KR20080063371A - Heat treatment method of thick wall forging - Google Patents

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.

Description

Heat treatment method of back wall forging {METHODS FOR HEAT TREATING THICK-WALLED FORGINGS}

FIELD OF THE INVENTION The present invention relates generally to the field of heat treatment of thick wall forgings, and more particularly to the heat treatment and controlled tempering and quenching processes of thick wall forgings using low alloyed steels of a specific composition.

Good control is important for oil and gas exploration. For example, when drilling boreholes in oil and gas exploration operations, safety devices must be deployed to prevent injury to personnel and equipment damage from accidental accidents associated with the excavation work.

Excavating boreholes in oil and gas exploration involves excavating various surface geologic structures or “layers”. Sometimes a wellbore will pass through a layer having a formation pressure that is substantially higher than the pressure maintained in the wellbore. In this case, the borehole is said to "taken a kick." The pressure increase associated with such kicks is generally achieved by the inflow of forming fluid (which may be liquid, gas, or mixtures thereof). Relatively high pressure kicks tend to propagate from the inlet point of the wellbore uphole (from the high pressure region to the low pressure region). If the kick can reach the ground, digging fluid, borehole tools, and other digging structures can be ejected from the wellbore. These "blowouts" can result in catastrophic failure of the drilling rig (including the drilling rig) and the casualty of the operator.

Because of the risk of such ejection, blowout preventers (BOPs) are generally installed on the surface of the ground or deep sea drilling device to effectively seal the well bore until the active means can control the kick. BOPs work so that kicks are properly controlled and circulated in the system.

1 shows a prior art annular BOP 101. The annular BOP 101 includes a housing 102, which has a bore penetrating therein and disposed about the longitudinal axis 103. The packing unit 105 is disposed inside the annular BOP 101 and likewise centered on the longitudinal axis 103. The packet unit 105 includes an elastic annulus 107 and a plurality of metal inserts 109. The annular BOP 101 is operated by fluid pumped into the opening 113 of the piston chamber 112. The fluid presses on the piston 117 and moves upwards to compress the packing unit 105 around the longitudinal axis 103. If there is a rig along the longitudinal axis 103, the packing unit 105 will seal around the rig. When fluid is pumped into the opening 115 of the piston chamber, the annular BOP 101 will undergo a similar reverse motion. The fluid then translates a force downward relative to the piston 117 to expand the packing unit radially. The removable head 119 provides access to the packing unit 105 to enable inspection or replacement of the packing unit 105 as needed.

Since the BOP must withstand high pressure, it is important that the walls of the BOP are thick and uniform in mechanical properties such as tensile strength and hardness. Forgings, such as forgings used in BOPs, are generally made of low alloy steel that has been heat treated to increase strength and minimize certain mechanical properties. Heat treatment of low alloy steels is generally accomplished by normalizing, austenitizing, quenching, and tempering the steel. Soaking involves heating above the critical temperature for a sufficient time to refine the ferrite grain size of the steel, to reduce the remaining non-uniform stress, and to have more uniform mechanical properties. The forging is then cooled in air from the soak temperature. The metal is liquid quenched after austenitization to obtain maximum hardness. Austenitization involves heating the steel above a critical temperature for a sufficient time to convert the grain structure into austenite in preparation for quenching. During quenching, the austenitized metal is immersed in a quench bath of a quench medium, such as water, oil, or polymers, and in very rare cases, is critical to transform most of the bainite or martensite microstructures. It is immersed in a brine that can be sufficiently mixed to obtain a cooling rate, thereby increasing the hardness and mechanical strength of the metal. Finally, the low alloyed steels used in such treatments are always reheated forgings to temperatures below the low critical temperature, thereby tempering, reducing the high strength and hardness of the quenched metals and increasing the ductility and toughness of the metals. Tempering is also known as "drawing the temper" or, more simply, "drawing."

In the case of using large forgings to produce pressure vessels, it is important that the increased strength of the steel after heat treatment is as uniform as possible over the entire cross-sectional thickness of the forging. If the thickness of the steel is several inches, it is difficult to obtain uniform strength of the steel. When quenching a large forging, the outer surface in contact with the quenching medium of the forging may have a high cooling rate necessary to achieve maximum transformation and thus mechanical properties. However, the cooling rate of the metal mass inside the center side of the forging is gradually slowed down as the metal mass is located far from the surface and the quenching medium. Thus, in steels with cross-sectional thicknesses of several inches, the innermost metal mass of the forging cannot be quenched rapidly and in most cases does not have the minimum critical cooling rate at which phase transformation can occur. It is most difficult to increase the mechanical properties and hardness.

When large forgings are used to produce pressure vessels, it is important that the increased strength of the steel after heat treatment is as uniform as possible over the entire thickness of the forging. If the thickness of the steel is several inches, it is difficult to obtain uniform strength of the steel. When quenching a large forging, the outer surface in contact with the quenching medium of the forging may have the high cooling rate necessary to achieve maximum hardness. However, the cooling rate of the metal mass inside the forging is gradually slowed down as the metal mass is located far from the quenching medium. Therefore, in steels up to several inches in thickness, it is most difficult to increase the hardness of the metal because the innermost metal mass of the forging cannot be quenched rapidly.

The depth of hardenability is the ability to cope with heat treatment uniformly at relatively large cross-sectional thicknesses. Low alloy steels are well known in the field of good depth hardenability. Low-alloy steel, defined as American Steel Association standard AISI 4130, typically has a yield strength of 75 to 80 Ksi at a depth of hardening limited to about 2 inches, which yields a range of 2 inches from the heat treatment process. It is expected to be possible. Other low alloy steels, defined as AISI 4140, have a similar yield strength of 75 to 80 Ksi, and are generally limited to 6 inches of hardening depth.

For large BOP and pressure vessels, the steel cross section may have a thickness in excess of 20 inches. Therefore, a composition of steel with a large depth of hardening ability is required to obtain a high strength level.

In one aspect, the present invention relates to a method of heat treatment of a thick wall forging. The method includes heating the low alloy steel to an austenitization temperature, wherein the low alloy steel has about 0.05 to 0.2 wt.% Carbon, about 0.3 to 0.8 wt.% Manganese, and about 0.25 to 1.0 wt.% Contains nickel. The method also includes the step of quenching the low alloyed steel in the quenching medium, followed by tempering to less than about 30 minutes per critical inch thickness of one inch, followed by another two hours.

In another aspect, the present invention relates to a forging. The forging comprises about 0.05 to 0.2 wt.% Carbon, about 0.3 to 0.8 wt.% Manganese, and about 0.25 to 1.0 wt.% Nickel. The forging also has a cross-sectional thickness of at least about 8 inches, an internal yield strength of greater than about 85 Ksi, and a Brinell hardness value of up to about 237.

Other aspects and advantages of the invention will be apparent from the following description and claims.

1 is a cutaway view of a prior art annular spout protector.

2 is a flowchart illustrating a heat treatment method of a forging according to an embodiment of the present invention.

3 is a diagram showing the temperature of water compared to the cooling power of water.

4 is a plot showing the hardness results of steel quenched in water and brine.

In one aspect, the present invention provides a method for heat treatment of a rear wall forging. More specifically, the method can be used to produce BOPs that require high strength levels over the entire width of the wall.

As mentioned above, for thick wall forgings that must withstand high pressure environments, low alloyed steels with certain desirable compositions must be heat treated to increase hardness and strength.

The method according to one embodiment of the invention utilizes a low alloy steel comprising about 0.05 to 0.2 wt.% Carbon, about 0.3 to 0.8 wt.% Manganese, and about 0.25 to 1.0 wt.% Nickel. Due to this chemical composition, the low alloy steel, when heat treated according to one embodiment of the invention, may have a hardenability depth of greater than 8 inches. In another embodiment, the composition of nickel may be limited to about 0.5 to 1.0 wt.%. Low alloy steels, in addition to carbon, manganese, and nickel, have the chemical composition of one embodiment of up to about 0.04 wt.% Phosphorus, up to about 0.04 wt.% Sulfur, up to about 0.5 wt.% Silicon, about 2.0 to 2.5 wt.% chromium, and about 0.45 to 1.15 wt.% molybdenum. In one embodiment, molybdenum may be 0.90 to 1.10 wt.%.

Low alloy steels must have very high fracture toughness, in addition to having a large depth of hardenability. Fracture toughness measures the amount of energy absorbed by a material when the fracture strain is large. Highly tough materials absorb more energy than brittle materials. The low alloy steels of the present invention can provide the fracture toughness required for use in large high pressure vessels such as BOP.

Conventional general steel melting techniques can make the desired phosphorus and sulfur content of low alloyed steels much lower than the above mentioned maximums. Using less phosphorus and sulfur can increase the fracture toughness of the steel. In addition, low alloy steels can be calcium treated in the melting process when the alloy steels are melted with the original elements to improve sulfide morphology and fracture toughness. In addition, low alloy steels may include aluminum and / or vanadium for deoxidation and particle refinement.

In one embodiment of the present invention, the heat treatment of low alloy steel is performed according to standard methods for metal heat treatment such as soaking, austenitization, quenching, and tempering. Selective soaking treatment is generally achieved in which the low alloyed steel is controlled within a selected soak temperature of ± 25 ° F (± 14 ° C). The soak temperature is generally chosen to be 25-50 ° F. (14-28 ° C.) above the austenitization temperature. The forging is then reheated to form austenite at an austenitization temperature, such as at least 1725 ° F. (940 ° C.), and the selected temperature is controlled within about ± 25 ° F. (± 14 ° C.). The forge is quenched in a sufficiently mixed quench immersion bath after the austenitization, the initial temperature of the quenching medium does not exceed about 75 ° F. (24 ° C.). Maintaining the initial quenching medium temperature at less than about 75 ° F. at the onset of quenching increases the cooling rate of the low alloyed steel, resulting in better quenching efficiency. For forgings with cross-section thicknesses greater than about 8 inches (about 20 cm), the temperature of the quenching media should not exceed about 95 ° F. (35 ° C.) at the end of the quenching. For forgings up to about 20 inches (51 cm) thick, the temperature of the quenching media should not exceed about 75 ° F. (24 ° C.) at the end of the quenching. To this end, the selected temperature rise of the quenching medium will determine the minimum amount of quenching medium required for efficient and proper quenching. Smaller selected temperature rises of the quenching medium will require more quenching medium to obtain the same amount of heat from the forging. Optionally, the quench tank may overflow at 75 ° F. (24 ° C.), or the quench cooling medium or quench medium may be circulated to maintain a temperature below 75 ° F. (24 ° C.).

Controlling the initial temperature of the quenching medium below about 55 ° F. and above about 32 ° F. results in a greater depth of cure than when the forging is quenched in the higher temperature quench medium. Figure 3, taken from the Metals Handbook, 9th Edition, Vol. 4, page 35, shows the cooling power versus the quenching medium temperature of the cooling medium, where water was used as the quenching medium. As shown in Fig. 3, the cooling power of the water decreases rapidly as the initial temperature is increased, indicating that the water can quench the forging more quickly and the depth of cure can be increased if the initial temperature of the water is low. . However, as the initial temperature of the quenching medium is lowered, the forging increases the risk of cracking and rupture, also known as "quenching cracking." Therefore, in order to avoid hardening cracks and ruptures, it is necessary to ensure that the initial temperature of the hardening medium is not too low.

For low alloyed steels, brine is a preferred quenching medium than water, because brine can provide higher hardness in low alloyed steels than water. Brine produces fewer bubbles than water and can therefore wet the surface of low alloy steel. This allows the brine to cool the low alloy steel almost twice as fast as water, allowing the low alloy steel to have a higher hardness. FIG. 4, taken from the Metals Handbook, 9th Edition, Vol. 4, page 37, shows the results of steel quenched in water and brine. As shown in FIG. 4, when quenched at the same temperature of 180 ° F. (80 ° C.), the hardness by brine is higher than the hardness by water. Although brine allows for faster quenching to increase the depth of hardness of low alloy steels, brine is more corrosive than water. Thus, efforts are needed to protect the quenched material and the quenching equipment from brine.

The forging has an additional soak time of 1-2 hours after quenching and tempering to a selected tempering temperature of at least 30 minutes per inch of cross-sectional thickness. The selected tempering temperature should be maintained within ± 15 ° F (± 8 ° C).

In the prior art, low alloy steels are tempered for 45 minutes to 1 hour per inch of cross-sectional thickness and then maintained at a tempering temperature of 1-2 hours. However, this long tempering treatment time can lead to excessive tempering of the alloy, resulting in unnecessary loss of mechanical properties of the low alloy steel. As a result, the low alloy steel may not meet the conditions for tensile strength and hardness.

Soaking, austenitization, and tempering depend on the alloy and composition of the steel. Material specifications of a particular composition may be consulted to determine the appropriate soaking, austenitization, and tempering temperatures, and the appropriate quenching medium.

2 is a flowchart illustrating a heat treatment method of a forging according to an embodiment of the present invention. The low alloy steel forgings used in this method are made from the chemical composition of the present invention and generally have a cross-sectional thickness of more than 8 inches. This method begins with an optional soak process 210, in which the forging is heated to a soak temperature within about ± 25 ° F (± 14 ° C). After the optional soak process 210, the forge is reheated to an austenitization temperature range within about ± 25 ° F. (± 14 ° C.) of the austenitization temperature selected in the austenitic process 220. Then, the forged product is immersed in the quenching medium using a quenching bath which is actively stirred in the quenching process 230. The quench medium used in the quench process 230 should have an initial temperature of less than about 75 ° F. (24 ° C.). However, if the initial temperature of the quenching medium is controlled between about 55 ° F. (13 ° C.) and 32 ° F. (0 ° C.), the depth of hardening of the forging can be greater. In forgings less than about 12 inches thick, the quench bath should be large enough so that the end of quench does not exceed about 95 ° F. (35 ° C.). In forgings less than about 20 inches thick, the quench bath should be large enough so that the end of quench does not exceed about 75 ° F. (24 ° C.). Also in the quenching process 230, brine is the preferred quenching medium over water for quenching large forgings. The forging is subjected to the tempering process 240 after the quenching process 230. The forging is heated to a selected tempering temperature within about ± 15 ° F. (± 8 ° C.). In particular, the forging has an additional soak time of 1 to 2 hours after tempering for 30 minutes per inch of thickness. For example, a 10 inch thick forging with the chemical composition of the present invention should be tempered for about 6 to 7 hours. The forging formed by the method 200 will have a hardenability depth in excess of 8 inches and will be able to meet certain mechanical properties necessary to be safe to be used as a BOP. In particular, the forging will be able to meet the API's criteria for pressure receiving members, as described in API 16A / ISO 13533 Section 6.3.

To form a forging having an internal yield strength of at least about 85 Ksi, an ultimate strength of at least about 100 Ksi, an elongation of at least about 20%, an area reduction of at least about 70%, and a surface hardness of Brinell hardness values of about 217 to 237. A combination of chemical composition, heat treatment temperature control, quenching medium control, and tempering time control is possible. Yield strength refers to the applied stress that the low alloy steel can withstand before plastic deformation. Ultimate strength refers to the applied stress that can withstand low-alloy steels before failure. Elongation refers to the change in length from the original length until the length of the low alloy steel is broken. The area reduction rate means that the cross-sectional area of low-alloy steel is changed most significantly compared to the original cross-sectional area until it is broken by tension. Brinell hardness values above about 217 ensure that the low alloy steels meet the minimum mechanical properties with regard to yield strength and ultimate strength. Brinell hardness values of up to about 237 ensure that the conditions of NACE MR 0175 / ISO 15156 for low alloy steels intended to be used for sour service are met. BOPs are sometimes exposed to sour services and therefore must conform to the specifications of API 16A to meet the requirements of NACE MR 0175 / ISO 15156. Sour service refers to the use of the alloy in a wellbore fluid environment containing hydrogen sulfide (H ₂ S) at a concentration sufficient to cause sulfide stress corrosion cracking (SSCC) in the alloy.

Annular BOPs 101, which may be made of the low alloy steel of the present invention, which is a major component of the prior art, include a housing 102, a piston 117, and a removable head 119. Those skilled in the art should understand that the low alloyed steel of the present invention is not limited to pressure vessels. Another embodiment is combined with the use of rear wall forgings made of the low alloy steel of the present invention.

While the invention has been described above with respect to a limited number of embodiments, those skilled in the art may contemplate other embodiments within the scope of the invention. Accordingly, the scope of the invention should be limited only by the claims.

Claims (20)

In the heat treatment method of the rear wall forging, Heating the low alloy steel to an austenitization temperature, Quenching the low alloy steel in a quenching medium, and Tempering the low alloy steel to less than about 30 minutes per 1 inch critical section thickness followed by another 2 hours of tempering Including, The low alloy steel includes about 0.05 to 0.2 wt.% Carbon, about 0.3 to 0.8 wt.% Manganese, and about 0.25 to 1.0 wt.% Nickel. Heat treatment method. The method of claim 1, The low alloy steel has a maximum of about 0.04 wt.% Of phosphorus, a maximum of about 0.04 wt.% Of sulfur, a maximum of about 0.5 wt.% Of silicon, about 2.0 to 2.5 wt.% Of chromium, and about 0.45 to 1.15 wt.% Heat treatment method, characterized in that it further comprises molybdenum. The method of claim 1, Wherein the low alloy steel comprises from about 0.5 to 1.0 wt.% Nickel. The method of claim 1, The low alloy steel is characterized in that it further comprises aluminum, heat treatment method. The method of claim 1, The low alloy steel further comprises vanadium, heat treatment method. The method of claim 1, The low alloy steel, characterized in that the calcium treatment in the melting process, heat treatment method. The method of claim 1, Further comprising a step of soaking the low alloy steel prior to heating to the austenitization temperature. The method of claim 1, And the quenching medium is brine. The method of claim 1, Wherein the final temperature of the quenching medium is less than about 95 ° F. (35 ° C.). Blowout preventor made using the method of claim 1. About 0.05 to 0.2 wt.% Carbon, About 0.3 to 0.8 wt.% Manganese, About 0.25 to 1.0 wt.% Nickel, About 8 inches or more in section thickness, Internal yield strength greater than about 85 Ksi, and Brinell hardness value up to about 237 Forging comprising a. The method of claim 11, Further comprising up to about 0.04 wt.% Phosphorus. The method of claim 11, Further comprising up to about 0.04 wt.% Sulfur. The method of claim 11, Further comprising up to about 0.5 wt.% Silicon. The method of claim 11, Forgings further comprising about 2.0 to 2.5 wt.% Chromium. The method of claim 11, Further comprising about 0.45 to 1.15 wt.% Molybdenum. The method of claim 11, The forging further characterized by an ultimate strength (ultimate strength) of about 100 Ksi or more. The method of claim 11, A forging further characterized by an elongation of at least about 20%. The method of claim 11, The forging further comprises a section shrinkage of at least about 70%. The method of claim 11, And a Brinell hardness value of at least about 217.

Images