In order to prolong the service life of the induced draft fan of the boiler, the impeller suface of the fan shall be anti-wear treated and repaired for anti-wear after being worn. The main methods of anti-wear to impeller surface of the fan are compared and the the application of the methods in the industry is introduced. 

1. Surface sticking or welding ceramic: Stick the anti-wear engineering ceramics with high strength and high temperature resistant adhesives or special welding process on the fan blade surface. The anti-wear performance of anti-wear ceramic is the best, especially for the anti-wear ceramics with purity above 99% . At acouring angle around 30 ° angle, its performance is better than that of any other anti-wear materials. Compared with metal material, the anti- wear ceramic has small friction coefficient, high wear resistance, good heat resistance, small specific gravity, small thermal expansion coefficient and high temperature strength; It has excellent anti-corrosion resistance. In acid medium, the anti-corrosion resistance of anti-wear ceramics of aluminum oxide is better than that of stainless stee for tens of thousands of times. In alkaline medium, the performance of 92% alumina oxide anti-wear ceramics is roughlt the same with that of stainless steel, while 99% aluminum oxide anti-wear ceramic almost can not be seen to be corroded. But anti-wear ceramics at the same time has the shortcomings of being unable to bear large tensile stress and bending stress, especially unable to bear large mechanical and thermal stress gradient or mutational stress gradient. And it is strictly required in process, such as with little careness in construction, the construction is not carried out strictly according to the requirement of process, the wear-resistant ceramic pieces will shed, making  the impeller out of balance, also the sticking of anti-wear ceramics will increase the weight of the impeller,  the start-up of the motor and the power consumption of the fan. 

2. Surface coating: On the blade surface wear parts, coat or stick the polymer wear-resisting material. As for the applicable wear-resisting coating materials of the wear-resistanting coating materials, only one coating less than 1mm is painted on the blade surface, and it doesn't cause any damage to the parent metal. The weight of coating materials is light, the  coating process is simple, the wear-resisting performance  is good, the coated parts are not restrained by geometric. The wearing performance of the anti-wear coating is far less than the ceramic, but during the period of overhaul and repair to the unit, it is easy and convenient to repair the worn parts, in order to keep the blade " forever " not be worn. This is gradually concerned by departments concerned at home and abroad. The shortcoming of coating anti-wear coating on the surface is  that because of wear-resisting material is organic matter, which is limited in ful-filling on the surface of impeller, its heat resistance is poor and is easy to age. These factors can not make the anti-wear coating reach longer service life under the harsh working condition of electric power operation, thus  increasing the workload to workers. 

3.  Surfacing: Using the wear-resisting welding electrode and wear-resisting powder block, the wear-resisting alloy is sufacing welded on the fan blades. Using wear-resisting electrode for surfacing on blade is  a relatively simple measures, the surfacing welding way can be in strips, intermittent small pieces or directly and fully surfacing at the easily worn parts. The thickness of the blade itself is increasing due to  surfacing, a part of abrasion can be resisted, at the same time, this kind of welding electrode is special wear-proof electrode, making erosion- resistance ability of this part of the surfacing layer increase,  so as to improve the service life of the impeller. The application of wear-resistant electrode is more convenient, but the heat- affected zone of welding is bigger, it will cause welding deformation, and during welding, the impeller will be weightlessness and out of balance, it is suitable to use the welding method only if the blade has  a certain thickness. And if the quality of wear-resistant electrode is not selected well,  the wear-resisting performance and the effect will not be ideal. 

4. Thermal Spraying (welding) : Using plasma spraying method or oxyacetylene flame, spray ceramic or  tungsten carbide  or spray-weld nickel base + tungsten carbide alloy on the the blade wear surface. Due to the limitation of coating materials and coating process to thermal spray wear-resisting technology, the biding force of flame spraying- coating is low, the coating peeling and falling off phenomenon easily occur, and because a large amount of heat is input, the impeller is likely caused  to deform, and now due to less application, the flame spraying technology application is restricted in terms of wear- resistance of blade. 

5. The arc spraying: Using special electric arc spraying equipment, fully molten, atomize and spray the metal wire materials of wear resistance and corrosion resistance in a very short time on the blade surface. The electric arc spraying coating thickness can  reach 0.1-0.1 mm, the strength of combination of coating and base metal is high and the service life is longer. Before spraying, the preheating is not necessary,  the heat affected zone is relatively small, especially the electric arc spraying is suitable for thin-walled parts, such as fan blades and thin parts. The electric arc spray chromium alloy coating has excellent adhesion and wear resistance, it is suitable for  new impeller blade surface strengthening or restrengthening of the repaired old impeller blade of the power plant. 

To sum up,  some common anti-wear surface treatment methods to impeller blade surface have their own advantages and disadvantages. The technology of sticking wear-resisting ceramics and painting wear-resisting coating technology are used to treat the surface of the impeller on condition that the impeller and its internal organization are not damaged, they really have played an role for protecting the impeller and can continue repairing, in order to ensure the parent material to be always in a good state, but the wear-resisting layer is not firm, however the other several methods will damage the parent metal, making the  internal organization structure of parent metal of the impeller  change, so that the balde crisps, has big tendency to crack, its deformation can not be controled, it can't be repaired after wear and it can be used only once.