The traditional electroplating process for repairing the surface of hydraulic support columns has problems such as high labor intensity, serious electroplating pollution and short life. The traditional electroplating process for repairing hydraulic support columns is studied. The shortcomings and disadvantages of the electroplating process are pointed out, and the laser cladding repair process for hydraulic support columns is proposed. The experimental research method is adopted, and the piston rod of the hydraulic support column is used as the test object. The advantages and disadvantages of laser cladding repair technology and traditional electroplating technology are compared and analyzed, and the hardness, corrosion resistance and service life of the parts after the two repair processes are studied. The results show that:
The surface hardness of the traditional electroplating process is 25~30HRC, while the hardness value of the laser cladding sample reaches 50~62HRC, the surface hardness value is high, the wear resistance, corrosion resistance and high temperature resistance are better, the service life of the equipment is extended, and a satisfactory comparison effect is achieved, which verifies the advantages of laser cladding technology.
Hydraulic support is the most important electromechanical equipment in coal mines, and it is also the key equipment to achieve effective support for the roof and coal wall of the fully mechanized working face in coal mines. It plays a very important role in the efficient mining process of coal mines. Hydraulic supports are important bearing components in coal mines. During the support process, the columns of hydraulic supports are directly exposed to the outside. Affected by the harsh working conditions in coal mines, they are in a humid, dusty and acid-base corrosive environment for a long time. Column parts such as hydraulic support columns are very prone to corrosion and scratch failures. The occurrence of these faults will directly affect the sealing effect of the entire cylinder piston rod, and will also affect the supporting force, and may even cause pressure relief in the cylinder, unable to ensure sufficient supporting force, and bury safety hazards for reliable support in coal mines. For this reason, it is necessary to regularly repair the surface of hydraulic support columns. The traditional repair method is to electroplating a layer of wear-resistant material chrome on the surface of the hydraulic support column. This chrome plating process has a relatively short service life and often causes surface shedding, thereby shortening the service life of the entire hydraulic support column. In addition, the chrome plating process will also cause certain pollution to the environment.
With the continuous improvement of environmental protection requirements in all walks of life in my country, it is urgent to change the traditional pollution-prone process in the repair process of hydraulic support column surface wear, and find new column repair technology to extend the service life of the column and ensure that it does not pollute the environment. To this end, this paper proposes laser cladding technology to repair the surface of hydraulic support columns, forming high-performance solid alloys on the surface to improve the strength and hardness of the surface, meet the requirements of on-site use, and avoid pollution and damage to the environment. After comparison, laser cladding technology has obvious advantages and effects over traditional electroplating technology.
1 Analysis of traditional electroplating process for repairing hydraulic support columns
Hydraulic support is an important mechanical equipment in the comprehensive mining working face of coal mines. Hydraulic support columns are very prone to wear during use. The traditional wear treatment process is electroplating. The electroplating process flow is shown in Figure 1.
From the analysis of Figure 1, it can be seen that the electroplating process for repairing hydraulic support columns requires multiple processes, including inspection before electroplating, installation of hangers, chemical degreasing, water washing and reverse engraving, followed by electroplating of hydraulic support columns. In the actual electroplating process, the quality of electroplating needs to be controlled. It can be seen from the requirements of on-site electroplating process that when electroplating the hydraulic support column, it is necessary to first grind the Q235 steel plate to control the surface thickness to 0.4 μm, then electroplate a layer of 30μm copper-tin alloy on the surface, and then electroplate a layer of 40μm hard chrome alloy on the surface, and the total thickness of the external plating alloy reaches 70μm. The traditional electroplating process adds a certain strength of hard chrome alloy to the surface of the hydraulic support column, so that the hardness of the hydraulic support column itself is improved to a certain extent. However, the electroplating process will cause certain pollution to the environment, and the electroplating chromium technology is a surface treatment technology that uses an electrolytic process to deposit chromium on the surface of the substrate to form a chromium plating layer. This method is to repair the column by physically combining the plating layer and the base part, which is very easy to cause shedding and bulging. In many cases, the effect of electroplating is relatively poor, and the time of each process needs to be guaranteed, otherwise the quality of the final electroplating will be poor. In order to ensure the life of the hydraulic support column and reduce the impact on the environment, the traditional electroplating process cannot meet the current requirements.
2 Principle and repair process of laser cladding technology
Laser cladding technology is a new type of surface improvement technology proposed by my country, which is widely used in the repair of coal mine hydraulic supports. This technology can strengthen the outer surface of the hydraulic support column and piston rod. From the principle of laser cladding technology, laser cladding uses a high-energy laser beam as a heat source to melt the alloy powder with high strength and good corrosion resistance together with the outer surface of the hydraulic support column and jack, and produce a laser cladding layer with strong wear resistance and corrosion resistance through metallurgical reaction. After the laser cladding layer is formed, the service life of the column can be extended. The laser cladding process route is shown in Figure 2.
From the analysis of Figure 2, it can be seen that the laser cladding process first requires surface pretreatment of the substrate, including rust removal and cleaning of the surface, and then preheating after pre-setting the cladding material, melting the material after laser scanning, and then adding the high-strength alloy to the molten surface in the form of wire or powder after the material is melted, so as to form a layer of coating material on the surface to improve the hardness and strength of the surface. Laser cladding technology is a new surface improvement technology that can strengthen the surface of hydraulic support columns, improve the corrosion resistance and wear resistance of the surface, and thus increase the life of the hydraulic support columns. The key process requirements for laser cladding repair are as follows:
(1) Surface pretreatment process. The surface needs to be cleaned and the bent and deformed columns need to be corrected to ensure that the bending degree is ≤2%. When pretreating the surface, it is necessary to ensure that the accessories on the surface do not affect the laser cladding before the surface can be cleaned.
(2) During laser cladding, the single-side thickness must be greater than 0.3mm, and the hardness of the cladding surface must meet 48~58HRC. The surface hardness after cladding must be relatively uniform, and defects such as hard thorns, bubbles, and burns are not allowed. The diameter deviation of the repaired column must be guaranteed to be less than 8mm.
From the perspective of laser cladding process, it can be seen that the use of laser cladding technology uses a high-power laser beam to achieve energy concentration, instantly melt the surface of the hydraulic support column, so that the alloy powder and the workpiece surface can be fully integrated together, so the bonding force is relatively strong. The wear resistance and corrosion resistance of the workpiece surface after laser cladding treatment are significantly improved, and the accuracy of the workpiece after laser cladding basically meets the use requirements, and the accuracy requirements of the workpiece are always maintained during the processing.
3 Comparison of the effects of traditional electroplating process and laser cladding
Through the analysis of the traditional surface electroplating process and laser cladding treatment process of hydraulic support columns, it can be seen that the laser cladding treatment of hydraulic support has certain advantages. In order to verify the effect of traditional electroplating process and laser cladding technology on the surface treatment of hydraulic support columns, the piston rod of the hydraulic support column was selected as the test object, and the same material and piston rod were uniformly selected. The size of the piston rod was Φ300mm×1200mm, and the two repair processes were ensured in the same working environment. The following experiments were carried out.
(1) Surface electroplating treatment. The traditional electroplating process was used to treat the piston rod of the hydraulic support column. The specific electroplating process flow is: polishing → hanging → degreasing and degreasing → water washing → electrolytic polishing or chemical polishing → pickling and activation → pre-plating → electroplating → water washing → post-treatment → water washing and drying. During the entire electroplating process, each link needs to be strictly controlled to ensure the quality of the surface after electroplating.
(2) Laser cladding technology treatment. HUIRUI UHS-3000W laser cladding machine was selected, and the cladding powder was selected as the hydraulic support special powder JG-3, whose main chemical components are Fe, C, Cr, Mn, Si, Ni, etc. The C content of the cladding powder used was 0.07%, and the Cr content was 18.3%. The operating process parameters of the laser cladding machine were adjusted: the power was 1460kW, the scanning speed was 5mm/s, the protective gas flow rate was set to 15L/min, and the powder feeding amount was set to 4.6g. The hydraulic support column piston rod was laser clad. The laser cladding machine equipment site is shown in Figure 3.
Hardness test was performed on the coatings under different surface treatments. The HXD-1000 microhardness tester was used to measure the surface hardness of the two processes, and the MLS-25 wear tester was used to detect the wear of the piston rod surface obtained by the traditional electroplating process and laser cladding technology; the LJ-60 salt spray tester was used to conduct the surface salt spray corrosion experiment, and the surface was sprayed continuously for 1800 hours to observe the pitting on the surface. The high temperature tester was used to test the hydraulic support column after electroplating and laser cladding treatment. By observing the data after the test, the performance comparison results of the traditional electroplating process and laser cladding can be obtained as shown in Table 1.
Through the test, it can be concluded that the surface hardness of the traditional electroplating process is 25~30HRC, while after laser cladding, the material is more tightly bonded and the hardness value reaches 50~62HRC; the powder used in laser cladding has a low carbon content and the cladding layer on the surface is fine after the cladding technology. In addition, the corrosion resistance is verified by using salt spray corrosion experiments. The surface is sprayed with salt spray continuously to simulate the chemical corrosion under real working conditions. After 1800h, the two surface conditions are observed. It can be seen that there are a lot of rust spots on the surface of the electroplated sample, and the corrosion is very serious. It is observed that there are only small rust spots on the surface of the laser cladding sample, and during the experiment, the electroplated sample first began to have rust spots at 36h, while the laser cladding sample did not have rust spots until 256h. It can be seen that the sample obtained by laser cladding has better resistance to chemical corrosion. The comparison of experimental results shows that the surface hardness value after laser cladding is high, and the wear resistance, corrosion resistance and high temperature resistance are better.
4 Conclusions
(1) The traditional electroplating process technology is to deposit chromium elements on the surface of the substrate to strengthen the substrate. The electroplating process is prone to fall off and bulging, and the surface effect after electroplating is poor.
(2) The laser cladding surface improvement technology was designed and the laser cladding process route was formulated. Due to the low powder content and the fine cladding layer on the surface after cladding, the theoretical analysis shows that the use of laser cladding technology effectively extends the service life of the hydraulic support column.
(3) The laser cladding technology and process technology were used to treat the surface of the hydraulic support column through two groups of samples. The results show that the surface hardness of the hydraulic support column piston rod after laser cladding reached 50~62HRC, while the surface hardness after traditional electroplating was only 25~30HRC. It is concluded that the surface hardness value after laser cladding is high, and the wear resistance, corrosion resistance and high temperature resistance are better. Laser cladding technology is more environmentally friendly, and the life of the hydraulic support column after laser cladding is more than 5 times that of traditional electroplating: it has strong promotion and application value.
