JPS63250452A - Production of steel pipe having chromized inside surface - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method of manufacturing internally chromized steel pipes used mainly for superheaters and reheaters of boilers in thermal power plants, and more specifically relates to a method for manufacturing internally chromized steel pipes that are used in boilers such as superheaters and reheaters in thermal power plants. The present invention relates to a method for producing an inner chromized steel pipe having a uniform chromized layer on the inner surface, by filling the pipe with chromized steel and performing chromized treatment.

Technical Background Steel pipes used in superheaters and reheaters in thermal power plants are oxidized as high-temperature steam flows through them, and the outer layer is made of magnetite (FL304>) and the inner layer is made of spinel ((Fe, C).
r) A two-layered oxide scale consisting of 30t) is generated. This scale is subjected to thermal strain during temperature cooling or during the subsequent heating process, so it flakes off from the inner surface of the tube, and in the case of stainless steel, the tube is blocked by the flaked scale, and in the case of Cr-MO8Jll, the scale peels off and falls off. The peeled off scale is scattered into the turbine section, causing problems such as severe erosion and thinning of the turbine blades. There are currently few cases of Cr-MoW4 in Japan, and they are most noticeable in the United States, but in the future it is inevitable that boiler usage times in Japan will reach the same level as in the United States.
- It is expected that scale peeling accidents of Mo1ll will occur frequently in Japan and become a problem, so sufficient countermeasures are required.

Prior Art As a technique aimed at preventing the formation of scale due to steam oxidation on the inner surface of the steel pipe for boiler tubes, a method of applying talomization treatment to the inner surface of the steel pipe has been studied.

This chromizing treatment method includes: ■ immersing the entire steel pipe in a penetrant and applying chromizing treatment to both the inner and outer surfaces;
■A method is known in which a penetrant is filled into a steel pipe and the inner surface is chromized while the pipe remains stationary. In these methods, a mixture of metal chromium powder and alumina powder is used as a penetrant, and a powder to which a small amount of halide is added is added to
00% filled and used in hydrogen or inert gas,
Generally, it is heat treated at high temperature.

Problems to be Solved by the Invention In the conventional chromizing treatment method, the FθCR2 gas generated in the chromizing reaction described below becomes saturated on the steel surface, and the chromizing reaction rate decreases significantly. If the thickness is set to 100 μm, there is a drawback that a processing time of 10 hours or more is required.

Fa+CrCb=FeCb+Cr Roughly speaking, there is a drawback that the reaction rate differs in the circumferential direction and the variation in the chromized layer thickness increases. Note that the filling rate is the area ratio of the penetrant to the cross section of the pipe.

This invention was made in order to eliminate the above-mentioned drawbacks of the conventional method, and it is possible to produce a chromized steel pipe with no variation in the thickness of the chromized layer in the circumferential direction and longitudinal direction without requiring a long time as in the conventional method. The purpose of this paper is to propose a method that can be manufactured in a short time without any problems.

Means for Solving the Problems The present invention is a method for solving the above-mentioned drawbacks of the conventional technology.As a result of various studies, it was found that rotating the tube makes the growth of the chromized layer uniform and promotes it, and that the penetrating agent is Filling rate 1
It was discovered that the chromized layer thickness grows at a sufficient speed even when the thickness is less than 0.00%, and the invention was completed.

The gist of this invention is to fill a steel pipe with a penetrant consisting of metal chromium powder, an alumina sintering inhibitor, and a halide flux, and to chromize the inner surface of the pipe at high temperature. By setting the filling rate to 25% or more and rotating the tube to perform the chromization treatment, the influence of saturated Fs Cfl 2 gas generated on the steel surface is reduced, the chromization reaction rate is improved, and the chromization reaction rate is increased in a short time in the circumferential direction. A steel pipe with a uniform and high quality chromized layer was obtained.

In this invention, the purpose of laying the tube horizontally is to make the filling rate of the penetrating agent uniform in the length direction of the tube.

In addition, the filling rate of the penetrant was limited to 25% or more, as shown in Figure 3, which shows the influence of the filling rate of the penetrating agent on the thickness of the chromized layer.When the filling rate is less than 25%, the growth of the chromized layer thickness is reduced. This is because the growth of the chromized layer thickness increases rapidly at 25% or more. Note that if the filling rate exceeds 90%, some voids will be generated in the chromized layer and a uniform chromized layer will not be obtained, so the filling rate of the penetrant is preferably 30 to 90%.

In addition, the purpose of rotating the steel pipe filled with the penetrant is to make the reaction rate uniform in the circumferential direction of the pipe and eliminate variations in the thickness of the chromized layer that is generated. As shown in Fig. 2, which shows the influence of the circumferential speed of the inner surface of the tube on the thickness of the chromized layer, as the inner rotational circumferential speed of the steel pipe filled with the penetrant increases,
The thickness of the chromized layer grows quickly, and the effect saturates as the speed increases, so it is preferably 0.005 to 2 m.
/s.

As illustrated in FIG. 1, the apparatus for carrying out the method of the present invention includes a steel pipe (1) filled with a penetrant (21), which is heated in a heat treatment furnace (3) having an inlet (6) and an outlet (7) for atmospheric gas. ), and both ends of the steel pipe (1) are connected to the rotating shaft (4
) and rotated by a motor (5) can be used.

That is, metal Cr powder and M2O3 are placed inside the lll tube (1).
The steel pipe is filled with a penetrant (2) consisting of an anti-sintering agent and N 84 Cff flux at a filling rate of 25% or more, and is rotatably supported in a heat treatment furnace (3), and a motor (5)
While rotating the furnace, H2 or Ar is supplied into the heat treatment furnace (3) from the inlet (6), and chromization treatment is performed in this H2 or Ar air flow.

By performing the chromizing treatment in this manner, a chromized layer can be uniformly formed on the inner surface of the steel pipe in a short time.

Example 1 5TBA 24 (6
After filling a penetrant into a pipe of 0.3φ did. Further, it was found that although a filling rate of 100% causes no practical problem in the chromized layer, some voids were generated, so that the upper limit of the filling rate is preferably 90%.

Implementation Example 2 After filling the same steel pipe as in Example 1 with a penetrant, chromization treatment was performed under the conditions shown in Table 2 to examine the rotation effect of the steel pipe. As a result, the chromization layer thickness was determined so that the steel pipe did not rotate. It was found that the method of the present invention, in which the steel pipe is rotated, increases the thickness of the pipe in the same processing time compared to the conventional method.

Further, Table 3 shows the variation in the chromized layer thickness in the circumferential direction of Test No. 1 (example of the present invention) and Test No. 4 (conventional example) in which the chromized treatment was performed under the conditions of 1050'Cx 10 hours. From Table 3, it was found that the variation in the chromized layer thickness was significantly reduced by rotating the M tube.

Example 3 The same steel pipe as in Example 1 was filled with a penetrant and then subjected to chromization treatment, and the effects of changes in rotational circumferential speed were investigated. The results are shown in Table 4.

From Table 4, as the circumferential speed increases, the reaction speed increases,
It turned out to be saturated. Therefore, it has been found that the preferable range of the circumferential velocity of the inner surface of the tube is o, oos to 2 m/s.

In addition, the chromized layer thickness in Examples 1 to 3 above is as follows:
It is the average value of the values measured at 8 points (equally spaced) in the circumferential direction.

The following margins are shown below. Since steel pipes can be manufactured in a short time, internally chromized steel pipes can be manufactured with high productivity and at low cost, and high quality internally chromized steel pipes can be provided at low cost.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of an apparatus for carrying out the method of the present invention. FIG. 2 is a diagram showing the influence of the circumferential speed of the inner surface on the thickness of the chromized layer. FIG. 3 is a diagram showing the influence of the filling rate of the penetrant on the thickness of the chromized layer. 1... Steel pipe, 2... Penetrant, 3...
Heat treatment furnace, 4... Rotating shaft, 5... Motor,
6...Entrance, 7...Exit. Applicant Sumitomo Metal Industries Co., Ltd. Agent Press 1) Good Kurogou Printing Figure 1

Claims (1)

[Claims] In a method of filling a steel pipe with a penetrant consisting of metallic chromium powder, an alumina sintering inhibitor, and a halide flux, and chromizing the inner surface of the pipe at high temperatures, the pipe is laid on its side and the filling rate of the penetrant inside the pipe is adjusted. 25% or more, and the method for producing an internally chromized steel pipe, which comprises rotating the pipe to perform the chromizing treatment.