Annealing method for a D406A ultra-high strength steel combustion chamber shell

Abstract

The present invention provides an annealing method for a D406A ultra-high strength steel combustion chamber shell. The cylinder after spinning between passes is placed in an induction coil, heated to 760 - 820 °C, held for 5 - 30 s and then cooled to 650 °C, held for 10 - 30 s and then air-cooled; the cylinder after final spinning is loaded into the induction coil, heated to 700 - 780 °C, held for 1 - 2 s and then air-cooled; after the front and rear connectors are welded to the cylinder, the induction coil is aligned with the weld, heated to 300 - 350 °C, held for 30 - 60 s and then heated to 750 - 770 °C, held for 10 - 15 s and then cooled to 600 °C, held for 10 - 30 s and then air-cooled; the combustion chamber shell after the support is welded is coated and protected and then hoisted into a resistance furnace for annealing, held at 750 - 770 °C for 30 min - 60 min, furnace-cooled to 600 °C and then taken out of the furnace for air-cooling. The annealing method of the present invention has a short annealing time, low energy consumption, can quickly improve production efficiency, refine grains, reduce deformation, and has a simple process and convenient operation.

Description

Technical Field

[0001] The present invention belongs to the technology of metal material processing and relates to an annealing method. Background Art

[0002] The combustion chamber shell of a solid rocket motor generally uses ultra-high strength steel D406A material, which is usually welded by a front connector, a rear connector, a cylinder body and a support. The forming process flow of the combustion chamber shell is as follows: machining of each workpiece → spinning between passes of the cylinder body → softening annealing between passes of the cylinder body → final spinning of the cylinder body → stress relief annealing of the cylinder body → butt welding of the front and rear connectors and the cylinder body → annealing → welding of the support → annealing → quenching + tempering → machining. That is, the main processing technologies in the forming process of the combustion chamber shell are machining, spinning forming, welding and heat treatment, and the annealing processes that the shell forming process needs to go through are annealing between spinning passes of the cylinder body, stress relief annealing after final spinning of the cylinder body, annealing after butt welding of the front and rear connectors and the cylinder body, and annealing after welding of the support. At present, the annealing during the forming process of the cylinder body and the annealing after welding are both carried out in a resistance furnace, the annealing temperature is between 650°C and 720°C, and the annealing duration of each process ranges from 6h to 24h. This results in high energy consumption and a long cycle in the entire annealing process of the combustion chamber shell. In addition, the combustion chamber shell belongs to a thin-walled part, and there are phenomena such as decarburization oxidation, large deformation, unstable mechanical properties and uneven stress elimination during multiple annealings. Therefore, it is imperative to seek an annealing method for the combustion chamber shell with high efficiency, low energy consumption, stable mechanical properties and uniform stress elimination. Summary of the Invention

[0003] In order to overcome the deficiencies of the prior art, the present invention provides an annealing method for a combustion chamber shell made of D406A ultra-high strength steel. By replacing the annealing in a resistance furnace between spinning passes of the cylinder body, after final spinning of the cylinder body, and after butt welding of the front and rear connectors and the cylinder body with induction heating annealing, the induction heating annealing has a short time, low energy consumption, can quickly improve production efficiency, refine grains, reduce deformation, and has a simple process and convenient operation.

[0004] The technical solution adopted by the present invention to solve its technical problems includes the following steps:

[0005] 1) Place the cylinder body after spinning between passes into the induction coil, heat it up to 760 - 820°C at a rate of 60 - 200°C / s, and hold for 5 - 30s; then cool it to 650°C at a rate of 10 - 20°C / s, hold for 10 - 30s, and after the holding ends, air cool it;

[0006] 2) Load the cylinder body after final spinning into the induction coil, heat it up to 700 - 780°C at a rate of 100 - 300°C / s, hold for 1 - 2s, and after the holding ends, air cool it;

[0007] 3) After the front and rear connecting pieces are welded to the cylinder body, align the induction coil with the weld seam, heat it up to 300 - 350 °C at a rate of 100 - 200 °C / s, and keep it warm for 30 - 60 s; then heat it up to 750 - 770 °C at a rate of 10 - 20 °C / s and keep it warm for 10 - 15 s; after the heat preservation ends, cool it to 600 °C at a rate of 5 - 10 °C / s, then keep it warm for 10 - 30 s, and after the heat preservation ends, cool it in air;

[0008] 4) After the support is welded, protect the combustion chamber shell with coating and then lift it into a resistance furnace for annealing. Keep it warm at 750 - 770 °C for 30 min - 60 min, cool it in the furnace to 600 °C and then take it out of the furnace and cool it in air to complete the annealing of the combustion chamber shell.

[0009] In the steps 1) - 3), during the heating, heat preservation and cooling processes, the cylinder body rotates at 15 - 30 revolutions per minute.

[0010] In the step 3), the width of the induction coil is 30 - 50 mm.

[0011] The beneficial effects of the present invention are as follows:

[0012] First of all, D406A ultra-high strength steel has the characteristic of low stress tearing. When the strength of a certain point of the material is broken through, a tearing effect will occur. Therefore, when the cylinder body is spun, softening annealing needs to be carried out between spinning passes to eliminate stress, restore plasticity, and reduce hardness, so as to facilitate subsequent re-spinning forming. The wall thickness of the cylinder body after spinning between passes is between 5 - 10 mm. Put the cylinder body spun between passes into the induction coil, and quickly heat it up to the softening temperature of 760 - 820 °C at a heating rate of 60 - 200 °C / s, keep it warm for 5 - 30 s. The rapid heating rate can inhibit austenite transformation. At the same time, the rapid heating rate expands the softening temperature range of the D406A ultra-high strength steel spun cylinder body, that is, compared with ordinary annealing, the lowest softening temperature will decrease and the softening termination temperature will increase. In this temperature range, with a higher softening temperature and a shorter heat preservation time, the cold deformation texture of the spun cylinder body is quickly transformed into equiaxed grains, and the formed grains are finer. Then cool it to 650 °C at a rate of 10 - 20 °C / s, keep it warm for 10 - 30 s and then cool it in air. Slowly cooling from the softening temperature to 650 °C and keeping it warm and then cooling it in air is to prevent large thermal stress from being formed by rapid cooling at high temperature;

[0013] Second, after the final spinning of the cylinder body, it is heated at an induction heating rate of 100 - 300 °C / s to 700 - 780 °C, held for 1 - 2 s, and then air-cooled after the heating of the cylinder body is completed. The wall thickness of the cylinder body after the final spinning is about 0.8 - 3 mm. The rapid heating and short-time holding can avoid decarburization of the thin-walled cylinder body, reduce the deformation of the cylinder body, and eliminate the spinning stress. In addition, the deformation amount of the cylinder body after the final spinning is larger than that between passes. The larger the cold deformation amount, the lower the softening temperature, and the lower the temperature for stress elimination and the formation of equiaxed grains, and the shorter the time. At the same time, due to the relatively thin wall thickness of the cylinder body after the final spinning, a short holding time is sufficient to ensure temperature uniformity and prevent grain coarsening due to long-time holding.

[0014] Third, after the front and rear connectors are welded to the cylinder body, the induction coil is aligned with the weld. The width of the induction coil is 30 - 50 mm. It is heated at a rate of 100 - 200 °C / s to 300 - 350 °C, held for 30 - 60 s, then heated at a rate of 10 - 20 °C / s to 750 - 770 °C, held for 10 - 15 s. After the holding is completed, it is cooled to 600 °C at a rate of 5 - 10 °C / s, then held for 10 - 30 s, and then air-cooled after the holding is completed. The width of the weld and the heat-affected zone does not exceed 30 mm after welding. Only the weld and the heat-affected zone are subjected to induction annealing, which has an accurate heating depth and heating area, is easy to control during the heating process, and causes little deformation of the combustion chamber shell. At the same time, hard and brittle martensite and upper bainite structures often appear during air-cooling after welding of D406A ultra-high-strength steel. Post-weld annealing is required in a timely manner. If the placement time is too long, cracks and deformation will occur, while induction annealing can achieve timely post-weld annealing. In addition, defects such as hot cracks, cold cracks, inclusions, and pores will inevitably appear in the weld after welding. The weld must be repaired by welding, and re-annealing is required after repair welding. If resistance furnace annealing is used, multiple annealings will cause many problems such as deformation and decarburization. However, if induction annealing is used, only the weld and the heat-affected zone need to be annealed. The annealing speed is fast and the above problems will not occur. In addition, rapid heating to 300 - 350 °C and holding can make the weld temperature uniform and avoid heating in the temper embrittlement zone. While holding at 750 - 770 °C can avoid heating and holding in the reheat crack sensitive zone of 500 - 700 °C to prevent the occurrence of reheat cracks. At the same time, it can prevent heating and holding in the temper embrittlement zone of 380 - 550 °C to reduce the toughness of the weld. Similarly, cooling to 600 °C and holding and then air-cooling are also to quickly pass through the temper embrittlement zone to prevent the occurrence of temper embrittlement. In addition, holding at 750 - 770 °C for 10 - 15 s, this temperature distribution range is just below the austenite phase transformation point of 775 °C of D406A material. The relatively slow heating rate of induction heating can ensure that the phase transformation temperature will not be increased in this temperature range, making the transformation speed of martensite and upper bainite structures in the weld faster, which is beneficial to improving the plasticity of the weld. At the same time, it can also accelerate the precipitation rate of alloy carbides, refine the weld structure and prevent its coarsening, which is beneficial to improving the strength and toughness of the weld.

[0015] Fourth, after the supports are welded, the coated combustion chamber shell is hoisted into a resistance furnace for annealing. It is kept at 750 - 770 °C for 30 - 60 minutes, and then cooled in the furnace to 600 °C and taken out of the furnace for air cooling. Since there are many supports welded on the shell, with different sizes and positions, the stress distribution of the shell after welding is different. Using a higher annealing temperature and a shorter holding time in the resistance furnace can quickly eliminate the uneven local stress distribution after the supports are welded. At the same time, it can eliminate the stress distribution after machining of the front and rear connectors, and the stress distribution in the transition section of the weld induction annealing, making the stress distribution of the entire combustion chamber shell uniform. In addition, cooling in the furnace to 600 °C and then taking it out of the furnace for air cooling can reduce the thermal stress caused by high-temperature air cooling out of the furnace, reduce the residual stress. Also, cooling in the furnace to 600 °C and then taking it out of the furnace for air cooling can reduce the time of the workpiece in the furnace and improve production efficiency. By adopting the above relevant measures, both the quenching deformation of the subsequent combustion chamber shell is reduced, and the stability and production efficiency of the combustion chamber shell are improved.

[0016] Fifth, rotate evenly during the heating process to ensure that the workpiece is heated evenly, with good tissue uniformity, more uniform mechanical property distribution, small deformation of the cylinder body and more uniform stress distribution.

[0017] Sixth, the induction annealing process does not require heat treatment coatings or inert gas protection, without oxidation and decarburization. The entire annealing process is completed within a few minutes, with a fast annealing speed, low energy consumption, low cost, and clean and pollution-free.

[0018] Seventh, compared with the conduction, convection, and radiation heat transfer of the resistance furnace, induction heating generates eddy currents through the alternating magnetic field formed by the induction coil. The eddy currents are blocked by the resistance of the workpiece itself and heat up to form the skin effect. This heating method makes the consistency of all workpieces very high. Specific Embodiments

[0019] The present invention will be further described below in conjunction with embodiments. The present invention includes but is not limited to the following embodiments.

[0020] For the annealing of the combustion chamber shell made of D406A ultra-high strength steel, it is carried out in a resistance furnace, with low production efficiency, high energy consumption, and at the same time, there are problems such as large annealing deformation, uneven grain size, and uneven mechanical property distribution. The present invention provides an annealing method for the combustion chamber shell made of D406A ultra-high strength steel that can not only improve production efficiency and reduce energy consumption, but also refine grains and reduce heat treatment deformation, including the following steps:

[0021] 1) Annealing between spinning passes of the cylinder: Put the cylinder after spinning between passes into the induction coil, heat it to 760 - 820 °C at a rate of 60 - 200 °C / s, hold for 5 - 30 s, then cool to 650 °C at a rate of 10 - 20 °C / s, hold for 10 - 30 s, and after the holding ends, air cool. During the heating and cooling processes, the workpiece rotates at 15 - 30 revolutions per minute;

[0022] 2) Stress relief annealing of the cylinder body: Place the cylinder body after final spinning into the induction coil, heat it up to 700 - 780 °C at a rate of 100 - 300 °C / s, hold for 1 - 2 s, rotate the cylinder body at 15 - 30 revolutions per minute during the heating process, and then air cool it after the heating of the cylinder body is completed;

[0023] 3) Annealing after welding the front and rear connectors to the cylinder body: Align the induction coil with the weld seam, the width of the induction coil is 30 - 50 mm, heat it up to 300 - 350 °C at a rate of 100 - 200 °C / s, hold for 30 - 60 s, then heat it up to 750 - 770 °C at a rate of 10 - 20 °C / s, hold for 10 - 15 s, after the holding is completed, cool it to 600 °C at a rate of 5 - 10 °C / s, then hold for 10 - 30 s, and after the holding is completed, air cool it. The workpiece rotates at 15 - 30 revolutions per minute during the heating and cooling processes;

[0024] 4) Annealing after welding the support: After coating and protecting the combustion chamber shell after welding the support, lift it into the resistance furnace for annealing, hold at 750 - 770 °C for 30 min - 60 min, cool it in the furnace to 600 °C and then take it out of the furnace for air cooling to complete the annealing of the combustion chamber shell.

[0025] The annealing method for the combustion chamber shell made of D406A ultra-high strength steel provided by the present invention improves production efficiency, reduces energy consumption, and solves problems such as large workpiece deformation, unstable mechanical properties, and uneven stress elimination.

[0026] Example 1:

[0027] The annealing method for the combustion chamber shell made of D406A ultra-high strength steel in this example is specifically carried out according to the following steps: Place the cylinder body with a wall thickness of 7.3 mm and a thinning rate of 50% after 2 passes of spinning in the induction coil, with a 10 mm spacing between the inner diameter of the induction coil and the outer diameter of the cylinder body. Heat the cylinder body to 820 °C at a rate of 100 °C / s, hold for 5 s, then cool it to 650 °C at a rate of 10 °C / s, hold for 20 s, and after the holding is completed, air cool it to room temperature. The cylinder body rotates at 30 revolutions per minute during the heating and cooling processes.

[0028] After the treatment in Example 1, the hardness of the D406A ultra-high strength steel spun cylinder body is between HV 215 - HV 220, and the roundness of the cylinder body is between 0.15 - 0.25 mm; the annealing duration is less than 5 min.

[0029] Comparative Experiment 1:

[0030] Heat the cylinder body with a wall thickness of 7.3 mm and a thinning rate of 50% after 2 passes of spinning in the resistance furnace to 700 °C, then hold at 700 °C for 120 min, and cool it in the furnace to 600 °C and then take it out of the furnace for air cooling; among them, protective coatings are used to protect the inner and outer surfaces of the cylinder body during annealing.

[0031] After the treatment of Comparative Experiment 1, the hardness of the D406A ultra-high strength steel spun cylinder is between HV 220 and HV 230, and the roundness of the cylinder is between 0.20 and 0.52 mm; the annealing duration is about 360 min, of which the coating spraying and drying time is about 90 min, and the heating, holding and furnace cooling time is about 270 min.

[0032] Example 2:

[0033] An annealing method for a D406A ultra-high strength steel combustion chamber shell described in this example is specifically carried out according to the following steps: The cylinder with 2 passes of spinning, a wall thickness of 7.3 mm and a reduction rate of 50% is induction heated and then subjected to 3 passes of spinning. After spinning, the wall thickness of the cylinder is 2.5 mm and the reduction rate is 66%. Then, stress relief annealing is carried out on the cylinder after 3 passes of spinning. The distance between the inner diameter of the induction coil and the outer diameter of the cylinder is 10 mm. The cylinder is heated to 700 °C at a rate of 200 °C / s, held for 2 s, and then air-cooled to room temperature after the holding ends. During the heating process, the cylinder rotates at 30 revolutions per minute.

[0034] After the treatment of Example 2, the hardness of the D406A ultra-high strength steel spun cylinder is between HV 238 and 245, and the roundness of the cylinder is between 0.58 and 0.97 mm; the annealing duration is less than 5 min.

[0035] Comparative Experiment 2:

[0036] The cylinder with 2 passes of spinning, a wall thickness of 7.3 mm and a reduction rate of 50% is induction heated and then subjected to 3 passes of spinning. After spinning, the wall thickness of the cylinder is 2.5 mm and the reduction rate is 66%. Then, the cylinder after 3 passes of spinning is heated to 650 °C in an electric resistance furnace, held at 650 °C for 120 min, and then furnace cooled to 600 °C and taken out of the furnace for air cooling; among them, protective coating is used to protect the inner and outer surfaces of the cylinder during annealing.

[0037] After the treatment of Comparative Experiment 2, the hardness of the D406A ultra-high strength steel spun cylinder is between HV 240 and HV 255, and the roundness of the cylinder is between 0.65 and 1.15 mm; the annealing duration is about 330 min, of which the coating spraying and drying time is about 90 min, and the heating, holding and furnace cooling time is about 240 min.

[0038] Example 3:

[0039] An annealing method for a D406A ultra-high strength steel combustion chamber shell according to this embodiment is specifically carried out according to the following steps: The front and rear connectors are welded to the cylinder body. The cylinder body is the one that undergoes induction annealing between passes and after final spinning. Align the induction coil with the weld. The width of the induction coil is 40 mm. Heat up to 350 °C at a rate of 100 °C / s, hold for 60 s, then heat up to 770 °C at a rate of 20 °C / s, hold for 10 s. After the holding ends, cool to 600 °C at a rate of 5 °C / s, hold at 600 °C for 20 s. After the holding ends, air cool. During the heating and cooling process, the cylinder body rotates at 30 revolutions per minute.

[0040] After the treatment in Example 3, the hardness of the weld of the D406A ultra-high strength steel combustion chamber shell is between HV 308 and HV 336, and the roundness of the combustion chamber shell is distributed between 0.70 and 0.92 mm; the annealing duration is less than 3 min.

[0041] Comparative Experiment 3:

[0042] The front and rear connectors are welded to the cylinder body. The cylinder body is the one that undergoes resistance furnace annealing between passes and after final spinning. Then, after the welded combustion chamber shell is heated to 700 °C in the resistance furnace, hold at 700 °C for 120 min, and cool in the furnace to 600 °C and then take it out for air cooling; among them, protective coatings are used for annealing to protect the inner and outer surfaces of the combustion chamber.

[0043] After the treatment in Comparative Experiment 3, the hardness of the weld of the D406A ultra-high strength steel combustion chamber shell is between HV 295 and HV 344, and the roundness of the combustion chamber shell is distributed between 0.75 and 1.45 mm; the annealing duration is about 360 min, among which the time for spraying coatings and drying is about 90 min, and the time for heating, holding, and furnace cooling is about 270 min.

[0044] Example 4:

[0045] An annealing method for a D406A ultra-high strength steel combustion chamber shell according to this embodiment is specifically carried out according to the following steps: After the combustion chamber shell with the support welded is protected by coatings, it is hoisted into the resistance furnace for annealing, hold at 750 °C for 60 min, and cool in the furnace to below 600 °C and then take it out for air cooling.

[0046] After the treatment in Example 4, the roundness of the D406A ultra-high strength steel combustion chamber shell is distributed between 0.80 and 1.0 mm; the annealing duration is about 300 min, among which the time for spraying coatings and drying is about 90 min, and the time for heating, holding, and furnace cooling is about 210 min.

[0047] Comparative Experiment ④:

[0048] After the combustion chamber shell with the support welded is protected by coatings, it is hoisted into the resistance furnace for annealing, hold at 720 °C for 120 min, and cool in the furnace to below 600 °C and then take it out for air cooling.

[0049] After the fourth comparative experiment, the roundness of the D406A ultra-high strength steel combustion chamber shell is distributed between 0.80 and 1.68 mm; the annealing duration is about 360 min, of which the spraying and drying time is about 90 min, and the heating, heat preservation and furnace cooling time is about 270 min

[0050] Example 5:

[0051] The annealing method of a D406A ultra-high strength steel combustion chamber shell described in this example is specifically carried out according to the following steps: Quench and temper the combustion chamber shell annealed by the processes of Examples 1 to 4.

[0052] After the treatment of Example 5, the tensile strength of the matrix sample of the D406A ultra-high strength steel combustion chamber shell is between 1730 MPa and 1780 MPa, and the elongation after fracture is between 9.53% and 10.65%; the tensile strength of the welded sample is between 1740 MPa and 1770 MPa; the roundness of the five cross-sections of the shell after quenching is 0.50, 1.2, 1.55, 1.1, and 0.8 mm respectively.

[0053] Comparative Experiment 5:

[0054] The annealing method of a D406A ultra-high strength steel combustion chamber shell described in this example is specifically carried out according to the following steps: Quench and temper the combustion chamber shell annealed by the processes of Comparative Experiment 1 to Comparative Experiment 4.

[0055] After the treatment of Comparative Experiment 5, the tensile strength of the matrix sample of the D406A ultra-high strength steel combustion chamber shell is between 1690 MPa and 1770 MPa, and the elongation after fracture is between 8.55% and 10.30%; the tensile strength of the welded sample is between 1700 MPa and 1750 MPa; the roundness of the five cross-sections of the shell after quenching is 0.75, 1.5, 2.1, 1.15, and 0.80 mm respectively.

[0056] Analysis of the implementation effect of the present invention:

[0057] Compared with Comparative Experiments 1 to 4, the hardness distribution of the workpieces after annealing treatment in Examples 1 to 4 is more uniform, the deformation is smaller, and all annealing times are shorter. At the same time, compared with Comparative Experiment 5, the tensile strength, elongation after fracture and tensile strength of the welded sample of the workpieces after the treatment of the shell in Example 5 are more uniformly distributed and stable, and the elongation after fracture is higher. At the same time, the change in the roundness of the shell after quenching is smaller.

[0058] It can be seen from this that for the D406A ultra-high strength steel shell processed by the method of the present invention, its hardness distribution is more uniform, the deformation is smaller, and all annealing times are shorter. The tensile strength and elongation after fracture are higher and more uniformly distributed, and the consistency of the product is better and more stable, which can be widely applied to the production of solid rocket motor shells. In addition, adopting the present invention can significantly shorten the cycle of traditional heat treatment processes and save energy.

Claims

1. An annealing method for a D406A ultra-high strength steel combustion chamber shell, characterized in that, It includes the following steps: 1) Place a cylinder body that has undergone two passes of spin forming, with a wall thickness of 7.3 mm, a thinning rate of 50%, and a material of D406A, inside an induction coil. Heat it to 760 - 820 °C at a rate of 60 - 200 °C / s and hold for 5 - 30 s; then cool it to 650 °C at a rate of 10 - 20 °C / s and hold for 10 - 30 s. After the holding ends, air cool it; 2) Load the cylinder body after final spin forming into the induction coil. Heat it to 700 - 780 °C at a rate of 100 - 300 °C / s and hold for 1 - 2 s. After the holding ends, air cool it; 3) After the front and rear connectors are welded to the cylinder body, align the induction coil with the weld seam. Heat it to 300 - 350 °C at a rate of 100 - 200 °C / s and hold for 30 - 60 s; then heat it to 750 - 770 °C at a rate of 10 - 20 °C / s and hold for 10 - 15 s; after the holding ends, cool it to 600 °C at a rate of 5 - 10 °C / s, then hold for 10 - 30 s. After the holding ends, air cool it; 4) After coating and protecting the combustion chamber shell after welding the supports, lift it into a resistance furnace for annealing. Hold at 750 - 770 °C for 30 min - 60 min, cool in the furnace to 600 °C and then take it out of the furnace for air cooling to complete the annealing of the combustion chamber shell.

2. The annealing method of the D406A ultra-high strength steel combustion chamber housing according to claim 1, characterized in that, In the steps 1) - 3), the cylinder body rotates at 15 - 30 revolutions per minute during the heating, holding, and cooling processes.

3. The annealing method of the D406A ultra-high strength steel combustion chamber housing according to claim 1, characterized in that In the step 3), the width of the induction coil is 30 - 50 mm.