In the manufacture of tissue paper, the cast iron Yankee dryers are being replaced by the dryers made from steel gradually, because the latter has obvious advantages in toughness, heat conduction and safety. The surface of steel Yankee dryer cylinder sprayed with cemented carbide coating is harder than the surface of cast iron, which can improve the quality of paper products, extend the life of the dryer and the maintenance cycle, and help to complete chemical coating and wrinkling process. The grade of steel Yankee dryer cylinder would not be reduced with the decrease in thickness or thinning of a metallic coating. Shandong Xinhe has developed and produced the steel Yankee dryer cylinder with φ30 0 0m m and Φ3660mm. Nowadays, Xinhe has had the ability to produce the steel Yankee dryer cylinder with size of Φ6000mm×5000mm, with the spraying implemented by RIF corporation in Italy.
The steel used in the Yankee dryer cylinder produced by Xinhe is medium-strength steel plate, which can better endure sand blasting roughening surface, activate the surface by increasing surface free energy, and also enlarge the surface area of spraying particles adhered. The principle of thermal spray can be seen in Figure 1.
Coating material: It can be linear or sticky which depends on the techniques of thermal spray. The coating material of the steel Yankee dryer cylinder is featured by: (1) high surface strength, over 55HRC; (2) wear resistant and long service life; (3) good heat conduction and corrosion resistance; (4) dense and smooth coating; (5) good bonding strength and not easy to fall off; (6) uniform material and chemically inert.
Techniques of thermal spray: The Yankee dryer with thermal spray coating process is wire arc spraying. In the process, two metal wires with positive and negative charges and usually the same
chemical composition are used to produce the arc, which can instantly generate heat to melt the metal wire tip, and the compressed air can make the spray material melting atomization, then accelerate spray into the cylinder surface. The spraying speed of wire can be control by adjusting the wire feed speed appropriately, so a constant arc can be maintained (Figure 2). At present, the Yankee dryer, with coating thickness of about 0.75mm and roughness of 0.3 ~ 0.6μm, can endure harder scraper operation and improve the softness of the paper, thus making a wider space for the operation and promotion of dryers. Steel dryer cylinder must be pre-grinded before spraying, and the appropriate camber should be calculated according to the structure of Yankee and vacuum roller.
The specific process of steel Yankee dryer spraying is shown in Figure 3.
The Yankee dryer cylinder after spraying has high thermal conductivity, thus saving 30% steam consumption compared with cast iron cylinder.
Coating measurement: In addition to coating properties, there are some features that are relatively easy to test and have standard quality control inspection methods, including visual inspection, measurement of coating thickness and surface roughness measurement. If necessary, the test sample can be sprayed at the same time and under the same conditions, and the coating can be tested in time to determine other properties of the coating quality. In order to accurately determine the coating thickness, porosity and structure, as well as the microstructure of the melted particles and oxide inclusions, the coatings were subjected to lateral cross sectional testing. What’s more, the combination method of strength determination can also
be promoted by using a special glue and test equipment. In addition, some other methods are also useful, including the analysis of various elements, scratch test, tribology properties research, stress analysis, and research of corrosion and abrasion characteristics.