Preparation And Research Of High Adhesion Thick Coating

Rolls are bulk consumable tools in metallurgical production,and their quality and life have an important impact on product quality,production efficiency and cost.Repairing technology applied to repair the failed roll not only produces huge economic benefits,but also reduces the carbon emission in the casting process of the roll,which has good social benefits.However,the current surfacing technology can repair steel rolls with good weldability,but high alloy steel and cast iron rolls cannot be repaired.In this paper,the prefabricated coating was prepared on the ductile iron substrate by the soluble anode high-speed nickel brush plating process,and then the coating was strengthened by electrical contact strengthening technology.In view of the difficulty of thickening the cast iron substrate and the inability to strengthen the thick coating made of alkali copper.It is proposed an experiment plan.It is to first brush a 0.2mm thin nickel coating,after strengthened by electrical contact strengthening technology,and then perform brush plating to prepare a 1.4mm thick coating,and finally strengthen the coating again.Via this technical solutions,a 1.6mm thick coating on cast iron has been successfully prepared.Undertaking the coating analysis before and after electric contact strengthening,it has been found that the cracks,holes and other defects generated in the process of preparation pressed,and the structure became denser.Non-dense zone only had a little residue on the outermost layer with a thickness of 0.2mm,which could be removed by processing.Mutual fusion and bridging appear at the interface between the coating and the substrate,indicating a metallurgical bond.The success of this study provides a new technical approach for the repair of cast iron rolls.In the process of research on high-bond thick coatings on ductile iron substrates,the main findings and achievements are as follows:（1）It is impossible to directly brush a thick coating on cast iron with a special nickel primer and soluble anode nickel coating.The main manifestation is that when the thickness of the coating exceeds 0.2mm,the thickening is very slow,and phenomena such as peeling,dropping,and slag appear.Electrochemical tests show that the corrosion potential of ductile iron in special nickel solution is-0.376V,and the corrosion current density is 1.067x10^-4m A·cm^-2.It shows that ductile iron has poor corrosion resistance in special nickel solution,and is weakly combined with the substrate,so it is easy to peel off.（2）The use of alkali copper as a primer can improve the corrosion resistance of ductile iron in soluble anode solution,and can prepare a thick nickel plating layer.The test proves that it is suitable when the brush plating voltage is 8V,and the brush plating temperature is kept at the range from45℃to 65℃.When the surface burrs increase and it cannot continue to thicken,the method of brushing and polishing is adopted to make it thicker.In the test,a nickel-plated layer with a thickness of 1.6 mm was prepared,which has good process stability and repeatability.（3）The thick coating prepared with alkali copper primer is not suitable for electrical contact strengthening,and the coating will be arched,cracked and peeled off.Annealing test and theoretical analysis confirmed that the expansion coefficient of cast iron,copper and nickel is12.2x10^-6 m/℃,17.5x10^-6 m/℃and13.0x10^-6 m/℃respectively.When heated,the expansion between the coating and the substrate is not the same.High thermal stress will be generated,and when the strength limit is exceeded,it will lead to cracking and peeling of the coating.（4）A technical scheme of using special nickel as primer,first brushing 0.2mm nickel coating,strengthening by electrical contact strengthening,and then brushing thick nickel coating is proposed.It overcomes the technical problem above and successfully prepares a 1.6mm thick coating with high bonding force on cast iron,and there was no phenomenon of arching and peeling during electrical contact strengthening.Analysis the tissue before and after electrical contact strengthening found that:（1）Electrical contact strengthening can effectively improve the structure of the brush-plated thick coating.The structure of the brush-plated thick coating above 0.2mm is divided into three areas:0.4mm"crack area"above the coating after 0.2mm electrical contact strengthening,0.6mm"pore area"in the middle,and 0.4mm"loose area"on the surface.These are inherent defects of brush plating thick coatings,which have been analyzed and discussed in previous studies.After electrical contact strengthening,the regional characteristics basically disappear,the defects are pressed and the structure is dense,which is able to meet the requirements of ductile iron rolls.Non-dense zone only had a little residue on the outermost layer with a thickness of 0.2mm,which could be removed by processing.Defects in the original coating have larger contact resistance and smaller flow resistance.Therefore,due to the hot press coupling effect of electrical contact strengthening,the defects in the original coating and the interface have large contact resistance and small flow resistance.Therefore,under the combined action of resistance heat and pressure,it is most likely to be filled and pressed,the defects disappear,and the density increases.Since the outermost layer can flow axially and circumferentially,a thinner non-dense zone is left.（2）After the electrical contact strengthening,the interface between the strengthened0.2mm nickel coating and the subsequent brush-plated thick coating disappears,indicating that the nickel coating that has undergone electrical contact strengthening can be continuously thickened,so that it can be used in applications that require thick repair coatings.（3）After the electrical contact strengthening,the interface between the substrate and the coating is further deepened,and the phenomenon of mutual fusion and bridging occurs,which is beneficial to the combination of the substrate and the coating.This is because under the condition of resistance heating,as the temperature increases,the deformation resistance of the tissue decreases,the plasticity increases,and the point contact causes local overheating to produce fusion welding.This helps to improve resistance to high stress and thermal fatigue during rolling.