A heat treatment production method of AISI8630 alloy steel

Abstract

The application discloses a heat treatment production method of AISI8630 alloy steel, and is characterized in that the method comprises the following steps: (1) forging forming; (2) normalizing: after the forging blank is air-cooled to room temperature, the forging blank is reheated to 870-890 DEG C for normalizing, and is kept for 2-4 hours; (3) quenching: the forging blank after normalizing is reheated to 860-880 DEG C, kept for 2-4 hours, and then rapidly immersed into a quenching medium to perform variable flow rate quenching; (4) tempering: the workpiece after quenching is heated to 560-640 DEG C for tempering, and is taken out of water when the workpiece is water-cooled to the interval of 150-200 DEG C, and the workpiece is air-cooled to form a film by using the residual heat. By optimizing the quenching flow field control and the cooling medium, the water quenching soft point defects and the cracking tendency of the large forging are effectively reduced, and the method is suitable for the industrialized production of key stress components.

Description

Technical Field

[0001] This invention relates to the field of alloy steel heat treatment technology, and in particular to a heat treatment production method for AISI8630 alloy steel. Background Technology

[0002] Subsea wellheads and oil production equipment are crucial components in offshore oil and gas field development and key equipment in subsea oil production systems. International deep-sea oil and gas reserves are enormous, and international oil and gas companies are increasing their investment in deep-sea exploration and production technologies year by year. Currently, my country's investment in deep-sea oil and gas resource exploitation is still in its early stages. Furthermore, the world's three major deep-sea oil production equipment manufacturers largely do not use forgings produced in China, primarily due to substandard mechanical properties and internal defects in the products.

[0003] AISI8630 material is mainly used in key components such as high-pressure wellhead bodies, casing hangers, joints, and protectors. Its comprehensive mechanical properties are crucial to the safety of the entire deep-sea oil production system. In addition, the high humidity and high salinity of the deep-sea environment require large forgings to have a certain initial corrosion resistance during processing and transfer.

[0004] Therefore, the technical problem to be solved by rationally designing the alloy composition and improving the heat treatment process, so as to solve the problems of surface dead corner soft spots and core thermal stress phase transformation microcracks that are easy to generate when water quenching large, thick, deep-sea forgings, while giving the workpiece surface certain anti-corrosion passivation properties to meet the comprehensive performance requirements of high strength and high toughness, is to solve the problem. Summary of the Invention

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A heat treatment production method for AISI 8630 alloy steel includes the following steps: (1) Forging billet: According to the preset AISI8630 alloy steel composition, it is smelted and cast into an ingot, and the ingot is forged into a forging billet; (2) Normalizing: Heat the forging billet to 870-890℃ for normalizing, hold for 2-4 hours, and then remove it from the furnace and air cool to room temperature; (3) Quenching: After normalizing, the forging billet is reheated to 860-880℃ and held for 2-4 hours. Then, the forging billet is quickly immersed in the quenching medium for quenching. (4) Tempering: Heat the quenched workpiece to the range of 560-640℃ and keep it at that temperature for 4-8 hours. After taking it out of the furnace, immerse it directly in the tempering cooling medium for water cooling. When the surface temperature of the workpiece drops to the range of 150-200℃, lift it out of the liquid surface and use the residual heat inside the workpiece to air cool it to room temperature and solidify it into a film.

[0006] Preferably, the chemical composition of the AISI8630 alloy steel, by mass percentage, includes: C: 0.28%–0.33%, Si: 0.15%–0.35%, Mn: 1.10%–1.40%, P≤0.035%, S≤0.035%, Cr: 0.40%–0.60%, Ni: 0.40%–0.70%, Mo: 0.15%–0.25%, Al: ≤0.08%, with the balance being Fe and unavoidable trace elements.

[0007] Preferably, the chemical composition of the AISI8630 alloy steel is as follows: C: 0.32%, Si: 0.20%, Mn: 1.30%, P: 0.018%, S: 0.015%, Cr: 0.50%, Ni: 0.55%, Mo: 0.19%, Al: 0.08%, with the balance being Fe and unavoidable trace elements.

[0008] Preferably, the quenching medium in step (3) is a polyalkylene glycol (PAG) water-based quenching liquid. By mass percentage, the quenching liquid includes: a PAG stock solution concentration of 5% to 8%, a sodium silicate concentration of 0.1% to 0.5%, and the remainder being water; the initial temperature of the quenching medium is controlled to be no higher than 40°C.

[0009] Preferably, in step (3), after the forging billet is immersed in the quenching medium, a three-stage variable flow rate quenching is performed by controlling the frequency conversion circulation system in the quenching tank: First stage: Within 0 to 15 seconds after the forging blank is immersed in water, the relative flow velocity of the quenching medium on the surface of the forging blank is controlled to be 1.0 to 1.5 m / s; Second stage: After the forging billet has been immersed in water for 15 seconds until the surface temperature of the workpiece drops to 350℃, reduce the power of the frequency conversion circulation system and control the relative flow rate of the quenching medium to 0.4~0.8m / s. The third stage: During the period when the surface temperature of the workpiece drops below 350℃ until it is completely cooled, the frequency conversion circulation system is turned off, so that the quenching medium is in a natural convection state and the forging billet is allowed to cool slowly in the medium.

[0010] The beneficial effects of this invention are as follows: 1. This invention uses PAG water-based quenching fluid in conjunction with a variable frequency circulation system to provide high power and high flow rate (1.0~1.5m / s) flushing in the early stage of quenching, effectively peeling off the initial high temperature insulation vapor film on the surface, steps and blind holes of large cross-section forgings, forcing the workpiece to quickly enter the boiling heat transfer stage, greatly improving the initial cooling uniformity and reducing the generation of non-martensitic soft spots.

[0011] 2. This invention employs a three-stage variable frequency flow rate quenching control model based on the martensitic transformation point (Ms point). When the surface temperature of the forging reaches 350℃ (close to the Ms point of AISI 8630 steel), the forced liquid flow is promptly stopped, and the process is switched to static slow cooling. This operation effectively reduces the cooling rate during the martensitic transformation period, achieving a smooth self-tempering transformation of martensite, avoiding the superposition of phase transformation stress and thermal stress, and significantly reducing the tendency of water quenching microcracks in thick forgings.

[0012] 3. In step (4) of the present invention, after high-temperature tempering and heat preservation before exiting the furnace, a composite cooling process of water cooling followed by air cooling is adopted. This not only allows the second type of tempering brittle temperature zone of 500℃~600℃ to be quickly passed in the early stage of water exit, effectively suppressing the segregation of impurity elements at the grain boundary; but also utilizes the residual heat of the surface at 150~200℃ to promote the dehydration and solidification of the trace amount of silicate attached to the surface into a film, thereby taking into account both the high impact toughness of marine engineering components and the corrosion resistance between processes. Attached Figure Description

[0013] Figure 1 The image shows the metallographic structure of AISI 8630 alloy steel after heat treatment in Example 1 of this invention. Figure 2 The image shows the metallographic structure of AISI 8630 alloy steel after heat treatment in Comparative Example 1. Detailed Implementation

[0014] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] This invention addresses the problems of cracking caused by dead-angle soft spots, uneven cooling, and excessive thermal and structural stress in the core of large-section AISI8630 alloy steel forgings used in deep-sea equipment during traditional water quenching. The main improvements are made in the composition of the quenching medium and the control of the cooling flow rate.

[0016] In the initial stage of quenching in water, within 0-15 seconds, the cooling medium surrounding the surface of the high-temperature workpiece rapidly vaporizes and forms a vapor film. This vapor film hinders direct heat exchange between the workpiece surface and the cooling medium. This invention utilizes a variable frequency circulation system in the quenching tank to provide a high-velocity circulating liquid flow, controlling the relative velocity of the medium on the workpiece surface at 1.0-1.5 m / s. The shearing and scouring effects of the high-velocity liquid flow rapidly break down the vapor film at complex morphologies such as steps and blind holes, thereby improving the heat exchange uniformity during the initial cooling stage.

[0017] After immersion in water for 15 seconds until the workpiece surface temperature drops to approximately 350°C, the circulating fluid flow rate is adjusted to a medium speed to maintain stable heat exchange and promote heat transfer from the core to the outside. Once the workpiece surface temperature drops below 350°C, the circulating water pump is turned off, allowing the quenching medium to be in a state of natural convection. Since 350°C is close to the martensitic transformation temperature range of this type of alloy steel, reducing the cooling intensity at this point helps alleviate the stress concentration caused by the phase transformation volume expansion, thereby reducing the risk of quenching microcracks in large-section forgings. Example 1

[0018] This embodiment provides a heat treatment production method for AISI 8630 alloy steel, specifically including the following steps: (1) Forging: by mass percentage, C:0.32%, Si:0.20%, Mn:1.30%, Cr:0.50%, Ni:0.55%, Mo:0.19%, Al:0.08%, S:0.015%, P:0.018%, with the balance being Fe. The mixture is smelted and cast into ingots, and then forged into thick, large blind-hole flange forgings at 1200℃. (2) Normalizing: After the forging billet is air-cooled to room temperature, it is reheated to 880°C and held for 3 hours. Then it is taken out of the furnace and air-cooled to room temperature to refine the grains and eliminate forging stress. (3) Composite dynamic quenching: The normalized forging billet is heated to 870℃ and held for 3 hours. A composite water-based quenching medium is prepared, with a PAG concentration of 6.0% and a sodium silicate concentration of 0.3% by mass percentage, and an initial water temperature of 30℃. The forging billet is quickly immersed in the medium after being taken out of the furnace, and a three-stage variable flow rate control is performed by controlling the variable frequency circulation system in the quenching tank: (a) Strong flow film breaking stage: Within 0 to 15 seconds after water entry, turn on the high power of the variable frequency circulating pump and control the relative flow velocity of the medium on the surface of the workpiece to be 1.2 m / s; (b) Stable heat exchange stage: After 15 seconds of water immersion, the frequency of the circulation system is reduced, the relative flow velocity of the medium on the surface of the workpiece is controlled at 0.6 m / s, and this medium flow velocity is maintained until the surface temperature of the workpiece drops to about 350°C; the surface temperature of the workpiece can be determined by a preset surface thermocouple, a contact temperature measuring device, or a verified cooling time-temperature correspondence. (c) Slow cooling and crack prevention stage: When the surface temperature of the workpiece drops below 350°C until it is completely cooled and drained from the water, the frequency conversion circulation system is turned off, and the forging billet is allowed to cool slowly and naturally in the medium.

[0019] (4) Tempering and in-situ film formation: Heat the quenched workpiece to 560°C and hold for 6 hours. After removing it from the furnace, immerse it in the above medium for water cooling. When the surface temperature of the workpiece drops to about 180°C, lift it off the liquid surface and suspend it in the air for cooling. Use the residual heat conducted from the inside of the workpiece to make the surface moisture evaporate quickly, promote the solidification of trace silicates to form an inorganic anti-corrosion passivation film, and finally air cool to room temperature. Example 2

[0020] In step (3), the quenching temperature is raised to 860℃ and held for 3 hours. The PAG concentration is 5.0%, the sodium silicate concentration is 0.2%, and the water temperature is 38℃. The three-stage flow rate control is as follows: (a) stage relative flow rate 1.0 m / s; (b) stage 0.4 m / s; (c) stage pump off and allowed to stand. In step (4), the tempering temperature is 640℃, held for 6 hours, and then cooled to about 150℃ by water cooling before being lifted from the liquid surface and air-cooled to form a film. The remaining processes are the same as in Example 1. Example 3

[0021] In step (3), the quenching temperature is raised to 880℃ and held for 3 hours. A composite water-based quenching medium is prepared, with a PAG concentration of 6.0% and a sodium silicate concentration of 0.3% by mass percentage, and an initial water temperature of 30℃. The three-stage flow rate control is as follows: (a) stage relative flow rate 1.5 m / s; (b) stage 0.8 m / s; (c) stage pump off and allowed to stand. In step (4), the tempering temperature is 580℃, held for 6 hours, and then cooled to about 200℃ by water cooling before being lifted from the liquid surface and air-cooled to form a film. The remaining processes are the same as in Example 1.

[0022] Comparative Example 1 Ordinary pure water was used as the quenching and tempering medium. Flow rate control was implemented as follows: the circulating water pump was kept running continuously from the time the workpiece was immersed in water until it was completely cooled, and the relative flow rate of the medium was maintained at a constant 0.8 m / s. Three-stage flow rate regulation was not implemented, and the pump was not stopped for slow cooling below 350°C.

[0023] Comparative Example 2 The quenching and tempering cooling medium is the same as in Example 1, and the tempering temperature and film-forming operation are the same as in Example 1. The difference is that the quenching flow rate control method is the same as in Comparative Example 1, that is, after the workpiece is immersed in water until it is completely cooled, a constant forced liquid flow of 0.8 m / s is maintained throughout the process, and the circulating water pump is not turned off for static slow cooling below 350°C.

[0024] Comparative Example 3 The quenching and tempering cooling medium is ordinary pure water, but the variable flow rate control method is exactly the same as in Example 1 (three-stage control: strong convection in the first 15 seconds, medium convection in the middle, and slow cooling in the later stage).

[0025] Test Experiment Mechanical sampling was performed on the thick forgings prepared in the above embodiments and comparative examples according to national standards such as GB / T228.1. Ultrasonic testing (UT) and fluorescent magnetic particle testing (MT) were used to detect the scrap rate of surface and subsurface quenching microcracks. Corrosion resistance was assessed using a neutral salt spray test (NSS), recording the time it took for large areas of red rust to appear on the surface of the bare parts without rust-preventive oil. The test results are shown in Table 1. Table 1: Comparison of overall performance and defect rate of each embodiment and comparative example Grouped items Yield strength (MPa) Tensile strength (MPa) Elongation (%) Impact energy at -20°C (J) Surface hardness scatter (HB) Microcrack scrap rate Time to red rust (h) for NSS Example 1 660 810 18.5 65 12 0% 144 Example 3 640 780 21 72 13 0% 168 Example 2 590 720 24.5 95 14 0% 120 Comparative Example 1 645 795 14.5 38 35 18.5% 8 Comparative Example 2 520 660 6.5 45 15 8.5% 24 Comparative Example 3 655 800 17.5 50 28 4.5% 8 As shown in Table 1, compared to the ordinary pure water constant flow cooling process, this invention, by using high flow rate for film breaking in the initial stage of water entry, medium flow rate for stable heat exchange in the middle stage, and slow cooling with pump stoppage below 350℃, combined with a silicate-containing composite water-based medium, not only reduces surface hardness variation and microcrack scrap rate, but also significantly extends the salt spray corrosion resistance time of bare parts. Although Comparative Example 2 uses a silicate-containing composite water-based medium, forced convection cooling is maintained below 350℃, resulting in greater structural stress and microcracks in the martensitic transformation zone of the workpiece, leading to poor continuity of the surface passivation film. Therefore, its corrosion resistance time is significantly lower than that of Example 1. This invention can balance the hardenability, crack resistance, structural uniformity, and short-term rust prevention performance of large-section AISI 8630 alloy steel forgings.

[0026] Combination Figure 1 and Figure 2 As can be seen, the microstructure of the AISI8630 alloy steel forging obtained in Example 1 is mainly composed of uniform and fine tempered sorbite, with a small amount of tempered martensite. The microstructure is uniformly distributed, and no obvious coarse network ferrite, grain boundary cracks, or quenching microcracks were observed. In contrast, Comparative Example 1, which uses ordinary pure water with constant forced convection cooling throughout the process, exhibits poor microstructure uniformity, with localized areas showing coarsening and uneven transformation characteristics. This indicates that ordinary pure water constant flow cooling cannot simultaneously ensure the initial cooling uniformity of complex structural parts and stress control during the martensitic phase transformation stage. Therefore, the composite dynamic quenching process of the present invention can reduce phase transformation stress concentration while ensuring sufficient core quenching, thus providing a microstructure basis for large forgings to achieve a good balance of strength, plasticity, and low-temperature impact toughness.

[0027] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A heat treatment method for producing AISI 8630 alloy steel, characterized in that, Includes the following steps: (1) Forging billet: According to the preset AISI8630 alloy steel composition, it is smelted and cast into an ingot, and the ingot is forged into a forging billet; (2) Normalizing: Heat the forging billet to 870-890℃ for normalizing, hold for 2-4 hours, and then remove it from the furnace and air cool to room temperature; (3) Quenching: After normalizing, the forging billet is reheated to 860-880℃ and held for 2-4 hours. Then, the forging billet is quickly immersed in the quenching medium for quenching. (4) Tempering: Heat the quenched workpiece to the range of 560-640℃ and keep it at that temperature for 4-8 hours. After taking it out of the furnace, immerse it directly in the tempering cooling medium for water cooling. When the surface temperature of the workpiece drops to the range of 150-200℃, lift it out of the liquid surface and use the residual heat inside the workpiece to air cool it to room temperature and solidify it into a film.

2. The heat treatment production method for AISI 8630 alloy steel according to claim 1, characterized in that, The chemical composition of the AISI8630 alloy steel, by mass percentage, includes: C: 0.28%–0.33%, Si: 0.15%–0.35%, Mn: 1.10%–1.40%, P: ≤0.035%, S: ≤0.035%, Cr: 0.40%–0.60%, Ni: 0.40%–0.70%, Mo: 0.15%–0.25%, Al: ≤0.08%, with the balance being Fe and unavoidable trace elements.

3. The heat treatment production method for AISI 8630 alloy steel according to claim 2, characterized in that, The chemical composition of the AISI8630 alloy steel is as follows: C: 0.32%, Si: 0.20%, Mn: 1.30%, P: 0.018%, S: 0.015%, Cr: 0.50%, Ni: 0.55%, Mo: 0.19%, Al: 0.08%, with the balance being Fe and unavoidable trace elements.

4. The heat treatment production method for AISI 8630 alloy steel according to claim 1, characterized in that, The quenching medium mentioned in step (3) is a polyalkylene glycol (PAG) water-based quenching liquid. By mass percentage, the quenching liquid includes: a PAG stock solution concentration of 5% to 8%, a sodium silicate concentration of 0.1% to 0.5%, and the remainder being water; the initial temperature of the quenching medium is controlled to be no higher than 40°C.

5. The heat treatment production method for AISI 8630 alloy steel according to claim 4, characterized in that, In step (3), after the forging billet is immersed in the quenching medium, a three-stage variable flow rate quenching is performed by controlling the frequency conversion circulation system in the quenching tank: First stage: Within 0 to 15 seconds after the forging blank is immersed in water, the relative flow velocity of the quenching medium on the surface of the forging blank is controlled to be 1.0 to 1.5 m / s; Second stage: After the forging billet has been immersed in water for 15 seconds until the surface temperature of the workpiece drops to 350℃, reduce the power of the frequency conversion circulation system and control the relative flow rate of the quenching medium to 0.4~0.8m / s. The third stage: During the period when the surface temperature of the workpiece drops below 350℃ until it is completely cooled, the frequency conversion circulation system is turned off, so that the quenching medium is in a natural convection state and the forging billet is allowed to cool slowly in the medium.

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