Keywords: Hardening, Defects, Oxidation, Decarburisation, Deformation, Cracking, Overheating, Overcooking, Insufficient hardness, Soft spots,

　　Quenching common defects are oxidation, decarburisation, deformation, cracks, overheating, overburning, insufficient hardness, soft spots, soft spots, etc., of which oxidation, decarburisation, overheating, deformation and cracking are the most serious.

        The main reason for oxidation, decarburisation phenomenon is due to the heating medium contains oxygen, carbon dioxide, water vapour and other oxidising substances, when heated, they have a chemical reaction with the iron or carbon on the surface of the parts, generating iron oxide or making the surface of the carbon combustion and turn into poor carbon.

        How to prevent oxidation and decarburisation? In the box-type electric furnace and pit-type electric furnace towel, there are through the protective gas, there are also drops of paraffin, methanol, or through the ammonia, nitrogen and other gases to be protected; a simpler way is to put some firewood to produce co and other gases to be protected. At present, with the form of paint on the surface of the parts coated with some kind of protective agent, also can effectively prevent parts oxidation, decarburisation. In the salt bath furnace generally use silica gel to deoxidise can also get good results. 

        Insufficient hardness is often caused by insufficient heating, short holding time or slow cooling. High carbon steel parts heating temperature is too high will also make the hardness is not enough. So to target the use of reasonable heating specifications and the correct operating methods (such as heating the parts to leave a certain gap between the cooling to be stirred, etc.), you can prevent this defect. 

         Organisation of coarse grains, mainly due to the heating temperature is too high and the formation of. Only a strict grasp of the temperature, the first heat treatment, in order to effectively prevent the phenomenon of overheating. 

        Deformation and cracking is the most serious defect of heat treatment, the root cause of this defect is excessive internal stress. When the internal stress is greater than the yield limit of the material, the part produces plastic deformation; internal stress exceeds the strength limit, the part will crack. Specifically, the shape of the part is not symmetrical, the cross-section thickness is not uniform hook, the original organisation is poor, improper heating and cooling, there are sharp edges and sharp corners of the shape of the parts will cause deformation and cracking. Only targeted measures, especially the selection of reasonable process parameters, in order to effectively prevent the generation of such defects.

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