The crack and cavitation parts of the propeller blade must be polished before repair welding

Repair welding and repair technology of propeller
1. Before welding, the crack and cavitation parts of the blade must be polished, and the number, length and direction of the crack must be determined by dye penetrant inspection.
2. Before welding repair of cracks, the cracks shall be thoroughly excavated (ground), all the cracks shall be removed, and appropriate welding junctions shall be selected for welding repair. Repair welding of blade edge cracks shall be carried out from the central area to the edge side.
3. The oxide and dirt in the edge notch and denuded hole shall be completely removed to expose the metal luster before welding repair.
4. The proper form of welded junction shall be selected for cracks and broken blades.
5. For the broken blades in zone C, the chemical composition B and mechanical property test report of the newly cast and welded block must be approved by the surveyor when the new block is used for butt welding. During welding, the pressure side of the blade should be downward, and the lining plate should be placed under the butt joint. The welded joint should be double-sided U-shaped or V-shaped, and should be fully welded. The welding shall be carried out from the center to the two edges.
6. The preheating device can adopt coke oven, flexible far-infrared heating blanket and propane soft flame spray gun.
7. It is recommended to use tungsten argon arc (TIG) and molten argon arc (MIG), or manual arc welding and other effective welding repair methods. Oxyacetylene welding can only be used for repairing the blade edge in zone C with thickness less than 32mm. Manual argon tungsten arc welding generally adopts AC, but DC can also be used for repair welding of ZHMn55-3-1 alloy. The diameter of tungsten electrode is 4 mm~6 mm, the diameter of nozzle is 16 mm~24 mm, and the flow of protective gas is 12 L/min - 18 L/min. The gas flow should be appropriately increased, and DC reverse connection should be adopted for arc welding.
8. During welding, corresponding measures shall be taken to reduce the welding temperature, and the interpass temperature at the molten pool shall be kept at about 300 ℃ to reduce the evaporation and oxidation of zinc. During welding, the weld meat shall be continuously hammered intensively and evenly to make it extend and reduce the shrinkage of the crater, so as to ensure that there will be no re-cracks between the weld meat and the base metal.
9. After repair welding, the propeller shall be subject to annealing treatment. For repair welding in areas A and B or other large areas, if conditions permit, the propeller shall be heated at a speed not greater than 100 ℃ per hour to the annealing temperature. The insulation time of the whole propeller shall be 4~8 hours according to the size of the propeller, and the cooling rate shall not exceed 55 ℃ per hour. After the temperature is reduced to below 200 ℃, the furnace can be opened for cooling.
10. For example, after a blade or partial repair welding, if the furnace annealing condition is not available, the heat preservation time can be calculated as 20 min/25 mm at the section thickness of the repaired part, but it should not be less than 1 hour, and then the heat preservation material is used for binding, and the natural annealing treatment is carried out as the room temperature drops. After annealing, the welding surface shall be polished and the dye penetrant inspection shall be carried out to ensure that there are no cracks, pores, slag inclusions, looseness and other phenomena.
Summary of propeller blade thickness calculation
The thickness of propeller blades is mainly to ensure strength, and blade strength is one of the most important factors to ensure the safety of ship navigation. The calculation of propeller thickness is mainly based on the radial force of the blade, and also considers the performance of propeller efficiency and cavitation. The thickness of a section along the radial direction of the blade is equal to the difference between the section thickness at 0.2R and 1.0R multiplied by the coefficient plus the tip thickness, that is, Tx=Sx (T0.2-T1.0)+T1.0, where Tx is the section thickness at a certain radius; T1.0 is the thickness of blade tip section; T0.2 is the section thickness at the radius of 0.2R; Sx is the thickness factor.
The blade of large propeller has high internal strength requirements
The blades of large propellers can not be formed by forging. At present, the production methods that can be used in industry are mainly high-precision casting, which can be cast once and then polished mechanically to meet the requirements. Copper has a low melting point and is easy to cast. With the cost of precision welding and hard surface polishing added, the interest is much more expensive, so the stainless steel propeller is not widely used. Compared with steel materials, copper alloy has its advantages as propeller: low chemical activity, not easy to oxidize, especially nickel-copper alloy, commonly known as white copper, which will not corrode for decades and remain bright as before. The hot copper liquid has good fluidity, less bubbles and high finish, so the propeller material is almost monopolized by copper alloy, and more nickel aluminum bronze is used. It is also possible to use stainless steel casting, but the overall casting technology of stainless steel parts is too difficult. The melting point of stainless steel is high, so the fire resistance of casting mould should also be improved.