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Casting repair

Repair of casting

 For some casting defects, they can be repaired economically. Below are the measures:

 1. Welding repai

 The welding can be used to repair cracks, below holes, shrinkage, sand inclusions, cold lap and so on. It can be divided into gas welding and electric welding.

  1) Gas welding
  The gas welding is always used for iron casting and without machining. Before welding, the casting is usually preheated to 500-550℃. After welding, the casting is cooled slowly. For some complex parts, they need the annealing heat treatment for stress relieving.

  2) Electric welding
  The electric welding is used for steel casting, and usually, it’s not preheated. But below situations need preheating: big casting, the defect in important place, big size of defect and a great amount of welding. It needs thermal insulation and annealing heat treatment after the welding.
  The electric welding for iron casting can be divided into cold welding and hot welding.
  The cold welding is used for repairing the defects on without machining surface. It always use nickel base or copper base cast iron electrode. Using low current can avoid white iron and reduce stress.
  For hot welding, the casting need to be preheated to 450-55℃, and it’s even higher for big and complex casting. It’s always used for repairing defects on machining surface. The high silicon cast iron electrode is commonly used. It takes slow cooling and thermal insulation after welding to get gray structure.

  Before welding, we must clear the defective place to expose the metal base, which is easier for welding and to make sure the full welding. On the welding for cracks, it needs to drill two stop-crack holes of ¢8-10mm, which are located on both ends of crack and 5-10mm from the ends.

 2. Putty filler repair

 If the cavities are appeared on some unimportant place, we can fill putty of same color with casting to repair these defects. Below are some usual putty:
  1) Iron powder 75%, sodium silicate 20%, cement 5%.
  2) Magnesium oxide 2.5%, water 3%, iron powder 81%, sulfur 1%, acetic acid 2.5%.
  3) Cast iron fillings 80%, magnesium oxide powder 15-20%, then mixed together in saturated magnesium chloride solution to lumps.
 On repairing, clear the defective place first, then use scraper to press the putty into the cavity, finally equate it with the casting surface.

 3. Impregnation repair

 For the casting part working under pressure, if it has the porosity defects like dispersed shrinkage, it will have leakage in the water pressure test or air pressure test. If the defects can’t be repaired by welding or its working temperature is lower than 250℃, and its working pressure is not big, we can use the repairing method of impregnation.
 The method of impregnation is pouring impregnant into the casting cavity or putting the casting in the impregnant. After some time, the casting cracks will be filled by impregnant, which will hardened finally.
 For some porosity defects, the holes are too small to let impregnant in. If so, we can create the high pressure for the impregnant or low pressure on the other side of casting to let impregnant in.
 The common impregnant materials are epoxy resin, lead, copper sulfate, ammonium chloride aqueous solution etc.

 4. Molten metal repair

 For the method of molten metal repair, it means put molten metal on the repairing place to make the defective place melt and using new mold to create new place. Firstly, it needs to make a sand mold according to the shape of repairing place. As the solidification will cause some shrinkage, the riser should be set in the right place for feeding. The first poured molten metal is easy to cold, so it needs to set an outflux to let the cold molten metal flow out. Check the melting condition of the defective surface, once it is melted, block up the outflux immediately, then fill the sand mold.

 Notice: to avoid the solidification of the first poured molten metal, we can pour more molten metal firstly, then pour slowly. Further more, the ladle can be lifted higher to bring strong impact for better stir in molten metal, which will better avoid the cold. The casting can be preheated to reduce inner stress. Make the repaired place cold slowly to solidify. It needs stress relief heat treatment after the solidification.

No Casting inspection

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