JPS62224627A - Oxidation resistance treatment for heat resisting steel - Google Patents

Abstract

PURPOSE:To improve oxidation resistance to high temp. steam, by cold working the inner surface of heat resisting steel pipe made of high Cr steel or austenitic stainless steel through which high temp. steam is communicated, then irradiating laser beam and rapidly cooling the surface. CONSTITUTION:The inner surface of a pipe material 1 for passing high temp. steam such as heat transfer pipe of boiler for thermal power generation made of high Cr heat resisting steel contg. >=13% Cr or Ni-Cr austenitic stainless steel, etc., is subjected to cold workings such as roller finishing, shot blasting and shot peening. Tightly closing plates 4 having an inlet 2 and an outlet 3 for gas are attached to both ends of the pipe 1, gaseous Ar is fed in the pipe 1 from the inlet 2 and exhausted from the outlet 3 to obtain gaseous Ar atmosphere. Successively, a laser beam 5 is introduced in the pipe 1 through a crystal lense 6 made of KCl, inner surface of steel pipe is irradiated with a reflecting mirror 7 to heat it to 800-1,050 deg.C. The surface is locally melted and water cooled from outer part of steel pipe. Inner surface of the pipe 1 is made amorphous or fine grain layer is formed to improve oxidation resistance to high temp. steam.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a heat exchanger tube used in a superheater and a reheater of a boiler for thermal power generation, and a chemical plant.

This article relates to an oxidation-resistant treatment method for heat-resistant steel pipes such as piping used in the petroleum refining 2 petrochemical industry, etc., for circulating high-temperature steam, and heat transfer tubes used in heat exchangers that use high-temperature steam as a heat source.

[Conventional technology]

With the rise in temperature and pressure of boilers for thermal power generation.

Superheaters and reheaters use a large amount of austenitic steel pipes (hereinafter referred to as heat steel pipes).However, even stainless steel pipes do not come in contact with high temperature and high pressure steam for long periods of time. Oxidized scale forms on the inner surface of the tube.

The growth rate of this oxide scale depends on the temperature of water vapor.

It is known that it changes depending on the time it is in contact with water vapor and is significantly affected by the grain size and processing conditions of the stainless steel pipe. There is no problem with the formation of oxide scale when it is formed thinly on the inner surface of the pipe, but when it grows thickly, the CrNi content is higher than the base material content, and the lower layer and the lower layer are in close contact with the base material, which has a dense structure. Oxidized scale is formed on the upper layer mainly composed of porous Fe3O4, not only during boiler operation, but also when there are large temperature changes such as when the boiler is stopped and started.

The upper layer of oxide scale flakes off and accumulates in the U-bends of superheaters and reheaters, blocking the flow of water vapor and causing these pipes to overheat and explode. be. Moreover, even if it does not blow out, it is scattered along with steam to the turbine, causing damage to the nozzle and blades.

As a countermeasure against this problem, the following method is used.

(1) Fine-grained stainless steel pipes are used because the steam oxidation scale rate is slower for fine-grained pipes than for large-grained pipes. (This is because Cr in the base metal quickly diffuses to the surface through grain boundaries, forming a strong oxide film, which has the effect of significantly slowing down the oxidation rate thereafter.) (2) Grinding or shot peening the inner surface of the tube When a large amount of missing parts are created in the crystalline part through processing and come into contact with water vapor.

By using these defective parts as passageways, Cr in the base material is selectively diffused to the surface, and an oxide scale layer with high oxidation resistance is quickly generated.

(3) Perform Cr diffusion/penetration treatment or diffuse oxidation-resistant elements such as An and Si into the inner surface of the tube.

(4) Usually used stainless steel pipe (5US30
4.5US316.5US321, 5US347), stainless steel pipes with higher Cr content (e.g. 25
Cr-2ONi steel, 25Cr-12Ni steel, etc.) to improve oxidation resistance.

[Problem to be solved]

However, although method (1) improves oxidation resistance,
Fine-grained pipes have poor high-temperature strength. In the method (2), stress corrosion cracking is induced during the boiler construction period, for example, during a hydraulic test, and the effect of the processing is lost if solution treatment is performed after these cold workings. Method (3) is expensive, and diffusion of these oxidation-resistant elements generally requires heating the entire steel pipe to over 900°C, which poses a problem of deterioration of the strength of the base material, which requires reheat treatment. There are many uneconomic points, such as trying to recover the strength by using the same method. Furthermore, method (4) has serious economic problems, and a complete countermeasure has not been implemented at this stage.

Due to the above situation, when the boiler is stopped, the U-bent part of the superheater tube is irradiated with gamma rays, and the state of oxide scale accumulation is photographed to determine the presence or absence of oxide scale. Uneconomical methods such as cutting the oxide scale to remove the oxide scale are being used.

Furthermore, in recent years, research has been carried out to put boilers with steam temperatures as high as 650° C. into practical use, and the development of steel pipes and treatment methods that are superior to steam oxidation is expected.

[Means for solving problems]

In order to prevent the occurrence of failures caused by oxide scale by significantly reducing the growth rate of oxide scale, the present invention cold-works the inner surface of stainless steel with large grains in advance, and then irradiates it with a laser beam. Provided is a method for oxidation-resistant treatment of heat-resistant steel, which is characterized by carrying out oxidation-resistant treatment of heat-resistant steel and rapidly cooling it.

[Effect]

The surface irradiated with the laser beam is subjected to cold processing such as shot blasting, shot peening, and grinding, or by passing a roller through the inner surface of the steel pipe, and then heated to 800 to 1050°C by laser beam irradiation, and then
．． 01-1.5 Melt the surface layer of the eyebrows. Thereafter, the first surface layer is rapidly cooled, and the second surface layer is made amorphous or a fine crystal grain layer is formed in the surface layer.

〔Example〕

Example 1 Test material: 5US304, 5US316SUS321,
5US347 Both 5US310, 5TBA24
JIS symbol 5TBA26, 5US405 US430 Sample material dimensions: Width 5 Q m

Test laser: Carbon dioxide laser, output 3kW.

The diameter of the laser beam is 2. The laser beam is irradiated on only one side of the specimen, and there are two cases in which a part of the irradiated part melts at intervals of 3M, and a case where a part of the irradiated area is melted at an angle of 800 to 1050 degrees.
The output and moving speed of the irradiation device were adjusted in advance in case C was reached.

Furthermore, during irradiation, the cooling rate of the sample material was adjusted by cooling it from the back side of the irradiated surface using cooling water or liquid nitrogen.

Evaluation method of O oxidation resistance The test material after laser irradiation was placed in a tubular electric furnace, and a test was conducted at 650° C. for 300 hours while water vapor was flowing. Thereafter, the test material was taken out, a cross section was cut, and the oxidation resistance was evaluated by observing the formation of oxide scale using an optical microscope. Also, regarding evaluation.

The back side of the irradiated surface (cooled surface) was used as the non-irradiated surface, and the thickness of the oxide scale layer formed on this surface was taken as 100, and the ratio was expressed as 100.

Regarding the crystal grain size of the sample material, the austenitic stainless steel had ASTM No. 5 to 6, but when cooled with liquid nitrogen, no clear crystal grains were observed and it became amorphous. . The water-cooled material was so fine that it could not be indicated by ASTM No.

○ Oxidation resistance test results Table 1 below shows the oxidation resistance test results. Even if the type of sample material and cooling method change, the thickness of the oxide scale generated on the laser beam irradiated surface will change over time. It was also found that it was thin and had extremely high oxidation resistance.

Note that those in parentheses in Table 1 indicate those cooled with liquid nitrogen.

On the other hand, in the case of Cr steel, although the irradiation effect is lower than that of the former, it still shows much better oxidation resistance than the non-irradiated part. For steel types with a low Cr content in the base material (for example, 5TBA24, 5TBA26), almost no effect of laser irradiation is observed. The effect of laser beam irradiation on the surface of the specimen is 0.01 to 0.05 in the case of heating at 800 to 1050°C, and 0.5 to 1 in the case of melting heating.
．． It reached the inside of each of 511gIt.

Table 1 (Remarks) Items in parentheses are nitrogen-cooled Example 2 (Applied to the inner surface of a stainless steel tube The inner surface of an actual stainless steel tube was irradiated with a laser to test its oxidation resistance.

Test material: 5US304HTB, 5US316HTB
.

5US321HTB, 5US347HTB Test steel pipes: Outer diameter 74 m x Thickness 6 m x Length 2007 m The inner surface of the steel pipes had been roller-processed in advance.Inner surface finish of the test steel pipes: They were tested as they were on the market.

Test laser: Same irradiation method as in Example 1: As shown in Fig. 1, a sealing plate 4 having an Ar gas inlet 2 and an Ar gas outlet 3 was attached to each end of a stainless steel pipe 1 to create an Ar gas atmosphere. A laser beam 5 generated from a carbon dioxide laser source (not shown) is introduced into the steel pipe through a crystal lens 6 made of KCL. An Ag-plated reflector 7 is installed inside the steel pipe, and the beam is radiated to the inside of the steel pipe with a 90° angle change.

Irradiation conditions: Same as Example 1. The temperature of the irradiation part is 800-1
The temperature was adjusted to 050° C., which locally caused a melting state, and cooling was performed by water cooling from the outside of the steel pipe.

Evaluation of oxidation resistance After irradiation, a test piece measuring 50 g long x 202 g wide was cut out from the test steel pipe, and subjected to a steam oxidation test under the same conditions as in Example 1 and evaluated. In this case, the non-irradiated surface is the outside of the steel pipe (water-cooled part), and the source of oxide scale generation on this surface is
00, and the oxidation resistance of the irradiated surface was evaluated.

O Oxidation Resistance Test Results The test results are summarized in Table 2 below, and the effect of laser irradiation was clearly recognized even when large steel pipes were used, and the oxidation resistance of all irradiated areas was significantly improved. .

Table 2 Example 3 (When applied to the inner surface of a stainless steel pipe) Using the same test steel pipe as in Example 2, the oxidation resistance was investigated with only the laser irradiation interval being changed. That is, 2
In Example 2, the interval between the laser beams was 3 vrm, but here the interval was made even smaller so that the areas locally melted by the laser partially overlapped.

When the irradiated part was heated to 800 to 1050°C, no melting phenomenon was observed in the irradiated part, but the irradiation was performed so that the beams partially overlapped. In this example, the diameter of the beam is approximately 2W.
Because it was.

The movement of the test steel pipes was adjusted so that the distance was 2 mm. The method for cooling the steel pipe, the method for collecting test pieces after irradiation, the oxidation resistance test conditions, etc. are the same as in Example 2.

Table 3 below shows the oxidation resistance test results in this example. In Table 2, irradiation method (A) is the one in which the irradiation method was applied at three-frame intervals as in Example 2, and (B) is the irradiation method in this example. It is something. As is clear from these results, when the laser beams overlap, it is recognized that the oxidation resistance is somewhat reduced, especially when the laser beams are irradiated to the melted state. This is thought to be due to the fact that the molten state continued for a long time due to the weight of the laser beam, which promoted the growth of crystal grains. But 5oo-ios
It can be seen that the samples, including those heated at 0° C., maintain better oxidation resistance than the non-irradiated portions.

Third coating Example 4 (example applied to the outer surface of a stainless steel pipe) Using the same test material and steel pipe dimensions as those used in Example 2, the outer surface of the steel pipe was shot peened with a steel ball, as shown in Figure 2. Laser irradiation was performed using the apparatus shown in . That is, an Ar gas inlet portion 12 is provided in a stainless steel pipe 11.

A container 13 for constructing a high-level atmosphere is attached in close contact with the steel pipe. Since the steel pipe 11 rotates during laser beam irradiation,
A part of the injected Ar (as it leaks to the outside from a small gap between the atmosphere configuration container 13 and the stainless steel pipe 11)
A non-oxidizing atmosphere is created by supplying r. The laser beam 14 is
It becomes possible to irradiate the surface of the steel pipe through the Cβ crystal.

The irradiation conditions are the same as in Example 2. On the downstream side, water was separated into the steel pipe for cooling during laser beam irradiation.

Table 4 below shows the oxidation resistance test carried out in this case, and as in Table 2, the effect of laser fish meat was recognized, and the oxidation resistance improved without flowing inside and outside the steel pipe. It was clear that this would happen.

[Effect of the invention]) The surface of high Cr steel and austenitic stainless steel is cold-worked, irradiated with a laser beam, and then rapidly cooled to make the irradiated part amorphous. Or, since an ultrafine crystal grain layer is formed in the irradiated area, the carbon contained in the base material is removed during germination in an oxidizing atmosphere such as water vapor.
Oxidation resistance can be improved by selectively oxidizing r to form a strong protective film.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a state in which the inner surface of a stainless steel pipe is treated, and FIG. 2 is a diagram showing a state in which the outer surface of a stainless steel pipe is treated. 1.11 stainless steel pipe, 2,12: Ar gas inlet,
3: Ar gas outlet, 4: Sealing plate, 514: Laser beam, 6.15: KCu crystal lens, 7 two reflecting mirrors, 13: Atmosphere configuration container. 5 Seam Diagram 1

Claims (1)

[Claims] A method for oxidation-proofing treatment of heat-resistant steel, characterized in that the surface of high Cr steel of 13% or more and austenitic stainless steel is irradiated with a laser beam after cold working and rapidly cooled.