Ni/c-BN reinforced 316L stainless steel composite cladding layer was prepared on the surface of martensitic stainless steel (ZG06Cr13Ni4Mo) by laser cladding technology. The effects of different Ni/c-BN contents on the microstructure and corrosion resistance of the cladding layer were analyzed by XRD, SEM, EDS, electrochemical test and erosion test. The results show that the phase of the composite cladding layer is composed of FeCr0.29Ni0.16C0.06 solid solution. With the increase of Ni/c-BN addition, the number of equiaxed crystals in the top structure decreases and the number of cellular dendrites increases. The corrosion resistance of the composite cladding layer increases first and then decreases with the increase of Ni/c-BN addition. When the Ni/c-BN addition is 5wt.%, the corrosion resistance of the cladding layer is the best. Excessive Ni/c-BN causes pores on the surface of the cladding layer and a small amount of large-sized residual c-BN inside the cladding layer, which reduces the corrosion resistance and erosion resistance of the cladding layer.
Martensitic stainless steel has excellent hardness and wear resistance and is the main material for making turbine flow parts. However, under long-term liquid immersion and sediment erosion, martensitic stainless steel flow parts will undergo electrochemical corrosion and erosion, resulting in the need to replace the flow parts regularly. Therefore, surface modification technology is needed to improve the service life of the flow parts. Through laser cladding technology, a 316L austenitic stainless steel corrosion-resistant cladding layer is prepared on the surface of martensitic stainless steel, and nickel-coated cubic boron nitride (Ni/c-BN) is added to the cladding powder to increase the content of N in the cladding layer, so as to further improve the structure and corrosion resistance of the cladding layer.
This paper focuses on the corrosion resistance of martensitic stainless steel, studies the effects of different Ni/c-BN on the structure and corrosion resistance of 316L cladding layer, and explores the corrosion resistance mechanism of Ni/c-BN on the cladding layer.
1 Sample preparation and method
The cast ZG06Cr13Ni4Mo martensitic stainless steel was selected as the substrate, and the stainless steel cladding layer used aerosolized spherical 316L stainless steel powder with a particle size of 150~325 meshes. Its chemical composition is shown in Table 1. Ni/c-BN powder was purchased from Funike Superhard Materials Co., Ltd. with a particle size of 200~350 meshes. The nickel plating method used was chemical plating. The laser cladding equipment is RFL-A2000D coupled semiconductor laser. The test parameters are: laser power 1 100 W, scanning speed 6 mm/s, powder feeding rate 8 g/min, overlap rate 45%, and 316L austenitic stainless steel corrosion-resistant cladding layer with different Ni/c-BN contents (0, 5wt.%, 10wt.% and 15wt.%) is prepared. The overall length of the cladding layer is 150 mm and the width is 50 mm; the cladding layer has a melting height of 0.787 mm, a melting depth of 0.228 mm, a melting width of 2.555 mm, and a dilution rate of 22.5%. The effect of Ni/c-BN on the microstructure of the 316L cladding layer is analyzed by XRD, SEM, EDS and other methods.
The corrosion resistance of the sample was tested using a CHI660E electrochemical workstation. The sample surface was used as the working electrode, the platinum electrode as the auxiliary electrode, and the saturated calomel electrode (SCE) as the reference electrode. The polarization curve test was carried out in a 3.5% NaCl solution. The scanning potential range was -1.1~1.1 V, and the scanning speed was 1 mV/s. The erosion test was carried out using a slurry tank type rotary erosion tester. The angle between the erosion surface and the horizontal line was 45° as the erosion angle. The height of the plane where the sample was located from the bottom was 60 mm. The erosion liquid was distilled water. The erosion test conditions are shown in Table 2. The erosion rate was used to evaluate the erosion resistance of the substrate and the cladding layer, and the surface morphology after erosion was observed and analyzed. The erosion rate expression is shown in Formula 1: E=Δm/S*t (1) Where: Δm is the wear mass loss, g; S is the erosion area, m2; t is the erosion time, h.
2 Experimental results and analysis
2.1 Nondestructive testing results of cladding layer
Figure 1 shows the nondestructive testing results of the cladding layer surface. When the Ni/c-BN addition is less than 10wt.%, there are no defects such as pores and cracks on the surface of the cladding layer (Figure 1a, b); when the Ni/c-BN addition is 10wt.% and 15wt.%, a small number of holes appear on the surface of the cladding layer (Figure 1c, d). With the increase of Ni/c-BN addition, the N element in the molten pool increases, and the reaction produces N2, which is not discharged in time during the subsequent cooling process, thus forming pores.
2.2 Cladding layer phase analysis
Figure 2 shows the XRD spectrum of the 316L composite cladding layer with different Ni/c-BN additions. The composite cladding layer is composed of FeCr0.29Ni0.16C0.06 austenite structure, and no new phase is produced. This is because the addition of Ni/c-BN increases the content of N elements in the cladding layer, which can expand the austenite zone and stabilize the austenite, resulting in no precipitation of new phases such as carbides in the cladding layer. In addition, there is a preferred orientation in the grain growth of the 316L cladding layer. As the amount of Ni/c-BN added increases, the height of the diffraction peak of the FeCr0.29Ni0.16C0.06 solid solution changes, and the trend of the preferred orientation gradually weakens; when the Ni/c-BN addition is 15wt.%, the preferred orientation almost disappears.
2.3 Analysis of the cladding layer structure
Figure 3 shows the top microstructure morphology of the 316L cladding layer with different Ni/c-BN additions. The top structure of the 316L composite cladding layer without Ni/c-BN and with an addition of 5wt.% is mainly composed of fine equiaxed crystals and a small amount of cellular dendrites (Figure 3a, b); when the Ni/c-BN addition is more than 5wt.%, the number of cellular dendrites in the top structure increases, the number of equiaxed crystals decreases, and a small amount of residual c-BN particles appear (Figure 3c, d). The SEM results of the top of the 316L cladding layer with different Ni/c-BN additions are shown in Figure 4. When Ni/c-BN is not added, the austenite grains in the cladding layer are mainly cladding grains with a certain directionality (Figure 4a). The addition of Ni/c-BN refines the grains and causes black blocks to appear at the grain boundaries of the cladding layer (Figure 4b-d). EDS element analysis of the top structural characteristic area of the cladding layer is shown in Table 3. Points 1, 3, 5 and 8 are all inside the austenite grains, points 2, 4, 6 and 9 are grain boundaries, and points 7 and 10 are large black blocks.
As shown in Table 3, the element addition ratios of the top structures of the cladding layers with different Ni/c-BN additions are similar. The grains are rich in Fe, Cr, and Ni elements, which are FeCr0.29Ni0.16C0.06 solid solutions; Cr elements are only slightly segregated at the grain boundaries; the black matter mainly contains B and N elements, which are incompletely decomposed c-BN particles. Due to their low content, they are not detected by XRD. In addition, with the increase of Ni/c-BN addition, the N addition in the austenite in the cladding layer increases. Since the tendency of N element phase to segregate at the grain boundary is greater than that of C element, the crystallization and precipitation of carbon chromium compounds such as Cr23C6 are inhibited.
2.4 Corrosion resistance analysis of cladding layer
2.4.1 Electrochemical corrosion analysis
Figure 5 shows the Tafel polarization curves of the substrate and the 316L composite cladding layer with different Ni/c-BN additions in 3.5% NaCl solution. As shown in Figure 5, both the substrate and the cladding layer showed obvious passivation behavior in the corrosive solution, and the corrosion resistance of the cladding layer was better than that of the substrate; with the increase of Ni/c-BN addition, the corrosion resistance of the cladding layer showed a trend of first increasing and then decreasing. When the Ni/c-BN addition was 5wt.%, the passivation interval width of the cladding layer was the largest, and the cladding layer had the best corrosion resistance. This is mainly because the addition of Ni/c-BN increases the content of N in the cladding layer, which can inhibit the precipitation of intergranular chromium carbide and avoid the formation of chromium-poor areas, reduce the tendency of intergranular corrosion, inhibit the dissolution of the passivation film, and improve corrosion resistance. In addition, the addition of Ni/c-BN inhibits the growth of grains and plays a role in refining grains (Figure 4), further improving the corrosion resistance of the cladding layer. However, when the addition amount of Ni/c-BN is greater than 5wt.%, the pores on the surface of the cladding layer allow the corrosive liquid to enter the interior of the cladding layer (Figure 1c, d), increasing the corrosion area and forming micro-galvanic cells inside the cladding layer, increasing the corrosion rate and reducing the corrosion resistance of the cladding layer [10-11].
2.4.2 Analysis of cladding layer erosion test
The erosion wear rate of the substrate and the 316L composite cladding layer with different Ni/c-BN additions is shown in Figure 6. In acidic media, the average erosion rate of the cladding layer is lower than that of the substrate, and with the increase of Ni/c-BN addition, the erosion rate of the composite cladding layer first increases and then decreases, showing the same trend as the corrosion resistance. When the Ni/c-BN addition is 5wt.%, the erosion rate of the composite cladding layer is 33.3 g·m’-2·h’-1, which has the best erosion resistance.
Figure 7 shows the erosion surface morphology of 316L composite cladding layers with different Ni/c-BN additions in acidic media. The results show that the surface damage of the cladding layer without Ni/c-BN addition is more serious, with a large number of grooves and impact pits, accompanied by material extrusion (Figure 7a); when the Ni/c-BN addition is 5wt.%, the surface of the cladding layer is the smoothest as a whole, no cutting grooves appear, and a small number of small-sized impact pits are distributed on the surface (Figure 7b); when the Ni/c-BN addition is 10wt.%, the impact pits of the cladding layer increase, the size and depth increase, and the local material loss increases (Figure 7c); when the Ni/c-BN addition is 15wt.%, many black pits appear on the surface of the cladding layer (Figure 7d). During the erosion process, the surface of the cladding layer is simultaneously eroded and worn by the gravel in the erosion medium and corroded by the acidic medium. Therefore, the improvement of the corrosion resistance of the cladding layer can effectively improve its erosion resistance. However, when the addition amount of Ni/c-BN is greater than 5wt.%, the erosion resistance of the cladding layer decreases and the surface morphology deteriorates. On the one hand, due to the decrease in the corrosion resistance of the cladding layer (Figure 5), the resistance of the cladding layer surface to the acidic medium is reduced; on the other hand, the c-BN particles remaining in the cladding layer fall off during the erosion process, increasing the loss of material.
3 Conclusion
(1) The composite cladding layer is composed of austenite phase solid solution (FeCr0.29Ni0.16C0.06) and a small amount of undecomposed c-BN; the addition of Ni/c-BN weakens the preferred orientation of the crystals in the cladding layer.
(2) With the increase of Ni/c-BN addition, the top structure of the cladding layer is continuously refined, the number of columnar crystals and equiaxed crystals decreases, and the number of cellular dendrites increases.
(3) With the increase of Ni/c-BN addition, the corrosion resistance and erosion resistance of the composite cladding layer both increase first and then decrease. The composite cladding layer with a Ni/c-BN addition of 5wt.% has the best corrosion resistance.
