As coal mining conditions become increasingly harsh, laser cladding technology with excellent performance is widely used in the remanufacturing of hydraulic support cylinders. However, there are many types of cladding powders on the market, and the chemical composition and preparation process are obviously different, which leads to obvious fluctuations in the performance of laser cladding layers. Four commercially available iron-based cladding powders were selected as raw materials, and iron-based cladding layers were prepared on the outer wall of 27SiMn hydraulic cylinders using laser cladding technology. The composition of cladding powders, microstructure and corrosion resistance of cladding layers were analyzed. The results show that the commercially available cladding powders are mainly made by water atomization and gas atomization, and the composition differences are mainly Mo and Nb elements; the four laser cladding layers are mainly martensite, with a small amount of residual austenite and carbon/boride. At the same time, the microstructure of the cladding layer changes significantly in the thickness direction, from the planar crystal zone and columnar dendrite zone above the fusion line to the equiaxed dendrite zone on the upper part of the cladding layer, and the composition segregation is also alleviated. The corrosion resistance test found that the corrosion resistance of the cladding layer is affected by the chemical composition and processing technology: first, the processing defects become weak corrosion areas; second, with the increase of Mo element, the corrosion resistance of the cladding layer is improved.
Hydraulic support is an important part of coal mining equipment and has been in service for a long time in corrosive mine atmospheres such as Cl- and H2S. During the coal mining process, the hydraulic support is repeatedly loaded and unloaded, resulting in long-term corrosion-wear coupling damage to the hydraulic cylinder, which becomes the main component affecting the service safety of the hydraulic support. For the damage of the hydraulic cylinder, remanufacturing becomes an effective way to restore its function, especially laser cladding technology has become the first choice for repairing and remanufacturing of failed hydraulic cylinders. For the laser cladding remanufacturing of hydraulic cylinders, cladding equipment, process and powder are the main factors affecting the performance of the cladding layer. Especially under the conditions of selected equipment, suitable cladding powder becomes the key factor in determining the performance of the cladding layer. However, the development time of laser cladding remanufacturing technology for hydraulic cylinders is short, and there are many problems such as insufficient technical accumulation and inconsistent cladding powder system, which have caused obvious fluctuations in the performance of the cladding layer. Therefore, it is crucial to explore the differences between different cladding powders and analyze their effects on the microstructure and performance of the cladding layer in order to improve the service safety of hydraulic cylinders. In view of this, this paper selects four commonly used hydraulic cylinder iron-based cladding powders as raw materials to analyze the differences in their chemical composition and their effects on performance. Subsequently, the iron-based cladding layer was prepared on the outer wall of the 27SiMn hydraulic cylinder using laser cladding technology to study the effects of the material system on the microstructure and corrosion resistance of the cladding layer, and to explore the evolution relationship between the material composition and its performance, in order to obtain an iron-based alloy cladding layer with excellent service performance.
1 Experimental Materials and Methods
1.1 Experimental Materials
Four commercially available hydraulic support cladding powders were selected as raw materials (named X1, X2, X3 and X4, respectively), and the chemical composition of the four powders was analyzed using an X-ray fluorescence spectrometer and an elemental analyzer, as shown in Table 1. The results show that the four iron-based powders all use Cr as the main alloying element, accompanied by different contents of Ni, Mo, Nb, Si, B and other elements. Combined with literature analysis, when the Cr content reaches about 12 wt%, the pitting potential of stainless steel changes from negative to positive. As the Cr content continues to increase, the pitting potential continues to move positively. This is because Cr can form a stable and dense protective film on the surface of the steel, and the passivation effect produced can prevent the steel from further corrosion. Ni is an excellent corrosion-resistant element and an indispensable element in super martensitic stainless steel. It is an element that forms austenite in steel [14]. In a corrosive environment, Mo can promote the passivation of Cr, improve the corrosion resistance of chromium-nickel stainless steel in sulfuric acid, hydrochloric acid, and phosphoric acid, and effectively inhibit the pitting tendency of chloride ions. As a strong carbide-forming element, Nb can form stable and fine carbides and nitrides at high temperatures, which significantly inhibits grain growth, refines grains, and pins grain boundaries. In addition, the addition of Nb also inhibits the formation of intergranular chromium carbides and improves intergranular corrosion performance. Finally, elements such as Si and B mainly improve the mechanical properties of the material.
1.2 Preparation of cladding layer
27SiMn hydraulic cylinder (GB/T 700-2006) was selected as the base material. First, the outer surface of the 27SiMn hydraulic cylinder was ground and cleaned for standby. Subsequently, the cladding layer was prepared using the JGRFJ-10000 laser cladding equipment. The laser was a 10 kW semiconductor fiber-coupled laser, and the laser spot was a rectangular spot of 30mm×2mm. The specific cladding process was: laser power 9500W, scanning speed 8mm/s, overlap rate 45%, and powder feeding speed 60 g/min.
1.3 Characterization of cladding layer
After grinding and polishing the laser cladding sample, it was cleaned with ultrasound and dried with a cold air blower for standby.
The XRD-7000 X-ray diffractometer (XRD) was used for phase composition analysis. The measurement parameters were: target Cu-Ka ray, scanning voltage 40kV, scanning current 40 mA, step length 0.02°, scanning rate 5°/min and diffraction angle 20°～90°. The microstructure morphology of the longitudinal section of the cladding layer was observed using an Olympus GX53 inverted optical metallographic microscope (OM) and a JSM-6390A scanning electron microscope (SEM), and the composition was analyzed using an energy dispersive spectrometer (EDS) with a scanning electron microscope.
The electrochemical performance of the cladding layer was tested using a Wuhan Koster electrochemical workstation. The electrochemical parameters were: 3.5wt% NaCl solution, sampling frequency 10 Hz, scanning speed 0.3mV/s, the sample was the working electrode, the saturated calomel electrode (SCE) was the reference electrode, and the platinum sheet was the auxiliary electrode. The hardness of the material was measured using a TMHVS-1000-XYZ microhardness tester. The friction coefficient and wear rate of the cladding layer in a 3.5wt% NaCl medium were measured using an MRT-R4000 friction and wear tester. The test conditions were: friction pair GCr15 steel ball (quasi 6mm, 63 HRC), wear scar length 10mm, load 30N, frequency 2Hz, time 60min.
2 Results and discussion
2.1 Microstructure of iron-based cladding layer
Four iron-based cladding layers were prepared using laser cladding technology, and the phase composition of the cladding layers was analyzed using XRD, as shown in Figure 1. It can be seen that the four iron-based cladding layers are composed of α’ martensite phase and residual austenite phase, which indicates that the cladding structure of the four cladding layers is a martensite/austenite composite structure. Secondly, from the perspective of chemical composition, although the carbon content Wc of the four powders X1, X2, X3 and X4 is less than 0.2wt%, due to the high content of alloy elements (22wt%~25wt%) and the extremely fast cooling rate of laser cladding, the austenite formed after the solidification of the iron-based powder melt has high high-temperature stability, and is easy to transform into martensite at low temperatures. However, due to the phase transformation stress and thermal stress of the austenite transformation process to martensite, the martensite phase transformation has a certain inhibitory effect, resulting in part of the austenite unable to transform into martensite and existing in the cladding layer as residual austenite. Secondly, during the cladding process, the alloy powder melts to form a multi-component system such as Fe-C-Cr-Ni-B, and physical metallurgy and chemical metallurgy occur between the elements. In addition to forming an iron-based supersaturated solid solution (α’ phase), some C, Cr, and B elements form alloy carbon/boride phases.
Figure 2 is the metallographic structure diagram of the cross section of four iron-based cladding layers. It can be seen that the thickness of the four cladding layers after machining is 385, 630, 520, and 470 μm, respectively. According to the crystal growth morphology, the four cladding layers all have obvious columnar crystal structures, and the characteristics of X4 are more obvious. There is a long dendrite structure that runs through the cladding layer, which causes the component segregation and directionality of the mechanical properties of the cladding layer. In addition, the four cladding layers all show that except for the fusion line, they are mainly martensite, while the top is martensite and residual austenite. The results show that the coexistence of martensite and residual austenite causes microscopic galvanic corrosion in the cladding layer, which leads to the deterioration of the corrosion resistance of the cladding layer.
The point composition of different cladding layers was analyzed by EDS, as shown in Table 2. The results show that the chromium content around the fusion line, such as X1-2, X1-3, X2-2 and X2-3 of the cladding layer, is significantly different. Along the depth direction to the surface of the cladding layer, it was found that the segregation of chromium elements at positions X1-6, X1-7, X2-6 and X2-7 of the cladding layer was significantly slowed down, which is beneficial to reducing corrosion caused by component segregation.
2.2 Hardness and corrosion resistance of iron-based cladding layer
2.2.1 Hardness
The hardness of the cladding layer is related to the microstructure of the cladding layer. Figure 3 is a hardness curve of four cladding layers X1, X2, X3 and X4 and 27SiMn steel substrate. It can be seen that the hardness of the four cladding layers varies significantly along the depth direction. The surface hardness of the X4 cladding layer is the highest, and the hardness of X2 and X3 is constant. At the same time, it can be seen that the hardness of X4 decreases rapidly, while the hardness of X3 decreases rapidly. This means that differences in the thickness of the cladding layer and the depth of the heat-affected zone will affect the corrosion resistance and wear resistance of the cladding layer.
2.2.2 Electrochemical corrosion behavior
Self-corrosion voltage is an indicator for evaluating the corrosion tendency of a material. The larger the self-corrosion potential, the smaller the corrosion tendency. Figure 4 and Table 3 are the polarization and impedance curve results of different iron-based cladding layers. As shown in Figure 4 (a) polarization curve and Table 3, the self-corrosion potential of X2 cladding layer is the largest, the self-corrosion potential of X1 cladding layer is close to that of X4 cladding layer, and that of X3 is the smallest. The increase of self-corrosion potential indicates that the corrosion tendency of the three cladding layers is weakened. Secondly, self-corrosion current is an indicator for evaluating the corrosion rate of materials. The smaller the self-corrosion current, the smaller the corrosion rate. As shown in Table 3, the self-corrosion current of X2 cladding layer is the smallest, the self-corrosion current of X4 cladding layer is slightly smaller than that of X1 cladding layer, and that of X3 is the largest. It can be seen that X2 cladding layer has a lower corrosion rate. Therefore, considering the self-corrosion voltage and self-corrosion current results of the polarization curve, it can be seen that X2 cladding layer has better corrosion resistance. In addition, impedance is also one of the indicators to characterize the corrosion resistance of materials. The higher the impedance value (radius of the impedance arc), the stronger the ability of the material to withstand the corrosive environment. From the impedance curve of Figure 4 (b), it can be seen that the impedance value of the X2 cladding layer is the largest, followed by the impedance values of the X1 and X4 cladding layers, and the impedance value of the X3 cladding layer is the smallest, indicating that the corrosion resistance of the four cladding layers is significantly different.
2.2.3 Salt spray corrosion performance
Figure 5 is a macroscopic morphology of the four cladding layers under 1000 h of neutral salt spray conditions. It can be seen that the X3 cladding layer has relatively dense brown corrosion points; the X1 and X4 cladding layers have relatively few brown corrosion points on the surface; the X2 cladding layer has a smooth surface and only a small number of corrosion points. In summary, the X2 cladding layer has the best corrosion resistance, which is consistent with the electrochemical test results.
The average content of Cr in the X1 cladding layer is relatively high, but the chromium carbide formed between dendrites causes the intracrystalline chromium content to be far lower than the average content, thereby reducing the corrosion resistance of the cladding layer. The average content of Cr in the X4 cladding layer is similar, and it contains a higher Ni element, which improves the corrosion resistance. However, the corrosion resistance of the X3 cladding layer is slightly weaker due to component segregation. Secondly, the content of Mo in the X2 cladding layer is relatively high, and Mo has the function of purifying the grain boundary, forming a stable and dense passivation film during the corrosion process, thereby improving the corrosion resistance of the cladding layer.
According to “GBT 6461-2002 Rating of samples and specimens of metal and other inorganic coatings on metal substrates after corrosion tests”, the neutral salt spray corrosion sample rating was carried out. The results show that after 1000 h of neutral salt spray corrosion, the corrosion resistance of the X2 cladding layer is obvious, while the corrosion resistance of the X3 cladding layer is poor, and the corrosion resistance of X1 and X4 is close. Therefore, the chemical composition and phase composition of the material have a significant effect on the corrosion resistance of the cladding layer. At the same time, the comparison results also show that optimizing the cladding process and obtaining a cladding layer with fewer defects is the key to improving the protective performance of the cladding layer.
In summary, under the same cladding process conditions, the microstructure and chemical composition of the material have a close influence on the corrosion resistance of the cladding layer. Therefore, combined with the phase and chemical composition analysis, the following conclusions can be drawn: (1) The cladding layer should minimize the residual austenite, especially avoid the appearance of δ ferrite, and reduce the tendency of intergranular corrosion;
(2) The chemical composition of the cladding layer should contain a certain amount of Mo element to reduce the chromium depletion caused by sensitization and further improve the corrosion resistance of the cladding layer;
(3) The cladding process needs to be further optimized to eliminate defects such as cracks and pores in the cladding layer to ensure that there are no weak areas.
3 Conclusion
Four commercially available iron-based cladding powders were used as raw materials. The iron-based cladding layer was prepared on the outer wall of 27SiMn hydraulic cylinder by laser cladding technology. The composition of different cladding powders, the microstructure of the cladding layer and the corrosion resistance were analyzed. The following main conclusions were obtained:
(1) The commercially available cladding powders are mainly made by water atomization and gas atomization. The composition difference is mainly Mo and Nb elements.
(2) The four laser cladding layers are mainly martensite, with a small amount of residual austenite and carbide. At the same time, the microstructure of the cladding layer changes significantly in the thickness direction, from the planar crystal zone and columnar dendrite zone above the fusion line of the cladding layer to the equiaxed dendrite zone on the upper part of the cladding layer, and the composition segregation is alleviated.
(3) The corrosion resistance of the cladding layer is affected by processing defects and chemical elements, and can be modified from two aspects. First, by improving the process to reduce cladding defects; second, for cladding layers dominated by pitting corrosion, the powder composition can be adjusted to make the cladding layer more martensitic, and the content of pitting corrosion resistant elements can be appropriately increased.
