JPS6353211A - Treatment of stainless steel tube body for existing boiler - Google Patents

Abstract

PURPOSE:To surely remove the scale and to prevent the development of scale, which peels easily, a long time by executing the shot blast working aiming descaling and cold working layer formation to the removed titled pipe body. CONSTITUTION:There are superheater tube, reheater tube, etc., as the object in the steel tube for boiler, and these are removed at each tube unit or the panel unit connecting plural tube bodies by a spacer. To the inner face of tube body removed under such state, the shot blast working is executed. In this way, at the first half part of this working, the scale is removed and also at the second half part of the working, the cold working is executed to the inner face of tube, and so the working layer forming protecting film having excellent high temp. steam oxidizing resistance is formed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for treating the inner surface of stainless steel tubes used as superheater tubes, reheater tubes, etc. in existing boilers. The object of the present invention is to provide a treatment method that can remove steam oxidation scale from surfaces and appropriately improve high-temperature steam oxidation resistance.

[Conventional technology and its problems]

Iron-based alloy tubes such as austenitic stainless steel are used as steel tubes for large boilers such as boilers for thermal power generation, but the inner surface of such tubes is exposed to high-temperature steam of 500 to 650 degrees Celsius, so they tend to scale. The formation of is remarkable.

For example, scale on the inner surface of a tube may grow to a size of 100μ or more in a few years after the boiler is used, and this scale layer is due to the physical properties of the inner layer of the tube containing alloying elements and the outer layer containing almost no alloying elements. Because the outer scale is composed of inner and outer scale layers with different levels, the outer scale is likely to peel off due to temperature changes such as when the boiler is stopped, and such peeled off scale is carried to the steam turbine along with the steam, causing damage and wear to the turbine nozzle blades, etc. Cause. In addition, if this peeling scale becomes large, it accumulates inside the pipe and causes pipe blockage in curved pipe parts such as superheaters, obstructs the steam flow, and causes the pipe wall temperature to rise abnormally, causing a pipe blowout accident. It may happen that
To address these problems, methods for manufacturing boiler steel pipes with enhanced high-temperature steam oxidation resistance have been proposed, but countermeasures using such steel pipes are not suitable for newly manufactured boilers. Although effective, existing boilers that are already in continuous use and have not been addressed will not be involved at all. In general, for existing boilers like this, measures are taken to detect scale blockage through periodic radiological inspections, etc., and depending on the situation, cut off the bent pipe section, remove the scale therein, and rejoin it by welding. However, it is extremely troublesome to perform such work on a regular basis, and the maintenance costs involved are also variable (depending on various conditions). There are cases where it cannot be applied depending on the vehicle condition between the scales, and it is difficult to say that it is a perfect countermeasure. In this way, for existing boilers that have not been countered, the current situation is that the only measures taken are to remove the scale that has formed and flaked off from the outside of the pipe, and the contents are complicated as described above. Since this method is incomplete and incomplete, it has been desired to put into practical use an effective treatment means for such existing boilers.

[Structure and Embodiments of the Invention] The present invention has been made in view of the above-mentioned conventional problems, and it removes scale generated on the tube body of an existing boiler, improves high-temperature steam oxidation resistance, and eliminates scale formation. The basic feature is that the stainless steel pipe body removed from the existing boiler is treated with a metallurgical treatment that can be properly prevented. The reason is that shot blasting is applied.

The present invention will be specifically explained below.

The steel tubes for boilers that are the object of the present invention include superheater tubes, reheater tubes, etc., and these can be removed in units of individual tubes or in units of panels formed by connecting multiple tube bodies with spacers. The treatment of the present invention is applied.

The inner surface of the tube body removed in this way is subjected to shot blasting, which removes scale in the first half of the process, and then coats the inner surface of the tube in the second half of the process. Cold working is performed to form a processed layer that can form a protective film with excellent high temperature steam oxidation resistance.

In shot blasting, steel balls or other hard particles made of carbon steel, alloy steel, stainless steel, etc. are sprayed onto the inner surface of a tube at an appropriate pressure and amount. A method of inserting an introductory pipe with a A method such as moving the nozzle in the longitudinal direction of the tube while rotating the tube body can be adopted. Note that the particles used in this shot blasting process are those with an angular outer surface shape (such as alumina) that facilitates grinding of scale when considering descaling as the main ingredient, but also those with an angular outer surface shape (e.g. alumina), which are suitable for forming a strained layer due to cold working. When considered as a main body, it can be said that type 4 with a smooth outer surface shape such as the above-mentioned steel ball is suitable.For this reason, for example, the shot blasting process is divided into two stages, and the first stage focuses on descaling. Particles such as alumina are suitable for this [particles with a squared outer surface shape]
It is possible to use a method such as using particles such as steel balls (particles with a smooth outer surface shape) suitable for cold working in a later stage.

As mentioned above, the scale is almost completely removed by this shot blasting process, and a processed layer is formed on the inner surface of the tube in which Cr is easily diffused by heating. A protective film is formed on the layer, and this film provides a high degree of high temperature steam oxidation resistance. Here, 4. In order to obtain sufficient oxidation resistance with such a processed layer, it is preferable to form the processed layer uniformly with a certain thickness, and usually the processed layer has a thickness of about 10μ or more. Forced to form. In addition, in order to obtain such a layer uniformly, 4. OK9
/cn? For spraying above, the pressure, o, ozKr/αshimi
For spraying of n or more, shot particles are sprayed in quantity.

In addition, the formation of a processed layer by shot blasting can also be evaluated by its surface hardness K, and usually a portion of a predetermined layer thickness (for example, 40μ) has a Vickers hardness of 280 (
It is preferable to process the material so that it has a Hv of about 280) or higher.

Next, one embodiment of the present invention will be described. A superheater tube (SUS 321 HTB) that has been used for about 5 years from the secondary superheater of an existing large boiler for thermal power generation.

A sample with a measured average outer diameter of 45.0 m and a wall thickness of 8.2 mm was taken out and treated by the method of the present invention. The average scale layer on the inner surface of the tube was 52μ for the inner layer and 18μ for the outer layer (presumably due to repeated peeling several times). After descaling the inner surface of the tube by spraying alumina grains toward the entire circumference at a spray pressure of 6.5Kp7-, steel shot is sprayed from a 20-hole nozzle moving in the lengthwise direction of the tube. Crab esKg/cyt? , Spray amount: 10
Cold working was performed by jetting in the entire circumferential direction at Kf/min.

Figure 1 shows the cross-sectional microstructure near the inner surface after shot blasting (after precipitation treatment at 650°C for 2 hours, magnified 300x with a microscope). ) is clearly formed.

The high-temperature steam oxidation resistance of the above test tubes was evaluated at 650℃
A 500 hour accelerated test was conducted. For comparison, a new pipe (untreated) of similar material and size was also tested. As a result of this accelerated test, a scale layer with an average size of about 50 μm was generated in the untreated comparative material, whereas a scale layer with an average size of about 2 to 3 μm was observed in the treated material according to the present invention. It was found that high temperature steam oxidation resistance was obtained. From the comparison between the material treated according to the present invention and the comparative material through such accelerated tests, it is estimated that the formation of scales that would otherwise occur in Mt-treated tubes treated with the method of the present invention can be suppressed over a long period of time. be done.

〔Effect of the invention〕

According to the present invention described above, it is possible to reliably remove scale from stainless steel tubes such as superheater tubes and reheater tubes in existing boilers, and improve the high-temperature steam oxidation resistance of the stainless steel tubes, thereby eliminating scale that may cause peeling. It is possible to reliably prevent scale generation over a long period of time, and it can be said that prevention of scale generation is extremely effective as a measure against scale in existing boilers where no measures have been taken.

[Brief explanation of drawings]

FIG. 1 is a photograph (magnification: 300 times) of a cross-sectional microstructure near the surface of a tube in which a precipitation treatment was performed at 650° C. for 2 hours after shot blasting in an example of the present invention. Figure 1:

Claims (1)

[Claims] A method for treating a stainless steel tube body of an existing boiler, characterized by subjecting the stainless steel tube body removed from the existing boiler to shot blasting for the purpose of descaling the inner surface and forming a cold worked layer.