In the mainland, waste incineration power generation has gradually replaced the traditional landfill, becoming one of the main waste disposal methods in the mainland, but the flue gas produced during waste incineration is different from thermal power generation, containing a large amount of chloride and sulfide, the corrosion of the boiler tube is much greater than that of thermal power generation, often causing boiler leakage and other accidents. In order to improve the service life of boiler tubes, a layer of corrosion-resistant nickel-based alloy Inconel625 is often welded on the outer surface of the tube wall. However, the commonly used TIG/MIG overlay welding technology due to the excessive heat input to the matrix, resulting in the dilution rate of Fe in the overlay layer is as high as 5%~15%, and the enrichment of Fe in the molten layer greatly reduces the high temperature corrosion resistance of Inconel625 alloy, and the residual thermal stress of the overlay welding method is very large, and the large-area melting will cause the workpiece stress field distribution to be extremely uneven, resulting in warpage deformation and even cracking. Laser melting technology melts and solidifies the functional layer in the form of metallurgical combination on the surface of the matrix, which has the advantages of low dilution rate, fine grain, uniform and dense structure, small heat affected zone of the matrix, and controllable thickness. By using laser melting technology to prepare Inconel625 melting layer on the surface of 20G, under the premise of ensuring the metallurgical combination of the melting layer and the substrate, the thickness and dilution rate of the molten layer are strictly controlled, which greatly improves the service life of the workpiece in the high temperature corrosion environment. In addition, when the laser power and speed do not match, pores will be generated in the composite layer, and the melting parameters of the melting process of Inconel625 alloy have a great influence on the microstructure and properties of the molten layer.
Due to the low hardness, the commonly used Inconel625 alloy cannot effectively prevent the erosion corrosion of flue gas during use, so that the corrosion rate of boiler tubes is accelerated. Alloy686 on the basis of Inconel625 adds an appropriate amount of W, Si and other elements for alloying between components to generate wear-resistant hard phase, which greatly improves the hardness and wear resistance of the material under the premise of meeting corrosion resistance, and the WC of Ni625 and WC composite coating can also improve wear resistance in laser additive remanufacturing. And Alloy686 alloy contains more Mo, and W and Mo have high thermal conductivity, respectively 173W/(m·K) and 138W/(m·K) at room temperature, so the molten alloy has high thermal conductivity, which is beneficial to improve heat transfer efficiency, reduce the surface temperature of the tube row molten layer, and improve corrosion resistance. Although Inconel625 and Alloy686 nickel-based materials can meet the needs of boiler tube corrosion resistance, but it has not been widely used because its cost is too high, and the use of Inconel625 Mo nickel-based powder as the corrosion-resistant molten layer of the pipe row, the expensive Mo in Inconel625 is removed to reduce costs, but will reduce the corrosion resistance of the molten layer. In this paper, the best conditions for the use of three nickel-based alloy powders were sought by comparing the microstructure, mechanical properties and corrosion resistance of the molten layer by laser melting Inconel625 and its Mo and Alloy686 nickel-based alloys on a 20G surface.
【Test materials and methods】
Laser-6000W laser is used for laser melting, the base material is 200 mm× 150 mm× 12 mm 20G steel plate, the melting material is Inconel625, Inconel625 Mo, Alloy686 and other three high-temperature nickel-based alloy gas atomized spherical powders, Figure 1 shows the particle size and morphology of the three alloy powders. The chemical composition is shown in Table 1. From Figure 1a and Figure 1c, it can be seen that the size of Inconel625 and Alloy686 powder is between 45~135μm, while the size of Inconel625 Mo powder is slightly larger than 70~150μmSee Figure 1b. Observing the surface of a single powder, it was found that there were adherent satellite powders and uneven bumps on the surface, especially the non-spherical powder in Alloy686 powder, see Figure 1c, the existence of such powders affected the fluidity and forming quality of the alloy to a certain extent, and there were potential defects.
The 20G steel plate is ultrasonically cleaned, scrubbed with absolute ethanol and dried. The powder was dried in a vacuum drying oven at 140 °C for 2 h. Argon is used as the carrier gas and shielding gas, and the carrier gas is 4LPM. The laser focal length WDH=278 mm, the circular spot diameter is 4.0mm, and the lap rate is 67.5%. Through the process parameter test, the molten layer without cracks, pores and other defects is obtained.
(a) Inconel625 (b)Inconel625去Mo (c)Alloy686
Figure 1: Powder particle size and morphology
The specimen was intercepted by wire cutting perpendicular to the direction of laser head movement, and a 10mm×10mm×13mm molten layer specimen was obtained, and after the specimen was polished, the cross-sectional microhardness of the specimen was measured by HXD-1000TMC/LCD microhardness tester, the loading load was 2N, and the holding time was 10s; The tensile strength of the molten specimen was tested by the MTS tensile testing machine, and the dimensions of the tensile specimen are shown in Figure 2. Using the molten salt corrosion method that simulates the corrosion environment, the molten layer is cut out of 20mm×10mm×3mm specimen with an electric wire EDM machine, placed in ZnCl2, KCl, CaCl2, Na2SO4 mass ratio of 2:2:3:3 salt powder and kept warm in a muffle furnace at 550 °C for 360h, after corrosion, the degumming agent is used to soak and ultrasonic cleaning to remove the impurities bonded on the surface of the corroded sample, the weight loss rate before and after corrosion is measured, and the average of 3 samples is taken for each molten layer, and the corrosion results are shown in Table 2. HITACHI S3400N scanning electron microscope was used to test the thickness of the molten layer, the fe dilution rate, the fracture morphology of the tensile specimen and the corrosion morphology characteristics of the corroded specimen. And using 10% perchloric acid ethanol solution for electrolytic polishing, the voltage is 20V, the electrolysis time is 20s, EBSD test, observe the size and orientation of the grains in the molten layer.
Fig. 2 Tensile specimen size
【Graphic results】
Inconel625 and Inconel625 Mo alloy powder require similar energy density, Alloy686 alloy powder because it contains higher W, Mo and other high melting point elements, its melting process requires the highest energy density. Inconel625 has a small transition of Fe dilution rate at the bonding of the Mo melting layer and the substrate, and the power needs to be increased; The dilution rate of Fe in the Alloy686 melt layer is high, and the power needs to be reduced.
The melting layers of Inconel625, Inconel625 Mo alloy and Alloy686 nickel-based alloy are all face-centered cubic structures, and the optimal grain growth orientation is <001 >. Among them, the grain of Inconel625 is more concentrated in the direction of <001>, followed by the grain of Inconel625 to Mo, although some of the grains of Alloy686 are biased towards the <101 > direction, but the main direction is still <001> direction and the grain size is more uniform.
The hardness (HV) of Inconel625 alloy molten layer is 268.1, the hardness (HV) of Inconel625 deMo molten layer is 246.5, and the hardness (HV) of Alloy686 molten layer is 362.1, which is 35.6% higher than that of Inconel625 melted layer. In the tensile test, it was found that the tensile strength of Inconel625, Inconel625 Mo and Alloy686 molten layer samples was 19.9%, 29.5% and 33.5% higher than that of the matrix, respectively, but the elongation of the Alloy686 molten layer specimen was only 10.8%, which was much lower than that of the matrix (30%).
When corroded at high temperature at 550°C, the weight loss rates of Inconel625 molten layer, Inconel625 de-Mo molten layer and Alloy686 molten layer were 9.75%, 32.02% and 8.43%, respectively, and the corrosion pits of Inconel625 de-Mo molten layer were very large. Inconel625 and Alloy686 molten layers have good corrosion resistance and meet the working conditions of boiler tubes.
(a) Inconel625 Condensed Layer (b) Inconel625 De-Mo Coagulation Layer (c) Alloy686 Coagulation Layer
Figure 3: Macroscopic morphology of the molten layer
(a) Inconel625 Condensed Layer (b) Inconel625 DeMo Coagulation Layer (c) Alloy686 Coagulation Layer
Figure 4: Colored flaw detection of molten layer
(a) Inconel625 Condensed Layer (b) Inconel625 DeMo Coagulation Layer (c) Alloy686 Coagulation Layer
Figure 5: Dilution rate of Fe in the molten layer
(a) grain (b) grain orientation
Fig. 6 Inconel625 Condensed layer EBSD grain and grain orientation
(a) grain (b) grain orientation
Fig. 7 Inconel625 de-Mo melted layer EBSD grain and grain orientation
(a) grain (b) grain orientation
Figure 8 EBSD grain and grain orientation of Alloy686 molten layer
Figure 9: Hardness of the molten layer
(a) Inconel625 Condensed Layer (b) Inconel625 DeMo Coagulation Layer (c) Alloy686 Coagulation Layer
(a)20G基材(b)Inconel625 (c)Inconel625去Mo (d)Alloy686
Figure 10 Inconel625 and its stress-strain curves for nickel-based alloys de-Mo and Alloy686
(a)20G基材 (b)Inconel625 (c)Inconel625去Mo (d)Alloy686
Fig. 11 Morphology of tensile specimens of Inconel 625 and its de-Mo and Alloy 686 nickel-based alloys
(a)试样SEM(b)图a方框放大(c)Ni(d)Cr(e)Mo(f)Nb
Fig. 12 Corrosion morphology and composition distribution of Inconel625 alloy specimens
(a) Specimen SEM (b) Fig. A box magnified (c) Ni(d) Cr(e)Nb
Fig. 13 Corrosion morphology and composition distribution of Inconel625 de-Mo molten layer specimen
(a)试样SEM (b)图a方框处放大(c)Ni(d)Cr(e)Mo(f)W
Fig. 14 Corrosion morphology and composition surface sweep distribution of molten layer
【Literature Information】
WANG Jincai,HUO Xiaoyang,LI Lei,et al.Study on laser melting properties of Inconel625 and its deMo and Alloy686 powder[J].Special Casting and Nonferrous Alloys,2022,42(10):1248-1254.)
WANG J C,HUO X Y,LI L,et al. Laser melting solidification properties of Inconel625,Mo free Inconel625 and alloy686 powders[J]. SpecialCasting & Nonferrous Alloys,2022,42(10):1248-1254.