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A STUDY OF MICRO-CHANGES IN HARDENED ;
HIGH CARBON STEEL AT ELEVATED TMPERATURES
UNDER A REDUCED PRESSURE’

Thesis
Submitted to the Faculty
of
Michigan State College
of Agriculture and Applied Science
in Partial Fulfillment
of the
Requirements for the Degree
of

'Maater of Science

/
Kenneth Leon girl: )V 410563.): 1"

June .1935 Ni“ :WC

THES'E

 

 

ACKNOWLEDGMENT

For his sound advice and
friendly suggestions I do most
humbly thank Professor Henry E.
Publov under whom this work was

done .

CONTENTS..."...........o...o........I’age

I Introduction
A. Scope of Research.......... 1
II Experimental Work
A. Apparatus.................. 6
B. Procedure and Results...... 12
0. Conclusions................ 23
III Appendix

A. NOtQSOoccceeecooeoceeecceee 86

I INTRODUCTION

- 1 -
A. SCOPE OF RESEARCH

In general, microscopic examination has
been applied to the study of'metals at room
conditions. These conditions are almost
requisite inasmuch as the difficulties of
manipulation.make it very nearly impossible to
do otherwise. If a microscope were focused
upon a polished and etched section of steel and
the specimen then heated by some means or
other, the clearly defined.microstructure would
be obliterated by the formation of oxide films
before the temperature had been raised suffi-
ciently to shmw any structural change.

Nevertheless, it is well established that
very important structural changes do occur in
steel and.many other alloys at temperatures
exceeding that of room.conditions. These
changes have been studied dynamically by thermal
mmthods, magnetic methods, and other means.

The resulting structures upon repolishing and
reetching after nearly every heating and cooling
condition possible have been studied extensively
with the microscope. But as brought out in the
first paragraph it is nearly impossible to make
a.microscopic examination while the changes are
actually occurring.

There are exceptions. Some investigators

-2-

have heated polished specimens in a vacuum,
admitted into the containing furnace some etch-
ing gas such as chlorine, evacuated the furnace,
and then cooled the specimen in a vacuum.

They interpreted the resulting structure on the
polished surface as that having existed at

the elevated temperature. Rawdon and Scott
‘utilized a similar heat etching:method*.

Rogers and van Wert" photoed a wave passing
over the surface of pure iron, heated in a
miniature electric vacuum-furnace (hydrogen
atmosphere), through a quartz window. This

wave was interpreted as the A3 transformation.
Knight*'fiesigned an apparatus to show the low-
temperature transformation of austenite to
martensite by using alloy steels. Parker**"‘
designed and constructed a small vacuum

furnace to set upon the stage of a metallurgical

microscope. The design was such that the

* "Mierostructure of Iron and Mild Steel at High
Temperatures“, Bureau of Standards Scientific
Paper #356.

*" "Movie of Metals at High Heat", Metal Progress,
vo1.zz, #z,p.46.

-3-

polished surface of a specimen fell Just within
a window in the top portion of the furnace.
It was possible to focus a low power objective
upon the surface of the specimen and observe
as it was heated in a vacuum. Parker worked
with normalized samples, mostly of low and
medium carbon steel. Although he was unable to
see a dissolving reaction at the alpha-gamma
transformation, he did note several interesting
phenomena concerning volume changes within the
critical range, loci of carbide reactions,
effects of reduced pressureupon allotropic
transformations and the volatilization of
carbon from surfaces upon heating in a vacuum.
This same method of investigation with
some modifications was adopted in this work in
an attempt to discover new clues to the age-old
secret of reactions involved when steel is
quenched and when the resultant hardened steel

is tempered.

*** “Apparatus and Method for Metallographic Work
at Low Temperatures”, Metals and Alloys, pp.256-8,
Vol.5; Nov.1934.

**** "The Structure of Steel at Elevated
Temperature Under Reduced Pressure".

M.S. Thesis, Michigan State College, 1953.

Figure 1

The apparatus which was used
in a previous investigation.

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-4-

The mechanism of the hardening of steel
might be correctly called the caption of an
eternal debate. As long as scientific methods
have been employed in the study of metals,
metallurgists (basing their conclusions upon
experimental evidence most convincing to
themselves) have advanced theories to account
for the phenomenon so useful to the evolution
of civilization. No particular theory has
ever gained universal acceptance. This fact
clearly indicates that there is some link
missing. New methods of research are necessary
to answer the question. Dr. Sauveur writes,
"It would seem as if the methods used to date
for the elucidation of this complex problem
have yielded all they are capable of yielding
and that further straining of these methods
will only serve to confuse the issue, a
point having been reached when this juggling,
no matter how skillfully done, with allotropy,
solid solutions, and strains is causing weari-
ness without advancing the solution of the
problem, The tendency of late has been to
abondon the safer road of experimental facts
and to enter the maze of excessive speculations,

in which there is great danger of some becoming .

hopelessly lost."f

The work related by this thesis has been
done in an effort to bring light from a
different angle upon this most interesting
problem. The facts and data recorded in the
following pages are not offered as the solution
to the enigma. The conclusions reached may be
erroneous, but the experimental work is presented
in sincerity. It is heped that this investi-
gation will be a means, if nothing more, of
showing to those experimenters more skilled and
experienced in the study of metals than is the
author, the possibilities or the hopelessness
of this method of studying the reaction in-
volved in the hardening of steel and the tem-

pering of hardened steel.

I"Sauveur; "The Metallography and Heat Treatment
of Iron and Steel", University Press, Cambridge,
Mass; 1926.

II EXPERIMENTAL WORK

- 6 -
A. APPARATUS

The furnace designed by Parker (See
Figure l) was available so several trial
heats were made with it. Two difficulties
were outstanding. The illumination system
was cumbersome, being difficult to adjust
and keep in adjustment. The vacuum seal be-
tween the body and shell of the furnace was
rather tempermental. At times it held
exceptionally well and sometimes it leaked.
Consequently, a new furnace was designed to
eliminate these difficulties. Details of
the furnace are shown in the accompanying
blue-prints.

The furnace is cylindrical in shape
having been turned from.a billet of 0.4%
carbon steel. It rests upon the stage of a
Bausch and Lomb large metallographic micro-
scOpe of the Le Chatelier type. The cut-
away portion of the base of the furnace body
is sufficiently large to allow the objective
to approach the observation window. It is
necessary to have the furnace carefully placed
upon the stage with the objective centered

before the stage is lowered.

The observation.window mentioned in the
preceeding paragraph.is made of optical glass,
having a thickness of one-sixteenth inches
and a diameter of fifteen-sixteenth inches.

It is cemented in place with De Khotinsky
cement over the drilled-hole which is centered
in the thin section of the base. The window
is cooled by a stream of air from.which the
oil (contamination resulting from the com-
pressor) is removed by a trap packed with
glass wool.

The heating element, wound in spiral from
B a 3 gauge #18 chromel wire, is held in
position by alundum cement cast in a steel
supporting ring. Sheet-mica, which has been
rolled into a cylindrical shell to fit inside
the element, serves to insulate the specimen
from the heating element. The combined weight
of the assembled element and specimen is about
one-hundred-ninety grams so it sets firmly in
place without clamps.

The specimen used in this work is made in
the form of a cylinder, having a diameter of
one-fourth inch and a length of one-and three-
fourths inches. A three-thirty-seconds inch

hole was drilled along the longitudinal axis

-8-

of the piece to a depth of one-and three-
eights inches. This is to permit insertion of
a minute thermocouple for measuring the
temperature of the specimen. The piece is
threaded for a distance of one-eighth inch on
the end from which the thermocouple hole is
drilled.

A thin asbestos-board disc, of sufficient
diameter to overlap the supporting ring and
resting upon it, is drilled at the center to
take the shank of a small steel fitting. This
fitting is drilled and tapped to accomodate
the threads of the specimen. (Originally, the
disc was made of steel and the specimen screwed
into it directly as shown in Blue-print 4.
Later the design was changed as described above
and shown in Blue-print 5. The purpose of the
asbestos was to reduce heat losses.) The
specimen, screwed into the fitting is thus
suspended through the loops of the spiral-
wound heating element along the vertical
diametrical axis of the furnace. The lower end
of the specimen is ground and polished by
metallOgraphic methods to a plane perpindicular
to the axis. The height of the supporting ring

and the length of the specimen are correlated

-9-

so that the surface to be observed is approxi-
mately the same distance from.the inside sur-
face of the observation.window as is the front
lens of the sixteen millimeter objective (used
in this work) from.the outside surface of the
window. Thus, minimum conduction and radiation
of heat to the window and to the objective is
established.

The leads for the heating-element are
brought in through outlets drilled in the wall
of the furnace. These leads are fabricated
from B a S #8 gauge copper wire which are
adequately large to prevent heating and to
provide rigidity. They are led around the wall
of the furnace, at sufficient distance to
prohibit shorting, until opposite the binding
posts of the element. They are connected with
semi-flexible jumpers of B & S #18 copper wire
so that the heavy wires cannot be loosened
from.their rigid position which is necessary to
make vacuumytight Joints. These Joints were
made in the following manner. Small cylinders
of insulating fiber, with flanges on the end
coned to fit against the shoulders in the
drilled outlets, were turned in a lathe and

holes were drilled along the diametrical axis

of a size to take the copper leads with a tight
fit. These fiber cylinders are held firmly in
place by bushings bearing against the flange.
The outer surfaces are covered with a thin coat-
ing of De Khotinsky cement to prevent leakage.

The thermocouple is made of B a S gauge
#25 iron and constantan wire. The leads are
brought through the side-wall of the furnace
near the tap by means of an outlet and Joint
made in the same manner as that used for the
element leads except that two tiny holes were
drilled in the fiber plug for the wires in-
stead of the one hole for B an S gauge #8 copper
wire. When the couple is inserted into the
specimen the wires are insulated from each
other by means of a long narrow section of
sheet-mica.

The furnace shell is cooled by circulation
of tap-water through a system of holes drilled
through the base as shown in the blue-prints.
The furnace is sealed ,by cap held in place with
eight cap-screws. A grooved circular tongue on
the lower surface of the cap fits into a
corresponding grooved circular key-way in the
shell. The seal is made perfect by a lead
gasket. This method of closing off the furnace

- 11 -

is very satisfactory, never giving trouble.

Other apparatus used in the investigation
includes a cenco hi-vac vacuum pump, mercury
manometer, Leeds and Northrup potentiometer,
rheostat, Bausch and.Iomb large metallographic
microscope and.water and air connections.

Direct current is used as a source of
energy for heating inasmuch as alternating
current causes slight vibration.

Figure 2 is a photograph of the furnace
and auxiliary apparatus in position used for

the experhmental work described in this thesis.

The‘apparatua which was used
in this investigation.

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. 12 -
B. PROCEDURE AND RESULTS

_It should be explained.at the outset that
all samples used in the investigation fell under
the specifications of S.A.E. 10120 steel so
they will be considered as such unless other-
wise designated. All samples were given a
preliminary heat-treatment which consisted of
heating to 1850°F., holding for 150 minutes,
and furnace cooling. This procedure was for
the purpose of thoroughly normalizing the
samples and making the grains sufficiently
large for resolution with a 16 mm, objective.

It was impossible to use an objective of greater
resolving power because the working distance
had to be adequate to accomodate the observation
window and to prevent the disfiguration of the
front lens of the objective by heat radiation.
Furthermore, several heats made with etched
specimens proved that chemical etching was unp
necessary and often gave an indecipherable
Pattern upon the surface after heating. Hence,
Practically all runs were made with unetched
Specimens.

Parker concluded from his observations of
ilnwbcarbon steels that the critical range is

'lowered from.one-hundred to one-hundred-and-fifty

-13-

degrees fahrenheit by heating in vacuum.) He
verified this statement by two methods,l ob-
taining cooling-curve data 21. w and observ- ‘
ing a double-network forming on the surface“ it
from heat-etchingand volume changes occurring
at the critical temperature. He observed that
carbon volatilizes very rapidly from the surface
when the specimen was heated into the critical
range in a vacuum. This reaction appeared to
occur most markedly at the grain-boundries,

the cementite of the pearlite proceeding into
minute globules which migrated to the boundries.
These globules evidently decreased in size
until the carbon reached an atomic state, giving
opportunity for volatilization.

Figure 5 is a photomicrograph of the
polished specimen at 100 diameters as it came
from the preliminary heat-treatment. There is
a faint outline of the cementite network around
the pearlite due to relief polishing. However,
this detail is very faint and otherwise nothing
is visible except the final polishing scratches.
Heating to 500°F. caused no change to occur but
' as the temperature was slowly raised another
tWenty or thirty degrees, the cementite boundries

El-'bl'tlll>’61y appeared as dark lines surrounding the

9
Magnificat ion: 100 Diameters

FI CURE 4

Magnification: 100 Diameters

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-14-

pearlite grains. This phenomenon occurred
repeatedly within a 50°F. range (500°F. to
550°F.) and it assumes significance in the
light of other evidence to be presented later.
Figure 4 gives the appearance of the surface
magnified 100 diameters at 600°F.

Upon raising the temperature further
another reaction starts, a roughening of the
pearlite grains becoming evident between 650°F.
and 700°F. This condition was pronounced at
855°F. when the surface was photographed at
100 diameters. See Figure 5. Whatever the
occurrence was, the activity must have been
progressive inasmuch as the surface disturbance
becomes more marked without any change in de-
tail by the time 1040°F. is reached. It was
that temperature at which Figure 6 (100
diameters) was taken.

If the conditions were favorable, further
heating into the critical range caused the
surface to become brighter. No sharp change was
observed at either the A3_2_1 point or the ‘cm
Point. The surface gradually assumed the
appearance as shown in Figure 7 taken at 300
diameters. The detail is rather indistinct.
hamination of the surface with an objective

FIGURE 5

Magnification: 100 Dismet ere

FIGURE 6

Magni that ion: lOO Diameters

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FIGURE 7

Magnificat ion: 2500 Disasters

FIGURE 8

Hagni ficat ion: 700 Diameters

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-15-

of higher resolving power (such as 5.5 mm. with
which Figure 8 was obtained at approximately
700 diameters) after ecoling in a vacuum re-
vealed areas nearly devoid of cementite. This
confirms Parker's observation that rapid
volatilization of carbon takes place from
surfaces heated into the critical range.
Cementite globules are concentrated in the
grain-boundries in the same manner as found
by Parker in low-carbon steels. Since the free
cementite of a hyper-eutectoid steel occurs in
the grain-boundries naturally, no parallelism
can be drawn concerning accelerated carbide
movements at the grain-boundries. Figure 9
gives the appearance of the surface through a
5.5 mm. objective after filing away approxi-
mately one millimeter of metal and then polish-
ing and etching with 2.0% nital. Decarburiza-
tion is still evident at this depth. Figure 10
was taken at 100 diameters along the edge of
the piece, indicating that the carbon was
nearly all removed along the free surface and
that the carbon content increased with the
distance from.that surface.

Now considering a sample that had been

quenched in ice-brine from.l?OO°F. subsequent

FIGURE 9

Magnificat ion: 500 Diameters

hgnificat ion:

100 Diameters

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. 15 -

to its prelhminary heat-treatment, it was
observed that upon heating in vacuum there was no
reaction upon the surface until a temperature
of 480°F. to 530°F. was reached. Within that
range there appeared the roughening effect
which continued to grow more pronounced as the
temperature was raised. Figure 11 is a
photomicrograph at 100 diameters shmwing the
heat-etch which was imparted by heating to
718°F. The heating-rate was the important
factor affecting the initial appearance of
the heat-etched pattern. Extremely slow

heats caused it to appear as low as 480°F.
while with normal heats it usually appeared
from 520°F. to 530°F.

The close agreement in respect to temp
perature between the appearance of acicular
structure upon the surface of the quenched
sample and the free cementite upon the sur-
face of the normalized sample is evident.
This fact taken alone might indicate that
similar volume changes were occurring in the
two pieces. However, such a conclusion is
refuted by certain dilation experiments per-
formed by L.L. Clark and Prof. H.E. Publow
at Michigan State College. They found that

FIGURE 11

Magnificat ion: 100 Diameters

PI CURE 12

Magni ficat i on: 300 Diameters

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when a quenched S.A.E. 10120 steel is heated
very slowly in a dilatometer, two minor arrests
in expansion occur below the AZ-Z-l temperature
whereas normalized S.A.E. 10120 steel expands
smoothly until the A3_3_1 temperature is
reached. Typical dilation curves for quenched
and normalized samples are shown in Figure 15.
The difference in lengths at room temperature
was due to failure of the piece upon quenching
to return to its original length. The first
arrest, occurring at approximately 250°F., was
very slight. The second arrest was very
noticable. The specimen stopped expanding at
about 480°F. and recovered at about 600°F. to
continue normal expansion until the critical
range was reached. The normalized specimen
expanded as a straight-line function throughout
the entire range from.room-temperature to the
A3_2_1 point.

Condensing this experimental evidence we
have the cementite net-work of normalized
samples and the acicular structure of quenched
samples appearing at very nearly the same tem-
peratures. Dilation experiments show that
normalized samples expand uniformly below
A whereas quenched samples show an arrest

3-2-1
in their normal expansion curves. Also, it is

FIGURE 13

Dilation Curves

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-18-

quite generally accepted by metallurgists
that upon severe quenching (such as was given
the samples in this work) the gamma-alpha
transformation is depressed to a temperature
approximating 550°F. The inertia of the iron
atoms and iron-carbide molecules (or carbon
atoms, allowing the views held by Jeffries
and Archer*, and others) within the face-
centered space-lattice must be sufficient to
withstand the directional change of atomic
attraction or repulsion, set up by the

shift of energy-levels within the iron atoms
at the normal allotropic transformation tem-
perature until that specific lower temperature
is reached.

Consideration of this evidence suggests
that there is some sort of spontaneous reaction
on heating within the temperature range of
500°F. to 550°F. At this temperature atomic
mobility is low and thus the effect is masked

to make closer isolation impossible. Therefore,

* "The Science of Metals", McCraw-Hill Book Co.,

New York; 1924.

-19..

any direct attempt to analyze the reaction
would be a speculation. However, it is be-
lieved by the author that this reaction is
connected.with a molecular change, one which
permits molecular association and which causes
a change in atomic volume of the constituent
which we ordinarily consider cementite.

So far, the discussion of quenched samples
has been limited to the range of temperature
below 7000F. As the temperature was raised ,
above that point the surface distortion in-
creased, the acicular veins becoming more and
more prominent. Figure 12 is a photomicro-
graph at three-hundred diameters when the
specimen was at 1065°F. Figure 14, also
photographed at 300 diameters, shows the sur-
face of a quenched sample which has been etched
with nital. By comparing it with Figure 12,
we note that the patterns produced by the two
types of etching are quite different.

Figure 15 and Figure 16, both taken at three-
hundred diameters, show the progressive)
changes in microstructure at 1290°F. and
15350F. as the specimen was heated steadily
into the critical range. There was no abrupt

change at the critical temperature; the lines

FIGURE 14

Magnification: 300 Diameters

FIGURE 15

Magnification: zoo Diameters

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FIGURE 17

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- 20 -
slowly lost their distinct appearance and the
microstructure became increasingly messy.

The change which.was apparent even below
the A343”1 temperature was that which might
have been anticipated. Such a change occurs
by heat-treatment for spheroidization of
cementite. The atomic mobility has increased
to the point where the cementite particles
diffuse readily. Such a condition.might
logically be expected to be more accentuated
at and near a free surface where internal
strains are minimum.

Figure 17 is a photomicrograph at three-
hundred diameters of the surface of a specimen
which was held at 1045°F. for thirty minutes.
The micro-structure shows the effect of holding
at temperature as contrasted.with that of
Figure 12 which was taken when the specimen
first reached 1065°F. Similarly, Figure 18
shows the effect of holding at 1250°F. for
one hour as contrasted.with Figure 15 which
was taken when the specimen first reached 1290°F.
In both oases_the acicular structure has begun
to lose its sharpness of detail. This suggests
a movement of cementite particles which.takes

place at these temperatures where the well-known

FIGURE 18

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FIGURE 19

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Figure 19, Figure 20; and Figure 21
were all taken at a magnification of three-
hundred diameters. The first shows the sur-
face as it appeared at 1655°F. and the second
and third show the surfaces of pieces which
have been heated to 16600F. and 17400F. re-
spectively and cooled in vacuum. The identity
of the acicular structure has vanished.
Examination with greater resolution gives
further evidence that the condition produced
by quenching has been effaced. Figure 22 is
the same surface given by Figure 20 as it
appeared.with an oil-immersion objective
(magnification: 2200 diameters). This shmws
very plainly that the ferrite grains have formed
and that carbon has volatilized from the sur-
face. Also, the majority of the cementite
spheroids are found in the grain boundries
‘which substantiates Parker's conclusion that
grain-boundries are the loci of greatest
activity.

Figure 23 is a photomicrograph (taken at
2000 diameters with an oil-immersion objective)
of a specimen that had been held at 800°F. for

FIGURE 20

Magnification: 300 Diameters

FIGURE 21

Ingnification: 300 Diameters

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two hours and cooled in a vacuum and followed
by polishing and etching. The cementite part-
icles have started to form.and they are very
numerous. This evidence also supports Parker's
work in showing that the carbon tends to
volatilize:much.more rapidly above the critical
than below it.

All in all, we may summarize the reactions
of quenched high carbon steels above the A3_2_1
temperature as behaving in.a manner identical

with normalized high carbon steels.

-23-
C. CONCBUSIONS

When a high-carbon steel is heated to
500°F. - 550°F. in a vacuum, there appears
upon the surface a heat-etched pattern. Such
a pattern appears upon either a normalized steel
or a quenched steel, being a characteristic
cementite netdwork'upon the former or being a
mass of acicular needles upon the latter.

Such a reaction may be associated with the
arrest, at that temperature, in expansion
which takes place when a quenched steel is
heated in the dilatometer. A normalized
steel does not show an arrest in expansion
below the A3_2_1 point. It does, however,
show evidence of a microstructural change as
does also a quenched steel. Is it not then
reasonable that the reaction,whatever it may
be, takes place in a normalized steel, and
the condition resultant from.quenching:mere1y
serves to accentuate the reaction? Regarding
its mochanism.we can only speculate. However,
if we accept the views held by many metallurg-
ists that quenched steels are composed of
extremely fine grains of ferrite and particles
of iron-carbide in fine dispersion, we must

confine our conception of the reaction to some

- 24 -

change that would affect either ferrite grain-
size or iron-carbide particle size and dispersion.
Since there was a reaction in the normalized
steels and since there is no lOgical reason to
believe that a change in ferrite grain-size
would occur there, we should.most reasonably
look to the cementite for an explanation. It

has been shown by many investigators that
tempering promotes growth of cementite particles.
However, just when and where this growth.begins
has never been determined. iMight we not con-
clude that this reaction, occurring at 500°F. -
550°F., suggests a molecular change of the
iron-carbide affecting the properties of
molecular mobility, molecular association, and
most likely atomic volume? Whether or not

there is any gamma iron retained at room-
temperature upon quenching is a debatable question
Assuming that there is, we might account for

the reaction at 500°F. - 550°F. as a residual
gamma-alpha transformation but for one fact.

That fact is the corresponding appearance of

the cementite netawork of normalized steels

when heated to that temperature. According

to the evidence of this investigation, the

only probable temperature of this transformation

is at about 250°F. where the first arrest in

-25-

dilation appears upon heating a quenched steel.

Further heating of the normalized samples
showed a progressive roughening action upon
the surface above 500°F. This was undoubtedly
caused by a spheroidizing effect upon the
cementite in the pearlite. A close examination
of Figure 5 and Figure 6 shows the cementite
to be spheroidal in nature. Holding*within
the critical range for short periods of time .
caused the surface to lose cementite. This
loss was most likely caused by volatilization
which seemed to take place primarily from the
grain-boundries.

The changes in.micro-structures of
quenched steels upon further heating were
analagous to those occurring in normalized
steels. The heat-etch pattern became in-
creasingly fuzzy, indicating that carbide
particles were most likely moving and coalescing
to form.increasingly larger spheroids. Holding
at temperature within the critical range
caused a reaction to occur very nearly identical
with that of normalized steels. Figure 20 shows
grain outlines and.much less carbon than we would
expect in a S.A.E. 10120 steel. Evidently, there
has been a loss of carbon through volatilization
in this case as there was with the normalized

sample 8 o

III APPENDIX

A. NOTES

It would not be fitting to close this
thesis without mentioning some of the observa-
tions and incidents in connection.with this
investigation which do not fall directly under
the title.

One of the most significant observations
of the entire investigation was that concerning
a particular characteristic of heat-etching.
That characteristic was the permanence of the
heat-etch imparted at the highest temperature
reached. That is, no reversal of reaction
was noted upon a surface at any time as samples
were cooled after their maximum.temperatures
had been reached.

In an attempt to reduce volatilization
and to prevent minor oxidation that sometimes
occurred, a carefully controlled hydrogen
atmosphere was used in the furnace for several
trials. No improvement resulted and since the gene
eration and purification of the hydrogen re-
quired no little amount of time, the use of hydrogen
was discontinued.

An attempt was made to observe the mechanism
of recrystallization of cast brasses and bronzes.

The idea was soon discarded, however, when it

-27-

was found that zinc, tin, lead, or any similar
constituent would volatilize from the surface
at 600°F. - 800°F. and the vapors would
condense upon the observation window, making

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