A welding method for a 1900 MPa grade low alloy ultra-high strength steel high-pressure vessel shell

Abstract

A welding method of 1900MPa grade low alloy ultra-high strength steel high pressure vessel shell, when welding 300M steel welding parts with diameter of 100-1000mm and wall thickness of 1.0-5.0mm by tungsten argon arc welding, different welding modes of clockwise two forward and one backward, 30° helical tooth, counterclockwise two forward and one backward are adopted for multi-pass combination welding; through the heat treatment scheme of "vacuum air quenching + cold treatment + twice tempering", the performance of high pressure vessel weld and linear precision meet the requirements, and the preparation of 1900MPa grade low alloy ultra-high strength steel shell is realized. Before the welding process, the welding position is preheated in a temperature range different from that of traditional steel, which slows down the cooling speed after welding, realizes the escape of hydrogen in the weld metal, greatly reduces the hardening degree of the weld, effectively avoids the generation of cracks during welding, ensures that the tensile strength of the 300M steel shell reaches more than 1900MPa, the shell deformation is small, the linear degree is not more than 2mm per meter, the heat treatment size precision of the high pressure vessel shell is ensured, and the surface finish is good.

Description

Technical Field

[0001] This invention relates to a welding method for a 1900MPa grade low alloy ultra-high strength steel shell used in the field of machining. Background Technology

[0002] The steel high-pressure vessel shells used in the machinery industry are generally made of high-strength steel materials such as 30CrMnSiA and D406A. The general manufacturing process is as follows: typical pre-welding components of high-pressure vessels, such as the front head, cylinder, and rear head, are assembled. The front and rear heads are machined, while the cylinder is formed by high-pressure spinning. These pre-welding components are then assembled and welded together using argon arc welding. The entire assembly is then subjected to air heat treatment furnace tempering, and finally machined to form the high-pressure vessel shell. The tensile strength of the welds in this type of steel high-pressure vessel shell is generally lower than 1600MPa, and the product has poor dimensional accuracy, with straightness generally exceeding 2mm per meter, which cannot meet the performance and accuracy requirements of a 1900MPa high-pressure vessel shell. The main reasons are: 1. The materials used cannot achieve the required tensile strength after heat treatment; 2. Traditional welding process parameters are difficult to match, which can easily cause defects such as cracks and porosity, resulting in subsequent performance not meeting the requirements; 3. The heat treatment process uses an ordinary air furnace. During the heat treatment process, the inner and outer surfaces of the high-pressure vessel shell react with oxygen and other gases in the air, producing a certain proportion of decarburized layer, which leads to a decrease in the performance of the high-pressure vessel shell weld. At the same time, the surface cleanliness of the shell is poor after heat treatment, and further treatment such as sandblasting is required to meet the surface cleanliness requirements. The shell is severely deformed and requires strong straightening treatment to meet the requirements.

[0003] The invention patent ZL202210420958.0 discloses an efficient production method for 1900MPa grade suspension spring steel. This method relates to the field of iron and steel metallurgy technology. By optimizing the spring steel production process, it eliminates the shrinkage cavity in the center of the billet, improves the center segregation of the billet, and eliminates the high-temperature long-time heating process of the large billet in continuous casting and the billet peeling and finishing process after billet opening. After the continuous casting billet passes the flaw detection, it is rolled into the required high-quality hot-rolled coil wire rod of 1900MPa grade suspension spring by roughing, intermediate rolling, pre-finishing rolling and finishing rolling mills. This meets the processing and use requirements of automotive suspension springs, shortens the processing process and production time. This method only involves smelting, casting and billet making and rolling into spring steel wire. Spring steel wire is used to make elastic elements. Due to its material properties, it cannot be welded or spun into shape, and is unrelated to the welding method of high-pressure vessel shells described in this article.

[0004] Invention patent ZL 202010670350.4 discloses a 1900MPa grade high-strength and toughness hot stamping aluminum-silicon coated steel sheet and its welding method. The 1900MPa grade high-strength and toughness hot stamping aluminum-silicon coated steel sheet is a composite steel sheet consisting of a steel substrate and an aluminum-silicon coating. The tensile strength is improved through the hot stamping process. It is a welding method for high-performance composite steel sheets. However, because its surface has an aluminum-silicon coating mixed with steel, it is a multi-component mixture that cannot be welded and is irrelevant to the production of container products.

[0005] Invention patent ZL 201810454283.5 discloses a flux-cored welding wire for welding ultra-high strength alloy steel with a strength of 1900MPa, relating to the field of welding materials in materials processing. To ensure the alloy possesses high strength and toughness, the carbon content in the flux-cored welding wire is strictly controlled, and elements such as titanium, vanadium, molybdenum, and cobalt, with scientifically optimized proportions, are added to improve the strength and high-temperature strength of the welding flux core. This is a novel flux-cored welding wire, unrelated to the welding method described in this article.

[0006] In November 2011, Pei Jian published "The Influence of Ni Content and Tempering Process on the Microstructure and Properties of 300M Steel" (Issue 11, 2011) at Yanshan University. In order to improve the toughness of 300M steel, the influence of Ni content on the continuous cooling transformation, microstructure evolution and mechanical property changes of 300M steel was studied by adjusting the content of the toughening element Ni. The study also investigated the influence of high Ni content on the microstructure and properties of 300M steel, which was not related to the welding method of 1900MPa grade low alloy ultra-high strength steel high pressure vessel shell.

[0007] In 2008, Liu Zhenbao et al. published "Development of 1900MPa Grade Ultra-High Strength Stainless Steel" in Volume 32, Issue 3 of *Mechanical Engineering Materials*. They proposed a 1900MPa chromium-nickel-calcium-platinum series large-size bar (φ200mm) ultra-high strength martensitic aging stainless steel. By studying the influence of heat treatment processes on the mechanical properties of eight different compositions of chromium-nickel-calcium-platinum series martensitic aging stainless steel, they further optimized the alloy composition, achieving the development of 1900MPa stainless steel bars. This paper focuses on high-performance stainless steel bars. Martensitic aging stainless steel and low-alloy ultra-high strength steel are not the same type of steel. Martensitic stainless steel has a high hardening tendency and low thermal conductivity, resulting in poor weldability and a high susceptibility to cold cracking. Furthermore, the weld area is prone to embrittlement, and it is generally not used as a material for high-pressure vessels. Therefore, it is not relevant to this invention. Summary of the Invention

[0008] To overcome the limitation of existing technologies where high-performance steel cannot be used to manufacture high-pressure vessel shells, this invention proposes a welding method for high-pressure vessel shells made of 1900MPa grade low-alloy ultra-high-strength steel.

[0009] The welding process for the 1900MPa grade low-alloy ultra-high strength steel high-pressure vessel shell proposed in this invention is as follows:

[0010] Step 1, Pre-welding preparation:

[0011] Step 2, Assembly of the parts to be welded:

[0012] According to the design requirements, the front end cap to be welded is assembled with the cylinder, and the cylinder is assembled with the rear end cap to form a whole, resulting in a high-pressure vessel shell that is not fixedly connected.

[0013] During the assembly of the parts to be welded, the gap between adjacent welded parts shall be <0.5mm; the straightness of the shell after assembly shall be <1mm / m. The welding bevels between the front end cap and the cylinder, and between the cylinder and the rear end cap, shall be coaxially aligned; the misalignment of this alignment shall be <0.2mm.

[0014] Step 3, welding the end cap and cylinder:

[0015] The specific process is as follows:

[0016] The assembled high-pressure vessel shell is clamped on a ring welding machine, and the circular runout is ≤0.2mm.

[0017] The areas to be welded on the assembled high-pressure vessel shell are preheated; the preheating temperature is 120–160°C. The heating rate is 50°C / min.

[0018] Welding is carried out after the area to be welded reaches the preheating temperature. During the welding process, the connection between the front end cap and the cylinder is welded first. The welding arc voltage is 11V, the arc current is 45A, and the pulse current is 135A; the weld is formed through three passes.

[0019] Before argon arc welding, a 99.99% pure argon gas shield is provided to the back of the weld. The argon gas flow rate is 18 L / min, and the input direction is maintained at 90° with the welding torch. Argon gas is continuously supplied during the welding process.

[0020] The specific process of the three-pass welding is as follows:

[0021] The first welding pass is performed. The arc starting point for welding is located at the 12 o'clock position on the cross-section of the cylinder.

[0022] Move the welding torch 2mm above the arc initiation point and perform the first pass of welding clockwise towards the front end cap, maintaining a 50° angle between the welding torch and the cylinder axis, and a 75° angle with the welding direction. Use a two-in-one-out technique during welding, advancing 2mm and retreating 1mm, while maintaining a welding speed of 110mm / min. Complete the first pass of welding.

[0023] The second welding pass is performed. The welding torch is moved to the starting point of the first welding pass to perform the second welding pass. The angle between the welding torch and the axis is adjusted to 55°, while the angle with the welding direction is 75°. The welding speed is 55 mm / min. A 30° bevel welding method is used, and welding is performed clockwise until the welding is completed.

[0024] The third welding pass is performed. Adjust the welding torch to a 50° angle with the axis and a 75° angle with the welding direction. The welding speed is 110 mm / min. Use a two-in-one-out technique (2mm forward, 1mm back) to complete the third pass counter-clockwise.

[0025] The 30° oblique tooth method refers to welding in a reciprocating oblique direction at a 30° angle between the welding torch and the weld normal.

[0026] Step 4: Weld the end cap and cylinder together.

[0027] When welding the rear end cap and the cylinder, repeat the welding process of the front end cap and the cylinder in step three to complete the welding of the cylinder and the rear end cap. The resulting high-pressure vessel shell is a semi-finished product.

[0028] Step 5, vacuum quenching treatment:

[0029] The obtained high-pressure vessel shell semi-finished product is subjected to vacuum quenching treatment. The entire process of vacuum quenching treatment consists of vacuum quenching, cold treatment, and secondary tempering. The resulting high-pressure vessel shell semi-finished product is then obtained after vacuum quenching treatment.

[0030] During the vacuum quenching treatment:

[0031] The first step is to place the obtained high-pressure vessel shell semi-finished product into a vacuum furnace and evacuate it to a vacuum degree of 30×10⁻⁶. ～5 Pa. The vacuum furnace is heated to 880°C at a heating rate of 28°C / min and held at that temperature for 1 hour.

[0032] The second step, after the heat preservation is completed, is to fill the furnace with liquid nitrogen of 99.999% purity. The filling of liquid nitrogen is stopped when the nitrogen pressure inside the furnace reaches 5 bar.

[0033] The third step involves using nitrogen gas in the furnace to cool the semi-finished high-pressure vessel shell; then, gas quenching begins, reducing the temperature inside the vacuum furnace to 0-30°C over 3 hours to complete the quenching process. The vessel is then removed from the furnace.

[0034] The cold treatment temperature is -40℃ and the cold treatment time is 60 minutes.

[0035] During the second tempering:

[0036] The first tempering treatment is performed on the semi-finished high-pressure vessel shell after cold treatment. Specifically, the semi-finished high-pressure vessel shell is placed in a room temperature environment and allowed to warm up to 0-30°C within 2 hours. Then, the semi-finished high-pressure vessel shell is subjected to the first tempering treatment. First, the semi-finished high-pressure vessel shell is heated to 290°C at a heating rate of 20°C / min and held at this temperature for 60 minutes. After the holding time is completed, it is removed from the furnace. The first tempering treatment is then complete.

[0037] Immediately after the first tempering process is completed, the second tempering process shall be carried out.

[0038] During the second tempering process, the process of the first tempering is repeated to complete the second tempering of the high-pressure vessel shell semi-finished product.

[0039] Step Six, Machining:

[0040] As per design requirements, the heat-treated high-pressure vessel shell was machined into the finished high-pressure vessel shell using traditional machining methods. This completes the welding of the 1900MPa grade low-alloy ultra-high-strength steel high-pressure vessel shell.

[0041] This invention relates to a welding method for the shell of a 1900MPa grade low alloy ultra-high strength steel high-pressure vessel with a diameter of φ100-φ1000mm, a wall thickness of 1.0-5.0mm, and a straightness of no more than 2mm per meter.

[0042] The high-pressure vessel shell is made of 300M low-alloy ultra-high-strength steel and consists of three parts: front head 1, cylinder 2, and rear head 3. The front head 1 and rear head 3 are machined or stamped into a bowl shape, while the cylinder is a straight part with a wall thickness of 1.0 to 5.0 mm, formed by high-pressure spinning. The front head, cylinder, and rear head are welded into an integral vessel shell using tungsten inert gas welding. After vacuum quenching treatment, the tensile strength of the shell weld reaches more than 1900 MPa, thus realizing the preparation of the high-pressure vessel shell.

[0043] Before tungsten inert gas (TIG) welding, tooling is used to ensure the coaxial alignment of the weld bevels of the front head and cylinder, guaranteeing the straightness of the welding process meets requirements. Argon protection is required during welding; the shielding gas is 100% argon with a purity of 99.99%. The gas flow rate is strictly controlled at 12-20 L / min according to the size of the high-pressure vessel to ensure a clean weld joint and prevent oxygen and other impurities from entering the welding area, corroding the weld and forming inclusions, porosity, and other defects, resulting in poor weld quality and ultimately preventing the high-pressure vessel from reaching its 1900 MPa performance. The arc voltage is controlled at 9-12V, the arc current at 30-50A, and the pulse current at 120-150A. The welding speed is 80-100 mm / min, and multiple passes are used to form the weld, achieving 100% X-ray flaw detection and appearance compliance. After the front head and cylinder welds are completed, the cylinder and rear head welds are performed sequentially, using the same operating methods and parameter settings.

[0044] The assembled shell is subjected to vacuum quenching treatment, and the vacuum degree is strictly required to be 20-30×10 during the vacuum quenching process. ～5 Within the Pa range, to prevent the oxidation of the inner and outer surfaces of the shell by impurities in the air, ensuring no decarburized layer after heat treatment; the assembled shell is placed in a vacuum furnace and heated to 850-970℃ at 20-30℃ / min, and held for 1-2 hours; after the assembled shell reaches the holding requirement, liquid nitrogen (99.999% purity) is injected into the furnace, followed by gas quenching and air cooling to room temperature, with strict control over the cooling rate, to induce a phase transformation in the weld structure of the assembled shell, forming retained austenite and martensite; after vacuum quenching, a cold treatment process at -40 to 60℃ is performed on the high-pressure vessel shell after cold treatment. The material undergoes tempering treatment at 250–320℃ to transform the residual austenite and martensite composite into martensite and tempered martensite, respectively, followed by another tempering treatment at 250–320℃. This process transforms the entire high-pressure vessel shell material composite into tempered martensite, achieving a tensile strength of over 1900 MPa for the weld seam of the 300M steel high-pressure vessel shell. Simultaneously, the shell exhibits minimal deformation, a straightness not exceeding 2 mm per meter, and excellent surface finish. Subsequently, the material is machined into a finished high-pressure vessel shell, thus realizing the preparation of a 1900 MPa grade low-alloy ultra-high-strength steel high-pressure vessel shell.

[0045] The shell material of the high-pressure vessel is 300M steel, which is a low-alloy ultra-high-strength steel with good comprehensive mechanical properties. It is assembled by tungsten inert gas welding of multiple parts and achieves a tensile strength of over 1900MPa through vacuum quenching heat treatment.

[0046] Traditional high-strength steel welding involves a gas flow rate of 8–12 L / min, welding process parameters of 160–220 A welding pulse current, 14–16 V arc voltage, and 50–70 mm / min welding speed.

[0047] The key technical point of this invention is that when welding 300M steel weldments with a diameter of 100 to 1000 mm and a wall thickness of 1.0 to 5.0 mm using tungsten inert gas welding, different welding methods are used and the welding is performed in a multi-pass combination in a certain order; through the heat treatment scheme of "vacuum quenching + cold treatment + two tempering", the performance and straightness accuracy of the high-pressure vessel weld are met, and the 1900 MPa grade low alloy ultra-high strength steel shell is prepared.

[0048] This invention employs tungsten inert gas (TIG) welding in the fabrication of high-pressure vessel shells made of 300M steel. Argon gas protection strictly limits impurity erosion. Preheating of the weld area within a temperature range significantly different from that of traditional steel before welding slows down post-weld cooling, facilitating hydrogen escape from the weld metal and significantly reducing weld hardening, effectively preventing crack formation during welding. Appropriate process parameters are used to minimize welding deformation. The high-pressure vessel shell welding process requires controlling the pulse current at 120–150A and the welding speed at 80–120 mm / min to prevent weld defects. Three different welding methods—clockwise two-pass advance / one-pass, 30° helical teeth, and counterclockwise two-pass / one-pass—are combined in a specific sequence of multiple passes. The relationship between welding passes and wall thickness is shown in Table 1, preparing the high-pressure vessel for a pressure of 1900 MPa.

[0049] Table 1 Relationship between Welding Passes and Wall Thickness

[0050]

[0051] The high-pressure vessel shell made of 300M steel is manufactured using vacuum quenching heat treatment technology to eliminate the oxidation effect of impurities in the air on the inner and outer surfaces of the shell, ensuring that there is no decarburized layer at the weld joints. After vacuum quenching, a cold treatment process of -40 to 60℃ is performed. The heat treatment regime adopts appropriate process parameters and strictly controls the cooling rate. The high-pressure vessel shell undergoes two tempering treatments at 250 to 320℃, which is significantly different from the traditional low-temperature tempering and medium-temperature tempering regimes. The tempering temperature and other parameters are controlled as shown in Table 2, ensuring that the tensile strength of the 300M material shell reaches more than 1900MPa. The shell deformation is small, meeting the requirement that the straightness is no more than 2mm per meter, ensuring the dimensional accuracy of the high-pressure vessel shell after heat treatment, and the surface finish is good.

[0052] Table 2 Tempering Heat Treatment Parameters

[0053] Attached Figure Description

[0054] Figure 1 It is a high-pressure vessel shell made of 1900MPa grade low alloy ultra-high strength steel.

[0055] Figure 2This is a schematic diagram of the front end cap.

[0056] Figure 3 This is a schematic diagram of a cylinder.

[0057] Figure 4 This is a schematic diagram of the rear end cap.

[0058] Figure 5 This is a flowchart of the present invention.

[0059] In the diagram: 1. Front end cap; 2. Cylinder; 3. Rear end cap; 4. Weld.

[0060] This embodiment describes a welding method for the shell of a high-pressure vessel made of 1900MPa grade low-alloy ultra-high-strength steel. Detailed Implementation

[0061] This embodiment describes a welding method for the shell of a high-pressure vessel made of 1900MPa grade low-alloy ultra-high-strength steel.

[0062] like Figure 1 As shown, the high-pressure vessel shell structure in this embodiment is composed of a front end cap 1, a cylinder 2, and a rear end cap 3 welded together. The 1900MPa-level high-pressure vessel shell is prepared by tungsten inert gas welding followed by vacuum quenching.

[0063] Step 1, Pre-welding preparation:

[0064] The pre-welding preparation refers to the preparation of the front end cap 1, cylinder 2, and rear end cap 3 to be welded. Both the front end cap 1 and rear end cap 3 are machined from 300M steel forgings, with 45° bevels at the weld joints to the cylinder 2. The cylinder 2 is spun from 300M steel forgings, forming a 1000mm long cylinder with a 3.0mm wall thickness, with 45° bevels machined at both ends for welding.

[0065] The 60mm width of the area to be welded on the front end cap 1, cylinder 2, and rear end cap 3 was cleaned by oxidizing with cotton yarn soaked in anhydrous ethanol. The area to be welded was then sanded with a wire wheel.

[0066] Step 2, Assembly of the parts to be welded:

[0067] According to the design requirements, the front end cap 1 to be welded is assembled with cylinder 2, and cylinder 2 is assembled with rear end cap 3 as a whole to obtain a high-pressure vessel shell that is not fixedly connected; the gap between adjacent welded parts is <0.5mm; the straightness of the assembled shell is <1mm / m.

[0068] During assembly, tooling is used to ensure that the welding bevels of the front end cap 1 and the cylinder 2, and the welding bevels of the cylinder 2 and the rear end cap 3 are coaxially aligned; the misalignment of this alignment is <0.2mm. The tooling uses existing technology.

[0069] Step 3: Weld the end cap and cylinder together.

[0070] The specific process is as follows:

[0071] The first step is to clamp the unfixed high-pressure vessel shell onto the ring welding machine, and ensure that the circular runout is ≤0.2mm with the welding machine shaft as the center.

[0072] The second step involves preheating the area to be welded on the unfixed high-pressure vessel shell; the preheating temperature is 150°C, with a heating rate of 50°C / min. Traditional alloy steel preheating temperatures are typically below 80°C. This invention relates to a material preheating range of 120–160°C, significantly reducing the weld cracking rate. Welding is then performed after the area to be welded reaches the preheated temperature.

[0073] Before argon arc welding, provide 99.99% pure argon gas protection to the back of the weld. The argon gas flow rate is 18L / min, and the input direction is kept at 90° to the welding torch. Argon gas is continuously supplied during the welding process. Argon gas protection is stopped only after welding is completed to ensure that the weld joint is fully and completely protected during the welding process, preventing oxygen and other impurities from entering the welding area.

[0074] During the welding process, the connection between the front end cap 1 and the cylinder 2 is welded first. During welding, the arc voltage is 11V, the arc current is 45A, the pulse current is 135A, and the weld is formed through 3 passes.

[0075] During welding, the welding of the end cap 1 and the cylinder 2 is completed in three passes. Specifically,

[0076] The first welding pass is performed. The arc starting point for welding is the highest point of the cross-section of the cylinder, i.e., the 12 o'clock position.

[0077] Move the welding torch to a position 10mm above the arc ignition point and turn on the torch switch. The high-voltage electric pulse ionizes the argon gas, igniting the welding arc. Immediately after arc formation, lower the torch to a position 2mm above the arc ignition point and perform the first pass of welding clockwise towards the front end cap 1, maintaining a 50° angle between the welding torch and the cylinder axis, and a 75° angle with the welding direction. Use a conventional two-forward, one-backward technique (forward 2mm, backward 1mm) and maintain a welding speed of 110mm / min. The first pass yields a weld width of 7mm.

[0078] The second welding pass is performed. The welding torch is moved to the starting point of the first pass for the second pass, and the angle between the welding torch and the axis is adjusted to 55°, while simultaneously forming a 75° angle with the welding direction. The welding speed is 55 mm / min. A 30° oblique tooth welding method is used, welding clockwise until the weld is completed; the 30° oblique tooth method refers to a reciprocating oblique line direction at a 30° angle between the welding torch and the weld normal. The weld width is 10 mm. During the second pass welding process, the high-temperature weld pool formed is fully agitated and uniformly shaped with the weld structure of the first pass at a 30° angle, offsetting the welding internal stress and achieving full and uniform fusion of the structures of the first and second passes.

[0079] The third welding pass is performed. Adjust the welding torch to a 50° angle with the axis and a 75° angle with the welding direction. The welding speed is 110 mm / min. Use a two-in-one-out technique (2mm forward, 1mm back) to complete the third pass counter-clockwise. The weld width for the third pass is 10 mm.

[0080] The third pass increases the weld density, ensuring the final weld quality and morphology. The order of these three passes must be strictly followed; changing the order will increase the difficulty of fusion, leading to numerous weld porosity and lack of fusion defects, and increasing the risk of welding failure for 300M steel.

[0081] Step 4: Weld the end cap and cylinder together.

[0082] When welding the rear end cap and the cylinder, repeat the welding process of the front end cap 1 and the cylinder 2 in step three to complete the welding of the cylinder 2 and the rear end cap 3.

[0083] The obtained high-pressure vessel shell semi-finished product.

[0084] Step 5, vacuum quenching treatment:

[0085] The obtained high-pressure vessel shell semi-finished product is subjected to vacuum quenching treatment. The entire process of vacuum quenching treatment consists of vacuum quenching, cold treatment, and secondary tempering, specifically:

[0086] The first step is to place the obtained high-pressure vessel shell semi-finished product into a vacuum furnace and evacuate it to a vacuum degree of 30×10⁻⁶. ～5 Pa. The vacuum furnace is heated to 880°C at a heating rate of 28°C / min and held at that temperature for 1 hour.

[0087] The second step, after the heat preservation is completed, is to fill the furnace with liquid nitrogen of 99.999% purity. The filling of liquid nitrogen is stopped when the nitrogen pressure inside the furnace reaches 5 bar.

[0088] The third step involves using nitrogen gas in the furnace to cool the semi-finished high-pressure vessel shell; then, gas quenching begins, reducing the temperature inside the vacuum furnace to 0-30°C over 3 hours to complete the quenching process. The vessel is then removed from the furnace.

[0089] The semi-finished high-pressure vessel shell is transferred to a cold box furnace at -40℃ within 1 minute for 60 minutes of cold treatment before being removed from the furnace. The cold treatment makes the grain boundary membrane in the weld structure of 300M steel disappear, reduces the residual austenite, and prepares the subsequent tempering process to improve performance. At the same time, it eliminates the residual stress of the material during the vacuum quenching process, which is beneficial to stabilizing the size of the high-pressure vessel shell.

[0090] The first tempering treatment is performed on the semi-finished high-pressure vessel shell after cold treatment. Specifically, the semi-finished high-pressure vessel shell is placed in a room temperature environment and allowed to warm up to 0-30°C within 2 hours. Then, the semi-finished high-pressure vessel shell is subjected to the first tempering treatment. First, the semi-finished high-pressure vessel shell is heated to 290°C at a heating rate of 20°C / min and held at this temperature for 60 minutes. After the holding period, it is removed from the furnace, allowing the residual austenite and martensite complex to transform into martensite and tempered martensite, respectively. This achieves the first increase in the tensile strength of the weld of the semi-finished high-pressure vessel shell, reaching 1600-1700 MPa. The first tempering treatment is then completed.

[0091] Immediately after the first tempering process is completed, the second tempering process shall be carried out.

[0092] During the second tempering process, the process of the first tempering treatment is repeated to complete the second tempering treatment of the high-pressure vessel shell semi-finished product. This process transforms the weld of the high-pressure vessel shell into tempered martensite, resulting in a tensile strength of over 1900MPa at the welded part of the 300M steel high-pressure vessel shell. At the same time, the shell deformation is small, the straightness is no more than 2mm per meter, and the surface finish is good.

[0093] Step Six, Machining:

[0094] As per design requirements, the heat-treated high-pressure vessel shell was machined into the finished high-pressure vessel shell using traditional machining methods. This completes the welding of the 1900MPa grade low-alloy ultra-high-strength steel high-pressure vessel shell.

[0095] In this invention, the shell material of the high-pressure vessel is 300M steel, which is a low-alloy ultra-high-strength steel with good comprehensive mechanical properties. It is assembled by tungsten inert gas welding of multiple parts and achieves a tensile strength of over 1900MPa through vacuum quenching heat treatment.

[0096] Traditional steel welding uses a conventional gas flow rate of 8–12 L / min, with welding process parameters including a welding pulse current of 160–220 A, an arc voltage of 14–16 V, and a welding speed of 50–70 mm / min. The preheating temperature range for traditional alloy steel is less than 80℃. In the fabrication of high-pressure vessel shells made of 300M steel, tungsten inert gas (TIG) welding is employed. This ensures that the preheating temperature at the weld site reaches 120–160℃ before welding, and argon gas protection strictly limits impurity erosion. Appropriate welding process parameters are used, controlling the pulse current at 120–150 A and the welding speed at 80–120 mm / min. Depending on the different thicknesses and structural requirements of the vessel shell, three different welding methods—clockwise two-pass advance / one-pass, 30° helical teeth, and counterclockwise two-pass / one-pass—are combined in a specific sequence of multiple passes. This results in effective fusion of the weld microstructure, producing a fine and dense weld, improving weld quality, reducing the risk of weld defects, and preparing the vessel for a pressure rating of 1900 MPa.

[0097] In this invention, the order of the three welding passes must be strictly followed. Changing the order will increase the difficulty of fusion, generate a large number of weld defects such as porosity and lack of fusion, and cause the risk of welding failure of 300M steel. It will also cause microstructure problems and affect the performance of subsequent heat treatment.

[0098] This invention relates to the post-weld vacuum quenching heat treatment of 300M steel high-pressure vessel shell semi-finished products. Through a "vacuum quenching + cold treatment + double tempering" heat treatment scheme, the vacuum quenching process eliminates the oxidation effect of impurities in the air on the inner and outer surfaces of the shell, ensuring no decarburized layer after heat treatment and minimizing the impact of surface oxidation on the performance of the high-pressure vessel shell. The cold treatment eliminates the residual austenite grain boundary membrane in the weld and microstructure from the vacuum quenching process, preparing for the microstructure transformation during the subsequent two tempering processes. It also eliminates residual stress from the vacuum quenching process, which is beneficial for stabilizing the dimensions of the pressure vessel shell. The heat treatment regime employs optimized process parameters, strictly controls the cooling rate, and performs two consecutive tempering treatments at 250–320℃ on the high-pressure vessel shell to ensure that the tensile strength of the weld in the 300M steel high-pressure vessel shell reaches over 1900 MPa.

[0099] The vacuum quenching heat treatment process and procedure for 300M steel welded shells involved in this invention differs significantly from the traditional heat treatment process for ultra-high strength steel. Taking typical ultra-high strength steels such as 30CrMnSiA and D406A as examples, their heat treatment process is quenching plus one tempering treatment. Generally, cold treatment and secondary tempering are not allowed because adding cold treatment and secondary tempering cannot improve the tensile strength of 30CrMnSiA and D406A steel. At the same time, secondary tempering will cause structural hardening and the risk of reverse grain enlargement, resulting in a decrease in tensile strength, hardness, and plasticity, causing the product performance to fail to meet the requirements and be scrapped.

[0100] The welding and heat treatment technology adopted in this invention has little impact on the deformation during the manufacturing process of the high-pressure vessel shell, meets the requirement that the straightness is no more than 2 mm per meter, ensures the dimensional accuracy of the heat treatment of the weld seam of the high-pressure vessel shell, and has a good surface finish. The tensile strength of the weld seams in high-pressure vessel shells made of traditional 30CrMnSiA and D406A ultra-high-strength steel is generally lower than 1600MPa, and the product's dimensional accuracy is poor, with straightness typically exceeding 2mm per meter. This is mainly due to the use of a two-step forward, one-step backward welding process, with multiple passes using the same welding method, resulting in uneven weld microstructure, inability to offset welding internal stress, and large deformation near the weld. Furthermore, the use of an air furnace for heat treatment causes the inner and outer surfaces of the high-pressure vessel shell welds to react with oxygen and other gases in the air, producing a certain proportion of decarburized layer, which degrades the weld performance. The oxidation reaction causes a surge in internal stress, leading to severe deformation of the weld seams. This requires intensive reshaping to meet requirements. Additionally, the surface cleanliness of the high-pressure vessel after heat treatment is poor, necessitating a sandblasting process to remove oxide scale, making the rework process cumbersome and difficult.

Claims

1. A welding method for the shell of a high-pressure vessel made of 1900MPa grade low-alloy ultra-high-strength steel, characterized in that, The specific process is as follows: Step 1: Preparation before welding; Step 2, Assembly of the parts to be welded: According to the design requirements, the front end cap to be welded is assembled with the cylinder, and the cylinder is assembled with the rear end cap to form a whole, resulting in a high-pressure vessel shell that is not fixedly connected. Step 3: Weld the end cap and cylinder together. The specific process is as follows: The assembled high-pressure vessel shell is clamped on a ring welding machine, with a circular runout ≤ 0.2 mm; The weldable areas of the assembled high-pressure vessel shell are preheated; the preheating temperature is 120~160℃; the heating rate is 50℃ / min. Welding is carried out after the area to be welded reaches the preheating temperature; during the welding process, the connection between the front end cap and the cylinder is welded first. The welding arc voltage is 11V, the arc current is 45A, and the pulse current is 135A; the weld is formed through three passes. The specific process for forming the three-pass weld seam is as follows: The first welding pass is performed with the 12 o'clock position on the cross-section of the cylinder as the starting point for the welding arc. Move the welding torch to 2mm above the arc starting point, and perform the first pass of welding in a clockwise direction towards the front end cap, keeping the welding torch at a 50° angle to the cylinder axis and at a 75° angle to the welding direction; during welding, use a two-in-one-out technique, advancing 2mm and retreating 1mm, and maintain a welding speed of 110mm / min; complete the first pass of welding. The second pass of welding is performed; the welding torch is moved to the starting point of the first pass of welding to perform the second pass of welding. The angle between the welding torch and the axis is adjusted to 55°, and at the same time, it is at a 75° angle with the welding direction. The welding speed is 55 mm / min. Welding is performed using a 30° helical tooth method, and the welding is performed clockwise until the welding is completed. The third pass of welding is performed; the angle between the welding torch and the axis is adjusted to 50°, and at the same time, the angle with the welding direction is 75°; the welding speed is 110mm / min; the welding process is carried out by advancing two mm and retreating one mm, and the third pass of welding is completed counterclockwise. The 30° oblique tooth method refers to welding in a reciprocating oblique direction at a 30° angle between the welding torch and the weld normal. Step 4: Weld the end cap and cylinder together. When welding the rear end cap and the cylinder, repeat the welding process of the front end cap and the cylinder in step three to complete the welding of the cylinder and the rear end cap; the resulting high-pressure vessel shell semi-finished product; Step 5, vacuum quenching treatment: The obtained high-pressure vessel shell semi-finished product is subjected to vacuum quenching treatment; the entire process of vacuum quenching treatment consists of vacuum quenching, cold treatment and secondary tempering; and the high-pressure vessel shell semi-finished product after vacuum quenching treatment is obtained. During the second tempering: The first tempering treatment is performed on the semi-finished high-pressure vessel shell after cold treatment. Specifically, the semi-finished high-pressure vessel shell is placed in a room temperature environment and allowed to warm up to 0~30℃ within 2 hours. Then, the semi-finished high-pressure vessel shell is subjected to the first tempering treatment. First, the semi-finished high-pressure vessel shell is heated to 290℃ at a heating rate of 20℃ / min and held at this temperature for 60 minutes. After the holding time is completed, it is removed from the furnace, thus completing the first tempering treatment. Immediately after the first tempering process is completed, the second tempering process shall be carried out. During the second tempering, the process of the first tempering is repeated to complete the second tempering of the high-pressure vessel shell semi-finished product. Step Six, Machining: According to the design requirements, the heat-treated high-pressure vessel shell is processed into a finished high-pressure vessel shell using traditional machining methods; thus, the welding of the 1900MPa grade low-alloy ultra-high-strength steel high-pressure vessel shell is completed.

2. The welding method for the shell of a 1900MPa grade low-alloy ultra-high strength steel high-pressure vessel as described in claim 1, characterized in that, When assembling the parts to be welded, the gap between adjacent welded parts is <0.5mm; the straightness of the shell after assembly is <1mm / m; the welding bevels of the front end cap and the cylinder, and the welding bevels of the cylinder and the rear end cap are all coaxially aligned; the misalignment of this alignment is <0.2mm.

3. The welding method for the shell of a 1900MPa grade low-alloy ultra-high strength steel high-pressure vessel as described in claim 1, characterized in that, Before argon arc welding, 99.99% pure argon gas is provided to the back of the weld for protection; the gas flow rate of this argon gas is 18L / min, the input direction is kept at 90° with the welding torch, and argon gas is continuously input during the welding process.

4. The welding method for the shell of a 1900MPa grade low-alloy ultra-high strength steel high-pressure vessel as described in claim 1, characterized in that, During the vacuum quenching treatment: The first step is to place the obtained high-pressure vessel shell semi-finished product into a vacuum furnace and evacuate it to a vacuum degree of 30×10⁻⁶. ~ 5 Pa; The vacuum furnace is heated to 880°C at a heating rate of 28°C / min and held at that temperature for 1 hour; The second step is to fill the furnace with 99.999% pure liquid nitrogen after the heat preservation is completed; stop filling the furnace with liquid nitrogen when the nitrogen pressure inside the furnace reaches 5 bar. The third step involves using nitrogen gas in the furnace to cool the semi-finished high-pressure vessel shell; gas quenching begins, reducing the temperature inside the vacuum furnace to 0-30°C over 3 hours to complete the gas quenching; the vessel is then removed from the furnace.

5. The welding method for the shell of a 1900MPa grade low-alloy ultra-high strength steel high-pressure vessel as described in claim 1, characterized in that, The cold treatment temperature is -40℃ and the cold treatment time is 60 minutes.

Images