On 2018-08-08 14:48:54
Gas quenching by heat treatment
Heat treatment usually uses oil or water-soluble quenchant for quenching. In the past 10 years, the gas quenching method has been applied to the quenching of medium and low hardenability steels. High pressure gas or high flow rate gas quenching is used to enlarge the application field of gas quenching and reduce the deformation at the same time. In order to minimize the deformation of workpiece, some quenching methods which can adjust the local quenching rate and simulate the shape of workpiece should be adopted. Gas quenching and spray quenching may meet these requirements. Therefore, the gas quenching method is attracting people's attention.
Quenching determines the final microstructure and properties of the heat treated workpiece. On the one hand, in order to obtain ideal microstructure (generally martensite), the quenching cooling rate must be high enough. On the other hand, in order to reduce the deformation, the cooling rate should be as slow as possible, and the cooling rate on the whole workpiece section should be as uniform as possible. Generally, the cooling rate of a liquid medium can not be adjusted in the quenching process. It has a fixed characteristic. Especially when the deformation sensitive workpiece is quenched, it is not easy to control the quenching rate by quenching with oil or water-soluble medium.
As a quenching medium, the heat transfer coefficient of gas is smaller than that of oil and water-based medium under normal conditions, but it can adjust the quenching process according to the special requirements of workpiece shape and material, and can make its heat transfer coefficient reach the level of oil or water-based medium by increasing the gas flow rate and pressure. It is generally believed that gas quenching will increase the cost, thus limiting its widespread application. However, to evaluate the gas quenching process from the overall cost accounting, although the cost of gas quenching itself is high, it can be compensated by reducing the cleaning cost, improving the reproducibility of the gas quenching process, reducing the grinding of subsequent processing and reducing the depth of the hardened layer.
There are different ways to improve the cooling capacity of gas quenching: increasing the pressure of gas quenching; increasing the flow rate of gas; selecting different types of gas (such as nitrogen, helium, hydrogen); optimizing the flow state, increasing the air stirring, etc. During high pressure gas quenching, the type and pressure of gas can be selected, and the gas flow speed is related to the furnace structure and charging capacity. When the workpiece is placed in a high velocity position (such as gas inlet) in the furnace, high heat conduction can be obtained. If the workpiece is facing the direction of air flow and the flow rate is the maximum, then the heat transfer may be the maximum. When the charging structure and influence parameters are fixed, the results of gas quenching can be repeated. By reasonable design, the uniformity of cooling can be improved. At present, the problems such as gas reverse flow passing through the charging parts vertically and horizontally have been solved to make the cooling as uniform as possible. Another method is to install a plurality of nozzles radially around the furnace to form a vortex field in the cooling chamber and the furnace, so as to increase the air mixing degree. The basis of gas quenching in nozzle field is to increase the quenching intensity by increasing the flow rate and stirring degree, rather than by increasing the pressure. In the adjustable nozzle field, the complex workpiece is cooled uniformly by a given airflow inlet. This process can achieve local different cooling rate, and ultimately minimize the residual stress. The cooling uniformity can be further improved by rotating the workpiece or nozzle, especially when the symmetrical workpiece is hardened. The quenching intensity can be increased by using a special cooling chamber, and the size of the cooling chamber can be adjusted according to the charging capacity, especially under high quenching pressure, the gas loss is very low. At present, the single-layer charging capacity of multi chamber furnace with cooling chamber technology has reached 100 kg.