U.S. patent application number 11/239035 was filed with the patent office on 2007-03-29 for method for heat treating thick-walled forgings.
Invention is credited to Philip A. Huff.
Application Number | 20070068607 11/239035 |
Document ID | / |
Family ID | 37892420 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070068607 |
Kind Code |
A1 |
Huff; Philip A. |
March 29, 2007 |
Method for heat treating thick-walled forgings
Abstract
A method for heat treating thick-walled forgings including
heating a low alloy steel to an austenitizing temperature, wherein
the low alloy steel comprises carbon of about 0.05-0.2 wt. %,
manganese of about 0.3-0.8 wt. %, and nickel of about 0.25-1.0 wt.
%. The method further includes quenching the low alloy steel in a
quench media, and then tempering the low alloy steel for less than
about thirty minutes per inch of critical section thickness plus
about two hours.
Inventors: |
Huff; Philip A.; (Spring,
TX) |
Correspondence
Address: |
OSHA LIANG L.L.P.
1221 MCKINNEY STREET
SUITE 2800
HOUSTON
TX
77010
US
|
Family ID: |
37892420 |
Appl. No.: |
11/239035 |
Filed: |
September 29, 2005 |
Current U.S.
Class: |
148/663 ;
148/649; 420/112; 420/119 |
Current CPC
Class: |
C21D 1/56 20130101; C21D
8/00 20130101; C21D 1/18 20130101 |
Class at
Publication: |
148/663 ;
148/649; 420/112; 420/119 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Claims
1. A method for heat treating thick-walled forgings, the method
comprising: heating a low alloy steel to an austenitizing
temperature, wherein the low alloy steel comprises carbon of about
0.05-0.2 wt. %, manganese of about 0.3-0.8 wt. %, and nickel of
about 0.25-1.0 wt. %; quenching the low alloy steel in a quench
media; and tempering the low alloy steel for less than about thirty
minutes per inch of critical section thickness plus about two
hours.
2. The method of claim 1, wherein the low alloy steel further
comprises phosphorus of greater than 0 up to about 0.04 wt. %,
sulfur of greater than 0 up to about 0.04 wt. %, silicon of greater
than 0 up to about 0.5 wt. %, chromium of about 2.0-2.5 wt. %, and
molybdenum of about 0.45-1.15 wt. %.
3. The method of claim 1, wherein the low alloy steel comprises
nickel of about 0.5-1.0 wt. %.
4. The method of claim 1, wherein the low alloy steel further
comprises aluminum.
5. The method of claim 1, wherein the low alloy steel further
comprises vanadium.
6. The method of claim 1, wherein the low alloy steel is calcium
treated during a melting process.
7. The method of claim 1, further comprising normalizing the low
alloy steel prior to heating the low alloy steel to an
austenitizing temperature.
8. The method of claim 1, wherein the quench media is brine.
9. The method of claim 1, wherein the finish temperature of the
quench media is less than about 95 degrees Fahrenheit (35 degrees
Celsius).
10. A blowout preventer made using the method of claim 1.
11. A forging comprising: carbon of about 0.05-0.2 wt. %; manganese
of about 0.3-0.8 wt. %; nickel of about 0.25-1.0 wt. %; a
cross-section thickness of at least about 8 inches; an internal
yield strength of greater than about 85 Ksi; and a Brinell hardness
value of at most about 237.
12. The forging of claim 11, further comprising phosphorus of
greater than 0 up to about 0.04 wt. %.
13. The forging of claim 1, further comprising sulfur of greater
than 0 up to about 0.04 wt. %.
14. The forging of claim 11, further comprising silicon of greater
than 0 up to about 0.5 wt. %.
15. The forging of claim 11, further comprising chromium of about
2.0-2.5 wt. %.
16. The forging of claim 11, further comprising molybdenum of about
0.45-1.15 wt. %.
17. The forging of claim 11, further comprising an ultimate
strength of at least about 100 Ksi.
18. The forging of claim 11, further comprising an elongation of at
least about 20%.
19. The forging of claim 11, further comprising a reduction of area
of at least about 70%.
20. The forging of claim 11, further comprising a Brinell hardness
value of at least about 217.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to the field of heat
treating thick-walled forgings. More specifically, the invention
relates to heat treating thick-walled forgings using low alloy
steel of a specific composition and a controlled tempering and
quenching process.
BACKGROUND OF INVENTION
[0002] Well control is an important aspect of oil and gas
exploration. When drilling a well, for example, in oil and gas
exploration applications, safety devices must be put in place to
prevent injury to personnel and damage to equipment resulting from
unexpected events associated with the drilling activities.
[0003] Drilling wells in oil and gas exploration involves
penetrating a variety of subsurface geologic structures, or
"layers." Occasionally, a wellbore will penetrate a layer having a
formation pressure substantially higher than the pressure
maintained in the wellbore. When this occurs, the well is said to
have "taken a kick." The pressure increase associated with the kick
is generally produced by an influx of formation fluids (which may
be a liquid, a gas, or a combination thereof) into the wellbore.
The relatively high pressure kick tends to propagate from a point
of entry in the wellbore uphole (from a high pressure region to a
low pressure region). If the kick is allowed to reach the surface,
drilling fluid, well tools, and other drilling structures may be
blown out of the wellbore. These "blowouts" may result in
catastrophic destruction of the drilling equipment (including, for
example, the drilling rig) and substantial injury or death of rig
personnel.
[0004] Because of the risk of blowouts, blowout preventers ("BOPs")
are typically installed at the surface or on the sea floor in deep
water drilling arrangements to effectively seal a wellbore until
active measures can be taken to control the kick. BOPs may be
activated so that kicks are adequately controlled and "circulated
out" of the system.
[0005] FIG. 1 shows a prior art annular BOP 101. The annular BOP
101 includes a housing 102, with a bore 102 extending therethrough
and disposed about a longitudinal axis 103. A packing unit 105 is
disposed within the annular BOP 101 and is also about the
longitudinal axis 103. The packing unit 105 includes an elastomeric
annular body 107 and a plurality of metallic inserts 109. The
annular BOP 101 is actuated by fluid pumped into opening 113 of a
piston chamber 112. The fluid applies pressure to a piston 117,
which moves the piston 117 upward to compress the packing 105 about
the longitudinal axis 103. In the event a drillpipe is present
along the longitudinal axis 103, the packing unit 105 will seal
about the drillpipe. The annular BOP 101 goes through an analogous
reverse movement when fluid is pumped into opening 115 of the
piston chamber. The fluid then instead translates downward force to
the piston 117, allowing the packing unit to radially expand. A
removable head 119 also enables access to the packing unit 105,
such that the packing unit 105 can be serviced or changed if
necessary.
[0006] Because of the high pressures that BOPs must sustain, it is
important that the walls of the BOP are thick and of uniform
mechanical properties; tensile strength and hardness. Forgings,
such as the forgings used in BOPs, are generally made of low alloy
steel which has been heat treated to increase strength and meet
specific minimum mechanical properties. Heat treatment of the low
alloy steel is typically done by normalizing, austenitizing,
quenching, and tempering the steel. Normalizing involves heating
the steel above a critical temperature for a sufficient period of
time to refine the ferritic grain size of the steel, reduce
residual non-uniform stresses and to produce more uniform
mechanical properties. The forging is then allowed to cool in still
air from the normalizing temperature. In order to achieve maximum
hardness, the metals are liquid quenched after austenitizing.
Austenitizing involves heating the steel above a critical
temperature for a sufficient period of time to transform the grain
structure to austenite preparatory to quenching. During quenching,
the austenitized metal is immersed into a quench bath of a quench
media, such as water, oil or polymer and in very rare cases brine
which may be vigorously agitated to achieve a critical rate of
cooling to achieve transformation to a predominantly bainitic or
martensitic microstructure, to increase the hardness and mechanical
strength of the metal. Finally, the low alloy steed used for this
application is always tempered by reheating the forging to a
temperature below the lower critical temperature, which reduces the
high strength and hardness of the as quenched metal and increases
the ductility and toughness of the metal. Tempering is also known
as "drawing the temper" or more simply "drawing."
[0007] When using large forgings to produce pressure vessels, it is
important that after heat treatment the increased strength of the
steel is as uniform as possible throughout the forging's entire
section thickness. Uniform steel strength can be difficult to
achieve when the steel is many inches thick. When quenching a large
forging, the outer surfaces of the forging in contact with the
quench media can have the necessary high cooling rate to achieve
maximum transformation and the attendant mechanical properties.
However, the cooling rate of the metal mass inside toward the
center of the forging becomes progressively slower as the metal
mass is located further from the surface and the quench media.
Thus, in steel with several inches of section thickness, the metal
mass deepest inside of the forging will be most difficult to
increase the metal's mechanical properties and hardness because the
mass cannot be quenched as rapidly and in many cases fails to meet
the minimum critical cooling rate for phase transformation to
occur.
[0008] When using large forgings to produce pressure vessels, it is
important that after heat treatment the increased strength of the
steel is uniform throughout the forging's entire thickness. Uniform
steel strength can be difficult to achieve when the steel is many
inches thick. When quenching a large forging, the outer surfaces of
the forging in contact with the quench media can have the necessary
high cooling rate to maximize hardness. However, the cooling rate
of the metal mass inside of the forging becomes progressively
slower as the metal mass is located further from the quench media.
Thus, in steel with several inches of thickness, the metal mass
deepest inside of the forging will be most difficult to increase
the metal's hardness because the mass cannot be quenched as
rapidly.
[0009] Depth of hardenability is the ability of a metal to respond
to heat treatment uniformly in relatively large section
thicknesses. Low alloy steel has been known to the industry for
good depth of hardenability. The low alloy steel AISI 4130 has a
yield strength range from 75 to 80 Ksi with a depth of
hardenability generally limited to about two inches, meaning the
given yield strength range can be expected to be maintained in a
region of two inches from the heat treatment process. AISI 4140,
another low alloy steel, has a similar yield strength range from 75
to 80 Ksi and a range generally limited to six inches for depth of
hardenability.
[0010] For large BOPs and pressure vessels, cross-sections of steel
can be more than twenty inches thick. Therefore, steel compositions
with large depths of hardenability in order to achieve high
strength levels are desired.
SUMMARY OF INVENTION
[0011] In one aspect, the present invention relates to a method for
heat treating thick-walled forgings. The method includes heating a
low alloy steel to an austenitizing temperature, wherein the low
alloy steel comprises carbon of about 0.05-0.2 wt. %, manganese of
about 0.3-0.8 wt. %, and nickel of about 0.25-1.0 wt. %. The method
further includes quenching the low alloy steel in a quench media,
and then tempering the low alloy steel for less than about thirty
minutes per inch of critical section thickness plus about two
hours.
[0012] In another aspect, the present invention relates to a
forging. The forging includes carbon of about 0.05-0.2 wt. %,
manganese of about 0.3-0.8 wt. %, and nickel of about 0.25-1.0 wt.
%. The forging further includes a cross-section thickness of at
least about 8 inches, an internal yield strength of greater than
about 85 Ksi, and a Brinell hardness value of at most about
237.
[0013] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 shows a cutaway view of a prior art annular blowout
preventer.
[0015] FIG. 2 shows a flow chart illustrating one method of heat
treating a forging in accordance with an embodiment of the present
invention.
[0016] FIG. 3 shows a graph of the cooling power of water versus
the temperature of water.
[0017] FIG. 4 shows a graph of the hardness results for steel
quenched in water and in brine.
DETAILED DESCRIPTION
[0018] In one aspect, the present invention provides methods for
heat treating thick-walled forgings. More specifically, the methods
disclosed can be used to create BOPs which require high hardness
levels throughout the entire width of the walls.
[0019] As explained above, for thick-walled forgings to sustain
high pressure environments, the low alloy steel of a certain
preferred composition must be heat treated to increase hardness and
strength.
[0020] A method in accordance with one embodiment of the invention
uses a low alloy steel comprising carbon of about 0.05-0.2 wt. %,
manganese of about 0.3-0.8 wt. %, and nickel of about 0.25-1.0 wt.
%. With such a chemical composition, the low alloy steel can have a
depth of hardenability of greater than eight inches when heat
treated in accordance with one embodiment of the present invention.
In another embodiment, the percent of nickel may be limited to
about 0.5-1.0 wt. %. In addition to carbon, manganese, and nickel,
the low alloy steel chemical composition of one embodiment can also
include phosphorus of greater than 0 up to about 0.04 wt. %, sulfur
of greater than 0 up to about 0.04 wt. %, silicon of greater than 0
up to about 0.5 wt. %, chromium of about 2.0-2.5 wt. %, and
molybdenum of about 0.45-1.15 wt. %. In one embodiment, the
molybdenum may be 0.90-1.10 wt. %.
[0021] In addition to a large depth of hardenability, the low alloy
steel should exhibit a very high fracture toughness. Fracture
toughness measures the amount of energy absorbed by the material
during a high strain fracture. Tougher materials absorb more energy
than brittle materials. The low alloy steel of the present
invention can provide the fracture toughness needed for use in
large high pressure vessels, such as BOPs.
[0022] General state of the art steel melting technology makes
available low alloy steels with a preferable phosphorus and sulfur
content much lower than the maximum provided above. The use of
reduced amounts of phosphorus and sulfur helps to obtain a high
fracture toughness of the steel. Also, the low alloy steel may be
calcium treated in the melting process when the alloy steel is
originally melted together from the original elements to provide
sulphide morphology control and improve fracture toughness.
Further, the low alloy steel may include aluminum and/or vanadium
for deoxidation and grain refinement.
[0023] In one embodiment of the invention, heat treatment of the
low alloy steel is performed in accordance with the standard
practice for heat treating metals: normalizing, austenitizing,
quenching, and tempering. The optional normalizing treatment is
typically performed so that the low alloy steel is controlled to
within about .+-.25.degree. F. (.+-.14.degree. C.) of the selected
normalizing temperature. The normalizing temperature is typically
chosen to be 25-50.degree. F. (14-28.degree. C.) above the
austenitizing temperature. The forgings are then re-heated to form
austenite at an austenitizing temperature, such as at least
1725.degree. F. (940.degree. C.), with the selected temperature
controlled to within about .+-.25.degree. F. (.+-.14.degree. C.).
After austenitizing, the forgings are then quenched in a vigorously
agitated immersion quench bath with the initial temperature of a
quench media not exceeding about 75.degree. F. (24.degree. C.).
Holding the initial quench media temperature to less than about
75.degree. F. at the start of the quench provides a more efficient
quench by increasing the cooling rate of the low allow steel. For
forgings with section thickness greater than about 8 inches (about
20 cm), the temperature of the quench media should not be allowed
to exceed about 95.degree. F. (35.degree. C.) at the end of the
quench. For forgings of up to about 20 inches (51 cm) of thickness,
the temperature of the quench media should not be allowed to exceed
about 75.degree. F. (24.degree. C.) at the end of the quench. To
accomplish this, the selected temperature rise of the quench media
would determine the minimum amount of the quench media necessary
for effective and adequate quenching. A smaller selected
temperature rise in the quench media would require larger amounts
of quench media to receive the same amount of heat from the
forgings. Optionally, the quench tank would have to be overflowed
with 75.degree. F. (24.degree. C.) or cooler quench media, or the
quench media would have to be circulated through a cooling system
to maintain the temperature below 75.degree. F. (24.degree.
C.).
[0024] Control of the initial quench media temperature to less than
about 55.degree. F. and above about 32.degree. F. should result in
an even greater depth of hardening than when the forgings are
quenched in a warmer, higher temperature, quench media. FIG. 3,
from Metals Handbook, Ninth Edition, Volume 4, page 35, shows the
cooling power of the quench media versus the initial quench media
temperature, in which water was used as the quench media. As shown
in FIG. 3, the cooling power of water decreases rapidly as the
initial temperature increases, indicating water can quench forgings
more rapidly and allow greater depth of hardening in lower initial
temperature water. However, as the initial quench media temperature
is reduced, the forgings become more susceptible to cracking and
fracturing, also known as "quench cracking." Therefore, efforts
should be taken to not allow too low of an initial quench media
temperature as to avoid quench cracking and fracturing.
[0025] For low alloy steels, brine is a preferable quench media to
water because brine is able to provide higher hardness results in
low alloy steels than water. Brine produces less gas bubbles than
water, and therefore can wet the surface of the low alloy steel.
This allows brine to cool the low alloy steel almost twice as fast
as water, enabling the low alloy steel to have higher hardness
results. FIG. 4, from Metals Handbook, Ninth Edition, Volume 4,
page 37, shows the results of steel quenched in water and in brine.
As shown in FIG. 4, the hardness results for brine are higher than
that of water when quenching at the same temperature of 180.degree.
F. (80.degree. C.). Though, brine allows for a faster quench to
increase the depth of hardening of the low alloy steel, brine is
more caustic and corrosive than water. Therefore, efforts would
also need to be taken to protect the quenched materials and the
quenching equipment from the brine.
[0026] After quenching, the forgings are tempered at a selected
tempering temperature for at least thirty minutes per inch of
section thickness plus one or two hours of additional soak time.
The selected tempering temperature should be maintained within
about .+-.15.degree. F. (.+-.8.degree. C.).
[0027] In the prior art, low alloy steels are tempered for forty
five minutes to one hour per inch of section thickness, plus one to
two hours time at the tempering temperature.
[0028] However, such long tempering hold times can result in over
tempering of the alloy, resulting in an unnecessary loss of
mechanical properties of the low alloy steel. As a result, the low
alloy steel may fail to meet the requirements for tensile strength
and hardness.
[0029] The temperatures for normalizing, austenitizing, and
tempering is dependent upon the alloys and the composition of the
steel. Material specifications for particular compositions may be
referred to in order to determine appropriate normalizing,
austenitizing, and tempering temperatures, and the appropriate
quench media.
[0030] FIG. 2 shows a flow chart illustrating a method of heat
treating a forging in accordance with an embodiment of the present
invention. The low alloy steel forging used in the method is made
from the chemical composition of the present invention and is
typically greater than 8 inches in cross-section thickness. The
method begins with the optional normalizing process 210, in which
the forging is heated to a normalizing temperature within about
.+-.25.degree. F. (.+-.14.degree. C.). After the optional
normalizing process 210, the forging is then re-heated into the
austenite temperature range in the austenitizing process 220 within
about .+-.25.degree. F. (.+-.14.degree. C.) of a selected
austenitizing temperature.
[0031] Then, using a vigorously agitated immersion quench bath, the
forging is immersed in a quench media in the quenching process 230.
The quench media used in the quenching process 230 should have an
initial temperature less than about 75.degree. F. (24.degree. C.).
However, a greater depth of hardening of the forging can be
achieved if the initial temperature of the quench media is
controlled between about 55.degree. F. (13.degree. C.) and
32.degree. F. (0.degree. C.). For a forging less than about twelve
inches of thickness, the quench bath should be large enough as to
not allow the quench media to exceed about 95.degree. F.
(35.degree. C.) by the end of the quench. For a forging less than
about twenty inches of thickness, the quench bath should be large
enough as to not allow the quench media to exceed about 75.degree.
F. (24.degree. C.) by the end of the quench. Also, in the quenching
process 230, brine is a preferable quench media over water for
quenching the large forging. The tempering process 240 of the
forging follows the quenching process 230. The forging is heated to
a selected tempering temperature within about .+-.15.degree. F.
(.+-.8.degree. C.). Specifically, the forging is tempered for
thirty minutes per each inch of thickness, plus an additional one
or two hours of soak time. For example, a forging of ten inches of
thickness comprised of the chemical composition of the present
invention should be tempered for about six to seven hours. A
forging created by method 200 will have a depth of hardenability
greater than eight inches, and will be able to meet the specific
mechanical properties necessary for safety for use as a BOP.
Specifically, the forging will be able to meet the standards of the
American Petroleum Institute ("API") for pressure containing
members, as indicated in the API Specification 16A/ISO 13533
section 6.3.
[0032] The combination of the disclosed chemical composition, heat
treatment temperature control, quench media control, and tempering
time control is able to produce forgings with at least about 85 Ksi
internal yield strength, at least about 100 Ksi ultimate strength,
at least 20% elongation, at least 70% reduction of area, and a
surface hardness range of about 217 to 237 Brinell hardness value.
Yield strength refers to the applied stress that the low alloy
steel can experience before plastic deformation. Ultimate strength
refers to the applied stress the low alloy steel can experience
before failing or breaking. Elongation refers to the change in
length the low alloy steel can experience relative to the original
length of the steel before failing in tension. Reduction in area
refers to the largest change in cross-sectional area the low alloy
steel can experience relative to the original cross-sectional area
of the steel before failing in tension. The Brinell hardness value
of at least about 217 is to ensure that the low alloy steel meets
the minimum mechanical properties with regard to yield strength and
ultimate strength. The Brinell hardness value of at most about 237
is to ensure the low alloy steel meets the provisions of NACE
MR0175/ISO 15156 for low alloy steels intended to be used in sour
service. BOPs are sometimes exposed to sour service and therefore
are required by API 16A to meet necessary requirements of the
provisions of NACE MR0175/ISO 15156. Sour service refers to the use
of metallic alloys in wellbore fluid environments that contain
Hydrogen Sulfide, H.sub.2S, in concentrations great enough to cause
SSCC, Sulfide Stress Corrosion Cracking of susceptible metallic
alloys exposed to those environments.
[0033] The major components of the prior art annular BOP 101 that
can be created from the low alloy steel of the present invention
include the housing 102, the piston 117, and the removable head
119. Those having ordinary skill in the art will appreciate that
the low alloy steel of the present invention is not limited to
pressure vessels. Other embodiments which incorporate the use of
thick-walled forgings may be manufactured from the low alloy steel
of the present invention.
[0034] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *