Ferritic stainless steel thin-wall spiral welded pipe and manufacturing method thereof

Abstract

The invention belongs to the technical field of stainless steel welding processing, and relates to a ferrite stainless steel thin-wall spiral welded pipe and a manufacturing method thereof. The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe comprises the following steps that reinforcing ribs with arc-shaped band edges are welded to the middle of a ferritic stainless steel band in the length direction of the steel band; and the steel strip welded with the reinforcing ribs is fed into spiral welded pipe forming equipment, double-face welding is conducted in the spiral forming process, the ferrite stainless steel thin-wall spiral welded pipe is obtained, a front welding seam facing the outer side of the steel strip is welded through a TIG self-melting welding technology, and a back welding seam facing the inner side of the steel strip is welded through an MIG welding method. According to the manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the economical ferritic stainless steel is adopted to replace expensive austenitic stainless steel to manufacture the thin-wall spiral welded pipe, good corrosion resistance and mechanical performance are kept, meanwhile, the cost of raw materials is greatly reduced, and the market competitiveness of products is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of stainless steel welding processing, and particularly relates to a ferritic stainless steel thin-wall spiral welded pipe and a manufacturing method thereof. BACKGROUND

[0002] The spiral welded pipe is a kind of welded steel pipe widely used in the fields of conveying pipe, pipe pile and mechanical structure. The manufacturing process thereof takes a steel strip coil as a raw material, and the steel strip coil is formed into a spiral shape and then welded by single-sided or double-sided automatic welding. At present, the spiral welded pipes at home and abroad are mostly made of austenitic stainless steel materials such as SUS304 and 316. Although the welding process of the austenitic stainless steel is mature and the comprehensive performance is excellent, the material cost is relatively high, and the thermal expansion coefficient is large and the thermal conductivity is poor, which easily causes large welding stress and deformation at high temperature.

[0003] On the other hand, ferritic stainless steel such as SUS409L, 439 and 441 has potential application value in the field of thin-wall structure due to its high chromium content, good corrosion resistance, small thermal expansion coefficient and relatively low cost. However, the ferritic stainless steel has a significant grain growth tendency during welding, especially in the heat-affected zone (HAZ), which easily leads to the decrease of joint plasticity and toughness, thereby limiting its wide application in welded structures. At present, there is no mature process or public report on the manufacture of thin-wall large-diameter spiral welded pipes made of ferritic stainless steel. The main product form of the ferritic stainless steel welded pipe is a thin-wall straight seam welded pipe, and the welding methods adopted include TIG, plasma and high-frequency induction welding.

[0004] In the prior art, for the manufacture of large-diameter spiral welded pipes, the wall thickness is usually increased or austenitic stainless steel materials are used to ensure the structural rigidity, but this will lead to the increase of material cost and weight, and the corrosion resistance and economic advantages of ferritic stainless steel cannot be fully exerted. In addition, the existing welding processes mostly adopt a single welding method, which is difficult to take into account the welding forming quality and joint comprehensive performance, especially for the welding of thin-wall ferritic stainless steel, and there is a lack of systematic and suitable process parameters and structural strengthening means.

[0005] Therefore, how to provide a ferritic stainless steel thin-wall spiral welded pipe manufacturing method capable of effectively controlling welding heat input, inhibiting grain growth and ensuring the mechanical performance of the welded pipe has become a technical problem to be solved in the field. SUMMARY

[0006] In view of the defects of the prior art, the present application provides a ferritic stainless steel thin-wall spiral welded pipe and a manufacturing method thereof.

[0007] Specifically, the manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application comprises the following steps: (1) welding a reinforcing rib along the arc-shaped edge of the ferritic stainless steel strip in the middle of the strip along the length direction of the strip; (2) feeding the strip with the welded reinforcing rib into a spiral pipe forming device to perform double-sided welding during spiral forming to obtain a ferritic stainless steel thin-wall spiral welded pipe, wherein the front weld facing the outer side of the strip is welded by using a TIG self-melting welding process, and the back weld facing the inner side of the strip is welded by using a MIG welding method.

[0008] The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the wall thickness of the reinforcing rib is 1.8-3 mm, and the ratio of the arc height to the chord length is 0.2-0.4.

[0009] The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the reinforcing rib is welded by using a TIG wire filling welding method to weld the fillet weld, the fixed inclination angle of the welding gun is 35-55°, the welding current is 150-180 A, and the welding speed is 0.9-1.1 m / min.

[0010] The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the current of the TIG self-melting welding process is 180-260 A, and the welding speed is 0.9-1.1 m / min.

[0011] The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the MIG welding method uses an ER308L welding wire, the arc voltage is 21-24 V, the welding current is 110-160 A, and the welding speed is 0.9-1.1 m / min.

[0012] The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe, the wall thickness of the ferritic stainless steel thin-wall spiral welded pipe is 1.8-3 mm, and the diameter is 273-478 mm.

[0013] The ferritic stainless steel thin-wall spiral welded pipe provided by the application comprises a pipe body formed by spiral rolling of a ferritic stainless steel strip, and the outer surface of the pipe body is provided with a spiral arc-shaped reinforcing rib extending in the axial direction.

[0014] The ferritic stainless steel thin-wall spiral welded pipe, the weld of the pipe body comprises a TIG self-melting weld on the outer side and a MIG filling weld on the inner side.

[0015] The ferritic stainless steel thin-wall spiral welded pipe, the ferritic stainless steel is a single-phase ferritic stainless steel.

[0016] The ferritic stainless steel thin-wall spiral welded pipe, the grade of the ferritic stainless steel is SUS439M, SUS409L, SUS429, SUS441, SUS443, SUS445 or SUS446.

[0017] The technical scheme of the present application has the following beneficial effects: (1) The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application, by using economic ferritic stainless steel to replace expensive austenitic stainless steel to manufacture thin-wall spiral welded pipe, greatly reduces the raw material cost while maintaining good corrosion resistance and mechanical properties, and improves the product market competitiveness; (2) The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application, by pre-welding arc-shaped strip edge stiffeners on the steel strip, significantly improves the deformation resistance and overall stiffness of the thin-wall welded pipe, so that the welded pipe with a wall thickness of only 1.8-3 mm can be stably manufactured to a diameter of 273-478 mm, realizing reliable production of large-diameter-to-wall ratio spiral welded pipes and widening the application of ferritic stainless steel in the field of structural pipes; (3) The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application, by using a double-face double-process welding method combining front TIG self-melting welding and back MIG wire filling welding, and strictly controlling the welding heat input parameters, effectively inhibits the excessive growth of ferrite grains in the weld and heat affected zone, ensuring good mechanical properties and corrosion resistance of the welded joint; (4) The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application, since ferritic stainless steel has the characteristics of small thermal expansion coefficient and single-phase structure at high temperature and room temperature, the thermal stress and organizational stress during welding are very small, and no heat treatment is required after welding, simplifying the process flow and reducing energy consumption and production cost; (5) The manufacturing method of the ferritic stainless steel thin-wall spiral welded pipe of the present application, through systematic welding theory analysis, process test and example verification, it is fully confirmed that the welded pipe manufactured by this method has good weld formation, joint performance meets the standard, excellent corrosion resistance, and has clear industrialization feasibility and technical reliability. BRIEF DESCRIPTION OF DRAWINGS

[0018] Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The accompanying drawings are included only to illustrate preferred embodiments and are not to be considered as limiting of the present application.

[0019] Figure 1 The reinforcing rib and its welding schematic diagram of the present application; Figure 2 SUS439M stainless steel 1350℃ simulation sample bending test results; Figure 3 Weld metal with a fusion ratio of 25% in the Schaeffler diagram; Figure 4 Schematic diagram of the welded thin-wall spiral welded pipe of the present application; Figure 5 Microstructure of the welded joint of Example 1; Figure 6 Tensile specimen of welded joint for Example 1; Figure 7 Inside bend specimen of welded joint for Example 1; Figure 8 Outside bend specimen of welded joint for Example 1; Figure 9 72h salt spray corrosion results of front weld for Example 1; Figure 10 72h salt spray corrosion results of back weld for Example 1; Figure 11 is the electrochemical corrosion results of the weld of Example 1. DETAILED DESCRIPTION

[0020] In order to fully understand the purposes, features and effects of the present application, the following specific embodiments are used to make a detailed description of the present application. The process of the present application uses conventional methods or devices in the art except for the following content. The following terms have the meanings commonly understood by those skilled in the art unless otherwise specified.

[0021] The present application is based on a deep understanding of the composition, structure and properties of ferritic stainless steel, through a large number of welding process tests and welding mechanism research, so as to obtain a comprehensive and systematic understanding of the welding process and weldability of ferritic stainless steel, determine the scientificity and feasibility of replacing austenitic stainless steel, and formulate a reasonable welding process parameter range.

[0022] Specifically, the manufacturing method of the ferritic stainless steel thin-walled spiral welded pipe of the present application includes the welding of arc-shaped strip edge reinforcing ribs and double-sided double-process welding method. According to the relevant national standards, the diameter of the steel pipe with a wall thickness of 2-3mm is 63.5-219mm, and by using the method of reinforcing rib spiral welded pipe, a welded pipe with large diameter-to-wall ratio can be manufactured. The thin-walled spiral welded pipe has a large diameter-to-wall ratio, so the stiffness is poor. In order to improve the stiffness of the welded pipe, arc-shaped reinforcing ribs are welded on the outer surface of the steel strip to improve the stiffness of the finished welded pipe, as shown in Figure 1 The steel strip is processed into an arc-shaped strip edge shape by using a forming roller, the thickness of the rib plate is the same as the wall thickness of the welded pipe, the fillet weld is welded by using TIG wire filling welding method, the welding torch is fixed, the inclination angle α is 35-55°, the welding current is 150-180A, the welding speed is 0.9-1.1m / min, the protective gas is industrial pure argon, and the welding speed is the steel strip transmission speed.

[0023] In order to maximize the stiffness, the ratio of the arc height of the reinforcing rib to the chord length is t / lThe control is between 0.2 and 0.4, and too large or too small will weaken the effect. The ferritic stainless steel spiral welded pipe is manufactured by using the reinforcing rib process, the wall thickness range is 1.8-3mm, and the corresponding diameter range can reach 273-478mm.

[0024] The present application improves the rigidity and deformation resistance of the thin-wall large-diameter spiral welded pipe by welding the reinforcing rib of the arc-shaped strip edge on the steel strip before spiral forming, so as to obtain the spiral welded pipe with large diameter-wall ratio, reduce the cost and unit weight of the welded pipe, and fully utilize the corrosion resistance and wear resistance of stainless steel.

[0025] In order to quantitatively analyze the influence of coarse grains in the HAZ of the ferritic stainless steel welded joint on the mechanical properties, the inventors designed and implemented the following test: after the ferritic stainless steel SUS439M with a thickness of 1.8mm is processed into tensile and bending samples, the samples are taken out after being kept at 1350℃ in a box-type resistance furnace for 5min and air-cooled to room temperature, so as to obtain the overall simulation sample with grain size close to the HAZ of the welded joint. Then, the mechanical property test is performed, and the results can represent the characteristics of the HAZ micro area. The test results are shown in Table 1. Compared with the mechanical properties of the base material in Table 2, the strength and elongation after fracture decrease by 8-14%, the hardness increases due to the formation of more carbonitrides, the maximum bending part of the bending sample presents an "orange peel" pattern, but no cracks are generated (see Figure 2 ), and the above results show that the coarse grains will reduce the mechanical properties of the stainless steel, so it is necessary to use the welding process of the filler wire, form the weld reinforcement, and avoid the fusion line parallel to the thickness direction, so as to ensure the transverse tensile property of the weld.

[0026] Table 1 Mechanical properties of SUS439M stainless steel simulation HAZ

[0027] Table 2 Mechanical properties of SUS439M stainless steel

[0028] Therefore, when the welding process of the welded pipe is formulated, the double-sided welding process is used, the front weld adopts the TIG self-melting welding process, so as to ensure the beautiful appearance of the surface weld; in order to improve the overall performance of the welded joint and avoid the formation of coarse ferrite structure, the back weld (the inner side) adopts the MIG welding process, and the austenitic stainless steel wire ER308L is selected. The phase composition of the weld can be calculated by using the Schaeffler diagram, assuming that the fusion ratio of the weld is 25%, when the ER308L austenitic stainless steel welding material is used, the phase composition of the obtained weld is located in the three-phase coexistence zone of austenite + martensite + high-temperature ferrite, and the Schaeffler diagram ( Figure 3) and the star point on the red line, so the microstructure of the weld is complex, but mainly austenite, containing about 20% ferrite and a small amount of martensite. The principle of determining the welding process parameters is to control the welding heat input under the premise of obtaining good weld forming, and at the same time, in order to consider production efficiency, so the large current rapid welding process is determined. The specific welding method and process are that the first weld is the front (outer) weld, and the TIG self-melting welding process is adopted to obtain beautiful forming, and the specific process parameters are that the welding current is 180~260A, the welding speed is synchronous with the steel strip transmission speed, and is 0.9~1.1 m / min, and the protective gas is industrial pure argon. The second weld is the back (inner) weld, and the MIG welding method is adopted to ensure the joint performance, and the specific process parameters are that the welding wire ER308L, the arc voltage is 21~24V, the welding current is 110~160A, the welding speed is synchronous with the steel strip transmission speed, and is 0.9~1.1 m / min, and the protective gas is industrial pure argon.

[0029] In order to ensure the welding quality, all the welding methods adopt horizontal position welding, as shown in Figure 4 The spiral welded pipe forming equipment is pushed forward while rotating, so as to complete the manufacture of the welded pipe and cut into a certain length.

[0030] The present application adopts the double-face double-process welding method combining the front TIG self-melting welding with the back MIG wire filling welding, and strictly controls the welding heat input parameters, effectively inhibits the excessive growth of the ferrite grains in the weld and the heat affected zone, and ensures the good mechanical properties and corrosion resistance of the welded joint. Further, the large current rapid welding is adopted to obtain smaller heat input and welding deformation, so that it is possible to obtain the finished welded pipe with the wall thickness range of 1.8~3mm and the diameter range of 273~478mm.

[0031] The ferritic stainless steel has the characteristics of small thermal expansion coefficient, the elastic modulus of typical ferritic stainless steel is 220 KN / mm 2 (20 DEG C), the thermal expansion coefficient is 11.9*10 -6 / K (500 DEG C), the thermal conductivity is 23 W / m*K, and the specific heat capacity is 430 J / kg*k. The elastic modulus of typical austenitic stainless steel is 200 KN / mm 2 (20 DEG C), the thermal expansion coefficient is 18.0*10 -6 / K (500 DEG C), the thermal conductivity is 15 W / m*K, and the specific heat capacity is 500 J / kg*k. In addition, the ferritic stainless steel is single-phase at high temperature and room temperature, so the thermal stress and microstructure stress in the welding process are small, and no heat treatment is needed after welding.

[0032] In another aspect, the present application also provides a thin-walled ferritic stainless steel spiral welded pipe prepared according to the above method, comprising a pipe body formed by spiral winding of a ferritic stainless steel strip, and a spiral arc-shaped reinforcing rib arranged on the outer surface of the pipe body and extending along the axial direction of the pipe body.

[0033] Preferably, the weld of the pipe body comprises a TIG self-fluxing weld on the outer side and a MIG filling weld on the inner side, wherein, Preferably, the ferritic stainless steel is a single-phase ferritic stainless steel with a grade of SUS439M, SUS409L, SUS429, SUS441, SUS443, SUS445 or SUS446.

[0034] According to the chemical composition and physical properties of the ferritic stainless steel, the present application selects appropriate welding methods, welding processes, welding parameters and welding materials to manufacture thin-walled spiral welded pipes with excellent performance and low cost on a general spiral welded pipe production line, replacing the high-cost austenitic stainless steel welded pipes, and has the advantages of simple production process, high production efficiency and low cost.

[0035] Embodiment The present application will be further described by way of examples below, but the present application is not limited to the scope of the examples. The experimental methods in the following examples are not specified, and are carried out according to conventional methods and conditions.

[0036] Embodiment 1 The test stainless steel is SUS439M ferritic stainless steel, which is supplied in an annealed and pickled state, with a thickness of 1.8 mm, and the mechanical properties are shown in Table 2, and the chemical composition is shown in Table 3.

[0037] Table 3 Chemical composition of SUS439M stainless steel (wt. %)

[0038] The width of the steel strip is 400 mm, and after uncoiling and leveling, it is transmitted at a speed of 1 m / min. First, a reinforcing rib is welded in the middle of the steel strip along the length direction of the steel strip. The reinforcing rib is made of the same material and is processed into an arc-shaped strip edge shape by a forming roller, with a thickness of 1.8 mm, a chord length of 100 mm, and an arc height of 30 mm. The TIG welding current is 180 A, the fixed inclination angle of the welding gun is 45°, the welding speed is 1 m / min, the protective gas is industrial pure argon, and the filler wire is an ER308L austenitic stainless steel wire with a specification of Φ 1.2 mm.

[0039] The welding process used is TIG + MIG double-sided double-process automatic welding, and the welding of the spiral welded pipe is completed by online welding. The front (outer) weld is TIG self-melting welding, with a welding current of 190 A; the back (inner) weld is MIG welding method, with welding wire ER308L, arc voltage 21 V, welding current 110 A, welding gun fixed, welding speed (i.e. steel strip transmission speed) 1 m / min, and the protective gas is industrial pure argon. The finished welded pipe specification is Φ 273x1.8mm, single length 6m.

[0040] Performance test 1. Weld appearance The weld appearance was detected, and the detection showed that the inner and outer welds were well formed and had no welding defects.

[0041] 2. Microstructure Figure 5 is the microstructure of the welded joint, the upper part is the back weld and the heat affected zone (HAZ), and the lower part is the front weld and the heat affected zone (HAZ). The coarsening of ferrite grains in the front weld will cause a decrease in performance, but the back weld is mainly composed of austenite and the HAZ has less grain growth, so the joint has good overall performance, which is verified by the mechanical property test.

[0042] 3. Mechanical properties Table 4 is the mechanical property test result obtained from two samples cut from the finished welded joint, Figure 6 is the tensile sample of the welded joint after detection, Figure 7 is the front bend sample of the welded joint, Figure 8 is the back bend sample of the welded joint, the fracture position is in the base material, the elongation after fracture is greater than 30%, and no cracks are found in the front and back bend samples of the weld, so the welded joint has good performance and fully meets the use requirements.

[0043] Table 4 Mechanical properties of SUS439M stainless steel spiral welded pipe welded joint

[0044] 4. Corrosion resistance 4.1 Salt spray corrosion test The salt spray corrosion test is carried out according to GB / T 10125-1997 "Artificial Atmosphere Corrosion Test-Salt Spray Test Method"; the reagents used in the test are all reagents of chemical purity or above; the test is carried out at room temperature 25℃, and the pH value of the collected spray solution is determined by using pH test paper, so that the solution is neutral, and the pH value is kept between 6.5-7.2; the settlement rate of the salt spray is closely observed during the test, and is kept at about 1.5ml / h.

[0045] Figure 9 is the front weld 72h salt spray corrosion result, Figure 10 is the back weld 72h salt spray corrosion result, and Figures 9~10 It can be seen that only a small amount of surface rust is easily removed. The corrosion resistance test result shows that the welded joint has good corrosion resistance.

[0046] 4.2 Electrochemical corrosion test The electrochemical corrosion test uses a CS350 electrochemical workstation, the electrolytic cell is a standard three-electrode system, the sample is the working electrode, the auxiliary electrode is a platinum electrode, the reference electrode is a saturated calomel electrode (SCE), the dynamic potential scanning range is-0.5-0.5V, the scanning rate is 2mV / s, the scanning frequency is 1Hz, the selected corrosion solution for the test is a 3.5wt% sodium chloride solution, and the test temperature is 25℃.

[0047] Figure 11 is the electrochemical corrosion test result, and the result shows that, due to the use of austenitic stainless steel welding material, the corrosion resistance of the back weld (corrosion potential-0.11V) is higher than that of the front weld (-0.25V). As a comparison, the corrosion potential of the 439M stainless steel base material is-0.15V, so the welded joint has excellent corrosion resistance.

[0048] The preferred embodiments of the present application are described in detail above, but the present application is not limited to the specific details in the above-described embodiments, and various simple modifications can be made to the technical solutions of the present application within the technical concept of the present application, and these simple modifications all belong to the protection scope of the present application.

[0049] In addition, it should be noted that, in the above-described specific embodiments, various specific technical features can be combined in any appropriate manner without contradiction, and in order to avoid unnecessary repetition, the present application will not further describe various possible combinations.

[0050] In addition, various different embodiments of the present application can also be combined in any appropriate manner, as long as they do not deviate from the idea of the present application, and they should also be considered as disclosed by the present application.

Claims

1. A method of manufacturing a thin-walled ferritic stainless steel spiral welded pipe, characterized by, The method comprises the following steps: (1) welding a reinforcing rib along the arc-shaped edge of the ferritic stainless steel strip in the middle part of the strip in the length direction of the strip; (2) feeding the strip with the welded reinforcing rib into a spiral pipe forming device to perform double-sided welding during spiral forming to obtain a ferritic stainless steel thin-wall spiral welded pipe, wherein the front weld facing the outer side of the strip is welded by using a TIG self-melting welding process, and the back weld facing the inner side of the strip is welded by using a MIG welding method.

2. The production method according to claim 1, characterized by The reinforcing rib has a wall thickness of 1.8-3 mm, and the ratio of the arc height to the chord length is 0.2-0.

4.

3. The production method according to claim 1, characterized by The reinforcing rib is welded by using a TIG wire filling welding method to weld the fillet, the fixed inclination angle of the welding gun is 35-55°, the welding current is 150-180 A, and the welding speed is 0.9-1.1 m / min.

4. The production method according to claim 1, characterized by The TIG self-melting welding process has a current of 180-260 A and a welding speed of 0.9-1.1 m / min.

5. The production method according to claim 1, characterized by The MIG welding method uses an ER308L welding wire, an arc voltage of 21-24 V, a welding current of 110-160 A, and a welding speed of 0.9-1.1 m / min.

6. The production method according to claim 1, characterized by The ferritic stainless steel thin-wall spiral welded pipe has a wall thickness of 1.8-3 mm and a diameter of 273-478 mm.

7. A ferritic stainless steel thin walled spiral welded pipe manufactured according to the method of any one of claims 1 to 6, characterized in that, The pipe body formed by spiral winding of the ferritic stainless steel strip is provided with a spiral arc-shaped reinforcing rib extending in the axial direction of the pipe body.

8. The ferritic stainless steel thin walled spiral welded pipe according to claim 7, characterized in that, The weld of the pipe body comprises a TIG self-melting weld on the outer side and a MIG filling weld on the inner side.

9. The ferritic stainless steel thin walled spiral welded pipe according to claim 7, characterized in that, The ferritic stainless steel is a single-phase ferritic stainless steel.

10. The ferritic stainless steel thin walled spiral welded pipe according to claim 7, characterized in that, The ferritic stainless steel has a grade of SUS439M, SUS409L, SUS429, SUS441, SUS443, SUS445, or SUS446.

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