Casting iron stress relief annealing heat treatment
Stress relief annealing is to keep the casting at a certain temperature and then slowly cool it to eliminate the casting residual stress in the castings. For gray cast iron, stress relief annealing can stabilize the casting geometry and reduce deformation after cutting. For white cast iron, stress relief annealing can prevent castings from deforming or even cracking when they are subjected to vibration or environmental changes during storage, transportation and use.
Generation of casting residual stress
The volume of the casting will shrink or expand during the solidification and subsequent cooling processes. This volume change is often constrained by the outside world and the various parts of the casting and cannot proceed freely, thus producing casting stress. If the cause of the stress is eliminated, the casting stress will be eliminated. This stress is called temporary casting stress. If the casting stress still exists after the cause of the stress is eliminated, this stress is called casting residual stress.
During the solidification and subsequent cooling process of the casting, due to different wall thicknesses and different cooling conditions, the temperature and phase change degree of each part will be different, resulting in different volume changes of each part of the casting. If the cast alloy is already in an elastic state at this time, there will be phase constraints between the various parts of the castability.
Casting residual stress is often the stress caused by different temperatures and different degrees of phase transformation. In order to improve the actual effect of stress relief annealing process, the heating temperature can reach the one-time complete unloading temperature of the casting and lower than the one-time complete unloading temperature. When the heating temperature is higher, the stress relief is more complete.
However, if the heating temperature is too high, it will cause changes in the casting structure, thus affecting the performance of the casting. For gray iron castings, if the heating temperature is too high, the eutectoid cementite will be graphitized and the strength and hardness of the casting will be reduced. For white iron castings, if the heating temperature is too high, the eutectoid cementite will decompose, greatly reducing the hardness and wear resistance of the casting.
The heating temperature for stress relief annealing of ordinary gray cast iron is 550°C. When the cast iron contains alloy elements that stabilize the matrix structure, the stress relief annealing temperature can be appropriately increased.
For low-alloy gray cast iron, it is 600°C, and for high-alloy gray cast iron, it can be increased to 650°C. The heating speed is generally 60~100. The holding time can be calculated by the following empirical formula: H=casting thickness/25+H where the unit of casting thickness is millimeters, the unit of holding time is hours, H” can be selected in the range of 2 ~ 8.
The shape is complex and requires full stress relief Castings should take a larger H” value. The cooling rate with the furnace should be controlled below 30°C/h. Generally, castings are cooled to 150~200% and castings with complex shapes are cooled to 100%.
Machine tool castings are the most widely produced among various castings such as mechanical castings, and are used in the production of machine tool beds. Annealing is a metal heat treatment process. Bed castings need to be heat treated to improve their performance.
It refers to slowly heating the metal to a certain temperature, maintaining it for a sufficient time, and then cooling it at an appropriate speed. The purpose is to reduce hardness and improve machinability; eliminate residual stress, stabilize dimensions, and reduce deformation and crack tendencies; adjust the structure and eliminate structural defects. Common annealing processes include: stress relief annealing, spheroidizing annealing, complete annealing, etc.
Annealing of machine tool bed castings is also called low-temperature annealing. This annealing is mainly used to eliminate residual casting stress in machine tool castings, such as machine tool beds, machine tool tables, machine tool components, machine tool castings, etc.
If these stresses are not eliminated, they will cause castings such as machine tool beds and columns to deform or crack during subsequent cutting or use or after a certain period of use. In order to reduce the tendency of deformation and cracking of castings during use and subsequent processing, annealing is usually performed for the purpose of removing casting stress after demolding.
To sum up, after tempering, the mechanical properties of machine tool castings are greatly improved in all aspects, and their internal stress, stability, workpiece size, etc. are all improved.
