Artificial aging refers to the process of heating the alloy after solution treatment to an appropriate temperature above room temperature and holding it for a certain time to change the properties of the alloy.During artificial aging, the precipitation rate of new phase is faster than that of natural aging, but the hardening peak is notNatural prescriptionHigh.If the heating temperature is too high or the holding time is too longOveragingThe hardness is reduced.
Chinese name
Artificial aging
Foreign name
Artificial aging
Classification
Complete aging, incomplete aging, over aging, etc
Related parameters
Stress, yield strength, plasticity and toughness, etc
Aging treatmentIt can be divided into natural aging and artificial aging.Natural aging is to put the workpiece outdoors and other natural conditions, so that the internal stress of the workpiece can be naturally released to eliminate or reduce the residual stress.Artificial aging is an artificial method. Generally, heating or cold treatment is used to eliminate or reduce the micro stress and machining residual stress in the quenched workpiece to prevent deformation and cracking.Stabilize the structure to stabilize the shape and size of parts.The method is to heat the workpiece to a certain temperature, cool it with the furnace after holding for a long time (5~20 hours), or cool it in air.It saves time compared with natural aging, and the residual stress is removed more thoroughly, but the stress release is not complete compared with natural aging.
Artificial aging classification
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Artificial aging is an operation to improve mechanical properties by holding at a certain temperature higher than room temperature for a certain time.The artificial aging strength is higher, and the yield strength increases more significantly (Rp0.2/σb0.8~0.95), but the plasticity, toughness and corrosion resistance are generally poor.
Artificial aging can also be divided into complete aging, incomplete aging, over aging, stabilizing aging, etc.The strength obtained by full aging is the highest, reaching the peak of aging strengthening;The aging temperature of incomplete aging is slightly lower or the aging time is shorter to retain higher plasticity. Compared with complete aging, the decrease of strength is compensated by less plastic decline;On the contrary, the aging degree exceeds the strengthening peak value, and the corresponding comprehensive properties are better, especially the corrosion resistance;The temperature of stabilizing aging is higher than that of overaging, so as to stabilize the properties of alloy and the size of parts.
The natural aging process is relatively slow, and the artificial aging process is relatively fast.[1]
Artificial aging treatment
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Relationship between aging time and yield strength HIP: 510 ℃ × 4h, 68.6MPa
The heat treatment process in which the alloy after solution treatment is heated and kept warm to precipitate the strengthening phase to improve its performance.After the alloy is solution treated (quenched), because the solute elements (alloy elements) dissolved in the solid solution have no time to precipitate, an unstable supersaturated solid solution is formed.At room temperature, the effect of precipitation of strengthening phase in solid solution with the extension of storage time is called natural aging;When heated to a certain temperature and kept warm, the precipitation process can be accelerated, and the precipitation phase different from natural aging can be obtained, so that the alloy can be strengthened and toughened, and the service performance can be improved, which is called artificial aging treatment, or "precipitation treatment".The type, quantity and rate of precipitated phase can be controlled by artificial aging.The artificial aging temperature of aluminum alloy is generally 75~250 ℃.After treatment, the yield strength increased more than the tensile strength, and the plasticity and toughness decreased accordingly.It is widely used in various alloys.[2]
① Check whether the furnace and lifting equipment are damaged, and repair them in time if damaged;
② In the combustion chamberslagAnd the ash in the furnace pit shall be cleaned;
③ Check whether the ventilation equipment such as blower, gate, air duct, etc. are normal;
④ The small flue shall be cleaned once a month, the main flue shall be cleaned once a season, and it shall be cleaned at any time when necessary.
3. Preparation of temperature measuring equipment
① Make sure toThermocoupleAnd millivoltmeter, and put thermocouples at appropriate positions in the furnace close to the casting temperature;
② A sufficient number of thermocouples shall be prepared, and at least two thermocouples shall be prepared to measure the maximum and minimum temperature in the furnace;
③ Prepare the furnace temperature record sheet or record book.
4. Charging
The rationality of casting charging is directly related to the quality of castings after aging, so the following points should be paid attention to during charging:
① For castings aged in the same furnace, the wall thickness and size of the castings shall be close to each other so as to select the aging process;
② Castings with the same aging specification shall be installed in one furnace;
Layered loading diagram 1-1
③ Castings installed in the furnace shall be placed stably.In order to avoid the deformation of castings under the action of self weight or other weight when the furnace temperature rises, the offset of castings should be properly selected. If the offset is too large, sizing block should be added in the middle;
④ When the casting is loaded in several layers, the sizing block position between each layer of casting shall be the same.It is not allowed to stagger the sizing blocks between the upper and lower layers to avoidBending stress。The schematic diagram of layered loading is shown in 1-1:
⑤ The casting shall be placed on the special sizing block.In order to ensure uniform heating, the distance between the casting and the trolley surface shall not be less than 150 mm, and the distance between the casting and the furnace top shall not be less than 500 mm.The distance between two adjacent castings shall not be less than 200mm;
⑥ When selecting the plane to load the casting in the furnace, the rigidity of the part under heating shall be taken into account.That is to say, the surface with greater rigidity of the casting shall be placed vertically on the furnace bottom during loading;
⑦ When loading, avoid making the casting directly face the fire mouth (especially thin-walled casting) to prevent overheating.The distance between casting and furnace wall shall not be less than 200mm, and small parts shall not be placed in large parts during aging;
⑧ Generally, the temperature is uniform at the middle height of the furnace or in the center of the furnace, so important castings or important parts of castings should be co located at this height or placed in the center of the furnace as far as possible;
⑨ After the casting is loaded, close the furnace door and seal the gaps around with mud and sand.
5. Inspection of lifting equipment
① Before each furnace loading, check whether the trolley is firm, whether it operates flexibly, whether the steel wire ropeGuide wheelWhether it is normal;
② When pulling or loading, check whether the crane or lifting equipment is normal.[3]
Application on castings
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When casting, internal stress is often generated in the casting due to uneven wall thickness and different cooling speed of each part.The existence of internal stress may cause deformation, distortion or crack of castings.Therefore, for castings that do not need special heat treatment, especially those with complex shapes, artificial aging (also called low-temperature annealing) is often used to eliminate internal stress.
The internal stress of iron castings is mainly generated in the transition stage from plastic deformation to elastic deformation, that is, between 620 ℃ and 400 ℃.Therefore, in this temperature range, the slower the cooling rate, the smaller the internal stress.
The low temperature annealing process to eliminate the internal stress of iron castings is to charge the castings in the furnace at a temperature lower than 200 ℃, and the heating rate is 50-150 ℃/h.For small castings with simple shape and structure and thin wall thickness, the temperature rise can be faster;For complex shape and thick wallLarge castingsThe temperature needs to rise slowly.Increasing the heating temperature can shorten the holding time. When the heating temperature is 500~600 ℃, the internal stress can be effectively eliminated.However, excessive heating temperature may cause partial stressCementiteThe hardness is reduced by decomposition and granulation of.The heat preservation time is generally 2~8 hours. The time for thin and small parts can be shorter, and the time for thick and large parts should be longer.It must be ensured that all parts of the casting are evenly heated to the required temperature.The cooling speed should be slow. According to the complexity of the casting, it should be cooled in the furnace at the rate of 20~50 ℃/hour. When it is cooled to below 200~150 ℃, it can be taken out of the furnace and cooled in air.
