A heat treatable 800mpa grade pressure vessel electrode

Abstract

The application discloses a heat-treatable 800MPa-grade pressure container electrode, which is composed of an electrode core and a coating, the electrode core is a low-alloy carbon steel core wire, the coating is coated on the outer wall of the electrode core, the weld metal is jointly transitioned by the core wire and the coating, and a low-C, low-Si, medium-Mn-Ni-Mo, Cu-containing, micro-Ti and V alloy system is adopted. The weld metal of the electrode still has high strength and high toughness after being in a welded state and being heat-treated at 580+ / -20 DEG C for 2-8 hours, and the weld metal in the welded state and the heat-treated state both meet the requirements of yield strength being greater than or equal to 690MPa, tensile strength being greater than or equal to 800MPa and impact at-60 DEG C being greater than or equal to 80J, and can be used for the construction of 800MPa-grade ultra-high-strength pressure containers.

Description

Technical Field

[0001] This invention belongs to the field of welding materials, and specifically relates to a heat-treatable welding electrode for 800MPa pressure vessels. Background Technology

[0002] The research and development of low-alloy high-strength steel (HSLA) is of paramount importance in the history of materials science. HSLA steel was developed to meet the needs of reducing structural weight, improving reliability, and conserving steel in large engineering components. In today's era of energy shortages and resource scarcity, HSLA steel, as a high-efficiency material, has demonstrated strong development potential due to its lower cost and excellent performance. It has been widely used in offshore platforms, ships, bridges, machinery, and storage tanks. The strength levels of HSLA steel are also continuously improving, with the market usage of 800MPa series high-strength steel plates gradually increasing.

[0003] In recent years, with the rapid development of my country's energy, petroleum, and chemical industries, the production of high-strength, high-toughness pressure vessel steel has made significant progress. The storage of petrochemical, hydrogen energy, and related products is crucial to national energy security and plays an irreplaceable role. my country's storage tanks are increasingly trending towards larger sizes. Unlike high-strength bridges and steel structures, pressure vessels are sealed containers capable of withstanding pressure. Due to the various complex conditions they must withstand during use, pressure vessels place extremely high demands on safety and reliability. Therefore, the reliability of welded joints is also extremely important, and the quality of the welding materials must be guaranteed. For pressure vessel welding, using materials with a strength level of 800MPa significantly increases the strength, toughness, and welding difficulty, making the safety and reliability of the welded structure a major challenge.

[0004] Large pressure vessels and storage tanks, due to their large size, thick walls, and high restraint stress, often require high heat input welding during manufacturing to improve efficiency, leading to even greater residual stress and compromising the safety and reliability of the welded structure during service. Furthermore, while quenched and tempered steels of 800MPa and above have low carbon equivalents and excellent overall performance, they still exhibit a significant tendency to harden, which must be eliminated and mitigated through post-weld heat treatment. Studies show that the heat treatment time for weld metals of low-alloy high-strength steels needs to be extended with increasing plate thickness. Prolonged holding time during high-temperature tempering at 450℃-650℃ easily causes high-temperature tempering brittleness. This is mainly because prolonged holding time is accompanied by the growth and coarsening of the metallographic structure, precipitate growth, and compositional segregation, ultimately leading to a deterioration in the mechanical properties of the weld metal. The higher the strength grade, the more significant the deterioration in the toughness of the weld metal after heat treatment. Currently, some heat-treatable welding materials are available domestically, with heat treatment typically at 500℃-650℃ for 1-2 hours. However, the strength of these materials is relatively low (generally below 650MPa), and there are no matching welding electrodes available for heat-treatable 800MPa grade steel.

[0005] Furthermore, high-strength steel also presents technical challenges such as a high yield-to-tensile strength ratio (above 0.90), which has long hindered the design and construction of high-strength pressure vessels. As strength increases, the yield-to-tensile strength ratio of the weld metal continuously rises. A high yield-to-tensile strength ratio is detrimental to the safety and reliability of welded structures. When the yield strength reaches 690 MPa and the tensile strength exceeds 800 MPa, the yield-to-tensile strength ratio reaches as high as 0.90-0.95, severely restricting the design and application of steel for 800 MPa-level pressure vessels. Moreover, its toughness decreases sharply after heat treatment, and this deterioration becomes more pronounced with increasing plate thickness and longer heat treatment time. Therefore, there is an urgent need to develop heat-treatable, high-toughness welding materials with a low yield-to-tensile strength ratio to meet the construction requirements of 800 MPa-level high-strength pressure vessels.

[0006] Therefore, the design and development of matching welding materials for heat-treatable 800MPa pressure vessel steel is of great significance to meet market demand, reduce manufacturing costs and improve construction efficiency. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a heat-treatable welding electrode for 800MPa pressure vessels. It employs a CaO-MgO-BaF2-SiO2-Zr2O3 slag system, a low-alloy carbon steel core, and a low-C, low-Si, medium-Mn-Ni-Mo, Cu-containing, and micro-Ti, V alloy system. The coating has a weight coefficient of 0.45-0.65 of the total electrode weight, with alloying elements transitioning between the core and coating. It exhibits excellent all-position welding performance. The weld metal retains high strength and toughness in both the as-welded state and after heat treatment at 580±20℃ for 2-8 hours. The yield strength ratio of the weld metal is between 0.82 and 0.88. Both the as-welded and heat-treated weld metals meet the following requirements: yield strength ≥690MPa, tensile strength ≥800MPa, and impact strength ≥80J at -60℃. It is compatible with 800MPa ultra-high strength pressure vessels.

[0008] To achieve the above technical objectives, the technical solution of the present invention is: a heat-treatable welding electrode for 800MPa pressure vessels, consisting of a core and a coating, wherein the coating is applied to the outer wall of the core and accounts for 0.45-0.65% of the total weight of the electrode; the core is an ultra-low carbon low alloy carbon steel core; and the coating adopts a CaO-MgO-BaF2-SiO2-Zr2O3 slag system; the alloying elements of the weld metal of the electrode are transitioned by both the core and the coating.

[0009] (a) Based on the total weight of the welding core, the composition of the low-alloy carbon steel welding core, expressed as a percentage by weight, is as follows:

[0010] C: ≤0.04%;

[0011] Si: ≤0.10%;

[0012] Mn: 0.35-0.6%;

[0013] P: ≤0.008%;

[0014] S: ≤0.008%;

[0015] P+S: ≤0.015%;

[0016] Ni: 3.0-3.75%;

[0017] V: 0.08-0.26%;

[0018] Ti: 0.015-0.050%;

[0019] Fe: Balance;

[0020] (b) Based on the total weight of the drug coating, the composition of the drug coating, expressed as a percentage by weight, is as follows:

[0021] Dolomite: 26-40%;

[0022] Barium fluoride: 8-15%;

[0023] Cryolite: 2-4%;

[0024] Yttrium-based rare earth fluorides: 1-3%;

[0025] Quartz: 6-12%;

[0026] Zirconium sand: 3-8%;

[0027] Manganese metal: 5-8%;

[0028] Rare earth ferrosilicon: 2-5%;

[0029] Nickel powder: 3-5%;

[0030] Ferromolybdenum; 1.2-2.6%;

[0031] Copper powder: 0.4-1.5%;

[0032] Talc: 0.5-1.0%;

[0033] Alginate: 0.5-2.5%;

[0034] Iron powder: Balance.

[0035] The composition of the weld metal of the heat-treatable 800MPa pressure vessel welding electrode, by weight percentage, is as follows:

[0036] C: 0.030-0.060%;

[0037] Si: 0.10-0.25%;

[0038] Mn: 0.80-1.50%;

[0039] P: ≤0.008%;

[0040] S: ≤0.005%;

[0041] P+S: ≤0.012%;

[0042] Ni: 3.0-5.5%;

[0043] Mo: 0.3-0.6%;

[0044] Cu: 0.15-0.75%;

[0045] Ti: 0.008-0.030%;

[0046] V: 0.05-0.15%;

[0047] Fe: Balance.

[0048] The weld metal of the heat-treatable 800MPa pressure vessel welding electrode also has the following characteristics:

[0049] (Ni%+0.83×Cu%) / (C%)≥70;

[0050] Mn% + Mo% ≤ 1.8%;

[0051] (1.57×C 0.5 %+7.86×Si%+23.23×P%+8.17×S%)×(1.21×Mn%+2.65×Mo%)≤5.25.

[0052] The weld metal yield strength ratio of the heat-treatable 800MPa pressure vessel welding electrode is between 0.82 and 0.88.

[0053] This invention also provides a method for preparing the heat-treatable 800MPa grade welding electrode for pressure vessels, comprising the following steps:

[0054] 1) Mix all components of the medicine coating evenly according to the proportions; among them, the silicate minerals, including quartz, zircon sand and talc powder, are first dried at 800-850℃;

[0055] 2) Add 15-30% of the total weight of potassium silicate and sodium silicate mixed water glass, stir and mix evenly, and then use a hydraulic coating machine to evenly coat the coating powder onto the welding core.

[0056] 3) After the welding rod is coated, it is baked at low temperature and high temperature respectively. The low temperature baking temperature is 60-100℃×2h, and the high temperature baking temperature is 300-400℃×1h to obtain heat-treatable welding rods for pressure vessels of grade 800MPa.

[0057] The potassium silicate and sodium silicate mixed water glass has a modulus of 2.6-3.2 and a concentration of 38-43Be′.

[0058] This invention uses a low-alloy carbon steel core, and the weld metal is transitioned by both the core and the flux coating. On the one hand, compared with a pure flux coating transition, it has better chemical and structural uniformity during welding, thus providing a more fundamental guarantee for the joint quality; it also avoids excessive alloy addition in the flux coating, resulting in an overly thick flux coating that affects the controllability of all-position welding; on the other hand, some trace key alloying elements are supplemented by the flux coating, which has more significant economic benefits than smelting them separately in a furnace.

[0059] The key to the design of this invention's welding electrode is its weld metal alloy system, which consists of low C, low Si, medium Mn-Ni-Mo, Cu, trace Ti, and V. The low C and Si composition reduces the formation of metamorphic oxide (MA) islands in HLSA steel during tempering, significantly improving the low-temperature toughness of the tempered state. Furthermore, the low C content prevents excessive precipitation of brittle carbides during heat treatment. While Mn, Cr, and Mo significantly lower the black-white point (BS point) of HLSA steel, promote MA island formation, and prevent MA island decomposition during tempering (which damages weld impact), the alloy cementite formed by these elements and C also enhances resistance to tempering softening. Therefore, to balance the strength and toughness of the weld metal, this invention employs a medium Mn, Cr-free, and low Mo composition. Trace amounts of V and Ti refine the grain, and together with trace amounts of C, they enhance weld strength through solid solution strengthening. They also refine the grain, improve the temperability of the steel during tempering, and enhance the impact toughness of the weld after tempering. A high proportion of Ni is the key to ensuring the tempering toughness of HSLA steel. Through repeated and extensive experiments, it has been found that a higher proportion of Ni combined with a small amount of Cu can significantly improve the impact toughness of HSLA steel after tempering.

[0060] Furthermore, the design and control of the weld metal alloy composition are key to this invention and must meet the following requirements:

[0061] (Ni%+0.83×Cu%) / (C%)≥70;

[0062] Mn% + Mo% ≤ 1.8%;

[0063] (1.57×C 0.5 %+7.86×Si%+23.23×P%+8.17×S%)×(1.21×Mn%+2.65×Mo%)≤5.25.

[0064] The rationale for the above ingredient design is as follows:

[0065] ① It reduces the degree of conjugate segregation of elements such as C, Mn, Mo, P, and S in the weld during solidification, inhibits the formation of high-C martensite during welding and heat treatment, and improves the low-temperature toughness of the weld.

[0066] ②Increase the critical phase transformation cooling rate of martensite to promote the formation of low-carbon tempered martensite, so that the weld can obtain good tempering toughness.

[0067] ③ Reduce the number and size of the heat-affected zone and MA islands during the heat treatment process under the action of welding thermal cycle, and improve the impact toughness of the heat-affected zone and weld after heat treatment.

[0068] The weld metal of this invention still has high strength and high toughness after being in the welded state and after heat treatment at 580±20℃ for 2-8 hours. Both the weld metal in the welded state and the heat-treated state meet the following requirements: yield strength ≥690MPa, tensile strength ≥800MPa, and impact strength ≥80J at -60℃.

[0069] The weld metal of the electrode of the present invention has a low yield strength ratio (<0.9), which is between 0.82 and 0.88.

[0070] In this invention, the electrode coating primarily functions to generate gas, form slag, deoxidize, and transfer alloys to the weld. The coating design employs a CaO-MgO-BaF2-SiO2-Zr2O3 slag system with moderate basicity and the addition of 9-20% acidic oxides, resulting in excellent all-position weldability. Controlling the oxygen content of the weld metal to 400-500 ppm ensures sufficient oxide nucleation sites, refines grains, and improves weld microstructure and toughness. Small amounts of rare earth fluorides and rare earth alloys further purify and refine the weld, refine grains, and improve weld microstructure.

[0071] The following is a detailed analysis of the roles played by the main components of the coating in the welding electrode.

[0072] The main functions of carbonates in welding electrodes are slag formation and gas generation. The CaO and MgO alkaline oxides produced by decomposition can increase the basicity of the slag, refine the molten droplets, and remove sulfur and phosphorus, thereby improving the crack resistance of the weld metal. They also regulate the melting point, viscosity, surface tension, and cross-sectional tension of the slag. In this invention, dolomite is used as the carbonate, with its content controlled at 26-40%.

[0073] In this invention, the fluorides used are BaF2, cryolite, and yttrium-based rare earth fluorides. These fluorides can adjust the melting point of the slag, playing a crucial role in reducing weld porosity, improving the physical properties of the slag, and enhancing weld formation and slag removal. Fluorides can also reduce the surface tension of liquid metal, adjusting the viscosity of the molten slag and improving its coverage, resulting in a more aesthetically pleasing weld and reducing porosity defects. The fluoride content in the welding electrodes of this invention is 11-22%.

[0074] Silicon oxides obtained from quartz, zirconium sand, and sodium potassium silicate water glass can adjust the viscosity of molten weld slag, resulting in good slag coverage and improving weld appearance and shape. However, an excessively high proportion of silicon oxides will lead to an excessively high oxygen content in the weld, thereby reducing the mechanical properties of the weld, especially low-temperature impact toughness. Therefore, the proportion of silicon oxide components in the flux coating is controlled between 9-20%.

[0075] The main functions of ferroalloys and other metal powders are deoxidation and transition alloying, ensuring the alloy element composition in the weld, balancing weld strength, crack resistance, fatigue resistance, and impact toughness, and achieving optimal strength and toughness matching in the heat-treated state through reasonable element design. Rare earth ferrosilicon and yttrium-based rare earth fluorides, in addition to deoxidation, also transition rare earth elements into the weld, thus purifying and removing impurities, and also have a certain dehydrogenation effect, but the cost is very high. In this invention, the content of yttrium-based rare earth fluorides and rare earth alloys is controlled at 3-8%.

[0076] The binder uses a mixture of potassium silicate and sodium silicate water glass, combined with alginate and talc to improve the coating properties of the welding rod. In addition to coating and binding the welding rod, the water glass also plays a role in slag formation, adjusting the state of the molten slag, and stabilizing the arc.

[0077] The above explains the limitations on the composition of the coating material in this invention. The residue consists of iron and unavoidable impurities.

[0078] This invention employs a CaO-MgO-BaF2-SiO2-Zr2O3 slag system, a low-alloy carbon steel core, and a low-C, low-Si, medium-Mn-Ni-Mo, Cu-containing, and micro-Ti, V alloy system. The electrode alloy composition is transitioned jointly by the core and coating, allowing for more precise composition control and greater economic benefits compared to pure core wire transitions. This enables the heat-treatable 800MPa pressure vessel welding electrode of this invention to possess excellent all-position welding performance. The weld metal retains high strength and high toughness in both the as-welded state and after heat treatment at 580±20℃ for 2-8 hours. Both the as-welded and heat-treated weld metals meet the following requirements: yield strength ≥690MPa, tensile strength ≥800MPa, and impact strength ≥80J at -60℃. This makes it suitable for the construction of 800MPa-class ultra-high strength pressure vessels. Detailed Implementation

[0079] The technical solutions of this application will be further described below with reference to specific embodiments, but this application is not limited to these embodiments.

[0080] This invention consists of a welding core and a flux coating. The flux coating is applied to the outside of the welding core, which is made of low-alloy carbon steel. The composition (weight percentage %) of the welding core is shown in the table below:

[0081] Table 1: Components of the solder core of this invention (weight percentage %)

[0082]

[0083] To better understand the present invention, the following examples 1-5 are provided for further illustration. The composition of the solder core in examples 1-5 is shown in Table 2.

[0084] Table 2: Core Composition of Examples (Weight Percentage %)

[0085]

[0086]

[0087] The coating uses a CaO-MgO-BaF2-SiO2-Zr2O3 slag system, and the coating accounts for 0.45-0.65% of the total weight of the electrode. The core wire diameter is 2.6mm, 3.2mm, and 4.0mm. Examples of the coating composition are shown in Table 3.

[0088] Table 3. Composition of the drug coating in the examples (weight percentage %)

[0089]

[0090] The chemical composition of the weld metal in each embodiment is shown in Table 4.

[0091] Table 4. Weld metal composition (weight percentage %) of the examples

[0092]

[0093] Note: 1. Coefficient A = (Ni% + 0.83 × Cu%) / C% ≥ 70;

[0094] 2. Coefficient B = 1.57 × C 0.5 (% + 7.86 × Si% + 23.23 × P% + 8.17 × S%)

[0095] (1.21×Mn%+2.65×Mo%)≤5.25;

[0096] 3. C coefficient = Mn% + Mo% ≤ 1.80%

[0097] Table 5 shows the test results of mechanical properties, low-temperature impact, and yield strength ratio of weld metal in the as-welded and heat-treated states for each embodiment.

[0098] Table 5 Mechanical properties of weld metals from examples

[0099]

[0100]

[0101] As can be seen from the above embodiments, the welding electrode of the present invention has excellent weldability, exhibiting high strength, a low yield strength ratio, and high low-temperature impact toughness in both the welded and heat-treated states. Its core wire uses low-alloy carbon steel, with alloying elements transitioning between the core wire and the coating. The weld metal adopts a low-C, low-Si, medium-Mn-Ni-Mo, Cu-containing, micro-Ti, and V alloy system. Compared to similar 800MPa grade steel welding materials, its weld metal has a lower yield strength ratio of 0.82-0.88 (<0.9). Even after welded conditions and heat treatment at 580±20℃ for 2-8 hours, the weld metal still possesses high strength and high toughness. Both the weld metal in the welded and heat-treated states meet the following requirements: yield strength ≥690MPa, tensile strength ≥800MPa, and impact strength ≥80J at -60℃, thus meeting the construction requirements for 800MPa grade pressure vessels.

[0102] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept of this application, and these all fall within the protection scope of this application.

Claims

1. A heat-treatable welding electrode for 800MPa pressure vessels, comprising a core and a coating, wherein the coating is applied to the outer wall of the core and comprises 0.45-0.65% of the total weight of the electrode, characterized in that... The core electrode is made of ultra-low carbon low alloy carbon steel, and the flux coating adopts the CaO-MgO-BaF2-SiO2-Zr2O3 slag system. The alloy composition of the weld metal of the electrode is transitioned by both the core electrode and the flux coating. (a) Based on the total weight of the core, the composition of the core is as follows by weight percentage: C: ≤0.04%, Si: ≤0.10%, Mn: 0.35-0.60%, P: ≤0.008%, S: ≤0.008%, P+S: ≤0.015%, Ni: 3.0-3.75%, V: 0.08-0.26%, Ti: 0.015-0.050%, Fe: balance; (b) Based on the total weight of the drug coating, the composition of the drug coating by weight percentage is as follows: dolomite: 26-40%, barium fluoride: 8-15%, cryolite: 2-4%, yttrium-based rare earth fluoride: 1-3%, quartz: 6-12%, zircon sand: 3-8%, metallic manganese: 5-8%, rare earth ferrosilicon: 2-5%, nickel powder: 3-5%, ferromolybdenum: 1.2-2.6%, copper powder: 0.4-1.5%, talc: 0.5-1.0%, alginate: 0.5-2.5%, iron powder: balance.

2. The heat-treatable welding electrode for 800MPa pressure vessels according to claim 1, characterized in that, The weld metal composition of the electrode, by weight percentage, is as follows: C: 0.030-0.060%, Si: 0.10-0.25%, Mn: 0.80-1.50%, P: ≤0.008%, S: ≤0.005%, P+S: ≤0.012%, Ni: 3.0-5.5%, Mo: 0.3-0.6%, Cu: 0.15-0.75%, Ti: 0.008-0.030%, V: 0.05-0.15%, Fe: balance.

3. The heat-treatable welding electrode for 800MPa pressure vessels according to any one of claims 1-2, characterized in that, The weld metal of the heat-treatable 800MPa pressure vessel welding electrode also has the following characteristics: (Ni%+0.83×Cu%) / (C%)≥70; Mn% + Mo% ≤ 1.80%; (1.57×C 0.5 %+7.86×Si%+23.23×P%+8.17×S%)×(1.21×Mn%+2.65×Mo%)≤5.25。 4. The heat-treatable welding electrode for 800MPa pressure vessels according to any one of claims 1-2, characterized in that, The weld metal yield strength ratio of the welding electrode is between 0.82 and 0.

88.

5. The method for preparing heat-treatable welding electrodes for 800MPa pressure vessels as described in claim 1, characterized in that, The preparation method includes the following steps: 1) Mix all components of the medicine coating evenly according to the proportions; among them, the silicate minerals, including quartz, zircon sand and talc powder, are first dried at 800-850℃; 2) Add 15-30% of the total weight of potassium silicate and sodium silicate mixed water glass, stir and mix evenly, and then use a hydraulic coating machine to evenly coat the coating powder onto the welding core. 3) After the welding electrode coating is completed, it is baked at low temperature and high temperature respectively. The low temperature baking temperature is 60-100℃×2h, and the high temperature baking temperature is 300-400℃×1h to obtain the heat-treatable 800MPa grade pressure vessel welding electrode.

6. The method for preparing heat-treatable welding electrodes for 800MPa pressure vessels according to claim 5, characterized in that, The potassium silicate and sodium silicate mixed water glass has a modulus of 2.6-3.2 and a concentration of 38-43Be′.

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