In the school forge shop, the tool steel used should be of an inexpensive kind. High priced steel should not be used as more or less is wasted by the pupils in working. A carbon steel should be used for all forge shop tools. About 75 to 95 point is suitable. High-speed tool steel should be used only to give the pupils instruction in its handling and use, and to familiarize them with the different kinds of steel and their treatment.
To the steel maker, temper means the percentage of carbon in the steel. The word point means one-hundredth of one per cent, thus 10 point carbon means ten one-hundredths of one per cent. One hundred and fifty point carbon contains one and one-half per cent. This is about as high as is generally made. One hundred and fifty point is known as high temper; low temper is about 40 point. Steel containing less than 40 point does not harden to advantage and is classed with machinery steel. There is a range of tempers between high and low point which are used for different kinds of tools.
In the forge shop the term temper means the degree of hardness given to a piece of tool steel. As an example, a piece of steel is heated to a dark red color and cooled in water or oil. This is called hardening. If this piece is too hard for the purpose intended, it is then tempered to reduce some of its hardness, and to give the steel elasticity and strength. In doing this, it is subjected to heat, (the more heat the softer the piece becomes). In the forge shop, in tempering steel, the metal is polished bright after hardening. If it is a small piece, it is then held on or near a piece of hot iron. As the piece becomes heated, the steel heated in the air assumes colors; at first a very faint yellow and gradually darker, until all of the color has disappeared leaving the steel without any trace of hardness.
These different colors as they appear on the surface of hardened steel represent different degrees of hardness. The following simple list of colors applies to the different tools and carbon to use:
Light straw—430° F. Lathe tools—130 point carbon.
Dark straw—470° F. Taps and dies—120 point carbon.
Purple gray—530° F. Chisels and blacksmiths’ tools, 75 to 95 point carbon.
Of course there are other colors than these. As the heat advances every few degrees the color keeps changing to a darker which indicates the tool is becoming softer.
The hardening heat is about 1300 to 1400 degrees Fahrenheit, or a cherry red. About 400 degrees Fahrenheit relieves the strain in a hardened piece of steel; 600 degrees leaves a trace of hardness and is about right for springs.
In order to know the results of heating and cooling steel one should take a small bar and cut nicks in it with a chisel every half inch. The bar is then heated from a white heat at the end to a very dark red some inches back. It is then cooled in water, the pieces broken and the grain noted. The heat that leaves the steel file hard and a very fine grain is the hardening heat of that steel. The hardening heat is a dark red. The hotter it was when cooled the coarser the grain shows on the end of the broken pieces.