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Effect of applied tensile stress on phase transformations in steel
EFFECT OF APPLIED TENSILE PHASE TRANSFORMATIONS L. F. PORTER7 Using a specially designed apparatus
STRESS ON IN STEEL*
and P. C. ROSENTHAL:
capable of making simultaneous
measurements
of the electrical
resistance and the extension of a wire specimen during rapid quenching or isothermal transformation, the transformation characteristics of a eutectoid steel have been studied under dead-weight tensile loading. Data obtained on transformation to pearlite, bainite, and martensite indicate that there is a threshold stress above which transformation is accelerated. When transforming under load, extensive plastic deformation is noted coincident with transformation. There also appears to be a threshold stress In the case of transformation to pearlite and bainite, increased associated with the plastic deformation. On transformation rates of transformation occur at the same threshold stress as the plastic deformation. to martensite gross plastic deformation occurs at very low stress, while the M, temperature is not raised until stresses on the order of 28,500 lb/in2 are reached. To account for the coincidence of transformation and plastic extension it is proposed that dislocations piled up at grain boundaries and other barriers produce stress fields which result in increased rates of nucleation. Moreover, when the nucleus loses coherency with the parent austenite, the advancing interface acts as a sink for the piled-up dislocations and thus plastic deformation is observed. According to this picture, the threshold stress is the stress necessary to move free dislocations out from their sources t,o produce the piled-up arrays. The difference in the behavior of the martensite transformation under load can be explained on the basis of the differences between the mechanism of formation of martensite and that of pearlite and bainite. EFFET
DUNE DEFORMATION TRANSFORMATIONS DE
PAR TRACTION SUR PHASE DE L’ACIER
LES
Les auteurs etudient les characteristiques de la transformation dun acier eutectoide soumis a une deformation par traction. A cette fin, ils utilisent un montage special permettant de mesurer simultanement la resistance electrique et l’allongement d’un fil au tours dune trempe Bnergique ou d’une transformation isotherme. Les rirsultats obtenus pour les transformations perlitiques, bainitiques et martensitiques indiquent qu’il existe un seuil de tension au-dessus duquel la transformation est accBl&ee. Lorsque la transformation a lieu sous charge, une deformation plastique importante se marque simultanement & la transformation. 11 semble exister Bgalement un seuil de tension associi! a la deformation plastique. Dans le cas des transformations perlitiques et bainitiques, l’acceleration de la vitesse de la transformation se produit pour le m&me seuil de tension que pour la deformation plastique. Pour le transformation martensitique, une deformation plastique importante apparait pour une tension tres faible tandis que la temperature M, n’est accrue que pour des tensions de l’ordre de 28,500 lb/in2. Les auteurs proposent d’interpreter cette coincidence de la transformation et de la deformation plastique par un empilement de dislocations aux frontiirres granulaires. Ces empilements et autres barriiires similaires produisent des champs de tensions d’oh resulteront des vitesses de germination accrues. En outre, lorsque le germe n’est plus coherent vis-a-vis de la matrice austenitique, l’interface en mouvement agit comme un puits pour l’empilement des dislocations et la dOformation plastique en resulte. D’apres ce modele, le seuil de tensions correspond a la tension necessaire pour deplacer les dislocations libres de leurs sources afin de provoquer des reseaux d’empilement. La difference de cornportement de la transformation martensitique sous charge peut Btre expliquee sur la base des differences entre le mecanisme de la deformation de la martensite et celui de la ferrite et de la bainite. BEEINFLUSSUNG
VON
PHASENUMWANDLUNGEN
VON
STAHL
DURCH
ZUGSPANNUNG
Mit Hilfe eines besonders konstruierten Apparates, der es gestattet, wiihrend schnellen Abschreckens oder isothermer Umwandlung einer Drahtprobe gleichzeitig deren elektrischen Widerstand und Verliingerung zu messen, wurde der Verlauf der Umwandlung van eutektoidem Stahl unter konstanter Zuglast untersucht. Bei der Umwandlung zu Perlit, Bainit und Martensit lassen die Messungen auf eine Schwellenspannung schliessen, oberhalb deren die Umwandlung beschleunigt ist. Verlauft die Umwandlung unter Last, so ist sie von grossen plastischen Verformungen begleitet. Auch fur die plastische Verformung scheint es eine Schwellenspannung zu geben. Im Fall van Perlit und Bainit ist die Schwellenspannung fur die Umwandlung und die plastische Verformung dieselbe. Bei der Martensitumwandlung tritt bei sehr niedriger Spannung bereits grosse plastische Verformung auf, die M,-Temperatur steigt jedoch erst von Spannungen der Grossenordnung 28.500 lb/in2 (=20 kg/mm%) ab an.
* Based on a thesis submitted in partial fulfillment of the requirements for a Ph.D. at the University Received June 4, 1958; revised version December 29, 1958. t Now with the Applied Research Laboratory, U.S. Steel Corp., Monroeville, Penn. $ Department of Mining and Metallurgical Engineering, University of Wisconsin, Madison, Wisconsin. ACTA
METALLURGICA,
VOL.
7, JULY
1959
504
of Wisconsin.
PORTER
AND WOSESTHAL:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
IN STEEL
85
Zur Erklarung des gleichzeitigen Auftretens von Umwandlung und plastischer Dehnung wird vorgeschlagen, dass die Spannungsfeldar von Versetzungen, die sich an Korngrenzen und anderen Hindernissen aufstauen, verstiirkte Keimbildung znr Folge haben. Wenn der Keim dann die Koharenz mit der Austenitmatrix verliert, bilden die entstehenden Phasengrenzflachen Senkon fur die aufgesKach dieser Vorstellung ist also die tauten Versetzungen, und man beobachtet plastische Verformnng. S~h~~elIenspannnng diejenige Spannung, die notwendig ist., urn freie Versetzungen von ihren Quellen zn l&en und so die aufgestauten Gruppen zu bilden. Das unterschiedliehe Verhalten der Martensiturnwandlung unter Last l&St sich auf Grund der Unterschiede zwischen den Bildungsmechanismen von Martensit einerseits und Perlit und Bainit andererseits verstehen.
1. INTRODUCTION
approximation
Until recently it was assumed by most metallurgists that stress would have little influence on phase trans-
transformed
formations
equal
in solids.
one considers phase
This view is indeed juststilled if
only the possible
equilibria.
For
effects
instance
of stress on
applying
Chatelier principle to the A, equilibrium
the
Le
in a eutectoid
that the percentage
of the total resis-
tance difference between austenite and the completely structure
to the volume
phases present. was checked
at a given
temperature
was
per cent of the transformed
The validity
of this approximatio~l
using metallographic
found to be within the accuracy
methods
and was
of the metallographic
carbon steel, it can be shown that it takes a triaxial
estimation
stress
range. In order to obtain the required data, apparatus had
of
12,750
temperature
lb/in2
to
change
however,
experimental
and Kehlt2) indicate
and pearlite transformation applied uniaxial
stress.
found
of stress on the martensite
in iron-nickel systems
of
alloys
by
transformation and
lithium-magnesium. The
present
to obtain
applied
tensile
was
stress on the
martensite transformations
hainite,
material
used
and
theory.
C
MI1
0.90%
0.4.5;/, 0.005~0
which
apparatus
evolved
described
primary advantage of
the
investigation
music wire having the P
Si 0.25%
the specimen
quenchant
MO
V
0.010%
nil
cooling.
from an austenitizing Instead,
furnace to a
the wire specimen
tizing time by passing a high amperage the high amperage
current is replaced
with a direct
current resistance measuring circuit, and the specimen is quenched with a high velocity stream of helium gas isothermal temperature
perature is maintained with a wire-wound central
by surrounding
the specimen
tube furnace having a controlled
zone of uniform
temperature.
In order to
under stress, arrangenlents
are made for
a dead-weight
tensile load to the specimen. with the speci-
0.08%
When making isothermal tests, the IX drop through
homogeneous
the specimen is recorded on a photoelectric meter.
In order to study the martcnsite
Reproductions
6
corrected for any plas-
during the test, were con-
transformation
using
the
first
potentio-
transforma-
tion, it is necessary to record five variables on a sixchannel record.ing oscillograph operating at a chart during an isothermal
percent,
or to room
In isothermal tests, the isothermal tem-
speed of 1 in./seo.
to
alternating
electric current through it. After proper austeIlitizing,
resistance
verted
to austeni-
and held for the required austeni-
The resistivity
occurring
The
of the system is that no movement
The progress of the transformation was obtained by recording the electrical resistivity of the specimen. tic deformation
of an
men assembled and in place is shown in Fig. 1.
as received and was used without further homogenizing heat treatment.
measurements,
of
The system
is an adaptation
A schematic diagram of the apparatus
CU
The wire was judged metallo~aphically
recording
variables
is heated in a purified helium atmosphere
applying
0.010%
other
by Colner and Zmeskalo”).
is necessary.
transform
Ni
0.0100/
and the
or during continuous
was finally
temperature.
composition:
Cr
formation
to a predetermined
METHOD
throughout
consisted of 0.040 in. diameter following
pearlite
and an explanation
based on dislocation
2. EXPERIMENTAL The
in an
of the effect of
in a eutectoid carbon steel.
The results have been analyzed has been proposed
conducted
a unified picture
of simultaneously
resistance
tizing temperature
investigation
attempt
transformation
interest which were changing dnring isothermal trans-
a marked
indiuni-thallium
capable
of the
and
as well as in the non-ferrous
gold--cadmium,
to be developed
most
temperature,
Since the work of Scheil(3) a have
throughout
The
that the bainite
in steel is accelerated
of investigators(4-g)
influence
of phase transfor-
the picture is quite different.
evidence of Jepson and Thomsoncr)
~hattacharyya
number
equilibriunl
1°C.
When one looks at the kinetics mation,
this
the change continuous
recorded
in the five variables
of the changes in test and of
recorded
during a
cooling test are shown in Fig, 2. Careful
506
ACTA
METALLURGICA,
VOL.
5,
1959
AUSTENITIZING~ CURRENT
LEAD
MEASURING
CIRCUITS
HELIUM OUT SPECIMEN
SUPPORT
BRACKET
ISOTHERMAL CHEATING
HOLDING
FURNACE
c01Ls /MEASURING BEHIND HEAT
CIRCUITS STAINLESS
PASS STEEL
SHIELD
SPECIMEN THERMOCOUPLE STAINLESS GRADED
STEEL TUBE WITH OPENINGS OPPOSITE
SPECIMEN
FLEXISLE
HELIUM
QUENCH
S -STRAIN
HELlUM
FOR
GAGE
EXTENSOMETER
IN
AUSTENlTlZlNG/ CURRENT LEAD
STRAIN DEAD
GAGE
LOAD
WEIGHT
LOAD
CELL
AzzEik PEDESTAL
RELEASE
PIN
SCHEMATIC
DASH
DIAGRAh3
OF
TEST
PAT
APPARATUS
FIG. 1. Schematic transformation
diagram of apparatus for observing under applied dead-weight tensile loads.
FIN. 2. Reproductions of records showing changes in variables during continuous cooling test (top) and the change in resistance during isothermal test (bottom).
PORTER EFFECT 260”
AND ROSENTHAL: OF C-
STRESS 500’
ON THE
F
TENSILE
BAINITE
ISOTHERM
STRESS
AND
PHASE THE
TRANSFORMATION
AUTOCATALYTIC
TRANSFORMATIONS DECREASE
ISOTHERMAL
PLOT
IN TIME
IN STEEL
TO TRANSFORM
TRANSFORMATION
UN[LR
507
SO%, STRESS
600
500 :: g
1
400
” :
f
300
? t 200
IO0
0 0
IO
20
M
40
50
STRESS IN POUNDS FW SOUARE INCH x IO‘= 200
100
1000
500
300 TIME
2000
3000
IN SECONDS
3. Typical autocatalytic plot showing applied tensile stress on bainite transformation
FIG.
effect of at 260°C.
calibration and checking of the apparatus indicated that, during test, temperatures remained uniform along the specimen and that the accuracy of the recorded information was limited primarily by the ability to read the charts. 3. RESULTS
The results are most conveniently divided into three groups: results obtained on isothermal tests in the bainite region, less complete results obtained isothermally and by continuous cooling in the pearlite region, and results obtained for the martensite transformation on quenching. Transformation
to bainite
In the bainite region the transformation characteristics were determined for a series of increasing applied uniaxial tensile stresses at isothermal temperatures of 260°C (500”F), 316°C (600°F) and 371°C (700°F). The results indicate that above a certain threshold stress applied tensile stress markedly accelerates the transformation. It appears that the degree of acceleration, as measured by the decrease in time to achieve a given amount of transformation, is a linear function of the applied stress, especially at the lower stress levels. Instead of plotting per cent transformation vs. log time to produce the typical sigmoid curve, the resistance measurements were converted to percent transformation and plotted on an autocatalytic plot. The type of plot obeys the following equation: log (P/100
-
P) =
K log t +
G
FIG. 4. The manner in which the applied stress decreases Note the threshold at the transformation time. 600043000 lb/in2
where P is the amount transformed and t is time, K and C being constants. Its use for representing the bainite transformation was first suggested by Austin and Rickettol) who showed that the original data of Davenport and Bain(12)for the bainite transformation lay on a straight line when plotted in this way. While the fact that a straight line results is probably without real significance, such a plot does aid in establishing start and completion times for the reaction (on the basis of 1 per cent and 99 per cent transformation) and promotes confidence in the experimental results when they plot on a straight line. Fig. 3 is an autocatalytic plot showing the effect of applied stress on the kinetics of the bainite transformation at 260°C. One notes that the experimental results plot as straight lines at all but the highest stress level. It is apparent that applied tensile stress, while markedly increasing transformation rates, has a greater effect on the early stages of the transformation than on the Iater stages. Results at 316°C and 371°C show the same characteristics as those at 260°C but of course transformation occurs in shorter times. Furthermore, if one examines the decrease in transformation time as a function of applied stress, another important effect is noted. In Fig. 4 the decrease in time to achieve 50 per cent transformation has been plotted against applied stress. The decrease is found to be proportional to applied stress and, on the basis of the extrapolated plots, the minimum stress needed to produce accelerated transformation is found to be 8000 lb/in2 at 26O”C, approximately 7000 lb/in2 at 316”C, and 6000 lb/in2 at 371°C.
_4CTA
50s
METALLURGICA,
VOL.
i,
OF
APPLIED
EFFECT
DURING
1959 STRESS
ON
ISOTHERMAL
PLASTIC
EXTENSION
TRANSFORMATION
.020 688°C-12700F
0
200
100
FIG.
FIG. 5. The progress of the deformation coincident with isothermal transformation 260°C.
which occurs to bainite at
in electrical
began to extend
resistance,
plastically
at a rather rapid rate.
The extension continued throughout of the transformation, tivity measurements was complete.
the major portion
ceasing somewhat
The progress
of the rapid extension at 260°C is shown in
Fig. 5. Again these results are typical tures.
curves obtained
Extension measurements made during isothermal transformation to pearlite at 6SS”C.
7.
mately 35 times higher than that resulting from transformation
without applied stress.
of the form of
at the higher tempera-
line relationship
the same as the thresholds of the transformation
APPLIED STRESS vs MAXIMUM EXTENSION PRODUCED OURlNG ISOTHERMAL TFJ.ANSFORMATlON TO BAINITE
500°
F -260°
tion rates.
on extrapolation
relationship
There
between
the
noted and the increased transforma-
Indeed,
the onset of the rapid extension
under load is found to be a more sensitive indication of the start of transformation
than are the resistivity
since it is observed before a change in
Transformation
to pearlite
Only a few isothermal tests were carried out in the pearlite range at 688°C (1270°F). measurements
made
transformation
characteristics
ally unsatisfactory,
C
obtained
appears to be an intimate rapid extension
straight
time vs. stress curves.
resistance is detected.
is approxi-
of
values which are approximately
measurements,
stresses investigated,
as a function
exists and the extrapolated
lines have threshold
suffered by the specimen stress and,
extension
stress (Fig. 6) one finds that at lower stresses a straight
to the applied
The total extension
is found to be proportional at the maximum
before resis-
indicated that the transformation
occurring during transformation the extension
was noted
the specimen
600
500
IN SECONDS
If one plots maximum
Shortly before evidence of transformation by a change
400
300 TIME
ISOTHERM
the isothermal
the
kinetics
isothermal
under load were gener-
owing to difficulties
in attaining
and to the rapid change with temperature
in this
The results did indicate that transformation
was accelerated
by stresses below
again rapid extension before
Results of resistance
determine
temperature
in transformation region.
to
was observed
the resistance
transformation
measurements
4000
lb/in2
and
to occur slightly indicated
that
was in progress.
The results shown in Fig. 7 are typical of the extension measurements made during isothermal transfor-
OO”F-371°C
mation in the pearlite region. At a stress of 7900 lb/in2 it is seen that rapid extension occurs and is completed in about 300 set, after which a steady rate of creep STRESS FIG.
and
IN POUNDS
PER SQUARE
INCH
IO-’
6. The relationship obtained between applied stress Note that the same thresholds are extension. observed as in Fig. 4.
continues.
It is interesting
to note that under the
conditions
of this test the pearlite transformation
completed
in somewhat
is
under an hour and that in
PORTER
.%SD ROSENTHA1L:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
TEMPERATURE,
IN
STEEL
509
‘C
FIG. 10. Results
of resistance and extension measurements made during quenching to form martensite at low applied stresses. The lower plot gives the percentage of the total load applied in the temperature range indicated.
FIG. 8. Reproductions of records of extension obtained during continuous cooling through the pearlite region.
300 see the specimen cent transformed. rapid extension
is somewhat
less than 50 per
At the lower stress of 3950 lb/in2 and its subsequent
is noted again.
early completion
In this case no creep is observed after
the rapid extension has ceased. Additional through
data
obtained
by
continuous
the pearlite region substantiate
mal results.
cooling
the isother-
Here again data obtained from extension
measurements
prove to be most reliable. the applied
the extension
during transformation.
occurring
EFFECT OF APPLIED STRESS ON EXTENSION OCCURRING DURING MARTENSITE TRANSFORMATIO,,
Fig. 8 shows
the effect of increasing amination
FIG. 11. Results of resistance and extension measurements made at high applied tensile stresses. The lower plot gives the percentage of the total load applied in the temperature range indicated.
.05
tensile stress on Ex-
lb/in2 and below do not alter the temperature recalescence
formation
range of
E
.03
these low stresses do
B 5 z
.02
in excess of the normal trans-
z
Likewise,
not result in extension dilatation.
acceleration
Y
and therefore do not accelerate the pear-
lite transformation.
.04
I
of Fig. 8 shows that applied stresses of 1575
At higher stresses, evidence
of the transformation
of
9
as well as plastic
t: APPLIED STRESS vs EXTENSION ON TRPNSFORMATION TO PEARLITE DURING CONTINUOUS COOLING
.Ol
E I.oo
0
IO APPLIED
20 STRESS
30 40 IN PSI x IO-’
FIG. 12. Applied stresses in excess of 1000 lb/in2 cause extension during the martensite transformation. EFFECT OF APPLIED STRESS ON M, z E
g
.ooo 1
0
I
I
I
I
I
2
4
6
8
IO
APPLIED
FIG. 9. The relationship extension
STRESS IN PSI
x lO-3
between applied stress and the obtained during continuous cooling through the pearlite region.
50
= 2
250
TEMPERAWE
I(
,__I
1
1)1_
/ITHR~WJ
1
I,3
L 3 e-
150 c -0 APPLIED
IO
20 30 STRESS IN PSI x lO-3
40
50
FIG. 13. The M, temperature is not affected until stresses over 28,500 lb/in2 are applied.
510
ACTA
extension during transformation the bainite
data,
METALLURGICA,
is observed.
extrapolated
linear
VOL.
As with
plots
7,
1959
02% OFFSET YIELD STRENGTH AUSTENITE “5 SQUARE ROOT
of the
extension data (Fig. 9) indicate a threshold for plastic extension
coincident
with accelerated
In the case of pearlite, stress of lb/in2
2000
lb/in2
deduced
for
the
threshold
as compared the
bainite
A
transformation. occurs
with
the
0
8000
transformation
DATA FROM GUARNIERI 8, KANTER 5% Cr , I/P%Mo .CAST STEEL 09%
at a
OF METASTABLE OF TEMPERATURE
CARBON
THRESHOLD
STEEL
VALUES
i
at
260°C. Martensite transformation Representative quenching
examples
of the data obtained
to form martensite
loads are given in Figs. 10 and 11. noted that rapid extension tion.
The
effect
transformation
of
on
under various applied Here again it is
occurs during transforma-
stress
on
the
extension
are presented in Figs. 12 and 13.
The results are quite different from those obtained previously
for bainite
and pearlite.
It is observed
that the threshold for increased extension is very low, about 1000 lb/in2, while no effect on the M, temperature is observed until a stress of approximately
28,500
It is also evident from Figs. 10 and
lb/in2 is reached.
11 that the beginning
of rapid extension is coincident
with the M, temperature,
as measured by resistance,
it as in the case of bainite and
instead of preceding
pearlite transformations.
Measurements
of hardness
and of retained austenite content by integrated X-ray intensity methods indicate that the applied stress has little
influence
formed. calculated
on the total
amount
of martensite
While the progress of transformation from
the
shapes of the resistivity
resistivity
that the stress-induced
occurring at temperatures
2OO”C, the normal
M, temperature,
much slower rate with decreasing does the normal transformation
in excess of
progresses temperature
at a than
occurring below 200°C.
In other words, stress-induced transformation and progresses slowly at temperatures above but when the normal M, temperature normal transformation
the
curves obtained under stresses
of over 28,500 lb/in2 indicate transformation
was not
measurements,
occurs 2OO”C,
is reached the
begins and progresses just as it
would if no stress were present. 4. DISCUSSION
with plastic
extension
100
200 300 TEMPERATURE
400 500 600 700 IN DEGREES CENTIGRADE
FIG. 14. An estimation of the yield strength of metastable austenite in the material used in this study. Note the low values of the thresholds compared with the yield strength.
Here plastic extension
during transformation
does not occur until a very high stress is reached, The threshold
stress is very low compared
to the
stress being sustained by the specimen up to the time transformation the probable
begins.
Indeed, it is much lower than
0.2 per cent offset yield stress.
A rather
good estimate of the yield strength of the metastable austenite
may be obtained
from the loads which it
was able to sustain during isothermal additional
pieces of information
studies.
Two
on this point are also
available. First, it is known that the yield stress should vary as the square root of the absolute temperatrue and secondly,
Guarnieri and Kanter(l*) have made 0.2
per cent offset yield stress measurements austenite which
in 5%
chromium-$“/o
can be compared
on metastable
molybdenum
with the present
steel
data.
Fig. 14, a stress vs. d/T plot of data derived isothermal with
and continuous
the data
estimated
cooling
of Guarnieri
temperature
stress
relationship
study
is indicated.
and
In from
data is compared Kanter,
and
the
vs. 0.2 per cent offset yield
for
the
Also
material
shown
The question one naturally stress
during transforma-
mechanism
used
in this
are the threshold
during
transformation,
and
the threshold stress for increased rates of transformation. Thus when the applied stress exceeds a certain
under load has been reported
rates are accelerated,
and when
occurs, the yield strength of the steel
is suddenly markedly reduced. Conditions are somewhat different in the case of martensite formation.
asks is:
“What
is the
by which the resistance to flow is lowered
threshold
transformation
occurs
at very low stress while an increase in M, temperature
tion, and in the pearlite and bainite regions this is also
value, transformation
8co
stresses in the pearlite, bainite and martensite regions.
OF RESULTS
The results indicate that there is a threshold associated
4
0
and
stress?”
detailed explanation
what
determines
the
Plastic flow during transformation previously(2y14J5) but a
is usually lacking.
Boasu6) states that it may be possible that the high mobility of the atoms at the interface between old and new phase during
transformation
plasticity
gives
rise to
weakness
and
of the metal in the same way as does the
PORTER
AND ROSENTHAL:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
high mobility at grain boundaries at elevated tempera-
work
tures.
against these stresses.
However,
whole story. forming
Boas’
can not tell the
It is difficult to see how isolated trans-
nuclei,
such as must be present
effect is first noted, boundary
suggestion
atoms
deformation
could,
by the mobility
account
alone,
when the
of the remaining
for
matrix.
of their
gross
plastic
Moreover,
on
must
be done
Transformation boundaries
in order to drive
could
but extensive dislocations
way.
of grain
Dislocations
sources at relatively
yielding
511
dislocations
alter the influence
in the following
out from dislocation
IN STEEL
move
low stresses,
is not observed
because
are held up at the gram boundaries.
the This
the basis of the results presented here, any complete
results in a piling-up of dislocations into an equilibrium
explanation
distribution
threshold
would have to account
for plastic deformation
for the observed
during transforma-
against the boundaries
as shown schema-
tically in Fig. 15. The nucleation
of the transforma-
tion and for the fact that the same threshold is observed
tion causes a collapse of the resistance offered by the
for increased rates of transformation.
grain boundaries,
If one rejects mobility
the explanation
that the increased
of the atoms at the transforming
interface, in
itself, can account for the plastic deformation panying transformation
accom-
under load, one must examine
with transformation If the picture shown
that,
blocked
which might be altered by the initiation
transformation
mation.
These
sources
solution,
precipitates,
are
interacting
the difficulty of transmitting another. In austenite, the alloying
alloying
dislocations,
contribution
would
nucleation.
especially
cubic lattice.
strength
tempera-
involved,
carbon
must
atmospheres
and nitrogen,
Precipitates,
be small.
is not strong because,
at the dislocations
Taking everything
of the
of solute
are probably
in the face-centered
as such, are not present.
into account,
it would appear that
rates.
of dislocations
will promote
It is generally
rates and that the increased
high
solubility
it remains to be
stress, an array
increased
conceded
that
the stresses involved can not greatly influence diffusion must
Cottrell anchoring of dislocations
not condensed
and
lead one to believe that their
to the yield
for the temperatures
in
arrays to
is noted.
at a grain boundary
slip from one grain to
elements present and the elevated
tures involved
atoms,
elements
the dislocation
is to be complete
under
the other possible sources of resistance to deformation of transfor-
allowing
move forward, and thus the rapid extension coincident
be mostly
associated
Using
the
rate of transformation with increased
results
of
rates of
Cottrell(ls)
and
et al. (lg), Koehlerc20) has shown that, in an
Eshelby
array of edge dislocations
by an obstacle
as
shown in Fig. 15, there can be a high concentration
blocked
of
tensile stress over a considerable of the dislocation dilatation
region in the vicinity
nearest the obstacle,
and extensive
of the lattice can occur in the region below
this dislocation.
The extraordinary
lattice
the formation
promotes
through
the
action
of
the
dilatation
of the
of the stable phases LeChatelier
principle.
it must be the release of the resistance to deformation
Moreover,
because of the number
offered primarily by the grain boundaries, and to some extent by the interacting dislocations, which is
produced,
more nuclei will be activated,
reduction
in the distance an atom must diffuse before
responsible
reaching
the nearest
for
the
plastic
flow
associated
with
Gram boundaries
offer resistance to slip because the
accelerate diffusion.(21)
direction of the plane of slip changes at the boundary.
rate
Moreover,
materially
in order to take into account
grain size, Nabarroo’) cm thick
the effect of
suggests that there be a region
at the grain
boundaries
where
the
of
nucleation
of
Finally,
the internal will tend to
For these reasons, the normal the
stable
phases
increased when dislocation
will
be
arrays pile up
at grain boundaries under the action of applied stress. The mechanism
whereby
transformation
applied tensile stress in-
coherence of the grains causes severe elastic distortion
fluences
and slip on unfavorable
regions may thus be described
planes, in which case, extra
arrays
resulting in a
stress gradient near the first dislocation
transformation.
lop4
nuclei.
of piled-up
in the bainite as follows:
and pearlite The thres-
hold stress represents the effective stress necessary to
A6 - SLIP PLANE P - OBSTACLE
lP
A_gI++
cause
dislocations
to
move
out
from
dislocation
sources and begin piling up at grain boundaries.
The
dislocation arrays thus formed produce a large concentration of tensile stress in the vicinity of the leading dislocation.
This stress and its accompanying
dilata-
tion of the crystal lattice result in increased rates of THE
PILING
OF DISLOCATIONS
AGAINST
AN OBSTACLE
FROM
COTTRELL
FIG. 16. A schematic representation of a pile-up array of dislocations after Cottrell.
nucleation and therefore in increased rates of transformation. When the nuclei lose coherency with the parent lattice, the dislocation arrays are freed from
ACTA
,512
The growing
their barriers.
interface
as well as a source for dislocations. movement
is again possible
occurs coincident can
continue
continuous.
acts as a sink
Thus dislocation
and rapid
with transformation.
until
the
phases
being
after
has been formed has
continuous
only
about
50%
and not until
been
formed,
lends support observed
not
95%
of the becomes the
and the grain
investigation
effective tensile stress necessary
comparable,
proposed
mechanism,
stress
is indeed
the
to move dislocations
out from their sources to form dislocation It is also interesting
arrays.
to note that, according either uniaxial
the dislocations
to influence
which the transformation
transformation
It will be remembered of applied
tension or com-
is greatest at
that in examining
at elevated
temperatures
and some plastic deformation
load.
Thus the austenite
temperature
the M, temperature, probably
was strained when the M,?
was reached, and for loads which raised some plastic
deformation
still in progress when transformation in the vicinity
of the normal MS (200°C) is
estimated as 32,000 lb/in2 and on the basis of a square root dependence one
between
1,000 lbjin2.
stress is not involved.
with
If
nucleation
throughout catalytic
Results
obtained
by Jepson
rates
were
accelerated
the transformation,
uniformly
the resulting
auto-
plot would be displaced to shorter times but
the
expect
stress and tem-
LO,000 lb/in2 whereas the actual threshold
stress along the slip direction and the sign of the shear
would
threshold
since the active component
indicate that this is indeed the case.
was
started.
From Fig. 15 the 0.2 per cent, offset yield strength of
perature
and Thomson(l)
during occurred
before the material became cool enough to support the
pression should show increased rates of transformation, of the stress is the shear
the effect,
stress on the ~nartensite transformations
rapid cooling,
au&mite
to the
is accelerated
the start of transformation.
loads were applied between
directly
However,
greatly
while the amount by
of plastic
to the idea that the threshold
in the present
appear
is greatest during the
pearlite
freed from its atmosphere, although
do not
middle stages of transformat,ion
lower yield point, the tensile st,ress necessary to move size in ferrite,
increased transformation.
the rate of plastic deformation
are
of the pearlite
work(22), on the relationship
a dislocation
1959
Deformation
much earlier than bainite.
Petch’s
7,
merely by the act of moving through the lattice, since
formed
about
since
VOL.
deformation
This accounts for the completion
deformation bainite
METALLURGICA,
a threshold
of
about
occurs
at
&‘o change in the M, is noted coincident
threshold
stress,
and
none
occurs
until
stresses over 30,000 are applied. The most extension
obvious
observed
transformed
explanation
for the increased
at low stress when austenite
to martensite
is that low applied
is
stress
would still be parallel to the original unstressed plot.
causes the martensite
plates to form with a preferred
In the present
orientation,
in the concomitant
case the slopes
change
as stress is
resulting
increased so that the plots have a larger separation at
length even though the transformation
the start of transformation
not been changed
transformation. of transformation than
during
creased
of
nucleation is accelerated much more
the later stages,
transformation formation
than at the completion
This indicates that in the early stages The early
stages of
are believed to be accelerated
of more and larger pile-ups stress
concentration
transformation
is under
way,
below
by the
and the in-
At high applied
to be associated
has been
by Chang and Readt7) in the case of the
gold-cadmium
system.
However,
in the present> ease, during the early
the pile-ups
stages of transformation
of the
This is believed
deformation
from stresses in excess of the yield stress. bands resulting from such deformation
tation during the martensite transformation reported
techniques failed
stresses the early portions
with plastic
Enhanced
to reveal any preferred
plots depart from linearity (see curve for
47,300 lb/in2 applied stress, Fig. 3).
amount.
under load as a result of preferred orien-
Once
begin to be released and their influence is soon lost. autocatalytic
a detectable
in
a careful search using metallographic
pile-ups.
however,
deformation
change
kinetics have
and X-ray preferred
diffraction orientation
orientation
to martensite
techniques
under stress,
failed to reveal any
in the completely
transforn~ed
martensite structure. The experimental results might better be explained
resulting
in a second way.
The slip
free energy which must be available to form a marton-
probably
pro-
site plate
It is well known that some of t,he
is released
as kinetic
energy
during
the
duce atomic configurations which act as additional nucleation sites within the grains. This type of
formation of the plate. This kinetic energy manifests itself as observable slip in the austenitc adjacent to
mechanism
the newly formed
has been reviewed by Averbachcz3) and is
well known in age hardening systems. In all cases it has been assumed movement trations
must occur to produce and
atomic
configurations
passes through that dislocation
the stress conoenwhich
lead
to
plate and in a shock wave which
the austenite,
often
resulting
audible click. Perhaps the “triggering existing martensite embryos supplies necessary to drive dislocations
in an
off” of prethe energy
through the austenite,
PORTER
forming
ANT) ROSENTHAL:
pile-ups
which
grain boundaries. transformation,
TENSILE
are forced
STRESS
hard against
the
Thus, in the case of the martensite the energy to move the dislocations
AND
PHSSE
TKANSFORM.4TIOSS
IN STEEL
studied in a eutectoid carbon steel.
It has been shown
that stress in excess of a threshold
value is capable of
accelerating
transformation
to pearlite
out from their sources and to force arrays against the
and of raising the M, temperature.
grain boundaries
of the specimen occurs coincident
is supplied
and only a small additional to activate
by the transformation, applied stress is required
slip in preferred directions
in the adjacent
An accurate
description
of the cause of the defor-
mation which is observed
to accompany
tion under stress probably
involves both the effects of
preferred
orientation
of the transformation
and the plastic deformation
mechanism
this paper, the relative importance nisms being dependent experimental
transformaproducts
described
of the two mecha-
on the system involved
conditions.
in
and the
In this case it would appear
that the latter mechanism
is predominant.
with transformation
Why applied stresses begin to influence M, only in
for plastic extension
for accelerated
mation.
to martensite,
On transformation
value while the applied
M, temperature
loads have no effect on the
until very high stresses are reached.
It is proposed
that on transformation
and bainite the threshold
required to move dislocations
out from their sources.
This
move to the grain boundaries or other barriers where they form piled-up arrays. It is further
suggested that the stress concentration the increased rates of transformation
it has been demonstrated
critical temperature
frequently
formed by plastic
To account
between M, and a higher
sion observed
known as M,.
to pearlite
stress is the effective stress
For example,
can be isothermally
transforthe thres-
hold stress for plastic extension has a surprisingly low
leading dislocation
at temperatures
the
is found to have the
same value as the threshold
influence cannot result solely from plastic deformation.
deformation
In
the case of pearlite and bainite transformation,
The dislocations
excess of 28,500 lb/in2 is not easily understood.
that martensite
and bainite,
Plastic extension
when the applied load exceeds a given threshold. threshold
grains and thus cause the observed extension.
,513
But if this mecha-
explanation
existing on the
of such an array is responsible
for the low threshold on transformation
for
observed. for plastic extento martensite,
the
offered is that the kinetic energy released
nism were active in the present case, one would expect
when a plate of martensite forms is capable of moving
that as soon as plastic
dislocations
deformation
occurred
below
out from their sources and forcing pile-ups
Md martensite would be produced and there would be
arrays against the grain boundaries,
a sudden jump in the temperature
small additional
of transformation
from the normal M, at 200°C to some higher tempera-
slip in adjacent
ture
deformation.
(M,)
instead
of the actual
gradual1 y rising transformation
experience
of the
temperature
as the
stress is raised above 28,500 lb/ins. It is apparent
grains
M, temperature
is difficult
results have
tensite formation. needed
on
deformation influence to martensite.
martensite
transformation
which
are
readily explained by our present understanding transformation. on martensite
not
of the
Further study of the effect of stress formation
plastic deformation
under varying
conditions
of
and grain size should prove to be
a fruitful field of investigation. From the practical the thresholds
point of view, consideration
for plastic deformation,
low threshold on transformation prove of importance tion of residual treatment,
on the
It is believed
especially
of
should
and
stress patterns
from
heat
just as an awareness of the effect of stress
on transformation kinetics is helpful in explaining anomalous internal structures often observed in heat treated steels.
special
transformation
high
speed
under applied
recording
to explain
on the basis of mar-
size
and
stress-induced
that consideration
prior
is
plastic
transformation of the thresholds
and accelerated transformation
structural
in studying anomalies
residual which
stress
occur
in
heat treated steels. ACKNOWLEDGMENTS
The
authors
Belt Company
are especially of Milwaukee,
ing this investigation
grateful
to the Chain
Wisconsin,
for sponsor-
and to J. J. Scales and the
members
of the metallurgical
Company
for their
helpful
staff
discussion,
at Chain
Belt
interest
and
encouragement. REFERENCES
5. CONCLUSIONS
Using
plastic
the
and interpreta-
resulting
gross
needed to raise the
of the mechanism
grain
be of importance
to martensite, should
in the prediction
way
for plastic deformation patterns
produce
It appears that more information
brought to light several features of the effect of stress the
and
The high threshold
of our present knowledge
that the experimental
so that only a
applied stress is necessary to activate
techniques,
tensile loads has been
1. M. D. JEPSON and F. C. THOMSON, Acceleration Rate of Isothermal
St. Inst.
Transformation
162,49 (1949).
of Au&mite,
of the J. Iron
514
ACTA
METALLURGICA,
2. S. BHATTACH~RYY~and G. L. KEHL, Isothermal Transformation of Austenite under Externally Applied Tensile Stress, Trans. Amer. Sot. Metals 47, 351 (1955). 3. E. SCHEIL,Uber die Umwandlung des Austenits in Martensit in Eisen-Nickel-Legierungen miter Belastung, 2. Artorg. Allgem. Chem. 207, 21 (1932). 4. A. W. MCREYNOLDS,Effect of Stress and Deformation on the Martensite Transformation, J. AppZ. Phys. 20, 896 (1949). 5. S. A. KULIN, M. COHEN and B. L. AVERBACH,Effect of Applied Stress on the Martensite Transformation, Trans. Amer. Inst. Min. (Metall.) Engrs. 194, 661 (1952). 6. J. R. PATEL and M. COHEN, Criterion for the Action of Applied Stress in the Martensite Transformation, Acta Met. 1,531 (1953). 7. L. C. CHANG and T. A. READ, Plastic Deformation and Diffusionless Phase Channes in Metals-The Gold-Cadmium BetaPhase, Trans. Amer. Inst. Min. (Metall.) Engrs. 191,47 (1951). 8. M. W. BURKART and T. A. READ, Diffusionless Phase Changes in the Indium-Thallium System, Trans. Amer. Inst. Min. (Metall.) Engrs. 197,1516 (1953). 9. C. S. BARRETT and 0. R. TRAUTZ, Low Temperature Transformation in Lithium and Lithiun-Magnesium Alloys, Metals Tech. 15, T.P. No. 2346 (1948). 10. W. H. COLNERand 0. ZMESKAL,An Electrical Resistance Apparatus for Studying Transformation in Stainless Steel, Trans. Amer. Sot. Metals 44, 1158 (1952). 11. J. B. AUSTIN and R. L. RICKETT,Kinetics of the Decomposition of Austenite at Constant Temperature, Metals Tech., 5, T.P. No. 964 (1938). 12. E. 8. D_&VENPORT and E. C. BAIN, Transformation of
VOL.
13. 14.
15. 16. 17. 18. 19. 20. 21. 22. 23.
7,
1959
Austenite at Constant Subcritical Temperatures, Trans. Amer. Inst. Min. (Met&Z.) Engrs. 99,117 (1930). A. KOCHEND~RFER, Plastische Eigenschaften van KristaZZen und Metallischen Werkstoffen. Springer-Verlag, Berlin (1941). G. J. GUA~NIERIand J. J. KANTER, Some characteristics of the Metaetable Austenite of 4-6% Chromium plus By, Molybdenum Cast Steel, Trans. Amer. Sot. Metals 40, 1147 (1948). A. SAUVEUR, The Metallography and Heat Treatment of Iron and Steel. The University Press, Cambridge, Massachusetts (19261. W. BOAS, Phyiics df Metals und Alloys, p. 121. Wiley, New York (1949). F. R. N. NABARRO.S~~~ Recent Develovments in Rheoloav. Y” British Rheologists’ Club, London (1950). A. H. COTTRELL, Theory of Dislocations, Progress in Metal Physics, Vol. 1, Chapter 2. Interscience, New York (1950). J. D. ESHELBY,F. C. FRANK, and F. R. N. NABARRO,The Equilibrium of Linear Arrays of Dislocations, Phil. Mag. 42, 351 (1951). J. S. KOEHLER,The Production of Large Tensile Stresses by Dislocations, Phys. Rev. 85, 480 (1952). F. R. N. NABARRO,Symposium on Internal Stress in Metals and Alloys, p. 237. Institute of Metals, London (1948). J. HESLOP and N. J. PETCR, The Stress to Move a Free Dislocation in Alpha Iron, Phil. Mag. 1, 866 (1956). B. L. AVERBACH,The Effect of Plastic Deformation on Solid Reactions, Cold Working of Metals, p. 262. smerican Society for Metals, Cleveland (1949).
STRESS ON IN STEEL*
and P. C. ROSENTHAL:
capable of making simultaneous
measurements
of the electrical
resistance and the extension of a wire specimen during rapid quenching or isothermal transformation, the transformation characteristics of a eutectoid steel have been studied under dead-weight tensile loading. Data obtained on transformation to pearlite, bainite, and martensite indicate that there is a threshold stress above which transformation is accelerated. When transforming under load, extensive plastic deformation is noted coincident with transformation. There also appears to be a threshold stress In the case of transformation to pearlite and bainite, increased associated with the plastic deformation. On transformation rates of transformation occur at the same threshold stress as the plastic deformation. to martensite gross plastic deformation occurs at very low stress, while the M, temperature is not raised until stresses on the order of 28,500 lb/in2 are reached. To account for the coincidence of transformation and plastic extension it is proposed that dislocations piled up at grain boundaries and other barriers produce stress fields which result in increased rates of nucleation. Moreover, when the nucleus loses coherency with the parent austenite, the advancing interface acts as a sink for the piled-up dislocations and thus plastic deformation is observed. According to this picture, the threshold stress is the stress necessary to move free dislocations out from their sources t,o produce the piled-up arrays. The difference in the behavior of the martensite transformation under load can be explained on the basis of the differences between the mechanism of formation of martensite and that of pearlite and bainite. EFFET
DUNE DEFORMATION TRANSFORMATIONS DE
PAR TRACTION SUR PHASE DE L’ACIER
LES
Les auteurs etudient les characteristiques de la transformation dun acier eutectoide soumis a une deformation par traction. A cette fin, ils utilisent un montage special permettant de mesurer simultanement la resistance electrique et l’allongement d’un fil au tours dune trempe Bnergique ou d’une transformation isotherme. Les rirsultats obtenus pour les transformations perlitiques, bainitiques et martensitiques indiquent qu’il existe un seuil de tension au-dessus duquel la transformation est accBl&ee. Lorsque la transformation a lieu sous charge, une deformation plastique importante se marque simultanement & la transformation. 11 semble exister Bgalement un seuil de tension associi! a la deformation plastique. Dans le cas des transformations perlitiques et bainitiques, l’acceleration de la vitesse de la transformation se produit pour le m&me seuil de tension que pour la deformation plastique. Pour le transformation martensitique, une deformation plastique importante apparait pour une tension tres faible tandis que la temperature M, n’est accrue que pour des tensions de l’ordre de 28,500 lb/in2. Les auteurs proposent d’interpreter cette coincidence de la transformation et de la deformation plastique par un empilement de dislocations aux frontiirres granulaires. Ces empilements et autres barriiires similaires produisent des champs de tensions d’oh resulteront des vitesses de germination accrues. En outre, lorsque le germe n’est plus coherent vis-a-vis de la matrice austenitique, l’interface en mouvement agit comme un puits pour l’empilement des dislocations et la dOformation plastique en resulte. D’apres ce modele, le seuil de tensions correspond a la tension necessaire pour deplacer les dislocations libres de leurs sources afin de provoquer des reseaux d’empilement. La difference de cornportement de la transformation martensitique sous charge peut Btre expliquee sur la base des differences entre le mecanisme de la deformation de la martensite et celui de la ferrite et de la bainite. BEEINFLUSSUNG
VON
PHASENUMWANDLUNGEN
VON
STAHL
DURCH
ZUGSPANNUNG
Mit Hilfe eines besonders konstruierten Apparates, der es gestattet, wiihrend schnellen Abschreckens oder isothermer Umwandlung einer Drahtprobe gleichzeitig deren elektrischen Widerstand und Verliingerung zu messen, wurde der Verlauf der Umwandlung van eutektoidem Stahl unter konstanter Zuglast untersucht. Bei der Umwandlung zu Perlit, Bainit und Martensit lassen die Messungen auf eine Schwellenspannung schliessen, oberhalb deren die Umwandlung beschleunigt ist. Verlauft die Umwandlung unter Last, so ist sie von grossen plastischen Verformungen begleitet. Auch fur die plastische Verformung scheint es eine Schwellenspannung zu geben. Im Fall van Perlit und Bainit ist die Schwellenspannung fur die Umwandlung und die plastische Verformung dieselbe. Bei der Martensitumwandlung tritt bei sehr niedriger Spannung bereits grosse plastische Verformung auf, die M,-Temperatur steigt jedoch erst von Spannungen der Grossenordnung 28.500 lb/in2 (=20 kg/mm%) ab an.
* Based on a thesis submitted in partial fulfillment of the requirements for a Ph.D. at the University Received June 4, 1958; revised version December 29, 1958. t Now with the Applied Research Laboratory, U.S. Steel Corp., Monroeville, Penn. $ Department of Mining and Metallurgical Engineering, University of Wisconsin, Madison, Wisconsin. ACTA
METALLURGICA,
VOL.
7, JULY
1959
504
of Wisconsin.
PORTER
AND WOSESTHAL:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
IN STEEL
85
Zur Erklarung des gleichzeitigen Auftretens von Umwandlung und plastischer Dehnung wird vorgeschlagen, dass die Spannungsfeldar von Versetzungen, die sich an Korngrenzen und anderen Hindernissen aufstauen, verstiirkte Keimbildung znr Folge haben. Wenn der Keim dann die Koharenz mit der Austenitmatrix verliert, bilden die entstehenden Phasengrenzflachen Senkon fur die aufgesKach dieser Vorstellung ist also die tauten Versetzungen, und man beobachtet plastische Verformnng. S~h~~elIenspannnng diejenige Spannung, die notwendig ist., urn freie Versetzungen von ihren Quellen zn l&en und so die aufgestauten Gruppen zu bilden. Das unterschiedliehe Verhalten der Martensiturnwandlung unter Last l&St sich auf Grund der Unterschiede zwischen den Bildungsmechanismen von Martensit einerseits und Perlit und Bainit andererseits verstehen.
1. INTRODUCTION
approximation
Until recently it was assumed by most metallurgists that stress would have little influence on phase trans-
transformed
formations
equal
in solids.
one considers phase
This view is indeed juststilled if
only the possible
equilibria.
For
effects
instance
of stress on
applying
Chatelier principle to the A, equilibrium
the
Le
in a eutectoid
that the percentage
of the total resis-
tance difference between austenite and the completely structure
to the volume
phases present. was checked
at a given
temperature
was
per cent of the transformed
The validity
of this approximatio~l
using metallographic
found to be within the accuracy
methods
and was
of the metallographic
carbon steel, it can be shown that it takes a triaxial
estimation
stress
range. In order to obtain the required data, apparatus had
of
12,750
temperature
lb/in2
to
change
however,
experimental
and Kehlt2) indicate
and pearlite transformation applied uniaxial
stress.
found
of stress on the martensite
in iron-nickel systems
of
alloys
by
transformation and
lithium-magnesium. The
present
to obtain
applied
tensile
was
stress on the
martensite transformations
hainite,
material
used
and
theory.
C
MI1
0.90%
0.4.5;/, 0.005~0
which
apparatus
evolved
described
primary advantage of
the
investigation
music wire having the P
Si 0.25%
the specimen
quenchant
MO
V
0.010%
nil
cooling.
from an austenitizing Instead,
furnace to a
the wire specimen
tizing time by passing a high amperage the high amperage
current is replaced
with a direct
current resistance measuring circuit, and the specimen is quenched with a high velocity stream of helium gas isothermal temperature
perature is maintained with a wire-wound central
by surrounding
the specimen
tube furnace having a controlled
zone of uniform
temperature.
In order to
under stress, arrangenlents
are made for
a dead-weight
tensile load to the specimen. with the speci-
0.08%
When making isothermal tests, the IX drop through
homogeneous
the specimen is recorded on a photoelectric meter.
In order to study the martcnsite
Reproductions
6
corrected for any plas-
during the test, were con-
transformation
using
the
first
potentio-
transforma-
tion, it is necessary to record five variables on a sixchannel record.ing oscillograph operating at a chart during an isothermal
percent,
or to room
In isothermal tests, the isothermal tem-
speed of 1 in./seo.
to
alternating
electric current through it. After proper austeIlitizing,
resistance
verted
to austeni-
and held for the required austeni-
The resistivity
occurring
The
of the system is that no movement
The progress of the transformation was obtained by recording the electrical resistivity of the specimen. tic deformation
of an
men assembled and in place is shown in Fig. 1.
as received and was used without further homogenizing heat treatment.
measurements,
of
The system
is an adaptation
A schematic diagram of the apparatus
CU
The wire was judged metallo~aphically
recording
variables
is heated in a purified helium atmosphere
applying
0.010%
other
by Colner and Zmeskalo”).
is necessary.
transform
Ni
0.0100/
and the
or during continuous
was finally
temperature.
composition:
Cr
formation
to a predetermined
METHOD
throughout
consisted of 0.040 in. diameter following
pearlite
and an explanation
based on dislocation
2. EXPERIMENTAL The
in an
of the effect of
in a eutectoid carbon steel.
The results have been analyzed has been proposed
conducted
a unified picture
of simultaneously
resistance
tizing temperature
investigation
attempt
transformation
interest which were changing dnring isothermal trans-
a marked
indiuni-thallium
capable
of the
and
as well as in the non-ferrous
gold--cadmium,
to be developed
most
temperature,
Since the work of Scheil(3) a have
throughout
The
that the bainite
in steel is accelerated
of investigators(4-g)
influence
of phase transfor-
the picture is quite different.
evidence of Jepson and Thomsoncr)
~hattacharyya
number
equilibriunl
1°C.
When one looks at the kinetics mation,
this
the change continuous
recorded
in the five variables
of the changes in test and of
recorded
during a
cooling test are shown in Fig, 2. Careful
506
ACTA
METALLURGICA,
VOL.
5,
1959
AUSTENITIZING~ CURRENT
LEAD
MEASURING
CIRCUITS
HELIUM OUT SPECIMEN
SUPPORT
BRACKET
ISOTHERMAL CHEATING
HOLDING
FURNACE
c01Ls /MEASURING BEHIND HEAT
CIRCUITS STAINLESS
PASS STEEL
SHIELD
SPECIMEN THERMOCOUPLE STAINLESS GRADED
STEEL TUBE WITH OPENINGS OPPOSITE
SPECIMEN
FLEXISLE
HELIUM
QUENCH
S -STRAIN
HELlUM
FOR
GAGE
EXTENSOMETER
IN
AUSTENlTlZlNG/ CURRENT LEAD
STRAIN DEAD
GAGE
LOAD
WEIGHT
LOAD
CELL
AzzEik PEDESTAL
RELEASE
PIN
SCHEMATIC
DASH
DIAGRAh3
OF
TEST
PAT
APPARATUS
FIG. 1. Schematic transformation
diagram of apparatus for observing under applied dead-weight tensile loads.
FIN. 2. Reproductions of records showing changes in variables during continuous cooling test (top) and the change in resistance during isothermal test (bottom).
PORTER EFFECT 260”
AND ROSENTHAL: OF C-
STRESS 500’
ON THE
F
TENSILE
BAINITE
ISOTHERM
STRESS
AND
PHASE THE
TRANSFORMATION
AUTOCATALYTIC
TRANSFORMATIONS DECREASE
ISOTHERMAL
PLOT
IN TIME
IN STEEL
TO TRANSFORM
TRANSFORMATION
UN[LR
507
SO%, STRESS
600
500 :: g
1
400
” :
f
300
? t 200
IO0
0 0
IO
20
M
40
50
STRESS IN POUNDS FW SOUARE INCH x IO‘= 200
100
1000
500
300 TIME
2000
3000
IN SECONDS
3. Typical autocatalytic plot showing applied tensile stress on bainite transformation
FIG.
effect of at 260°C.
calibration and checking of the apparatus indicated that, during test, temperatures remained uniform along the specimen and that the accuracy of the recorded information was limited primarily by the ability to read the charts. 3. RESULTS
The results are most conveniently divided into three groups: results obtained on isothermal tests in the bainite region, less complete results obtained isothermally and by continuous cooling in the pearlite region, and results obtained for the martensite transformation on quenching. Transformation
to bainite
In the bainite region the transformation characteristics were determined for a series of increasing applied uniaxial tensile stresses at isothermal temperatures of 260°C (500”F), 316°C (600°F) and 371°C (700°F). The results indicate that above a certain threshold stress applied tensile stress markedly accelerates the transformation. It appears that the degree of acceleration, as measured by the decrease in time to achieve a given amount of transformation, is a linear function of the applied stress, especially at the lower stress levels. Instead of plotting per cent transformation vs. log time to produce the typical sigmoid curve, the resistance measurements were converted to percent transformation and plotted on an autocatalytic plot. The type of plot obeys the following equation: log (P/100
-
P) =
K log t +
G
FIG. 4. The manner in which the applied stress decreases Note the threshold at the transformation time. 600043000 lb/in2
where P is the amount transformed and t is time, K and C being constants. Its use for representing the bainite transformation was first suggested by Austin and Rickettol) who showed that the original data of Davenport and Bain(12)for the bainite transformation lay on a straight line when plotted in this way. While the fact that a straight line results is probably without real significance, such a plot does aid in establishing start and completion times for the reaction (on the basis of 1 per cent and 99 per cent transformation) and promotes confidence in the experimental results when they plot on a straight line. Fig. 3 is an autocatalytic plot showing the effect of applied stress on the kinetics of the bainite transformation at 260°C. One notes that the experimental results plot as straight lines at all but the highest stress level. It is apparent that applied tensile stress, while markedly increasing transformation rates, has a greater effect on the early stages of the transformation than on the Iater stages. Results at 316°C and 371°C show the same characteristics as those at 260°C but of course transformation occurs in shorter times. Furthermore, if one examines the decrease in transformation time as a function of applied stress, another important effect is noted. In Fig. 4 the decrease in time to achieve 50 per cent transformation has been plotted against applied stress. The decrease is found to be proportional to applied stress and, on the basis of the extrapolated plots, the minimum stress needed to produce accelerated transformation is found to be 8000 lb/in2 at 26O”C, approximately 7000 lb/in2 at 316”C, and 6000 lb/in2 at 371°C.
_4CTA
50s
METALLURGICA,
VOL.
i,
OF
APPLIED
EFFECT
DURING
1959 STRESS
ON
ISOTHERMAL
PLASTIC
EXTENSION
TRANSFORMATION
.020 688°C-12700F
0
200
100
FIG.
FIG. 5. The progress of the deformation coincident with isothermal transformation 260°C.
which occurs to bainite at
in electrical
began to extend
resistance,
plastically
at a rather rapid rate.
The extension continued throughout of the transformation, tivity measurements was complete.
the major portion
ceasing somewhat
The progress
of the rapid extension at 260°C is shown in
Fig. 5. Again these results are typical tures.
curves obtained
Extension measurements made during isothermal transformation to pearlite at 6SS”C.
7.
mately 35 times higher than that resulting from transformation
without applied stress.
of the form of
at the higher tempera-
line relationship
the same as the thresholds of the transformation
APPLIED STRESS vs MAXIMUM EXTENSION PRODUCED OURlNG ISOTHERMAL TFJ.ANSFORMATlON TO BAINITE
500°
F -260°
tion rates.
on extrapolation
relationship
There
between
the
noted and the increased transforma-
Indeed,
the onset of the rapid extension
under load is found to be a more sensitive indication of the start of transformation
than are the resistivity
since it is observed before a change in
Transformation
to pearlite
Only a few isothermal tests were carried out in the pearlite range at 688°C (1270°F). measurements
made
transformation
characteristics
ally unsatisfactory,
C
obtained
appears to be an intimate rapid extension
straight
time vs. stress curves.
resistance is detected.
is approxi-
of
values which are approximately
measurements,
stresses investigated,
as a function
exists and the extrapolated
lines have threshold
suffered by the specimen stress and,
extension
stress (Fig. 6) one finds that at lower stresses a straight
to the applied
The total extension
is found to be proportional at the maximum
before resis-
indicated that the transformation
occurring during transformation the extension
was noted
the specimen
600
500
IN SECONDS
If one plots maximum
Shortly before evidence of transformation by a change
400
300 TIME
ISOTHERM
the isothermal
the
kinetics
isothermal
under load were gener-
owing to difficulties
in attaining
and to the rapid change with temperature
in this
The results did indicate that transformation
was accelerated
by stresses below
again rapid extension before
Results of resistance
determine
temperature
in transformation region.
to
was observed
the resistance
transformation
measurements
4000
lb/in2
and
to occur slightly indicated
that
was in progress.
The results shown in Fig. 7 are typical of the extension measurements made during isothermal transfor-
OO”F-371°C
mation in the pearlite region. At a stress of 7900 lb/in2 it is seen that rapid extension occurs and is completed in about 300 set, after which a steady rate of creep STRESS FIG.
and
IN POUNDS
PER SQUARE
INCH
IO-’
6. The relationship obtained between applied stress Note that the same thresholds are extension. observed as in Fig. 4.
continues.
It is interesting
to note that under the
conditions
of this test the pearlite transformation
completed
in somewhat
is
under an hour and that in
PORTER
.%SD ROSENTHA1L:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
TEMPERATURE,
IN
STEEL
509
‘C
FIG. 10. Results
of resistance and extension measurements made during quenching to form martensite at low applied stresses. The lower plot gives the percentage of the total load applied in the temperature range indicated.
FIG. 8. Reproductions of records of extension obtained during continuous cooling through the pearlite region.
300 see the specimen cent transformed. rapid extension
is somewhat
less than 50 per
At the lower stress of 3950 lb/in2 and its subsequent
is noted again.
early completion
In this case no creep is observed after
the rapid extension has ceased. Additional through
data
obtained
by
continuous
the pearlite region substantiate
mal results.
cooling
the isother-
Here again data obtained from extension
measurements
prove to be most reliable. the applied
the extension
during transformation.
occurring
EFFECT OF APPLIED STRESS ON EXTENSION OCCURRING DURING MARTENSITE TRANSFORMATIO,,
Fig. 8 shows
the effect of increasing amination
FIG. 11. Results of resistance and extension measurements made at high applied tensile stresses. The lower plot gives the percentage of the total load applied in the temperature range indicated.
.05
tensile stress on Ex-
lb/in2 and below do not alter the temperature recalescence
formation
range of
E
.03
these low stresses do
B 5 z
.02
in excess of the normal trans-
z
Likewise,
not result in extension dilatation.
acceleration
Y
and therefore do not accelerate the pear-
lite transformation.
.04
I
of Fig. 8 shows that applied stresses of 1575
At higher stresses, evidence
of the transformation
of
9
as well as plastic
t: APPLIED STRESS vs EXTENSION ON TRPNSFORMATION TO PEARLITE DURING CONTINUOUS COOLING
.Ol
E I.oo
0
IO APPLIED
20 STRESS
30 40 IN PSI x IO-’
FIG. 12. Applied stresses in excess of 1000 lb/in2 cause extension during the martensite transformation. EFFECT OF APPLIED STRESS ON M, z E
g
.ooo 1
0
I
I
I
I
I
2
4
6
8
IO
APPLIED
FIG. 9. The relationship extension
STRESS IN PSI
x lO-3
between applied stress and the obtained during continuous cooling through the pearlite region.
50
= 2
250
TEMPERAWE
I(
,__I
1
1)1_
/ITHR~WJ
1
I,3
L 3 e-
150 c -0 APPLIED
IO
20 30 STRESS IN PSI x lO-3
40
50
FIG. 13. The M, temperature is not affected until stresses over 28,500 lb/in2 are applied.
510
ACTA
extension during transformation the bainite
data,
METALLURGICA,
is observed.
extrapolated
linear
VOL.
As with
plots
7,
1959
02% OFFSET YIELD STRENGTH AUSTENITE “5 SQUARE ROOT
of the
extension data (Fig. 9) indicate a threshold for plastic extension
coincident
with accelerated
In the case of pearlite, stress of lb/in2
2000
lb/in2
deduced
for
the
threshold
as compared the
bainite
A
transformation. occurs
with
the
0
8000
transformation
DATA FROM GUARNIERI 8, KANTER 5% Cr , I/P%Mo .CAST STEEL 09%
at a
OF METASTABLE OF TEMPERATURE
CARBON
THRESHOLD
STEEL
VALUES
i
at
260°C. Martensite transformation Representative quenching
examples
of the data obtained
to form martensite
loads are given in Figs. 10 and 11. noted that rapid extension tion.
The
effect
transformation
of
on
under various applied Here again it is
occurs during transforma-
stress
on
the
extension
are presented in Figs. 12 and 13.
The results are quite different from those obtained previously
for bainite
and pearlite.
It is observed
that the threshold for increased extension is very low, about 1000 lb/in2, while no effect on the M, temperature is observed until a stress of approximately
28,500
It is also evident from Figs. 10 and
lb/in2 is reached.
11 that the beginning
of rapid extension is coincident
with the M, temperature,
as measured by resistance,
it as in the case of bainite and
instead of preceding
pearlite transformations.
Measurements
of hardness
and of retained austenite content by integrated X-ray intensity methods indicate that the applied stress has little
influence
formed. calculated
on the total
amount
of martensite
While the progress of transformation from
the
shapes of the resistivity
resistivity
that the stress-induced
occurring at temperatures
2OO”C, the normal
M, temperature,
much slower rate with decreasing does the normal transformation
in excess of
progresses temperature
at a than
occurring below 200°C.
In other words, stress-induced transformation and progresses slowly at temperatures above but when the normal M, temperature normal transformation
the
curves obtained under stresses
of over 28,500 lb/in2 indicate transformation
was not
measurements,
occurs 2OO”C,
is reached the
begins and progresses just as it
would if no stress were present. 4. DISCUSSION
with plastic
extension
100
200 300 TEMPERATURE
400 500 600 700 IN DEGREES CENTIGRADE
FIG. 14. An estimation of the yield strength of metastable austenite in the material used in this study. Note the low values of the thresholds compared with the yield strength.
Here plastic extension
during transformation
does not occur until a very high stress is reached, The threshold
stress is very low compared
to the
stress being sustained by the specimen up to the time transformation the probable
begins.
Indeed, it is much lower than
0.2 per cent offset yield stress.
A rather
good estimate of the yield strength of the metastable austenite
may be obtained
from the loads which it
was able to sustain during isothermal additional
pieces of information
studies.
Two
on this point are also
available. First, it is known that the yield stress should vary as the square root of the absolute temperatrue and secondly,
Guarnieri and Kanter(l*) have made 0.2
per cent offset yield stress measurements austenite which
in 5%
chromium-$“/o
can be compared
on metastable
molybdenum
with the present
steel
data.
Fig. 14, a stress vs. d/T plot of data derived isothermal with
and continuous
the data
estimated
cooling
of Guarnieri
temperature
stress
relationship
study
is indicated.
and
In from
data is compared Kanter,
and
the
vs. 0.2 per cent offset yield
for
the
Also
material
shown
The question one naturally stress
during transforma-
mechanism
used
in this
are the threshold
during
transformation,
and
the threshold stress for increased rates of transformation. Thus when the applied stress exceeds a certain
under load has been reported
rates are accelerated,
and when
occurs, the yield strength of the steel
is suddenly markedly reduced. Conditions are somewhat different in the case of martensite formation.
asks is:
“What
is the
by which the resistance to flow is lowered
threshold
transformation
occurs
at very low stress while an increase in M, temperature
tion, and in the pearlite and bainite regions this is also
value, transformation
8co
stresses in the pearlite, bainite and martensite regions.
OF RESULTS
The results indicate that there is a threshold associated
4
0
and
stress?”
detailed explanation
what
determines
the
Plastic flow during transformation previously(2y14J5) but a
is usually lacking.
Boasu6) states that it may be possible that the high mobility of the atoms at the interface between old and new phase during
transformation
plasticity
gives
rise to
weakness
and
of the metal in the same way as does the
PORTER
AND ROSENTHAL:
TENSILE
STRESS
AND
PHASE
TRANSFORMATIONS
high mobility at grain boundaries at elevated tempera-
work
tures.
against these stresses.
However,
whole story. forming
Boas’
can not tell the
It is difficult to see how isolated trans-
nuclei,
such as must be present
effect is first noted, boundary
suggestion
atoms
deformation
could,
by the mobility
account
alone,
when the
of the remaining
for
matrix.
of their
gross
plastic
Moreover,
on
must
be done
Transformation boundaries
in order to drive
could
but extensive dislocations
way.
of grain
Dislocations
sources at relatively
yielding
511
dislocations
alter the influence
in the following
out from dislocation
IN STEEL
move
low stresses,
is not observed
because
are held up at the gram boundaries.
the This
the basis of the results presented here, any complete
results in a piling-up of dislocations into an equilibrium
explanation
distribution
threshold
would have to account
for plastic deformation
for the observed
during transforma-
against the boundaries
as shown schema-
tically in Fig. 15. The nucleation
of the transforma-
tion and for the fact that the same threshold is observed
tion causes a collapse of the resistance offered by the
for increased rates of transformation.
grain boundaries,
If one rejects mobility
the explanation
that the increased
of the atoms at the transforming
interface, in
itself, can account for the plastic deformation panying transformation
accom-
under load, one must examine
with transformation If the picture shown
that,
blocked
which might be altered by the initiation
transformation
mation.
These
sources
solution,
precipitates,
are
interacting
the difficulty of transmitting another. In austenite, the alloying
alloying
dislocations,
contribution
would
nucleation.
especially
cubic lattice.
strength
tempera-
involved,
carbon
must
atmospheres
and nitrogen,
Precipitates,
be small.
is not strong because,
at the dislocations
Taking everything
of the
of solute
are probably
in the face-centered
as such, are not present.
into account,
it would appear that
rates.
of dislocations
will promote
It is generally
rates and that the increased
high
solubility
it remains to be
stress, an array
increased
conceded
that
the stresses involved can not greatly influence diffusion must
Cottrell anchoring of dislocations
not condensed
and
lead one to believe that their
to the yield
for the temperatures
in
arrays to
is noted.
at a grain boundary
slip from one grain to
elements present and the elevated
tures involved
atoms,
elements
the dislocation
is to be complete
under
the other possible sources of resistance to deformation of transfor-
allowing
move forward, and thus the rapid extension coincident
be mostly
associated
Using
the
rate of transformation with increased
results
of
rates of
Cottrell(ls)
and
et al. (lg), Koehlerc20) has shown that, in an
Eshelby
array of edge dislocations
by an obstacle
as
shown in Fig. 15, there can be a high concentration
blocked
of
tensile stress over a considerable of the dislocation dilatation
region in the vicinity
nearest the obstacle,
and extensive
of the lattice can occur in the region below
this dislocation.
The extraordinary
lattice
the formation
promotes
through
the
action
of
the
dilatation
of the
of the stable phases LeChatelier
principle.
it must be the release of the resistance to deformation
Moreover,
because of the number
offered primarily by the grain boundaries, and to some extent by the interacting dislocations, which is
produced,
more nuclei will be activated,
reduction
in the distance an atom must diffuse before
responsible
reaching
the nearest
for
the
plastic
flow
associated
with
Gram boundaries
offer resistance to slip because the
accelerate diffusion.(21)
direction of the plane of slip changes at the boundary.
rate
Moreover,
materially
in order to take into account
grain size, Nabarroo’) cm thick
the effect of
suggests that there be a region
at the grain
boundaries
where
the
of
nucleation
of
Finally,
the internal will tend to
For these reasons, the normal the
stable
phases
increased when dislocation
will
be
arrays pile up
at grain boundaries under the action of applied stress. The mechanism
whereby
transformation
applied tensile stress in-
coherence of the grains causes severe elastic distortion
fluences
and slip on unfavorable
regions may thus be described
planes, in which case, extra
arrays
resulting in a
stress gradient near the first dislocation
transformation.
lop4
nuclei.
of piled-up
in the bainite as follows:
and pearlite The thres-
hold stress represents the effective stress necessary to
A6 - SLIP PLANE P - OBSTACLE
lP
A_gI++
cause
dislocations
to
move
out
from
dislocation
sources and begin piling up at grain boundaries.
The
dislocation arrays thus formed produce a large concentration of tensile stress in the vicinity of the leading dislocation.
This stress and its accompanying
dilata-
tion of the crystal lattice result in increased rates of THE
PILING
OF DISLOCATIONS
AGAINST
AN OBSTACLE
FROM
COTTRELL
FIG. 16. A schematic representation of a pile-up array of dislocations after Cottrell.
nucleation and therefore in increased rates of transformation. When the nuclei lose coherency with the parent lattice, the dislocation arrays are freed from
ACTA
,512
The growing
their barriers.
interface
as well as a source for dislocations. movement
is again possible
occurs coincident can
continue
continuous.
acts as a sink
Thus dislocation
and rapid
with transformation.
until
the
phases
being
after
has been formed has
continuous
only
about
50%
and not until
been
formed,
lends support observed
not
95%
of the becomes the
and the grain
investigation
effective tensile stress necessary
comparable,
proposed
mechanism,
stress
is indeed
the
to move dislocations
out from their sources to form dislocation It is also interesting
arrays.
to note that, according either uniaxial
the dislocations
to influence
which the transformation
transformation
It will be remembered of applied
tension or com-
is greatest at
that in examining
at elevated
temperatures
and some plastic deformation
load.
Thus the austenite
temperature
the M, temperature, probably
was strained when the M,?
was reached, and for loads which raised some plastic
deformation
still in progress when transformation in the vicinity
of the normal MS (200°C) is
estimated as 32,000 lb/in2 and on the basis of a square root dependence one
between
1,000 lbjin2.
stress is not involved.
with
If
nucleation
throughout catalytic
Results
obtained
by Jepson
rates
were
accelerated
the transformation,
uniformly
the resulting
auto-
plot would be displaced to shorter times but
the
expect
stress and tem-
LO,000 lb/in2 whereas the actual threshold
stress along the slip direction and the sign of the shear
would
threshold
since the active component
indicate that this is indeed the case.
was
started.
From Fig. 15 the 0.2 per cent, offset yield strength of
perature
and Thomson(l)
during occurred
before the material became cool enough to support the
pression should show increased rates of transformation, of the stress is the shear
the effect,
stress on the ~nartensite transformations
rapid cooling,
au&mite
to the
is accelerated
the start of transformation.
loads were applied between
directly
However,
greatly
while the amount by
of plastic
to the idea that the threshold
in the present
appear
is greatest during the
pearlite
freed from its atmosphere, although
do not
middle stages of transformat,ion
lower yield point, the tensile st,ress necessary to move size in ferrite,
increased transformation.
the rate of plastic deformation
are
of the pearlite
work(22), on the relationship
a dislocation
1959
Deformation
much earlier than bainite.
Petch’s
7,
merely by the act of moving through the lattice, since
formed
about
since
VOL.
deformation
This accounts for the completion
deformation bainite
METALLURGICA,
a threshold
of
about
occurs
at
&‘o change in the M, is noted coincident
threshold
stress,
and
none
occurs
until
stresses over 30,000 are applied. The most extension
obvious
observed
transformed
explanation
for the increased
at low stress when austenite
to martensite
is that low applied
is
stress
would still be parallel to the original unstressed plot.
causes the martensite
plates to form with a preferred
In the present
orientation,
in the concomitant
case the slopes
change
as stress is
resulting
increased so that the plots have a larger separation at
length even though the transformation
the start of transformation
not been changed
transformation. of transformation than
during
creased
of
nucleation is accelerated much more
the later stages,
transformation formation
than at the completion
This indicates that in the early stages The early
stages of
are believed to be accelerated
of more and larger pile-ups stress
concentration
transformation
is under
way,
below
by the
and the in-
At high applied
to be associated
has been
by Chang and Readt7) in the case of the
gold-cadmium
system.
However,
in the present> ease, during the early
the pile-ups
stages of transformation
of the
This is believed
deformation
from stresses in excess of the yield stress. bands resulting from such deformation
tation during the martensite transformation reported
techniques failed
stresses the early portions
with plastic
Enhanced
to reveal any preferred
plots depart from linearity (see curve for
47,300 lb/in2 applied stress, Fig. 3).
amount.
under load as a result of preferred orien-
Once
begin to be released and their influence is soon lost. autocatalytic
a detectable
in
a careful search using metallographic
pile-ups.
however,
deformation
change
kinetics have
and X-ray preferred
diffraction orientation
orientation
to martensite
techniques
under stress,
failed to reveal any
in the completely
transforn~ed
martensite structure. The experimental results might better be explained
resulting
in a second way.
The slip
free energy which must be available to form a marton-
probably
pro-
site plate
It is well known that some of t,he
is released
as kinetic
energy
during
the
duce atomic configurations which act as additional nucleation sites within the grains. This type of
formation of the plate. This kinetic energy manifests itself as observable slip in the austenitc adjacent to
mechanism
the newly formed
has been reviewed by Averbachcz3) and is
well known in age hardening systems. In all cases it has been assumed movement trations
must occur to produce and
atomic
configurations
passes through that dislocation
the stress conoenwhich
lead
to
plate and in a shock wave which
the austenite,
often
resulting
audible click. Perhaps the “triggering existing martensite embryos supplies necessary to drive dislocations
in an
off” of prethe energy
through the austenite,
PORTER
forming
ANT) ROSENTHAL:
pile-ups
which
grain boundaries. transformation,
TENSILE
are forced
STRESS
hard against
the
Thus, in the case of the martensite the energy to move the dislocations
AND
PHSSE
TKANSFORM.4TIOSS
IN STEEL
studied in a eutectoid carbon steel.
It has been shown
that stress in excess of a threshold
value is capable of
accelerating
transformation
to pearlite
out from their sources and to force arrays against the
and of raising the M, temperature.
grain boundaries
of the specimen occurs coincident
is supplied
and only a small additional to activate
by the transformation, applied stress is required
slip in preferred directions
in the adjacent
An accurate
description
of the cause of the defor-
mation which is observed
to accompany
tion under stress probably
involves both the effects of
preferred
orientation
of the transformation
and the plastic deformation
mechanism
this paper, the relative importance nisms being dependent experimental
transformaproducts
described
of the two mecha-
on the system involved
conditions.
in
and the
In this case it would appear
that the latter mechanism
is predominant.
with transformation
Why applied stresses begin to influence M, only in
for plastic extension
for accelerated
mation.
to martensite,
On transformation
value while the applied
M, temperature
loads have no effect on the
until very high stresses are reached.
It is proposed
that on transformation
and bainite the threshold
required to move dislocations
out from their sources.
This
move to the grain boundaries or other barriers where they form piled-up arrays. It is further
suggested that the stress concentration the increased rates of transformation
it has been demonstrated
critical temperature
frequently
formed by plastic
To account
between M, and a higher
sion observed
known as M,.
to pearlite
stress is the effective stress
For example,
can be isothermally
transforthe thres-
hold stress for plastic extension has a surprisingly low
leading dislocation
at temperatures
the
is found to have the
same value as the threshold
influence cannot result solely from plastic deformation.
deformation
In
the case of pearlite and bainite transformation,
The dislocations
excess of 28,500 lb/in2 is not easily understood.
that martensite
and bainite,
Plastic extension
when the applied load exceeds a given threshold. threshold
grains and thus cause the observed extension.
,513
But if this mecha-
explanation
existing on the
of such an array is responsible
for the low threshold on transformation
for
observed. for plastic extento martensite,
the
offered is that the kinetic energy released
nism were active in the present case, one would expect
when a plate of martensite forms is capable of moving
that as soon as plastic
dislocations
deformation
occurred
below
out from their sources and forcing pile-ups
Md martensite would be produced and there would be
arrays against the grain boundaries,
a sudden jump in the temperature
small additional
of transformation
from the normal M, at 200°C to some higher tempera-
slip in adjacent
ture
deformation.
(M,)
instead
of the actual
gradual1 y rising transformation
experience
of the
temperature
as the
stress is raised above 28,500 lb/ins. It is apparent
grains
M, temperature
is difficult
results have
tensite formation. needed
on
deformation influence to martensite.
martensite
transformation
which
are
readily explained by our present understanding transformation. on martensite
not
of the
Further study of the effect of stress formation
plastic deformation
under varying
conditions
of
and grain size should prove to be
a fruitful field of investigation. From the practical the thresholds
point of view, consideration
for plastic deformation,
low threshold on transformation prove of importance tion of residual treatment,
on the
It is believed
especially
of
should
and
stress patterns
from
heat
just as an awareness of the effect of stress
on transformation kinetics is helpful in explaining anomalous internal structures often observed in heat treated steels.
special
transformation
high
speed
under applied
recording
to explain
on the basis of mar-
size
and
stress-induced
that consideration
prior
is
plastic
transformation of the thresholds
and accelerated transformation
structural
in studying anomalies
residual which
stress
occur
in
heat treated steels. ACKNOWLEDGMENTS
The
authors
Belt Company
are especially of Milwaukee,
ing this investigation
grateful
to the Chain
Wisconsin,
for sponsor-
and to J. J. Scales and the
members
of the metallurgical
Company
for their
helpful
staff
discussion,
at Chain
Belt
interest
and
encouragement. REFERENCES
5. CONCLUSIONS
Using
plastic
the
and interpreta-
resulting
gross
needed to raise the
of the mechanism
grain
be of importance
to martensite, should
in the prediction
way
for plastic deformation patterns
produce
It appears that more information
brought to light several features of the effect of stress the
and
The high threshold
of our present knowledge
that the experimental
so that only a
applied stress is necessary to activate
techniques,
tensile loads has been
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