Heat treating and inspection of metals Page: 16
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8 AIR CORPS
austenite is produced. If slowly cooled from above its critical tem-
perature, the austenite is broken down and a succession of other con-
ditions are produced, each being normal for a particular range of
temperatures. Starting with austenite, these successive conditions
are: martensite, troostite, sorbite, and finally pearlite.
(1) The most important step in annealing is to raise the tempera-
ture of the metal to the critical point, as any hardness that may have
existed will then be completely removed. Strains which may have
been set up through heat treatment will be eliminated when the steel
is heated to the critical point and then restored to its lowest hardness
by slow cooling. In annealing, the steel must never be heated more
than 50 to 75 F. above the critical point and when large articles
are annealed, sufficient time must be allowed for the heat to penetrate
(2) Steel is usually subjected to the annealing process;for the fol-
(a) To increase its ductility by reducing hardness and brittleness.
(b) To refine the crystalline structure and remove stresses.
Steel which has been cold worked is usually annealed so as to in-
crease its ductility. However, a large amount of cold-drawn wire
is used in its cold-worked state when very high yield point and tensile
strength are desired and relatively low ductility is permissible, as in
spring wires, piano wires, wires for ropes, cables, etc. Heating to
low temperatures, as in soldering, will destroy these properties unless
(3) Assuming that the part to be annealed is heated to the proper
temperature, the required slow cooling may be accomplished in sev-
eral ways, depending on the metal and the degree of softness required.
The common methods are packing and furnace cooling. Packing
requires that the part be buried in some substance that does not con-
duct heat readily. For this purpose, a metal box containing slaked
lime, ashes, or powdered charcoal is satisfactory, and care must be
taken to keep the material perfectly dry. In furnace cooling, the
part is merely left to cool down with the furnace.
b. Normalizing, although involving a slightly different heat treat-
ment, may be classed as a form of annealing. This process removes
all strains due to machining, forging, bending, and welding. Nor-
malizing can only be accomplished with a good furnace, where the
temperatures and the atmosphere may be closely regulated and held
constant throughout the entire operation. A reducing atmosphere
will normalize the metal with a minimum amount of oxide scale, while
an oxidizing atmosphere will leave the metal heavily coated with
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/18/: accessed January 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.