JPH0140713B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for joining steel pipes by welding, and particularly to a method for joining steel pipes suitable for manufacturing hydraulic cylinders with uniform inner diameters. [Prior Art] When steel pipes such as hydraulic cylinders are joined by welding along the circumferential direction, the welded portion deforms due to contraction from the molten state. FIG. 1 schematically shows the cross-sectional shape of the welded part after such welding. That is, even if an appropriate welding wave 4 is normally obtained, a shrinkage portion 5 is generated in the base metal weld heat affected zone 3 on the inner diameter side, and the inner diameter becomes larger. For this reason, in order to create a tube with a uniform inner diameter even if the ridges 4 are removed, a shrinkage allowance corresponding to the depth of the shrinkage portion 5 must be provided in advance and the entire circumferential surface must be cut. The amount of mechanical cutting on the inner diameter increases. The reason why the shrinkage portion 5 is formed is due to the weight of shrinkage cavities and structural transformation due to cooling of the molten metal, residual stress, etc. Therefore, there is a method of using a backing metal member to achieve complete penetration and release the welding heat there, but in this case, it is easy to cause a difference in the inner diameter, and it is necessary to process the inner diameter of the entire length in the longitudinal direction after joining. Sexuality may also occur. For this reason, it has been difficult to join two steel pipes by welding to produce a steel pipe with a uniform inner diameter. [Objective of the Invention] The object of the present invention is to prevent deformation due to contraction of the welded heat-affected zone on the inner diameter side when joining steel pipes by welding, reduce the amount of mechanical cutting of the inner diameter, and achieve uniform dimensions in the longitudinal direction of the inner diameter. It is an object of the present invention to provide a method for joining steel pipes that can easily manufacture steel pipes. [Summary of the Invention] The present invention provides a steel pipe joining method for manufacturing a steel pipe with a uniform inner diameter by joining two steel pipes by welding.
The two steel pipes have different inner diameters, and a protrusion is provided along the circumference on the inner diameter side of the joint of the steel pipe with the smaller inner diameter, and the outer circumferential surface of the joint with the larger inner diameter is brought into close contact with the outer circumferential surface of this protrusion. , and the joining part of both steel pipes is characterized by forming a groove with the root gap substantially zero, and then joining by arc welding, and then cutting the inner peripheral surface of the steel pipe with a small inner diameter. It is something to do. [Embodiments of the Invention] The present invention will be described below with reference to the accompanying drawings. In FIG. 2, 1A is a base material of a steel pipe with a small inner diameter, and a protrusion 6 is provided along the circumference on the inner diameter side of the joint portion of the base material 1A. The outer circumferential surface of the joint portion of the base material 1B is in close contact with the outer circumferential surface of the protrusion 6, substantially eliminating the root gap. Both base materials 1
The abutting portions of A and 1B have a V-groove shape.
It is desirable that the l dimension of the protrusion 6 is at least 2 mm or more, and the t dimension of the protrusion 6 is 1.5 mm or more. l is 2
If it is smaller than mm, it will be difficult to align the central axes of the two base materials 1A and 1B, and if t is less than 1.0 mm, the molten pool will easily melt through. Such protrusions with appropriate dimensions appropriately increase heat capacity, making them difficult to heat and cool. In other words, the width of the thermal influence is reduced and the cooling time is lengthened. Moreover, it is clear from the shape that this protrusion functions as a restraining member and controls the deformation of the entire joint. Moreover, it is effective for correctly setting the central axes of the base materials 1A and 1B. Furthermore, when cutting the inner circumferential surface of the steel pipe after welding, it is only necessary to cut the inner circumferential surface on the side of the steel pipe with the smaller inner diameter, and there is no need to cut the entire inner circumferential surface of the steel pipe. Secondly, hydraulic cylinders are generally subject to high internal pressure,
Furthermore, since the inner surface is subject to sliding wear, a material with relatively high strength and hardness is used. That is, steel with a large carbon equivalent is used. Since such materials are prone to weld cracking, they are preheated during welding to slow down the cooling of the welded part. However, when preheating is performed, the area heated to a high temperature by welding heat becomes larger, resulting in greater deformation. The inventors decided to omit preheating in order to minimize this deformation even when joining steel pipes with a high carbon equivalent. This increases the risk of weld cracking, but the following measures are desirable to prevent it. That is, according to the inventors' experiments with S35C (carbon steel for mechanical structures), the higher the preheating temperature, the higher the critical cracking stress and the higher the cracking temperature. However, the time it takes for cracks to occur becomes longer. This is because a higher preheating temperature improves the structure, and a higher preheating results in slower cooling, which increases the time until cracking occurs.
In addition, in the case without preheating, the temperature of the welded part is approximately 50℃.
If the temperature is below ℃, the risk of cracking increases significantly. The incubation period until cracking occurs, that is, the time it takes for the weld to cool to 50°C, is approximately the amount of welding heat input.
Approximately 600 to 1500 seconds after welding is completed at 14000J/cm.
Therefore, when using steel with a high carbon equivalent, a joint without weld cracking can be obtained by performing the next weld one after another and completing the welding within the incubation period specific to that steel.
In order to reduce the deformation of the inner diameter of the welded part, it is necessary to omit preheating, and if the next welding is performed within the crack incubation period, weld cracking will be prevented even if preheating is omitted. Furthermore, when preheating is omitted as described above, if a welding material with a higher strength than the base metal is used, weld cracking is likely to occur, so it is desirable to use a welding material for low-strength mild steel with good elongation. The use of low strength welding materials can lengthen the incubation period of weld cracking and reduce cracking. Also, depending on the inner diameter of a steel pipe, such as a hydraulic cylinder, the time required for preheating is almost equal to the welding time, so being able to omit preheating will shorten the welding time, which, together with shortening the mechanical cutting process, will improve the welding time. Manufacturing costs can be reduced. Furthermore, since a sound weld can be obtained even with a low-strength welding material, manufacturing costs can be reduced from this point of view as well. In a joint tensile test with the protrusion removed, as shown in Figure 2, the inventors found that even if the strength of the welded member is lower than that of the base metal, the fractured part is the base metal as long as there is no defect in the weld metal, and the molten metal It has been confirmed that the toughness of the material is comparable to that of the base material. In the present invention, when the internal pressure changes in the longitudinal direction of the steel pipe, a high-strength steel material is used for the steel pipe on the side where the internal pressure is higher, and a steel material with lower strength than the high-strength steel material is used for the steel pipe on the side where the internal pressure is lower. By joining them by welding, it is possible to change the steel material in the longitudinal direction of the steel pipe. Especially in a hydraulic cylinder as shown in Figure 3,
Pipe 7A on the side with high oil pressure in a long cylinder in the axial direction
By making the length of the pipe 7B about 1/3 of the axial length of the whole pipe, and making the other pipe 7B about 2/3 of the whole pipe,
Since only about 1/3 of the total amount of high-strength steel is used, the weight of the hydraulic cylinder can be reduced. Examples of the present invention will be described below. Figure 4 shows the welding groove of the steel pipe that was carried out. Table 1 shows the base material and

[Table] Shows the chemical composition and mechanical properties of welding materials. The welding materials in Table 1 are all deposited metals in carbon dioxide arc welding materials. The steel pipes were welded using the above welding method, using a positioner to rotate the steel pipes and welding them downward. The welding conditions are shown in Table 2.

[Table] First, t in Figure 4 is 1.0, 1.5, 2.0, 3.0, 5.0mm.
1.0, 2.0, 2.0, 8.0, and 10.0, welded one layer using the base metal and welding material A in Table 1 under the conditions of W1 in Table 2, and then immediately welded 2 layers. Welding of the layers was completed with W2. Figure 5 shows the 4th axis on the horizontal axis.
The figure shows the range in which good welding is possible, with l in the figure taken and t taken on the vertical axis. If t is less than 1.0 mm, the molten pool will burn through and welding will be impossible. Also l
If it is less than 20 mm, it will be difficult to align the central axes of the two base materials. From this, t is 1.5 mm or more, l
2.0mm or more is required. Table 3 shows the joint tensile test results for the flat plate of this joint. The tensile strength is almost the same as base material 1B in Table 1, and exceeds the strength of welding material A.

[Table] Yes. This is because the strength of the weld metal increases as the weld metal undergoes plastic restraint due to the dilution of the base material 1A and the increase in hardness of the heat-affected zone. Figure 6 shows the hardness distribution of the weld. The locations where the hardness was measured are as shown in FIG. The hardness of the base material 1A of the first layer is significantly lower than that of the second layer, but this is because the martensite of the first layer is tempered by the welding heat of the second layer. The first layer of the weld metal is hard, which indicates that the dilution from the base metal 1A is large. From the results in Table 3 and Figure 6, the strength of the welding material can be lower than the base metal strength, and if the heat affected zone of the preceding weld is tempered by the heat of the subsequent weld, the hardness of high-strength copper will decrease. It can be seen that this decreases significantly. Table 4 shows the results of the Shapey impact test of the weld heat affected zone.

〔Effect of the invention〕

As described above, according to the present invention, a protrusion of an appropriate size is provided on the side of a steel pipe with a small inner diameter, and when the inner peripheral surface of a steel pipe with a large inner diameter is brought into close contact with this protrusion and joined, the protrusion causes melting. Reduce the heating width of the
A steel pipe with a uniform inner diameter can be obtained by suppressing deformation occurring in the back corrugated portion and only cutting the inner circumferential surface on the side of the steel pipe with a smaller inner diameter after welding.

[Brief explanation of drawings]

Figure 1 shows a conventional steel pipe joint (without backing metal)
2 is a schematic cross-sectional view of a steel pipe joint obtained by the present invention in a direction perpendicular to the weld line. FIG. 3 is an explanatory diagram of the present invention applied to a hydraulic cylinder. Figure 4 is a sectional view of the welding groove of the example, Figure 5 is a relationship between the length and thickness of the protrusion and welding conditions in the present invention, and Figure 6 is the hardness distribution of the welded part obtained in the example. The graph shown in FIG. 7 is an explanatory diagram showing the measurement locations of the hardness distribution in FIG. 6. 1A...Base metal, 1B...Base metal, 2...Weld metal, 3...Weld heat affected zone, 4...Uranami bead, 5
...Deformed part, 6...Protrusion, 7A...High pressure side hydraulic cylinder, 7B...Low pressure side hydraulic cylinder, 8
...Piston rod, 9...Piston, 10...
Cylinder head, 11...Cylinder joint.

Claims (1)

[Claims] 1. In a steel pipe joining method in which two steel pipes are joined by welding to produce a steel pipe with a uniform inner diameter, the two steel pipes have different inner diameters, and a circle is formed on the inner diameter side of the joint of the steel pipe with a smaller inner diameter. A protrusion is provided in the axial direction along the circumference, and the inner circumferential surface of the joint of the steel pipe with a large inner diameter is brought into close contact with the outer circumferential surface of this protrusion, and the joint between the two steel pipes is made so that the distance between the roots is substantially A method for joining steel pipes, which comprises forming a groove as a zero, joining by arc welding, and then cutting the inner circumferential surface of the steel pipe with a small inner diameter. 2. The method for joining steel pipes according to claim 1, wherein the protrusion has an axial length of 2 mm or more and a thickness of 1.5 mm or more. 3 In claim 1, the strength of the welding material is made lower than the strength of the base material, welding is started without preheating, and after the preceding welding is performed, within the incubation period of cracking in the welded part. A method of joining steel pipes characterized by performing subsequent welding. 4 In claim 1, the steel pipe is a hydraulic cylinder, the steel pipe on the side receiving high internal pressure is made of high-strength steel material, the other steel pipe is made of low-strength steel material, and these steel pipes are joined. A method for joining steel pipes. 5 In claim 1, the subsequent welding is performed when the temperature of the preceding weld metal does not fall below 50°C, and the heat affected zone of the preceding weld metal and the base metal is 600°C.
A method of joining steel pipes, characterized by welding so that the pipes are heated to at least ℃ or higher.