JPH0826448B2 - Steel pipe chromizing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a chromizing method for a steel pipe by a powder pack method, and more particularly to a chromizing method for a steel pipe which prevents sintering of a powder penetrant.

[Conventional technology]

In order to impart corrosion resistance and wear resistance to steel materials, a chromizing treatment for applying a chromized layer to the surface thereof is adopted.

There are various chromizing treatment methods, and one of them is a powder pack method using a powder penetrant.

In order to prevent the generation of scale due to steam oxidation, for example, by the powder pack method, the chromized layer was applied to the inner surface of the steel pipe for boiler as follows.

On the inner surface of the cleaned steel pipe, granular or powdered metal Cr and a halide such as NH ₄ C that promotes the permeation and diffusion of the metal Cr.
l (In addition to this, NaCl, HN ₄ Br, etc. can be used), and a powder penetrant mixed with alumina (Al ₂ O ₃ ) for preventing sintering is packed.

To prevent the powder penetrant from falling off the steel pipe end,
Install stoppers on both ends of the steel pipe. In this case, this plug is
Instead of hermetically sealing the inner surface of the steel pipe, the steel pipe is attached by being spot-welded so as to have air permeability.

This steel pipe is necessary for chromizing treatment in an atmosphere of H ₂ gas or Ar gas in order to prevent oxidation of Cr.
For example, it is heated and held in a temperature range of 800 ° C. or higher.

By this heating and holding, the powder penetrant reacts as follows.

Cr + 2NH ₄ Cl → CrCl ₂ + 2NH ₃ + H ₂ (1) CrCl ₂ + H ₂ → 2HCl + Cr (2) CrCl ₂ + Fe (Fe in steel pipe) → FeCl ₂ + Cr (3) That is, the reduction shown in formula (2) The activated Cr deposited by the reaction and the substitution reaction represented by the formula (3) diffuses from the inner surface of the steel pipe into the steel, and a high Cr alloy layer (chromized layer) is formed.

The heat of the chromized layer changes depending on the material of the steel pipe, the heating temperature, the holding time, and the like.

[Problems to be Solved by the Invention]

However, it has been discovered that when the chromizing treatment is performed by this method, a chromized layer is not formed near the tube end.

Therefore, when investigating the pipe end after chromizing treatment,
The powder penetrant in the part where the chromized layer is not formed is
It was found to be sintered.

The main factor is the case of using H ₂ gas as H ₂ and ambient gas iron oxide that is generating the steel pipe itself is generated by (1), is reduced by the H _2, H ₂ produced by this O and water in the air (H ₂ O) mixed in the powder penetrant react with activated Cr as shown in the following formula (4), and 2Cr + 3H ₂ O → 2Cr ₂ O ₃ + 3H ₂ (4 ) Cr ₂ O ₃ generated by this reaction reacts with alumina (Al ₂ O ₃ ) as shown in the following formula (5) to sinter.

Cr ₂ O ₃ + Al ₂ O ₃ → (Al, Cr) ₂ O ₃ (5) Then, the sintering according to the formula (5) proceeds from the tube end. This is because H ₂ O produced by the reduction of iron oxide produced on the outer surface of the tube invades from the tube end to the inside.

Therefore, a main object of the present invention is to provide a chromizing method for a steel pipe in which the portion where the chromized layer is not formed can be made as small as possible by preventing the sintering of the powder penetrant.

[Means for solving the problem]

The above-mentioned problem is to fill a steel pipe with a powder penetrant containing metal Cr, a halide and alumina, and form a stopper in a state where both ends of the steel pipe are permeable, and 800 ° C in a hydrogen gas atmosphere.
When the steel pipe inner surface is subjected to chromizing treatment by the powder pack method by heating and holding as described above, it is heated and held for 1 hour or more in a temperature range of 500 to 600 ° C. in a hydrogen gas atmosphere, and iron oxide generated at least on the outer surface of the steel pipe Can be solved by carrying out a preliminary treatment for removing the moisture produced by the reaction of oxygen and hydrogen in the atmospheric gas to the outside of the atmosphere, followed by a chromizing treatment.

[Action]

As described above, in the steel pipe, if the chromized layer is not formed at the pipe end, if it is caused by the sintering of the powder penetrant according to the above formulas (4) and (5), it becomes a source of sintering. It is understood that it is sufficient to prevent H ₂ O from contacting with activated Cr.

Specifically, it is conceivable to remove the air containing water contained in the powder penetrant using a steel pipe in which iron oxide, which is the source of H ₂ O, is not produced.

However, it takes a lot of man-hours to completely remove iron oxide, and even if iron oxide is completely removed, there is still time until the chromizing treatment, so iron oxide is generated during this period. However, it is virtually impossible to obtain a steel pipe in which iron oxide is not produced.

Therefore, as a result of various studies of the other methods, the present inventors. Rather than completely preventing the generation of iron oxide and preventing the generation of H ₂ O, the presence of iron oxide may be allowed and H ₂ O may be generated. It was found that, if preliminarily carried out, it can be carried out at the same time at this time, and therefore, it is possible to prevent H ₂ O from coming into contact with activated Cr during the subsequent chromizing treatment, thereby preventing sintering.

[Specific configuration of the invention]

 The present invention will be described in more detail below.

The present invention will be specifically described step by step in the case of providing a chromized layer on the inner surface of a steel pipe.

The scale and the like on the inner surface of the steel pipe are removed, the powder penetrant of the above example is packed into the cleaned inner surface of the steel pipe, and a breathable plug is attached.

This steel pipe is charged through the opening of a steel container provided with a gas supply port, a gas exhaust port, and an opening for charging and extracting the steel pipe.

 A steel plate is welded to the opening to seal the container.

An exhaust pump is connected to the gas exhaust port to evacuate the air in the container, and H ₂ gas is fed into the container from the gas supply port to replace the air in the container with the H ₂ gas.

 Place the container in the furnace.

While removing the gas in the container from the gas exhaust port, sending H ₂ gas from the gas supply port, while circulating the H ₂ gas in the container, heat the temperature in the container to the range of 500 ~ 600 ℃ , Hold in this temperature range.

 At this stage, the preliminary process according to the present invention is completed.

In this the step, the circulating of H ₂ gas, by feeding a new H ₂ gas, H ₂ O generated by this as well as reducing iron oxide, iron oxide with H ₂ produced by (1) reducing The H ₂ O generated as a result and the air containing water mixed in the powder penetrant are discharged to the outside of the container together with the H ₂ gas.

Furthermore, the heating and holding temperature is set to 500 to 600 ° C. because the temperature at which magnetite or hematite as iron oxide is reduced by H ₂ gas in a short time is approximately 500 ° C. and 800 ° C. as shown in FIG.
It is above ℃, but above 600 ℃, (2), (3)
The reaction of the formula becomes active and activated Cr is generated (4)
This is because the reactions represented by the formulas (5) occur, and if the temperature is lower than 500 ° C, the reduction reaction of iron oxide with H ₂ gas takes a long time.

In the present invention, the heating and holding time is 1 hour or more. This is because if the heating time is less than 1 hour, the iron oxide is not sufficiently reduced even if it is heated in the temperature range of 500 to 600 ° C.

Although the upper limit of the heating and holding time is not particularly limited, even if the heating and holding time is lengthened, the sintering prevention effect is saturated and uneconomical.
Time is enough.

When the above-mentioned pretreatment is completed, subsequently, the temperature in the container is further heated to a temperature necessary for the chromizing treatment, for example, a temperature of 800 ° C. or higher, usually within a range of 800 to 1200 ° C., and this temperature range is set. Hold in.

The holding temperature and holding time depend mainly on the material of the steel pipe and the thickness of the chromized layer to be obtained.
To apply a 100 μm chromized layer to the inner surface of the S-STBA24 steel pipe, it takes about 1090 ° C. for about 10 hours.

During this time, the inside of the container may be in an H ₂ gas atmosphere as in the pretreatment, or the gas in the container may be replaced with an inert gas at the time of temperature rise after the pretreatment to create an inert gas atmosphere. it can.

Both methods prevent the oxidation of Cr,
This is because NH ₃ generated by the formula (1) and HCl generated by the formula (2) are discharged to the outside of the container.

The FeCl ₂ generated by the formula (3) remains in the penetrant.

Then, when the container is heated to a temperature higher than 1000 ° C, the container is cooled to about 1000 ° C in the heating furnace and then taken out of the heating furnace.

Cooling to about 1000 ° C in the heating furnace is to prevent deformation of the steel container.

When H ₂ gas is used, it is cooled in the heating furnace or taken out of the heating furnace, and then the H ₂ gas is discharged from the gas discharge port to the outside of the container and is also inert from the gas supply port to the inside of the container. Put gas and replace. The reason for replacing the gas in the container with an inert gas is to prevent the explosion at the time of melting the steel sheet described in the item.

The steel plate with the opening of the container closed is melted and the steel pipe is taken out.

In the above example, the pretreatment is followed by heating to the temperature required for the chromizing treatment, but after the pretreatment, the chromizing treatment may be performed after once cooling to room temperature.

With the above configuration, when a steel container is used, it is possible to prevent the influence of iron oxide generated in the steel container.

〔Example〕

 Hereinafter, the effects of the present invention will be clarified by examples.

(Example 1) Material JIS-STBA24, dimension outer diameter 50.8 mm, wall thickness 6. On the inner surface of a steel tube with a wall thickness of mm, a penetrant (Cr: 90%, NH ₄ ClK: 1%, Al ₂ O ₃ : 9
%) Was used to perform a chromizing treatment in which it was held at 1090 ° C. for 10 hours in order to provide a 100 μm chromized layer.

During this temperature increase, while circulating H ₂ gas in the container,
Pretreatment was carried out by holding at 550 ° C for 0.5 to 2 hours. As the atmosphere gas, H ₂ gas was used even after the chromizing treatment.

The results of the relationship between the heating and holding time and the sintering depth are shown in FIG. Conventional method (without pretreatment according to the present invention)
The results are also shown in the same figure. The degree of sintering is indicated by the length (sintering depth) of the portion sintered from the tube end.

As is clear from Fig. 2, the sintering depth is 200 mm.
From (conventional method), if the holding time is set to 1 hour or more, it is significantly reduced to 70 mm. If the sintering depth is about 70 mm, there will be no particular inconvenience because it is within the cut-off range of the pipe end.

(Conventional Example 1) As a conventional method, a chromizing treatment was performed under the same conditions as in Example 1 except that only Ar gas was used without using H ₂ gas.

Results are shown in FIG. Thus, it can be seen that even if the chromizing treatment time is 5 hours, the sintering depth exceeds 100 mm, and a chromizing layer-unformed portion is generated due to the sintered portion at the tube end.

〔The invention's effect〕

As described above, according to the present invention, by preventing the sintering of the powder penetrant, the portion where the chromized layer is not formed can be made as small as possible.

[Brief description of drawings]

Figure 1 shows magnetite (a) and hematite (b) as iron oxides.
On the other hand, a correlation diagram between the reduction time and temperature with hydrogen gas and the reduction rate, FIG. 2 is a correlation diagram between the heating and holding time under the flow of the reducing gas and the sintering depth, and FIG. It is a correlation diagram of Ising processing time and sintering depth.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masayasu Yoshikawa 8 Oike-cho, Kosai-cho, Koga-gun, Shiga Nihon Calorie Industry Co., Ltd. Shiga factory (72) Koichi Okada 1 Nishino-cho, Higashimukaijima, Amagasaki-shi, Hyogo Sumitomo Metal Industries, Ltd. Steel Pipe Mill (72) Inventor Tomiichi Ikeda 1 Nishinocho, Higashimukaijima, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries Ltd. Steel Pipe Mill (72) Inventor Toshihiro Imoto Kitahama, Chuo-ku, Osaka City, Osaka Prefecture 4-5-33 Sumitomo Metal Industries, Ltd. (56) Reference JP-A-51-139533 JP, A) JP-B 48-17418 (JP, B2) JP-B 49-30898 (JP, B2)

Claims (1)

[Claims] 1. A steel pipe is filled with a powder penetrating agent containing metal Cr, a halide and alumina, and both ends of the steel pipe are capped in a gas permeable state at 800 ° C. in a hydrogen gas atmosphere.
When the inner surface of the steel pipe is subjected to chromizing treatment by the powder pack method while being heated and held as above, in a hydrogen gas atmosphere, in a temperature range of 500 to 600 ° C
After heating and holding for at least an hour, at least the iron oxide generated on the outer surface of the steel pipe is reduced, and the pretreatment is performed to remove the moisture generated by the reaction between oxygen and hydrogen in the atmosphere gas outside the atmosphere, and then chromizing. A method of chromizing a steel pipe, which comprises treating the steel pipe.