U.S. patent number 3,655,465 [Application Number 04/805,827] was granted by the patent office on 1972-04-11 for heat treatment for alloys particularly steels to be used in sour well service.
This patent grant is currently assigned to The International Nickel Company, Inc.. Invention is credited to Frank W. Schaller, Edwin Snape.
United States Patent |
3,655,465 |
Snape , et al. |
April 11, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
HEAT TREATMENT FOR ALLOYS PARTICULARLY STEELS TO BE USED IN SOUR
WELL SERVICE
Abstract
Steel characterized by high yield strength, e.g., over 90,000
p.s.i., is rendered greatly less susceptible to Sulfide Corrosion
Cracking in sour oil wells or comparable environments through the
use of a sequence of heat treating operations in which the steel is
heated within its A.sub.c1 and A.sub.c3 region and thereafter
heated below its A.sub.c1 temperature.
Inventors: |
Snape; Edwin (Monsey, NY),
Schaller; Frank W. (Ringwood Passaic, NJ) |
Assignee: |
The International Nickel Company,
Inc. (New York, NY)
|
Family
ID: |
25192612 |
Appl.
No.: |
04/805,827 |
Filed: |
March 10, 1969 |
Current U.S.
Class: |
148/621; 148/335;
148/622; 148/334; 148/336; 148/663 |
Current CPC
Class: |
C21D
1/18 (20130101); C21D 1/185 (20130101) |
Current International
Class: |
C21D
1/18 (20060101); C21d 001/18 () |
Field of
Search: |
;148/142,143,134,144,36,31,135,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dean; Richard O.
Claims
We claim:
1. A process for improving the resistance of steel to sulfide
corrosion cracking which comprises "bringing into contact with a
sulfide corrosion cracking environment a steel which has been, (a)
heated" above its A.sub.c1 temperature but below its A.sub.c3
temperature to effect a phase change in which part of the metal
structure transforms to austenite, the temperature being controlled
such that upon cooling not more than about 50 percent of an
austenite decomposition product is formed, (b) cooled such that a
metal matrix is formed containing the decomposition product of
austenite, (c) heated to a temperature below its A.sub.c1
temperature, and (d) thereafter cooled to form a metal matrix
containing relatively uniformly distributed carbide particles and a
tempered decomposition product of austenite.
2. A process in accordance with claim 1 in which the steel upon
cooling from the second stage heating has a yield strength of at
least about 90,000 psi.
3. A process in accordance with claim 2 in which the yield strength
of the steel is less than 120,000 psi.
4. A process in accordance with claim 1 in which the amount of
austenite decomposition product formed is at least about 5 percent
and the second stage heat treatment is conducted more than
100.degree. F. below the A.sub.c1 temperature.
5. A process in accordance with claim 1 in which the amount of
austenite decomposition formed is not more than about 40 percent
and the second stage heat treatment is conducted more than
100.degree. F. below the A.sub.c1 temperature.
6. A process in accordance with claim 4 in which the amount of
decomposition formed is not more than about 30 percent and the
second stage heat treatment is conducted more than 200.degree. F.
below the A.sub.c1 temperature.
7. A process in accordance with claim 1 in which nickel is present
in an amount up to about 10 percent.
8. A process in accordance with claim 1 in which the steel contains
at least one temper-resistant constituent in the following ranges:
up to 3% molybdenum, up to 4% chromium, up to 3% silicon, up to 3%
vanadium and up to 3% tungsten.
9. A process in accordance with claim 7 in which the nickel is from
1 to 7.5 percent.
10. A process in accordance with claim 8 in which the steel
contains at least one temper-resistant element in the following
ranges: 0.05 to 2% molybdenum, 0.5 to 3% chromium, 0.2 to 1%
silicon, 0.1 to 1% vanadium and 0.05 to 2% tungsten.
11. A process in accordance with claim 7 in which the steel
contains carbon in an amount of at least 0.2 percent.
12. A process in accordance with claim 8 in which the steel
contains carbon in an amount of at least 0.2 percent.
13. A process in accordance with claim 1 in which the steel
contains about 0.3 to 0.5% carbon, about 0.4 to 1% manganese, about
1.25 to 2.5% nickel, about 0.4 to 1.25% chromium and about 0.1 to
0.75% molybdenum.
14. A process in accordance with claim 1 in which the steel
contains about 0.05 to 0.2 percent carbon, about 1.75 to 2.75%
chromium and about 0.5 to 1.2% molybdenum.
15. A process in accordance with claim 1 in which the decomposition
product of austenite is substantially martensite.
16. A process in accordance with claim 5 in which the austenite
decomposition product is substantially martensite.
17. A process in accordance with claim 7 in which the austenite
decomposition product is substantially martensite.
18. A process in accordance with claim 8 in which the austenite
decomposition product is substantially martensite.
19. A process in accordance with claim 15 in which the
microstructure obtained upon cooling from below the A.sub.c1
temperature consists of a ferritic matrix containing relatively
uniformly distributed carbide particles and tempered
martensite.
20. A process in accordance with claim 16 in which the
microstructure obtained upon cooling from below the A.sub.c1
temperature consists of a ferritic matrix containing relatively
uniformly distributed carbide particles and tempered
martensite.
21. A process in accordance with claim 17 in which the
microstructure obtained upon cooling from below the A.sub.c1
temperature consists of a ferritic matrix containing relatively
uniformly distributed carbide particles and tempered
martensite.
22. A process in accordance with claim 18 in which the
microstructure obtained upon cooling from below the A.sub.c1
temperature consists of a ferritic matrix containing relatively
uniformly distributed carbide particles and tempered
martensite.
23. A process in accordance with claim 7 in which the second stage
heat treatment is conducted more than 100.degree. F. below the
A.sub.c1 temperature when the steel contains at least 5 percent
nickel.
24. A process in accordance with claim 9 in which the second stage
heat treatment is conducted more than 100.degree. F. below the
A.sub.c1 temperature when the steel contains at least 5 percent
nickel.
25. A process in accordance with claim 17 in which the second stage
heat treatment is conducted more than 100.degree. F. below the
A.sub.c1 temperature when the steel contains at least 5 percent
nickel.
26. A process in accordance with claim 11 in which the steel
contains about 1 to about 10 percent nickel.
Description
As those skilled in the art are well aware, the petroleum industry
has, for close to 20 years, been confronted with a perplexing
problem commonly referred to as "Sulfide Corrosion Cracking", a
problem of no little magnitude. In point of historical origin, the
dilemma seemingly first manifested itself (doubtless there were
others) circa 1950 in connection with the unexpected failures of
oil well tubing in sour oil wells located in Canada, the failures
being unexpected to the extent that the steel from which the tubing
was fabricated had given excellent service prior thereto, though,
it appears, in sweet condensate wells. In any event, since that
time investigations into the "causes" of such failures have been
many, and extensive as well as intensive. And there have been a
number of solutions advanced, many of which have been deemed
impractical by reason of economic considerations while others have
imposed such severe commercial limitations as to be unsuitable.
All the research notwithstanding, there ostensibly is not yet an
unanimously accepted theory as to the mechanism responsible for
Sulfide Corrosion Cracking. But there is a considerable body of
authoritative opinion which reflects that both hydrogen
embrittlement and stress corrosion cracking phenomena are involved.
And there does seem to be virtual accord that steels of high yield
strength, particularly above 90,000 pounds per square inch (psi),
are exceptionally prone to Sulfide Corrosion Cracking. Too, it
might be added that cracking of the type under consideration
appears to be relatively spontaneous in occurrence, thus rendering
detecting devices of doubtful value. This, of course, only
accentuates the gravity of the difficulty, which has been expressed
in terms that if either the casing or tubing in a high pressure
sour well should fail, apart from other damage, the well itself
might be lost.
As to the nature of the problem itself and assuming both hydrogen
and stress to be involved, it has been considered that hydrogen in
nascent form first penetrates the steel surface, steel exhibiting a
rather striking affinity for hydrogen. The initial point of
hydrogen entry seems concentrated at such sites as voids,
discontinuities, inclusions, or other points of imperfection. In
sour oil and/or gas wells and apart from other sources, the
hydrogen can be introduced by corrosive attack of the steel
(tubing, casing, etc.). Regardless of the source, the supply of
atomic hydrogen is unfortunately ample.
It has been postulated that upon penetration of the steel surface,
the nascent or atomic hydrogen tends to accumulate and form
molecular hydrogen. This in turn is thought to bring about a volume
expansion of hydrogen in the void, etc. Thus, a stress pattern is
set up (the hydrogen aspect of the problem) which together with
internally and externally induced pressures (the stress cracking
part) causes the formation or extension of a crack which with time
continues to propagate under pressure until failure.
As a practical matter, it is virtually impossible to prevent the
occurrence of either internal or external stresses. For example, it
is conventional to cold work tubing and the like simply to
straighten the same and cold work is a classical method of inducing
internal stress. Similarly, certain heat treatments are conducive
to internal stressing, quenching from austenitizing being a prime
example. As to external pressures, the weight of equipment and gas
pressures are exemplary.
Accordingly, it has been proposed to use various inhibitors,
coatings (prevent hydrogen penetration), liners (of special alloys
to allow for permeation of the liner by atomic hydrogen such that
the hydrogen transforms into the molecular state which is passive
to steel), and different metals such as stainless steels and
nickel-base alloys (proposals considered too expensive). None of
these solutions, which are but illustrative, seem to have attained
a point of acceptance at least with regard to steels having yield
strengths above 90,000 psi.
Therefore, there has been a distinct commercial need for low cost
steel characterized by high yield strength e.g., 90,000 psi or
more, which greatly resists Sulfide Corrosion Cracking. The
emphasis is on yield strength as well as resistance to cracking
because such strength levels are most desirable for the deeper sour
wells since the external pressures are greater. Of course, as
mentioned herein, it has been the higher strength steels which have
proven to be the most susceptible to attack. For this reason the
National Association of Corrosion Engineers (NACE) recommended that
steels to be used in sour wells be tempered such that the yield
strength thereof not exceed 90,000 psi, and the American Petroleum
Institute (API) introduced a specification to this effect in 1963.
Insofar as we are aware, this remains the practice currently
prevailing, a point seemingly confirmed by a recent article which
indicated that at yield strengths above 100,000 psi no known alloy
except a certain copper-nickel alloy completely resists the onset
and failure by way of Sulfide Corrosion Cracking. All steels
failed. It is to this problem which the present invention is
primarily addressed.
It has now been discovered that the capability of steels
characterized by high yield strengths, 90,000 psi and above, e.g.,
100,000 psi, to resist Sulfide Corrosion Cracking are markedly
enhanced provided the steels are subjected to a special sequence of
heat treating operations as described herein.
It is an object of the invention to provide steels of yield
strengths on the order of 90,000 psi and higher which, despite
their strength, display a greatly improved ability to resist the
degradation effects of Sulfide Corrosion Cracking.
Other objects and advantages of the invention will become more
apparent from the following description.
Generally speaking, the present invention contemplates subjecting a
steel to a two-stage heat treatment in which the steel is first
brought to a temperature within its A.sub.c1 and A.sub.c3 region
(often referred to herein as the intercritical temperature),
cooled, and thereafter again heated but to a temperature below its
A.sub.c1, the steel again being cooled. (It is to be understood, of
course, the first stage heat treatment can be preceded by other
treatments including such conventional treatments as normalizing or
austenitizing and quenching.)
Upon heating above the A.sub.c1 temperature, a phase change takes
place in which a portion of the metal structure is converted into
austenite which upon cooling transforms whereby a metal matrix is
formed containing a decomposition production of austenite. This
decomposition product, for example, martensite, should not
constitute more than about 50 percent, by volume, of the metal
matrix. For, in carrying the invention into practice, should the
first stage temperature be too high, the microstructure upon
cooling therefrom is one obtained in which the matrix is
predominantly influenced by the austenite decomposition product,
e.g., martensite, and it has been found, as will be illustrated
herein, that in such instances the steel can be rendered
susceptible to Sulfide Corrosion Cracking. Accordingly, it is
advantageous that the intercritical temperature be controlled such
that not more than about 30 percent or 40 percent of the austenite
decomposition product is formed upon cooling therefrom. (Of course,
the exact intercritical temperature will vary from steel to steel
since, as is well known to those skilled in the art, A.sub.c1
temperature (also A.sub.c3) depends upon composition. However, it
is merely a routine matter to determine the point at which, for
example, more than about 50 percent martensite forms for any given
composition.) On the other hand, the intercritical temperature
should be sufficiently high, i.e., above the A.sub.c1 temperature,
so as to provide a microstructure containing at least about 5
percent, and beneficially at least 10 percent, of the decomposition
product upon cooling.
With regard to the duration a steel should be held within its
A.sub.c1 - A.sub.c3 temperature range, long holding periods should
be avoided since they only add to cost. A suitable period would be
up to 4 hours, e.g., 15 minutes to 2 hours. Cooling from the
A.sub.c1 - A.sub.c3 temperature should be carried down past the
temperature necessary to transform the austenite, for example,
below the M.sub.s and preferably below M.sub.f temperature in the
case of martensite. Other operations can be carried out to effect
maximum transformation, e.g., cold treating as by refrigeration
down to below, say -100.degree. F.
Concerning the second stage treatment, the temperature used, of
course, should not exceed the A.sub.c1, lest the first stage be
simply repeated. Preferably, the temperature should be at least
25.degree. F. or 50.degree. F. below A.sub.c1, a range of
50.degree. F. to 300.degree. F. below A.sub.c1 being suitable.
However, for nickel steels, particularly high nickel steels, e.g.,
5 percent or more, a temperature of at least 100.degree. F. below,
and preferably at least 200.degree. F. below, A.sub.c1 should be
used. Cooling can be conducted by air, oil quenching, water
quenching, etc. (This also applies to cooling from the
intercritical temperature.)
While the exact mechanism which might explain the theory involved
is not yet completely at hand, the effect of the above-described
two-stage heat treatment might be considered unusual. To simply
double temper a steel below its A.sub.c1 temperature does not
result in any significant improvement in respect of Sulfide
Corrosion Cracking (failure obtains). Moreover, conventional double
tempering at best usually results in a loss of strength accompanied
by a slight increase in toughness. This focuses attention upon heat
treating between the A.sub.c1 and A.sub.c3 temperatures. From what
has heretofore appeared in the literature, it would seem that such
a heat treatment should not be employed. To explain -- it has been
previously said that the formation of martensite is an excellent
way to actually promote Sulfide Corrosion Cracking. However, the
intercritical treatment contemplated herein encompasses the
intentional formation of martensite. But when the matrix so formed
is tempered below the A.sub.c1 temperature, what might otherwise
have been a crack-prone steel becomes a steel greatly resistant to
sulfide corrosion cracking.
Actually, it has been found that in respect of certain steels the
dual step heat treatment in accordance herewith contributes to
higher strength levels notwithstanding the fact that the second
heating is a tempering treatment in which a loss in strength and an
increase in ductility would be expected. The increase in ductility
is readily understandable since hard austenite decomposition
products, such as martensite, formed upon cooling from the
intercritical temperature are softened by tempering. More difficult
to explain, however, is the simultaneous increase in strength. In
any case, it is considered that the mechanism involves
stress-strain behavior. It has been noted that in single tempering
below the A.sub.c1 temperature of such steels, a sharp yield point
is observed which disappears as the temperature is raised to just
above A.sub.c1, yield strength also decreasing. A further increase
in temperature above A.sub.c1, but well below A.sub.c3, results in
a substantial strength increase; however, the yield point does not
reappear. This behavior indicates a straining of the matrix by
transformation of the austenite region. Thereupon, tempering below
A.sub.c1 enables strain aging to occur in the plastically deformed
regions of the matrix and restores the yield point whereby strength
is increased. (This overall behavior is often referred to herein as
"intercritical strengthening".)
Intercritical strengthening occurs particularly in respect of
nickel-containing steels, and more particularly as to those steels
which also contain at least one temper resistant element such as
molybdenum, chromium, silicon, vanadium, tungsten, etc. The nickel
content can be as high as 10 percent although a range of from 1 to
5 or 7.5 percent is satisfactory. Up to 3% molybdenum, up to 4%
chromium, up to 3% silicon, up to 3% vanadium, up to 1% carbon (and
preferably at least 0.2% carbon) as well as other desired
constituents can be present in the steels. Such other constituents
include age hardening constituents such as copper, aluminum and
titanium in the following percentages: up to 3% copper, up to 2%
aluminum and up to 2% titanium. Further, columbium and boron may be
present in amounts up to 2% and up to 0.25%, respectively. A steel
containing from 1 to 10% nickel and at least one or more temper
resistant constituents in the following ranges is deemed suitable:
0.05 to 2% molybdenum, 0.5 to 3% chromium, 0.2 to 1% silicon, 0.1
to 1% vanadium, 0.1 to 0.5% carbon, 0.05 to 2% tungsten, the
balance being essentially iron. A particularly satisfactory steel
contains from 0.3 to 0.5% carbon, from 0.4 to 1% manganese, from
1.25 to 2.5% nickel, 0.4 to 1.25% chromium, 0.1 to 0.75%
molybdenum. Another illustrative steel contains about 0.05 to 0.2%
carbon, from 1.75 to 2.75% chromium, from 0.5 to 1.5% molybdenum.
As contemplated herein, the nickel content can be replaced in whole
or in part by an equivalent amount of manganese. Moreover, it is
considered that the subject invention could be used in connection
with certain stainless steels containing from about 11 to 14
percent chromium.
In order to give those skilled in the art a better understanding of
the invention the following illustrative data are given.
A series of commercially produced steels, C-75, AISI 4140 and AISI
4340, the composition of which are given in Table I, were heat
treated in accordance with the invention and, for purposes of
comparison, by other heat treatments, the heat treatments being set
forth in Table II. Alloy C75 was an open hearth heat from which
27/8 inch O.D. tubing had been formed. Specimens were
longitudinally cut therefrom for test. The AISI steels 4140 and
4340, which had been produced using an electric furnace, were hot
rolled from 1 1/8 inch round bar stock and 4.times. 4 inch squares,
respectively, to 3/8 inch plate from which specimen blanks were
cut.
After testing for mechanical properties, (results reported in Table
II) specimens were finished machined to a dimension of
approximately 3.times. 1/4 .times. 1/8 inch. Notch beam specimens
were then made, the notch being cut transversely to the direction
of hot rolling with an included angle of 45.degree. and a notch
radius of 0.010 inch. Two specimens for each condition of heat
treatment were deflected to the yield deflection in three-point
loaded fixtures, a common testing procedure. The deflection
necessary to approximately reach the onset of plastic deformation
was determined using instrumented bend tests.
The loaded specimens were then immersed in a 5-liter flask
containing an aqueous solution of 5% NaCl and 0.5% acetic acid,
nitrogen being passed through the solution for about 30 minutes to
purge the system of oxygen. The solution was then saturated with
H.sub.2 S which was continuously bubbled therethrough. Prior to
opening the flask for examination of the specimens, the nitrogen
purge was again repeated. Inspections were carried out after the
second and seventh days and every seven days thereafter (as to
uncracked specimens) until a predesignated period of thirty days
(total) elapsed, after which the test was discontinued. To avoid
accumulation of corrosion products (which might have otherwise
interfered with accuracy of the results) and to also keep the pH
constant at about 3.8, the solution was changed after each
inspection. In various instances threshold stress values were
determined, i.e. threshold values at or below yield deflection.
With respect to specimens which exhibited early failure, the
threshold evaluation was one of reducing the percent of yield
deflection to successively lower levels until a stress level was
reached (.+-. 5000 psi) at which no failure occurred within an
exposure period of 30 days. ##SPC1## ##SPC2##
In perusing the data given in Tables I and II it will be observed
that in each instance in which a steel specimen was treated in
accordance with the invention complete resistance to Sulfide
Corrosion Cracking obtained over the full 30 day period of test.
This is in marked contrast to the failure characteristic of all
specimens treated in a manner beyond the scope of the invention
(but not inconsistent with conventional practice). It should also
be particularly noted that in many instances the yield strength
(yield point or yield stress) exceeded 100,000 psi and yet Sulfide
Corrosion Cracking did not occur. This is thought to be quite
significant inasmuch as the testing procedure used is considered to
be one of considerable severity. Note should be taken of the AISI
4340 specimens heated to the intercritical temperature of
1,375.degree. F. While this temperature is below the A.sub.c3 for
the steel, nonetheless, it represents a situation referred to above
herein in which the amount of martensite formed upon cooling from
the 1,375.degree. F. temperature was excessive (above 50 percent)
such that upon subsequent tempering Sulfide Corrosion Cracking
resulted. A most desirable microstructure as contemplated herein
consists of a ferritic matrix containing relatively uniformly
distributed carbide particles and tempered martensite.
With particular regard to AISI 4340, a steel which contained nickel
together with such temper resistant constituents as molybdenum and
chromium, the yield strength actually increased upon the second
stage (tempering) treatment. This is reflected by the data
concerning the intercritical temperature of 1,350.degree. F., the
yield stress being raised by a value of about 7,000 psi. Normally,
as indicated previously a loss in strength would be expected as a
result of the tempering (softening) treatment below A.sub.c1.
Together with the increase in strength, ductility was also
considerably improved as can be seen from a comparison of the
tensile elongation (Elong., percent) and reduction in area (R.A.,
percent) figures.
This result whereby strength is improved by a tempering treatment,
or is not impaired but toughness is considerably enhanced, is also
reflected by the data given in connection with the steels set forth
in Tables III and IV. ##SPC3## ##SPC4##
Apart from the strength and/or toughness being improved, in
connection with the steels set forth in Tables III and IV it should
be particularly mentioned that with regard to the series of the 9
percent nickel steel the results were significantly better when the
second stage heat treatment was conducted more than 100.degree. F.
below the A.sub.c1 temperature. (The A.sub.c1 temperatures for
Alloys 9 Ni-A, 9 Ni-B and 9 Ni-C are on the order of 1050.degree.
F., 1025.degree. F. and 1000.degree. F., respectively.)
While the invention as described above has been primarily directed
to resisting Sulfide Corrosion Cracking in steels of yield
strengths of 90,000 psi or above, it is to be understood, of
course, that steels of lower yield strength can be treated in
accordance herewith. However, to minimize the possibility of
Sulfide Corrosion Cracking, it is preferred to use a steel in which
the yield strength is less than 120,000 psi. Castings may be given
the double stage heat treatment as well as wrought products. In
this regard, it is contemplated that cast irons may also be so
treated in which case the carbon content can be as high as 4 or 5
percent together with the conventional elements commonly found in
cast irons, e.g., nickel, manganese, chromium, molybdenum,
vanadium, etc.
Although the present invention has been described in conjunction
with preferred embodiments, it is to be understood that
modifications and variations may be resorted to without departing
from the spirit and scope of the invention, as those skilled in the
art will readily understand. Such modifications and variations are
considered to be within the purview and scope of the invention and
appended claims.
* * * * *