On 2018-08-08 14:48:54
Heat treatment and deformation treatment
Effective treatment of heat treatment deformation and prevention methods
In the conventional heat treatment, the main reason for the shape change of parts is the thermal stress and transformation stress during the heating and quenching of heat treatment. If the heating speed is too fast, the parts are too large compared with the heating furnace, and the temperature of each part is different, it will lead to thermal deformation. During heat preservation, the residual stress of processing will be released to produce deformation, and the self weight of the parts will also lead to deformation. When cooling, due to the different cooling speed of different parts of the parts, it will form thermal stress and deform the parts, even if the cooling speed is the same, the cooling speed is the same But always surface fast, heart slow.
In the process of heat treatment and austenitizing, the longer the holding time and the higher the temperature, the more carbon dissolved in austenite, and the larger the expansion of martensite. During cooling, the expansion of martensite is large, followed by upper bainite, and the volume changes of lower bainite and troostite are small. When tempered at low temperature, the martensite shrinks in direct proportion to the supersaturated carbon content. When heated at room temperature - 200 ℃, some of the retained austenite will turn into martensite and expand. However, due to the decomposition of martensite near 200 ℃, the expansion has little change.
There are two types of heat treatment deformation: one is the change of size, the other is the change of part geometry. With different heat treatment technology, the deformation and anti deformation methods of part size and geometry are also different.
In the heat treatment of forgings, the parts to reduce deformation are placed in the following ways: first, hang vertically as much as possible, second, place vertically at the bottom of the furnace, third, support horizontally with two points, the fulcrum position is between one third and one quarter of the total length, fourth, place horizontally on the heat-resistant steel tooling.
Therefore, the surface of the first phase transformation makes the center of the non phase transformation plastic deformation. If there is segregation of alloy components or decarburization on the surface of the material, the transformation stress will be more uneven and the deformation of the part will be more easily caused. In addition, if the thickness of parts is uneven, the cooling speed will be different.