English 
Spanish 
The hydraulic cylinders of large forging equipment, such as our 100MN and 40MN hydraulic presses, are prone to surface scratches and damage on the piston rods, leading to oil leaks, reduced pressing accuracy, and even debris contamination of the entire hydraulic system.
Due to the large size of this forging equipment, disassembling the cylinders is labor-intensive and time-consuming. Furthermore, problems like scratches are difficult to repair quickly. To address this recurring problem, our maintenance personnel have exhausted all available resources, utilizing their own maintenance capabilities and resources to repair some of these issues, enabling them to meet operational requirements.
Oversized plunger cylinders or cylinders with severe scratches or damage require the services of a professional cylinder repair company.
Combined with our maintenance methods, we briefly summarize the repair methods of hydraulic cylinder piston rod (plunger) surface damage, hoping to bring you some help!
Based on the piston size requirements, there are two main methods: repair by restoring size and repair by reducing size. The main implementation methods include machining, welding (surfacing, repair welding, brazing, etc.) and electroplating (low-temperature iron plating, chromium plating), polymer composite material spraying, etc.
Restoration method
1. For small, non-concentrated, and relatively small areas of localized corrosion on the piston rod, use stainless steel argon arc welding to repair the rusted area, followed by grinding and polishing. This method is simple, inexpensive, and can meet the requirements of use.
2. For piston rods with localized plating loss, shallow rust pits, and large areas, repair by stripping the chrome layer, polishing, and re-chrome plating. Tolerance requirements must be maintained after electroplating. If deep pits are observed in individual areas after polishing, brazing can be used to repair the area, followed by manual polishing. After repair, chrome plating is performed, followed by hard chrome plating (coating thickness 0.03-0.05mm) and polishing. For manual repair, it is best to machine a grinding sleeve based on the piston rod’s outer diameter and grind the repaired area to a surface roughness of Ra 0.1-0.2μm (although this is difficult).
Note: Advantages and disadvantages of commonly used welding repair methods
(1) Repair welding: Repair welding is when welding technology is used to repair local defects in components. Repair welding is characterized by ease of application, low cost, and a short repair time. Appropriate repair materials and processes should be selected based on the material type. For ordinary carbon steel, the repair welding method should be determined based on the material’s carbon equivalent (not its carbon content). For stainless steel, cast iron, aluminum, and aluminum alloys, repair welding requires particular attention to the material’s properties and the workpiece’s operating environment. A detailed analysis of substrate issues should be conducted, and careful consideration should be given to pre-weld treatment, welding, and post-weld treatment methods and application parameters.
Since repair welding is a specialized form of welding, the process inevitably creates a molten pool (generating localized high temperatures) at the repair site. Uneven heating from the molten pool to the workpiece inevitably generates thermal stresses in the weld zone and heat-affected zone, leading to defects such as deformation, cracking (such as cracking in cast iron and high-carbon steel parts), localized hardening, microstructure changes, and decreased fatigue performance. Repair welding can also lead to porosity, phase changes, and reduced mechanical properties in and around the molten pool.
(2) Brazing: To reduce the welding temperature during repair welding, hot melt welding is performed using a solder with a lower melting point, commonly referred to as brazing. The key difference between repair welding and brazing is that no molten pool forms on the workpiece during brazing. During the brazing process, only the brazing filler metal (which has a lower melting point) melts, while the substrate itself does not actually melt. The molten brazing filler metal adheres to the substrate and forms a repair layer at the brazed location. If the brazing filler metal and flux are properly selected, micro-diffusion between the filler metal and the substrate helps to strengthen the bond between the brazing layer and the substrate.
Therefore, compared to fusion welding, brazing reduces the thermal impact on the workpiece, minimizes part deformation, and significantly reduces mechanical properties. Currently, many people use a combined brazing and brush plating technique to repair dents. This method involves first brazing a tin-bismuth alloy filler metal (melting point 135-140°C), then scraping and brushing it with a wear-resistant coating to achieve the desired repair.
The major disadvantage of brazing is a soft, low-strength solder layer. Improper brazing filler metal or flux can lead to a weak bond between the brazing layer and the substrate. To improve the bonding strength between the brazing layer and the substrate, materials with casting defects or prone to oxide film formation (stainless steel, aluminum, and its alloys) should be brush-coated with copper before brazing, followed by a tin-bismuth alloy. The purpose of copper plating is to improve the brazing properties of the substrate.
(3) If the piston rod is severely corroded and de-chrome polishing does not meet the requirements for electroplating, the piston rod should be straightened, ground, and turned. Once the ground size is 0.5mm smaller than the original size and meets the electroplating requirements, copper plating followed by chrome plating can be used to meet the requirements.
If this method still fails to meet the requirements, the original size should be reduced by 4-5mm, then restored to its original size using thermal spraying or iron plating, followed by grinding and chrome plating. This method is costly and expensive. Moreover, when chrome is plated after iron plating, the chromium layer is easy to fall off and the spraying quality is difficult to control.
(4) Currently, some high-performance polymer composite materials are also used to repair minor piston scratches. High-performance two-component epoxy composite materials reinforced with nano-inorganic materials adhere well to the piston surface and offer excellent resistance to high temperatures and chemical corrosion. After spraying, they can be quickly machined. The technology is simple to operate, provides high-quality repairs, saves time, and is cost-effective.However, for high-pressure cylinders and cylinders with rapid and frequent reversing, the spray coating may peel off under pressure shocks. Therefore, use depends on the operating conditions.
Downsizing repair method
For pistons with severe surface corrosion or scratches, direct machining and downsizing can be used for repair. Generally, the diameter reduction involves correction, turning, and grinding of the piston rod. Even if the diameter is reduced by 5mm, the electroplating requirements can be met, and the repair cost is low, ensuring that the required performance is met.
A 5mm reduction in the piston rod is generally considered standard for the cylinder piston rod series, and standard seals can be used. If this method is used for repair, the piston sleeve must be redesigned and reassembled with the appropriate seals. (Remember: Be sure to save this data to the equipment file for future repairs.)
The above is a brief sharing of our practical work experience. I hope you will pay attention and offer your feedback.
The little bits of experience we gain from our work are like the stars in the night sky: seemingly insignificant, but when they come together, they illuminate our path forward.
