On 2018-08-08 14:48:54
Heat treatment heats and cools
Important processes of heat treatment: heating and cooling
Heat treatment plant said, heating temperature is one of the important process parameters of heat treatment process, the selection and control of heating temperature, is the main problem to ensure the quality of heat treatment. The heating temperature varies according to the metal being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain high-temperature microstructure. In addition, the transformation needs a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperature is the same, so that the microstructure changes completely, this period of time is called the holding time. When using density heating and surface heat treatment, heating speed is fast, there is no holding time, and chemical heat treatment holding time is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with different processes, mainly to control the cooling rate. In general, the cooling rate of annealing is slow, the cooling rate of normalizing is fast, and the cooling rate of quenching is fast. However, there are also different requirements for different types of steel, such as hollow steel can be quenched with normalized cooling rate.
The steel is heated to a certain temperature and kept warm for a period of time. Annealing of steel is a heat treatment in which the steel is heated to the temperature at which phase transition or partial phase transition occurs and then slowly cooled after heat preservation. The purpose of annealing is to eliminate microstructure defects, optimize microstructure, homogenize composition and refine grain, increase mechanical properties of steel, and reduce residual stress. At the same time can reduce the hardness, increase the plasticity and toughness, optimize the cutting performance. Therefore, annealing is not only to eliminate and optimize the tissue defects and internal stress left by the previous process, but also to prepare for the subsequent process, so annealing is a heat treatment of semi-finished products, also known as pre-heat treatment.