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Motion Picture Studies
of
The Grain Formation
In Steel
At Elevated Temperature
and

Under Reduced Pressure

Thesis
Submitted to the Faculty
of
Michigan.stete College
Of Agriculture and Applied Science
In Partial Fulfillment
Of The
Requirements for a Degree
of

master of Science

Frederick W3 Kerr

m

Junae 19 37

co ’9 fun .‘7.

I!!! .33.: -r

'”‘_'O:Q!q

ACKNCWLBDGNENT

I wish to express my appreciation to
Professor H. E. Publow; under whose
direction the present'work has been per-
formed, and whose assistance has been

generously given on this problem.

TABLE OF CONTENTS

IntrOductiO n.- ----- -------------
Apparatus --—-------—-------------
Procedure ----------u------ III-I----

Experimental ----------- ........ --

 

Summary ----—-- ------ - ............

Film.Index — —— _ —____— —_ __

 

Page 1

Page 5

Page 11
Page 12
Page 2h
Page 29
Page 30

Page 31

1

INTRODUCTION

Many very important changes take place both in the physical
properties and in the microstructure in steel at temperatures
exceeding that of rcem.temperature. The changes in the physical
properties such as the yield point, tensile strength, elongation,
etc., have been studied extensively by many investigators. In
the majority of cases test bars have been subjected to tensile
tests while maintained at some predetermined temperature.
The changes in the microstructure, or the allotropic trans-
formations in low, medium, and high carbon steels and in a variety
of alloy steels, have been studied by a great number of'workers.
There is yet no theory or explanation of these allotropic forms
or transformations that is universally accepted by all. A compara-
tively few have studied the transformations as they actually take
place, that is, by observing the transformations through a micro-
scope as it occurs in the metal. The changes in the microstructure
have been studied by heating steel in a vacuum.ar supposedly neutral
atmosphere of hydrogen or nitrogen, and the resulting structure
assumed to be that which existed at the highest temperature reached.
Other investigators have heated polished specimens in a vacuum.and
then admitted some etching gas (chlorine) allowing it to remain for

a few mements and then pumping it out. The specimens were then cooled
in the furnace and the structure assumed to be that which existed

at the temperature the etching gas was admitted into the furnace.

Others heated polished specimens in a vacuum while under microscoPic

observation and interpreted the surface changes which took place
as those brought about by a change in volume or by allotropic
transformation.

B. A. Roger (7), by means of a.motion picture film showed the
A} transformation in Armco iron as an eruptive wave which appeared
at one side of the field of view and swept over the surface. The
time of passage of such a wave being a fraction of a second or
several seconds, depending upon the rate of heating.

H. J.‘Wiester (9), at Technischen Hochschule, Berlin, suc-
ceeded in watching the crystallization of martensite directly in
the microscope and in photographing the transformation on a motion
picture film'.

'Wiester obtained austenite by quenching steel with 1.7 percent
carbon in a metal bath at'lOO C. The austenite is extremely stable
at this temperature, and when the steel was allowed to cool,marten-
site crystallized out from the austenite grains. The changes in-
volved an increase in volume, and was the result of two opposing
tendencies; the immobility of the carbon atoms which obstruct atomic
rearrangement, and the tendency of the iron atoms of the undercooled
austenite to expand into the alpha space lattice.

T. D. Parker (3) designed a small vacuum.electric furnace to
set on the stage of a microscope. The furnace was so constructed
that he could observe, through a small glass window in the top a

specimen as it was heated in a vacuum through the critical range.

Parker used normalised specimens of low and medium.carbon steel.
He observed several interesting facts, among'which'was the vola-
tilization of elements from the polished surface, or decarburiza-
tion, and the volume changes at temperatures within the critical
range. He did not observe any dissolving reaction of alpha and
gamma iron or the A3 transformation as an eruptive wave passing
over the surface.

K. L. Clark (3) followed the same method of investigation,
'with a few modifications, as did Parker. In his investigation
hardened high carbon steel was used, and when this steel was
heated in a vacuum, a heat-etched pattern appeared in a similar
manner as observed by Parker. Upon the normalized steel the char-
acteristic cementite netawork was formed and upon the quenched
steel a mass of acicular needles. Clark did not observe a wave
pass over the surface of the metal as it passed through the A}
transformation range.

Rawdon and Scott (6), in their work examined the structure of
iron and mild carbon steel at high temperatures by heating polished
specimens in a vacuum. They interpreted the heat-etched pattern
produced on the polished face as the structure corresponding to the
different allotropic forms. They also state that the pattern pro-
chxced by heating revealed not only the condition of the surface metal,
but also that of the interior.

The present investigation was performed in a similar manner as

L;

followed by Parker and Clark, i.e., heating a specimen in a vacuum
while under observation. Motion picture apparatus was used to

record the changes in the structure which took place on the polished

surface of the specimen.

5

APPARATUS

The apparatus used in this work consisted mainly of a small
vacuum electric furnace, microscope, Cine'-Kodak motion picture
machine, model A, and illumination system (Figure 1).

The furnace (Figure 2) was the one used by Parker in his work.
The method of supporting the specimen ‘was changed (Figure 3) so
that it would not be necessary to remove the heating element each
time a new specimen was used. The furnace was designed so that it
could be placed on the stage of a Bausch and Lomb microscope
(Figure 3a). Holes drilled in the base of the furnace provided for
the entrance of the thermocouple and heating element leads. A hor-
izontal hole was drilled through the base and connected to a water
line for cooling purposes. A silica ring supported by a small
flange on the furnace base held a small steel cylinder which had a
one-fourth to one-eighth inch tapered hole in the center. The spec-
imen was machined to fit this tapered hole, thus giving it a fairly
rigid support. Supporting the specimen in this manner facilitated
the removal and insertion of a new sample. Temperature measurements
were made by a Leeds and Northrup potentionmenter using an ironpcon-
stantine thermocouple made of B. and S. gauge 25 wire. The thermo-
couple passed through the silica ring and steel cylinder into a one-
eighth inch hole drilled in the steel specimen. The end of the couple
'was one—eighth inch or less below the polished surface of the speci-
men and the temperature recorded by the couple was assumed to be that

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The top of the furnace was equipped‘with a glass tindou'whick.made
the specimen visable frmu the outside. The height of the furnace
was such that the distance from the top of the specimen was less
than the working distance of a is mm. objective. ‘A gravity seal
was maintained between the base and the top of the furnace. The
contact surfaces were ground together with different grades of
abrasives to give a true planesurface. It was necessary to pass a
stream of air over the glass window to keep it cool when the fur-
nace was being heated.

It was difficult to maintain a vacuum.in the furnace as
designed by Parker. The contact surface between the top and bottom
would leak, unless perfectly true, and had to be frequently reground.
The method of inserting the heating element and thermocouple leads
was very unsatisfactory, since they were passed through rubber
stoppers cemented in the base of the furnace and covered with De-
Khotinski cement. Thus, whenever the lead wires were moved, the
cement was apt to be cracked and a vacuum.eould not be maintained.

In order to overcome these difficulties, a new furnace was de-
signed and constructed (Figure h). The details of construction are
shown in the accompanying blue prints. The furnace was made much
lighter and smaller, being only three inches in diameter and about
'bwo and one-half inches high. The top of the furnace was made in
‘the form of a flat plate with a small flange on the under side.
Tfliis flange was machined to fit into a small groove containing a

Ilead gasket, in the body of the furnace. The top was secured to the

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body by six machine screws. This method gave two contact surfaces
and also the additional advantage of the lead gasket.

The furnace was so designed that the heating element and thermo-
couple lead-ins are permanently sealed into the wall of the furnace.
A test sample may be removed or replaced without altering or touch-
ing the electrical or thermocouple leads on the inside of the fur-
nace. The blue print No. 5 shows the details of assembly. The ex-
ternal connections to the furnace are made by plugging into the
permanently embedded brass lead-ins.

The window in the top had to be changed from glass to fused
silica to withstand the temperature change and prevent breaking.
Transparent Vitreosil or fused silica discs were used which had been
ground and polished. (Discs obtained from the Thermal Syndicate
Ltd. 58 Schenectady Ave., Brooklyn; N. Y.)

Other details of the furnace, i.e., the heating element, method
of supporting the specimen and water cooling the base, etc., were
not altered.

The electric heating element, made of chromel resistance wire,
(B. & S. gage 18) was wound in a spiral. The spiral was about one-
fourth inch larger in diameter than the shank of the specimen. It
was cemented, with alundum refactory cement, into a silica tube and
placed around the shank of the specimen and supported on the base of
the furnace as shown in Figure 5.

The microscope used was of the type generally employed in.metal-

lography. (Manufactured by Bausch and Lomb.)

The apparatus for taking the motion pictures consisted of a
Cine'-Kodak Model A without lens, support for Cine'-Yodak with motor,
speed control, place for supporting microscope, optical connector,
and observation eyepiece. (l)

The support of the Cine-Kodak has a 110 volt 60 cycle synchron-
ous motor mounted on it, which, through a chain of gears, drives the
camera at one of eight different speeds. The gears are inclosed in
a sound proof box and the functioning of the gears and the running
of the motor induces but a negligible amount of vibration, which is
a very important factor. The speeds are controlled by an external
gear shift drive having eight rings numbered from one to eight. The
slowest speed permits pictures to be taken at the rate of one every
four minutes and sixteen seconds, while the highest speed is sixteen
pictures every second. The following table gives the different speeds
for the different gear positions.

Gear Position Picture Speed
1 16 per second
8 per second
2 per second
1 every 2 seconds
every 8 seconds
1 every 32 seconds

1 every 2 minutes and 8 seconds

CDN) ()th N
H

1 every h.minutes and 16 seconds

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The support carrying the camera, motor, and gear train is ad-
justable for height. Into the camera is fastened an optical con-
nector and observation eyepiece. The microscope sets on a base
plate so that the body tube and eyepiece adapter are directly under
the connector. Into the eyepiece adapter of the microscope is in-
serted an amplifier, either a 7.5 X, 12.5 x, or 15 X, depending on
the magnification required. The observation eyepiece is equipped
with a system of prisims so that observation and focusing adjustments
can be made while the camera is in Operation.

The electric motor is operated by 60 cycle current, the altera—
tions of which are constant. With the shift lever set at a certain
number of exposures per minute, or per second, there is very little
deviation from.this speed. The importance of this fact lies in the
advantage of being able to take a film when it is developed, and by
counting the number of frames during which a complete process or
change has taken place, determine the exact rate at which it took
place.

The following pieces of apparatus were also used:

Cenco Hyvac Pump
Mercury manometer
Leeds and Northrup potentiometer
Carbon arc lamp
Variable resistance

Electric heating element

10

Steel specimen

Air line.

Water connections

Mazda ribbon filament lamp

A. C. llO-volt, 60 cycle current

D. C. 220-volt line, used for heating

ll
PROCEDURE

The procedure for a characteristic experiment can be described
briefly in the following manner. In all experiments the specimens
were of 1020, thO, or 1050 steel. The samples were first heated
in a vacuum bomb at 1800 F for a period of three hours so as to pro-
duce a large grain structure and to remove the occluded gases. They
wmre then machined down to size. (One-fourth inch in diameter and
one and one-quarter inches in length.) The specimens were prepared
for metallographic examination by the usual procedure. The sample
was then placed in the furnace. The contact surfaces of the top and
base were wiped clean and then a thin film.of vaseline was applied
and.the top placed in position. The vacuum pump was started and
allowed to run for twenty minutes or more before the furnace was
heated. The cooling water was adjusted and camera placed in position.
When the specimen had reached a temperature of between 500°F and
800.? the microscope was focused and.the camera started. As the
temperature increased the microscope had to be refocused due to the
expansion of the specimen. During the experiment the temperature was
recorded each minute, and during several runs, for each foot of fiLm
used. From 25 to 35 minutes were required for the furnace to reach
maxtmwm temperature. The specimen was allowed to cool in the furnace
so that the surface would not become oxidized and it could be examined

when removed.

l2
EXPERIMENTAL

In this work a new method of recording the changes which take
place on the surface of heated steel was employed. Instead of the
customary still pictures, a motion picture camera was used to obtain
moving pictures. There was no literature or information available
concerning the time of exposure or the speed at which the camera
should be run for this method of operation. Thus considerable time
and experimentation were required to obtain a satisfactory combin-
ation of light intensity and shutter speed.

A short exposure of film.was made for each of the eight differ-
ent shutter speeds. A carbon arc lamp was used as the source of
light. The results of this first film showed the sixth speed (one
exposure every 32 seconds) gave the right time for exposure, but as
the change took place rapidly, this speed was too leW'to record the
details. The preceding speeds did not give sufficient time for ex-
posure, thus causing the film to be light. Speeds seven and eight
were slow, causing the film to be over-exposed and dark.

The camera is equipped with a small disk which has in it four
smoked glasses of different densities. The smoked glasses are
marked 0.0, 0.3, 0.6, 0.9, and henceforth will be referred to as
0.3 smoked glass, etc. The following table shows the amount of

light which passes through each glass:

Smoked glass Percent of light
o.o 1007:
0.3 70%
0.6 h0%

0.9 10%

15

A sample of film was made with the camera set as Number six
speed and using each of the smoked glasses. When this film was ex-
amined it was too dark, being over-exposed. A similar sample of
film was made for speeds five, four, three and two, using each of
the smoked glasses at each different speed. The portion of the film
obtained using Number four speed and the 0.8 smoked glass gave about
the right exposure. Also, Number three speed with the 0.0 and 0.3
smoked glass could be used, but it was too fast. The time of ex-
posure being one-half second, giving 120 pictures or frames per
minute and requiring three feet of film. Use of this speed would
require a great amount of film, involving unnecessary expense.

The lighting arrangement was changed to see what effect a less
intense source of light would have on the shutter speed. The carbon
arc lamp was replaced by a mazda ribbon filament lamp, and a film
made with this arrangement. The camera was run at number seven speed
and a short exposure made using each smoked glass. Upon developing
the film it was found to be over-exposed. Slight detail could be
seen on the portion made when the 0.9 smoked glass was used, but not
for the others. The same procedure was followed for speeds four,
five, and six. The developed film showed that speed five with 0.9
and 0.6 smoked glass gave about the right exposure. Also speed four
with the 0.0 and 0.3 smoked glass.

A film was made using an unetched sample of 1020 steel. The

camera was run at Number four speed (one exposure every mo seconds)

 

 

C7 1

1h

and the 0.3 smoked glass was used to filter out some of the light.
During the heating the temperature'was recorded each minute, and at
the end of each foot of film used. From this data a heating curve
was plotted. (Number 1) From this curve it was possible to determine
the temperature of the specimen at which each individual frame was
exposed. This is possible due to the fact that the camera runs at a
constant rate of speed. This film.did not shmw grain formation very

well because the specimen was slightly oxidized, obliterating the

heat-etc‘
Data for Curve No. I
Time Temperature Feet of Film. Temperature
1 200's 0 050%
2 510 1 885
3 h00 2 770
h two 3 860
5 515 h 91o
6 570 5 950
7 --- 6 1000
8 650 7 1015
9 690 8 1035
10 770 9 1100
ll 8&5 10 1170
12 885 11 1215
13 930 12 1230

In

Time Temperature Feet of Film Temperature
1b. 960 13 1260
15 990 11+ 1300
16 1010
17 1060
18 1100
19 1160
20 1205
21 1250
22 1250
23 1290
2h 1315
25 1530

A short film was made using an unetched sample of low carbon
steel (1020). The camera was run.at number four speed and the 0.5
smoked glass used, the same as in.the preceding trial. (The fiLm
obtained was not clear due to vibration. If the microscope and
adapter were not placed exactly under the observation eyepiece or
if the observation eyepiece was touching it, the vibration of the
motor was imparted to the sample, causing the film.to appear blurred.
The following is the time temperature data recorded for the trial
and the data for plotting heating curve Number 2:

Heating Curve No. 2
Time Temperature Feet of Film. Temperature
0 72'F 0 7 50'F

1 200 1 - 860

15

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Temperature Feet of Film Temperature
11805” 2 955°F
580 3 995
650 h 1030
750 5 1050
800 6 1070
860 7 1120
900 8 1185
9L0 9 1205
995 10 1270

1025 11 1310
10110
1060
1075
1155
1180
1200
1220
1250
1300

 

 

 

     

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

  
     

 

        

 

 

 

 

 

 

 

 

 

 

       

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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16

The two heating curves show that there was some change taking
place between 1000.? and 120an. It was impossible to observe any
detail on the film. During the run the microscope had to be read-
justed several times due to the eXpansion of the specimen. Very
often this resulted in bringing a different field under observation
that was originally observed. Thus, the film recorded a short
length of one field, and then another, and not a continuous picture
of the same field as desired.

Figure 5, taken at 100 It shows a sample of low carbon steel
after it was cooled from 1300.17. The grain structure is much smaller
than in the original nital etched specimen. The pearlitic areas
have completely disappeared. The grain boundaries are much larger
than in a specimen etched with acid. Figure 6 shows the same spec-
imen at l500 x. There appears to be small globules forms in the
grain boundaries.

The motion picture of this specimen showed a formation very
similar to that shown in Figure 6. The first appearance of a heat
etch was at about lOOOoF.

There was no heat-etched pattern on the specimen when the film
was started (BOOOF), but as the temperature increased, a faint pattern
appeared, and as the temperature continued to rise it became more and
more distinct until about 1250.1? when there seemed to be no further
change.

A sample of 0.14 carbon steel was placed in the furnace unetched

I

(D V t? o

17

and heated to MOO'F. The film obtained shows the heat-etch or net-
work as it first starts to form between 1000’F and 1250: At about

13500F the microscope had to be refocused and a different field was
obtained which did not appear to be the same as the original field.

Figure 7 shows the structure at 100 x of a specimen after
cooling frcm 15000F. It appears quite rough and the boundaries are
not distinct. The pearlitic areas have completely disappeared. In
Figure 8, photomicrograph taken at 500 X, there is some evidence of
a second netawork or set of grain boundaries. Figure 9 taken at
1000.1 of the same area shows more clearly the roughened condition
of the surface, and also the widening of the grain boundaries. There
are also small globules formed, some in the grain boundaries, and
some in the interior of the grains. These may be spheroids of cement-
ite or may be caused by some surface phenamina. The film obtained
during this trial shows the formation of the heat-etch or the recrys-
talization, but it did not have very good detail. The film was
started at BSd'F and was continuous until lSOOoF'was reached, after
which temperature there seemed to be no further change.

A double netawork or two sets of grain.boundaries are shown in
Figure 12, a photomicrograph taken at 1000 X. The focal points of
the two noteworks are different. ‘When the specimen was at the ele-
vated temperature there was just one netawork or set of grain bound-
aries and this appeared to be the one consisting of the wide grain

boundaries. This indicated that one set was formed as the specimen

 

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18

was heated up into the critical range and the other was formed as the
specimen cooled. Figures 10 and 11 are photomicrographs of the same
surface as above at 100 X and 500 X respectively. The entire surface
appears to be quite rough. The darker portion, or where there appears
to be more boundaries and where there seems to be more concentration
of the globule formation, was the area originally occupied by the
pearlite.

Film Number.h, which was taken as the specimen was heated from
850 F to 1560 F, shows the change in structure as it took place on
the surface. There is no evidence of a wave passing over the polished
surface as it passed through the A} transformation. The change is
gradual. The heat etch appears very faint at first and as the tem-
perature increases it appears more distinct and shows better detail.

Film Number 5 was taken as a sample of medium carbon steel was
heated from h50 F to 1550 F. Before fiiis specimen was placed in the
vacuum.flurnace it‘was etchedwwith a 2% solution of nital. Figure
13 is a photomicrograph of the specimen as it appeared before being
placed in the furnace. The first part of the film.shows the struc-
ture as brought out by the nital etch. It shows the pearlitic areas
and the grain boundaries the same as in Figure 13. As the specimen
was heated the pearlitic areas disappeared and the boundaries brought
out by the etchant became indistinguishable from those caused by
heating. The structure of the specimen when at 1500 F did not show
any relation to the original structure.

Figures 1h, 15, 16 and 17 are photomicrographs taken at 100 X,

 

 

 

 

 

 

 

 

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19

500 X, 1000 x, and 2000 X respectively, after the specimen had

cooled to room temperature. The structure shown in Figure 1h.bears.
no relation to that as Shown in Figure 13, which gives evidence

that a reaction had taken place at the surface. At the elevated
temperature the polished surface is silver-white and there is no
pearlitic formation. This may be due to the fact that all the pearl-
ite has changed to austenite. After the specimen was cooled, the
surface had the same silverdwhite appearance but no pearlite areas.
This indicated that the surface of the specimen has been decar-
burized.

Further evidence of this decarburization is shown in Figure
18, a photomicrograph taken at 150 x of a medium.carbon steel after
being heated in a vacuum.at 1560 F and cooled to room.temperature.
This photcmicrograph also shows the globular formation in the grain
boundaries and in the interior of the grains. Figures 19 and 20 are
photomicrographs of the same area taken at 500 X and 1000 X.

Film Number 6 is a record of the change in the structure of
the polished surface of a steel sample. It is a continuous picture
taken as the specimen was heated from 900 F to 1560 F. ‘At the
start of the film the surface of the specimen was entirely devoid
of detail except for the polishing scratches, but as the temperature
increased a faint netawork formed the same as in the preceding trials.

Film.Number 7 was taken in the same manner as the others.

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20

It shows the surface changes as the specimen was heated from.800°F

to lSOO’F. The film shows fairly good detail except for the end
portion, which is blurred due to vibration. In.most all of the films
obtained, some frames were lighter than others due to unavoidable
changes in the intensity of the light source which was operating on
A. C. current. This seemed to be more prevalent when partly consumed
carbon electrodes were used in the arc lamp.

Figure 20 is a photomicrograph taken at 100 X of the specimen
after it was removed from.the vacuum furnace. The surface was silver-
white and showed rather fine grain structure. It appeared similar
to ferrite, there being no pearlitic areas in evidence. The grain
boundaries are much wider than those which are brought out by some
acid etchant. Figuras22 and 23, taken at 500 X and 1000 x respect-
ively, show the grain boundaries in much better detail. Figure 23
shows very clearly the double netswork and also the globular forma-
tion in the grain boundaries. It shows that the spheriods are much
more concentrated in the grain boundaries, if they are such, than
through the interior of the grains. In the second set of grain
boundaries there is no evidence of this globular formation. The
boundaries appear similar to those brought by a liquid etchant.

The two different sets of grain boundaries are apparently indepen-
dent of each other. If the metal is crystalline or granular in
nature and the boundaries of these grains are visable through the

microscope, there must have been two reactions or changes which took

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21

place in order to give the two net-work formations.

Figure 2b and 214a, photomicrographs taken at 14500 X of the
same specimen as in Figures 21, 22 and 23, show more clearly the
double net-work formation, the independency of the two, and the
widened condition of one. Figure 25 is a photomicrograph of the
same sample after it had been etched in 3% solution of picric acid
for several seconds. The wide set of grain boundaries did not seem
to be effected by the etchant but the other set seemed to be attacked
and brought out more clearly and sharply. This indicated that the
carbon had been removed from the wide grain boundaries.

The structure of a specimen etched with a 2% solution of nital
is shown in Figure 26. After taking this photomicrograph, the spec-
imen was repolished and heated in a vacuum until a net—work could
just be seen. Then the specimen was cooled in the vacuum to room
temperature and examined. Figure 27 shows the net-work formed.
The net-work formed by heating, or the heat-etch, does not appear
to have any relation to the structure as brought out by the dzchant.
The temperature of the specimen at which the net-work appeared was
llOO.F. The sample was lightly polished with levigated alumina to
ranove the heat etch, and then etched with a 2% solution of nital.
Figure 28 shows the structure which is very similar to that shown in
Figure 27, but it has no relation to that shown in Figure 26. As
there is no relation between the structures shown in Figure 26 and

28, but both were brought out by etching with nital. there must have

 

 

 

 

 

 

 

 

 

 

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22

been some reaction which took place*when.the specimen was heated

in the vacuum. This reaction must have taken place below the crit-
ical-range because the highest temperature of the specimen was 1100’
F, unless the temperature at which.the A3 transformation takes
place is lowered to llOO‘F when a sample of steel is heated in a
vacuum. Figure 29 is a photomicrograph of the same specimen as in
Figure 23 after it had been more deeply polished and etched with
nital. The structure is not the same as in the preceding micro-
graph, but there are some small pearlitic areas similar to those in
Figure 26. As there has been a change in the surface metal and
there is no evidence of carbon after the specimen was heated in a
vacuum, the carbon must have been volatilized off. In Figure 29
there is no evidence of a second netawork formation or grain struc-
ture.

FiLm Number 8 was made as a specimen.of medium carbon steel
was heated in a vacuum from 900.F to IhSOfF. It shows the netdwork
formation as it started to form at about 10000F and continued until
the specimen reached lDSOOF when there seemed to be no further
change. It was attempted to obtain pictures as the specimen cooled,
but this portion of the film proved unsatisfactory. The specimen
contracted rather rapidly and the microscope had to be continually
readjusted.

The magnification on all films is approximately 100 x. This

a
was determined by placing a stage micrometer on the top of the

 

 

 

 

 

 

23

furnace and taking a short exposure. The ruled lines on the micro-
meter are 0.1 mm. and 0.01 mm. apart and the fiLm shows them.tc be
about 1 cm, and 1 mm. apart respectively, indicating a magnifica-

tion of approximately 100 diameters.

cryst
gtma

about

us a
iron ‘

criti

2h
DISCUSSION

If iron is cast, it forms grains of'gamma iron above 167OPF
crystallizing with a face-centered cubic lattice. On cooling,
gamma iron changes completely into a body-centered lattice at
about 167d'F, and this transformation gives rise to entirely new
grains. If the iron is kept below 1670’F, it behaves much like
any other metal in a similar temperature range. On heating above
about 1670'F, however, the structure again changes completely in-
to new grains of gamma iron. Every time iron is caused to change
from.one crystal lattice to the other a new set of grains is pro-
duced. Thus every time a sample was heated up in a vacuum there
was a complete change from the face-centered lattice of the alpha
iron to the body-centered lattice of gamma iron if it passed the
critical range. (8)

The properties of iron are greatly influenced by small
amounts of carbon. As most of the specimens used in this work con-
tained 0.h.percent carbon, the temperature at which this change
takes place is lowered to about 1390°F. ‘When this change takes
place the metal expands because the body-centered arrangement of
the atoms is not so closely packed as the face-centered arrangement.
During the volume change the metal cools a slight degree. If a
curve is plotted from time temperature data taken while a specimen
is being heated at a constant rate, there will be a place in the
curve where the temperature will remain constant or rise at a slower

rate. This shows that the heat applied is being absorbed by the

sreci

.‘rcm
parai
berm
heate
highs
atior

time

01' se
8th
taken
Wit

ties

25

specimen more rapidly than before, and that a change is taking place
in the metal. This is shown by the two preceding curves, plotted
from data taken while a specimen was heated in a vacuum at a com-
paratively slow rate. The curves show that the change takes place
between 1100° F and 1250° F while theoretically 1: th. metal was
heated in air this change should take place from 100° to 200° F
higher. This gives evidence that pressure effects the transform-
ation temperature. Many unsuccessful attempts were made to obtain
time temperature data as the specimen cooled, but the rate of
cooling was too rapid and it could not be controlled with the
apparatus used. Parker, in his work, showed that the transforma-
tion temperature was lowered when the pressure was reduced.

When a specimen was heated in a vacuum a distinct net-work
or set of grain boundaries was formed which had no relation to the
structure of the specimen before heating. Thus a change must have
taken place. It might have been a volume change, but as the tem-
perature had not reached the critical range so that the space lat-
tice would change, there must have been some other reason for it.

When foreign atoms dissolve in a metal to form solid solutions,
if the lattice expands or the cube edge is increased, the solid
solution is termed interstitial. If it is of the substitution
type, however, it expands if the volume of the foreign atoms is
greater than the volume of the atoms of the solvent, and contracts

if the volume of the foreign atom is analler.

locat
latti
solut
stiti
isz

have

inter

mid

C.
O
’
(IE:

buck]

Poun:

26

In a substitution solid solution the atoms of the solute may
replace sane of the atoms of the solvent in the space lattice.

In an interstitial solid solution the atoms of the solute may be
located at random, except at the lattice points within the space
lattice of the solvent. There is a small amount of carbon in
solution if ferrite or alpha iron. Carbon in iron forms an inter-
stitial solid solution. As the surface of the sample. is decarbur-
ized when heated, the space lattice of the iron remaining must
have a decreased cube edge and thus a decrease in volume. The
interior of the specimen is not decarburized and when heated
‘would increase in volume due to normal expansion. These volume
changes acting in different directions might account for the
buckled appearance of the surface below the critical range.

It is held that it is impossible for an.intermetallic comp
pound, as such, to dissolve in a metal, as there is no room with-
in the space lattice for admission of molecules. The carbide
Fegc could not, as such, dissolve in iron. The carbon.must be
present in atomic dispersion which implies the dissociation of
F630 previous to the formation of a solid solution and necessarily
the formation of that compound as carbon comes out of solution.
As carbon is almost insoluble in alpha iron or ferrite, it must
be in the carbide form, and in normalized steel it is located in
the grain.boundaries and pearlite areas. 'When.a sample of steel

is heated in a vacuum decarburization takes place. This may

the
he
“‘1!
one

ls1

tom
the

tall

27

account for the globular formation in the grain boundaries as shown
in Figure 21;. If the carbon leaves the metal as carbon, and not as
the carbide, it must leave the iron that it was combined with in the
grain boundaries. If this is the case it is apparent that the glob-
ular formation would be concentrated in the grain boundaries because
that is where the carbide is concentrated.

When steel is heated the carbide must become more and more un-
stable until the transformation range is reached and it decomposes.
The carbon is in atomic dispersion so that it can go into solution
in game iron. As the carbon is in atomic dispersion and unstable
at the transformation range, decarburization would take place more
rapidly than if the temperature were above the transformation range,
since the carbon is then in solution in the gamma iron and more
stable. The assumption is made that there is a certain amount of
carbon that goes into solution.

After a sample of steel was heated in a vacuum above the A3
transformation and cooled there were two grain structures present.
One had wide grain boundaries and the other small and sharp bound-
aries. Whm observing a specimen as it was heated, there was but
one visable structure, the grain boundaries of which became wider
as the temperature increased. The widening of the boundaries may
have been due to the decarburization. If the second net-work was
formed as the specimen was heated, it should appear wide as well as
the first, but as it did not, it may have formed due to the recrys-

tallisation above the transformation point. The second net-work may

28

also have formed as the specimen cooled. It would form.a different
pattern because the composition of the surface was not the same when
the specimen.cooled as it was on heating, due to the loss of carbon.
Figure 30 is a photomicrograph taken at 2500 x of a section perpen-
dicular to a heat-etched surface. Nickel was plated on the surface
to preserve the existing structure. The decarburized layer and
roughened surface are plainly visible. The roughened condition of
the surface may be due to the following: volume change, decarbur-
isation, volatilization of the surface metal, or to the escaping

of "occluded" gases.

ry1
.

‘

‘. ,
.—

. I
r.—
s
I
-.
fl 1'

29

SUMMARY

This method of investigation can not be readily applied to
alloys because of the surface changes. The surface may not show
the condition of the interior of the metal. It could be applied
more easily to pure metals and much better results obtained.

When a sample of steel passed through the A; transformation
there was no evidence of a wave passing over the surface.

There are two distinct and independent net-works or grain
structures formed. One as the sample was heated and the other
as it cooled.

A thin layer of the surface metal is decarburised. Decar-
burisation takes place more rapidly within the critical range.

Two suggestions that might help in carrying out mrther
investigation are: (a) A variable resistance should be used so
that the heating and cooling rate can be more closely controlled,
and (b) a means of preventing the camera from vibrating due to
the motor and gear train, should be devised. This might be accom-
plished by placing small pieces of rubber sponge between the cam-

era and the camera wpport.

l.

2.

3.

5.

6.

7.

8.

9.

10.

30

BIBLIOGRAPHY

Bausch.and Lomb Optical Company. Motion Pictures of Micror-
ganisms.

Epstein, Samuel. The Alleys of Iron and Carbon. McGrawa
Hill Book Company, New York, 1930.

Clark, K. L. A Study of Micro-Changes in Hardened High
carbon Steel at Elevated Temperature under a Reduced Pres-
sure. M. S. Thesis; Library, Midnigan State College, East
Lansing, 1935.

Howe, H. H. The Metallography of Steel and Cast Iron.
McGraw-Hill Book Co., New York, 1916.

Jefferies, Zay and Archer, R. S. Science of'Metals.
McCraw-Hill Book Co., NeW'York, l92h.

Parker, T. D. The Structure of Steel at Elevated Temper-
ature Under a Reduced Pressure. M. S. Thesis, Library,
Michigan State College, East Lansing, 1933.

Rawdon and Scott. Microstructure of Iron and Mild Steel
at High Temperature, Bureau of standards, Scientific
Paper NO e 8556.

Roger, B. A. Metallographic Examination of Specimens at
High Temperature; Metal and.Alloys Vol. 2 No. l, 1931.

Sauveur, Albert. The Metallography and Heat Treatment of
Iron and Steel. McGraw-Hill Book Co., New York, 1935.

Wiester, H. J. Martensite Forms Instantly, Metal Progress
Vol. xx111 No. 2, 1933.

N00 1

NO. 2

NO. 3

No. L;

No. 5

N00 6

31
FILM INDEX

Shutter speed No. In 0.3 smoked glass, temperature
0 0
range 950 F to 111,00 F. Unetched sample of 0.1;

carbon steel. Arc lamp. Green filter.

Shutter speed No. 14; 0.3 smoked glass, temperature
range 8C1). F to 1500. F. Sample of 0.14 carbon steel

unetched. Arc lamp. Green filter.

Shutter speed No. b; 0.3 smoked glass, temperature
range 850.? to 15000 F. Sample of 0.1; carbon steel

unetched. Arc lamp. Green filter.

Shutter speed No. 14.; 0.3 smoked glass, temperature
range 850.F to 1550.17‘. Sample of 0.14 carbon steel

etched 2% nital. Arc lamp. Green filter.

shutter speed No. LL; 0.3 smoked glass, temperature
0
range 800,1? to 1550 F. Sample of 0.14 carbon steel

etched 2:74 nital. Arc lamp. Green filter.

Shutter speed No. )4; 0.3 smoked glass, temperature
0
range 900'F to 1560 F. Sample of 0.14 carbon steel

unetched. Arc lamp. Green filter.

 

 

 

 

N00 7

NOe 8

NOe 9

NOe 10

frame or one exposure every two seconds.

32

Shutter speed No. LL; 0.3 smoked glass, temperature
range 800.F to 15600 F. Sample of 0.11 carbon steel

unetched. Arc lamp. Green filter.

Shutter speed No. 1;; 0.3 smoked glass, temperature
0
range 900. F to 1.14.50 F. Sample of 0.1), carbon steel

unetched. Arc lamp. Green filter.

Shutter speed No. b; 0.3 smoked glass, temperature
0 0
range 650 F to D400 F. Sample of 0.14 carbon steel

unetched. Mazda ribbon filament lamp. No filter.

Short film taken at the different shutter speeds
using each smoked glass at each speed. Arc lamp as

source of light and green filter used.

The Number four speed used in all the films takes one

The 0.3 smoked

glass allows 70 percent of the light to pass.

office of the Chenical Engineering Department, Room M3 Olds

All of the above listed films are available at the main

Hall, Michigan State College, East Lansing.

Blue Print

No.1

‘3 body

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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