JP2016112574A - Crack repair method for cast steel member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reliably repair a crack, which appears in a cast steel member, on a job site.SOLUTION: In a method, a weld material is welded to a recess 13 that is formed by removing a surface part 11 of a cast steel member including a crack 12 appearing on a surface of the cast steel member, and the crack is repaired by filling the recess 13. The method includes a TIG welding step of introducing the weld material into the recess 13 and joining the introduced weld material and the recess 13 together by TIG welding.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a technique for reliably repairing a crack generated in a cast steel member on site in a crack repair method for a cast steel member.

  Some cast steel members in power plant facilities, such as steam turbine casings and valves, are subject to thermal stress during start-up and stop, and this causes cracks mainly. If cracks occur, they are ground and removed before repairing the weld. When the wall thickness after removing the crack becomes thinner than the minimum required wall thickness, the member becomes unusable and must be replaced with a new one.

  As a crack repair method, for example, Patent Document 1 discloses a first method in which a buttering weld is formed by performing buttering welding using covered arc welding on a groove formed in a weld repair target portion of a base material. A process, a TIG remelt treatment, a second step of melting the buttering weld and then solidifying it to form a TIG remelt treatment, and a buttering welding by covering arc welding, and a first main welding on the TIG remelt treatment A welding method according to a third step of forming a part and a fourth step of performing a main welding by covering arc welding and forming a second main welding part on the first main welding part is described.

JP 2010-20491 A

  Thus, generally, covering arc welding is used for repair of a crack. However, in the case of the cast steel member in the power plant facility mentioned above, there are many large-sized members, so it cannot be taken back to the factory to perform welding work, and there is a situation that repairs must be performed on site, It is difficult to perform post-weld heat treatment at the site, and there is a possibility that the welded portion is rapidly cooled after welding and cracking occurs.

  This invention is made | formed in view of such a subject, The objective is to provide the crack repair method of the cast steel member for repairing the crack which generate | occur | produced in the cast steel member on-site reliably.

  In order to achieve the above-mentioned object, the present invention provides a welding material welded to a recess formed by removing a surface portion of the cast steel member including a crack generated on the surface of the cast steel member, thereby filling the recess. A method for repairing a crack, comprising: a TIG welding step of installing the welding material in the recess and joining the welding material and the recess by TIG welding.

  When repairing cast steel members, welding materials containing nickel may be newly produced. However, if covered arc welding is used, oxygen may be mixed during the production of the welding material (welding rod). While this leads to a decrease in quality, if TIG welding is employed as in the present invention, there is no such adverse effect, and it is possible to prevent the formation of blow holes in the welded portion.

  In the present invention, in the TIG welding step, the TIG welding may be performed while laminating the welding material in the concave portion.

  In TIG welding, it is easy to control the amount of heat supplied during welding, so it is easy to reduce the thickness of each layer even in the lamination welding. Therefore, by performing TIG welding while laminating the welding materials as in the present invention, for example, a portion (heat affected zone (HAZ (Heat-Affected Zone)) that has been thermally altered with the formation of the first layer of the welded portion, It can be relaxed by the heat of the welding material of the second layer introduced on the first layer, whereby a structure improving method (for example, temper bead method, remelt method) when post-weld heat treatment cannot be performed by covered arc welding or the like. It is possible to prevent the weld from being hardened and the reoccurrence of cracks caused thereby.

  Moreover, in this invention, after implementing the said TIG welding process, it is good also as including the heat treatment process after welding which heats the junction part of the welding material introduced into the said recessed part, and the said cast steel member. By heating the joint, residual stress (particularly tensile residual stress) in the weld can be reduced, and cracks in the weld can be prevented from recurring.

  In the present invention, the welding material is preferably a welding material containing nickel. Since nickel has a high tensile strength, even if time elapses after welding, there is little risk of so-called delayed cracking in the weld. In addition, even if the welding is not performed sufficiently at the site and there is insufficient control of the heat supply to the welded part, and the tensile residual stress remains in the welded part, the welded part will remain in the steam turbine casing, valve, etc. If it is a site | part which receives the compressive stress more than a plastic region at the time of use like the inner surface of this, the result with respect to a stress will increase on the contrary by the tensile residual stress of the grade which remains by field work. That is, according to the present invention, delayed cracking of a cast steel member to which a large compressive stress is applied can be prevented more reliably.

  According to the present invention, a crack generated in a cast steel member can be reliably repaired on site.

It is a figure explaining the outer shell member 10 of a steam turbine casing. 4 is a flowchart for explaining a repair method for the outer shell member 10. FIG. 3 is a cross-sectional view of the vicinity of the surface of the outer shell member 10 including a crack 12 cut in the thickness direction. It is a figure which shows a mode that the 1st layer 21 was formed in the recessed part 13 by TIG welding. It is a figure which shows a mode that the 2nd layer 23 was formed in the recessed part 13 by TIG welding. It is the figure which showed a mode that the buildup part 20 was formed in the recessed part 13 by TIG welding. It is a figure explaining an example of the method of the heat processing after welding.

  FIG. 1 shows an outer shell member 10 of a steam turbine casing provided in a thermal power plant or a nuclear power plant. The outer shell member 10 of the steam turbine casing is made of CrMo cast steel or the like, and receives thermal stress (particularly compressive stress) when the steam turbine is started and stopped. As a result, a crack 12 is generated on the inner surface 11 of the outer shell member 10. The method for repairing a cast steel member according to this embodiment can be used for repairing a crack in a cast steel member such as the outer shell member 10. Hereinafter, the method for repairing the cast steel member of the present embodiment will be described using the outer shell member 10 as an example.

  FIG. 2 is a flowchart illustrating a method for repairing the outer shell member 10 according to the present embodiment.

  As shown in the figure, first, a recess is formed on the surface of the outer shell member 10 including the crack 12 (S1). Specifically, as shown in FIG. 3, the crack 12 generated on the surface 11 of the outer shell member 10 is ground by using a grinder or the like so as to include all the cracks 12. The surface portion is removed, and the recess 13 is formed.

  Subsequently, as shown in FIG. 2, TIG welding is performed on the recess 13 formed in S1 (S2).

  4 to 6 are diagrams for explaining an example of the procedure of TIG welding. In the present embodiment, TIG welding is performed while a welding material is stacked on the recess 13. First, as shown in FIG. 4, an arc is generated by using a torch or the like (not shown) in a state where the welding material is installed on the bottom surface 14 of the recess 13 to join the recess 13 and the welding material. For example, a nickel-based alloy is used as the welding material.

  In addition, when the 1st layer 21 of a welding part is formed by this, the surrounding temperature will change, and the structure | tissue of the 1st layer 21 will change and harden | cure with the temperature change. That is, the heat affected zone 15 (HAZ (Heat-Affected Zone) portion) is generated.

  Therefore, before the first layer 21 is cooled, a welding material (preferably the same welding material as the first layer 21) is further installed on the upper surface 22 of the first layer 21, as shown in FIG. By performing and joining with the recessed part 13, the 2nd layer 23 is formed.

  At this time, the first layer 21 is heated by the heat of the second layer 23, whereby the same effect as that obtained by annealing the first layer 21 is obtained. Next, when the second layer 23 is heated by the heat of the third layer 24 formed on the upper surface of the second layer 23, the same effect as that obtained by annealing the second layer 23 is obtained.

  Thereafter, as shown in FIG. 6, by repeating the lamination welding until the height of the weld reaches at least the surface 11 of the outer shell member 10, a plurality of weld layers (first layer 21, second layer 23, third The build-up part 20 which consists of the layers 24, ...) is formed.

  Finally, a post-weld heat treatment is performed on the vicinity of the joint between the welding material (the built-up portion 20) and the outer shell member 10 (S3 in FIG. 2).

  FIG. 7 is a diagram for explaining an example of the post-welding heat treatment method. This post-weld heat treatment is performed under conditions of time and temperature that can be carried out on site. For example, as shown in FIG. 7, the weld is heated to about 700 ° C. with a burner or the like, and then maintained at a temperature between 600 ° C. and 800 ° C. for about 15 minutes, and then slowly cooled. Thereby, the tensile residual stress in a welding part is reduced.

  As described above, since this post-weld heat treatment is performed at a time and temperature that can be carried out on site, there is a possibility that the amount of heating is small and some tensile residual stress remains in the weld. However, since the outer shell member 10 is subjected to a compressive stress (for example, about 300 MPa) that is equal to or higher than the plastic region when the steam turbine is started, the durability of the member is rather different if the residual tensile stress remains in the field work. improves. In particular, when the above-described nickel-based alloy is used as the welding material, the tensile strength of nickel is originally high, so that cracks (delayed cracks) appearing several days after welding are less likely to occur.

  As mentioned above, in the repair method of the cast steel member of this embodiment, welding material and a recessed part are joined not by covering arc welding but by TIG welding. If covered arc welding is used, oxygen may be mixed in during the production of the welding material (welding rod), which leads to a deterioration in the quality of the welded part. If welding is employed, there is no such harmful effect and it is possible to prevent the formation of blow holes in the welded portion.

  In addition, in TIG welding, it is easy to control the amount of heat supplied during welding, so it is easy to reduce the thickness of each layer in the lamination welding. For this reason, by performing TIG welding while laminating the welding materials as in the present embodiment, for example, the vicinity of the welded portion (heat affected zone 15) that has deteriorated with the formation of the first layer 21 of the welded portion is changed to the first layer. The heat of the welding material of the second layer 23 introduced on 21 can be relaxed. By repeating this process to form the build-up portion 20, the welded portion can be hardened without performing a structure improvement method (for example, treatment by a temper bead method or a remelt method) when post-weld heat treatment cannot be performed by covered arc welding or the like. And the recurrence of cracks caused by it can be prevented.

  The above description of the embodiment is for facilitating the understanding of the present invention, and does not limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and the present invention includes equivalents thereof.

DESCRIPTION OF SYMBOLS 10 Outer shell member, 11 Surface, 12 Crack, 13 Recessed part, 14 Bottom face, 15 Heat affected zone, 20 Overlaying part, 21 1st layer, 22 Upper surface, 23 2nd layer, 24 3rd layer

Claims (4)

Welding a welding material to a recess formed by removing a surface portion of the cast steel member including a crack generated on the surface of the cast steel member, and repairing the crack by filling the recess,
A crack repairing method for a cast steel member, comprising a TIG welding step of installing the welding material in the recess and joining the welding material and the recess by TIG welding.   The method for repairing a crack in a cast steel member according to claim 1, wherein in the TIG welding step, the TIG welding is performed while the welding material is laminated in the concave portion. The cast steel member according to claim 1, further comprising a post-weld heat treatment step of heating a joint portion between the welding material introduced into the recess and the cast steel member after performing the TIG welding step. Crack repair method.   The method for repairing a crack in a cast steel member according to claim 3, wherein the welding material contains nickel as a main component.

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