Bending method for reducing corrosion layer of high alloy steel double-layer pipeline

Abstract

The invention relates to a bending method for reducing a corrosion layer of a high-alloy steel double-layer pipeline, and belongs to the technical field of metal corrosion. The bending method comprises the steps that the inner circumferential face of the outer pipe and the outer circumferential face of the inner pipe are machined, the roughness is controlled, and coaxial sleeving is conducted; the Al-Mg series low-melting-point alloy material is heated and melted, and low-melting-point alloy liquid is obtained; preheating the outer pipe and the inner pipe; quickly filling the low-melting-point alloy liquid between the outer pipe and the inner pipe, and bending after cooling; heating the bent outer pipe and the bent inner pipe to melt the solidified alloy liquid again; and finally, the low-melting-point alloy liquid rapidly flows out between the outer pipe and the inner pipe, and the double-layer pipeline is obtained. Through the control, the contact of the alloy liquid with the outer pipe and the inner pipe and the Fe-Al reaction time can be shortened, then it is guaranteed that the corrosion depth is 170 micrometers or below, and the service life of the double-layer pipeline is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of metal corrosion, and in particular to a bending method for reducing the corrosion layer of a high-alloy steel double-layer pipeline. Background Art

[0002] Double-layer pipes are mainly used in the field of transportation engineering. They are mainly composed of an outer pipe and an inner pipe. The outer pipe mainly plays a protective role, and the inner pipe plays a transportation role. The materials of the outer pipe and the inner pipe are both high-alloy steel.

[0003] For pipes with special angles, the pipes need to be bent. The existing bending method is mainly to fill the outer pipe and the inner pipe with low melting point alloy liquid, cool it, bend the sleeve into shape, and then melt the low melting point alloy to take it out.

[0004] However, after the low melting point alloy liquid flows out of the high alloy steel double-layer tube, it leads to a decline in performance and reduces the service life of the double-layer tube. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline in order to control the depth of the corrosion layer and ensure the service life of the double-layer pipe.

[0006] In one aspect, the present invention provides a bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline, comprising the following steps:

[0007] S1: Process the inner circumference of the outer tube and the outer circumference of the inner tube, control the roughness, and coaxially sleeve them;

[0008] S2: heating and melting the Al-Mg series low melting point alloy material to obtain a low melting point alloy liquid;

[0009] S3: preheating the outer tube and the inner tube;

[0010] S4: quickly filling the low melting point alloy liquid between the outer tube and the inner tube, and bending after cooling;

[0011] S5: heating the bent outer tube and the inner tube to melt the solidified alloy liquid again;

[0012] S6: The low melting point alloy liquid flows out quickly from between the outer tube and the inner tube to obtain a double-layer pipeline.

[0013] Furthermore, the chemical composition of the low-melting-point alloy liquid, by mass percentage, includes Zn: 2%-7%, Mg: 1.0%-2.9%, Cu: 1.5%-3.5%, Si: 0.15-0.35%, La: 0.027-0.085%, Ce: 0.2-0.5%, and the balance is Al.

[0014] Furthermore, the chemical composition of the low-melting-point alloy liquid, by mass percentage, includes Zn: 2%-4%, Mg: 1.8%-2.9%, Cu: 2.2%-3.5%, Si: 0.25-0.35%, La: 0.044-0.085%, Ce: 0.35-0.5%, and the balance is Al.

[0015] Furthermore, the roughness of the inner circumferential surface of the outer tube and the outer circumferential surface of the inner tube is Ra0.4-Ra12.5.

[0016] Furthermore, the preheating temperature of the outer tube and the inner tube is 300-400°C. 。

[0017] Furthermore, the temperature of the low melting point alloy liquid is 670-700°C.

[0018] Furthermore, the pouring speed of the low melting point alloy liquid is 0.5-0.8 m / s.

[0019] Furthermore, in step S4, the outer tube and the inner tube are placed tilted, and the tilt angle is 30-60°.

[0020] Furthermore, in step S6, the heating temperature is 670-720°C, and the bent outer tube and inner tube are placed at an angle of 60-90°.

[0021] Furthermore, the depth of the corrosion layer is below 170 μm.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0023] 1. In the preparation stage of the present invention, the parts of the outer tube and the inner tube that are in contact with the low-melting-point alloy liquid need to be processed to control their roughness, and the Al-Mg series low-melting-point alloy material is selected, and then the low-melting-point alloy liquid is quickly filled between the inner tube and the outer tube, and the solidified alloy liquid is melted again after bending, and quickly flows out from between the inner tube and the outer tube. Through the above control, the contact between the alloy liquid and the outer tube and the inner tube and the Fe-Al reaction time can be reduced, thereby ensuring that the corrosion depth is below 170μm.

[0024] 2. In the present invention, the roughness of the outer tube and the inner tube needs to be controlled within the range of Ra0.4-Ra12.5. On the one hand, the roughness affects the depth of the corrosion layer. It has been verified that the greater the roughness, the greater the corrosion depth of the inner circumference of the outer tube and the outer circumference of the inner tube by the low-melting-point alloy liquid. On the other hand, the roughness within the above range facilitates the rapid flow of the low-melting-point alloy liquid and reduces the contact time.

[0025] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be achieved and obtained through the contents specifically pointed out in the description. DETAILED DESCRIPTION

[0026] The following is an explanation of the principles of the present invention in conjunction with the preferred embodiments of the present invention, but is not intended to limit the scope of the present invention.

[0027] During the bending process of the double-layer pipe, a low-melting-point alloy liquid is filled between the outer pipe and the inner pipe. After cooling, the pipe is bent into shape as an inner support. After bending, the inner support is not easy to remove due to the deformation of the pipe, so the low-melting-point alloy needs to be removed by melting.

[0028] However, after the low melting point alloy liquid flows out of the high alloy steel double-layer tube, it leads to a decline in performance and reduces the service life of the double-layer tube. After research, it was found that when the aluminum alloy liquid contacts the high alloy steel double-layer tube, a strong Fe-Al chemical reaction occurs, and both the outer tube and the inner tube have a certain corrosion depth. When the corrosion depth is too large, such as above 170μm, the continuity and stability of the double-layer tube structure are destroyed, thereby reducing the performance and life of the double-layer tube.

[0029] In order to control the corrosion depth below 170 μm, the present invention provides a bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline, comprising the following steps:

[0030] S1: Process the inner circumference of the outer tube and the outer circumference of the inner tube, control the roughness, and coaxially sleeve them;

[0031] S2: heating and melting the Al-Mg series low melting point alloy material to obtain a low melting point alloy liquid;

[0032] S3: preheating the outer tube and the inner tube;

[0033] S4: quickly filling the low melting point alloy liquid between the outer tube and the inner tube, and bending after cooling;

[0034] S5: heating the bent outer tube and the inner tube to melt the solidified alloy liquid again;

[0035] S6: The low melting point alloy liquid flows out quickly from between the outer tube and the inner tube to obtain a double-layer pipeline.

[0036] Compared with the prior art, the present invention needs to process the parts of the outer tube and the inner tube that are in contact with the low-melting-point alloy liquid in the preparation stage, control their roughness, and select Al-Mg low-melting-point alloy materials to ensure the strength during the bending process; and preheat the outer tube and the inner tube, and then quickly fill the low-melting-point alloy liquid between the inner tube and the outer tube, and melt the solidified alloy liquid again after bending, and quickly flow out from between the inner tube and the outer tube. Through the above control, the contact and reaction time of the alloy liquid and the outer tube and the inner tube can be reduced, thereby ensuring that the corrosion depth is below 170μm.

[0037] Specifically, the material of the inner tube and the outer tube is high alloy steel, which belongs to austenitic stainless steel (ASS), and its chemical composition is C≤0.035%, Si≤0.1%, Mn≤2.00%, Cr: 15.00-20.00%, Ni: 11.00-14.00%, N≤0.2%, and the balance is Fe.

[0038] Specifically, the chemical composition of the low-melting-point alloy liquid, by mass percentage, includes Zn: 2%-7%, Mg: 1.0%-2.9%, Cu: 1.5%-3.5%, Si: 0.15-0.35%, La: 0.027-0.085%, Ce: 0.2-0.5%, and the balance is Al.

[0039] It should be noted that in the present invention, the low melting point alloy liquid is aluminum alloy liquid, and Fe-Al chemical reaction occurs when it contacts the inner tube and the outer tube. When Si element is added to the low melting point alloy liquid, silicon is easily enriched at the grain boundary of the intermetallic compound (IMC) in the Fe / Al interface reaction, which inhibits the growth of IMC and reduces the chemical reaction of Fe-Al; adding Cu element can improve the strength of the low melting point alloy liquid after solidification, ensure smooth progress during the bending process, and not easily produce defects; on the other hand, adding copper element can also reduce the melting point of the low melting point alloy liquid, thereby reducing the melting temperature and reducing the intensity of the Fe-Al reaction, but its content cannot exceed 3.5%, otherwise it will increase the tendency of cracking. Adding rare earth elements La and Ce to the low-melting point alloy liquid, on the one hand, improves the tensile strength and elongation of the low-melting point alloy liquid after solidification, thereby ensuring that no defects occur during the bending process; on the other hand, the role of rare earth elements in the low-melting point alloy liquid is to combine with impurity atoms to form stable high-melting point compounds, and selectively adsorb on the growing intermetallic compounds or grain interfaces, acting as a modifier to inhibit and then inhibit the Al-Fe reaction, so that the corrosion depth meets the requirements.

[0040] Specifically, the roughness of the inner circumferential surface of the outer tube and the outer circumferential surface of the inner tube is Ra0.4-Ra12.5.

[0041] It should be noted that in the present invention, the inner circumference of the outer tube and the outer circumference of the inner tube need to be processed before pouring the low melting point alloy to control their roughness. The roughness is controlled within Ra0.4-Ra12.5. On the one hand, the roughness affects the depth of the corrosion layer. It has been verified that the greater the roughness, the greater the corrosion depth of the inner circumference of the outer tube and the outer circumference of the inner tube by the low melting point alloy liquid; on the other hand, the roughness within the above range facilitates the rapid flow of the low melting point alloy liquid and reduces the contact time.

[0042] Specifically, the preheating temperature of the outer tube and the inner tube is 300-400°C.

[0043] It should be noted that the purpose of preheating the outer tube and the inner tube is to reduce the temperature difference between the outer tube and the inner tube and the low melting point alloy liquid, so as to ensure that the space between the outer tube and the inner tube is filled with the low melting point alloy liquid, which is convenient for subsequent bending work. However, when the preheating temperature exceeds this temperature range and the temperature is kept for a long time, the outer tube and the inner tube made of austenitic stainless steel are prone to precipitate M 23 C6, causing chromium depletion at grain boundaries and increasing intergranular corrosion sensitivity.

[0044] Specifically, the temperature of the low melting point alloy liquid is 670-700°C.

[0045] Specifically, in step S4, the outer tube and the inner tube are placed tilted, and the tilt angle is 30-60°.

[0046] It should be noted that, in the present invention, before pouring the low-melting-point alloy liquid, the outer tube and the inner tube need to be placed at an angle of 30-60° to the horizontal plane. The purpose of the inclined setting is to facilitate the filling of the low-melting-point alloy liquid, and to remove the gas in the outer tube and the inner tube, so as to avoid defects such as cold shut, and ensure the success of the bending. When the inclination angle is too low, it is not convenient to fill the low-melting-point alloy liquid; when it is too high, it is not conducive to operation.

[0047] Specifically, in step S4, the pouring speed of the alloy liquid is 0.5-0.8 m / s.

[0048] It should be noted that the pouring speed needs to be reasonably controlled in the present invention, and the pouring is performed quickly. The faster the pouring speed, the better. On the one hand, it reduces the flushing time of the low-melting-point alloy liquid on the outer tube and the inner tube, and on the other hand, it reduces the time for the Al-Fe reaction. However, the pouring speed cannot be higher than 0.8m / s, otherwise the gas cannot be discharged in time in the outer tube and the inner tube, resulting in defects and affecting the bending strength. After the pouring is completed, the solidification is accelerated during the cooling process.

[0049] Specifically, in step S6, the heating temperature is 670-720°C, the time is within 10 hours, and the bent outer tube and inner tube are placed at an angle of 60-90°.

[0050] It should be noted that in step S6, the double-layer pipe needs to be heated after bending to melt the solidified low-melting-point alloy liquid again and flow out quickly from between the outer tube and the inner tube. During the melting process, the heating temperature is controlled at 670-720°C, and the insulation time is controlled within 10 hours. During the placement of the outer tube and the inner tube, the inclination angle of the outer tube and the inner tube is 60-90°, which is convenient for the outflow of the low-melting-point alloy liquid and reduces the reaction of Al-Fe.

[0051] In order to more clearly describe the present invention, it is further illustrated by the following examples and comparative examples.

[0052] Example 1

[0053] The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline comprises the following steps:

[0054] S1: The inner circumference of the outer tube and the outer circumference of the inner tube are processed to control the roughness to Ra6.3, and the outer tube and the inner tube are coaxially sleeved; the material of the outer tube and the inner tube is high alloy steel, and the chemical composition thereof is C: 0.035%, Si: 0.1%, Mn: 2.00%, Cr: 15.00-20.00%, Ni: 11.00-14.00%, N: 0.2%, and the balance is Fe;

[0055] S2: heating and melting the Al-Mg series low melting point alloy material to obtain a low melting point alloy liquid, the temperature of which is 670°C; the composition of which is shown in Table 1;

[0056] S3: preheating the outer tube and the inner tube at a preheating temperature of 350°C;

[0057] S4: quickly fill the low melting point alloy liquid between the outer tube and the inner tube, the outer tube and the inner tube are placed tilted, the tilt angle is 30 degrees, and the pouring speed is 0.5m / s, and then bend after cooling;

[0058] S5: heating the bent outer tube and the inner tube to melt the solidified alloy liquid again, the heating temperature is 670°C;

[0059] S6: The outer tube and the inner tube are placed tilted at an angle of 60°, and the low melting point alloy liquid flows out quickly from between the outer tube and the inner tube to obtain a double-layer pipeline.

[0060] Table 1 Chemical composition of aluminum alloy liquid (unit: wt%)

[0061] Group Zn Mg Cu Si La Ce Al Example 1-1 2 1.0 1.5 0.15 0.027 0.2 margin Example 1-2 4 1.8 2.2 0.25 0.044 0.35 margin Examples 1-3 7 2.9 3.5 0.35 0.085 0.5 margin Comparative Example 1-1 4 1.8 2.2 0.1 - - margin

[0062] The double-layer pipe is processed and removed, 10μm to 20μm each time. Each time, the aluminum content is accurately analyzed using XRF (X-ray fluorescence spectrometry). The test can be stopped when it is confirmed that the aluminum content is basically consistent with the aluminum content in the double-layer pipe, and the depth of the corrosion layer of the sample can be accurately measured.

[0063] Table 2 Test results

[0064] Group Comparative Example 1-1 Example 1-1 Example 1-2 Examples 1-3 Processing volume 175μm 140μm 125μm 110μm

[0065] Combining Examples 1-1, 1-2, 1-3 and Comparative Example 1-1 and Table 2, it can be seen that the composition of the low-melting-point alloy liquid can affect the thickness of the corrosion layer. Using the composition provided by the embodiment of the present invention, the corrosion depth of the double-layer pipe by the low-melting-point alloy liquid is below 170 μm, which can ensure the service life of the double-layer pipe.

[0066] Example 2

[0067] The process of Example 2 is substantially the same as that of Example 1. Example 2 adopts the chemical composition of Example 1-2 and verifies the influence of pouring temperature, speed and tilt angle on the depth of the corrosion layer, as shown in Table 3.

[0068] Table 3 Casting temperature, speed, tilt angle and corrosion depth

[0069] Group Pouring temperature Pouring speed Tilt Angle Processing volume Example 1-2 670℃ 0.5m / s 30° 125μm Example 2-1 700℃ 0.5m / s 30° 130μm Example 2-2 720℃ 0.5m / s 30° 155μm Example 2-3 670℃ 0.65m / s 30° 110μm Embodiment 2-4 670℃ 0.8m / s 30° 85μm Embodiment 2-5 670℃ 0.5m / s 45° 115μm Embodiment 2-6 670℃ 0.5m / s 60° 100μm Comparative Example 2-1 740℃ 0.5m / s 30° 180μm Comparative Example 2-2 670℃ 0.3m / s 20° 185μm

[0070] In combination with Examples 1-2, 2-1 to 2-6 and Comparative Examples 2-1, 2-2 and Table 3, it can be seen that the pouring temperature, pouring speed and inclination angle of the low melting point alloy liquid can affect the thickness of the corrosion layer. It is necessary to strictly control the pouring temperature to 670-700°C, the pouring speed to 0.5-0.8m / s and the inclination angle to 30-60° to ensure that the corrosion depth of the double-layer pipe by the low melting point alloy liquid is below 170μm, which can ensure the service life of the double-layer pipe.

[0071] Example 3

[0072] The process of Example 3 is substantially the same as that of Example 1. Example 3 adopts the chemical composition of Example 1-2 and the casting temperature is 670° C. to verify the effect of roughness on the depth of the corrosion layer.

[0073] Table 4 Roughness and corrosion layer depth

[0074] Roughness Ra 25 12.5 6.3 3.2 1.6 0.4 Processing amount / μm 180 165 125 150 120 100

[0075] Combining Example 3 and Table 4, it can be seen that the roughness of the double-layer pipe also affects the corrosion degree of the low-melting-point alloy liquid. The roughness of the inner circumference of the outer tube and the outer circumference of the inner tube in the double-layer pipe needs to be strictly controlled within the range of Ra0.4-Ra12.5 to ensure that the corrosion depth is below 170 μm, thereby ensuring the service life of the double-layer pipe.

[0076] Example 4

[0077] The process of Example 4 is substantially the same as that of Example 1. Example 4 adopts the chemical composition of Example 1-2 and the pouring temperature is 670° C. to verify the influence of the heating temperature and the tilt angle on the depth of the corrosion layer.

[0078] Table 5 Heating temperature, tilt angle and corrosion layer depth

[0079] Group Heating temperature Tilt Angle Processing volume Example 1-2 670℃ 60° 125μm Example 3-1 700℃ 60° 140μm Example 3-2 720℃ 60° 165μm Example 3-3 670℃ 70° 115μm Embodiment 3-4 670℃ 85° 95μm Comparative Example 3-1 740℃ 60° 180μm

[0080] Combining Examples 1-2, 3-1 to 3-4 and Comparative Example 3-1 with Table 5, it can be seen that when the low melting point alloy liquid flows out quickly from between the outer tube and the inner tube, the heating temperature and the tilt angle will also affect the thickness of the corrosion layer. According to the results in Table 5, it is necessary to strictly control the heating temperature to 670-720°C and the tilt angle to 60-90° to ensure that the corrosion depth of the low melting point alloy liquid to the double-layer pipe is below 170 μm, which can ensure the service life of the double-layer pipe.

[0081] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline, characterized in that: The following steps are involved: S1: Process the inner circumference of the outer tube and the outer circumference of the inner tube, control the roughness, and coaxially sleeve them; S2: heating and melting the Al-Mg series low melting point alloy material to obtain a low melting point alloy liquid; S3: preheating the outer tube and the inner tube; S4: quickly filling the low melting point alloy liquid between the outer tube and the inner tube, and bending after cooling; S5: heating the bent outer tube and the inner tube to melt the solidified alloy liquid again; S6: The low melting point alloy liquid flows out quickly from between the outer tube and the inner tube to obtain a double-layer pipeline.

2. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: The chemical composition of the low melting point alloy liquid, calculated by mass percentage, includes Zn: 2%-7%, Mg: 1.0%-2.9%, Cu: 1.5%-3.5%, Si: 0.15-0.35%, La: 0.027-0.085%, Ce: 0.2-0.5%, and the balance is Al.

3. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 2 is characterized in that: The chemical composition of the low melting point alloy liquid, calculated by mass percentage, includes Zn: 2%-4%, Mg: 1.8%-2.9%, Cu: 2.2%-3.5%, Si: 0.25-0.35%, La: 0.044-0.085%, Ce: 0.35-0.5%, and the balance is Al.

4. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: The roughness of the inner circumference of the outer tube and the outer circumference of the inner tube is Ra0.4-Ra12.

5.

5. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: The preheating temperature of the outer tube and the inner tube is 300-400°C.

6. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: The temperature of the low melting point alloy liquid is 670-700°C.

7. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: The pouring speed of the low melting point alloy liquid is 0.5-0.8 m / s.

8. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: In step S4, the outer tube and the inner tube are placed tilted, and the tilt angle is 30-60°.

9. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1 is characterized in that: In step S6, the heating temperature is 670-720°C, and the bent outer tube and inner tube are placed at an angle of 60-90°.

10. The bending method for reducing the corrosion layer of a high alloy steel double-layer pipeline according to claim 1, characterized in that: The depth of the corrosion layer is below 170 μm.