by tempering in different ways is shown in the Table, page 325.