Surface Protection Technology: Surfacing welding of sealing surface

Surface Protection Technology of Waste Incinerator Tube Wall (HY-industry technical centre)

At present, in view of the thermal corrosion problem of the tube wall of the heat exchange parts of the waste incinerator, the surface protection technologies studied or adopted mainly include ceramic patch, surfacing, laser cladding and thermal spraying.

Ceramic patch (Surface Protection Technology)

Ceramic sheets with high SiC or Al2O3 content have very good high temperature performance and stable chemical properties. They are ideal materials for thermal corrosion protection. It is arranged and installed on the fire-facing surface of the heat exchange tube wall, which can effectively resist the thermal corrosion and erosion wear of high-temperature gas, thereby prolonging the service life of the tube. This technology has been tested or practically applied in some waste incinerators and has achieved certain results. However, the inherent brittle nature of ceramic materials and the complexity of the corrosive environment make ceramic patches have extremely strict requirements on the material component design, preparation process, product form and installation method in the application, and their actual performance and The service life is also unpredictable. Once individual patches are cracked or peeled off, it will cause rapid failure of the overall protective layer. In addition, ceramic materials have low thermal conductivity, which blocks the effective conduction of heat energy and reduces energy conversion efficiency. At the same time, hindering heat transfer will increase the temperature of the gas in the furnace, thereby aggravating the thermal corrosion of other components in the gas passage. Therefore, this technology has not been widely applied.

(2) Surfacing (Surface Protection Technology)

The use of surfacing technology to prepare a heat-resistant corrosion-resistant deposited layer on the surface of the pipe wall is one of the more effective technical methods. It has been adopted and used since the 1990s. In the early days of waste incinerator water walls and partial superheaters The application shows a good protective effect. Among them, the most mature application is the overlay welding of Inconel625 alloy (Ni-21Cr-9Mo-3.5Nb), C-276M (Ni-18Cr-14Mo-4W), HC-2000 (Ni-23Cr-16Mo-1.6Cu) and so on. Compared with the thermal spray coating and ceramic patch technology, the surfacing cladding layer can form a strong metallurgical bond with the substrate, the structure is more uniform, the thickness can reach a few centimeters, and its performance is stable and durable under proper conditions of use The protective effect has obvious advantages.
However, the actual construction requires high welding equipment and technology, and the heat input needs to be strictly controlled to avoid problems such as weld penetration or pipe deformation. The construction efficiency is low and the cost is high. At the same time, in-situ repair is also a problem that is difficult to overcome by surfacing technology. During the second surfacing repair, it is easy to cause embrittlement of the original cladding layer structure, causing cracks and extending to the base material to cause overall failure, which also causes a large number of materials The waste and cost of use are further increased, so surfacing repair is not widely recommended.

In addition, studies have shown that the performance of the Inconel625 alloy cladding layer is closely related to the service temperature. When the service temperature is below 400℃, its thermal corrosion resistance is excellent and stable; when the service temperature reaches above 400-420℃, the cladding layer The protective effect is basically lost; if the use temperature exceeds 540℃, the corrosion rate of the cladding layer can even reach as high as 0.2μm/h. This greatly limits the application of surfacing Inconel625 alloy, especially in the face of high ambient temperature superheaters, it is difficult to meet the needs of use. With the continuous development of waste incineration technology, the demand for improving energy conversion efficiency and limiting secondary pollution emissions has also become higher. Further increasing the combustion temperature and reducing construction costs have become the mainstream trend. Therefore, the development of more suitable high The high-performance low-cost surfacing material is an urgent problem that this technology faces.

(3) Laser cladding (Surface Protection Technology)

Laser cladding is a surface technology that has developed extremely rapidly in recent years and has been widely used in many practical engineering fields. Compared with the surfacing technology, the laser cladding technology has less thermal influence on the base material, and can effectively improve the problem of pipe welding deformation. At the same time, the deposition dilution rate is low, and the designed alloy composition can be realized under the limited coating thickness, which reduces the use of materials and improves the heat exchange efficiency. However, the extremely high cooling rate of laser cladding can often obtain extremely fine and uniform cladding layer structure, which is beneficial to improve the thermal corrosion resistance of the cladding layer. In addition, the construction efficiency of laser cladding is high, which can significantly improve working hours and labor intensity. The study of laser cladding CoCrW Stellite6 L alloy found that the surface layer of the cladding layer produces a uniform fine-grained structure. The refinement of the structure improves the strength while also effectively improving the high-temperature performance. Although the laser cladding technology has unique advantages in solving the thermal corrosion of the tube wall of the waste incinerator, there are still some problems that limit its practical application, such as high equipment investment, complex construction technology, and difficulty in on-site construction. Therefore, this technology is still mainly researched in this field and has not been put into practical application.

(4) Thermal spraying (Surface Protection Technology)

Thermal spraying technology is a very active subject branch in the field of material surface engineering. It has unique technical advantages in solving the problem of thermal corrosion of actual components, so it has been widely used. In particular, the successful application of thermal spray coatings on the “four tubes” of coal-fired power plant boilers provides a good reference for the thermal corrosion protection of the heat exchange tube walls of waste incinerators that also aim at combustion and power generation. In recent years, the research, development and application of thermal spraying technology and materials in waste incinerators has become a hot spot in this field, and it is also considered to be one of the most effective and appropriate technical means to solve the problem of tube wall thermal corrosion. Among them, supersonic flame spraying, atmospheric plasma spraying, explosive spraying and arc spraying are several representative technical solutions.

① Supersonic flame spray
High velocity oxygen fuel (HVOF) has the characteristics of relatively low flame temperature and fast particle flight speed. It has obvious technical advantages in the preparation of metal alloys and cermet coatings with high density and low oxygen content. The compactness of the coating and the uniformity of the coating structure have a significant impact on the thermal corrosion resistance of the coating. It is precisely because of the advantages of supersonic flame spraying in the preparation of high-quality metal coatings that it has become a foreign research waste incinerator tube wall. The first choice and main push technology for protection. While achieving a large number of research results, it has also been widely promoted and applied in actual projects, greatly improving and extending the service stability and maintenance and replacement cycle of heat exchange components such as water walls and superheaters in waste incinerators, especially It performs better in surface protection of high-temperature components such as superheaters, and effectively solves the shortcomings of traditional surface surfacing technology in high-temperature thermal corrosion protection.

At present, the spraying materials researched, developed and used are also mainly Ni-based alloys, including Ni80Cr20, Ni50Cr50, Ni-18Cr-5Fe-5Nb-6Mo, Ni-17Cr-4Fe-3.5B-4Si, Cr3C2-NiCr, etc. Among them, Cr is the most important alloying element. The Cr2O3 generated on the surface of the coating during service has high chemical stability and protection under this type of corrosive environment and temperature, and can effectively prevent the penetration of corrosive media into the coating. The resulting spinel structure phase (NiCr2O4) also played a beneficial role in promoting the corrosion resistance of the coating. However, although supersonic flame spraying performs better in terms of coating performance, its process cost is also extremely expensive, including large equipment investment, frequent replacement of worn-out parts, amazing gas consumption, high powder quality requirements, and coating deposition rate Lower level, which largely limits its further promotion in actual use, especially in the Chinese market where cost requirements are more stringent. Therefore, under the current situation that it is difficult to change the high cost of the supersonic flame spraying process, seeking suitable alternative technologies is also one of the development trends of thermal corrosion protection of the tube wall of waste incinerators.

②Atmospheric plasma spraying
Atmospheric plasmaspray (APS), as one of the most representative technologies of thermal spraying, has the advantages of high beam temperature, good coating quality, and wide material adaptability. It can prepare metals, alloys, ceramics and their composite materials, etc. A variety of coatings provide a broader design idea for the protection of tube wall thermal corrosion, including the introduction of chemically more stable ceramics. In this regard, a lot of research work has been carried out outside China, including plasma spraying NiCr, NiCrSiB, NiCrAlY, NiCr-Cr3C2, ZrO2/Alloy625, etc. The research on the coating structure and thermal corrosion behavior shows that it can be A high-quality surface coating is obtained, and its performance is similar to that of the supersonic flame spray coating, which can effectively and stably protect the tube wall of the heat exchange component of the waste incinerator.
However, this technology also faces considerable resistance when it is promoted in practice, such as expensive equipment, high energy consumption, and high process complexity. Studies have shown that there are hundreds of factors that affect the quality of the coating in the plasma spraying process. Very high requirements are put forward on equipment stability and the technical experience of operators. Therefore, the plasma spraying technology is still based on research in the thermal corrosion of the tube wall of the waste incinerator, and has not been applied in a wide range of practical applications.

③Explosive spraying
Explosive spraying (Detonationgun, D-gun) is a very characteristic spraying technology in thermal spraying technology. The generation of high-speed detonation waves endows the powder particles with a high flying speed, which can form a very dense coating structure and reduce the coating. Defects are conducive to the improvement of the heat and corrosion resistance of the coating. At the same time, its intermittent spraying method has little thermal effect on the substrate, which can ensure that the mechanical properties of the pipe are not affected. Therefore, under similar conditions, coatings prepared by explosive spraying can often obtain better corrosion resistance than supersonic flame sprayed coatings, which can better protect the tube wall of the heat exchange component of the waste incinerator. At present, some research work has been carried out in this regard, including explosive spraying of NiCr, NiCr-Cr3C2, Cr/Ni50Cr50, etc. However, the practicability of this technology is low, especially in China, mainly due to Praxair’s blockade of high-efficiency explosive spraying technology. China’s investment in this area is also relatively limited, resulting in the stability and work efficiency of China’s existing equipment. Low, difficult to meet the needs of large-scale engineering applications and ensure stable coating quality.

④ Arc spraying
Arc spraying (Arcspray, AS) has become one of the most suitable spraying methods for large-scale on-site construction due to its simple equipment, flexible operation, high deposition efficiency, and low cost. At present, in the field of protection of the “four tubes” of coal-fired power station boilers, arc spraying Fe-based and Ni-based thermal corrosion and erosion resistant coatings has become one of the most important surface protection methods, which are widely used in practical projects outside China . However, due to the limitations of the process characteristics, compared with the aforementioned spraying methods, arc spray coatings tend to have relatively high porosity and oxide content. At the same time, due to the wire drawing process, the adjustment space of the coating alloy composition is relatively small. As a result, the application of arc spraying technology in more severely corrosive waste incinerators has been greatly restricted, which is also the main reason why relevant researchers have not paid enough attention. In recent years, the rapid development of powder core wire technology has provided greater selectivity for the composition design of the coating. On the basis of a large number of systematic studies, it has been found that by adding an appropriate amount of “deoxidation” elements to the wire, It can significantly reduce the generation of sprayed oxides and effectively improve the thermal corrosion resistance of the coating.

For example, the NiCrB-based powder-cored wire developed by this research group and HY-Industry is based on the NiCr-based material with an appropriate amount of B element that is beneficial to deoxidation, thereby reducing the oxygen content of the sprayed NiCrB coating to 2% Below, it is significantly lower than 9% of the commercial NiCrTi (45CT) coating. Comparative studies have shown that although the Cr content in NiCrB coatings (25%-30%) is lower than that in NiCrTi coatings (Cr: 43%-45%), the reduction of oxides has significantly improved its use in similar waste incinerators. The thermal corrosion resistance under the condition is shown in Figure 1. In addition, further comparative studies have also found that the thermal corrosion resistance of arc sprayed NiCrB coatings is even close to or better than that of Ni80Cr20 and NiCrSiB coatings prepared by supersonic flame spraying, which makes arc spraying gradually become feasible in this field. technology.
Therefore, with the continuous optimization of alloy composition design and the continuous development of new spray wire materials, it is expected to further improve the thermal corrosion resistance of arc spray coatings. At the same time, giving full play to the characteristics of low cost and suitable for in-situ construction of arc spraying, this technology will surely become one of the important methods for the protection of the heat exchange tube wall of the waste incinerator.

The thermal corrosion protection of the tube wall of the heat exchange component of the waste incinerator needs to be comprehensively considered from the aspects of coating performance, process convenience and construction cost. Through the analysis of several existing surface protection methods, supersonic flame spraying and arc spraying technology will be the main trends in the development of this field in the future. In an extremely corrosive environment, the preparation of high-quality coatings by supersonic flame spraying is a relatively reliable technical solution; and within an acceptable range, the comprehensive advantages of arc spraying in terms of coating performance and process cost will make it Have greater competitiveness.

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