When you buy steel for tools from a merchant he will assure you that
the steel he sells you will harden at a cherry-red heat. This is true
provided the metal has not been spoiled either by overheating it or
working it at too low a heat during the making. This causes cracks or
internal fractures.
If these directions for working steel are followed out a tool should
harden at a cherry-red heat when plunged into water. When the steel is
heated to the proper temperature, usually a cherry red, and plunged
into a hardening solution, it will be very brittle, so that a file will
not cut it. One test often used is to take a fine mill cut file and try
to cut the hardened part of the tool. If it slips over the surface
without cutting, the steel is considered hard; if the file cuts, the
steel is not hard enough. Re-heat the steel hotter than before, cool
it off in water, and test again. All cutting tools should possess
a certain amount of hardness or toughness. When the steel has been
plunged into cold water it is too hard for use. It is necessary to then
reduce this hardness so that one can use the tool for the particular
kind of work it is made to do. This process of reducing the hardness is
commonly called drawing the temper, and the colour scheme plays a very
important part in this operation. Perhaps the steps will be clearer if
the process of drawing the temper on a cold chisel is explained: After
the chisel is forged the proper shape, place the body of the tool in
the fire, heat it red hot back of the point. Now heat the point to a
uniform cherry-red heat, plunge 1-1/2 in. of this hot point into the
water and hold it there until it is quite cold. This is determined by
water clinging to the point when the chisel is taken out. Polish the
part cooled off with a piece of emery stone, an old brickbat, or any
rough polishing material. You will notice a group of temper colours
starting from the point where the tool came into contact with the
water. The heat in the body of the tool gives rise to these colours as
it is conducted through the cold point of the steel. In this group of
colours the first will be (1) pale yellow, (2) a full straw colour,
(3) brown, (4) purple, (5) dark blue, (6) full blue, (7) light blue,
(8) gray. This colour scheme corresponds to varying temperatures in
the metal. The first colour (pale yellow) accompanies a temperature of
430°, while the last colour, gray, means a temperature of 700°. The
colours show, too, a varying in the hardness or toughness of the steel.
A cold chisel should be tempered a blue; so when the blue reaches the
cutting end of the tool the end should be plunged immediately into
water and cooled off. This is the principle of hardening and tempering
all common tools.
[Illustration: Colors, left to right: pale yellow, straw color, brown,
purple, dark blue, full blue, light blue, gray]
However, these colours mean nothing so far as tempering is concerned
unless the cutting edge of the steel has been thoroughly hardened.
Then the colours have a real value. To prove that these colours are no
test unless hardening precedes, take a piece of brass, or copper, or
soft iron. Polish, then heat the piece in the fire to the temperature
given here. You find the same set of colours, but you cannot use any of
these metals for cutting tools.
The table of temper colours given in this book shows the colour
required for tempering tools most commonly used.
_Tempering of springs:_ Under the head of springs we may include every
variety, from the small spring used in locks and fire-arms to the
largest springs in use. They are made of spring steel of the required
thickness, forged into shape, then hardened and tempered.
This hardening and tempering of springs is done in some cases by
polishing and heating over a fire and drawing them to a blue colour.
Then they are plunged into oil to fix the temper. Sometimes springs are
heated red hot and cooled in oil, then held over the fire until the oil
burns with a bright flame on the spring. It is then allowed to cool in
the air. If the spring is found too hard, more oil is put on it and the
operation is repeated until the desired spring movement is obtained.