In order to improve the high temperature oxidation resistance of P92 steel, Ni-Al intermetallic compounds were laser clad on the surface of P92 steel substrate using line spot, the cladding process parameters were screened, and the microstructure and composition of the sample were analyzed. The results show that no cracks were found in the sample cladding layer under the screened cladding process, and there was no cracking tendency; the P92 steel substrate was less melted, and the Fe element in the substrate diffused to the top with a content of less than 5%. The main phases in the cladding layer were Ni3Al and NiAl, and the microstructure from the top of the cladding layer to the substrate part changed from equiaxed crystals to dendrites, columnar crystals and planar crystals. When the laser power was 1 800 W, the scanning speed was 2 m/min, the powder feeding speed was 17 r/min, and the powder feeding gas flow rate was 8 L/min, the substrate melting and Fe element diffusion could be reduced, which provided ideas for solving the cracking of Ni-Al intermetallic compound cladding layer and the diffusion of Fe elements in the steel substrate during laser cladding.
At present, most thermal power plants at home and abroad use supercritical thermal power generating units, which have the advantages of low cost, low energy consumption, and green environmental protection. P92 steel is the main steel used for high-temperature components such as the main steam pipe of the boiler of the supercritical unit, and has excellent high-temperature strength and high-temperature creep resistance. However, P92 has obvious disadvantages as a ferritic steel. Its high-temperature oxidation resistance is poor. It is easily corroded by boiler steam in a high-temperature environment, thereby shortening the service life, which will lead to a reduction in the service life of the boiler pipe. Ultra-supercritical technology has more stringent performance requirements for steel than supercritical technology. It is difficult for P92 steel to continue to meet the higher temperature and service pressure requirements of ultra-supercritical units. Therefore, it is necessary to clad a high-temperature oxidation-resistant coating on P92 steel.
Ni-AI intermetallic compounds have good oxidation resistance, and at the same time have the advantages of low density, high melting point, and good thermal conductivity. They are widely used in the aerospace field and are considered to be a new generation of high-temperature structural materials. However, Ni-AI intermetallic compounds also have disadvantages. Although they have excellent high-temperature oxidation resistance, their atomic arrangement structure leads to their room temperature brittleness. When the Ni-AI intermetallic compound coating is prepared by the traditional thermal spraying process, there are through-pore defects in the coating, which seriously reduce the high-temperature oxidation resistance of the Ni-AI coating. The laser cladding process is a relatively complex physical and chemical process. As a surface modification technology that has developed rapidly in recent years, its forming principle is to use a high-energy laser beam as a heating source to melt the treated alloy powder on the surface of the substrate to become the main component of the cladding layer. At the same time, the substrate itself is melted by laser irradiation, so that the substrate and the cladding material form a metallurgical bond.
When laser cladding is used to prepare Ni-AI intermetallic compound coatings, due to the large heat input of laser cladding and the extremely fast cooling rate, after the cladding layer is formed during the melting-solidification process, due to the different thermal physical properties between the substrate and the cladding powder, the thermal conductivity is greatly different, which will produce a large residual stress, which will cause the Ni-AI intermetallic compound to form surface cracks due to the residual stress, thereby significantly reducing the high-temperature corrosion resistance of the cladding layer. In order to solve the problem of cracking of Ni-AI intermetallic compound cladding layer during laser cladding, Cheng Guangping et al. used laser cladding technology to prepare crack-free NiAI-Fe intermetallic compound coating by using matrix Fe melt alloying. However, too high Fe content will reduce the good high-temperature chemical resistance of Ni-AI intermetallic compound. Zeng Chaoliu et al. found that when the atomic ratio of Fe element is less than 30%, the high-temperature oxidation resistance of Ni-AI intermetallic compound is reduced but not obvious, but when the atomic percentage of Fe element in Ni-AI intermetallic compound reaches 30%, the high-temperature oxidation resistance of Ni-AI intermetallic compound is seriously reduced.
This study uses line spot laser cladding technology to study the crack-free Ni-AI intermetallic compound cladding layer on P92 steel using line spot laser cladding. By adjusting various process parameters in the laser cladding process, including laser power, powder feeding speed, and powder feeding gas flow, the cracking tendency of Ni-AI intermetallic compounds is reduced, the melting of the P92 steel matrix and the diffusion of Fe elements are reduced, and the influence of Fe elements on the high-temperature oxidation resistance of the cladding layer is reduced.
1 Experimental materials and process parameters
1.1 Experimental materials
P92 steel with a sample size of 100 mmx15 mmx8 mm is used as the matrix material. The chemical composition of the matrix material is shown in Table 1. Ni powder with a purity of 99% and A powder with a purity of 99% are selected as cladding materials, and the powder particle size is 15~40μm. Before the cladding test, the P92 surface test plate was sanded with 800-grit sandpaper, and Ni powder and AI powder were put into the powder mixing tank and mixed in the powder mixer for 24 hours to make the powders evenly mixed. Before the test, the surface of the test plate was wiped with alcohol to remove the surface stains on the test plate. During the test, considering that the surface of the test plate has a reflective effect on the laser, the laser head was tilted 5° to protect the laser head.
1.2 Test method
The laser cladding system for this test was independently built by the laboratory, including a laser with a maximum power of 2,500 W, a matching special laser cladding head, a remote-controlled robotic arm, a cooling water circulation system, a powder feeder, etc. The sample was placed on the working platform. Ar gas was required to be introduced during the working process to ensure the normal operation of the laser cladding system. The carrier gas flow rate was 1 m’3/h, the working distance was 20 mm, and the laser focus was 3 mm from the workpiece surface. The movement of the laser cladding head was controlled by the remote-controlled robotic arm. Seven groups of laser cladding tests were carried out using the laser cladding test platform. The mass ratio of cladding powder was Ni∶Al=80∶20, the line spot size was 1 mm×5 mm, and the process parameters were shown in Table 2.
After the laser cladding was completed, the samples were obtained by vertical cutting along the laser scanning direction using wire cutting. After being polished to 2000 mesh using sandpaper of different particle sizes, aqua regia (V concentrated nitric acid: V concentrated hydrochloric acid = 3∶1) was used for corrosion, and the corrosion time was controlled at 4~5 min. The micromorphology of the cladding layer and the structure of each part of the cladding layer after corrosion were observed using a SU6600 thermal field emission scanning electron microscope. The samples were X-ray tested using an XRD-7000S SHIMADZU instrument, and the scanning range of the X-ray was 20°~90°.
2 Test results and discussion
2.1 Macromorphology of coating
Figure 1 shows the cladding layer of Ni-Al intermetallic compound laser cladding under different process parameters (samples 1~7 in the figure correspond to serial numbers 1~7 in Table 2). The thickness of the sample is closely related to the Fe content in the cladding layer and the powder feeding speed, laser power, and powder feeding gas flow rate. The thickness of the cladding layer of samples 1~4 is too low to meet the thickness requirements of the anti-oxidation coating. The thickness of the cladding layer is related to the amount of powder melted per unit time. Compared with the process parameters of samples 2~3, samples 4~6 increase the laser power and powder feeding speed. Although the obtained cladding layer meets the thickness requirements, the Fe content in the cladding layer is too high, indicating that the degree of laser melting on the substrate is still high. Increasing the powder feeding speed to 17 r/min, so that the powder absorbs more laser energy, can reduce the melting effect of the laser on the substrate. This method reduces the content of Fe in the cladding layer, indicating that the laser has less influence on the substrate at this time. The cladding layer of sample 7 prepared by this process has moderate thickness, continuous flatness, and low Fe content in the cladding layer. This shows that the Ni-Al intermetallic compound coating prepared under this process parameter is an ideal high-temperature anti-oxidation coating.
2.2 Analysis of cladding layer composition
Figure 2 is the X-ray diffraction diagram of the cladding layer of sample 7. It can be seen from Figure 2 that after adjusting the various process parameters of laser cladding, the substrate melts less, and the cladding layer is composed of NiAl and Ni3Al. Since a small amount of Fe elements reach the upper part from the substrate under the condition of molten pool flow, Ni3Al and NiAl can both dissolve a certain amount of alloying elements. Fe can occupy both Al and Ni positions in Ni3Al, and has a large solubility in NiAl. Since the incorporation of Fe elements will cause lattice distortion, thereby changing the molecular crystal plane spacing in the sample, this will cause a slight shift in the NiAl peak and the Ni3Al peak. The relative content of NiAl and Ni3Al in the surface layer of the cladding layer can be roughly estimated from the height of the diffraction peak. Since the height of the diffraction peak of Ni Al is relatively high, it can be judged that the phase composition of the cladding layer is mainly NiAl, and Ni3Al is supplementary.
2.3 Microcrack observation and microstructure
Figure 3 shows the cross-sectional micromorphology of sample 7. The melting depth of the cladding layer is about 250 μm, and the melting width of the cladding layer is 3974 μm. Because the heat source of the line spot laser cladding is a flat-top heat source, the height of the cladding layer changes smoothly, showing a trend of low on both sides and high in the middle. The cladding layer is well bonded to the substrate, and no cracks are found at the junction of the substrate and the cladding layer. At the same time, there are no through cracks and microcracks from the upper part to the lower part of the cladding layer. Under this process parameter, the cracking tendency of Ni-Al intermetallic compounds cladding by line spot laser is small, and no cracks are generated. Figure 4 (a) is the overall microstructure of the cladding layer. It can be found that the structure of the cladding layer changes from equiaxed crystals to dendrites, columnar crystals and planar crystals from top to bottom.
Figure 4 (b) is the junction of the cladding layer and the substrate. This area is a bright band area, mainly composed of planar crystals and columnar crystals. It can be judged that there is a good metallurgical bond between the cladding layer and the substrate. The microstructure of the cladding layer is related to the solidification rate R and temperature gradient G of the cladding layer during the solidification process. G/R reflects the microstructure of the solidified structure. The upper part of the cladding layer is in contact with the air and has a fast heat dissipation rate. Therefore, its temperature gradient G is small, the solidification rate R is large, and the G/R is small. Therefore, the solidified structure is mainly composed of equiaxed crystals. In the lower cladding layer, which is in contact with the substrate, during the solidification process of the laser cladding molten pool, the low heat dissipation rate at the junction leads to a large temperature gradient G at the junction, a low solidification rate, and a large G/R. Therefore, the solidified structure is mainly composed of planar crystals and columnar crystals. The heat dissipation rate in the middle of the cladding layer is between the two, so the middle of the cladding layer is mainly dendrites.
2.4 Element distribution and energy spectrum analysis
Figure 5 is the energy spectrum scanning result of sample 7. Figure 5 (a) is the element peak of the energy spectrum scanning, and Figure 5 (b) is the mass percentage distribution curve of the element with the increase of melting depth calculated according to the strength of the element peak. From Figure 5 (b), it can be observed that the Al content in the upper part of the cladding layer is relatively high. At about 0~50 μm, the Al content exceeds the Al content of the original powder, reaching a maximum of 28%. As the depth increases, at about 50~200 μm, the Al content remains at 20%. It can be seen from the element mass distribution curve that the elements of the cladding layer are evenly distributed in the cladding layer. Since the Al content in the upper part is high, the upper part has good oxidation resistance as a high-temperature oxidation-resistant coating. The Fe content of the cladding layer increases with the distance from the top of the cladding layer, which is caused by the melting of the matrix Fe. However, the Fe content in the upper part of the cladding layer is less than 5%. This process provides ideas and methods for solving the problems of cracking of laser cladding Ni-Al intermetallic compound cladding layer and diffusion of Fe elements in steel matrix.
3 Conclusions
(1) Line spot laser cladding of Ni-Al intermetallic compound was performed on P2 steel. The mass ratio of cladding powder was Ni∶Al=80∶20. The laser cladding process parameters were: laser power 1 800 W, powder feeding speed 17 r/min, powder feeding gas flow rate 8 L/min, scanning speed 2 m/min. The melting depth of the cladding layer was about 250 μm, and the melting width was 3 974 μm. Under this process, the cracking tendency of the cladding layer was relatively small, and no cracking phenomenon was found in the cladding layer after microscopic observation.
(2) The main components of the cladding layer are NiAl and Ni3Al, and part of the NiAl and Ni3Al lattices are replaced by Fe atoms. The heat dissipation rates of different parts of the cladding layer are different, resulting in differences in G/R. The microstructure morphology from the top of the cladding layer to the part in contact with the substrate changes from equiaxed crystals to dendrites, columnar crystals and planar crystals.
(3) The Al content at the top of the cladding layer at 0-50 μm is 28%, and the Fe content is less than 5%. As the distance from the top of the cladding layer increases, at a depth of 50-200 μm, the Al content remains at around 20%, and the Fe content continues to increase. The elements in the cladding layer are evenly distributed.
