Tempering, as a term, is used to comprehend both hardening and drawing; as a process it depends mainly upon judgment instead of skill, and has no such connection with forging as to be performed by smiths only. Tempering requires a different fire from those employed in forging, and also more care and precision than blacksmiths can exercise, unless there are furnaces and baths especially arranged for tempering tools.
A difficulty which arises in hardening tools is because of the contraction of the steel which takes place in proportion to the change of temperature; and as the time of cooling is in proportion to the thickness or size of a piece, it follows, of course, that there is a great strain and a tendency to break the thinner parts before the thicker parts have time to cool; this strain may take place either from cooling one side first, or more rapidly than another.
The following propositions in regard to tempering, comprehend the main points to be observed:
The permanent contraction of steel in tempering is as the degree of hardness imparted to it by the bath.
The time in which the contraction takes place is as the temperature of the bath and the cross section of the piece; in other words the heat passes off gradually from the surface to the centre.
Thin sections of steel tools being projections from the mass which supports the edges, are cooled first, and if provision is not made to allow for contraction they are torn asunder.
The main point in hardening and the most that can be done to avoid irregular contraction, is to apply the bath so that it will act first and strongest on the thickest parts. If a piece is tapering or in the form of a wedge, the thick end should enter the bath first; a cold chisel for instance that is wide enough to endanger cracking should be put into the bath with the head downward.
The upflow of currents of warmed water are a common cause of irregular cooling and springing of steel tools in hardening; the water that is heated, rises vertically, and the least inclination of a piece from a perpendicular position, allows a warm current to flow up one side.
The most effectual means of securing a uniform effect from a tempering bath is by violent agitation, either of the bath or the piece; this also adds to the rapidity of cooling.
The effect of tempering baths is as their conducting power; chemicals except as they may contribute to the conducting properties of a bath, may safely be disregarded. For baths, cold or ice water loaded with salt for extreme hardness, and warm oil for tools that are thin and do not require to be very hard, are the two extremes outside of which nothing is required in ordinary practice.