Induction heat treatment preparation method of double-strengthened thin-wall metal corrugated pipe

Abstract

The invention relates to an induction heat treatment preparation method of a double-strengthened thin-wall metal corrugated pipe. The induction heat treatment preparation method comprises the following steps: S1, providing a precursor corrugated pipe with uniform grain size space distribution after forming; s2, the formed corrugated pipe is subjected to induction heating treatment, the temperature rise rate is 300-500 DEG C / s, heating is started from the room temperature, the peak heating end temperature is 1050-1150 DEG C, and the valley heating end temperature is 980-1080 DEG C; the heat preservation time is 2-10 s; and S3, the corrugated pipe is firstly subjected to forced cooling to 250-350 DEG C in a jet-propelled multi-stage forced cooling mode, and then air cooling is carried out. According to the corrugated pipe prepared through the method, the grain size is in gradient distribution, the grain size is gradually reduced from the wave crest to the wave trough, and the effect of controlling grain growth in the wave trough area is obvious. And meanwhile, a large number of fine strip martensite distributed in the corrugation direction exists in the corrugated pipe, and cracks can be induced to expand in the preset corrugation direction during bending deformation. And under a double strengthening strategy of a gradient structure and prefabricated martensite, the bending resistance is greatly improved.

Description

Technical Field

[0001] The invention relates to the technical field of structural gradient materials, and in particular to an induction heat treatment preparation method for a double-reinforced thin-walled metal bellows. Background Art

[0002] Metal bellows is a flexible tube shell part with U-shaped, S-shaped, Ω-shaped and other cross-sectional structural forms of corrugation. It has the advantages of good sealing performance, high reliability, strong flexible compensation ability, good sound-absorbing and shock-absorbing effects, etc. As flexible connectors, elastic elements and seals, it is widely used in aerospace, petrochemical, nuclear power, metallurgy and other fields. However, in the actual service process, in order to compensate for the displacement of the pipeline, the bellows will be in a state of repeated bending for a long time, resulting in the phenomenon of fracture failure at the trough, which seriously restricts the quality of the product.

[0003] There are many ways to improve the bending resistance of bellows, such as changing the waveform parameters, introducing reinforcing hoops into the bellows, etc. However, the above methods are difficult to produce and the process means are relatively complicated. Compared with adjusting the macrostructure, it is more convenient and feasible to improve the bending resistance of the bellows by adjusting the microstructure of the bellows. Gradient strengthening, as a non-uniform strengthening strategy, can provide spatial gradient distribution characteristics of the matrix by adjusting the grain size. Unlike composite materials, the microstructure of gradient materials has a natural transition, and there is no obvious interface inside, which avoids the sudden change of performance caused by structural changes, and can improve one or more properties of the material without sacrificing other properties.

[0004] At present, the methods for preparing gradient materials are mainly physical deposition, chemical deposition and surface mechanical methods. However, the above methods have high requirements on the size of the sample. At the same time, due to the special thin-wall characteristics and corrugated structure of the bellows, the above methods are difficult to be used for the preparation of grain gradient structure bellows. Therefore, how to produce grain gradient structure bellows has become a problem that needs to be solved urgently.

[0005] In addition to grain size adjustment, phase adjustment is also one of the feasible methods to improve the bending resistance of bellows. Martensite, as one of the most common phases in austenitic stainless steel, can significantly improve the strength of the material and is widely used in the design of strong and tough steel in engineering. However, the existing microstructure regulation is mainly focused on regular materials such as plates and bars, and its application in thin-walled structural parts such as bellows is relatively scarce. Therefore, it is particularly important to achieve rational directional regulation of martensite in thin-walled stainless steel bellows. Summary of the invention

[0006] In view of the above problems, the purpose of the present invention is to provide an induction heat treatment preparation method for a double-strengthened thin-walled metal bellows. By using an induction heating method, appropriate heating parameters are applied to the crests and troughs respectively, and then the crests and troughs are graded and cooled using appropriate cooling rates. A bellows with a grain size gradient structure can be successfully prepared, and the grain size gradually refines from the crest to the trough. The refined grain size can make the trough more uniformly stressed. At the same time, under the refined regulation of the heat treatment process, a large number of fine banded martensite is distributed in the matrix along the corrugation direction. The brittleness of martensite is used to guide the cracks to expand along the preset corrugation direction, thereby slowing down the expansion rate of the main crack along the wall thickness direction. Under the dual strengthening strategy of gradient structure and prefabricated martensite, the bending performance of the bellows is greatly improved.

[0007] The technical solution adopted by the present invention is as follows:

[0008] The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows proposed by the present invention comprises the following steps:

[0009] S1. Provide a precursor bellows with uniform grain size distribution after forming;

[0010] S2. The formed corrugated pipe is subjected to induction heating treatment, with a temperature rise rate of 300-500°C / s, heating starts from room temperature, the peak heating termination temperature is 1050-1150°C, and the trough heating termination temperature is 980-1080°C; the holding time is 2-10s;

[0011] S3. The bellows is first cooled by jet multi-stage forced cooling, forced cooling to 250-350°C and then slowly cooled.

[0012] Furthermore, in step S2, induction heating is performed using a spiral induction heating coil, and the heating termination temperatures and average temperature rise rates of the peaks and troughs are as shown in the following formulas:

[0013]

[0014] K 谷 =T 谷 ×v / L;

[0015] K 峰 =T 峰 ×v / L;

[0016] Where: T 谷 is the heating termination temperature of the trough, in °C; T 峰 is the heating termination temperature of the peak, in °C; K 谷 is the average temperature rise rate of the trough, unit: ℃ / s; K 峰is the average temperature rise rate of the crest, in units of ℃ / s; v is the movement speed of the bellows, in units of cm / s, ranging from 2 to 6 cm / s; I is the magnitude of the external current, in units of A, ranging from 1200 to 1500A; d1 is the distance from the trough to the inner wall of the coil, in units of mm, ranging from 2 to 20mm; d2 is the distance from the crest to the inner wall of the coil, in units of mm, ranging from 1 to 19mm; f is the current frequency, in units of kHz, ranging from 10 to 40kHz; L is the length of the coil, in units of cm, ranging from 5 to 30cm; N is the number of coil turns, ranging from 3 to 18.

[0017] Furthermore, the cross section of the induction heating coil is circular, square or rectangular; the wall thickness of the induction heating coil is 0.5 to 3 mm.

[0018] Furthermore, in step S3, the jet multi-stage forced cooling is adjusted according to the actual layout of the production line, and the default is 3 levels; wherein, the peak cooling rate of the first stage forced cooling is 180-270°C / s, the trough cooling rate is 160-240°C / s, and the cooling time is 2-3s; the peak cooling rate of the second stage forced cooling is 100-145°C / s, the trough cooling rate is 80-130°C / s, and the cooling time is 1-2s; the peak cooling rate of the third stage forced cooling is 60-100°C / s, the trough cooling rate is 40-90°C / s, and the cooling time is 1-2s.

[0019] Furthermore, in the bellows cooling process, except for the first stage cooling time of 2 to 3 seconds, the remaining stages are all 1 to 2 seconds; the variation law of the cooling rate of the peak and the trough with the cooling stage is shown in the following formula:

[0020]

[0021] Where: C 峰 is the cooling rate of the peak, in ℃ / s; C 谷 is the cooling rate of the trough, in °C / s; N is the number of stages set for multi-stage cooling, and N is an integer greater than zero.

[0022] Furthermore, in step S3, the grain size of the cooled bellows is gradually refined from the crest to the trough in the form of a continuous gradient, and the bellows contains a large amount of fine banded martensite distributed along the corrugation direction.

[0023] Furthermore, the cooling gas used in the jet-type multi-stage forced cooling is nitrogen or hydrogen or NH3 decomposition gas at 0 to 25°C.

[0024] Furthermore, the wall thickness of the bellows is 0.1-0.5 mm, and the material is 304, 316 or 321 stainless steel.

[0025] Furthermore, the bellows is formed by combined spinning and rolling forming, hydraulic forming or mechanical bulging.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. Through induction heating and graded strengthening cooling, a thin-walled metal bellows with gradient structure and prefabricated martensite dual strengthening was successfully prepared. Compared with the bellows heat-treated in the existing mesh belt heating furnace, the bending resistance is significantly enhanced, and the number of repeated bending times is increased by nearly 300%.

[0028] 2. The thin-walled metal bellows with gradient structure and prefabricated martensite dual strengthening prepared by the present invention is less affected by the pipe diameter and the corrugation form of the cross-section of the bellows, and has simple process means, low cost, and high economic benefits in industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the microstructure of a double-reinforced thin-walled metal bellows;

[0030] Figure 2 It is a schematic diagram of geometric parameters during the induction heating process of the bellows;

[0031] Figure 3 It is a schematic diagram of the EBSD characterization results of the wave crests and troughs after the combined forming of spinning and rolling;

[0032] Figure 4 is a schematic diagram of the microstructure of the corrugated tube after corrosion in an embodiment of the present invention, taken under an optical microscope;

[0033] Figure 5 This is a microstructure diagram of the corrugated pipe in the comparative example of the present invention taken under an optical microscope after corrosion;

[0034] Figure 6 Schematic diagram of grain sizes at different positions of comparative examples and embodiments of the present invention;

[0035] Figure 7 This is a schematic diagram of the phase distribution results after the troughs of an embodiment of the present invention are characterized by EBSD;

[0036] Figure 8 The EBSD characterization result diagram of cracks at the side wall of the trough of the comparative example and the embodiment of the present invention;

[0037] Fig. 9 Schematic diagram of crack propagation of the trough matrix during the bending process of the comparative example and the embodiment of the present invention;

[0038] Fig.10 It is a schematic diagram showing the comparison results of the bending times of the bellows before breaking of the comparative example and the embodiment of the present invention. DETAILED DESCRIPTION

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0040] The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows proposed in the present invention specifically comprises the following steps:

[0041] S1. Providing a precursor bellows with uniform spatial distribution of grain size after forming; wherein the bellows is formed by a forming method including combined spinning and rolling forming, hydraulic forming, mechanical bulging, etc.

[0042] S2. The formed corrugated pipe is subjected to induction heating treatment, with a temperature rise rate of 300-500°C / s, heating starts from room temperature, the peak heating termination temperature is 1050-1150°C, and the trough heating termination temperature is 980-1080°C; the holding time is 2-10s;

[0043] Induction heating uses a spiral induction heating coil for heating, the cross section of the induction heating coil is circular, square, rectangular, etc.; the wall thickness of the induction heating coil is 0.5 to 3 mm;

[0044] The changes in the heating termination temperature and temperature rise rate of the peak and trough are achieved through the following formula:

[0045]

[0046] K 谷 =T 谷 ×v / L;

[0047] K 峰 =T 峰 ×v / L;

[0048] Where: T 谷 is the heating termination temperature of the trough, in °C; T 峰 is the heating termination temperature of the peak, in °C; K 谷 is the average temperature rise rate of the trough, unit: ℃ / s; K 峰is the average temperature rise rate of the crest, in units of ℃ / s; v is the movement speed of the bellows, in units of cm / s, ranging from 2 to 6 cm / s; I is the magnitude of the external current, in units of A, ranging from 1200 to 1500A; d1 is the distance from the trough to the inner wall of the coil, in units of mm, ranging from 2 to 20mm; d2 is the distance from the crest to the inner wall of the coil, in units of mm, ranging from 1 to 19mm; f is the current frequency, in units of kHz, ranging from 10 to 40kHz; L is the length of the coil, in units of cm, ranging from 5 to 30cm; N is the number of coil turns, ranging from 3 to 18.

[0049] S3. The bellows is first cooled by jet multi-stage forced cooling to 250-350℃ and then air-cooled;

[0050] The cooling gas used for the jet-type multi-stage forced cooling is nitrogen or hydrogen or NH3 decomposition gas at 0-25°C;

[0051] Among them, the jet multi-stage forced cooling is adjusted according to the actual layout of the production line, and the default is 3 levels; among them, the peak cooling rate of the first level forced cooling is 180-270℃ / s, the trough cooling rate is 160-240℃ / s, and the cooling time is 2-3s; the peak cooling rate of the second level forced cooling is 100-145℃ / s, the trough cooling rate is 80-130℃ / s, and the cooling time is 1-2s; the peak cooling rate of the third level forced cooling is 60-100℃ / s, the trough cooling rate is 40-90℃ / s, and the cooling time is 1-2s.

[0052] During the bellows cooling process, except for the first stage cooling time of 2 to 3 seconds, the remaining stages are all 1 to 2 seconds; the variation of the cooling rate of the peak and trough with the cooling stage is shown in the following formula:

[0053]

[0054] Where: C 峰 is the cooling rate of the peak, in ℃ / s; C 谷 is the cooling rate of the trough, in °C / s; N is the number of stages set for multi-stage cooling, and N is an integer greater than zero.

[0055] The grain size of the cooled bellows is gradually refined from the wave crest to the wave trough in the form of a continuous gradient, and the inside of the bellows contains a large amount of fine band-shaped martensite distributed along the wave direction.

[0056] The cross-sectional corrugation of the bellows can be U-shaped, S-shaped, C-shaped, or Ω-shaped, etc. The present invention is less restricted by the shape of the corrugations. The wall thickness of the bellows is 0.1-0.5 mm, and the material is 304, 316, or 321 stainless steel.

[0057] The present invention will be further described below by specific examples and comparative examples:

[0058] Example

[0059] S1. Provide a precursor DN12 corrugated tube with uniform grain size distribution and an average grain size of 20μm after combined spinning and rolling. The trough is the main deformation area, the dislocation density trough is greater than the peak, and the matrix structure of the peak and trough is mainly austenite, containing a small amount of martensite;

[0060] S2. The formed bellows is subjected to induction heating treatment, wherein the induction heating is performed by a spiral induction heating coil, the cross section of the induction heating coil is rectangular, the coil wall thickness is 1 mm, the distances from the crest and trough to the inner wall of the coil are 1.5 mm and 3.5 mm respectively, the moving speed of the bellows is 3.5 cm / s, the applied current is 1500 A, the current frequency is 33 kHz, the coil length is 10 cm, the number of turns is 6 turns, the crest temperature rise rate is 387°C / s, heating starts from 20°C, the heating end temperature is 1125°C, the trough temperature rise rate is 363°C / s, and the heating end temperature is 1056°C; the holding time is 5s;

[0061] S3. The bellows first adopts a jet-type three-stage forced cooling method to force the peaks and troughs to about 250-350°C and then air-cool. The cooling gas used for the jet-type three-stage forced cooling is nitrogen at 25°C.

[0062] Among them, the first stage forced cooling peak cooling rate is 200℃ / s, the trough cooling rate is 180℃ / s, and the cooling time is 3s; the second stage forced cooling peak cooling rate is 115℃ / s, the trough cooling rate is 95℃ / s, and the cooling time is 1.5s; the third stage forced cooling peak cooling rate is 70℃ / s, the trough cooling rate is 55℃ / s, and the cooling time is 1.2s.

[0063] The grain size of the cooled bellows decreases from 70μm at the crest to 45μm at the waist, and then to 20μm at the trough, and the grain size shows a continuous gradient evolution law. In addition, the matrix of the bellows contains a large amount of fine martensite distributed along the corrugation direction. During the bending process, this banded martensite can induce cracks to expand along the corrugation direction, thereby delaying the expansion speed of the main crack along the wall thickness direction.

[0064] The cross-sectional corrugation of the bellows is U-shaped, the wall thickness is 0.25 mm, and the material is 321 stainless steel.

[0065] Comparative Example

[0066] A heat treatment process of a thin-walled metal corrugated pipe mesh belt type heating furnace comprises the following steps:

[0067] S1. Provide a precursor DN12 bellows with uniform grain size distribution and an average grain size of 20μm after combined spinning and rolling forming;

[0068] S2. Use a mesh belt heating furnace to heat the formed bellows. Since the bellows are heated and cooled by convection heat transfer, the temperature difference between different parts of the bellows is very small. It can be approximately considered that the temperature of the bellows is uniform during the heat treatment process. Start heating from room temperature and heat to 1050°C within 240s, and keep warm for 60s.

[0069] S3, introduce 25°C NH3 decomposition gas into the heating furnace, cool the bellows along with the furnace, and cool the bellows to room temperature within 420s;

[0070] The cross-sectional corrugation of the bellows is U-shaped, the wall thickness is 0.25 mm, and the material is 304 stainless steel.

[0071] like Figure 3 and Figure 6 As shown, under the premise that the spatial distribution of the grain size of the precursor bellows is uniform and the average grain size is similar, the bellows prepared in the embodiment of the present invention has a gradually refined grain size along the transition from the crest to the trough, showing an obvious grain size gradient structure, while the grains at different positions of the bellows in the comparative example are coarse and uniform, with no gradient characteristics.

[0072] like Figure 4 , Figure 5 and Figure 7 As shown, the bellows prepared in the embodiment of the present invention has a large amount of fine banded martensite distributed along the corrugation direction, while the comparative example is almost entirely austenite structure, without banded martensite distributed along the corrugation direction. RD is the corrugation depth.

[0073] like Figure 8 As shown, due to the coarse grain size of the trough in the comparative example, the matrix is ​​unevenly stressed and there are local high dislocation density areas. These areas will undergo strain-induced martensitic transformation. Due to the brittleness of martensite, a large number of cracks initiate and propagate in the martensite, showing obvious transgranular fracture characteristics. As the grain size of the trough in the embodiment is significantly refined, the stress is more uniform, the high dislocation density areas are significantly reduced and the distribution is more uniform, and cracks also initiate and propagate in the martensite, showing obvious intergranular fracture characteristics.

[0074] like Fig. 9 and 10As shown, in the embodiment of the present invention, the refined grain size of the corrugated tube trough can improve the force uniformity of the matrix. At the same time, due to the brittleness of martensite, the banded martensite can induce cracks to expand along the corrugation direction during the bending process, thereby delaying the expansion of the main crack along the wall thickness direction. Under the dual strengthening strategy of gradient structure and prefabricated martensite, the bending resistance of the bellows is significantly improved. Compared with the bellows heat-treated in an ordinary heating furnace in the comparative example, the bellows prepared by the present invention has an improved bending resistance of nearly 300%. The results show that the present invention has a significant effect on improving the bending resistance of the bellows, and the bending test process is consistent with the GB / T 35990-2018 experimental standard.

[0075] Matters not described in detail in the present invention are all known technologies.

[0076] The embodiments described above are merely descriptions of preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a double-strengthened thin-walled metal bellows by induction heat treatment, characterized in that: The method comprises the following steps: S1. Provide a precursor bellows with uniform grain size distribution after forming; S2. The formed corrugated pipe is subjected to induction heating treatment, with a temperature rise rate of 300-500°C / s, heating starts from room temperature, the peak heating termination temperature is 1050-1150°C, and the trough heating termination temperature is 980-1080°C; the holding time is 2-10s; S3. The bellows is first cooled by jet multi-stage forced cooling, forced cooling to 250-350°C and then slowly cooled.

2. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 1 is characterized in that: In step S2, induction heating is performed using a spiral induction heating coil, and the heating termination temperatures and average temperature rise rates of the peaks and troughs are as shown in the following formula: K 谷 =T 谷 ×v / L; K 峰 =T 峰 ×v / L; Where: T 谷 is the heating termination temperature of the trough, in °C; T 峰 is the heating termination temperature of the peak, in °C; K 谷 is the average temperature rise rate of the trough, unit: ℃ / s; K 峰 is the average temperature rise rate of the crest, in units of ℃ / s; v is the movement speed of the bellows, in units of cm / s, ranging from 2 to 6 cm / s; I is the magnitude of the external current, in units of A, ranging from 1200 to 1500A; d1 is the distance from the trough to the inner wall of the coil, in units of mm, ranging from 2 to 20mm; d2 is the distance from the crest to the inner wall of the coil, in units of mm, ranging from 1 to 19mm; f is the current frequency, in units of kHz, ranging from 10 to 40kHz; L is the length of the coil, in units of cm, ranging from 5 to 30cm; N is the number of coil turns, ranging from 3 to 18.

3. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 2, characterized in that: The cross section of the induction heating coil is circular, square or rectangular; the wall thickness of the induction heating coil is 0.5-3 mm.

4. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 1 is characterized in that: In step S3, the jet multi-stage forced cooling is adjusted according to the actual layout of the production line, and the default is 3 levels; wherein, the peak cooling rate of the first stage forced cooling is 180-270°C / s, the trough cooling rate is 160-240°C / s, and the cooling time is 2-3s; the peak cooling rate of the second stage forced cooling is 100-145°C / s, the trough cooling rate is 80-130°C / s, and the cooling time is 1-2s; the peak cooling rate of the third stage forced cooling is 60-100°C / s, the trough cooling rate is 40-90°C / s, and the cooling time is 1-2s.

5. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 4, characterized in that: During the bellows cooling process, except for the first stage cooling time of 2 to 3 seconds, the remaining stages are all 1 to 2 seconds; the variation of the cooling rate of the peak and trough with the cooling stage is shown in the following formula: Where: C 峰 is the cooling rate of the peak, in ℃ / s; C 谷 is the cooling rate of the trough, in °C / s; N is the number of stages set for multi-stage cooling, and N is an integer greater than zero.

6. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 5, characterized in that: In the step S3, the grain size of the cooled bellows is gradually refined from the crest to the trough in the form of a continuous gradient, and the bellows contains a large amount of fine banded martensite distributed along the corrugation direction.

7. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 1, characterized in that: The cooling gas used for the jet-type multi-stage forced cooling is nitrogen or hydrogen or NH3 decomposition gas at 0-25°C.

8. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 1, characterized in that: The wall thickness of the bellows is 0.1-0.5 mm, and the material is 304, 316 or 321 stainless steel.

9. The induction heat treatment preparation method of a double-strengthened thin-walled metal bellows according to claim 1, characterized in that: The bellows is formed by combined spinning and rolling forming, hydraulic forming or mechanical bulging.

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