JPH0676655B2 - Heat resistant and corrosion resistant member and its manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a heat-resistant erosion resistant member that can be used as a surface hardening method for a nozzle of a steam turbine for a thermal power plant and a nuclear power plant, and a manufacturing method thereof.

[Conventional technology]

In steam turbine nozzles for thermal power plants, fine powder of oxide scale (boiler tubes or those that have been oxidized by superheated steam) is mixed in the superheated steam sent from the boiler. There are many cases where erosion is caused by the oxide scale.

As a countermeasure against these, a method of hardening the surface of the member is adopted.

As a means to harden the steel surface and improve wear resistance,
Carburizing, nitriding, quenching, hard metal overlaying, and ceramics thermal spraying are known, and the method of diffusing and permeating the boron compound referred to in the present invention is also one of the well-known processing methods.

Various methods are known for the boron diffusion and permeation method (hereinafter referred to as “boron dipping method”), and are classified into powder method, liquid method, gas method, molten salt method, electrolytic method in molten salt, etc. according to the state of the treating agent. Has been done. At present, a fragile boron permeation layer is known by any of the methods, but the powder method is most suitable from the viewpoints of processing cost, mass production, applicability to large parts.

In the treatment by the powder method, a powdery penetrant containing a boron compound is coated on the surface of the object to be treated, or the object to be treated is immersed in the penetrant, and is heated at a high temperature state, for example, 900 to 1050 ° C, and argon gas is applied. It is obtained by heating in a stream of air or hydrogen.

The boron permeation layer formed in general steel materials is composed of two layers, Fe ₂ B and FeB, and has a Vickers hardness of 1700 or more, which improves wear resistance. However, the permeation layer with Fe ₂ B and FeB layers is very brittle and its elongation is small, so cracks of various sizes often occur in the permeation layer during the cooling process during boron permeation treatment. This is because the steel material that is the object to be processed has a very different coefficient of linear expansion from the boron infiltration layer, so a large stress occurs between the two when the temperature is lowered after the diffusion treatment, and the hard and brittle boron infiltration layer cannot withstand the stress. This is because.

On the other hand, the steel material that has been subjected to the boron infiltration treatment is kept at a high temperature for a long time, and is also cooled with a relatively gentle temperature gradient during the subsequent temperature lowering process, resulting in an annealed state of the steel material. Mechanical strength usually deteriorates. Therefore, although quenching is performed after the treatment to recover the strength, rapid cooling of the object to be treated in this case also causes cracking of the boron-penetrated layer. To prevent this, gentle cooling may be used, but with this, the metallic structure of the object to be processed does not become martensite, and recovery of strength cannot be expected.

For this reason, it is relatively easy to infiltrate a test piece or a small object to be treated with boron, but it is practically difficult to treat a large member.

[Problems to be solved by the invention]

 The conventional method has the following problems as described above.

(1) When a hard and fragile boron permeation layer is formed on an object to be treated, a large number of large and small cracks are generated in the permeation layer mainly composed of Fe ₂ B and FeB due to the difference in coefficient of linear expansion from the object to be treated. Occur.

(2) Since the object to be treated after the boron permeation layer treatment is kept at a high temperature for a long time to form the permeation layer, it is in an "annealed" state and the mechanical strength deteriorates. For this reason, reheat treatment is performed, but this treatment generally requires rapid cooling from a high temperature state. At this time, for the same reason as in the item (1), a large number of cracks are generated in the boron permeation layer, and the boron penetration layer is easily peeled off, resulting in deterioration of wear resistance performance.

(3) In the boron permeation layer, the FeB layer is formed on the outer layer and the Fe ₂ B layer is formed on the inner layer. Of these, the FeB layer is very hard and has excellent wear resistance, but cracks are most likely to occur. The cracks generated in this layer grow during the infiltration treatment and the subsequent heat treatment process, and promote the generation of cracks in the inner Fe ₂ B layer. Therefore, it is necessary to select the conditions such that the FeB layer is not formed during the permeation treatment, or even if it is formed, the layer thickness is extremely thin.

The present invention has been made in view of the above problems of the conventional method, and provides a heat-resistant erosion-resistant member and a method for manufacturing the same, while sufficiently maintaining the strength of the base material, moreover, cracking is less likely to occur and the coating portion is less likely to peel off. To aim.

[Means for solving problems]

 Therefore, the present invention has the following configuration.

(1) A heat-resistant steel is used as a base material, the vicinity of the surface of the heat-resistant steel has a martensite structure, and a diffusion and penetration layer consisting essentially of Fe ₂ B is formed as a surface layer on the surface of the martensite structure layer. A heat-resistant and erosion resistant member characterized by

(2) Si is applied to the surface of the object to be processed made of heat-resistant martensitic steel.
Cover with C powder, and use B as a boron source on the outside of the SiC powder cover.
_A composite of ₄ C and KBF ₄ is placed and subjected to a boron diffusion and infiltration treatment by heating and cooling, and then at 20 ° C / m 2 with a non-oxidizing gas.
A method for producing a heat-resistant and erosion resistant member, which comprises quenching at a cooling rate of not less than in.

[Action]

When performing the boron infiltration treatment by the powder method, as a boron source, amorphous boron (B), ferroboron (Fe-B), boron carbide (B ₄ C), borofluoride boron salt (MBF ₄ where M is Na or K), borax (Na ₂ B ₂ O ₇ ) and the like. Usually, a mixture of ammonium chloride (NH ₄ Cl), sodium chloride (NaCl) and alumina (Al ₂ O ₃ ) as a bulking agent is used as a penetration enhancer.

In the present invention, B ₄ C and KBF ₄ were used as the boron source and carbon silicon (SiC) was used as the extender, but the boron source agent is
The minimum amount required. An example is shown below.

By reducing the boron supply source in this way, the distribution of boron concentration on the surface of the object to be treated is made uniform, and since the boron concentration is low, it is brittle and easily cracks.
It was decided to suppress the formation of layer B.

Furthermore, when a large-sized member is subjected to boron infiltration treatment, SiC powder is applied to the surface of the object to be treated, and the above penetrant is placed around it to perform non-uniform temperature distribution, which tends to occur in large-sized members. The non-uniformity of the permeation layer thickness that accompanies it could be compensated. (Because the boron concentration is low, the permeation rate does not increase even if the temperature rises locally, and a uniform permeation layer can be obtained.) Further, after performing the boron permeation treatment at a predetermined temperature, the rate of temperature decrease is set. The difference in linear expansion coefficient between the permeation layer and the object to be treated was set to 25 ° C / h or less (FeB: 23 × 10 ^-6 / ° C, 13Cr steel: 11.5
× 10 ^-6 / ° C, Fe ₂ B: 7.8 × 10 ^-6 / ° C) has succeeded in reducing the stress generated between the two and preventing cracks in the permeation layer (FeB).

As a quenching method for recovering the strength of the object to be treated after the permeation treatment, after heating in a vacuum at 950 to 980 ℃ for 2 hours, nitrogen gas, helium gas, argon gas, or hydrogen gas is placed in the same container. Was used to set the cooling rate at a position of 20 mm just below the permeation treatment layer to be 15 ° C. ± 5 ° C. or more per minute. Further, after that, as a tempering condition, heating was performed at 640 to 680 ° C. for 2 hours, and cooling was performed under the same conditions with the gas described above. However, since the cooling rate in this case does not need to be as fast as the quenching condition, it is appropriate for “quenching”. The objective was achieved by determining the cooling rate.

By maintaining the above heating conditions and cooling rate, the boron permeation layer was free from oxidation consumption and cracks, and the microstructure of the object to be treated became martensite and succeeded in recovering the mechanical strength.

〔Example〕

Example (Part 1) Examination of appropriate amount of boron compound in penetrant Examining proper amount of boron compound in penetrant using B ₄ C and KBF ₄ as the penetrant with the remainder being SiC did. As the object to be treated, four kinds of 13Cr steel materials as shown in Table 1 were finished to the shape dimensions (called small test pieces) of 40 mm in diameter and 100 mm in length, and the content of KBF ₄ was 3 % constant and then, examined the thickness of the boron penetration layer in the case of changing the content of B ₄ C 1 to 7%, the proportion physicians FeB layer and Fe ₂ B layers to form a permeation layer. The conditions for permeation treatment are 900
The atmosphere is argon gas at ℃ × 2h.

Figure 1 shows this result, and the permeation layer is almost constant even when the B ₄ C concentration changes from 1% to 7%, and the ratio of Fe ₂ B and FeB layers Fe ₂ B with little change
The ratio was 40 to 45 μm, and the remaining 15 to 20 μm was the FeB layer. Also, no significant difference was observed in the penetration layer thickness and Fe ₂ B ratio of the tested specimens, but this has the effect that the chemical compositions of the tested materials, especially the Cr content, are almost the same. It is considered to be a thing.

Next, B ₄ C was kept constant at 3%, and treatment was performed under the same conditions as above with a penetrant in which the amount of KBF ₄ was changed to 1 to 7%. FIG. 2 shows this result, and almost the same result as in FIG. 1 is obtained.

As is clear from the results shown in FIGS. 1 and 2, it was found that boron infiltration treatment was insufficient with B ₄ C and KBF ₄ alone as boron compounds, and that both compounds must coexist.

However, in the permeation layer obtained by this method, the amount of hard and brittle FeB layer generated is large, so there are many cracks, which suggests that it cannot be put to practical use as it is.

1 and 2, the total permeation layer FeB layer thickness is shown, but the thickness of the Fe ₂ B layer is the total permeation layer minus the thickness of the FeB layer. Hardness of FeB layer in Vickers hardness 1800 or more, Fe ₂ since B layer is in the range of 1600 to 1790, what can be expected sufficient abrasion resistance alone Fe ₂ B layers without generation of FeB layer Is.

Example (Part 2) Generation of Penetration Layer with FeB Layer and No Crack Generation According to the experiment of Example (Part 1), although a boron penetration layer was obtained, many cracks were generated with the formation of the FeB layer. , Here, an experiment was conducted to obtain a crack-free permeation layer with less FeB layer formation. From the experiment of (No. 1), it was revealed that all the specimens shown in Table 1 exhibit almost the same permeation behavior. Therefore, in this example,
SUS403 was used and heated at 900 ° C for 5 hours in an argon atmosphere. However, the composition of the penetrant and the method of contact with the object to be treated were changed as follows.

(1) The object to be treated was immersed in a mixture of B ₄ C 3%, KBF ₄ 3% and SiC 94%.

(2) Cover 100% SiC powder on the surface to be treated to a thickness of 2-3 mm,
The periphery thereof was covered with the composition of (1) to a thickness of 10 mm.

In addition, the temperature decrease rate per hour after the treatment at 900 ℃ × 5h
The temperature was adjusted to 80 ° C ± 7 ° C, 50 ° C ± 6 ° C, 25 ° C ± 5 ° C, 15 ° C ± 5 ° C (hereinafter indicated by the center value).

Table 2 shows the microstructure observation and hardness measurement results of the cross section of the permeation part of the object to be treated obtained in this example. The one using the penetrant having the composition of (1) is hard and brittle.
Although the B layer is often generated, in the penetrant having the composition of (2), the Fe ₂ B layer having the FeB layer having a thickness of several microns at the outermost layer is 50 to 60.
Had produced a micron thickness. However, even with this composition, cracks were observed at cooling rates of 80 ° C and 50 ° C per hour. On the other hand, 25 ℃ per hour, 10
No cracks were observed at the cooling rate of ℃, and a good permeation layer was obtained.

The reason for this is that the same penetrant as in (1) is used, but the boron compound that permeates the surface of the object to be treated is blocked by the SiC film coating the surface of the object to be treated. As a result of the delayed arrival, the diffusion rate of boron into the object to be processed becomes faster, the B content is lower than that of FeB, and the ductility is good.
It is considered that the Fe ₂ B layer was easily generated.

Figures 3 and 4 both show a cooling rate of 20 ° C after the infiltration treatment.
It is a cross-sectional microstructure photograph of the permeation layer obtained by controlling to / h. Photo 1 shows Si on the surface of the object to be processed in advance.
Due to the effect of the present invention coated with C powder, no crack is found in the permeation layer. On the other hand, in Photo 2, a large number of cracks occurred at the same cooling rate,
It can be seen that it has partially peeled off.

From this reason, it is estimated that B ₄ C and KBF ₄ in the penetrant are
Even if the amount of boron compound is reduced, the same result as in (2) may be obtained. However, when the size of the object to be treated becomes large, the source of boron is reduced in the penetrant with a low content of boron compound. It lacks and cannot obtain a uniform penetration layer. In this respect, if a penetrant with a relatively large amount of boron source is placed around it and SiC is used as a diffusion barrier for the penetration of boron compounds, it will always have a sufficient boron source even for large-scale members and for long-term infiltration treatment. Therefore, there is an advantage that a processed layer with stable quality can be obtained.

Example (Part 3) Examination of Minimum Boron Compound Amount in Penetrant According to the method of Example (Part 2) (2), a Fe ₂ B layer having no crack is formed even on a large object to be treated. It was found that a large number of Tsutomi layers can be obtained. However, it is economically advantageous to repeatedly use this type of penetrant.

In the method of (2) of the embodiment (No. 2), the surface of the object to be treated is
Since it is coated with SiC powder, it is mixed with SiC and boron compounds such as B ₄ C and KBF ₄ placed around it when it is taken out after the permeation treatment. Therefore, when a new object to be treated is coated with SiC powder on the surface again, the content of boron in the penetrant containing B ₄ C and KBF ₄ placed around it is reduced, and the infiltration treatment is repeated by this method. The more it is returned, the lower the boron content in the penetrant.

Therefore, it was decided to experimentally determine the minimum amount of boron compound in the penetrant packed around the object to be treated. As the object to be processed, the diameter is 40 mm and the length is 100 mm, which is the same as that of the embodiment (No. 1).
500 mm, 500 mm wide, 300 mm wide SUS403 (referred to as large test piece)
Was used. The composition of the penetrant and its use are as follows.

(1) A large object surface is coated with SiC to a thickness of 2 to 3 mm, and a mixture of B ₄ C, KBF ₄ and SiC is 10 mm around it.
Coated to be thick.

(2) For small size tests, SiC is coated in a thickness of 2 to 3 mm, and one that is directly immersed in a boron compound (B ₄ C + KBF ₄ ) is categorized into (1) and (2) 950 ° C × 5 h argon. Heated in gas.

The evaluation after the treatment was carried out by visual observation of the appearance and presence or absence of cracks by microscopic observation of the cross section of the permeation layer.

Table 3 summarizes these results, and there is a difference in the properties of the permeation layer obtained when a large test object and a small test piece are used. That is, in the former case, when the B ₄ C and KBF ₄ concentrations are high, FeB is often generated in the compositions (A) and (B) unless the surface is covered with SiC, and cracks are often generated on the surface cross section. If the concentration of the boron compound becomes thin, a relatively good permeation layer can be obtained, but if there is no SiC coating (F)
Even the composition has some cracks. On the other hand, when SiC is coated, the proportion of the FeB layer in the permeation layer is very small, and even if the amount of boron compound in the permeation agent is small [(C), (D), (E ) Like] Fe
A layer B and a permeation layer with few cracks were obtained. Under these various conditions, the permeation layer thickness did not change significantly. In the case of the penetrants of the compositions (F) and (G) having a very small amount of boron compound, the effect of covering the object to be treated with SiC was not recognized, but under these conditions, the permeation layer itself was thin and ) And (B), the thickness was only 30%. In the case of the small test piece, the effect of the SiC coating extends to the compositions of (D), (E) and (F), and the effect of the coating cannot be recognized until the composition of (G).

From the above experimental results, the effect of SiC coating on large-sized objects was confirmed at a concentration of boron penetrant (B ₄ F + KBF ₄ = 1.5%) or higher, and for small test pieces, B ₄ F + KBF ₄ of 1% or higher. It has been found to be effective for mixed penetrants.

Example (Part 4) Selection of appropriate heat treatment conditions after boron permeation treatment An object to be permeated by SUS403 having a length of 500 mm, a width of 500 mm and a height of 300 mm by the method of (2) of Example (Part 2) The conditions for recovering the strength of the base metal were obtained by measuring the hardness of the base metal. Since the boron permeation layer itself is poor in high temperature oxidation resistance, all of the present examples were carried out in a vacuum vessel, and the conditions are as follows.

(1) After the object to be treated is placed in a vacuum container, the pressure inside the container is set to 10 ^-4 torr, heating is performed at 960 ° C for 5 hours, and then the base material is converted to martensite by the following cooling method. , Strength was recovered. However, because the size of the object to be treated is large, a thermocouple was inserted 20 mm below the permeation treatment layer, and the cooling rate at this position was 1
It was cooled to 50 ° C. per minute.

Put into oil (so-called oil quenching) Introduce a large amount of hydrogen gas into the container and cool it. Introduce a large amount of helium gas into the container and cool it. Introduce a large amount of nitrogen gas into the container and cool it. Argon gas in the container. The optimum heat treatment conditions were determined from the presence or absence of cracks on the surface of the permeation layer by visual observation, the presence of cracks on the cross section of the permeation layer, and the hardness measurement results of the base metal for the object to be treated that had been introduced into I asked.

Table 4 shows the results of the above investigations. In oil quenching, the hardness of the base metal was high and martensite was sufficiently obtained, but numerous cracks occurred on the surface and cross section of the permeation layer. On the other hand, in the case of using gas cooling, cracks in the permeation layer did not occur, and even if they did occur, they were extremely small, but generally had low hardness and insufficient martensite formation, and there was a problem in strength recovery. , It became clear that it is not suitable for use in places where high strength is required.

Therefore, in order to further clarify the cooling conditions with hydrogen, helium, nitrogen, and argon, the same investigation was performed by changing the cooling conditions using these four kinds of gases as follows. Measurement position quenching procedure of cooling rate in this experiment,
All the survey items are the same as above, but the types of steel to be treated are the four types shown in Table 1 and the steel types (1) to (3) are 960.
The steel grades of ℃ and (4) were maintained at 1020 ℃ for 5 hours each. The cooling rate was 25 ° C / min and 15 ° C / min. Table 5 shows the properties of the permeation layer, and Table 6 shows the measured hardness of the base metal.
It was found that when the cooling rate was 15 ° C / min, cracks did not occur in the permeation layer, but the martensite of the base material was insufficient, the hardness was low, and the strength was not sufficiently recovered.

On the other hand, with a cooling rate of 25 ° C / min, there are almost no cracks in the permeation layer, and the generated ones are extremely small and the number is small. Moreover, it was confirmed that the hardness of the base material was strong and the structure was sufficiently transformed into martensite.

The cooling rate in this example was set as the temperature change at a position 20 mm immediately below the permeation layer, and the hardness of the base material was measured at this position for the following reason. That is, in the case of a normal steel material such as the object to be processed used in this example, the cooling rate of the central portion is slow and martensite formation is performed even if a material having a large cooling performance such as oil quenching is used. Absent. The hardening phenomenon caused by the boron permeation treatment occurs on the surface of the object to be treated (50 to 100 μm), and the base material that supports it can be sufficiently put into practical use if it is made into martensite at 20 mm from the treated surface. This is because I made a decision.

Also, the heat treatment for "annealing" after "quenching" is performed in a vacuum container in accordance with JIS G4 for the objects to be treated (1), (2) and (3) in Table 1.
The object to be processed of 304 (stainless steel bar), (3) is JIS G4311
Heat-resistant steel bar Heated under the conditions described, then hydrogen, helium,
Although cooled with nitrogen and argon gases, the permeation layer, which showed good properties in the "quenched" state, did not change at all when "annealed".

[Effect of the Invention] According to the method of the present invention, it is possible to obtain a member having a boron-penetrating layer with few cracks and less fragile FeB layer generation and excellent wear resistance, and having sufficient base metal strength. The resulting member exhibits excellent heat and erosion resistance, so that the life of the device can be greatly extended by using it, for example, in the nozzle of a steam turbine.

[Brief description of drawings]

1 and 2 are diagrams showing the experimental results of the thickness of the permeation layer according to the present invention, and FIG. 3 is a microscopic photograph of the microstructure of the heat-resistant erosion-resistant member according to one embodiment of the present invention. The figure is a microscopic metallographic photograph of a cross section of a member according to a comparative example.

Claims (2)

[Claims] 1. A heat-resistant steel as a base material, the vicinity of the surface of the heat-resistant steel has a martensite structure, and a diffusion and penetration layer consisting essentially of Fe ₂ B is formed as a surface layer on the surface of the martensite structure layer. A heat and erosion resistant member characterized by being formed. 2. A surface of an object to be treated made of heat-resistant martensitic steel is covered with SiC powder, and a composite of B ₄ C and KBF ₄ as a boron source is placed outside the cover of SiC powder and heated and cooled. Boron diffusion permeation treatment is performed and then non-oxidizing gas is used.
A method for producing a heat-resistant erosion member, which comprises quenching at a cooling rate of 20 ° C / min or more.