JP3517086B2 - Method for manufacturing curved pipe with stainless steel build-up welded on inner surface of steel pipe - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a curved tube used for petroleum refining equipment and the like, which has a stainless steel build-up welded on its inner surface (generally referred to as an elbow, The present invention relates to a method of manufacturing an elbow or an inner surface stainless steel build-up elbow), and more particularly to a method of manufacturing an integrated inner surface stainless steel build-up elbow. 2. Description of the Related Art Elbows used in places where corrosion resistance is required, such as piping materials for petroleum refining equipment, are overlaid with stainless steel on the inner surface. As shown in FIG. 1, such an elbow is prepared by first forming a divided curved pipe having a small bending angle, performing build-up welding of stainless steel on the inner surface of the curved pipe, and performing circumferential welding to perform the divided welding. It was common to join curved pipes and manufacture them to have a desired bending angle. [0003] The reason for adopting such a manufacturing method is that, first of all, it is technically difficult to weld the inner surface of an elbow having a large bend angle, and a large-scale apparatus is required for carrying out the welding. Secondly, even if a technically simple method of performing internal surface build-up welding in a straight pipe state and then bending is adopted, stainless steels commonly used for internal surface build-up welding of this type of elbow (SUS347, SUS30
8, SUS316) has a two-phase structure of austenite and delta ferrite, and therefore has low ductility in the hot forming temperature range of carbon steel or low alloy steel as a base material. This is because cracking, peeling, and the like occur in the stainless steel layer. [0004] In view of the above, an intermediate method of the methods described above for each of the above-mentioned reasons, that is, a size having a small bending angle such that the overlay welding can be performed relatively easily even after bending. This necessitated the adoption of a process in which the tube was divided into curved tubes (hereinafter sometimes simply referred to as "base tubes") and manufactured according to the above procedure. [0005] In the above-mentioned manufacturing method, a plurality of pipes need to be formed by hot bending, then build-up welding, and girth welding for connecting each bent pipe, and it takes a long time and effort to manufacture an elbow. Costs are also high. Further, even if the bend angle is small, the overlay welding of the inner surface of the raw pipe itself is difficult, so that a problem is likely to occur in the bead shape. Furthermore, since the finished elbow performs girth welding, the heat treatment conditions for solutionizing or stabilizing the stainless steel layer performed after the bending may be limited, and in such a case, the elbow may not be formed. Quality is also affected. [0006] From such a situation, an improvement study has been conducted on a method of manufacturing an elbow integrally by overlay-welding stainless steel at the time of a straight pipe and thereafter performing bending to a desired angle. . For example, Japanese Patent Application Laid-Open No. 56-144868 discloses that the composition of a stainless steel to be welded by overlaying is adjusted and the amount of ferrite in a Schaeffler structure diagram is set to 10% or less so that after welding, A method for manufacturing an elbow that prevents the stainless steel layer from cracking even after bending is introduced. However, even with the elbow manufactured by this method, the stainless steel layer sometimes cracked. This is because the amount of ferrite estimated in the Schaeffler structure diagram does not always coincide with the amount of ferrite actually formed, and not only the amount of ferrite but also the distribution state, segregation, etc. affect ductility. Further, even when the amount of ferrite is 10% or less, another problem may occur if the amount is extremely small, such as high temperature cracking during welding. SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object of the present invention is to perform bending without causing cracking or peeling of a stainless steel layer. It is an object of the present invention to provide a method for manufacturing an internal stainless steel cladding elbow that can be integrally formed. [0010] The above object has been achieved.
The method for manufacturing an elbow according to the present invention is a method for manufacturing an elbow by overlay-welding stainless steel to the inner surface of a steel pipe that is substantially in a straight pipe state and then bending the same to manufacture an elbow. After build-up welding stainless steel on the inner surface of the steel pipe, this build-up welded steel pipe is once heated to a temperature of not less than ₃ points Ac and not more than 1000 ° C. of the base material.
The overlaid welded steel pipe is bent in a state where the temperature is 0 ° C. or more and the temperature of the overlay-welded stainless steel layer is 700 ° C. or less. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have focused on the high-temperature ductility behavior of a stainless steel overlay metal to be welded in the development of an integral type internal stainless steel overlay elbow. As a result of the study by the present inventors, the stainless overlay metal has low ductility in a temperature range from around 700 ° C. to over 1000 ° C., but bend forming is performed in a lower temperature region and a higher temperature region. It has been found that it has sufficient ductility to withstand the stress. As mentioned above, it was originally known that stainless overlay metal has low ductility in a high temperature range exceeding 800 ° C. due to its structure (two-phase structure of austenite and delta ferrite). However, there is no finding that ductility is improved at a temperature of 700 ° C. or lower. Conventionally, such a finding was not obtained because in order to bend the carbon steel or the low alloy steel as the base material, the base material was heated to a temperature of _three or more Ac to austenitize the structure, It is necessary to perform bending in a state in which the deformation resistance is lowered, and since the temperature at which such hot forming is usually performed is a temperature exceeding 900 ° C., the inventors of the present invention in a temperature range below that. It is probable that this was because the detailed examination as was conducted was not performed. Based on the above findings, the present inventors proceeded with the development of an integrated inner surface stainless steel overlay elbow while further considering the workability and workability of the base material, and completed the present invention. It is. From the viewpoint of preventing cracking and peeling, 700 ° C. or lower, or 1000 ° C., at which the ductility is improved as described above.
It is effective to carry out bending at super. However,
At a high temperature exceeding 1000 ° C., the overlaid welded stainless steel layer is affected by the coarsening of the crystal grains, and the movement of carbon between the stainless steel layer and the base metal is so severe that the structure is changed. Therefore, the mechanical properties of the final elbow may be deteriorated. Also, it is difficult to raise the temperature of the entire elbow to more than 1000 ° C. by the high-frequency bending method.
It is difficult to maintain temperatures above ℃. On the other hand, when performing bending at a temperature of 700 ° C. or less, heating and temperature control become easy, but simply raising the temperature to 700 ° C. or less results in high deformation resistance of the base material and makes it difficult to form. However, when the base material is once heated to a temperature of _three or more Ac to austenite the structure, and then cooled, the base material structure is austenitic even if the temperature is lower than the conventional hot forming temperature. In a situation in which some ferrite phase is formed in the phase or austenite phase, although the deformation resistance is slightly higher than in the case of the conventional hot forming temperature, it is at a level sufficient to manufacture an elbow. It was found to be deformation resistance. However, if the temperature is excessively cooled, the ferrite phase increases to increase the strength of the base material and increase the deformation resistance. Therefore, the temperature of the base material needs to be 550 ° C. or higher in order to perform bending. At a high temperature exceeding 1000 ° C., as described above, the crystal grains of the overlay-welded stainless steel layer become coarse, and the movement of carbon between the stainless steel layer and the base metal becomes severe, and the structure changes. Temperature once heated to 1000 ° C.
It is necessary to: Therefore, after the stainless steel is weld-welded to the inner surface of the steel pipe, the weld-welded steel pipe is once heated to a temperature of not less than _three points Ac and not more than 1000 ° C. of the steel pipe. ℃ or more, and if the bead-welded steel pipe is bent under the condition that the temperature of the build-up welded stainless steel layer is 700 ° C. or less, the stainless steel layer has high ductility and is cracked. -Since there is no peeling or the like and the deformation resistance of the steel pipe is low enough to enable bending, it is possible to manufacture an integrated inner stainless steel cladding elbow. If such a manufacturing method is adopted, it is not necessary to consider the amount of delta ferrite in the build-up metal layer. In addition, since the integrated elbow can be manufactured, the internal surface build-up welding can be performed in a straight pipe state, thereby enabling high welding efficiency and excellent automatic bead shape welding, and a stainless steel layer formed after bending. Since the heat treatment for solution or stabilization of the steel can be determined according to the properties of the stainless steel layer without being restricted by girth welding of the base metal, the quality of the elbow can be improved. I can do it. Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples. It is included in the technical scope of the present invention. (Experimental Example 1) The ductility of the stainless steel having the composition shown in Table 1 at a high temperature was examined by a tensile test. The tensile test is performed by using a high-frequency heating type high-temperature high-speed tensile tester to raise the temperature of a welded metal test piece having a parallel portion of φ8 × 35 mmL in an air atmosphere, and a strain rate of 20 mm / sec two minutes after reaching a predetermined temperature. The test was carried out by tensile breaking. Each steel type shown in Table 1 satisfies the composition of SUS347, and the amount of ferrite is a value obtained from a Schaeffler micrograph. [Table 1] FIG. 2 shows the experimental results. 7 for each steel type
It can be seen that when the temperature is lower than 00 ° C., the material has stable high ductility. (Experimental Example 2) SU on a Cr-Mo steel sheet
A specimen was prepared by welding S309L on the lower side and each steel type shown in Table 1 as the upper side, and a stainless steel layer was welded by two layers, and a high-temperature bending test was performed to examine the state of cracking of the stainless steel layer after the test. . Each of the test pieces had a thickness of 28 mm including the stainless steel layer, and the test piece was used as a base material C
Heat the r-Mo steel in the furnace to 950 ° C, which is more than ₃ points of Ac, 1
After holding for a while, remove it from the furnace and set it on the bending mold,
After cooling to a temperature of 600 to 900 ° C., bending was performed under the condition that the bending R was 56 mm as shown in FIG. SUS309L used for base material Cr-Mo steel and underlay
Table 2 shows the composition of the stainless steel, and Table 3 shows the results of the cracking status of each test piece. In addition, the temperature of the test piece at the time of performing the bending test was substantially uniform throughout the test piece, and dropped by about 30 ° C. during the bending process. [Table 2] [Table 3] Regardless of the type of the upper steel plate, no crack was observed in the stainless steel layer if the bending start temperature was 700 ° C. or lower. (Experimental Example 3) A trial production experiment (sample Nos. 1 and 2) for producing an actual elbow standard product was performed. Table 4 shows the composition of the base metal and the build-up welded stainless steel of each sample. In all samples, overlay welding was performed on the base metal in a straight pipe state using automatic TIG welding, and S
One layer of US309L stainless steel, SUS as top
Two layers of 347 stainless steel were formed. The size of the elbow was 90 ° 12Bsch160LR, and the build-up welded surface was left as-welded. In the molding, a mold bending method is adopted, in which heating and cooling temperature control is easy, and the material is heated in the furnace to the temperature shown in Table 4 which is at least _three points of Ac of the base material, and is held at the temperature for 2 hours. It was taken out, placed on a bending die, and waited until the molding start temperature shown in Table 4 was lowered before press molding. The molding temperature is a value obtained by measuring the outer surface of the base material and the surface of the overlay welding layer. The bending R is R = 1.
37D (D is the diameter of the elbow). [Table 4] Sample No. No cracks were found in the stainless steel layers which were overlay-welded in both 1 and 2, and no delamination at the interface with the base material was seen. In addition, after the above bending,
The elbows 1 and 2 were subjected to normalizing and tempering treatments for refining the base material, and the following characteristics were also examined. Bending test at normal temperature (ASME Sec.
IX) Both Nos. 1 and 2 had good ductility and no cracks occurred. In both the structure observation and hardness measurement No. 1 and No. 2 of the build-up welded layer, the Vickers hardness of the upper SUS 347 part was a normal value of Hv 200 to 210, and the structure was a two-layer structure of austenite and ferrite. The carbide was mainly NbC and no sigma phase was found. Sulfuric acid / copper sulfate corrosion test of overlay welding layer (ASTM A2
62) No. Pitting and cracking did not occur in both 1 and 2. As a result, as shown in FIG. It was confirmed that Elbows 1 and 2 had good properties. As described above, according to the method for manufacturing a curved pipe according to the present invention, after the stainless steel is weld-welded to the inner surface of the steel pipe, the weld-welded steel pipe is welded to the Ac _{3 of the} steel pipe. Points above 10
Heating to a temperature of 00 ° C. or less, and thereafter, in a state where the temperature of the steel pipe portion is 550 ° C. or more and the temperature of the overlay-welded stainless steel layer is 700 ° C. or less, Since the steel pipe is configured to be formed by bending, the stainless steel layer is not cracked or peeled off even after the bending, and as a result, the inner surface stainless steel overlay elbow can be integrally manufactured. It has become possible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining a method of manufacturing a conventional inner surface stainless steel overlay elbow. FIG. 2 is a graph showing the results of examining the hot ductility behavior of SUS347 stainless steel. FIG. 3 is a diagram illustrating a state of bending in a high-temperature bending test performed in Experimental Example 2.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-5022 (JP, A) JP-A-8-47724 (JP, A) JP-A-8-294729 (JP, A) JP-A-9-99 52174 (JP, A) JP-A-59-83717 (JP, A) JP-A-59-232225 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21D 5 / 00-9 / 18 B23K 9/04

Claims (1)

(57) [Claims 1 After the stainless steel on the inner surface of the steel pipe as the base material steel was overlay welding, Ac ₃ to the overlay welding steel pipe once the preform
It was heated to 1000 ° C. or less of a temperature above a point, then the mother
Steel pipe temperature of wood portions is at 550 ° C. or higher, and, which is characterized in that the bending of the overlay welded steel pipe temperature of the overlay welding stainless steel layer is in such a state is 700 ° C. or less Method of manufacturing a curved tube with stainless steel build- up welded on the inner surface of the tube.