Quenching is a heat treatment process in which steel castings are heated to a temperature above Ac3 or Ac1, and then rapidly cooled after holding for a period of time to obtain a complete martensitic structure. The steel castings should be tempered in time after the hottest to eliminate the quenching stress and obtain the required comprehensive mechanical properties.
The quenching heating temperature of hypoeutectoid steel is 30℃-50℃ above Ac3; the quenching heating temperature of eutectoid steel and hypereutectoid steel is 30℃-50℃ above Ac1. Hypoeutectoid carbon steel is heated at the above-mentioned quenching temperature in order to obtain fine grained austenite, and fine martensite structure can be obtained after quenching. The eutectoid steel and hypereutectoid steel have been spheroidized and annealed before the quenching and heating, so after heating to 30℃-50℃ above Ac1 and incompletely austenitized, the structure is austenite and partially undissolved fine-grained infiltration Carbon body particles. After quenching, austenite is transformed into martensite, and undissolved cementite particles are retained. Due to the high hardness of cementite, it not only does not reduce the hardness of steel, but also improves its wear resistance. The normal quenched structure of hypereutectoid steel is fine flaky martensite, and fine granular cementite and a small amount of retained austenite are evenly distributed on the matrix. This structure has high strength and wear resistance, but also has a certain degree of toughness.
Cooling Medium for Quenching Heat Treatment Process
The purpose of quenching is to obtain complete martensite. Therefore, the cooling rate of the cast steel during quenching must be greater than the critical cooling rate of the cast steel, otherwise the martensite structure and corresponding properties cannot be obtained. However, too high a cooling rate can easily lead to deformation or cracking of the casting. In order to meet the above requirements at the same time, the appropriate cooling medium should be selected according to the material of the casting, or the method of staged cooling should be adopted. In the temperature range of 650℃-400℃, the isothermal transformation rate of supercooled austenite of steel is the largest. Therefore, when the casting is quenched, rapid cooling should be ensured in this temperature range. Below the Ms point, the cooling rate should be slower to prevent deformation or cracking. Quenching medium usually adopts water, aqueous solution or oil. In the stage quenching or austempering, the commonly used media include hot oil, molten metal, molten salt or molten alkali.
The cooling capacity of water in the high temperature zone of 650℃-550℃ is strong, and the cooling capacity of water in the low temperature zone of 300℃-200℃ is very strong. Water is more suitable for quenching and cooling of carbon steel castings with simple shapes and large cross-sections. When used for quenching and cooling, the water temperature is generally not higher than 30°C. Therefore, it is generally adopted to strengthen the water circulation to keep the water temperature within a reasonable range. In addition, heating salt (NaCl) or alkali (NaOH) in water will greatly increase the cooling capacity of the solution.
oil. The main advantage of oil as a cooling medium is that the cooling rate in the low temperature zone of 300℃-200℃ is much lower than that of water, which can greatly reduce the internal stress of the quenched workpiece and reduce the possibility of deformation and cracking of the casting. At the same time, the cooling capacity of oil in the high temperature range of 650℃-550℃ is relatively low, which is also the main disadvantage of oil as a quenching medium. The temperature of quenching oil is generally controlled at 60℃-80℃. Oil is mainly used for the quenching of alloy steel castings with complex shapes and the quenching of carbon steel castings with small cross-sections and complex shapes.
In addition, molten salt is also commonly used as a quenching medium, which becomes a salt bath at this time. The salt bath is characterized by a high boiling point and its cooling capacity is between water and oil. Salt bath is often used for austempering and stage quenching, as well as for the treatment of castings with complex shapes, small dimensions and strict deformation requirements.