JPH0442521B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a nozzle box provided in a high-pressure first stage of a steam turbine.

[Background of the invention]

Conventional turbine nozzle boxes of this type include:
During annealing to remove residual stress after welding, cracks may occur in the welded portion, resulting in a problem of reduced reliability.

The problems of this prior art will be explained below with reference to the drawings.

FIGS. 1 and 2 show typical shapes of this type of nozzle box. FIG. 1 is a perspective view thereof, particularly showing the steam chest 1 thereof. FIG. 2 is a view taken in the direction of arrow A in FIG. The high-temperature, high-pressure steam generated in the boiler is introduced into the turbine casing, first passes through this nozzle box, and then rotates the blades installed in the rotor to do work. Therefore, the nozzle box is exposed to high temperatures and
Used under severe high pressure steam conditions. For this reason, particularly high reliability is required of the nozzle box. Conventionally, this request has been dealt with as follows.

Please refer to FIG. 3, which corresponds to the BB sectional view of FIG. 2. In FIG. 3, reference numeral 1 indicates a steam chest, which is usually made of cast steel and has a Cr-Mo-V composition. When manufacturing a conventional nozzle box, such a steam chest 1,
The nozzle ring 2 made of 10Cr steel is assembled, and a weld metal 3 is formed using a welding rod having approximately the same composition as the nozzle ring 2, thereby joining the steam chest 1 and the nozzle ring 2, and then annealing is performed to remove residual stress. I'm going. Next, the nozzle ring 2 is machined to create a groove 4, the nozzle blades 5 are assembled, and the nozzle ring 2 and the nozzle blades 5 are integrated with weld metal 6. In this case, the steam chest 1 is usually made of Cr--Mo--V steel, as described above, and this is a material that is prone to annealing cracks during residual stress removal annealing after welding. However, technology has been established for a long time, and the occurrence of annealing cracks can be prevented by processing under appropriate preheating conditions.

On the other hand, with the recent decline in electricity demand, there is a need in the electric power industry for so-called middle-sized turbines, in which turbines are operated not at maximum output but at lower output. This is a request for so-called variable voltage operation, which means that since power demand is decreasing, there is no need to operate at maximum output, and that it is desired to operate at a lower output. To show a specific operating method, in normal operation, steam is introduced into the nozzle box from two places, as shown by numerals 7 and 8 in Figure 2, but with the shift to middle-sized nozzles, sometimes steam is introduced into the nozzle box from one place (7 and 8). or 8). Its control is carried out by a steam valve (not shown) which is a front stage of the nozzle box. However, in such variable pressure operation, the stress acting on the steam chest becomes high, and the conventional Cr-Mo-
V steel will not be able to withstand the strength (nozzle rings are usually made of 10Cr steel, which is several steps higher in strength than Cr-Mo-V steel, so there is no problem). In order to deal with this, it is essential to increase the strength of the steam chest.
- Changing from V steel to 10Cr steel, which is similar to the nozzle ring, increases the material cost and is not practical.
Therefore, a small amount, e.g. 5-50ppm, is added to Cr-Mo-V steel.
Cr-Mo-V-B steel containing B is about to be adopted as a material for steam chests. Since B is an element that improves the hardenability of the material after normalization, it can increase the strength and is considered suitable for this purpose. However, since the nozzle box has a structure as shown in FIG. 3, the following various problems occur when using a material to which B is added. Namely, (1) B-added Cr-Mo-V steel is extremely susceptible to annealing cracking during residual stress relief annealing performed after welding.

(2) Steam Chest's welded joints of B-added Cr-Mo-V steel and nozzle ring 10Cr steel are dissimilar metal welded joints that use welding rods of 10Cr steel composition, so annealing cracks cannot be detected by ultrasonic testing or other methods. Difficult to detect through destructive inspection.

The problem arises.

Originally, annealing cracking was caused by Cr as an inherent property of the material.
This phenomenon is unique to -Mo steel or Cr-Mo-V steel, and is a fine intergranular crack that occurs in the heat-affected zone of the weld during residual stress removal insensitivity. In the conventional technology, Cr-Mo, which may cause annealing cracks in the steam chest of the nozzle box,
-V steel is used, but in this case, heat handling technology during welding that does not cause annealing cracks has already been established, and it is actually being operated at power plants without any accidents. However, B-added Cr-Mo-V
As for steel, welding technology has not yet been established, and there is a possibility of annealing cracking, and if a nozzle box is installed and operated in a turbine with such annealing cracks still present, there is a risk of significantly reducing the reliability of the entire turbine. There is sex.

[Purpose of the invention]

An object of the present invention is to provide a turbine nozzle box that uses B-added Cr-Mo-V steel to have high strength, and in which case the above-mentioned annealing cracks do not occur, thus ensuring high reliability. There is a particular thing.

[Summary of the Invention] The turbine nozzle box of the present invention has a steam chest made of B-added Cr-Mo-V steel.
This steam chest is first subjected to a softening heat treatment.
For example, normalizing and tempering are performed, in which case at least the tempering is performed at a temperature higher than the normal temperature. The normal temperature here is the temperature at which the final target strength is obtained, and tempering at a temperature higher than this temperature means that a lower strength than the final target strength is obtained, that is, softening It will be heat treated. Next, after overlay welding with a welding rod of Cr-Mo-V steel composition, normalizing and tempering heat treatment is performed at a temperature selected to obtain the final target strength.
That is, the steam chest and the nozzle ring are integrated by performing regular heat treatment, assembling the nozzle ring, and welding the nozzle ring and the overlay metal using a welding rod of 10Cr steel composition.

In this way, in the first treatment, for example, the softening heat treatment was performed by tempering at a temperature higher than the normal temperature, so even in the case of steam chest, which is B-added Cr-Mo-V steel, the thermal influence This can prevent annealing cracks that easily occur in the parts.

In carrying out the present invention, in the initial normalizing and tempering process, tempering is performed at a temperature higher than the normal temperature, and also an embodiment is adopted in which normalizing is performed at a temperature lower than the normal normalizing temperature. I can do it. Since this is also a softening heat treatment, the above results can be further ensured by using this together.

[Embodiments of the invention]

Next, one embodiment of the present invention will be described with reference to FIGS. 4 to 6. FIG. 4 shows the steam chest 1 in the nozzle box of this example, FIG. 5 is a sectional view taken along the line C-C, and FIG. 6 shows the steam chest 1 obtained by integrating the steam chest and the nozzle ring. An example nozzle box is shown.

Steam chest 1 in Fig. 4 is a B-added Cr
- Made of Mo-V steel. In this example, the specific chemical composition is C: 0.1-0.25% by weight, Mn: 0.4-1.0% by weight, Si: 0.15-0.8% by weight, Cr: 0.9-1.7% by weight, Mo: 0.8-1.3% by weight. , V: 0.15-0.35% by weight
Based on this, 5-50ppm of B is added. As a result of the addition of B, the hardenability is significantly improved, and as a result, the high-temperature strength of the material becomes higher than that of Cr-Mo-V steel without B addition.

As a result, the steam chest 1 has sufficient strength to withstand even if the power generation is performed in a middle voltage operation and a variable voltage operation is performed.

When this steam chest 1 is normalized and tempered, it is performed as follows. That is, heat treatment is performed by selecting a temperature at which a strength lower than the final target strength is obtained. Specifically, we selected a tempering temperature higher than the tempering temperature (1715℃) at which the final target strength was obtained.
Reduce strength. In this example, a normalizing temperature lower than the normalizing temperature (approximately 1050°C) at which the final target strength is obtained is selected, and this also results in a lower strength than the final target strength. I'm letting you do it. By performing such softening heat treatment, annealing cracks are prevented. As a standard, the strength is reduced to that of conventional Cr-Mo-V steel without B addition. If the strength is reduced to this level, the conventional B-free Cr-
Since annealing cracks do not occur in Mo-V steel, annealing cracks do not occur during welding of the steam chest in this example as well.

Next, overlay welding is performed on the groove surface 9 shown in FIG. This groove surface 9 is visible as an upper edge in FIG. 5 showing the CC cross section of FIG. 4 (that is, the view taken in the direction of arrow D in FIG. 5 corresponds to FIG. 4).
As shown in FIG. 5, overlay welding is performed on this groove surface 9 by overlaying 10 with a welding rod of Cr-Mo-V steel composition. In this example, as a welding rod, C: 0.04 to 0.22% by weight,
Si: 0.2-1.0% by weight, Mn: 0.3-0.7% by weight, Cr:
0.4-0.7% by weight, Mo: 0.9-1.25% by weight, V: 0.15
A Cr-Mo-V steel having a composition of ~0.35% by weight was used.
After overlaying 10, stress relief annealing is performed, but since the strength of the steam chest 1 itself has been reduced by the heat treatment, annealing cracks do not occur.

Thereafter, the steam chest 1 is normalized and tempered at a temperature that provides the final desired strength. As a result, the original high-temperature strength of B-added Cr-Mo-V steel can be obtained. In addition, here, as the metal for overlay 10
If a welding rod with a Cr-Mo composition is used, the strength of the overlay metal will decrease due to normalizing and tempering operations in the subsequent process, so the Cr-Mo welding rod is not suitable for use. However, the welding rod with a Cr-Mo-V composition used in this example is
Strength can only be obtained by normalizing and tempering after welding, so it can be used effectively.

In addition, the thickness of the build-up 10 is set to a certain level so that the heat effect during welding of the build-up metal 10 and the nozzle ring 2 in the next post process does not reach the steam chest 1. It's better. In this case, cracks are likely to occur due to the influence of heat when the overlay metal 10 is about 2 to 3 mm thick, so if the thickness is set to about 10 mm, it is thought that there is almost no risk of cracks occurring. In this example, in order to eliminate the thermal effect, a diameter of about 10 mm or more was required. Even if a crack were to occur near the boundary between the overlay metal 10 and the steam chest 1 due to this thermal influence, it would be easy to detect by ultrasonic flaw detection because the materials of the two are similar ( This will be discussed further below).

Next, the nozzle ring 2 made of 10Cr steel is assembled, and the nozzle ring 2 and the overlay metal 10 are welded using a welding rod made of 10Cr steel. As a result, the nozzle ring 2 and the steam chest 1 are integrated into a structure as shown in FIG. 6. In FIG. 6, reference numeral 3 indicates this welding rod. This welding rod 3 is formed by welding between a Cr-Mo-V weld metal that does not contain B and a 10Cr steel nozzle ring 2, so it cracks during annealing to remove residual stress after welding. does not occur. Special precautions, such as when using B-added steel, are unnecessary here.

The subsequent assembly of the nozzle blade 5 and welding with the nozzle ring 2 (the welded part is indicated by 6) are as follows:
This can be done in the same manner as in conventional structures.

As explained above, in the nozzle box of this embodiment, B-added Cr-Mo-V steel, which easily causes annealing cracking, is used as the material for the steam chest 1. Softening heat treatment was performed by normalizing at a temperature lower than the standard temperature and tempering treatment at a temperature higher than the standard temperature, followed by overlay welding, and then normalizing and tempering treatment at the standard temperature. The occurrence of annealing cracks in the welded portion 3 during annealing to remove residual stress is reliably prevented. In addition, the material of the steam chest 1 is B-added Cr-Mo-V steel, and the material of the overlay metal 10 is B.
Although it is additive-free, it is a Cr-Mo-V steel, so even if a crack were to occur near this boundary, the compositions of the two are almost the same, so it would be easily detected using normal ultrasonic flaw detection. On the other hand, since the overlay metal 11 and the welding part 3 are made of 10Cr steel, they are different materials, and if a crack occurs in or near the welding rod 3, it may be difficult to perform ultrasonic flaw detection, but as mentioned above, For this reason, there is no risk of annealing cracking here, and therefore there is no practical problem even if flaw detection is difficult.

〔Effect of the invention〕

As mentioned above, the turbine nozzle box of the present invention has a steam chest containing B-added Cr-Mo-V.
It is made of steel, subjected to softening heat treatment, and then Cr-
The nozzle ring is assembled by overlay welding with a welding rod of Mo-V composition, normalized and tempered at a regular temperature, and the nozzle ring is welded between the overlay metal and the nozzle ring. Since it is constructed by integrating the est, it is possible to avoid the occurrence of annealing cracking even though a steam est of B-added Cr-Mo-V steel, which has a possibility of annealing cracking, is used. Therefore, a highly reliable nozzle box can be obtained. And B-added Cr
-Mo-V steel has high strength, so even when middle-speed operation (change-speed operation) is required, it can sufficiently withstand it. Therefore, in any case, the use of this nozzle box has the effect of significantly improving the reliability of the turbine.

It should be noted that, as a matter of course, the present invention is not limited to the embodiments described above.

[Brief explanation of drawings]

FIG. 1 is an external view of a conventional general nozzle box. FIG. 2 is a view taken along arrow A in FIG. 1. Figure 3 shows the conventional welding structure.
It is a BB sectional view of the figure. 4 to 6 show one embodiment of the present invention, and FIG. 4 is an external view of the nozzle box chest. FIG. 5 is a diagram showing a case where overlay is applied to the material, and corresponds to the sectional view taken along the line C--C in FIG. 4. FIG. 6 is a sectional view of the nozzle box of this embodiment. 1... Steam chest, 2... Nozzle ring, 3... Welded part, 4... Groove surface, 5... Nozzle blade, 6... Welded part, 7, 8... Steam, 9... Overlay Welding groove surface, 10... Overlay metal.

Claims (1)

[Claims] 1. In a nozzle box provided in the high-pressure first stage of a steam turbine, a steam chest made of B-added Cr-Mo-V steel is subjected to a softening heat treatment, and then Cr-Mo
- Overlay welding with a welding rod of V composition, normalizing and tempering at a regular temperature, assembling the nozzle ring, and welding between the overlay metal and the nozzle ring, the nozzle ring and steam chest are assembled. A turbine nozzle box characterized by being configured by integrating the following. 2. The softening heat treatment is performed by normalizing and tempering the steam chest, in which case the tempering temperature is higher than the normal temperature. turbine nozzle box. 3. The softening heat treatment involves normalizing and tempering the steam chest, in which case the normalizing temperature is lower than the normal temperature and the tempering temperature is higher than the normal temperature. A turbine nozzle box according to claim 1, characterized in that: