Preparation before quenching:
- Before quenching, it is necessary to remove the greasy dirt and dirt attached to the surface during cold deformation processing. It is usually wiped with gasoline, acetone, banana water, etc., or it can be soaked in an alkaline solution. The composition of the alkaline solution is: each solution contains 50g of sodium phosphate, 1g of sodium hydroxide and 3g of water glass. Solution temperature 50~60ºC, soaking time 5~10min. It can also be formulated into a solution with detergent to remove grease.
- After the alkali bubble, the workpiece is cleaned in hot water or running cold water until it is rinsed clean.
- The workpiece should be dried or air-dried before being put into the furnace.
- The temperature of the heating furnace is uniform, and the temperature control accuracy is within the range of ±(2~3)ºC, and the maximum does not exceed ±5ºC.
- The quenching transfer time depends on the composition, shape and production conditions of the parts: generally, the transfer time for small parts does not exceed 25s, for large parts does not exceed 40s, for plates does not exceed 30s, and for super-hard aluminum does not exceed 15s.
- Aluminum strip, aluminum wire or iron wire for quenching and heating the workpiece. The iron wire should be free of galvanized layer, and should not be bundled with copper wire. The fixture should not be made of copper to prevent copper and zinc from diffusing into the parts, reducing the corrosion resistance of the workpiece and causing local melting.
- When heating in a nitrate furnace, the distance between the parts and the bottom of the tank, the tank wall and the depth of the parts immersed under the liquid surface are not less than 100mm. There should be a certain gap between the parts, not close.
- In the air furnace, the heating parts are more than 200mm away from the furnace door and more than 100mm away from the separator of the electric heating element. Parts should not be placed in a location that prevents the circulation of hot air. The plate is heated, and the distance between the plates is not less than 20mm.
- Welded aluminum parts shall not be heated in the nitrate tank to prevent the corrosion of the weld due to the penetration of nitrate.
- In order to prevent nitrate corrosion, the total mass fraction of impurities in the salt bath should not exceed 2%. The mass fraction of sodium carbonate in the alkali does not exceed 1%, and the mass fraction of chloride does not exceed 0.5%. If it is found that the parts are corroded, potassium dichromate, which accounts for about 3% of the total mass fraction of the salt solution, should be added to the salt bath, and potassium dichromate and nitrate should be mixed in a ratio of 1:5 and dried. Add at about 400ºC.
- The time for re-quenching and heating of the workpiece is 1/2 of the normal, and the number of re-quenching is not more than 2 times; if the thickness of the part is less than 1mm, the re-quenching time is not more than once.
- Parts with simple shapes are quenched with water, and the water temperature is generally 10~30ºC; for parts with complex shapes, the quenching water temperature is 40~50ºC. A polyether quenching medium can also be used to reduce distortion.
- The quenching water temperature for forgings with complex shapes is 80ºC. Large forgings can be tempered in nitrate or hot oil at 160~200ºC for a period of time, and then quenched into running tap water to cool. The volume of the quenching medium is more than 20 times the volume of the part, and the medium should be circulated or agitated.
Cleaning and Aging:
- After quenching in nitrate, it should be rinsed in flowing 50ºC water; the time should not exceed 3min.
The method to check whether the nitrate is washed thoroughly is: use a solution of 90mL sulfuric acid (density 1.84g/cm³) + 0.5g diphenylamine + 10mL water, drop 1~2 drops on the surface of the part, dry it after 10~15s, and dry it. Rinse in water, if there is a bright blue color, it means that it has not been washed and should be washed again.
- The artificial aging is interrupted and can be repeated. The aging time is the sum of the time periods. Using ultrasonic aging can shorten the time and improve the strength.
- The regression treatment should be carried out in a nitrate furnace, and the number of regression should not exceed 3 to 4 times (duralumin).
- The workpiece after quenching will be distorted and warped. Therefore, it needs to be corrected within 2~3h after quenching. If it is placed for a long time, it will harden with natural aging, and it is not easy to correct.