Quenching defects causes and Prevention
The common defects in quenching are oxidation, decarbonization, deformation, crack, overheating, overheating, lack of hardness, soft spots, soft spots and so on, in which oxidation, decarburization, overheating, deformation and cracking are the most serious.
The main reason for the oxidation and decarbonization is that oxygen, carbon dioxide, steam and other oxidizing substances in the heating medium are used to react with iron or carbon on the surface of the parts to produce iron oxide skin or to burn the surface of carbon into poor carbon.
How to prevent the production of oxidation and decarburization? In the box type electric furnace and the well type electric furnace, there are protective gases through into the coal, methanol, or through the ammonia, nitrogen and other gases. The more simple way is to put some wood and produce CO and other gases to protect it. At present, coating with some protective agent on the surface of the parts can also effectively prevent oxidation and decarburization of the parts. In the salt bath furnace, silica gel is generally used for deoxygenation, and good results can be obtained.
Hardness is often caused by insufficient heating, short holding time or slow cooling. Too high heating temperature of high carbon steel will make the hardness less. Therefore, it is necessary to prevent such defects by using reasonable heating specifications and correct operating methods (such as a certain gap between parts during heating and mixing when cooling).The coarse grain of the structure is mainly due to the high heating temperature. Only by strictly controlling the temperature and doing the first heat treatment can we effectively prevent overheating.
Deformation and cracking are the most serious defects in heat treatment. The basic reason for such defects is the excessive internal stress. When the internal stress is greater than the yield limit of the material, the part will produce plastic deformation. When the internal stress exceeds the limit of strength, the part will crack. In particular, the dissymmetry of the parts, the uneven cross section of the section, the poor original tissue, the improper heating and cooling, and the shape of the sharp sharp angle will cause the deformation and cracking of the parts. Only by taking specific measures, especially selecting reasonable technological parameters, can we effectively prevent such defects.
