On 2018-08-08 14:48:54
Common heat treatment methods
In the process of metal heat treatment and heating, at about 450 ℃, the steel changes from elastomer to plasticity, so it is easy to take up plastic deformation. At the same time, when the residual stress is higher than this temperature, it will disappear because of recrystallization. Therefore, during heating, due to the temperature difference between the inner and outer parts of the workpiece, the outer part reaches 450 ℃ and becomes a plastic area, which is deformed due to the residual stress at the lower temperature in the inner part. After cooling, the area is where the deformation occurs.
For heat treatment of parts, it's the deformation of parts that makes people puzzled! There are many factors that cause heat treatment deformation, but in summary, they are mainly three points: during solid-state transformation, the change of mass and volume of different phases will cause volume change, resulting in the size change of expansion and contraction of parts; thermal stress, including rapid thermal stress and rapid cold thermal stress, will cause plastic deformation of parts when they exceed the yield of parts at this temperature, Causes the part shape change, namely askew, or is called the distortion.
The commonly used heat treatment methods are integral heat treatment, surface heat treatment and deformation heat treatment. The whole heat treatment is often heard in the factory as "four fires", which refers to the four commonly used heat treatment methods: annealing, normalizing, quenching and tempering, in which quenching and tempering are closely related and often used together. With the difference of heating temperature and cooling mode, the "four fire" has evolved different heat treatment processes, including modulation, aging treatment and deformation heat treatment. Among them, due to the large scale of the market, the automotive parts and mechanical basic parts industry has a significant impact on the demand of the heat treatment industry; while the aerospace parts, engineering machinery parts, new energy parts and machine tool parts industry have developed rapidly in recent years, which has become a new increase point of the heat treatment industry.
About 80% of the investigated fastener enterprises have heat treatment equipment, and most of them adopt Taiwan heat treatment process line; the process line equipment is continuous mesh belt furnace with atmosphere protection, and the atmosphere, temperature and process parameters are controlled by computer. The existing problems are lack of measurement of cooling performance of quenching medium, unstable control of carbon potential, and long period of furnace temperature validation, which are easy to cause defects in heat treatment.