Heat treating and inspection of metals Page: 15
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HEAT TREATING AND INSPECTION OF METALS 7-8
hardened steel is, in reality, a transformation of the unstable condition,
known as martensite, to a more stable form. Hardened steel becomes
fully transformed or reaches a stable condition at approximately
1,100 F. This transformation results in the formation of troostite
and sorbite as the temperature increases. At approximately 400
F., martensite, which is the structure of properly hardened steel, begins
to transform into troostite and at 750 F., troostite gradually changes
into sorbite. At 1,100, the transformation of troostite into sorbite is
complete and the formative stage of pearlite begins. Pearlite is
obtained by slow cooling from the critical point as in annealing.
b. Steel that has been hardened by rapid cooling from a point
slightly above its critical range is often harder than necessary, and
generally too brittle for most purposes. In addition, it is under severe
internal strain. In order to relieve the strains and reduce the brittle-
ness, the metal is usually "tempered." This is accomplished in the
same types of furnaces as are used for hardening and annealing. How-
ever, less refined methods are sometimes used for tempering small
c. As in the case of hardening, tempering temperatures may be
approximately determined by color. These colors appear only on the
surface and are due to a thin film of oxide which forms on the metal
after the temperature reaches 450 F. In order to see the tempering
colors, the surface must be brightened and a buff stick consisting of a
piece of wood with emery cloth attached is ordinarily used for this
purpose. When tempering by the color method, an open flame or
heated iron plate is ordinarily used as the heating medium and, al-
though the results are not absolutely accurate, it provides a very
convenient means of tempering many small parts. The tempering
temperatures and corresponding oxide colors are given in table II.
TABLE II.-Color chart for various tempering temperatures of carbon steels
Metal tem- Metal tem-
Oxide color perature Oxide color perature
(0 F.) (0 F.)
Pale yellow________________ 428 Purple__ 531
Straw _ _ _ _ _ _ _ _ _ _ _ _ 446 Dark blue________________ 550
Golden yellow__ __________ 469 Bright blue________________ 567
.Brown _ -------------------- 491 Pale blue_________________ 610
Brown dappled with purple__ 509
8. Annealing and normalizing.-a. Annealing.-When steel is
heated to a point above its critical range a condition referred to as
heated to -a -p,%mt above itsc ritical range a cndition referred tonas
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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/m1/17/: accessed April 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.