As the heat from the uncooled part gradually heats the point again, the
color of the polished portion changes rapidly. When a certain color is
reached, the tool should be completely immersed in the water until cold.
For lathe, planer, shaper and slotter tools, this color should be a light
straw.
Reamers and taps should be cooled from an ordinary straw color.
Drills, punches and wood working tools should have a brown color.
Blue or light purple is right for cold chisels and screwdrivers.
Dark blue should be reached for springs and wood saws.
Darker colors than this, ranging through green and gray, denote that the
piece has reached its ordinary temper, that is, it is partially annealed.
After properly hardening a spring by dipping in lard or fish oil, it should
be held over a fire while still wet with the oil. The oil takes fire and
burns off, properly tempering the spring.
Remember that self-hardening steels must never be dipped in water, and
always remember for all work requiring degrees of heat, that the more
carbon, the less heat.
_Case Hardening._--This is a process for adding more carbon to the
surface of a piece of steel, so that it will have good wear-resisting
qualities, while being tough and strong on the inside. It has the effect of
forming a very hard and durable skin on the surface of soft steel, leaving
the inside unaffected.
The simplest way, although not the most efficient, is to heat the piece to
be case hardened to a red heat and then sprinkle or rub the part of the
surface to be hardened with potassium ferrocyanide.
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