Repair method for cast steel member

A repair method for repairing a crack generated to a cast steel member due to a thermal stress includes removing a part of the cast steel member including the crack at a surface of the cast steel member to form a recess, fitting into the recess a first repair member that is made of a metal that is more flexible than the cast steel member and that is weldable to the cast steel member, and spot welding the first repair member to the cast steel member.

TECHNICAL FIELD

The present invention relates to a repair method for repairing a crack that was generated to a cast steel member due to thermal stress.

BACKGROUND ART

Cast steel members used in thermal power equipment, nuclear power equipment or the like are kept at high temperature and high pressure conditions for a long time in many cases, and therefore such equipment suffers creep damage due to thermal stress which in turn has generates cracks as a cause thereof.

Repair of such cracks are commonly carried out by a method including scraping parts adjacent the crack surface, hardfacing thereon and performing heat treatment. PTL 1, for example, discloses a case of repairing a turbine blade surface by forming a cutting surface to the surface crack, heating the surroundings of this cutting surface, welding and hardfacing the heated cutting surface with a welding material such as nickel base alloy, cutting the built up part after cooling and performing heat treatment to precipitate a gamma prime (γ′) phase.

CITATION LIST

Patent Literature

However, many of the cast steel members used in thermal power equipment, nuclear power equipment or the like, such as a steam turbine room and a steam turbine blade, are formed so large that these members cannot be transported back to the plant for heat treatment. Therefore, the heat treatment has to be performed on site where the scale of the heat treatment which was performed was limited, and this has lead to insufficient heat treatment as a result and had rather caused a defective condition such as embrittlement of the repaired part making such liable to fracture.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention provide a repair method for certainly repairing on site, cracks that were generated on a surface of a cast steel member due to thermal stress.

One or more embodiments provide a repair method for repairing a crack generated to a cast steel member due to a thermal stress, including removing a part including the crack at a surface of the cast steel member and forming a recess, fitting into the recess a first repair member that is made of a metal that is more flexible with respect to the cast steel member and that is weldable to the cast steel member, and joining by spot welding to the cast steel member the first repair member that was fit in.

According to one or more embodiments of the present invention, fitting into the part that had the crack removed a first repair member made of a metal that is more flexible with respect to the cast steel and that is weldable to the cast steel as well, allows thermal stress on this first repair member itself (the recess having the crack removed) to be released to the surroundings. And hereby, cracks can be prevented from generating from the repaired part. Further, according to one or more embodiments of the present invention, welding the first repair member to the cast steel is performed by spot welding which allows easy work on site. Hereby, the crack generated on the cast steel surface due to thermal stress can be certainly repaired on site.

According to one or more embodiments, a strip-form second repair member that is made of a metal that is more flexible with respect to the cast steel member and that is weldable to the cast steel member is welded in a manner intersecting a lengthwise direction of the crack and spanning over the first repair member that was fit in.

The direction that intersects the lengthwise direction of the crack is the direction in which thermal stress is likely to apply. Therefore, welding using the second repair member that is more flexible with respect to the cast steel, in a manner such that the second repair member crosses over the first repair member to intersect the lengthwise direction of the crack, as in one or more embodiments of the present invention, can certainly prevent a crack from generating from the repaired part.

According to one or more embodiments, the mounted second repair member and a main body of the cast steel member are joined by spot welding.

According to one or more embodiments of the present invention, on site work can be easily carried out since spot welding is also performed when welding the second repair member.

According to one or more embodiments, the first repair member and the second repair member are made of a same material.

According to one or more embodiments, cracks can be efficiently suppressed from generating from the repaired part by using the same material for the first and the second repair members to allow the thermal stress on the repaired part to evenly disperse to the surroundings thereof.

According to one or more embodiments, metal members made of, for example, 99.9% purity nickel is used for the first and the second repair members in the above case.

Advantageous Effects of Invention

According to one or more embodiments of the present invention, cracks that were generated on a surface of a cast steel member due to thermal stress can be certainly repaired on site.

DESCRIPTION OF EMBODIMENTS

The repair method for a cast steel member according to one or more embodiments can be applied to repair cracks generated to a cast steel member due to thermal stress and, for example, can be applied to the shell member10of a steam turbine wheel chamber provided to a thermal power equipment, nuclear power equipment or the like, shown inFIG. 1.

The shell member10of a steam turbine wheel chamber is made of a cast steel member such as CrMoV cast steel and is subject to thermal stress that is accompanied by the start-stop operation thereof. Hereby, the shell member10of a steam turbine wheel chamber will suffer creep damage so that a crack12will be generated to the inner surface11. The crack12will appear in, for example, a zigzag form as illustrated inFIG. 1. The following describes a method for repairing a cast steel member according to one or more embodiments taking the repair method for this crack12as an example.

FIG. 2is an enlarged view of the crack12. The repair method according to one or more embodiments firstly determines for the crack12that was generated to the surface11, the lengthwise direction (first direction) of the crack12and the crosswise direction (second direction) of the crack12being the direction orthogonal to the lengthwise direction, as shown inFIG. 2. Thereafter, the length of the crack12in the lengthwise direction and the length of the crack12in the crosswise direction are measured with a caliper or the like.

The length in the lengthwise direction of the crack12which needs to be repaired is, for example, 5 mm to 100 mm and the length in the crosswise direction of the crack12is, for example, 0.1 mm to 10 mm. The length along the lengthwise direction of the crack12is set to L and the length along the crosswise direction of the crack12is set to W in one or more embodiments, as shown inFIG. 2.

Subsequently, this crack12is removed. For the sake of description, the rectangular cut line13that surrounds the entire crack12is assumed to be set to the parent material surface, as shown inFIG. 3. The direction along the long side13aof the cut line13is set parallel to the previously set lengthwise direction of the crack12and the direction along the short side13bof the cut line13is set parallel to the previously set crosswise direction of the crack12.

The length of the long side13aof the cut line13is, for example, approximately 1 mm to 120 mm. And the length of the short side13bof the cut line13is, for example, approximately 1 mm to 30 mm. The long side13aof the cut line13is set to L2and the short side13bof the cut line13is set to W2in one or more embodiments, as shown inFIG. 3.

Then the crack12and the surroundings thereof are removed with reference to this cut line13. In other words, the surface11is cut along the cut line13halfway in the depth direction to be removed, as illustrated inFIG. 3. This work is carried out using a tool such as, for example, a drill, an end mill or the like.

The depth of the surface11that is cut for removal requires to be deeper than the depth of the crack12that is recognized to have developed. Cutting should be repeated little by little while confirming the existence of the crack when the amount of crack12developed toward the inside is unknown.

The depth of the cutting for removal is specifically, approximately 1 mm to 100 mm, for example. The recess15is formed to be in an approximately rectangular parallelepiped space with the side14in the depth direction having a length of D2in one or more embodiments, as shown inFIG. 3.

Next, the recess15formed in the above manner is repaired with the repair member (hereinafter called, first repair member20) shown inFIG. 4. The first repair member20is formed in a shape same as that of the recess15. For the sake of description, the first repair member20is a rectangular parallelepiped member with the sides21having a length of L2, sides22having a length of W2and sides23having a length of D2.

Thereafter, this first repair member20is embedded into the recess15, as shown inFIG. 5. Specifically, the sides21of the first repair member20are made to correspond to the long sides13aof the cut line13, the sides22of the first repair member20are made to correspond to the short sides13bof the cut line13and the sides23of the first repair member20are made to correspond to the sides14along the depth direction of the recess15to fit the first repair member20into the recess15. And then the first repair member20is pushed into the recess15by tapping the surface side thereof.

Here, the material of the first repair member20is made of a metal material that is more flexible with respect to the material (cast steel) of the surface11. The reason for this is as follows. Although the surface11where the crack12was generated is a part to which thermal stress is likely to occur, it is unlikely that a crack would generate from the repaired part, because fitting therein the first repair member20that is more flexible with respect to the cast steel which is the material of the surface11allows the thermal stress on the first repair member20to be dispersed to the surroundings thereof.

Pure nickel can be given as an example of this material for such first repair member20. In one or more embodiments, the purity of the nickel is 99.9% and greater. Further, for example, chrome, steel, aluminum and the like are suitable for metals besides nickel and the purity thereof in these cases is 99.9999% and greater.

Subsequently, spot welding is performed to the first repair member20as indicated with the reference numbers24inFIG. 6. In other words, the corners of the first repair member20are joined to the parent material at the surface11by spot welding. Spot welding is performed to the four corners in the example ofFIG. 6. This spot welding can be performed by, for example, Tungsten Inert Gas (TIG) welding.

The first repair member20requires to be a metal material to which such spot welding can be performed. And the aforementioned pure nickel and the like satisfy this condition.

Then the part that was repaired using the first repair member20is further reinforced.FIG. 7illustrates the member (hereinafter, second repair member30) used for this reinforcement. As shown inFIG. 7, the second repair member30is a member in a strip form.

The second repair member30is mounted on the surface11of the parent material.FIG. 8is an explanatory view illustrating the manner in which the second repair member30is mounted to the parent material. As shown inFIG. 8, grooves16are formed to the surface11of the parent material when mounting the second repair members30.

A groove16is formed by cutting the surface11along the rectangular cut line17that is provided to the surface of the parent material. The groove16is formed such that the second repair member30fits therein when the second repair member30is mounted into the groove16. The groove16is formed using a tool such as, for example, a cutter, an end mill or the like. The groove16may be formed by removing the surface11by other machine work such as grinding.

As illustrated inFIG. 8, the groove16is provided to span over the first repair member20where the lengthwise direction of the groove16is set in a direction that intersects the lengthwise direction (first direction) of the first repair member20. The lengthwise direction of the groove16comes along the direction (parallel to the crosswise direction of the crack12) that forms an approximate right angle (80 to 90 degrees) between the lengthwise direction. The angle formed between the lengthwise direction of the groove16and the lengthwise direction is an approximate right angle in one or more embodiments.

The depth of the groove16matches the thickness of the second repair member30and is designed such that the second repair member30is mount flush with the parent material surface that is associated with the repair part when the second repair member30is mounted. The depth of the groove16and the thickness of the second repair member30match each other in one or more embodiments.

The lengthwise length of the groove16is, for example, 5 mm to 100 mm, the crosswise (in a direction parallel to the lengthwise direction of the first repair member20) length of the groove16is, for example, 1 mm to 20 mm, and the length along the depth direction of the groove16is, for example 1 mm to 20 mm. The present example assumes that the length along the lengthwise direction of the groove16is L3, the length along the crosswise direction of the groove16is W3and that along the depth direction (material thickness direction) of the groove16is D3, as shown inFIG. 8. Therefore, the length along the lengthwise direction of the second repair member30is L3, the length along the width direction of the second repair member30is W3and the length along the thickness direction of the second repair member30is D3.

Similar to the first repair member20, the material of the second repair member30is a metal material that can have performed spot welding thereto. Additionally, same as the first repair member20, the material of the second repair member30is a metal material that is more flexible with respect to the surface11. The material of the first repair member20being the same as that of the second repair member30allows thermal stress on the repair part to be evenly dispersed to the surroundings thereof so that crack generation from the repair part can be effectively suppressed.

The aforementioned pure nickel can be given as an example of this material for such second repair member30. In one or more embodiments the purity of the nickel is at least 99.9% and greater. Further, for example, chrome, steel, aluminum and the like are metals besides nickel that can be used and the purity thereof in these cases is 99.9999% and greater.

Lastly, the second repair member30that is fit into the groove16and the parent material are fixed by spot welding. That is, the four corners of the second repair member30and the four corners of the cut line17are respectively fixed by spot welding, as indicated with the reference numbers31inFIG. 9. This spot welding is performed by, for example, Tungsten Inert Gas (TIG) welding.

Hereby, mounting of the second repair member30in the direction intersecting the lengthwise direction of the first repair member20, that is, the direction intersecting the lengthwise direction of the crack12effectively prevents a crack from generating from the repair part. That is, because the direction that intersects the direction (lengthwise direction of the crack12) in which the crack12was generated is the direction in which thermal stress is likely to occur, cracks can be effectively prevented from generating by making the material of the second repair member30more flexible with respect to the cast steel member, similar to the case of the first repair member20.

And in this case, on site work can be easily carried out since spot welding is performed for welding the second repair member30to the parent material, similar to the case of the first repair member20.

As described above, in the repair method according to one or more embodiments, fitting into the recess15the first repair member20made of a metal that is more flexible with respect to the cast steel member being the member to be repaired as well as a metal that is weldable to the cast steel member allows this first repair member20to release to the surroundings the thermal stress applied to this repair part and therefore cracks can be prevented from generating from the repaired part. Hereby, cracks that were generated to the surface of the cast steel member due to thermal stress can be certainly repaired.

In particular, the cracks that were generated due to thermal stress are likely to appear on the surface of a member. And therefore, forming the first repair member20in a shape that fits into the recess15and allowing the first repair member20embedded in the recess15to contact the surface of the recess15can effectively prevent cracks from generating from the surface of the recess15.

Further, in the repair method according to one or more embodiments, welding the first repair member20and the second repair member30to the parent material is performed by spot welding which allows easy site work. Hereby, cracks can be certainly repaired on site even when the cracks were generated to large size cast steel members such as those equipped to thermal power plants, nuclear power plants or the like.

The aforementioned description of the embodiments is simply for facilitating the understanding of the present invention and is not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompasses equivalents thereof.

For example,FIGS. 8 and 9have been illustrated to provide a plurality of the second repair members30, however, only one second repair member30may be provided when the crack12is short.

REFERENCE SIGNS LIST