Heat treating and inspection of metals

TM 1-423
High tempering temperatures improve ductility at the sacrifice of
tensile, yield strength, and hardness.
(5) Several special treatments may be given to iron base alloys
to produce a hard, wear-resistant surface, and at the same time, leave
the core of the metal tough and resilient. Such treatments are
known as case hardening, and three common methods are as follows:
(a) Carburizing.-This method consists of holding the metal at
an elevated temperature while in contact with a solid, liquid, or
gaseous material rich in carbon. The process requires several hours,
as time must be allowed for the surface metal to absorb enough
carbon to become high carbon steel.
(b) Nitriding.-This process consists of holding special alloy steel
at temperatures below the critical point in anhydrous ammonia.
Absorption of nitrogen as iron nitride into the surface of the steel
produces a greater hardness than carburizing but the hardened area
extends to a lesser depth.
(c) Cyaniding.-This is a rapid method of producing surface
hardness on an iron base alloy of low carbon content. It may be
accomplished by immersion of the steel in a molten bath of cyanide
salt, or by applying powdered cyanide to the surface of the heated
steel. The temperature of the steel during this process should range
from 1,3000 F. to 1,600 F., depending upon the type of steel, depth
of case desired, type of cyanide compound, and time exposed to the
General.. ------------------------------------------------------ 2
Furnaces --------- --- --------------------------------------- 3
Quenching tanks and liquids---- ---------------------------------- 4
Temperature measurement and control-------------------------------- 5
2. General.-Equipment necessary for heat treating consists of a
suitable means for bringing the metal to the required temperature, a
temperature measuring device, and a quenching medium. Heat may,
in some instances, be supplied by means of a forge or welding torch;
however, for the treatment required in aircraft work a furnace is
3. Furnaces.-a. Heat-treating furnaces are of many designs and
no one size or type perfectly fills every heat-treating requirement. The
size and quantity of metal to be treated and the various treatments re-
quired determine the size and type of furnace most suitable for each

United States. War Dept. Heat treating and inspection of metals. Washington, D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc96657/. Accessed August 30, 2014.