Improvement of Electroplating Quality of Crystallizer Copper Tubes

Through the analysis of the problem of poor electroplating quality of copper tubes in the mold, combined with the actual production, the relevant parameters affecting the electroplating quality of the copper tubes in the mold were improved, which improved the electroplating quality of the copper tubes and met the production needs.

Keywords: mold steel tube; electroplating; process flow; quality parameters

1 Introduction

The mold copper tube is a vulnerable spare part in continuous casting production. The inner surface of the copper tube is chromium-plated. The chromium-plated layer has high hardness, wear resistance and good heat resistance. In 2003, in order to realize the professional production of crystallizer copper tubes within Tangshan Iron and Steel, the company’s electroplating workshop and the crystallizer copper tube workshop were merged to ensure the completeness and standardization of the crystallizer copper tube production process. With the continuous advancement of continuous casting technology and the continuous development of production, the shortcomings of the hard chromium plating process of mold copper tubes in the past have gradually been exposed; technical indicators are backward and product quality accidents are frequent, which seriously affects continuous casting production. In order to solve the above problems and improve the electroplating quality of the mold copper tubes, quality control issues such as copper tube coating roughness, coating adhesion and coating hardness, and coating buildup were improved and perfected in 2004. The process flow: degreasing → polishing → insulation → clamping → flushing → preheating → impurity removal → flushing → upper hanging fixture → plating → flushing → lower hanging fixture → post-plating treatment.

2 Improve content and process flow

2.1 Improve the roughness of chromium layer

The roughness quality of the chromium layer mainly depends on the composition of the bath liquid (see Table 1) and whether the process conditions meet the standards. Chromic anhydride is the main component in the chromium plating solution, and the concentration of chromic anhydride has a great influence on the properties of the coating. If the chromic anhydride concentration exceeds the standard, the chromium plating layer will turn white and have a fever. At this time, the electroplating bath liquid should be electrolyzed to reduce the chromic anhydride concentration. Sulfuric acid is a catalyst for chromium plating and promotes the deposition of chromium. The determination of its content is related to the ratio of chromic anhydride. When CrO₃/SO-2 is greater than 100:1, the SO-2 content is insufficient, the brightness of the coating and the current efficiency of the plating solution are reduced, large hydrogen bubbles are precipitated in the cathode area, the surface of the coating is rough, and black stripes are produced. When CrO₃/SO-2 is less than or equal to 50:1, the SO-2 content is high, the dispersion ability of the electrolyte is reduced, the deposition speed is slow, small hydrogen bubbles precipitate in the cathode area, and the coating becomes dark. In severe cases, there may be partial or complete absence of chromium deposition. At this time, an appropriate amount of barium carbonate or barium hydroxide should be added for neutralization to ensure the sulfate concentration. The plating solution is filtered three times a week. 3~3.5kg of chromic anhydride is added for every 5 tubes plated, and a test is performed once for every 20 tubes plated to ensure the composition of the bath solution. Different temperatures and current densities have a great impact on the appearance of the coating. At low temperatures and high current densities, the coating appears dark gray or burnt. The characteristics of this coating are high hardness, brittleness, and texture. At high temperature and low current density, the coating is milky white. This coating has a fine structure, good plasticity, low porosity, no cracks, and good protective performance, but has low hardness (HB250-750) and poor wear resistance. At medium temperature and medium current density, a bright coating can be obtained. This coating has high hardness (HB700-900). It has fine crystallization and a fine and dense network structure. Therefore, medium temperature and medium current density should be used under the condition that the appearance, quality and hardness of the coating can be ensured.

Table 1 Composition of chrome plating bath liquid

Chromic anhydride CrO₃Sulfuric acid H₂SO₄Temperature ℃ Current density A/dm²
hard chrome230~2702.3~2.755~6050~60

2.2 Improve coating adhesion and control coating hardness

The quality of coating adhesion mainly depends on the pre-plating treatment. The electroplating process is briefly summarized: pre-plating treatment → plating → post-plating treatment. Practice has proved that quality accidents such as shelling, blistering, or even failure of plating in the coating are not caused by the electroplating process itself, but due to improper and suboptimal pre-plating treatment. Whether the pre-plating treatment is appropriate plays a very important role in the quality of the coating. If the electroplating surface is not clean (with oxide film and oil stains), even under the most favorable electroplating conditions, the metal deposition will be uneven, and the bonding force will be obvious. decrease. Therefore, it is necessary to strengthen the pre-plating treatment to remove oil stains on the surface of the copper tube, and add a reverse plating process based on the molding degreasing (heating water friction with metallographic sandpaper) and impurity removal (sulfuric acid washing) process, which is more conducive to the adsorption of the chromium layer (Sulfuric acid washing only removes impurities on the surface of the copper matrix, and the reverse current in the reverse tank can remove deep impurities in the copper matrix). The advantages of the reverse plating process are to prevent impurities from adhering to the surface of the copper tube, remove surface oil stains and oxide layers caused by incomplete activation or other reasons; expose crystal particles, increase binding force, reduce contamination of the potion by copper impurities, and adjust the current Density, control bath temperature, find optimal process conditions, and improve chromium layer quality.

Tests show that as the oxygen content in the coating increases, the hardness of the chromium coating also increases accordingly. The mechanical wear resistance of chromium plating is very high, however this property is not necessarily due to high hardness alone. High hardness and high brittleness go hand in hand. For burnt hard chromium plating, its wear resistance is extremely poor, and it is very brittle and brittle. However, its hardness is as high as HB1050-1150. In fact, the hardness of the hard chromium layer with good wear resistance is often between HB700-900. Although hardness and wear resistance are the most important technical characteristics of chrome plating, and these two properties are related to a certain extent, they are not comparable. However, the hardness of chromium coating plays an extremely obvious role in maintaining the gloss of the coating for a long time and preventing the workpiece from being damaged and deformed due to high temperature.

Figure 1 Effect of temperature and current density on chromium layer

1-Milk white coating 2-Bright chrome layer 3-Gray white semi-bright chrome layer 4-Gray chrome layer

2.3 Inhibit chromium layer tumor

The principle of electroplating is a process that uses electrolysis to process the surface of parts. Chromium plating of copper tubes means that chromium ions are deposited on the inner surface of the copper tube under the action of direct current to form a uniform and dense metal coating. Use a copper tube as the cathode. Use lead, lead-antimony alloy and lead-tin alloy as the anode.

Anodic reaction CrO42-+8H++6e→Cr+4H₂O;2H++2e→H₂↑

The cathodic reaction is Cr₂O;+8H++6e→Cr₂O₃+4H₂O.

However, needle-like plating burrs will occur at the interface between the inner cavity of the copper tube and the upper and lower sections due to the unique electroplating process of the copper tube. Especially at the lower fracture of the inner cavity section of the copper tube, coupled with the deposition of the plating solution, electroplating nodules with a thickness of 3 to 4 mm are produced. The electroplating nodules are too thick. Coupled with the high wear resistance of the chromium layer, it seriously hinders the movement of the billet with the dummy bar during the steel drawing process.

This causes the cast slab to break, seriously affecting production. Improving the hanging tooling, changing the step-by-step operation method in the previous outdated process, and adopting the clamping method with the lower end of the copper tube facing up can avoid any defects in the plating of the small end of the copper tube in the crystallizer. Because the small end of the copper pipe is mainly dechromated during steel drawing, the clamping plate for fixing the anode adopts a U-shape on both sides. The clamping plate should be as small as possible, and it is better to not block the circulation convection of the plating solution at all. At the same time, the conductive method is improved, using soft copper rows and flexible fixed conductors immersed in insulating paint. This conductive method can increase the current density and make the crystallization fine, which is beneficial to improving the quality of electroplating. At the same time, add a thin iron plate to the upper section of the copper pipe and the hanger, the size of which matches the inner cavity of the copper pipe. And the best effect is to protrude the section 2~3mm, so that the nodules generated by the discharge on the upper section can be weakened by the iron plate.

3 Conclusion

Obtaining excellent performance of the chromium plating layer of copper pipes mainly involves many factors such as the performance of the copper base material, mechanical processing technology, heat treatment technology, design and manufacturing of electroplating racks, pre-plating treatment, hard chromium technology, and even the reasonable design of the electroplating production line. Analyze and study different chromium layer qualities. Find out the key points of the problem to continuously improve the quality of electroplating.

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