Method for vacuum electron beam welding of 1900mpa class high pressure vessel shells

Abstract

A method for vacuum electron beam welding of 1900MPa high-pressure vessel shell, by positioning welding, sealing welding, full welding of three different welding methods, according to a certain order and the optimization of process parameters of multi-pass combined welding, to reduce the welding deformation. The vacuum degree in the welding is 5-30×10 ‑2 Pa, to ensure the cleanliness of the welded joint, and strictly limit the impurity erosion. Before welding, the welding site is preheated in a temperature range different from that of traditional steel. Because a certain amount of heat can slow down the cooling speed after welding, it is easy to realize the escape of hydrogen in the weld metal, and also greatly attenuate the hardening degree of the weld, effectively avoid the generation of cracks during the welding process, to prevent defects in the weld. The focused current during the welding process of high-pressure vessel shell should be controlled at 1500-2200mA, the electron beam current is 40-80mA, and the welding speed is 6-30mm / s.

Description

Technical Field

[0001] This invention relates to a method for machining, specifically a method for vacuum electron beam welding of a 1900MPa high-pressure vessel shell. 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 for the large billet in continuous casting and the subsequent billet peeling and finishing process. After the continuous casting billet passes the flaw detection, it is rolled into the required high-quality hot-rolled coil wire rod for 1900MPa grade suspension springs by roughing, intermediate rolling, pre-finishing, and finishing rolling mills. This meets the processing and usage requirements of automotive suspension springs, shortens the processing flow and production time. This method only involves smelting, casting, and billet preparation, followed by 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 preparation method of the high-pressure vessel shell 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 preparation 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 method for preparing a high-performance composite steel sheet. However, because its surface has an aluminum-silicon coating mixed with steel, it is a multi-component mixture that cannot be welded and is unrelated 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 preparation method proposed in this invention.

[0006] In November 2011, Pei Jian published an article titled "The Influence of Ni Content and Tempering Process on the Microstructure and Properties of 300M Steel" (Yanshan University Electronic Journal, Issue 11, 2011). In order to improve the toughness of 300M steel, he adjusted the content of the toughening element Ni and studied the influence of Ni content on the continuous cooling transformation, microstructure evolution, and mechanical property changes of 300M steel. He also studied the influence of high Ni content on the microstructure and properties of 300M steel, but it was not related to the preparation method of the shell of 1900MPa grade low alloy ultra-high strength steel high pressure vessel.

[0007] In 2008, Liu Zhenbao et al. published an article titled "Development of 1900MPa Grade Ultra-High Strength Stainless Steel" in Volume 32, Issue 3 of *Mechanical Engineering Materials*. This article proposed a 1900MPa chromium-nickel-calcium-platinum series large-diameter bar (φ200 mm) 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, the alloy composition was further optimized, 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 shortcomings of existing technologies where high-performance steel cannot be welded to form high-pressure vessel shells, this invention proposes a method for vacuum electron beam welding of 1900MPa-grade high-pressure vessel shells.

[0009] The specific process of this invention is as follows:

[0010] Step 1, Pre-welding preparation:

[0011] The pre-welding preparation includes forming a weld bevel at the welding location of the shell to be welded, sanding the welding area, and demagnetizing the shell. The magnetic flux of the shell to be welded after demagnetization is <1×10⁻⁶. -4 T.

[0012] The shell to be welded includes a front end cap, a cylinder, and a rear end cap.

[0013] Step 2, Assembly of the parts to be welded:

[0014] The front end cap to be welded is assembled with the cylinder, and the cylinder is assembled with the rear end cap to obtain the assembled high-pressure vessel shell.

[0015] During the assembly of welded parts, the gap between adjacent welded parts shall be less than 0.2 mm; after assembly, the straightness of the housing shall be less than 1 mm / m.

[0016] During assembly, tooling is used to ensure that the welding bevels between the front end cap and the cylinder, and between the cylinder and the rear end cap, are coaxially aligned; the misalignment of this alignment is less than 0.1mm.

[0017] Step 3, welding the end cap and cylinder:

[0018] The specific process is as follows:

[0019] The first step is to clamp the assembled high-pressure vessel shell.

[0020] The second step is to preheat the areas of the assembled high-pressure vessel shell that are to be welded; the preheating temperature is 120~160℃, and the heating rate is 50℃ / min.

[0021] The third step is to evacuate the welding workspace until the vacuum level reaches 5–20 × 10⁻⁶. -2 Stop vacuuming after Pa; begin welding.

[0022] During welding, the number of welding passes is determined according to the wall thickness h. When the wall thickness of the high-pressure vessel shell is 1 < h ≤ 2.5 mm, the number of welding passes is 2, including 1 tack weld and 1 full weld. When the wall thickness of the high-pressure vessel shell is 2.5 < h ≤ 4.0 mm, the number of welding passes is 3, including 1 tack weld, 1 seal weld and 1 full weld. When the wall thickness of the high-pressure vessel shell is 4.0 < h ≤ 5.0 mm, the number of welding passes is 4, including 1 tack weld, 1 seal weld and 2 full welds.

[0023] Welding is performed using a workpiece rotation method with the welding machine head vertically downwards. The welding of the front end cap to the cylinder is completed in 2, 3, or 4 passes. The 2-pass welding consists of tack welding + 1 full weld; the 3-pass welding consists of tack welding + 1 sealing weld + 1 full weld; and the 4-pass welding consists of tack welding + 1 sealing weld + 2 full welds. The specific process is as follows:

[0024] Ⅰ First pass, tack welding.

[0025] Select the positioning welding point positions; the positioning welding point is the 12 o'clock position of the part of the cylinder to be welded as the first welding point, and one point is determined every 45° clockwise. Four groups of eight positioning welding point positions are determined around the cross-section of the part of the cylinder to be welded, which are the first welding point to the eighth welding point in sequence.

[0026] During welding, a clockwise symmetrical point method is used to complete the tack welding of 4 groups of 8 welding points, forming 8 tack welds with a diameter of 2-5mm.

[0027] Complete the first pass of tack welding.

[0028] The accelerating voltage for the first pass of tack welding is 30~40KV, the focusing current is 1500~2000mA, the electron beam current is 40~60mA, and the welding speed is 10~30mm / s.

[0029] II. Second pass, sealing weld.

[0030] When the wall thickness of the high-pressure vessel shell is 1 < h ≤ 2.5 mm, sealing welding is not performed;

[0031] When the wall thickness of the high-pressure vessel shell is 2.5 < h ≤ 4.0 mm or 4.0 < h ≤ 5.0 mm, sealing welding shall be performed.

[0032] During the sealing welding, keep the position of the welding machine head fixed, and rotate the first positioning welding point of the cylinder to directly below the welding machine head through the welding machine center frame; adjust the vertical height of the welding machine head and the working distance between the first positioning welding point to 400-500mm; turn on the welding machine switch, and weld clockwise along the circumference of the cross-section of the part of the cylinder to be welded, with a welding width of 2-6mm, to complete the sealing welding.

[0033] The accelerating voltage for the second pass of sealing is 30~50KV, the focusing current is 1500~2000mA, the electron beam current is 40~80mA, and the welding speed is 10~30mm / s.

[0034] III. Third pass, first full weld.

[0035] When the wall thickness of the high-pressure vessel shell is 1 < h ≤ 2.5 mm, or 2.5 < h ≤ 4.0 mm, or 4.0 < h ≤ 5.0 mm, it is fully welded in one operation.

[0036] During the first full weld, the position of the welding machine head remains fixed. Using the welding machine center frame, rotate the cylinder to the starting position of the sealing weld, placing it directly below the welding machine head. Adjust the vertical height of the welding machine head and the working distance between the starting weld point of the second pass and the starting weld point to 300-500mm. Turn on the switch and weld counter-clockwise around the circumference of the part of the cylinder to be welded, with a weld width of 3-10mm. This completes the full weld.

[0037] The accelerating voltage for the third pass is 30~80KV, the focusing current is 1500~2200mA, the electron beam current is 40~100mA, and the welding speed is 5~30mm / s.

[0038] IV. Fourth pass, second full weld.

[0039] When the wall thickness of the high-pressure vessel shell is 4.0 < h ≤ 5.0 mm, a second full weld is required.

[0040] During the second full weld, the welding machine head position remains fixed. Using the welding machine center frame, rotate the cylinder back to the starting position of the first full weld, placing it directly below the welding machine head. Adjust the vertical height of the welding machine head and the working distance between the starting weld point of the second pass and the starting weld point to 300-500 mm. Turn on the switch and weld clockwise around the circumference of the part of the cylinder to be welded, with a weld width of 3-10 mm. The full weld is then complete.

[0041] The accelerating voltage for the secondary full welding is 60~80KV, the focusing current is 2000~2200mA, the electron beam current is 80~100mA, and the welding speed is 5~15mm / s.

[0042] Step 4: Weld the end cap and cylinder together.

[0043] 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.

[0044] The obtained high-pressure vessel shell semi-finished product.

[0045] Step 5, vacuum quenching treatment:

[0046] The obtained high-pressure vessel shell semi-finished product is subjected to vacuum quenching treatment. The vacuum quenching treatment process consists of vacuum quenching, cold treatment, and secondary tempering treatment.

[0047] The specific process of vacuum quenching is as follows:

[0048] 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 20-30 × 10⁻⁶.~5 Pa. The vacuum furnace is heated to 850-970℃ at a rate of 20-30℃ / min and held at that temperature for 1-2 hours.

[0049] The second step, after the heat preservation is completed, is to fill the furnace with 99.999% pure liquid nitrogen. The filling of liquid nitrogen is stopped when the nitrogen pressure inside the furnace exceeds 5 bar.

[0050] 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 from 0 to 30°C over 1-3 hours to complete the quenching process. The vessel is then removed from the furnace.

[0051] The cold treatment involves transferring the semi-finished high-pressure vessel shell, which has undergone vacuum quenching, to a cold box furnace at a temperature of -40 to -60°C within 1 minute after exiting the furnace for 60 to 200 minutes; then removing it from the furnace.

[0052] During the secondary tempering process, the semi-finished high-pressure vessel shell that has undergone cold treatment is placed in a room temperature environment and allowed to warm up to 0-30°C within 1-3 hours; the semi-finished high-pressure vessel shell is then subjected to the first tempering process.

[0053] The specific process of secondary tempering is as follows:

[0054] The high-pressure vessel shell semi-finished product is heated to 250-320℃ at a heating rate of 20-30℃ / min and held at that temperature for 40-80 minutes; after the holding period, it is removed from the furnace; the first tempering treatment is completed.

[0055] Immediately after the first tempering process is completed, the second tempering process shall be carried out.

[0056] 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.

[0057] Step Six, Machining:

[0058] According to design requirements, the high-pressure vessel shell, which has undergone vacuum quenching treatment, is machined into a finished high-pressure vessel shell.

[0059] Thus, the vacuum electron beam welding of the 1900MPa high-pressure vessel shell was completed.

[0060] This invention discloses a method for manufacturing a 1900MPa grade 300M steel high-pressure vessel shell with a diameter of 100-1000mm, a wall thickness of 1.0-5.0mm, and a straightness of <2mm / m. Through this invention, multiple pre-welding parts are welded together using vacuum electron beam welding in a multi-pass combination with different welding methods in a specific sequence. Vacuum quenching is then employed, followed by a heat treatment scheme of "vacuum quenching + cold treatment + two tempering processes" to ensure that the weld performance and straightness accuracy of the high-pressure vessel meet the requirements, thus achieving the manufacturing of a 1900MPa grade low-alloy ultra-high-strength steel shell.

[0061] The high-pressure vessel shell is made of 300M low-alloy ultra-high-strength steel and consists of three parts: a front end, a cylinder, and a rear end. The front and rear ends are machined or stamped into a bowl shape, while the cylinder is a straight part with a wall thickness of 1.0~5.0mm, formed by high-pressure spinning. The front end, cylinder, and rear end are welded into an integral vessel shell using vacuum electron beam welding. After vacuum quenching treatment, the tensile strength of the shell weld reaches over 1900MPa, thus realizing the preparation of the high-pressure vessel shell.

[0062] In existing vacuum electron beam welding techniques, oxygen and other impurities can enter the welding area and erode the weld, resulting in inclusions, porosity, and other defects, leading to poor weld quality and ultimately preventing the high-pressure vessel from reaching its 1900 MPa performance. This invention evacuates the welding equipment to a vacuum level of 5–20 × 10⁻⁶ before vacuum electron beam welding. -2 Pa ensures the cleanliness of the weld joint, preventing oxygen and other impurities from entering the welding area and causing defects such as inclusions and porosity in the weld. Simultaneously, tooling is used to ensure the coaxial alignment of all welded parts, guaranteeing straightness during the welding process. The welding machine head working distance is 300–500 mm, the accelerating voltage is 30–80 KV, the focusing current is 1500–2200 mA, the electron beam current is 40–100 mA, and the welding speed is 5–30 mm / s. Through multi-pass weld formation, the weld quality meets 100% X-ray flaw detection and appearance requirements.

[0063] The welded shell is subjected to vacuum quenching treatment, during which a vacuum degree of 20–30 × 10⁻⁶ is strictly required. ~5To 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 required temperature, 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 -40 to 60℃ cold treatment process is performed, and the cold-treated high-pressure vessel shell is then... 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 1900MPa for the weld seam of the 300M steel high-pressure vessel shell. Simultaneously, the shell exhibits minimal deformation, a straightness not exceeding 2mm per meter, and excellent surface finish. Subsequently, the material is machined into a finished high-pressure vessel shell, thus achieving the preparation of a 1900MPa grade low-alloy ultra-high-strength steel high-pressure vessel shell.

[0064] The high-pressure vessel shell material in this invention is 300M steel, which is a low-alloy ultra-high-strength steel with good comprehensive mechanical properties. It is assembled by vacuum electron beam welding of multiple parts and achieves a tensile strength of over 1900MPa through vacuum quenching heat treatment.

[0065] The traditional high-strength steel welding process parameters for vacuum electron beam welding are 1000-1600mA focusing current, 10-30mA electron beam current, and 2-10mm / s welding speed.

[0066] This invention employs vacuum electron beam welding in the fabrication of high-pressure vessel shells made of 300M steel. During the welding process, the vacuum level within the welding space must be maintained at 5–30 × 10⁻⁶. -2 Within the specified Pa range, the weld joint is kept clean, and the corrosion of impurities is strictly limited. Before welding, the weld area is preheated at a temperature range significantly different from that of traditional steel. This preheating slows down the post-weld cooling rate, facilitating the escape of hydrogen from the weld metal and significantly reducing the hardening degree of the weld, effectively preventing cracks during welding. Appropriate process parameters are used to reduce welding deformation. During the welding of the high-pressure vessel shell, the accelerating voltage must be controlled at 30–80 kV, the focusing current at 1500–2200 mA, the electron beam current at 40–100 mA, and the welding speed at 5–30 mm / s to prevent weld defects. Three different welding methods—tack welding, sealing welding, and full welding—are combined in a specific sequence of multiple passes. The relationship between the number of welding passes and the wall thickness h is shown in Table 1, preparing for the subsequent achievement of a 1900 MPa high-pressure vessel.

[0067] Table 1 Relationship between Welding Passes and Wall Thickness

[0068]

[0069] 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.

[0070] Table 2 Tempering Heat Treatment Parameters

[0071] Attached Figure Description

[0072] Figure 1 It is a high-pressure vessel shell made of 1900MPa grade low alloy ultra-high strength steel.

[0073] Figure 2 This is a schematic diagram of the front end cap.

[0074] Figure 3 This is a schematic diagram of a cylinder.

[0075] Figure 4 This is a schematic diagram of the rear end cap.

[0076] Figure 5 This is a flowchart of the present invention.

[0077] In the diagram: 1. Front end cap; 2. Cylinder; 3. Rear end cap; 4. Weld. Detailed Implementation

[0078] This invention relates to a method for vacuum electron beam welding of the shell of a 1900MPa high-pressure vessel, and its technical solution will be described in detail through 12 embodiments.

[0079] like Figure 1 As shown, the high-pressure vessel shell is assembled and welded from a front end cap 1, a cylinder 2, and a rear end cap 3. The cylinder has a diameter of 1000mm and a wall thickness of 5mm. The 1900MPa-level high-pressure vessel shell is prepared by a combination of vacuum electron beam welding and vacuum quenching treatment.

[0080] The specific process of this invention is as follows:

[0081] Step 1, Pre-welding preparation:

[0082] 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 90° straight edge bevels at the connection points to be welded to the cylinder 2. The cylinder 2 is spun from 300M steel forgings, forming a 1000mm long cylinder with a 5mm wall thickness, with 90° straight edge bevels machined at both ends for welding.

[0083] The 60mm width of the areas 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 areas to be welded were then sanded using a wire wheel.

[0084] The front end cap 1, cylinder 2, and rear end cap 3 are subjected to overall demagnetization treatment to ensure that the magnetic flux of the workpiece before welding is <1×10⁻⁶. -4 T, to avoid the electron beam direction being deflected during the subsequent welding process due to residual magnetism in the workpiece before welding, which would cause welding deviation and incomplete fusion defects.

[0085] Step 2, Assembly of the parts to be welded:

[0086] 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 <0.2mm; the straightness of the assembled shell is <1mm / m.

[0087] 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 <0.1mm. The tooling adopts existing technology.

[0088] Step 3: Weld the end cap and cylinder together.

[0089] The specific process is as follows:

[0090] The first step is to clamp the assembled high-pressure vessel shell.

[0091] The second step is to preheat the weldable areas of the assembled high-pressure vessel shell; the preheating temperature is 150℃, and the heating rate is 50℃ / min.

[0092] The third step is to evacuate the welding workspace until the vacuum level reaches 20×10⁻⁶. -2 Stop vacuuming after Pa; begin welding.

[0093] During welding, the workpiece rotates while the welding machine head remains vertically downward. The welding of the front end cap and the cylinder is completed in four passes. These four passes consist of tack welding, one sealing weld, and two full welds. The specific process is as follows:

[0094] Ⅰ First pass, tack welding.

[0095] Select the positioning welding point positions; the positioning welding point is the 12 o'clock position of the part of the cylinder to be welded as the first welding point, and one point is determined every 45° clockwise. Four groups of eight positioning welding point positions are determined around the cross-section of the part of the cylinder to be welded, which are the first welding point to the eighth welding point in sequence.

[0096] During welding, a clockwise, symmetrical point-to-point method was used to complete the tack welding of 4 groups of 8 welding points. Specifically:

[0097] Starting from the first welding point located at the 12 o'clock position, move the welding machine head to a distance of 500mm from the first welding point; turn on the switch to allow the electron beam to weld the first welding point in a vertically downward direction; the welding time is 4 seconds, and after forming a positioning weld point with a diameter of 5mm, turn off the welding machine. Rotate the cylinder 180° to the fifth welding point symmetrical to the first welding point, turn on the welding machine switch, and weld the fifth welding point using the same method as the first welding point.

[0098] Rotate the cylinder to complete the welding of the remaining six welding points in pairs in sequence; the order of the six welding points is: the second welding point between the midpoint of 12 o'clock and 3 o'clock, the sixth welding point between 6 o'clock and 9 o'clock; the third welding point at the 3 o'clock position, the seventh welding point at the 9 o'clock position; the fourth welding point between 3 o'clock and 6 o'clock, and the eighth welding point between 9 o'clock and 12 o'clock.

[0099] Complete the first pass of tack welding.

[0100] The accelerating voltage for the first pass of tack welding is 40KV, the focusing current is 2000mA, the electron beam current is 60mA, and the welding speed is 10mm / s.

[0101] Table 3 Welding parameters for each embodiment of the first pass tack weld

[0102]

[0103] II. Second pass, sealing weld.

[0104] During the sealing welding, keep the position of the welding machine head fixed, and rotate the first positioning welding point of the cylinder to directly below the welding machine head through the welding machine center frame; adjust the vertical height of the welding machine head and the working distance of the first positioning welding point to 400mm; turn on the welding machine switch, and weld clockwise along the circumference of the cross-section of the part of the cylinder to be welded, with a welding width of 5mm, to complete the sealing welding.

[0105] The accelerating voltage for the second pass of the sealing weld was 43KV, the focusing current was 2000mA, the electron beam current was 78mA, and the welding speed was 15mm / s.

[0106] Table 4 Welding parameters for various sealing examples

[0107]

[0108] III. Third pass, full welding in one pass.

[0109] During the first full weld, the position of the welding machine head remains fixed. Using the welding machine center frame, rotate the starting point of the cylinder sealing weld to directly below the welding machine head. Adjust the vertical height of the welding machine head and the working distance between the starting point and the second weld pass to 300mm. Turn on the switch and weld counter-clockwise around the circumference of the part of the cylinder to be welded, with a weld width of 8mm. The full weld is then complete.

[0110] The acceleration voltage for the third pass was 72KV, the focusing current was 2100mA, the electron beam current was 92mA, and the welding speed was 7mm / s.

[0111] Table 5 Welding parameters for each embodiment of single full weld

[0112]

[0113] IV. Fourth pass, second full weld.

[0114] During the second full weld, the position of the welding machine head remains fixed. Using the welding machine center frame, rotate the starting point of the first full weld on the cylinder to directly below the welding machine head. Adjust the vertical height of the welding machine head and the working distance between the starting point of the second pass and the starting point to 300mm. Turn on the switch and weld clockwise along the circumference of the part of the cylinder to be welded, with a weld width of 8mm. The full weld is then complete.

[0115] The fourth pass has an accelerating voltage of 80KV, a focusing current of 2200mA, an electron beam current of 100mA, and a welding speed of 5mm / s.

[0116] Table 6 Welding parameters for each embodiment of secondary full welding

[0117]

[0118] Step 4: Weld the end cap and cylinder together.

[0119] 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.

[0120] The obtained high-pressure vessel shell semi-finished product.

[0121] Step 5, vacuum quenching treatment:

[0122] 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:

[0123] 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 900°C at a heating rate of 28°C / min and held at that temperature for 1.5 hours.

[0124] 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.

[0125] 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.

[0126] The semi-finished high-pressure vessel shell is transferred to a cold box furnace at -60℃ within 1 minute for 70 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.

[0127] 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 300°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.

[0128] Immediately after the first tempering process is completed, the second tempering process shall be carried out.

[0129] 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.

[0130] Step 5, Machining:

[0131] According to design requirements, the heat-treated high-pressure vessel shell is processed into a finished high-pressure vessel shell through machining.

[0132] 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 vacuum electron beam welding of multiple parts and achieves a tensile strength of over 1900MPa through vacuum quenching heat treatment.

[0133] The high-pressure vessel shell made of 300M steel is manufactured using a vacuum electron beam welding method, maintaining a vacuum level of 5–30 × 10⁻⁶. -2 Within the specified Pa range, strict limits are placed on impurity erosion. Appropriate welding process parameters are employed, utilizing high welding speeds and large electron beam currents to prevent cracks and porosity in the shell welds due to increased heat input per unit area. The high-pressure vessel shell welding process requires controlling the accelerating voltage at 30–80 kV, the focusing current at 1500–2200 mA, the electron beam current at 40–100 mA, and the welding speed at 5–30 mm / s. Three different welding methods—positioning welding, sealing welding, and full welding—are employed in a multi-pass combination according to a specific sequence to prevent weld defects and prepare the vessel for achieving a pressure of 1900 MPa.

[0134] 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.

[0135] 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 above 1900MPa.

[0136] 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.

[0137] 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 vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel, characterized in that, The high-pressure vessel shell is made of 300M low-alloy ultra-high-strength steel. The specific process is as follows: Step 1, Pre-welding preparation: The pre-welding preparation includes forming a weld bevel at the welding location of the shell to be welded, sanding the welding location, and demagnetizing the shell to be welded; the magnetic flux of the shell to be welded after demagnetization is <1×10. -4 T; The shell to be welded includes a front end cap, a cylinder, and a rear end cap; Step 2, Assembly of the parts to be welded: The front end cap to be welded is assembled with the cylinder, and the cylinder is assembled with the rear end cap to obtain the assembled high-pressure vessel shell. Step 3, welding the end cap and cylinder: The specific process is as follows: The first step is to clamp the assembled high-pressure vessel shell; The second step is to preheat the welding areas of the assembled high-pressure vessel shell; the preheating temperature is 120~160℃; the heating rate is 50℃ / min. The third step is to evacuate the welding workspace until the vacuum level reaches 8×10⁻⁶. -2 Stop vacuuming after Pa; begin welding. During welding, the number of welding passes is determined based on the wall thickness h. When the wall thickness of the high-pressure vessel shell is 1 < h ≤ 2.5 mm, the number of welding passes is 2, including one tack weld and one full weld. When the wall thickness of the high-pressure vessel shell is 2.5 < h ≤ 4.0 mm, the number of welding passes is 3, including one tack weld, one seal weld, and one full weld. When the wall thickness of the high-pressure vessel shell is 4.0 < h ≤ 5.0 mm, the number of welding passes is 4, including one tack weld, one seal weld, and two full welds. Welding is performed by rotating the workpiece and keeping the welding machine head vertically downward; the welding of the front end cap and the cylinder is completed through 2, 3, or 4 welding passes. The two-pass welding consists of a tack weld and a full weld; the three-pass welding consists of a tack weld, a seal weld, and a full weld; and the four-pass welding consists of a tack weld, a seal weld, and two full welds. The specific process of the tack welding is as follows: Select the positioning welding point position; the positioning welding point is the 12 o'clock position of the part of the cylinder to be welded as the first welding point, and one point is determined every 45° clockwise. A total of 8 positioning welding point positions are determined around the cross-section of the part of the cylinder to be welded, which are the first welding point to the eighth welding point in sequence. During welding, a clockwise symmetrical point method was used to complete the tack welding of 4 groups of 8 welding points; Specifically: Starting from the first welding point located at the 12 o'clock position, move the welding machine head to a distance of 400-500 mm from the first welding point; turn on the switch to allow the electron beam to weld the first welding point in a vertically downward direction; the welding time is 1-5 seconds, and after forming a positioning welding point with a diameter of 2-5 mm, turn off the welding machine; rotate the welding machine 180° to the fifth welding point symmetrical to the first welding point, turn on the welding machine switch, and perform welding on the fifth welding point according to the welding method of the first welding point; The rotary welding machine sequentially completes the welding of the remaining six welding points in pairs; the order of the six welding points is: the second welding point between the midpoint of 12 o'clock and 3 o'clock, the sixth welding point between 6 o'clock and 9 o'clock; the third welding point at the 3 o'clock position, the seventh welding point at the 9 o'clock position; the fourth welding point between 3 o'clock and 6 o'clock, and the eighth welding point between 9 o'clock and 12 o'clock; The specific process of the first full soldering is as follows: Keeping the welding machine head in a fixed position, rotate the first positioning welding point of the cylinder to directly below the welding machine head again using the welding machine center frame; adjust the vertical height of the welding machine head and the working distance between the starting welding point of the second pass to 300-500 mm; turn on the switch and weld counterclockwise along the circumference of the part of the cylinder to be welded, with a welding width of 6 mm; complete the full weld. The specific process of the second full welding is as follows: During the second full weld, the position of the welding machine head remains fixed. The starting position of the first full weld on the cylinder is rotated again to directly below the welding machine head using the welding machine center frame. The vertical height of the welding machine head and the working distance between the starting weld point of the second sealing weld are adjusted to 300-500 mm. The switch is turned on, and welding is performed clockwise along the circumference of the part of the cylinder to be welded, with a welding width of 3-10 mm. The full weld is then complete. The accelerating voltage for the secondary full welding is 60~80KV, the focusing current is 2000~2200mA, the electron beam current is 80~100mA, and the welding speed is 5~15mm / s; 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 obtained 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 vacuum quenching treatment process consists of vacuum quenching, cold treatment, and secondary tempering treatment in sequence; the specific process of vacuum quenching is as follows: 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; then gas quenching begins, reducing the temperature inside the vacuum furnace to 0-30°C over 3 hours to complete the gas quenching; finally, the vessel is removed from the furnace. Step Six, Machining: According to the design requirements, the high-pressure vessel shell, which has undergone vacuum quenching treatment, is machined into a finished high-pressure vessel shell. Thus, the vacuum electron beam welding of the 1900MPa high-pressure vessel shell was completed.

2. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, During the assembly of welded parts, the gap between adjacent welded parts shall be less than 0.2 mm; the straightness of the housing after assembly shall be less than 1 mm / m. During assembly, tooling is used to ensure that the welding bevels between the front end cap and the cylinder, and between the cylinder and the rear end cap, are coaxially aligned; the misalignment of this alignment is less than 0.1mm.

3. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, The positions of the first to the eighth welding points on the cylinder are, respectively, the 12 o'clock position, the midpoint between the 12 o'clock and 3 o'clock position, the 3 o'clock position, the midpoint between the 3 o'clock and 6 o'clock position, the 6 o'clock position, the midpoint between the 6 o'clock and 9 o'clock position, the 9 o'clock position, and the midpoint between the 9 o'clock and 12 o'clock position.

4. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, The specific process of the sealing weld is as follows: Keep the welding machine head in a fixed position, and rotate the first positioning welding point of the cylinder to directly below the welding machine head using the welding machine center frame; adjust the vertical height of the welding machine head and the working distance between the first positioning welding point to 400-500mm; turn on the welding machine switch, and weld clockwise along the circumference of the cross-section of the part of the cylinder to be welded, with a welding width of 3mm, to complete the sealing weld.

5. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, The acceleration voltage for the tack welding is 30-40KV, the focusing current is 1500-2000mA, the electron beam current is 40-60mA, and the welding speed is 10-30mm / s; the acceleration voltage for the sealing welding is 30-50KV, the focusing current is 1500-2000mA, the electron beam current is 60-80mA, and the welding speed is 10-30mm / s; the acceleration voltage for the first full weld is 30-80KV, the focusing current is 1500-2000mA, the electron beam current is 40-100mA, and the welding speed is 5-30mm / s; the acceleration voltage for the second full weld is 60-80KV, the focusing current is 2000-2200mA, the electron beam current is 80-100mA, and the welding speed is 5-15mm / s.

6. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, The cold treatment involves transferring the semi-finished high-pressure vessel shell, which has undergone vacuum quenching, to a cold box furnace at -40°C within 1 minute of exiting the furnace for 60 minutes; then exiting the furnace.

7. The vacuum electron beam welding method for the shell of a 1900MPa high-pressure vessel as described in claim 1, characterized in that, The specific process of secondary tempering is as follows: First tempering treatment: The high-pressure vessel shell semi-finished product is heated to 290℃ at a heating rate of 20℃ / min and held at this temperature for 60min; after the holding time is completed, it is removed from the furnace; the first tempering treatment is completed. Immediately after the first tempering process is completed, the second tempering process shall be carried out. Second tempering: Repeat the process of the first tempering treatment to complete the second tempering treatment of the high-pressure vessel shell semi-finished product.

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