Energy-saving boiler superheater and small-radius U-shaped T91 alloy pipe bending process thereof

Abstract

The invention relates to an energy-saving boiler superheater and a small-radius U-shaped T91 alloy pipe bending process thereof in the technical field of energy-saving boilers. The energy-saving boiler superheater comprises the steps that S1, annealing treatment is conducted before bending; s2, first cold roll forming treatment; s3, heating extrusion temperature control; s4, extruding, finishing and forming; and S5, normalizing and tempering heat treatment. In the T91 alloy pipe bending process, the relative bending radius R1 of the T91 alloy pipe is set to be 1.5 times smaller than the outer diameter of the pipe in the first cold bending process, then the bending radius R2 is 0.6-1.1 times of the outer diameter of the pipe through the extrusion finishing forming process, and the bent elbow part is uniform in deformation and smooth in transition through progressive deformation control; and the problems of goose egg-shaped deformation and the like are solved.

Description

Technical Field

[0001] The invention relates to the technical field of energy-saving boilers, and in particular to an energy-saving boiler superheater and a small-radius U-shaped T91 alloy tube bending process thereof. Background Art

[0002] T91 material is a martensitic heat-resistant steel, mainly used to make seamless steel pipes. It has excellent high temperature resistance and creep properties. It is suitable for headers and steam pipes under high-temperature steam conditions. It is used for heating surface pipes of low and medium pressure boilers (working pressure is generally not more than 5.88MPa, working temperature is below 450℃); it is used for heating surface pipes of high-pressure boilers (working pressure is generally above 9.8MPa, working temperature is between 450℃ and 650℃), economizers, superheaters, reheaters, petrochemical industry pipes, etc.

[0003] The room temperature tensile strength of T91 steel is ≥585MPa, the room temperature yield strength is ≥415MPa, and the hardness is ≤250HB. The critical cooling rate of T91 is low, and the austenite stability is very large. It is not easy to undergo normal pearlite transformation during cooling, and thus martensite transformation occurs when cooled to a lower temperature. Due to this, T91 pipes have a great tendency to harden and cold crack. In the first cold bending of T91 small R elbows, cracks or fractures occur locally in the elbows, affecting the quality of the elbows.

[0004] The relative bending radius (ratio of bending radius R to outer diameter D of the tube) of T91 material small radius elbow is R x = 0.6 ~ 1.1, the relative deformation of the elbow is large, the existing extrusion finishing bending die will appear inside the elbow bulge, the inner arc of the elbow is oval (such as Figure 1 As shown), the elbow pipe end is flat and not full and other surface forming problems. Summary of the invention

[0005] In view of the defects in the prior art, the object of the present invention is to provide an energy-saving boiler superheater and a small-radius U-shaped T91 alloy tube bending process thereof.

[0006] A small radius U-shaped T91 alloy tube bending process provided by the present invention comprises:

[0007] S1, annealing treatment before bending: annealing the T91 alloy tube to be bent, controlling the temperature of the T91 alloy tube entering the furnace, the temperature in the furnace and the temperature out of the furnace, and air cooling the T91 alloy tube to room temperature after it is out of the furnace;

[0008] S2, first cold bending forming treatment: placing the T91 alloy tube annealed in step S1 on a first bending tube die body for cold bending to form a primary U-shaped tube, wherein the bending radius R1 of the primary U-shaped tube is 1.3 to 1.5 times the outer diameter of the T91 alloy tube;

[0009] S3, heating extrusion temperature control treatment: heating the primary U-shaped tube formed in step S2, controlling the heating temperature at 950° C. to 1050° C., and maintaining the temperature before extrusion molding at above 800° C.;

[0010] S4, extrusion finishing molding: placing the primary U-shaped tube which has been heated in step S3 and maintained at a temperature above 800° C. on an extrusion finishing die device, and forming the primary U-shaped tube by the extrusion finishing die device to form an intermediate U-shaped tube, wherein the bending radius R2 of the intermediate U-shaped tube is 0.6 to 1.1 times the outer diameter of the T91 alloy tube, and the elbow part of the intermediate U-shaped tube is flat inside and outside, with a smooth transition;

[0011] S5, normalizing and tempering heat treatment: the intermediate U-shaped tube is subjected to normalizing and tempering heat treatment to form a finished U-shaped tube, and the finished U-shaped tube is subjected to hardness and metallographic tests.

[0012] In some embodiments, in step S1, the temperature of the T91 alloy tube entering the furnace does not exceed 300°C. After entering the furnace, the temperature is increased from 300°C to 760±15°C at a heating rate of 110-150°C / h, and then kept warm for 50-70 minutes, and then cooled to below 300°C at a rate of 110-150°C. The T91 alloy tube is taken out of the furnace and air-cooled.

[0013] In some embodiments, in step S2, the first tube bending die body includes a die base, a tube bending main die, a clamping die, an anti-shrinkage neck die, a guide plate die and a mandrel, the tube bending main die is installed on the die base, the T91 alloy tube is clamped between the tube bending main die and the clamping die, the anti-shrinkage neck die and the guide plate die are embraced on both sides of the T91 alloy tube, the mandrel is filled in the T91 alloy tube, and the end of the mandrel is located at a predetermined rear end position of the starting point of the bending deformation of the T91 alloy tube;

[0014] During the process of the T91 alloy tube being clamped and rotated and bent by the main tube bending mold and the clamping mold, the guide plate mold moves linearly with the T91 alloy tube, and one end of the anti-shrinkage neck mold continuously abuts against the bending starting point. The inner and outer sandwich structure formed by the anti-shrinkage neck mold, the mandrel and the guide plate mold can make the deformation of the bending part of the T91 alloy tube uniform.

[0015] In some implementations, the outer diameter of the mandrel is the inner diameter of the T91 alloy tube*0.99-2 mm, and the end of the mandrel exceeds the bending point by 3 to 5 mm.

[0016] In some embodiments, the extrusion finishing die device includes left and right extrusion dies, a back die, and upper and lower extrusion dies, the back die is located at the ends of the left and right extrusion dies, and the back die and the left and right extrusion dies are provided with sliding inclined surfaces that are linked to each other;

[0017] The two arms of the primary U-shaped tube that has been heat treated in step S3 are placed in the left and right mold cavities formed by the left and right extrusion modules, and the end portion of the primary U-shaped tube is placed in the end mold cavity formed by the back mold group. During the process of the left and right extrusion modules extruding the two arms of the primary U-shaped tube closer to each other, the left and right extrusion modules push the back mold group to move through the adaptive linkage sliding slopes to adapt to the deformation of the elbow. The extrusion action is stopped when the radius of the primary U-shaped tube changes to 0.6-1.1 times the outer diameter of the T91 alloy tube, and then the primary U-shaped tube is clamped and shaped into an intermediate U-shaped tube by the upper and lower extrusion modules in an upper and lower clamping manner.

[0018] In some embodiments, the left and right extrusion modules include an upper left extrusion template, a lower left extrusion template, an upper right extrusion template and a lower right extrusion template. The upper left extrusion template and the lower left extrusion template are connected up and down to form a left extrusion cavity with a semicircular cross-section. The upper right extrusion template and the lower right extrusion template are connected up and down to form a right extrusion cavity with a semicircular cross-section. The left extrusion cavity and the right extrusion cavity are provided with symmetrical shrinkage transition zones and shrinkage molding zones. The shrinkage transition zones and the shrinkage molding zones are arranged sequentially from the starting point to the end of the elbow, and the diameter from the shrinkage transition zone to the shrinkage molding zone decreases sequentially.

[0019] In some embodiments, the back mold group includes an upper back mold and a lower back mold, and the upper back mold and the lower back mold are connected up and down to form an end mold cavity, and the arcuate end of the primary U-shaped tube is accommodated in the end mold cavity. The middle part of the end mold cavity is provided with an end avoidance area, and the end avoidance area is an arc-shaped structure extending outward, and the end avoidance area is used to accommodate the deformation of the top end of the elbow of the primary U-shaped tube.

[0020] In some embodiments, the upper and lower extrusion die sets include a finishing upper die, a finishing lower die, a positioning pin and a base plate, the finishing lower die is mounted on the base plate, the positioning pin is connected to the finishing lower die, the finishing upper die and the finishing lower die are joined together by the positioning pins, and the primary U-shaped tube extruded to a preset radius value is clamped in the cavity after the finishing lower die and the finishing upper die are joined together to form the intermediate U-shaped tube.

[0021] In some embodiments, in step S5, the method of normalizing and tempering heat treatment is: first normalizing treatment is performed, the intermediate U-shaped tube is placed in a furnace with a furnace temperature not exceeding 300°C, and then the temperature is increased to 1040-1050°C at a heating rate of 110-150°C / h, and then kept warm for 30-35 minutes. After the insulation is completed, it is taken out of the furnace at a furnace temperature not lower than 1040°C and air-cooled to room temperature; then tempering treatment is performed, specifically, the intermediate U-shaped tube is placed in a furnace with a furnace temperature not exceeding 300°C, and when it reaches 300°C, the temperature is increased to 750-770°C at a heating rate of 150-300°C / h and kept warm for 70-80 minutes, and after the insulation, it is taken out of the furnace at a temperature not lower than 730°C and air-cooled to room temperature.

[0022] The present invention also provides an energy-saving boiler superheater, which is made of a U-shaped tube formed by the small-radius U-shaped T91 alloy tube bending process.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention adds a tube annealing process before the first cold bending to improve the mechanical properties of the material, reduce internal stress, lower hardness, and avoid bending defects such as cracking and breaking of the first bending tube.

[0025] 2. In the present invention, according to the characteristics of T91 material, the heating extrusion temperature and the uniformity of the heating temperature are strictly controlled, the extrusion temperature is controlled at 950℃~1050℃, and the final extrusion temperature is ≥800℃, so as to ensure that the metallographic analysis of the finished elbow has uniform grains and the grain boundary span meets the requirements.

[0026] 3. The left and right extrusion dies in the small R extrusion finishing die device of the present invention are both provided with shrinkage deformation zones in their die cavities. The shrinkage deformation zones include shrinkage transition zones and shrinkage forming zones. The corresponding diameter shrinkage values ​​are set according to the pipe diameter and the bending radius. The left and right extrusion dies can be gradually reduced in the die cavities of the shrinkage transition zones, and the range of the shrinkage deformation zones can be changed according to different pipe diameters, and the necking transition is smooth.

[0027] 4. The end die cavity end size of the back die in the extrusion finishing die device of the present invention is increased, and the elbow end is fully formed by reserving space. At the same time, an end avoidance area is set in the middle of the end die cavity to make the elbow end fuller and improve the appearance quality.

[0028] 5. In the T91 alloy tube bending process of the present invention, the relative bending radius R1 is set to be less than 1.5 times the outer diameter of the tube in the first cold bending process, and then the bending radius R2 is 0.6 to 1.1 times the outer diameter of the tube through the fine forming process. Through progressive deformation control, the elbow part after bending is deformed evenly and smoothly, eliminating deformation problems such as goose egg shape. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0030] Figure 1 It is a schematic diagram of the oval structure of the end of a small radius U-shaped tube in the prior art;

[0031] Figure 2 This is a schematic diagram of the top view of the structure of the first bending mold body of the present invention clamping the T91 alloy tube;

[0032] Figure 3 This is a schematic diagram of the cross-sectional structure of the first bending mold body of the present invention clamping the T91 alloy tube;

[0033] Figure 4 It is a schematic diagram of the structure of the left and right extrusion modules of the present invention;

[0034] Figure 5 It is a structural schematic diagram of the upper left and lower left extrusion templates of the present invention;

[0035] Figure 6 This is a schematic diagram of the back module structure of the present invention;

[0036] Figure 7 It is a structural schematic diagram of the upper back template of the present invention;

[0037] Figure 8 It is a structural schematic diagram of the upper and lower extrusion dies of the present invention;

[0038] Fig. 9 for Figure 8 AA section structural diagram;

[0039] Fig.10 It is a schematic structural diagram of a small radius U-shaped tube after molding according to the present invention. DETAILED DESCRIPTION

[0040] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0041] The present invention provides a small radius U-shaped T91 alloy pipe bending process, comprising the following steps:

[0042] S1, annealing treatment before bending: place the T91 alloy tube 300 to be bent into a heating furnace, and its temperature when entering the furnace does not exceed 300℃. After entering the furnace, the temperature starts to rise to 760±15℃ at a rate of 110-150℃ / h after reaching 300℃, and then keep warm for 50-70min. After keeping warm, cool down to below 300℃ at a rate of 110-150℃, take the T91 alloy tube 300 out of the furnace, and cool it to room temperature by air. Heat treatment of T91 alloy tube before bending can effectively improve the mechanical properties of the material, reduce internal stress, reduce hardness, and effectively avoid bending defects such as cracking and breaking during the first cold bending process.

[0043] S2, first cold bending process: Figure 2-3 As shown, the T91 alloy tube 300 annealed in step S1 is placed on the first bending die 100 to form a primary U-shaped tube through cold bending. The bending radius R1 of the formed primary U-shaped tube is 1.3 to 1.5 times the outer diameter of the T91 alloy tube. The first bending die 100 includes a die base 110, a bending main die 120, a clamping die 130, an anti-shrinkage neck die 140, a guide plate die 150 and a mandrel 160. The bending main die 120 is installed on the die base 110, the clamping die 130 is located on one side of the bending main die 120, and the T91 alloy tube 300 is clamped between the bending main die 120 and the clamping die 130. The front end of the anti-shrinkage neck die 140 has an arc surface adapted to the bending main die 120. The anti-shrinkage neck die 140 and the guide plate die 150 are respectively arranged on the side push mechanism of the bending equipment, and butt and embrace the T91 alloy tube from both sides. At this time, the anti-shrinkage neck mold 140 is located at the rear side of the main mold 120 of the bending pipe, and the guide plate mold 150 is located at the rear side of the clamping mold 130. The so-called rear side here refers to the front and rear position relationship determined by the forward direction of the T91 alloy tube during the bending process. The mandrel 160 is placed in the T91 alloy tube 300. The shape, size and position of the mandrel 160 inserted into the tube will affect the quality of the bending tube. In this embodiment, the mandrel 160 is set on the mandrel rod equipped with the bending equipment and inserted into the cavity of the T91 alloy tube. The end position of the mandrel 160 should exceed the predetermined distance of the bending point of the T91 alloy tube, and the excess amount is preferably controlled at 3-5mm. The diameter of the mandrel 160 is set according to the inner diameter of the T91 alloy tube, and the basis for its setting is: the outer peripheral surface diameter of the mandrel 160 is the inner peripheral surface diameter of the T91 alloy tube*0.99-2mm.

[0044] The T91 alloy tube is rotated and bent to form a U shape under the clamping of the tube bending main die 120 and the clamping die 130. During the rotational bending deformation process, the guide plate die 150 presses the T91 alloy tube and moves linearly with the T91 alloy tube. One end of the anti-shrinkage neck die 140 continuously abuts against the position close to the bending start point 301. The T91 alloy tube is bent and deformed when close to the tube bending main die 120 and separated. When the T91 alloy tube is deformed, the inner and outer sandwich structure formed by the anti-shrinkage neck die 140, the mandrel 160, and the guide plate die 150 can make the deformation of the bending part of the T91 alloy tube uniform.

[0045] S3, heating extrusion temperature control treatment: heat the primary U-shaped tube formed in step S2, strictly control the heating extrusion temperature and the uniformity of the heating temperature, control the heating temperature at 950℃~1050℃, and keep the temperature before extrusion molding above 800℃ to ensure that the metallographic analysis of the finished elbow has uniform grains and the grain boundary span meets the requirements.

[0046] S4, extrusion finishing molding: the primary U-shaped tube which has been heated in step S3 and maintained at a temperature above 800°C is placed on an extrusion finishing die device 200, and the primary U-shaped tube is shaped by the extrusion finishing die device 200 to form an intermediate U-shaped tube, wherein the bending radius R2 of the intermediate U-shaped tube changes from the radius R1 of the primary U-shaped tube, which is 1.3 to 1.5 times the outer diameter of the T91 alloy tube, to 0.6 to 1.1 times. The process is basically as follows: the two arms of the primary U-shaped tube that has been heated in step S3 are placed in the left and right mold cavities formed by the left and right extrusion modules 210, and the end of the primary U-shaped tube is placed in the end mold cavity formed by the back mold 220. When the left and right extrusion modules 210 extrude the two arms of the primary U-shaped tube to approach each other, the left and right extrusion modules 210 push the back mold 220 to move through the adaptive linkage sliding slope to adapt to the deformation of the elbow. When the radius of the primary U-shaped tube changes to 0.6-1.1 times the outer diameter of the T91 alloy tube, the extrusion action is stopped, and then the primary U-shaped tube is clamped and shaped into an intermediate U-shaped tube by the upper and lower extrusion modules 230 in an upper and lower clamping manner. Specifically:

[0047] like Figure 4-5As shown, the extrusion finishing die device 200 includes left and right extrusion dies 210, a back die 220 and upper and lower extrusion dies 230. The back die 220 is located at the ends of the left and right extrusion dies 210, and the back die 220 and the left and right extrusion dies 210 are provided with sliding inclined surfaces linked to each other. The left and right extrusion modules 210 include an upper left extrusion template 211, a lower left extrusion template 212, an upper right extrusion template 213 and a lower right extrusion template 214. The upper left extrusion template 211 and the lower left extrusion template 212 are butted up and down to form a left extrusion cavity with a semicircular cross-section. The upper right extrusion template 213 and the lower right extrusion template 214 are butted up and down to form a right extrusion cavity with a semicircular cross-section. The left extrusion cavity and the right extrusion cavity are provided with symmetrical shrinkage transition zones 2101 and shrinkage molding zones 2102. The shrinkage transition zones 2101 and the shrinkage molding zones 2102 are arranged sequentially from the bending point 301 to the elbow end, and the diameters from the shrinkage transition zone 2101 to the shrinkage molding zone 2102 decrease sequentially. In this embodiment, in the extrusion finishing die device 200, a shrinkage transition zone and a shrinkage molding zone are set in the die cavity of the left and right extrusion dies 210, and the corresponding diameter shrinkage value is set according to the tube diameter and the bending radius, so that the bending part of the T91 alloy tube can be gradually reduced in the die cavity of the left and right extrusion dies, thereby obtaining the technical effect of smooth necking transition.

[0048] like Figure 6-7 As shown, the back mold group 220 includes an upper back mold plate 221 and a lower back mold plate 222. The upper back mold plate 221 and the lower back mold plate 222 are butted up and down to form an end mold cavity 2201, and the arc-shaped end of the primary U-shaped tube is placed in the end mold cavity 2201. The mold cavity end size of the end mold cavity 2201 is larger than the end size of the primary U-shaped tube, and the middle part of the end mold cavity 2201 is provided with an end avoidance area 2202, and the end avoidance area 2202 is an arc-shaped structure extending outward. The back mold group 220 is arranged at the front end of the left and right extrusion mold groups 210, and the symmetrical inclined surface of the back mold group 220 matches and slides with the front end inclined surfaces of the left and right extrusion mold groups 210 to drive the back mold group 220 to move forward synchronously. When the primary U-shaped tube is extruded through the left and right extrusion modules 210, the outer tube wall of the elbow is stretched significantly and the inner tube wall is thickened. By setting the end size of the end die cavity 2201 larger than the end size of the primary U-shaped tube and leaving space, the end of the elbow is fully formed and the roundness meets the standard requirements. At the same time, an end avoidance area 2202 is set in the middle position of the end die cavity 2201 to accommodate the deformation of the top end of the primary U-shaped tube elbow, so that the end of the elbow can be full in shape and the appearance quality meets the requirements.

[0049] like Figure 8-9As shown, the upper and lower extrusion dies 230 include a finishing upper die 231, a finishing lower die 232, a positioning pin 233 and a bottom plate 234. The finishing lower die 232 is mounted on the bottom plate 234, the positioning pin 233 is connected to the finishing lower die 232, the finishing upper die 231 and the finishing lower die 232 are joined by the positioning pin 233, and the primary U-shaped tube extruded to a preset radius value is clamped in the cavity after the finishing lower die 232 and the finishing upper die 231 are joined to form an intermediate U-shaped tube. The mold cavity specifications formed by the upper and lower extrusion dies 230 are the same as the elbow specifications of the intermediate U-shaped tube. Considering the need for shaping the elbow shape, the transition arcs of the finishing upper die 231 and the finishing lower die 232 are ground to R≥3, which is convenient for demoulding and there is no scratch on the elbow surface.

[0050] S5, normalizing and tempering heat treatment: firstly, normalizing treatment is performed, after the intermediate U-shaped tube is placed in a furnace with a temperature not exceeding 300°C, the temperature is increased to 1040-1050°C at a heating rate of 110-150°C / h, and then the temperature is kept for 30-35 minutes. After the temperature is kept, the intermediate U-shaped tube is taken out of the furnace at a temperature not lower than 1040°C and air-cooled to room temperature; then tempering treatment is performed, specifically, the intermediate U-shaped tube is placed in a furnace with a temperature not exceeding 300°C, and the temperature is increased to 750-770°C at a heating rate of 150-300°C / h from 300°C and kept for 70-80 minutes. After the temperature is kept, the intermediate U-shaped tube is taken out of the furnace at a temperature not lower than 730°C and air-cooled to room temperature. Hardness and metallographic tests are performed on the finished product that is air-cooled to room temperature.

[0051] In the T91 alloy tube bending process of the present invention, the relative bending radius R1 is set to be less than 1.5 times the outer diameter of the tube in the first cold bending process, and then the bending radius R2 is 0.6-1.1 times the outer diameter of the tube through the extrusion finishing forming process. Through the progressive deformation control, the elbow part after bending is deformed uniformly and smoothly, eliminating the deformation problems such as goose egg shape, such as Fig.10 shown.

[0052] In the description of the present application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0053] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A small radius U-shaped T91 alloy tube bending process, characterized in that: include: S1, annealing treatment before bending: annealing the T91 alloy tube to be bent, controlling the temperature of the T91 alloy tube entering the furnace, the temperature in the furnace and the temperature out of the furnace, and air cooling the T91 alloy tube to room temperature after it is out of the furnace; S2, first cold bending forming treatment: placing the T91 alloy tube annealed in step S1 on a first bending die (100) for cold bending to form a primary U-shaped tube, wherein the bending radius R1 of the primary U-shaped tube is 1.3 to 1.5 times the outer diameter of the T91 alloy tube; S3, heating extrusion temperature control treatment: heating the primary U-shaped tube formed in step S2, controlling the heating temperature at 950° C. to 1050° C., and maintaining the temperature before extrusion molding at above 800° C.; S4, extrusion finishing molding: placing the primary U-shaped tube which has been heated in step S3 and maintained at a temperature above 800° C. on an extrusion finishing die device (200), and subjecting the primary U-shaped tube to a shaping process by the extrusion finishing die device (200) to form an intermediate U-shaped tube, wherein the bending radius R2 of the intermediate U-shaped tube is 0.6 to 1.1 times the outer diameter of the T91 alloy tube, and the elbow part of the intermediate U-shaped tube is flat inside and outside, with a smooth transition; S5, normalizing and tempering heat treatment: the intermediate U-shaped tube is subjected to normalizing and tempering heat treatment to form a finished U-shaped tube, and the finished U-shaped tube is subjected to hardness and metallographic tests.

2. The small radius U-shaped T91 alloy tube bending process according to claim 1 is characterized in that: In the step S1, the temperature of the T91 alloy tube entering the furnace does not exceed 300°C. After entering the furnace, the temperature is increased from 300°C to 760±15°C at a heating rate of 110-150°C / h, and then kept at this temperature for 50-70 minutes. Then, the temperature is decreased to below 300°C at a rate of 110-150°C. The T91 alloy tube is taken out of the furnace and air-cooled.

3. The small radius U-shaped T91 alloy tube bending process according to claim 1 is characterized in that: In step S2, the first tube bending die body (100) comprises a die base (110), a tube bending main die (120), a clamping die (130), an anti-shrinkage neck die (140), a guide plate die (150) and a mandrel (160); the tube bending main die (120) is mounted on the die base (110); the T91 alloy tube is clamped between the tube bending main die (120) and the clamping die (130); the anti-shrinkage neck die (140) and the guide plate die (150) are embraced on both sides of the T91 alloy tube; the mandrel (160) is filled in the T91 alloy tube; and the end of the mandrel (160) is located at a predetermined rear end position of a bending starting point (301) of the bending deformation of the T91 alloy tube; During the process of the T91 alloy tube being clamped and rotated and bent by the main tube bending mold (120) and the clamping mold (130), the guide plate mold (150) moves linearly with the T91 alloy tube, and one end of the anti-shrinkage neck mold (140) continuously abuts against the position of the bending starting point (301). The inner and outer sandwich structure formed by the anti-shrinkage neck mold (140), the mandrel (160) and the guide plate mold (150) can make the deformation of the bending part of the T91 alloy tube uniform.

4. The small radius U-shaped T91 alloy tube bending process according to claim 3 is characterized in that: The outer diameter of the mandrel (160) is the inner diameter of the T91 alloy tube*0.99-2 mm, and the distance that the end of the mandrel (160) exceeds the bending point is 3 to 5 mm.

5. The small radius U-shaped T91 alloy tube bending process according to claim 1 is characterized in that: The extrusion finishing die device (200) comprises left and right extrusion die sets (210), a back die set (220) and upper and lower extrusion die sets (230); the back die set (220) is located at the ends of the left and right extrusion die sets (210); the back die set (220) and the left and right extrusion die sets (210) are provided with sliding inclined surfaces that are linked to each other; The two arms of the primary U-shaped tube that has been heated in step S3 are placed in the left and right mold cavities formed by the left and right extrusion dies (210), and the end portion of the primary U-shaped tube is placed in the end mold cavity formed by the back mold group (220). During the process of the left and right extrusion dies (210) extruding the two arms of the primary U-shaped tube to approach each other, the left and right extrusion dies (210) push the back mold group (220) to move through the adaptive linkage sliding inclined surfaces to adapt to the deformation of the elbow portion, and the extrusion action is stopped when the radius of the primary U-shaped tube changes to 0.6-1.1 times the outer diameter of the T91 alloy tube, and then the primary U-shaped tube is clamped and shaped into an intermediate U-shaped tube by the upper and lower extrusion dies (230) in an upper and lower clamping manner.

6. The small radius U-shaped T91 alloy tube bending process according to claim 5 is characterized in that: The left and right extrusion mold groups (210) comprise an upper left extrusion template (211), a lower left extrusion template (212), an upper right extrusion template (213) and a lower right extrusion template (214); the upper left extrusion template (211) and the lower left extrusion template (212) are butted up and down to form a left extrusion mold cavity with a semicircular cross section; the upper right extrusion template (213) and the lower right extrusion template (214) are butted up and down to form a right extrusion mold cavity with a semicircular cross section; the left extrusion mold cavity and the right extrusion mold cavity are provided with symmetrical shrinkage transition zones (2101) and shrinkage molding zones (2102); the shrinkage transition zones (2101) and the shrinkage molding zones (2102) are arranged in sequence from the bending starting point (301) to the elbow end; and the diameters from the shrinkage transition zone (2101) to the shrinkage molding zone (2102) are reduced in sequence.

7. The small radius U-shaped T91 alloy tube bending process according to claim 5 is characterized in that: The back mold group (220) comprises an upper back mold plate (221) and a lower back mold plate (222); the upper back mold plate (221) and the lower back mold plate (222) are butted up and down to form an end mold cavity (2201); the arc-shaped end of the primary U-shaped tube is accommodated in the end mold cavity (2201); an end avoidance area (2202) is provided in the middle portion of the end mold cavity (2201); the end avoidance area (2202) is an arc-shaped structure extending outward, and the end avoidance area (2202) is used to accommodate the deformation of the top end of the elbow of the primary U-shaped tube.

8. The small radius U-shaped T91 alloy tube bending process according to claim 5 is characterized in that: The upper and lower extrusion dies (230) comprise a finishing upper die (231), a finishing lower die (232), a positioning pin (233) and a bottom plate (234); the finishing lower die (232) is mounted on the bottom plate (234); the positioning pin (233) is connected to the finishing lower die (232); the finishing upper die (231) and the finishing lower die (232) are joined together and positioned and docked by the positioning pin (233); the primary U-shaped tube extruded to a preset radius value is clamped in the cavity after the finishing lower die (232) and the finishing upper die (231) are joined together to form the intermediate U-shaped tube.

9. The small radius U-shaped T91 alloy tube bending process according to claim 1 is characterized in that: In step S5, the method of normalizing and tempering heat treatment is: first, normalizing treatment is performed, the intermediate U-shaped tube is placed in a furnace with a furnace temperature not exceeding 300°C, and then the temperature is increased to 1040-1050°C at a heating rate of 110-150°C / h, and then kept warm for 30-35 minutes. After the insulation is completed, it is taken out of the furnace at a furnace temperature not lower than 1040°C and then air-cooled to room temperature; then tempering treatment is performed, specifically, the intermediate U-shaped tube is placed in a furnace with a furnace temperature not exceeding 300°C, and when it reaches 300°C, the temperature is increased to 750-770°C at a heating rate of 150-300°C / h and kept warm for 70-80 minutes, and after the insulation, it is taken out of the furnace at a temperature not lower than 730°C and then air-cooled to room temperature.

10. An energy-saving boiler superheater, characterized in that: The U-shaped tube is made by using the small radius U-shaped T91 alloy tube bending process as described in any one of claims 1 to 9.

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