High-temperature hydrogen-present low-alloy steel forge piece and preparation process thereof

Abstract

The invention belongs to the technical field of metal forging of general mechanical equipment, and particularly relates to a high-temperature hydrogen low-alloy steel forge piece and a preparation process thereof. The steel comprises the following chemical components in percentage by mass: 0.08-0.15% of C, less than or equal to 0.10% of Si, 0.30-0.60% of Mn, 2.00-2.50% of Cr, 0.90-1.10% of Mo, 0.25-0.35% of V, less than or equal to 0.07% of Nb, less than or equal to 0.03% of Ti, less than or equal to 0.002% of B, less than or equal to 0.008% of P, less than or equal to 0.002% of S and the balance of Fe. By adding alloy elements such as chromium, molybdenum and vanadium and strictly controlling the content of sulfur and phosphorus, the properties such as high-temperature strength, hydrogen corrosion resistance, tempering embrittlement resistance and hydrogen-induced stripping crack resistance of the steel are improved. The preparation process comprises the steps of electric furnace smelting, dephosphorization, ladle refining, vacuum degassing, forging, normalizing, tempering and other heat treatment modes, so that the performance of the steel is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal forging for general mechanical equipment, and in particular relates to a high-temperature hydrogen-exposed low-alloy steel forging and a preparation process thereof. Background Art

[0002] Hydrogen damage in conventional non-alloy and low-alloy steels in high-temperature hydrogen environments is a complex and multifaceted phenomenon. In addition to surface decarburization and hydrogen embrittlement, the diffusion and accumulation of hydrogen within the steel can trigger a series of more serious microstructural changes and degradation of mechanical properties.

[0003] First, in a high-temperature, high-pressure hydrogen environment, hydrogen atoms have extremely strong permeability and can diffuse rapidly into the interior of the steel. These hydrogen atoms react with carbides (such as cementite Fe3C) in the steel to generate methane gas. Because methane molecules are large and cannot diffuse freely like hydrogen atoms, they accumulate at grain boundaries, inclusions, or microscopic defects to form local high-pressure areas. The accumulation of this high-pressure methane gas will produce microcracks inside the steel, which will gradually expand into macrocracks, seriously weakening the material's bearing capacity.

[0004] Secondly, hydrogen damage can also exacerbate the steel's tendency to brittle fracture. Hydrogen atoms gather in areas of stress concentration (such as crack tips), reducing the bonding strength between metal atoms and causing the material to fracture brittlely at stresses below the normal fracture stress. This phenomenon is called hydrogen-induced cracking (HIC). In addition, under cyclic loads or alternating stresses, the diffusion and accumulation of hydrogen accelerate the initiation and propagation of fatigue cracks, causing the steel to fail under conditions far below the conventional fatigue limit. This phenomenon is called hydrogen-induced fatigue (HEF).

[0005] In addition to the aforementioned mechanisms, high-temperature hydrogen environments can also induce stress corrosion cracking (SCC). When steel is simultaneously exposed to hydrogen, water, and stress, hydrogen atoms promote corrosion reactions and form cracks in areas of stress concentration. These cracks often propagate along grain boundaries, causing the material to suddenly break without significant plastic deformation, which is extremely harmful.

[0006] In practical industrial applications, such as petrochemicals, oil refining, and ammonia synthesis, hydrogen damage is a particularly prominent problem for ordinary carbon steel and low-alloy steel in high-temperature, high-pressure hydrogen environments. To alleviate this problem, the following measures are usually taken: selecting hydrogen-resistant steels (such as chromium-molybdenum steels), adding trace alloying elements (such as vanadium, titanium, and niobium to form stable carbides), performing heat treatment to optimize the microstructure, or using surface coating technologies (such as plating or aluminizing) to block hydrogen penetration. However, even with these measures, hydrogen damage remains one of the main causes of failure of these materials under extreme conditions, so special attention must be paid to design and material selection. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a high-temperature hydrogen-resistant low-alloy steel forging, 12Cr2Mo1V(H), which improves the steel's high-temperature strength, resistance to hydrogen corrosion, resistance to temper embrittlement, and resistance to hydrogen-induced delamination cracking by adding alloying elements such as chromium, molybdenum, and vanadium and strictly controlling its sulfur and phosphorus content.

[0008] The invention also provides a preparation process thereof, which includes electric furnace smelting, dephosphorization, ladle refining, vacuum degassing, and further heat treatment such as forging, normalizing, and tempering to improve its performance.

[0009] 12Cr2Mo1V(H) forgings are used in high temperature, high pressure, humid environments, etc. Its smelting process must strictly control the composition, purity and structural uniformity to ensure excellent high temperature strength and toughness.

[0010] The high-temperature hydrogen-exposed low-alloy steel forgings described in the present invention have the following chemical compositions by mass fraction: C: 0.08-0.15%, Si≤0.10%, Mn: 0.30-0.60%, Cr: 2.00-2.50%, Mo: 0.90-1.10%, V: 0.25-0.35%, Nb≤0.07%, Ti≤0.03%, B≤0.002%, P≤0.008%, S≤0.002%, and the balance is Fe.

[0011] The process for preparing high-temperature hydrogen-exposed low-alloy steel forgings of the present invention comprises the following steps:

[0012] (1) Ingot smelting: Steel ingots are prepared by electric arc furnace smelting, converter smelting, LF refining and VD vacuum degassing process;

[0013] (2) Billet heating: heating the pretreated steel ingot and keeping it warm;

[0014] (3) Billet forging: alternating upsetting and drawing;

[0015] (4) Heat treatment: normalizing and dynamic cycle tempering treatment.

[0016] The raw materials for smelting the steel ingot are high-quality scrap steel (low sulfur and phosphorus, S≤0.008%), pig iron (carbon content 0.20-0.30%) and molten iron, and alloying elements such as high-purity ferrochrome (Cr≥99.8%), ferromolybdenum (Mo≥60%), and ferrovanadium (V≥50%).

[0017] The raw materials are screened and pre-treated to remove impurities and large foreign objects, and the amount of alloying elements added is accurately measured.

[0018] The steel ingot smelting process comprises:

[0019] The electric arc furnace smelting process uses scrap steel, pig iron, and molten iron as the base charge, and adds slag-forming materials to smelt the steel into molten steel. The high-temperature arc generated by the electrodes melts the charge, effectively removing impurities such as sulfur and phosphorus. The slag-forming material is a mixture of calcium oxide, aluminum oxide, magnesium oxide, and silicon dioxide in a mass ratio of 55:24:8:13.

[0020] In the converter smelting process, molten steel is used as raw material, oxygen is blown into the furnace to oxidize carbon and other impurities, reduce carbon content, decarburize and dephosphorize, and adjust the alloy composition. The tapping temperature is not less than 1650°C, and slag blocking technology is used during tapping to reduce the introduction of inclusions.

[0021] During the LF refining step, high-purity alloying elements are added to adjust the composition, and sulfur and phosphorus impurities are removed by heating and bottom-blowing argon stirring. The composition is precisely controlled to 2.0-2.5% Cr, 0.90-1.10% Mo, and 0.25-0.35% V.

[0022] During the VD vacuum degassing step, the vacuum level and treatment time are controlled to reduce hydrogen and oxygen content, and alloying elements are added. Specifically, the vacuum level is controlled to ≤ 0.5 mbar (absolute pressure), the treatment time is ≥ 25 minutes, and the final hydrogen (H) content is ≤ 1.5 ppm, and the total oxygen (TO) content is ≤ 1.5 ppm. This removes hydrogen and other gases, reduces defects such as pores and cracks, and fine-tunes the composition. Ferrovanadium is added under vacuum, and the yield is ≥ 90%.

[0023] In the converter smelting step, the final carbon content is 0.08% to 0.15%, the dephosphorization rate is not less than 90%, and the phosphorus content does not exceed 0.008%.

[0024] In the LF refining step, the white slag basicity (CaO / SiO2) is not less than 3.0, and the final sulfur content does not exceed 0.002%.

[0025] In the VD vacuum degassing step, the vacuum degree is not higher than 0.5 mbar, the processing time is not less than 25 minutes, the final hydrogen content does not exceed 1.5 ppm, and the total oxygen content does not exceed 1.5 ppm.

[0026] Continuous casting is generally used. Refined molten steel is continuously poured into a mold, where it gradually solidifies into a shell. The shell is then pulled out through a straightening machine, cooled and cut to the desired length. For special requirements, die casting can also be used. The molten steel is poured into a mold of a specific shape, and after solidification, the ingot is removed from the mold to form the ingot.

[0027] As a high-temperature heat-resistant steel forging, the forging process of 12Cr2Mo1V(H) forgings needs to focus on controlling the heating temperature, deformation, cooling method and heat treatment process to refine the grains, uniform the structure and avoid crack defects.

[0028] Blank preparation: Remove impurities such as scale and oil from the blank surface, polish the blank surface, and eliminate surface defects such as burrs, flash, and cracks caused by blanking to prevent the expansion of defects during forging. If the blank is blanked from a steel ingot, sufficient removal should be made at the head and tail of the ingot to ensure that there are no defects such as shrinkage cavities, looseness, cracks, and severe segregation that affect forging quality.

[0029] The billet heating process includes: using a walking beam furnace or a chamber furnace to ensure temperature uniformity; an initial preheating temperature not exceeding 600°C to avoid thermal stress cracks; a heating stage: heating to 1050°C to 1100°C (homogenization temperature) at a heating rate not exceeding 100°C / h; and a holding time calculated according to the cross-sectional size of the billet (usually 1.5 to 2.0 minutes / mm) to ensure consistent core and surface temperatures.

[0030] During the billet forging process, the initial forging temperature does not exceed 1100°C to prevent overheating. High-precision forging equipment is used to forge the billet in an alternating process of upsetting and drawing to refine the grain size and eliminate anisotropy. The forging ratio of the main cross-section is no less than 3 to fully fragment the as-cast structure. Final forging is completed at a high strain rate to minimize grain growth. Complex forgings are completed in multiple firings, with intermediate reheating and holding at a temperature of ≥850°C.

[0031] Post-forging cooling: Small and medium-sized forgings use air cooling to avoid internal stress caused by excessive cooling. Large forgings use pit cooling or sand cooling and need to be cooled slowly (≤50℃ / h) to prevent white spots and cracks.

[0032] The heat treatment process includes:

[0033] Normalizing treatment, temperature is 930℃ to 960℃, keep warm (time is 1.5~2min / mm according to thickness) and then air cool, avoid strong wind blowing directly to cause large temperature difference, refine bainite + a small amount of ferrite, grain size ≥ level 5;

[0034] Dynamic cycle tempering treatment, including a first stage of 720°C holding for four hours followed by air cooling, and a second stage of 650°C holding for two hours followed by air cooling, significantly increases the strength and toughness of the forgings, significantly improves their hydrogen resistance, and doubles their high-temperature lifespan. After machining, the forgings are then tested for relevant properties.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The present invention significantly improves hydrogen resistance by optimizing alloy composition design. By strictly controlling the content range of key elements such as C, Cr, Mo, and V (C: 0.08-0.15%, Cr: 2.00-2.50%, Mo: 0.90-1.10%, V: 0.25-0.35%), and limiting P ≤ 0.008% and S ≤ 0.002%, hydrogen embrittlement and hydrogen-induced cracking are effectively suppressed, and the material stability in high-temperature hydrogen environments is improved.

[0037] (2) Innovative smelting technology achieves ultra-high purity, adopting electric arc furnace + converter + LF + VD four-stage refining. By strictly controlling the white slag basicity ≥3.0, vacuum degree ≤0.5mbar deep degassing, final hydrogen ≤1.5ppm, total oxygen ≤1.5ppm, and inclusion control (A+B+C+D+DS≤2.0 level), the problem of unqualified flaw detection caused by pores and inclusions in traditional processes is solved.

[0038] (3) Multi-directional alternating forging technology achieves breakthroughs in microstructure uniformity. Using alternating forging of upsetting and drawing (total forging ratio ≥ 3), with each pass rotating 90°, the grain size is refined to level 9 (Example 1), which is more than 40% higher than the conventional forging grain size (usually level 5-6) of the reference patent, significantly improving anisotropy.

[0039] (4) Dynamic cyclic tempering process achieves hydrogen resistance. Through two-stage tempering at 720°C × 4h + 650°C × 2h, a nano-scale Mo2C / VC dispersed precipitate phase is formed, locking hydrogen atoms in low-energy traps, and improving the hydrogen-induced cracking resistance (CLR ≤ 5%) by more than 3 times compared to the single tempering process (CLR = 18% in Comparative Example 1). DETAILED DESCRIPTION

[0040] The present invention will be further described below with reference to the examples.

[0041] Unless otherwise specified, all raw materials used in the examples are commercially available.

[0042] After heat treatment, 12Cr2Mo1V(H) forgings must undergo comprehensive testing to ensure that their structure, performance, and defect control meet standard requirements and meet safety requirements in high-temperature, high-pressure, or hydrogen-resistant environments. Depending on the grade of the forging, the grading standard for forgings is NB / T 47008-2017, and the inspection requirements will vary as shown in the following table:

[0043]

[0044] 1. Mechanical properties test: (1) The room temperature tensile test test standard is GB / T228.1-2021. The specimen is required to be sampled along the main deformation direction of the forging (usually longitudinal specimens). The test parameters for forgings with a nominal thickness of ≤300mm are yield strength (Rp0.2) 350-380MPa (because this material has no obvious yield phenomenon, Rp0.2 is used instead of ReL), tensile strength (Rm) 590-760MPa, elongation after fracture (A) ≥17%, and reduction of area (Z) ≥40%. The test parameters for forgings with a nominal thickness of >300-500mm are yield strength Rp0.2 value 340-370MPa, tensile strength (Rm) 580-750MPa, and elongation after fracture (A) ≥17%. (2) The high-temperature tensile test standard is GB / T228.2-2021, the sample temperature is 500℃ (simulating actual service conditions), and the test parameter is high-temperature yield strength (Rp0.2) 180-200MPa. (3) The impact toughness test standard is GB / T229-2020, the experimental temperature is -20℃, and three V-notch specimens are taken. The acceptance requirement is: The Charpy V-notch impact absorption energy value is the arithmetic mean of the test results of the three impact specimens. The impact absorption energy value of one specimen is allowed to be less than 60J, but it must not be less than 42J.

[0045] 2. Microstructure and Metallographic Analysis: (1) Grain size inspection standard is GB / T6394-2017, requiring a grain size ≥ 5 (fine grained structure, improved high temperature creep performance). (2) Microstructure inspection standard is GB / T13298-2015, no continuous network carbides, no banded segregation or Widmanstätten structure, the target structure is: bainite + a small amount of ferrite, carbides are dispersed. (3) Non-metallic inclusion inspection standard is GB / T10561-2023, A, B, C, D, DS are all ≤ 1.5, and A + C ≤ 1.0, B + D ≤ 1.0, A + B + C + D + DS ≤ 2.0.

[0046] 3. Ultrasonic testing: Standard number NB / T 47008-2017, inspection standards are shown in Table 1 below:

[0047] Table 1 Ultrasound examination standards

[0048]

[0049] 4. Hydrogen induced cracking (HIC) resistance test. The hydrogen cracking resistance test and assessment method are in accordance with GB / T8650-2015, using standard solution A. The test result grades are shown in Table 2 below:

[0050] Table 2 Hydrogen induced cracking resistance test results

[0051]

[0052] Example 1

[0053] Product name: tube sheet; specification: φ1788×208; material grade: 12Cr2Mo1V(H)Ⅲ; specimen: φ30×350 (quantity 2 pieces, required to be made from the same furnace batch number, forging ratio and heat treated in the same furnace as the tube sheet forgings).

[0054] The preparation process of the high-temperature hydrogen-exposed low-alloy steel forgings comprises the following steps:

[0055] 1. Ingot preparation:

[0056] (1) Ingot smelting: Steel ingots are prepared by electric arc furnace smelting, converter smelting, LF refining and VD vacuum degassing process:

[0057] The raw materials for the steel ingot smelting are high-quality scrap steel (low sulfur and phosphorus, S = 0.005%), pig iron (carbon content 0.25%), molten iron, and high-purity ferrochromium (Cr = 99.9%), ferromolybdenum (Mo = 65%), and ferrovanadium (V = 55%) alloying elements. The raw materials are screened and pre-treated to remove impurities and large foreign matter, and the amount of alloying elements added is precisely measured.

[0058] The steel ingot smelting process comprises:

[0059] The electric arc furnace smelting process uses scrap steel, pig iron, and molten iron as the base charge, and adds slag-forming materials to smelt into molten steel. A high-temperature arc generated by electrodes melts the charge at a temperature of 1625°C, effectively removing impurities such as sulfur and phosphorus. The slag-forming material is a mixture of calcium oxide, aluminum oxide, magnesium oxide, and silicon dioxide in a mass ratio of 55:24:8:13.

[0060] The converter smelting step uses molten steel as raw material, blows oxygen into the furnace to oxidize carbon and other impurities, reduce carbon content, and decarburize and dephosphorize. C = 0.10%, dephosphorization rate = 93% (P = 0.005%), and appropriate amounts of scrap steel and alloying elements are added to adjust the composition. The smelting temperature is 1650°C, and the tapping temperature is 1700°C. Slag blocking technology is used during tapping to reduce the introduction of inclusions.

[0061] In the LF refining step, the molten steel after primary refining is transferred to the ladle refining furnace, and alloy elements such as high-purity ferrochrome (Cr=99.8%), ferromolybdenum (Mo=65%), and ferrovanadium (V=55%) are added. Impurities such as sulfur and phosphorus are further removed through heating and bottom blowing argon stirring operations. The smelting temperature is 1590°C, and the composition is precisely controlled: Cr: 2.4%, Mo: 1.0%, V: 0.3%. Desulfurization: white slag basicity (CaO / SiO2) = 3.3, end point S = 0.002%.

[0062] VD vacuum degassing: The molten steel is placed in a vacuum environment with a vacuum degree of 0.45 mbar (absolute pressure) for 30 minutes, resulting in a final hydrogen (H) concentration of 1.2 ppm and a total oxygen (TO) concentration of 1.2 ppm. This removes hydrogen and other gases, reduces defects such as pores and cracks, and fine-tunes the composition. Ferrovanadium is added under vacuum, achieving a yield of 93%. The molten steel is poured into a 6.5-ton ingot mold using a die-casting method with an argon-sealed long nozzle to prevent secondary oxidation. After solidification, the ingot is demolded to obtain the ingot.

[0063] The chemical composition of the steel ingot of Example 1 is shown in Table 3 below:

[0064] Table 3 Chemical composition of steel ingot in Example 1

[0065]

[0066] The advantages of the steel ingots obtained by the above smelting method are: (1) Multi-stage dehydrogenation technology: LF pre-deoxidation + VD deep dehydrogenation, final hydrogen (H) = 1.2ppm, hydrogen embrittlement sensitivity reduced by 50%. (2) Precision alloying: Adding ferrovanadium under vacuum, the yield is 93%, avoiding oxidation loss, Cr (2.4%) is the core element of high-temperature oxidation resistance, forming a dense oxide film, reducing the oxidation rate, Mo (1.0%) is a solid solution strengthening matrix to improve creep resistance, V (0.3%) forms fine VC and VN particles, hindering grain boundary sliding at high temperatures, and trace Nb (0.04%) forms stable NbC, refining the grains, and improving the 500℃ endurance strength by 20%. (3) High cleanliness: white slag refining + protective casting, total oxygen = 1.2ppm, inclusion grade is 0.8. This steel ingot is evaluated for fine and coarse metal inclusions according to method B specified in GB / T10561. The sulfide type (A) is level 0.5, the alumina type (B) is level 0, the silicate type (C) is level 0, the fine and coarse spherical oxide type (D) are level 0, and the single spherical particle type (DS) is level 0.3. It meets the requirements of A, B, C, D, and DS types ≤1.5, and A+C≤1.0, B+D≤1.0, and A+B+C+D+DS≤2.0, and the metal inclusion level is high.

[0067] 2. Forging:

[0068] (2) Billet heating:

[0069] Blank preparation: φ1788×208 is the net size of the tube sheet. In order to ensure the net size of the tube sheet after turning, the blank size of the tube sheet is determined to be φ1820×230 based on empirical values. The calculated blank weight of the raw materials used for the tube sheet is 4930Kg. There must be enough cutting amount at the head and tail of the steel ingot to ensure that there are no defects such as shrinkage cavities, looseness, cracks, and severe segregation that affect the forging quality. Therefore, the raw material ingot shape is determined to be 6.5T steel ingot, fire consumption: 5%, blanking weight: 4930Kg, blanking size: 886×886×800.

[0070] Heating: The billet is placed into a chamber furnace using a discharge stage to support it. This ensures uniform billet temperature. Billet heating temperature: Initially, the furnace temperature is slowly raised to 580°C at a rate of 80°C / hour to allow the internal structure sufficient time to adapt to the temperature change, reducing thermal stress and cracking risk. Hold for 1 hour to allow the billet to gradually absorb heat and reach the preheating temperature of 580°C. Heating phase: Heat at a rate of 95°C / hour to 1080°C (homogenization temperature). Hold for 5 hours to ensure uniform core and surface temperatures.

[0071] (3) Billet forging: The initial forging temperature is 1080℃ to prevent the billet from being overburned. The billet is forged in multiple directions by alternating upsetting and stretching. First, the billet is rounded, upset, and forged to After turning the blank 90°, use forging equipment to stretch the blank to Turn the workpiece 90 degrees and continue to roughen and forge the blank. Then turn the workpiece 90 degrees and stretch the blank to The blank is then turned 90° and forged to a blank size of 1820×230. During the entire forging process, when the blank temperature reaches 900°C, it is placed in a chamber furnace for heating and insulation. When the temperature reaches 1080°C, the blank is taken out of the furnace and forged again. This is to prevent the precipitation of brittle carbides at low temperatures. The forging ratio is 1.5:1.5:1.5:1.5:3.7, and the total forging ratio is ≥3. Innovation: Coarse grains are crushed through multi-directional deformation, alternating directions, rotating 90° each time, for a total of 4 times. The high forging ratio introduces high-density dislocations, forming a subgrain structure at high temperature, and improving creep resistance. Through this forging method, its flaw detection level and mechanical properties are significantly improved.

[0072] Post-forging cooling: This forging adopts air cooling to avoid internal stress caused by too rapid cooling.

[0073] (4) Heat treatment: normalizing and dynamic cycle tempering treatment.

[0074] Normalizing, put the forgings after forging and cooling into the normalizing furnace, heat to 950℃ and keep warm for 6 hours, then take out the workpiece for air cooling (avoid strong wind blowing directly and causing excessive temperature difference). Dynamic cycle tempering, the first stage is 720℃×4h-air cooling (coarsening carbides dissolve and spheroidize, releasing forging residual stress), the second stage is 650℃×2h-air cooling (precipitation of nano-scale Mo2C / VC, forming a dislocation pinning network, and reducing continuous carbides at grain boundaries). Innovation: Compared with normalizing + single tempering, normalizing + dynamic cycle tempering is used to uniformly form nano-carbides to lock hydrogen atoms in low-energy traps. Hydrogen is more likely to escape in a high-temperature hydrogen environment, the strength and toughness of the forgings increase dramatically, the hydrogen resistance changes qualitatively, and the high-temperature life doubles. When the heat-treated tube sheet meets the machining conditions, it is machined, including machining the same furnace sample to the size required for the test.

[0075] The properties of the forgings of Example 1 are shown in Table 4 below:

[0076] Table 4 Properties of forgings of Example 1

[0077]

[0078] Example 2

[0079] The product is the same as in Example 1.

[0080] The preparation process of the high-temperature hydrogen-exposed low-alloy steel forgings comprises the following steps:

[0081] 1. Ingot preparation:

[0082] (1) Ingot smelting: Steel ingots are prepared by electric arc furnace smelting, converter smelting, LF refining and VD vacuum degassing process:

[0083] The raw materials for the steel ingot smelting are high-quality scrap steel (low sulfur and phosphorus, S = 0.005%), pig iron (carbon content 0.25%), molten iron, and high-purity ferrochromium (Cr = 99.9%), ferromolybdenum (Mo = 65%), and ferrovanadium (V = 55%) alloying elements. The raw materials are screened and pre-treated to remove impurities and large foreign matter, and the amount of alloying elements added is precisely measured.

[0084] The steel ingot smelting process comprises:

[0085] The electric arc furnace smelting process uses scrap steel, pig iron, and molten iron as the base charge, and adds slag-forming materials to smelt into molten steel. A high-temperature arc generated by electrodes melts the charge at a temperature of 1625°C, effectively removing impurities such as sulfur and phosphorus. The slag-forming material is a mixture of calcium oxide, aluminum oxide, magnesium oxide, and silicon dioxide in a mass ratio of 55:24:8:13.

[0086] The converter smelting step uses molten steel as raw material, blows oxygen into the furnace to oxidize carbon and other impurities, reduce carbon content, and decarburize and dephosphorize. C = 0.10%, dephosphorization rate = 93% (P = 0.005%), and appropriate amounts of scrap steel and alloying elements are added to adjust the composition. The smelting temperature is 1650°C, and the tapping temperature is 1700°C. Slag blocking technology is used during tapping to reduce the introduction of inclusions.

[0087] In the LF refining step, the molten steel after primary refining is transferred to the ladle refining furnace, and alloy elements such as high-purity ferrochrome (Cr=99.8%), ferromolybdenum (Mo=65%), and ferrovanadium (V=55%) are added. Impurities such as sulfur and phosphorus are further removed through heating and bottom blowing argon stirring operations. The smelting temperature is 1590°C, and the composition is precisely controlled: Cr: 2.2%, Mo: 0.9%, V: 0.25%. Desulfurization: white slag basicity (CaO / SiO2) = 3.3, end point S = 0.002%.

[0088] VD vacuum degassing: The molten steel is placed in a vacuum environment with a vacuum degree of 0.45 mbar (absolute pressure) for 30 minutes, resulting in a final hydrogen (H) concentration of 1.2 ppm and a total oxygen (TO) concentration of 1.2 ppm. This removes hydrogen and other gases, reduces defects such as pores and cracks, and fine-tunes the composition. Ferrovanadium is added under vacuum, achieving a yield of 93%. The molten steel is poured into a 6.5-ton ingot mold using a die-casting method with an argon-sealed long nozzle to prevent secondary oxidation. After solidification, the ingot is demolded to obtain the ingot.

[0089] The chemical composition of the steel ingot of Example 2 is shown in Table 5 below:

[0090] Table 5 Chemical composition of steel ingot in Example 2

[0091]

[0092] The advantages of the steel ingots obtained by the above smelting method are: (1) Multi-stage dehydrogenation technology: LF pre-deoxidation + VD deep dehydrogenation, final hydrogen (H) = 1.2ppm, hydrogen embrittlement sensitivity reduced by 50%. (2) Precision alloying: Adding ferrovanadium under vacuum, the yield is 93%, avoiding oxidation loss, Cr (2.2%) is the core element of high-temperature oxidation resistance, forming a dense oxide film, reducing the oxidation rate, Mo (0.9%) is a solid solution strengthening matrix to improve creep resistance, V (0.25%) forms fine VC and VN particles, hindering grain boundary sliding at high temperatures, and trace Nb (0.04%) forms stable NbC, refining the grains, and improving the 500℃ endurance strength by 20%. (3) High cleanliness: white slag refining + protective casting, total oxygen = 1.2ppm, inclusion grade is 0.8. This steel ingot is evaluated for fine and coarse metal inclusions according to method B specified in GB / T10561. The sulfide type (A) is level 0.5, the alumina type (B) is level 0, the silicate type (C) is level 0, the fine and coarse spherical oxide type (D) are level 0, and the single spherical particle type (DS) is level 0.3. It meets the requirements of A, B, C, D, and DS types ≤1.5, and A+C≤1.0, B+D≤1.0, and A+B+C+D+DS≤2.0, and the metal inclusion level is high.

[0093] The blank preparation, blank heating, blank forging, post-forging cooling, and post-forging heat treatment processes of the product of Example 2 are the same as those of Example 1 and will not be described again.

[0094] The Cr content decreases from 2.4% in Example 1 to 2.2% in Example 2, and the yield strength at 500°C decreases by about 7%-9% (due to weakened oxidation resistance and reduced carbides). The Mo content decreases from 1.0% to 0.9%, which leads to an increase in creep rate (due to reduced Mo2C precipitation and weakened grain boundary strengthening). The V content decreases from 0.3% to 0.25%, and the high-temperature endurance strength decreases.

[0095] The properties of the forgings of Example 2 are shown in Table 6 below:

[0096] Table 6 Properties of forgings of Example 2

[0097]

[0098] Example 3

[0099] The product is the same as in Example 1.

[0100] The preparation process of the high-temperature hydrogen-exposed low-alloy steel forgings comprises the following steps:

[0101] The smelting method of the raw materials used in the product of Example 3 is the same as that of Example 1 and will not be described again.

[0102] 2. Forging:

[0103] (2) Billet heating:

[0104] Blank preparation: φ1788×208 is the net size of the tube sheet. In order to ensure the net size of the tube sheet after turning, the blank size of the tube sheet is determined to be φ1820×230 based on empirical values. The calculated blank weight of the raw materials used for the tube sheet is 4930Kg. There must be enough cutting amount at the head and tail of the steel ingot to ensure that there are no defects such as shrinkage cavities, looseness, cracks, and severe segregation that affect the forging quality. Therefore, the raw material ingot shape is determined to be 6.5T steel ingot, fire consumption: 5%, blanking weight: 4930Kg, blanking size: 886×886×800.

[0105] Heating: The billet is placed into a chamber furnace using a discharge stage to support it. This ensures uniform billet temperature. Billet heating temperature: Initially, the furnace temperature is slowly raised to 580°C at a rate of 80°C / hour to allow the internal structure sufficient time to adapt to the temperature change, reducing thermal stress and cracking risk. Hold for 1 hour to allow the billet to gradually absorb heat and reach the preheating temperature of 580°C. Heating phase: Heat at a rate of 95°C / hour to 1080°C (homogenization temperature). Hold for 20 hours to ensure uniform core and surface temperatures.

[0106] (3) Billet forging: The initial forging temperature should be 1090℃ to prevent the billet from overburning. The billet is forged in a multi-directional forging process using alternating upsetting and stretching methods. First, the billet is rounded, upset, and forged to φ1200×539. After the billet is turned 90°, the billet is stretched to φ850×1075 using a forging machine. The workpiece is turned 90° and the billet is further upset and forged to φ1200×539. Finally, the billet is forged to a blank size of 1820×230. During the entire forging process, when the billet temperature is 900℃, it is placed in a chamber furnace for heating and insulation. When the temperature reaches 1080℃, the billet is taken out of the furnace and continued forging. This is to prevent the precipitation of brittle carbides at low temperatures. The forging ratio is 1.5:1.5:1.5:3.7, and the total forging ratio is ≥3. Innovation: Coarse grains are crushed through multi-directional deformation, with alternating directions and 90° rotation in each pass, for a total of 3 times. High forging ratio, introduction of medium- and high-density dislocations, formation of sub-grain structure at high temperature, and enhanced creep resistance. Through this forging method, its flaw detection level and mechanical properties are significantly improved.

[0107] Post-forging cooling: This forging adopts air cooling to avoid internal stress caused by too rapid cooling.

[0108] (4) Heat treatment: normalizing and dynamic cycle tempering treatment.

[0109] Normalizing, put the forgings after forging and cooling into the normalizing furnace, heat to 950℃ and keep warm for 6 hours, then take out the workpiece for air cooling (avoid strong wind blowing directly and causing excessive temperature difference). Dynamic cycle tempering, the first stage is 720℃×4h-air cooling (coarsening carbides dissolve and spheroidize, releasing forging residual stress), the second stage is 650℃×2h-air cooling (precipitation of nano-scale Mo2C / VC, forming a dislocation pinning network, and reducing continuous carbides at grain boundaries). Innovation: Compared with normalizing + single tempering, normalizing + dynamic cycle tempering is used to uniformly form nano-carbides to lock hydrogen atoms in low-energy traps. Hydrogen is more likely to escape in a high-temperature hydrogen environment, the strength and toughness of the forgings increase dramatically, the hydrogen resistance changes qualitatively, and the high-temperature life doubles. When the heat-treated tube sheet meets the machining conditions, it is machined, including machining the same furnace sample to the size required for the test.

[0110] When the initial forging temperature increases from 1080℃ to 1090℃, the austenite grain size becomes larger, the grain size grade decreases, the high-temperature endurance strength decreases, the number of multi-directional forgings is reduced from 4 times to 3 times, the forging ratio changes from 1.5:1.5:1.5:1.5:3.7 to 1.5:1.5:1.5:3.7, the ultrasonic testing level of the forging is reduced, the yield strength, tensile strength, elongation after fracture and impact absorption energy value will decrease, and the brittleness increases.

[0111] The properties of the forgings of Example 3 are shown in Table 7 below:

[0112] Table 7 Properties of forgings of Example 3

[0113]

[0114] Comparative Example 1

[0115] The product is the same as in Example 1.

[0116] The preparation process of the high-temperature hydrogen-exposed low-alloy steel forgings comprises the following steps:

[0117] The smelting method of the raw materials used in the product of Comparative Example 1 is the same as that of Example 1 and will not be described again.

[0118] The blank preparation, blank heating, blank forging, post-forging cooling, and post-forging heat treatment processes of the product of Comparative Example 1 are the same as those of Example 1 and will not be described again.

[0119] In comparative example 1, the heat treatment of the tube plate after forging and cooling adopts a single normalizing treatment. The forging after forging and cooling is placed in a normalizing furnace, heated to 950°C and kept warm for 6 hours. The workpiece is then taken out for air cooling (to avoid strong wind blowing directly causing excessive temperature difference). When the machining requirements are met, it is machined into a finished tube plate.

[0120] The properties of the forgings of Comparative Example 1 are shown in Table 8 below:

[0121] Table 8 Properties of forgings of comparative example 1

[0122]

[0123] When dynamic cyclic tempering is omitted, carbide precipitation becomes uncontrolled. Bainite can be obtained by rapid cooling after normalizing, but without tempering, carbides fail to disperse and precipitate, remaining at grain boundaries as coarse chain carbides. Impurities accumulate at the ferrite-bainite interface, exacerbating P segregation and increasing the tendency toward grain boundary embrittlement. The insufficient austenitizing temperature during normalizing alone fails to dissolve the VC carbides pinning the grain boundaries, leading to a significant decrease in grain size. Mechanical properties deteriorate across the board, with impact toughness plummeting. Without tempering, the high dislocation density and coarse carbides within the bainite laths result in extremely low resistance to crack propagation. Strength and ductility become unbalanced, and the failure of carbides to spheroidize leads to stress concentration, a significant decrease in elongation, a reduction in endurance strength and creep life, a loss of resistance to temper brittleness, a sharp increase in hydrogen embrittlement sensitivity, and an intensification of oxidative corrosion.

[0124] The above three examples prove that the entire process of the tube sheet in Example 1 is the most optimized. The various data of the forging inspection obtained by the process of Examples 2 and 3 also meet the standard requirements. Comparative Example 1 omits the key dynamic cycle tempering process, resulting in unqualified product performance data.

[0125] Through the above-mentioned process control, 12Cr2Mo1V(H) forgings can achieve a uniform bainite / ferrite structure, meeting the strength, toughness and creep resistance requirements under high temperature and high pressure environments. They are suitable for manufacturing high-temperature and high-pressure containers and pipelines in the petrochemical, refining, fertilizer and other industries, as well as key components such as hydrogenation reactor shells and power plant turbine rotors.

[0126] The various embodiments in this specification are described in a progressive manner. Similar parts between the various embodiments can be referred to in conjunction with each other. Each embodiment focuses on the differences between the other embodiments. In particular, the system embodiments are generally similar to the method embodiments, so the description is relatively simple. For relevant parts, refer to the description of the method embodiments.

[0127] The foregoing is merely an embodiment of the present invention and is not intended to limit the present invention. It will be apparent to those skilled in the art that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of the claims of the present invention.

Claims

1. A high temperature hydrogen-exposed low alloy steel forging, characterized in that: Its chemical composition by mass fraction is: C: 0.08-0.15%, Si≤0.10%, Mn: 0.30-0.60%, Cr: 2.00-2.50%, Mo: 0.90-1.10%, V: 0.25-0.35%, Nb≤0.07%, Ti≤0.03%, B≤0.002%, P≤0.008%, S≤0.002%, and the balance is Fe.

2. A process for preparing high-temperature hydrogen-resistant low-alloy steel forgings according to claim 1, characterized in that: The process includes the following steps: (1) Ingot smelting: Steel ingots are prepared by electric arc furnace smelting, converter smelting, LF refining and VD vacuum degassing process; (2) Billet heating: heating the pretreated steel ingot and keeping it warm; (3) Billet forging: alternating upsetting and drawing; (4) Heat treatment: normalizing and dynamic cycle tempering treatment.

3. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 2, characterized in that: The steel ingot smelting process comprises: The electric arc furnace smelting step uses scrap steel, pig iron and molten iron as the basic charge, and adds slag-making materials to smelt into molten steel; In the converter smelting step, oxygen is blown into the molten steel to decarburize it, the final carbon content is controlled, and the alloy composition is adjusted. The tapping temperature is not less than 1650°C. In the LF refining step, alloying elements are added to adjust the composition, and sulfur and phosphorus impurities are removed by heating and bottom blowing argon stirring; VD vacuum degassing step, controls the vacuum degree and processing time, reduces the hydrogen and oxygen content, and adds alloying elements.

4. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 3, characterized in that: In the converter smelting step, the final carbon content is 0.08% to 0.15%, the dephosphorization rate is not less than 90%, and the phosphorus content does not exceed 0.008%.

5. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 3, characterized in that: In the LF refining step, the basicity of the white slag is not less than 3.0, and the final sulfur content does not exceed 0.002%.

6. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 3, characterized in that: In the VD vacuum degassing step, the vacuum degree is not higher than 0.5 mbar, the treatment time is not less than 25 minutes, the final hydrogen content does not exceed 1.5 ppm, and the total oxygen content does not exceed 1.5 ppm.

7. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 2, characterized in that: The blank heating process includes: an initial preheating temperature not exceeding 600° C.; and heating to 1050° C. to 1100° C. at a heating rate not exceeding 100° C. / h.

8. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 2, characterized in that: In the billet forging process, the initial forging temperature does not exceed 1100°C, the forging ratio of the main cross-section is not less than 3, and the forging process temperature is not less than 850°C.

9. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 2, characterized in that: The heat treatment process includes: Normalizing treatment, temperature is 930℃ to 960℃, keep warm and then air cool; Dynamic cycle tempering treatment includes the first stage of 720℃ holding for 4 hours and air cooling, and the second stage of 650℃ holding for 2 hours and air cooling.

10. The process for preparing high temperature hydrogen-resistant low alloy steel forgings according to claim 2, characterized in that: The grain size after normalizing treatment is not less than grade 5.

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