JP2012201975A - Austenitic stainless steel pipe having water vapor oxidation resistance, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel pipe excellent in water vapor oxidation resistance by determining quality of shot processing based on measurement of surface roughness on the inner surface of the pipe.SOLUTION: In the austenitic stainless steel pipe 1 used for a heat transfer pipe in a boiler, the roughness of the pipe inner surfaces 5 after shot processing performed on the pipe inner surfaces is an arithmetic mean roughness (Ra) of 2 μm or less. In a water vapor oxidation scale layer 6 occurring on the inside of the steel pipe during its use, the thickness of an inner layer 6a consisting of (Cr,Fe)Ois 10 μm or less. The hardness of the pipe inner surface after the shot processing on the pipe inner surface is 350 Hv or greater.

Description

  The present invention relates to an austenitic stainless steel pipe used for a heat transfer pipe of a boiler, and in particular, an austenitic stainless steel pipe excellent in steam oxidation resistance suitable for use in a portion where high-temperature and high-pressure steam flows in the steel pipe. About.

The high-temperature part where the high-temperature gas of the boiler flows is provided with a superheater, a reheater, etc. in which high-temperature, high-pressure steam flows in the steel pipe.The heat transfer tubes constituting these from the viewpoint of high-temperature strength and corrosion resistance, An austenitic stainless steel pipe containing 18% or more of Cr is used. During boiler operation, a steam oxidation scale is generated on the inner surface of the steel pipe by contact and reaction with high-temperature and high-pressure steam flowing inside the steel pipe. The steam oxidation scale has a two-layer structure including an inner layer made of (Cr, Fe) ₃ O ₄ and an outer layer made of Fe ₃ O ₄ .

  Since austenitic stainless steel generally has a large coefficient of linear expansion, expansion or contraction of the steel pipe occurs due to changes in the load of the boiler and the temperature change of the fluid flowing inside the steel pipe when the operation is stopped or started. Since the difference in expansion from the steam oxidation scale forming a layer on the inner surface of the steel pipe is large, peeling of the steam oxidation scale is likely to occur. The peeled steam oxide scale causes steam blockage erosion caused by pipe blockage caused by accumulation in a bent part of a steel pipe or by scattering to a steam turbine part via a steam pipe. Therefore, the steel pipe used in the high temperature part of the boiler is required to have excellent steam oxidation resistance in addition to high temperature strength.

  Measures to improve the steam oxidation resistance of austenitic stainless steel include increasing the Cr content in the material, refining the crystal grains, and forming a hardened layer by shot processing of the inner surface of the steel pipe. The method is to form a hardened layer by shot machining of the inner surface of the steel pipe. For example, as shown in Patent Document 1, the shot processing is performed by performing shot processing on the inner surface of the steel pipe with particles of stainless steel or the like at a predetermined pressure and a spraying amount or more, and shots having a predetermined thickness or more on the inner surface of the steel pipe. A processed layer is formed. When the steel pipe processed in this way is used in a superheater for generating high-temperature steam, an extremely thin and dense scale is formed on the inner surface of the steel pipe to prevent the growth of the entire steam oxidation scale.

  In addition, as a method for determining whether or not shot processing in an austenitic stainless steel pipe is reliably performed on the entire surface, for example, in Patent Document 2, a light source is applied to the inner surface of the tube from one side of the shot processed tube, and the other end Describes a method of measuring a shot-processed area while moving a TV camera for observing the inner surface in a tube. In this case, the shot-processed surface becomes a non-glossy surface due to minute unevenness, and the unprocessed surface can be identified by being glossy. In addition, it is described that a shot condition in which a shot processed area is 70% or more of the whole is selected.

JP 52-8930 A International Publication Number WO2007 / 099949 (Japanese Patent Application No. 2008-502795)

  In recent large-scale boilers for thermal power generation, the length of austenitic stainless steel pipes used for superheaters and reheater pipes is more than several thousand meters per boiler, and shot processing is reliably applied to the entire inner surface of the steel pipe. It is necessary to assure the quality of the product, and a judgment technique for that purpose is required. Although the above Patent Document 1 describes that it is necessary to perform shot processing with a predetermined pressure and a spraying amount or more, it is described that it is necessary to determine that proper shot processing has been performed. Not.

  Further, in Patent Document 2 described above, as a method for determining whether or not the shot processing is reliably performed, the fact that the shot processing surface becomes a non-glossy surface due to unevenness and the unprocessed surface becomes a glossy surface is used. However, this method is not practical because it is easily affected by the state of the tube inner surface before shot processing and the measurement accuracy is not high. .

  An object of the present invention is to provide an appropriate determination method as a method for determining whether or not the shot processing is reliably applied, and further to the steam oxidation resistance obtained by the execution by this determination method. The object is to provide an excellent austenitic stainless steel pipe.

In order to solve the above problems, the present invention mainly adopts the following configuration.
An austenitic stainless steel pipe for a heat transfer pipe, wherein the roughness of the inner surface of the pipe after shot processing on the inner surface of the steel pipe is 2 μm or less in terms of arithmetic average roughness (Ra). Further, this steel pipe is a steel pipe having a thickness of an inner layer made of (Cr, Fe) ₃ O ₄ of 10 μm or less among the steam oxide scale layers generated inside the steel pipe during use.

Moreover, in the manufacturing method of an austenitic stainless steel pipe used for a heat transfer pipe of a boiler, the steel pipe is subjected to shot processing on the inner surface of the steel pipe, and the roughness of the inner surface of the pipe after the shot processing is 2 μm or less in terms of arithmetic average roughness (Ra) Manufacturing method. Further among the steam oxidation scale layer to produce steel pipe inside during use, a method for producing a steel pipe according to (Cr, Fe) 10 [mu] m or less of the inner layer thickness consisting of ₃ O _4.

  Conventionally, it has been known to suppress the formation of steam oxidation scale by forming a processing layer on the inner surface of a steel pipe, but there is no appropriate method for determining whether or not shot processing has been applied reliably. It was. According to the present invention, it is possible to determine whether or not shot processing is reliably performed by measuring the surface roughness of the inner surface of the tube, and it is possible to obtain a steel tube excellent in steam oxidation resistance.

It is a figure which shows schematic structure which performs shot processing to the inner surface of the austenitic stainless steel pipe which concerns on embodiment of this invention. It is a figure which shows the relationship between the steam oxidation scale thickness produced | generated on the pipe | tube inner surface after the shot process regarding the 1st Embodiment of this invention, and also shows the relationship between arithmetic mean roughness and the hardness in a pipe | tube inner surface depth of 50 micrometers. FIG. It is a schematic diagram explaining the relationship between the roughness of the pipe inner surface and the produced | generated steam oxidation scale after the shot process regarding 1st Embodiment. The hardness distribution of the inner surface of the tube at a depth position from the inner surface of the tube after the shot processing according to the second and third embodiments of the present invention was compared with the hardness distribution of the inner surface of the tube without the shot processing. FIG. It is the photograph of the cross-sectional microstructure which performed the etching process on the pipe inner surface after the shot process regarding the 4th Embodiment of this invention, and clarified the shot process layer. It is a photograph of the cross-sectional microstructure at a depth of about 50 μm from the inner surface of the tube where the inner surface of the tube after the shot processing according to the fifth embodiment of the present invention is etched to clarify the shot processing layer. It is a figure which shows the relationship between the number of the slide line | wire per 10 micrometers length in the 50 micrometers depth position from the pipe inner surface after the shot process regarding 5th Embodiment, and the hardness in the 50 micrometers depth position of the pipe inner surface.

  An austenitic stainless steel pipe having steam oxidation resistance and a method for producing the same according to first to fifth embodiments of the present invention will be described below with reference to the drawings.

  First, an austenitic stainless steel pipe having steam oxidation resistance and a method for producing the same according to a first embodiment of the present invention will be described with reference to FIGS. In the drawings, 1 represents an austenitic stainless steel tube, 2 represents a shot nozzle, 3 represents shot particles, 4 represents a shot processed layer, 5 represents an inner surface of the tube, and 6 represents a steam oxidation scale.

  In FIG. 1, shot processing to the inner surface of an austenitic stainless steel pipe is performed by causing shot particles such as small austenitic stainless steel pieces or steel balls to collide with the inner surface of the steel pipe with compressed air, and crystal near the inner surface of the steel pipe. It causes many slip deformations in the grains and hardens them. In order to perform shot processing uniformly on the inner surface of the steel pipe, the shape and hardness of the shot particles, the spray pressure and amount of shot particles, the rotational speed of the shot nozzle in the inner peripheral direction of the steel pipe, and the axial direction of the nozzle It is necessary to optimize the moving speed condition.

  As a result of extensive research, the inventor of the present invention has a steel pipe inner surface roughness after shot processing that is an index of steam oxidation resistance in an austenitic stainless steel made of 16-23% Cr. It was found that there is a relationship with the thickness of the steam oxidation scale, and the relationship was confirmed by experimental data as shown in FIG.

  FIG. 2 shows the 18Cr8Ni austenitic stainless steel pipe (fire SUS304J1HTB) after the 876 hours of arithmetic mean roughness (Ra) of the inner surface of the austenitic stainless steel pipe made of 18% Cr after shot processing and the temperature of water vapor flowing through the steel pipe at 650 ° C. ) Was measured for the thickness of the steam oxidation scale (the thickness of all layers and the inner layer). In addition, in FIG. 2, the measured value of the hardness in the position whose depth from the steel pipe inner surface is 50 micrometers is also shown. The arithmetic average roughness was adjusted by changing the shot processing conditions, and the roughness was measured by direct measurement using a contact-type surface roughness meter. The measuring instrument is not limited to a contact type, and a non-contact type roughness meter such as a laser microscope may be used. Actual measurement is desirably performed over the entire length of the steel pipe, but can also be performed by sample measurement if the shot processing conditions are constant. Moreover, about the thickness of the steam oxidation scale, what extracted the sample was measured with the microscope by the corrosion test.

  As a result of the measurement, when the arithmetic average surface roughness (Ra) of the inner surface of the steel pipe is 1.97 μm or less (present inventions G to J), the thickness of the steam oxidation scale becomes 20 μm (inner layer 10 μm), The generation of steam oxidation scale could be suppressed. On the other hand, when the arithmetic average surface roughness (Ra) of the inner surface of the steel pipe is 2.06 μm or more (Comparative Examples C to F), the thickness of the steam oxidation scale becomes larger than 20 μm, and further the arithmetic of the inner surface of the steel pipe When the average surface roughness (Ra) was 2.44 μm or more, the thickness of the steam oxidation scale was larger than 70 μm (inner layer 40 μm), and generation of the steam oxidation scale on the steel pipe inner surface could not be suppressed.

  In the non-shot tube (Comparative Examples A and B), the thickness of the steam oxidation scale was 150 μm (inner layer 75 μm). The arithmetic average surface roughness (Ra) of the inner surface of the steel pipe was 2.44 and 2.51 μm.

  FIG. 2 also shows the measured value of the hardness at a position where the depth from the inner surface of the steel pipe is 50 μm. When the arithmetic average surface roughness (Ra) of the inner surface of the steel pipe is 1.97 μm or less, It can be seen that the hardness is 300 Hv or more, and the shot layer is formed reliably. On the other hand, when the arithmetic average surface roughness (Ra) of the steel pipe inner surface is 2.06 μm or more, the hardness at the position of 50 μm depth from the pipe inner surface (the Vickers hardness of the material hardness using the pyramid indenter) Satisfactory test) is less than 300 Hv, indicating that a reliable shot layer is not formed.

  FIG. 3 shows the relationship between the roughness of the pipe inner surface 5 after shot processing and the generated steam oxidation scale 6. As shown in the left diagram of FIG. 3, when shot processing is not uniform, a hardened layer is partially formed on the inner surface of the steel pipe, so that the surface roughness becomes rough. On the other hand, as shown in the right diagram of FIG. 3, when shot processing is uniformly performed on the inner surface of the steel pipe, the shot processing layer 4 is formed on the entire surface, so that the surface roughness becomes smooth.

  When exposed to high-temperature steam, the inner surface 6a of the steam oxidation scale is only thinly formed on the surface of the shot processed layer on which the shot processing has been performed, and the outer layer 6b generated on the outside thereof is only thinly generated. The layer thickness is reduced. A thick inner layer 6a is formed in a portion where shot processing is insufficient, and an outer layer 6b on the outer side is also formed thick, so that a thick steam oxidation scale 6 is formed as a whole. The oxide scale thickness increases (left figure).

  On the other hand, when the shot processing is uniformly applied to the inner surface of the steel pipe, the shot processing layer 4 is formed on the whole, and the steam oxidation scale is generated only thinly in the inner layer 6a and the outer layer 6b. Accordingly, the thickness of the steam oxidation scale becomes small even with the arithmetic average roughness (right figure).

Here, the steam oxidation scale produced | generated on the inner surface of an austenitic stainless steel pipe is demonstrated. The steam oxidation scale 6 formed on the inner surface of the austenitic stainless steel pipe has a two-layer structure in which the outer layer 6b is made of Fe ₃ O ₄ and the inner layer 6a is made of (Cr, Fe) ₃ O ₄ regardless of the presence or absence of shots. ing. Here, an oxide containing a large amount of Fe (Fe ₃ O ₄ , (Cr, Fe) ₃ O ₄ : (Fe> Cr)) is an oxide containing a large amount of Cr (Cr ₂ O ₃ , ((Cr, Fe) ₃ O ₄ : (Cr> Fe)) is generally faster in the generation rate because the diffusion rate (migration rate) of ions (Fe, Cr, O ions) in the oxide is higher in the case of Fe oxide. This is because it is faster than the oxide.

  Shot processing has the effect of increasing the diffusion rate of metals (Fe, Cr) in the metal, and in SUS steel of 18 (16) Cr or higher, the diffusion rate of Cr is relatively higher than that of Fe. A scale with a large amount of Cr in the scale is formed in the initial stage compared to the case without, and the scale growth rate is significantly reduced (suppressed). Therefore, if shot processing is applied uniformly (entire surface), a thin scale with a large amount of Cr is generated uniformly (entire surface), but if shot processing is non-uniform (partial), partial As a result, a thick scale containing a large amount of Fe is generated, and thus scale growth cannot be completely suppressed. It should be noted that both Cr oxide and Fe oxide are steam oxidation scales produced by the participation of metal with steam.

  Therefore, by measuring the arithmetic average roughness of the pipe inner surface after the shot processing on the steel pipe 1, it is possible to determine whether or not the shot processing for suppressing the generation of the steam oxidation scale is reliably performed. In this embodiment, the arithmetic average roughness (Ra) is used for the roughness of the pipe inner surface, but the present invention is not limited to this, and other examples such as the maximum height (Rz) and the average square root height (Rq) are used. A roughness parameter may be used. The validity of using these roughness parameters was confirmed by measuring the hardness at a depth of 50 μm.

Thus, in this embodiment, if the arithmetic average roughness of the inner surface of the pipe after shot processing is 2 μm or less, the thickness of the inner layer 6a made of (Cr, Fe) ₃ O ₄ is 10 μm or less. Therefore, it is judged to be an austenitic stainless steel pipe excellent in steam oxidation resistance.

  Next, FIG. 4 shows a method for evaluating the steam oxidation resistance by inner surface shot machining of an austenitic stainless steel pipe according to the second embodiment of the present invention by using the hardness of the inner surface of the pipe before using the steel pipe. The description will be given with reference. FIG. 4 shows the result of comparing the tube inner surface hardness of an 18% Cr austenitic stainless steel tube after shot processing and the hardness without shot processing. According to FIG. 4, the increase in hardness due to shot processing is maximized on the inner surface of the tube, and gradually decreases at a position toward the inside of the tube. Therefore, the hardness of the inner surface of the tube is not affected by the measurement position unlike the inside of the tube, and a stable result can be obtained.

  In the present embodiment, if the hardness of the tube inner surface after shot processing is 350 Hv or more, it is determined that the austenitic stainless steel tube has excellent steam oxidation resistance. In this embodiment, since the hardness is directly measured from the inner surface of the tube, it can be said that the measurement is easier and more accurate than the measurement inside the tube from the tube cross section.

  Next, steam oxidation resistance by inner surface shot machining of the austenitic stainless steel pipe according to the third embodiment of the present invention is further determined using the hardness of the pipe inner surface (depth position in the pipe) before using the steel pipe. A method for evaluation will be described with reference to FIG. As shown in FIG. 4, the hardness of the austenitic stainless steel pipe made of 18% Cr after the shot processing is maximized on the pipe inner surface, and is observed from the pipe inner surface to a depth position of about 200 μm. If the hardness is measured within a range of a depth of about 200 μm from the inner surface of the tube, the influence of shot processing can be evaluated, but what is important is the hardness near the inner surface of the tube.

  There is also a method of defining the hardness at a position about 100 μm deep from the inner surface of the tube (see Patent Document 3 above), but as the distance from the inner surface of the tube increases, the variation in hardness value increases, Since the hardness may not be guaranteed, it is better to evaluate the hardness near the inner surface of the tube as much as possible.

  When measuring the inner surface hardness of a steel pipe using the Vickers test method, the steel pipe is cut in a plane perpendicular to its axial direction into a ring shape, and the hardness of the inner surface of the ring-shaped steel pipe is changed from the inner surface to the outer surface of the tube. When measuring toward the surface, a diamond-shaped indenter (for example, the shape of the indenter is a quadrangular pyramid, the angle is 136 degrees, and the diagonal length is 0 to several μm) deviates from the measurement site in the vicinity of the inner surface. Therefore, a position with a minimum depth of 50 μm from the inner surface of the tube is an appropriate position for hardness measurement. According to FIG. 4, in this embodiment, if the hardness at a depth of 50 μm from the inner surface of the pipe is 300 Hv or more, it is determined that the austenitic stainless steel pipe has excellent steam oxidation resistance.

  Next, the steam oxidation resistance by the inner surface shot processing of the austenitic stainless steel pipe according to the fourth embodiment of the present invention, and the shot processing layer 4 (formed by shot processing, which is measured from the microstructure before using the steel pipe). A method of evaluating using the depth of the layer will be described with reference to FIG. FIG. 5 is a cross-sectional microstructure photograph of an austenitic stainless steel pipe made of 18% Cr after shot processing. This photograph shows the steel tube after shot processing for 1 hour, heat treated at 650 ° C for 1 hour, embedded in resin, mirror-polished, and observed by electrolytic etching in chromic acid solution to clarify the shot processing layer 4 It is.

  On the inner surface of the tube, a large number of slip lines 7 (see FIG. 6 to be described later) generated by plastic deformation by shot processing are observed in black, and this is called a shot processing layer 4. The shot processing layer 4 becomes uniform and deep when the shot processing is reliably performed. In the present embodiment, if the depth of the shot processing layer from the inner surface of the pipe is 50 μm or more, it is determined that the austenitic stainless steel pipe has excellent steam oxidation resistance.

  Next, the steam oxidation resistance by inner surface shot machining of the austenitic stainless steel pipe according to the fifth embodiment of the present invention, and the density of the slip line 7 of the shot machining layer 4 measured from the microstructure before using the steel pipe A method of evaluating using (number) will be described with reference to FIGS.

  FIG. 6 is a cross-sectional microstructure photograph of an austenitic stainless steel pipe composed of 18% Cr after shot processing at a depth of 50 μm from the pipe inner surface. This photograph was observed by the same processing method as in FIG. It can be seen that slip lines produced by plastic deformation by shot processing are observed in the crystal grains. The relationship between the number of slip lines (per 10 μm length) and hardness is summarized in FIG. 7 as experimental data.

  According to FIG. 7, it can be seen that if the number of slip lines at a depth of 50 μm is 3 or less, the hardness can be 300 Hv or less. Therefore, in this embodiment, if the number of slip lines at a depth of 50 μm from the inner surface of the tube is 4 or more per 10 μm length, it is determined that the austenitic stainless steel tube has excellent steam oxidation resistance. .

  As described above, in the austenitic stainless steel pipe used for the heat transfer pipe of the boiler according to the embodiment of the present invention, the roughness of the inner surface of the pipe after shot processing, the hardness of the inner surface of the pipe, and the vicinity of the pole surface in the cross section of the pipe When each of the hardness and the shot processed layer depth obtained from the structure observation on the tube cross section satisfies a specific reference value, it can be determined that the austenitic stainless steel tube has excellent steam oxidation resistance.

  According to the present invention, it is possible to determine whether shot processing is reliably applied to the inner surface of an austenitic stainless steel pipe by a relatively simple method that does not require expensive measuring means, and a steel pipe having excellent steam oxidation resistance. Can provide.

DESCRIPTION OF SYMBOLS 1 Austenitic stainless steel pipe 2 Shot nozzle 3 Shot particle 4 Shot processing layer 5 Pipe inner surface 6 Steam oxidation scale layer 6a Inner layer 6b Outer layer
7 slip line

Claims (13)

In austenitic stainless steel pipes for heat transfer tubes,
An austenitic stainless steel pipe characterized in that the roughness of the inner surface of the pipe after shot processing on the inner surface of the steel pipe is 2 μm or less in terms of arithmetic average roughness (Ra). In the austenitic stainless steel pipe according to claim 1,
An austenitic stainless steel pipe characterized in that, among the steam oxide scale layers generated inside the steel pipe when the steel pipe is used, the inner layer made of (Cr, Fe) ₃ O ₄ has a thickness of 10 μm or less. In the austenitic stainless steel pipe according to claim 2,
Furthermore, the hardness of the inner surface of the tube after shot processing on the inner surface of the tube is 350 Hv or more. In the austenitic stainless steel pipe according to claim 2,
Furthermore, the austenitic stainless steel pipe is characterized in that the hardness after shot processing on the inner surface of the pipe is 300 Hv or more at a depth of 50 μm from the inner surface of the pipe. In the austenitic stainless steel pipe according to claim 2,
Furthermore, an austenitic stainless steel pipe characterized in that a shot processed layer observed from the microstructure of the inner surface of the tube after the shot processing on the inner surface of the tube reaches a depth of 50 μm or more from the inner surface of the tube. In the austenitic stainless steel pipe according to claim 2,
Furthermore, the number of slip lines observed from the microstructure of the steel pipe after shot processing on the inner surface of the pipe is 4 or more per 10 μm length at a depth of 50 μm from the inner surface of the pipe. Steel pipe.   A boiler using the austenitic stainless steel pipe according to any one of claims 1, 2, 3, 4, 5, and 6 for a superheater or a reheater. In the manufacturing method of the austenitic stainless steel pipe used for the heat transfer pipe of the boiler,
A method for producing an austenitic stainless steel pipe, characterized in that the inner surface of the pipe is subjected to shot processing, and the roughness of the inner surface of the pipe after shot processing is 2 μm or less in terms of arithmetic average roughness (Ra). In the manufacturing method of the austenitic stainless steel pipe described in claim 8,
A method for producing an austenitic stainless steel pipe, characterized in that a thickness of an inner layer made of (Cr, Fe) ₃ O ₄ is 10 μm or less among the steam oxide scale layers generated inside the steel pipe when the steel pipe is used. In the manufacturing method of the austenitic stainless steel pipe according to claim 9,
A method for producing an austenitic stainless steel pipe, wherein the inner surface of the pipe is further subjected to shot processing, and the hardness of the inner surface of the pipe after shot processing is 350 Hv or more. In the manufacturing method of the austenitic stainless steel pipe according to claim 9,
Further, a method for producing an austenitic stainless steel pipe, wherein the inner surface of the pipe is shot and the hardness after the shot is set to 300 Hv or more at a depth of 50 μm from the inner surface of the pipe. In the manufacturing method of the austenitic stainless steel pipe according to claim 9,
Further, shot processing is performed on the inner surface of the tube, and the microstructure of the inner surface of the tube after the shot processing is observed. From the observation, a shot processing layer that is observed in black at a plurality of slip lines caused by plastic deformation is 50 μm or more from the inner surface of the tube. A method for producing an austenitic stainless steel pipe characterized by having a depth. In the manufacturing method of the austenitic stainless steel pipe according to claim 9,
Further, the inner surface of the pipe is shot, the microstructure of the steel pipe after the shot is observed, and from the observation, the number of slip lines observed is 4 per 10 μm at a depth of 50 μm from the inner surface of the pipe. A method for producing an austenitic stainless steel pipe characterized by comprising at least this.