suppression of the yield point in iron by hydrogen aro discussed. L'INFLUENCE: DE L' ... as follows: Atoms of carbon and/or nitrogen diffuse to dislocat,ions ...
THE
INFLUENCE
OF
HYDROGEN H.
ON
THE
YIELD
POINT
IN
IRON*
C. ROGERS!
Armco iron normally exhibits a sharp yield point when tested in tension at room temperature nnd below. After electrolytically charging the iron with hydrogon, however, a yield point no longer is observed at room tomperaturc. When the tonsilo test temperature is lowered, a yield point returns at about - 12’C and increases in magnitude with decreesing tast temperature. Two related mechanisms for the suppression of the yield point in iron by hydrogen aro discussed. L’INFLUENCE:
DE
L’HYDROGBNE
SUR
LA LIMITE
I?LASTIQUE
DU FF,R
Le for Armco pr&ente normalement une limite Blastique bien definie 8U coura de l’essai de traction B la temp&eture ambiante et & des temp&atures infbrieures. Cependant. une telle limita n’est plus obsen+e B la tcmp&ature ordinaim quand le fer a 6th chargb ~lcctrolytiquement d’hydrog&ne. En abaissant la temp6rature d’essai, la limite Clastique reepparait vow - 120°C ot sa valeur augmente lorsque la tempCrature diminuc. Deux mkcenismes reletifs B la supprcsaion de la limito 6lastiquo du fer par l’hydrog&no sont discutk. DER
ISINFLUSS
VOS
WASSERSTOFF
AUF
DIE
STKECKGRENZE
VON EISEN
Armco-Eisen weiat im Zugversuch bei Raumtemperetur und derunter normalerweise oino scharfo (obere) Streckgrenzo auf. Wurde das Eison jcdoch \rorher clektrolytisch mit Wasserstoff beladen, so ist eino Streckgronze bei Ksumtemperatur nicht mehr zu beobachton. 13ei Emiedrigung der Temperatur des Zugversuchs kchrt die Strockgrenze bei -120°C wiedcr: ihre Griisse nimmt mit abnohmender Versuchstemperstur eu. Zwoi miteinander vorwandte Mechanismen fiir die durch Wasserstoff in Eiaen hervorgerufeno Unterdriickung der Streckgrenze werdcn diskutiert.
Hydrogen
has been found
tensile yield behavior and rimmed steel.
to affect strongly
in SAE-1020
t,he
steel, Armco iron,
The yield point normally observed
in a room-temperature
tensile test can be eliminated
partially
eliminated
completely the
after
eliminated
reduction
of
tests as a function
area
min
at fracture
that
point causes only 60% of the maximum
short-time
cathodic
charge
at
room
temperature.
obtained
Fig. for
of the charging time.
substantially
steel merely by a
charging,
30 min.
from Armco iron and from rimmed steel, and can be reduced in SAE-1020
the 30-min
16
after
charge
which
after a prolonged
and
2 shows
these
same
It is evident
eliminates
the yield
cmbritt.lement
charge.
Cracknell and Petch(l) have just reported the complete elimination of the yield point in fifteen
When similar Armco iron specimens were charged for four hours and tested at. successively lower tem-
annealed
steels
relatively
pure iron to a 0.62%
ranging
successful elimination
in
charging
from
peratures,
steel.
Their
-12°C.
due to the use of more
conditions
present experiments. The variation in yield temperature
content
carbon
of the yield point in the higher
carbon steels is undoubtedly severe
carbon
than
were
used
in the
increased
both
with
time upon
embrit.tlement
begin
test
the
t General Electric
Resrarch
METALI,C’RGICA,
Laboratory,
VOL.
Scheuoctady,
4, MARCH
1956
in magnitude
to appear
a.t about
iron, the yield
with decreasing
point
temperature
to
that the hydrogen appear
until
possibility
yield
about
of a yield
point
-100°C.
was presumed point
due
does not In
these
to eliminate to carbon
or
nitrogen.
was
Preliminary aging experiments between room temperature and 200°C indicate that t,he yield point returns, at least in part, after its elimination by electrolytic charging. Dislocation theory explains the yield point in steel as follows: Atoms of carbon and/or nitrogen diffuse to dislocat,ions during annealing of the steel, and during cooling while the steel is still warm. At. room
of 0.2 amperes per square inch. Fig. 1 shows that the room-temperature yield point in Armco iron is
.+CTA
began
earlier tests, prestraining
determined for fixed charging conditions. Cylindrical specimens, 0.200-in. in diameter, were electrolytically charged in 4()/h sulfuric acid, using a platinum anode and a current density
* Received September 2. 1955.
point
However, other work with SAE-1020 (Fig. 3). steelc2), prestmined about 10% prior to charging, has shown
behavior
and with charging time was studied for charged Armco iron. In addition, the
cathodically influence of charging
a yield
As in the uncharged
N. Y. 114
THE
ROGERS: 1600
INFLUENCE
r
OF
HYDROGEN
ON
THE
YIELD
POINT
IN
IRON
A
140
I200
i/
v)
2 i? z
)
IOOd
600
2 53 600 400
ELECTROLYTIC CHARGING
200
A
0 MIN.
B
16 MIN.
c 30
_!
TIME
MIN.
I
0
.I0
.20
.30
.40
0
ELONGATION
.I0 IN
.20
.30
.I0
1
i
.30
.20
INCHES
Fro. 1. The influence of increasing amounts of hydrogen on the yielding of Ammo iron. Hydrogen introduced by electrolytic charging.
PRONOUNCED
YIELD
POINT
a az
0
50
150
100 CHARGING
TIME
IN
200
MIN.
FIG. 2. The influence of electrolytic hydrogen on the ductility of Armco iron.
250
115
ACTA
116
METALLURGICA,
VOL.
4,
1956
tip
UNCHARGED
CHARGED
2600-
2400
-
2000
-
u) z” 2 IEOOL z 2
1600-
s
I
,..h!-d--0
.I0
.40
.30
.eo
.I0
.50
ELONGATION
IN
I
I .20
J .40
.30
INCHES
FIGI. 3. The influence of testing temperrtture on the ability of hydrogen to suppress the
yield point in Armco iron.
temperature
they
are firmly
bound
by
an elastic
interaction. When a stress is applied at room temperature or below, the dislocations are held up by the
at much lower temperatures, when hydrogen itself becomes sufficiently immobile that it a,lone could pin
the
dislocations,
then
dislocations can move
to break more
freely,
away,
and
the stress
drops, giving rise to a yield point. In terms of dislocation
theory there appear to be at
least two possible explanations hydrogen eliminates the carbon point
at room
hydrogen
temperature.
each
First,
is bound to dislocations
is carbon or nitrogen. dislocation
then
for the fact that or nitrogen yield it may
be that
more tightly
than
After charging with hydrogen, would
be surrounded
by
a
cloud of tightly bound hydrogen atoms that would displace most of the previously bound carbon or nitrogen atoms. Assuming that hydrogen atoms can diffuse rapidly at room temperature, dislocations and their surrounding hydrogen clouds could move readily together under an applied stress, and there would
be no yield point
at this temperature.
Only
carbon
aging
would
appear.
free energy is still lower than on a dislocation.
they
model,
point
cause the pinned since
this
a yield
According hydrogen
to
would
attached solute atoms which do not diffuse rapidly at low temperature. High stresses are required to
to diffuse out of the specimen, and nitrogen
atoms
then
would
the dislocations and pin them, causing point to return as in normal strain aging.
allow
the
where its The
diffuse the
A second explanation could be that hydrogen bound less tightly to dislocations t,han is carbon nitrogen.
Hydrogen
would
not
now
displace
to
yield is or any
significant number of carbon or nitrogen atoms, but would attach itself to loops of dislocations that are freed from carbon or nitrogen by thermal fluctuations, lowering the energy-and thereby the stress-necessary for formation of a critical-size loop. At room temperature, hydrogen could depress the yield stress sufficiently to reduce or eliminate a sharp yield point. Lowering the temperature then would raise the yield stress and cause the discontinuity in the flowcurve to increase or become apparent. The yield
ROGERS:
stress would
increase
as in uncharged of the
THE
with
uncharged
its free
OF
decreasing
HYDROGEN
temperature
iron, always being smaller than that iron
As with the previous the diffusion
INFLUENCE
at any
model,
of hydrogen
energy
given
temperature.
aging would occur with
out of the specimen
is lower
than
on the
where
dislocation.
However,
since carbon and nitrogen are not displaced,
the yield
point
appearance
would
return
bound
nitrogen-is
suggest that the second
to
the more likely.
near -12°C.
been displaced
than
by hydrogen,
develops
that
can be accomplished is that
gen from dislocations concentration
associated
are
more
The internal a wire
Electrolytic
If hydrogen
The
nitrogen,
as
does not
is added to
displaced
nitro-
and returned it to free solution, of
unbound
friction specimen
iron,
strongly
by hydrogen.
unbound
when hydrogen
internal friction
the
pletely
YIELD
dislocations. eliminated
lytic
charging
was
almost
nitrogen
and
and
demonstrate remained performed had
pendulum.
A damping
of nitrogen
a tensile test was
dislocation as expected.
no damping peak over a temperature or nitrogen
peak
motion
to be noted that the damping measurements
with carbon
tested
It is
detected
range of 25°C to
that hydrogen
interacts
to shift the temperature
of
the internal friction-peak. atom also were
negative, but less conclusively so, because no damping peak appeared upon straining after hydrogen charging. The reason for dislocation observable
amount
motion
of carbon
to free solution
be that too little strain could with both dependence specimen
carbon
problem
and nitrogen
of the damping, was
be introduced
Another
the wire fractured.
not
in
the
not returning
an may
before
encountered
was the amplitude
particularly
when the
strain-aged
condition.
ACKNOWLEDGMENTS
pendulum.
in wet hydrogen
to
at -78°C
with
I am indebted
to G. Ardley,
J. C. Fisher,
J. H.
Hollomon, and J. R. Low, jun., for their stimulating and helpful discussions, and to R. W. Powers for equipment
and
advice
for
the
internal
to retain maximum peak was observed.
The wire then was strained to provide a large number of dislocations,
amount
that
lOO”C, so that it is unlikely
no
As a final check to
returned nitrogen to free solution,
providing
then water-quenched
point
was
A smaller damping
showing
reappeared,
friction measurements.
nitrogen in solution.
peak.
there
and the strained wire immediately
hour at 500°C in hydrogen ammonia,
com-
After electro-
until the yield
eliminated,
to dislocations,
remove carbon and nitrogen, was nitrided for half an saturated
almost
peak.
that an observable
bound
117
IRON
treatment
with hydrogen completely
IN
the
were performed
a torsional
heat-treated
This
the damping
increase in the damping
should rise.
experiments
POINT
Tests with carbon as the interstitial
near -12°C
by internal friction measurements,
strained and aged steel.
with
is present
dislocations
increase in concentration
the
near -lOO”C,
yield point is known to appear.t2)
observation
revealed
had
there should be no yield
The fact that a yield point
second
or
a yield
and nitrogen
point until much lower temperatures,
demonstrates
is less
is carbon
The first observation
material
If all carbon
where the hydrogen
of the
in which hydrogen
dislocations
is that hydrogen-charged
held than
the dis-
to
THE
in the torsional
proposed mechanisms-that
point
upon
of the hydrogen.
Two observations strongly
merely
ON
and aged to allow the nitrogen to move
REFERENCES 1. A. CRACKNELL and N. J. PETCH, Acta. Met., 3, 200 (1955). 2. H. C. ROGERS, Acta. Met., 2, 167 (1954).
