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is the lowest of the composition range as given by the
equilibrium diagram for this temperature (1365 deg.)
more carbon will dissolve in it after change takes
place, and thus allow carbon to be carried by diffusion
to the iron not originally in contact with cementite.
In this way the change will progressively proceed
throughout the mass of each ferrite plate.
AiPthe change takes place in a short time and
the carbon is carried by diffusion within this time
from the cementite plates to those particles of ferrite
furthest from any cementite, a concentration gradient
of carbon must exist between these points up to the
instant of complete change to gamma iron. The resulting
austenite at tha instant will be minutely segregated
with respect to carbon having in the loci of
the center of each ferrite plate the lowest possible
percentage of carbon with reference to the equilibrium
diagram and the center of each cementite plates the
highest possible percentage. How long will this segregated
condition exist?
On etching a polished section of cast carbon steel
with Stead's reagent a dendritic structure will be revealed,
which shows the solidification structure. The
fact that it is possible to reveal this structure shows
that segregation as between axes and branches has
occurred at solidification. The cause of this segregation
is the repulsion between carbon and phosphorus,
producing parts of the dendrites that is richer in either
element than other parts. When the dendritic grains
are large this condition will persist even after extremely
long annealing above the critical range.
The segregation produced in eutectoid steel on heating
through the critical range is very minute and will
not last nearly as long as dendritic segregation, but
even in this case it is believed that some time is necessary
for complete diffusion to take place.
It is known that higher carbon steel will require
a less drastic quench to produce a given degree of
hardness than a lower carbon steel. If the carbon content
and rate of cooling is such that martensite is the
product and if a time-temperature cooling curve is
plotted, it will be found that there is no break in the
curve until a temperature of 570 deg. F. is reached.*
If troostite is the product, the lag occurs at 1100 deg.
F These lags are due to the change of gamma to
alpha iron and show that both martensite and troostite
form directly from austenite.
Figs. 5. 6 and 7 are micrographs of the centres of
sections of bars after quenching in oil from above the
critical range. Figs. 5 and 6 shows the structure
when quenched after a 5 min. treatment at 1600 deg.
F. Fig. 7 is the structure produced after a 30 min.
treatment at 1600 deg. F. The dark areas are troostite
and the lighter martensite. It is shown by X-ray
diffraction patterns that both martensite and troostite
consist of grains that belong to the body-centered
space latticed system. Xo lines corresponding to the
face-centered system are found in the patterns of
either martensite or troostite. Thus both martensite
and troostite are composed of alpha iron and cementite
in different states of segregation.
If the conditions are such that both martensite
and troostite appear in the quenched sample, it will
be found that the dark troostite areas have the appearance
of grains crystallizing about neuclei that are
situated at the grain boundaries of the original aus­
rorging-Sfamping- Heaf Seating
September, 1925
tenite. The troostite grains owe their dark appearance
to the colloidally dispersed cementite particles present.
Now, since the troostite grains start forming at
the austenite grain boundaries and grow by the transformation
of gamma to alpha iron, it is evident that
they consist of single grains of alpha iron crystallizing
about neuclei, that exist or are formed at the austenite
grain boundaries. The rate of growth of these
grains decreases as the carbon content increases.
It has been shown above that the preliminary condition
of eutectoid steel on heating through the critical
range will consist of a minutely segregated condition
of the austenite and that the probability is that this
condition will not be effaced until the lapse of some
time. The results of the hardness tests show that
continued heating above the critical point will reduce
the hardening power of the steel at quench and the
resulting proportion of martensite to troostite. For
the 30 min. treatment at 1600 deg. F. the steel in
nearly all cases was composed of pure troostite. It
thus appears that the higher carbon plates of the
minutely segregated austenite produced by heating
through the critical range is as effective a bar or
hindrance to the spread or growth of the troostite
grains across or through the austenite grains, as if
the latter had the higher carbon throughout.
•Jeffries and Archer, Science of Metals, 1924, p. 422.
Course in Industrial Engineering
Lehigh University has announced that beginning
next September a four-year course in industrial engineering
will be inaugurated. Recognizing that every
modern enterprise depends on sound financing, adequate
accounting, and intelligent forecasting of economic
developments, the faculty at Lehigh will undertake
to produce engineers as thoroughly grounded in
these fundamentals of business as in mathematics,
physics and scientific subjects. The curriculum is
primarily of an engineering character, and will equip
the student with sufficient technical knowledge to
make him at home in a highly technical environment.
In addition, however, it will include courses in economics
and business that will be of service to those
graduates who enter the less technical departments of
any of the various industries that are essentially technical
in character.
Recording Smoke Detector
An electrical instrument to be attached to power
boilers, to give immediate warning of smoke, has been
developed by the Engineering Corporation, Long
Beach, Cal. The instrument consists of an element
insterted in the boiler flue, which is essentially two
plates between which the flue gases pass. These
plates are in the primary circuit of a transformer; in
the secondary circuit is the sensitive indicating and
recording unit, a graphic instrument of the switchboard
type. The conductance between the plates increases
as the density of the smoke passing between
them, causing a flow of charging current which is
indicated and recorded. The instrument is known as
the Kingsbury recording smoke detector. Patents
have been applied for.