JP5235598B2 - Thermal spray powder made of iron-silicon based intermetallic compound and manufacturing method thereof, thermal spray coating made of thermal spray powder, and substrate coated with the thermal spray coating - Google Patents

Description

  The present invention relates to a method for producing a thermal spraying powder, a thermal spraying powder produced by the production method, a thermal spray coating formed on a substrate surface by spraying the thermal spraying powder, and the thermal spray coating coated More particularly, the present invention relates to a thermal spraying powder suitable for forming a thermal spray coating excellent in corrosion resistance and wear on the surface of a heat transfer tube and a water wall tube of a coal fired boiler by high-speed flame spraying and a method for producing the same.

  Corrosion resistance of members used in the furnaces of conventional coal-fired boilers and in the passages through which combustion gas from the furnace flows is due to high temperature corrosion and wear thinning caused by the combustion ash contained in the coal combustion gas. Further, cermet granulated and sintered powders such as chromium carbide and tungsten carbide having excellent wear resistance are used as thermal spraying powders. In addition, ferrosilicon-based materials, which are iron-based materials, are not coal-fired boilers but are used in waste incinerators as thermal spraying powders that are relatively inexpensive and have the same corrosion resistance and wear resistance as cermet-based materials.

  In the thermal spraying method, various heat sources such as plasma and a combustion flame are used. The thermal spraying powder is heated by these heat sources, and it is considered that the thermal spraying powder becomes a thermal spray coating by colliding with the sprayed material (hereinafter referred to as the base material) at a high speed in a semi-molten and softened state. It has been. In order to form a laminate thickness of a thermal spray coating (hereinafter referred to as a film thickness) that is necessary for a thermally sprayed member to operate safely for a certain period of operation, a single laminate (pass) is required. Although it is impossible and lamination in multiple passes is necessary, the temperature of the base material is overheated if the lamination is performed continuously. In order to prevent deformation of the base material due to overheating and the like, when lamination by a plurality of passes is necessary, it is necessary to cool the base material between passes.

  High-speed flame spraying, which is one of the thermal spraying methods, heats and accelerates the thermal spraying powder using a combustion flame that is a mixture of fuels such as kerosene, propylene, and ethylene and pure oxygen, which is an oxidizer, and collides with the substrate.・ It is a method of adhesion, and it is used in the furnaces of coal-fired boilers and in the flow channels through which combustion gas flows from the furnace because there are few pores contained in the sprayed coating and it has excellent corrosion resistance. It is generally used as a thermal spraying method used for thermal spraying on members.

  However, cermet-based thermal spray materials such as chrome carbide and tungsten carbide have a small heat capacity of the combustion flame only by heating with the flame flame of high-speed flame spraying, and the thermal spray powder passes through the thermal spray flame at high speed. Heating tends to be insufficient, and cermet materials are hard even at high temperatures and are not easily melted and melted. Therefore, they do not adhere sufficiently because they reach the base material in a state that is not sufficiently softened. Further, when the cermet material is laminated in multiple passes, there is a problem that particles that have been folded in the previous pass are removed by the newly supplied thermal spraying powder. For this reason, there exists a problem that the adhesion yield of the cermet type thermal spray material is as low as about 10%.

  As with the cermet material, the ferrosilicon-based thermal spray material is a material that is unlikely to be semi-melt softened by heating. For this reason, as a method for improving the adhesion of the ferrosilicon-based spray powder, the present inventors can easily increase the temperature of each powder by restricting the particle diameter to ensure adhesion, and contain Si. A technology for restricting the amount to ensure adhesion has been developed and a patent application has been filed (Japanese Patent Application No. 2007-237577). However, the fine powder in the method of the patent application has a problem in that the yield during production is low and the powder becomes expensive.

  As a method for improving adhesion of a ferrosilicon-based thermal spray material, a method for improving adhesion by mixing other material powders has been proposed. For example, as disclosed in Japanese Patent No. 3356959, a method of mixing with a low melting point Al—Si based alloy powder has been proposed. Since the Al—Si based alloy powder is melted by heating at the time of thermal spraying, there is an effect that the adhesion is remarkably improved.

Further, as disclosed in Japanese Patent Application Laid-Open No. 2005-272927, a method using a powder mixed with a powder having the same component as that of a base material has been proposed.
Japanese Patent No. 3356959 JP 2005-272927 A

  The method disclosed in Japanese Patent No. 3356959 is a method assuming corrosion prevention at a low temperature, and in an environment in a boiler that is constantly exposed to a high temperature environment of 400 ° C. or higher, the Al—Si alloy is melted. Therefore, it cannot be applied.

  Furthermore, the method disclosed in Japanese Patent Application Laid-Open No. 2005-272927 has a problem that particles having low corrosion resistance exist in the coating and are preferentially corroded. In order to avoid this problem, it is necessary to reduce the mixing ratio of the powder having the same composition as the base material to be mixed. However, this causes a problem that adhesion during thermal spraying is lowered.

  In view of the above, the present inventors aim to provide a method for improving the adhesion of an anti-corrosion / anti-abrasion spray material that is not based on an expensive granulation sintering method, pulverization, or powder mixing that may cause a decrease in corrosion resistance. As a result of earnest research, the primary powder obtained by atomizing a molten metal containing 18% to 22% Si by weight and the balance containing Fe and unavoidable impurities is heat-treated in an appropriate temperature range to obtain fine particles. It has been found that by depositing an Fe-Si compound, the hardness of each thermal spraying powder particle is lowered, and the semi-melt softening of particles essential for thermal spray coating formation is likely to occur and the thermal spray adhesion yield is improved. .

The present invention comprises the following means.
The invention according to claim 1 is mainly composed of Fe ₂ Si which is Fe—Si based intermetallic compound by atomizing a molten metal containing 18% to 22% Si by weight and the balance being Fe and inevitable impurities. And holding the primary powder in a vacuum or in an inert gas at a temperature of 825 ° C. to 400 ° C. for a certain period of time, thereby providing an Fe—Si intermetallic compound in the primary powder particles. A method for producing a thermal spraying powder, comprising producing secondary powder particles in which at least one of Fe ₃ Si compound, FeSi compound, Fe ₅ Si ₃ compound and Fe ₂ Si compound is finely precipitated.

The invention according to claim 2 is a thermal spraying powder produced by the method for producing a thermal spraying powder according to claim 1, and contains 18% to 22% Si by weight, with the balance being Fe and inevitable impurities A powder for thermal spraying having a structure in which at least one of Fe ₃ Si compound, FeSi compound, Fe ₅ Si ₃ compound, and Fe ₂ Si compound, which is an Fe—Si intermetallic compound, is finely precipitated inside the particles. It is.

  The invention described in claim 3 is a sprayed coating having excellent wear resistance at high temperatures formed on the surface of the substrate by spraying the thermal spraying powder described in claim 2.

  A fourth aspect of the present invention is a substrate excellent in wear resistance at high temperatures coated with the thermal spray coating according to the third aspect.

(Function)
The reasons why the component composition (% by weight), the chemical composition of the crystal, the average crystal grain size, and the production method of the thermal spraying powder of the present invention are limited to the above claims will be described below.
Fe and Si are abundant elements, and it is easy to obtain raw materials. Further, as apparent from the Fe—Si based alloy phase diagram of FIG. 1, the Fe ₂ Si compound is formed by atomizing a molten metal containing 18 to 22% by weight and the balance being Fe and inevitable impurities. A primary powder comprising the main component is obtained. Fe ₂ Si itself has a hardness of about HV980 as shown in the primary powder B in Table 4, and when it is directly laminated as a thermal spray coating, the lamination efficiency is poor as shown in Comparative powders 3 and 6 in Table 3. Alternatively, it is difficult to form a film.

  Therefore, the present invention further improves adhesion by holding the primary powder at an appropriate temperature, and is suitable for forming a sprayed coating excellent in corrosion resistance and abrasion resistance on the substrate surface. The next powder is obtained.

If the content of Si is atomization of the molten metal less than 18%, although the main component of the primary powder is a Fe ₃ Si compound is Fe-Si-based intermetallic compound, Fe ₃ Si compounds of Table 1 A As shown in Comparative powders 1 and 2 in Table 4, the hardness is as soft as HV350 and the adhesion is good, but the corrosion resistance and wear resistance are inferior, so the Si content is preferably 18% or more.

Further, if the content of Si was atomization 22% to 34% of the molten metal, the main component of the primary powder, in addition to Fe ₂ Si compound is FeSi-based intermetallic compound, FeSi compounds, Fe ₅ Si _Three compounds are produced.

Further, when an atomizing treatment is performed on a molten metal having a Si content of 34% or more, crystals of FeSi compound and αFeSi ₂ compound are precipitated in the primary powder. These FeSi compound and αFeSi ₂ compound deposited on the primary powder both have a hardness of HV900 or higher, and are hard as in the case of the Fe ₂ Si compound and have low adhesion during thermal spraying.

Further, as is apparent from the Fe—Si based alloy phase diagram of FIG. 1, the FeSi compound is stable in the temperature range of 1410 ° C. or less and the αFeSi ₂ compound is stable in the temperature range of 982 ° C. or less, and the composition change and softening due to heat treatment do not occur. Therefore, there is a problem that the effect of improving adhesion cannot be obtained.

As described above, the primary powder for producing the thermal spraying powder of the present invention needs to start with a molten metal containing Si in a weight ratio of 18% to 22% and the balance of Fe and inevitable impurities. The reason for this is that the Fe ₂ Si compound obtained only in this Si range is changed to other Fe—Si compounds by the subsequent heat treatment, that is, Fe ₃ Si compound, Fe ₂ Si compound, Fe ₅ Si ₃ compound, and FeSi compound. This is because it can be softened.

From the alloy phase diagram of FIG. 1, the Fe ₃ Si compound is a stable compound only at high temperatures, and the gas atomization method, which is a manufacturing process involving rapid solidification of the molten metal, is optimal for the production of the primary powder. FIG. 2 shows an optical micrograph of a cross section of the primary powder particles before the heat treatment, and the powder particles are formed of uniform Fe ₂ Si. This primary powder is obtained by gradually cooling a molten metal containing Si having a Si content of 18% to 22% and maintaining the Fe ₂ Si compound at 1060 ° C. or higher after which the Fe ₂ Si compound becomes stable. It can also be obtained by pulverizing an ingot of Fe ₂ Si compound obtained by quenching by attaching.

Next, the heat treatment holding time for producing the secondary powder is set to 1 hour or longer.
This is because the thermal spraying powder has a particle size as small as 100 μm or less (about 10 μm to 100 μm), so that the diffusion and rearrangement of atoms in the particles is completely completed if the heat treatment is held for 1 hour or longer. by.

FIG. 3 is an optical micrograph of the cross-section of the secondary powder after the heat treatment, and can be changed to a structure in which various Fe—Si intermetallic compounds are precipitated. This is a structure in which a large amount of Fe ₃ Si compound, Fe ₂ Si compound, Fe ₅ Si ₃ compound, and FeSi compound is precipitated on the powder particles. In addition, even if it heat-processes for 1 hour or more, there is no change in the crystal structure of secondary powder. Since a fine structure in which such various Fe—Si intermetallic compounds are deposited is formed inside the powder, it is shown in FIG. 4 due to rapid heating during thermal spraying of the secondary powder and impact during collision with the base material. Thus, it is considered that deformation and cracking of the thermal spray particles are easily induced, and an effect of improving adhesion is obtained.

The range of the heat treatment temperature when forming the secondary powder from the primary powder can be set as follows. First, regarding the upper limit of the heat treatment temperature, when the heat treatment temperature is 800 ° C. or higher, a large amount of coarse Fe ₅ Si ₃ compound is precipitated, and the effect of improving the adhesion characteristic of the present invention cannot be obtained. Since the particles are sintered, the heat treatment temperature needs to be less than 800 ° C. On the other hand, as for the lower limit of the temperature range, the influence of softening due to heat treatment is seen between 200 ° C. and 400 ° C. from the relationship between the cross-sectional hardness of the particles after heat treatment in FIG. 5 and the heat treatment temperature. Since the degree of softening is particularly remarkable at 600 ° C. or higher, it is considered that the heat treatment temperature range is preferably 600 ° C. to 800 ° C.

  In addition, although the thermal spray powder softening due to heat treatment occurs in the atmosphere, from the viewpoint of preventing Si consumption due to oxidation of particles, deterioration of adhesion due to oxide entrapment, or deterioration of adhesion due to oxide film, in vacuum or argon, nitrogen, etc. It is preferable to carry out in an inert gas atmosphere. Note that the same effects can be obtained in both atmospheres in vacuum and in an inert gas.

  According to the present invention, a powder for thermal spraying onto a base material, which can adhere coarse ferrosilicon particles without reducing corrosion resistance, which could not be laminated when used as a high-speed flame spraying material in the past. Is obtained. Furthermore, according to the present invention, the hardness of the cross section of the coating is higher than when fine powder is used, and the wear resistance of the thermal spray coating is improved.

Embodiments of the present invention will be described with reference to the drawings.
The ends of the high-purity Si wafer for semiconductors and high-purity electrolytic iron were melted at high frequency to prepare three types of Si-containing melts shown in Table 1. The molten metal was atomized and classified so as to have a normal distribution in a specific particle size range to obtain primary powders A to D. A secondary powder obtained by placing these powders in an alumina crucible and heat-treating them in a vacuum using a high-speed flame spraying apparatus: DJ-2700 manufactured by SulzerMetco, with a 10-point average roughness (Rz) of about 80 / μm by shot blasting. A film was formed on the outer surface of a low alloy steel pipe having a diameter of 38 mm (STBA20: 0.5Cr0.5Mo). Specific film forming conditions are: propylene flow rate: 35 scfh, oxygen flow rate: 30 scfh, air flow rate: 50 scfh, powder supply amount: 35 g / min, gun speed: 1000 mm / sec, feed pitch: 3 mm.

About the primary powder A shown in Table 1, the hardness of the particle | grain cross section after performing the heat processing for 1 hour each at the temperature from 200 degreeC to 900 degreeC was measured with the micro Vickers hardness test apparatus. As measurement conditions, a load of 50 gf and an average hardness of 5 particles were used. The measurement results are shown in FIG.

  As shown in FIG. 5, what was about HV980 before the heat treatment softens at a heat treatment temperature of 200 ° C. or higher as indicated by point 3, and about HV650 at a heat treatment temperature of 750 ° C. as indicated by point 4. became. Further, as indicated by point 5, at a heat treatment temperature of 800 ° C., the hardness of the particles increased and the particles were sintered.

Regarding the primary powders A to D shown in Table 1, changes in the compounds constituting the powder particles by heat treatment are shown in Table 2. In addition, the wide angle X-ray diffraction method was used for the identification of a compound. In the present invention powders 1-4, what was an Fe ₂ Si compound before the heat treatment became an Fe ₃ Si compound, an Fe ₂ Si compound, an Fe ₅ Si ₃ compound, and an FeSi compound after the heat treatment. On the other hand, only Fe ₃ Si was detected in the comparative powders 1 and 2 in which the Si content of the primary powder was 15% regardless of the heat treatment.

In the comparative powders 3 to 8 in which the Si content of the primary powder is 20%, the composition does not change at a heat treatment temperature of 200 ° C. or less, and at 900 ° C., as in the present invention, the Fe ₃ Si compound Fe ₂ Si compound, Fe ₅ Si ₃ compound and FeSi compound were detected. Further, in the comparative powders 9 and 10 having a Si content of 30%, only the Fe ₅ Si ₃ compound was detected from the γ powder particles before and after the heat treatment, and no change was observed.

Originally, Fe ₅ Si ₃ should be decomposed into Fe ₃ Si and FeSi by heat treatment from the Fe—Si based alloy phase diagram of FIG. 1, but the lower limit of the stable temperature is relatively low at 825 ° C. It is thought that it is difficult to be decomposed because it is low. From the above, it has been experimentally confirmed that only the Fe ₂ Si compound can be changed to another compound by the heat treatment of the present invention.

  Table 3 shows the results of thermal spraying the powders treated under the respective heat treatment conditions for the primary powders shown in Table 1. As an index of the film formation rate, the number of passes required to obtain a sprayed film having a thickness of 200 μm that is generally employed is shown as the thickness of the sprayed film for the boiler. Regarding the primary powder C, the comparative powders 6, 7, and 8 could not be laminated, but the powders 3 and 4 of the present invention could be laminated to 200 μm or more.

  In addition, for the primary powder B, a predetermined film thickness was obtained with the comparative powders 3 and 4, but the number of laminations was 20 or more and the efficiency was low. The number of passes was reduced. In Comparative Powder 8, particles were sintered and could not be sprayed. In addition, about the primary powder D whose Si content range is higher than the range of this invention, it did not adhere by thermal spraying irrespective of the presence or absence of heat processing. Moreover, about the primary powder A, adhesiveness was favorable irrespective of the presence or absence of heat processing.

Next, what measured the change of the diameter for every path | pass of this invention powder 3 and the comparison powder 3 and 6 with a caliper is shown in FIG. Comparative powders 3 and 6 are within the scope of the present invention in terms of the component ranges, but remain as primary powders and are not heat-treated. In the present invention powder 3 indicated by point 6, a film thickness of about 250 μm was obtained in 7 passes, whereas in comparative powder 3 indicated by point 7, about 140 μm in 7 passes and comparative powder 6 indicated by point 8 were Since the film laminated in the previous pass was destroyed by the spray particles sprayed on the film in the next pass, the film thickness did not increase to 100 μm or more.

  Table 4 shows the results of evaluating the thermal spray coatings of the present invention powders 1 to 4 and the comparative powders 1 to 4 in which the thermal spray particles can be laminated in Table 3. The abrasion resistance of the thermal spray coating can be evaluated by the hardness of the coating cross section, and it is known that practically sufficient abrasion resistance can be obtained when used in a boiler if it is about HV700 or more. Then, the average of the measured value in 10 points | pieces was evaluated as a representative value using the micro Vickers hardness test apparatus. As a result of the measurement, the hardness of the thermal sprayed coating was HV730 or higher in the thermal sprayed coating of the present invention powder, whereas the comparative powder used the primary powder A and had no heat treatment at HV300, 750 ° C. × 1 hour. HV280, primary powder B sprayed without heat treatment, HV560, 200 ° C. × 1 hour heat treatment, and remained at HV520.

From the above, higher wear resistance is expected than in the conventional method in which the powder heat treatment of the present invention is not performed. Furthermore, as an evaluation of corrosion resistance, the sprayed steel pipe was cut to produce a coupon test piece of 10 mm × 10 mm × 5 mmt, and this was installed in an atmosphere replacement type horizontal electric furnace to simulate a coal combustion atmosphere, Gas composition (volume%): 0.22% H ₂ S-14% SO ₂ -6.3% CO-13.0% CO ₂ -1.8% H ₂ -10% H ₂ O-bal. A laboratory level corrosion test was carried out using a sulfide corrosive gas of N 2 under the condition of 500 ° C. × 300 h. As a result of observing the corrosion state of the cross section of the test piece after the test, the powders 1 to 4 of the present invention and the comparative powders 3 and 4 were healthy, whereas the films of the comparative powders 1 and 2 were corroded. From the above results, it was confirmed that the powder of the present invention was able to obtain higher resistance to both wear and corrosion than before.

  FIG. 7 shows a preferred range of application of the coating with the thermal spraying powder of the present invention. In recent coal-fired boilers, NOx reduction in exhaust gas is performed by a two-stage combustion system in which a boiler burner 12 and an after air port 11 are combined. In such a combustion method, since a strong reducing atmosphere is generated in the region 9 in the boiler, the furnace wall is corroded and the corrosion products are worn by the ash, so that there is a problem that a high thinning rate is obtained. It has become. By applying the thermal spraying powder of the present invention to this part, the thinning is suppressed. Moreover, the area | region 10 in a boiler is a site | part called a hopper. In this region 10, since a large amount of ash generated by combustion flows on the surface of the member like an avalanche, it becomes a severe wear environment. By applying the thermal spraying powder of the present invention to this part, the wear thinning of the member is suppressed, contributing to the stabilization of the plant by extending the member life.

The molten metal obtained by high frequency melting JIS ferrosilicon (FeSi ₃₀ ) and high purity electrolytic iron in an alumina crucible was held at 1100 ° C. for 1 hour so that the Si content was 20% by weight, and then air-cooled with a blower. An ingot made of Fe ₂ Si compound was obtained. After the ingot was pulverized with a ball mill, a primary powder was obtained in a particle size range of 20 to 140 μm. As a result of heat-treating this primary powder under the condition of 750 ° C. × 1 h and spraying on the outer surface of the STBA20 steel pipe, as in the case of the present invention powder 3 in Table 3, the lamination required for obtaining a film thickness of 200 μm or more The number of times was 5 passes, the hardness of the thermal spray coating was HV850, the coating after the corrosion test was also sound, and it was confirmed that a thermal spraying powder equivalent to the atomized powder could be prepared by a pulverization method.

  According to the present invention, coarse ferrosilicon particles, which could not be laminated when used as a high-speed flame spraying material in the past, can be attached without reducing the corrosion resistance. Furthermore, according to the present invention, the hardness of the coating cross section becomes higher than when fine powder is used, and the wear resistance of the thermal spray coating is improved.

ASM HANDBOOK Vol. 3 Alloy Phase Diagram, Binary Alloy Phase Diagram / 2 P.M. 253 is an Fe—Si based alloy phase diagram quoted from H.253. It is an optical microscope image of the powder particle cross section before the heat processing of this invention. It is an optical microscope image of the powder particle cross section heat-processed by 750 degreeC * 1h of this invention. It is a schematic diagram which shows the method of adhesion to the base material of the thermal spraying of the particle | grains obtained by this invention. It is a figure showing the relationship between heat processing temperature and the Vickers hardness of particle | grains. It is the figure which compared the number of passes of thermal spraying, and the change of a film thickness by the Example and comparative example of this invention. It is the figure which showed the example of application to the coal burning boiler of the powder for thermal spraying of this invention, and looked at the side surface from the furnace inner side.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heat processing powder 2 Base material 9 Area | region where the membrane | film | coat by the powder for thermal spraying of this invention is applied 10 Area | region where the membrane | film | coat by the powder for thermal spraying of this invention is applied 11 After air port 12 Burner

Claims (4)

Primary powder particles mainly containing Fe ₂ Si, which is an Fe-Si intermetallic compound, are prepared by atomizing a molten metal containing 18% to 22% Si by weight and the balance being Fe and inevitable impurities. By holding the primary powder in a vacuum or in an inert gas at a temperature of 825 ° C. to 400 ° C. for a certain period of time, the Fe ₃ Si compound, FeSi, which is an Fe—Si intermetallic compound, is contained in the primary powder particles. A method for producing a thermal spraying powder, comprising producing secondary powder particles in which at least one of a compound, an Fe ₅ Si ₃ compound and an Fe ₂ Si compound is finely precipitated. A thermal spraying powder produced by the thermal spraying powder manufacturing method according to claim 1, comprising 18% to 22% Si by weight, the balance being Fe and inevitable impurities, Fe inside the particles A powder for thermal spraying having a structure in which at least one of Fe ₃ Si compound, FeSi compound, Fe ₅ Si ₃ compound, and Fe ₂ Si compound, which are Si-based intermetallic compounds, is finely precipitated.   A thermal spray coating formed on a substrate surface by spraying the thermal spraying powder according to claim 2.   A substrate coated with the thermal spray coating according to claim 3.

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