An important process in the heat treatment process is quenched by quenching treatment can make the performance of metal materials can be greatly improved, and give full play to the potential of metal materials, in order to meet the requirements of various parts of metal materials. During the quenching process, due to improper treatment, the metallographic structure or mechanical properties can not meet the requirements, and soft spots, deformation, cracking and other defects may occur. These defects, in addition to the relationship between the material composition, the original state, and the workpiece shape and size, heating process, heat treatment equipment and other factors, also have close relationship with the quenching medium and cooling method used when quenching. Therefore, the reasonable choice and proper use of quenching medium in quenching cooling process is an important factor to ensure the quality of workpiece quenching.
With the development of quenching technology, people gradually find out the influence of quenching medium on quenching quality. It is nearly 50 years to study the quenching media systematically, and it is still nearly 30 years since China began to study and use special quenching medium. Nowadays, more and more heat treatment workers have realized the importance of quenching medium to the quality of quenching.
When the workpiece is heated to quenching, austenitizing, quenching in medium for cooling in the cooling process, cooling through different stages (with different cooling rate) complete quenching process, due to the quenching medium, the cooling process is also different. The commonly used quenching medium such as water and oil, will produce a change of state in the cooling process of quenching in usually quenching medium cooling process can be divided into three stages: vapor phase I, phase II and boiling cooling convection stage III.
II convection cooling stage I cooling stage III in steam film stage
Fig. 3.1 schematic diagram of different stages of quenching cooling
Vapor stage: popular into thermal quenching medium, quenching medium heating vaporization occurs immediately and a layer of steam film is formed on the surface of the workpiece, the workpiece is mainly through the heat radiation and conduction heat transfer to the vapor film, the slow cooling rate of the workpiece, workpiece surface until the heat can not be sufficient to maintain the required form of steam film heat, steam membrane rupture, began to shift to the boiling cooling stage.
The temperature at which the steam film phase transforms to boiling cooling is called the characteristic temperature of the quenching medium. The characteristic temperature of the quenching medium is high, indicating that the quenched workpiece can enter the rapid cooling stage at higher temperature. The characteristic temperature is related to the physical properties of the medium, such as low viscosity and low vapor pressure, which can cause the characteristic temperature to rise. The relative motion of quenching medium and workpiece, such as liquid agitation and workpiece movement can make steam membrane rupture, make characteristic temperature, cooling speed, to prevent the decomposition of austenite and non martensite. Characteristic temperature is an important index to evaluate the cooling performance of quenching medium.
Boiling cooling stage: the workpiece directly contact with the quenching medium, the quenching medium in the workpiece surface boiling strongly, the heat of the workpiece is absorbed by the medium vaporization, the heat dissipation rate is accelerated, the cooling rate reaches the maximum value. The temperature of the workpiece surface decreases rapidly. When the surface temperature of the workpiece is lower than the boiling point of the liquid, the boiling cooling stage is finished.
Convection cooling stage: when the surface temperature of the workpiece is lower than the boiling point of liquid, it enters the convection cooling stage, at this time the heat transfer between the workpiece and the medium is mainly conducted by convection conduction.
The workpiece during quenching, in order to avoid the change types of pearlite austenite in the higher temperature region, so it must be in the supercooled austenite in the most unstable region, with greater than the critical cooling rate (Vc) for cooling, in order to obtain martensitic organization, but also the cooling medium speed is not as fast as possible, if the cooling rate fast has cooled to room temperature, will enable the workpiece by thermal stress in the high temperature region in the workpiece in the impact plastic state, when the workpiece cooling to the martensite start point below Ms, but also due to the occurrence of organizational change is produced when the stress, which will lead to the great deformation, and even cracking. Therefore, slow cooling is needed in the martensitic transformation region. It should be the ideal cooling curve of the workpiece austenite unstable region (critical cooling area) with fast cooling speed (greater than the critical cooling rate), in the martensitic transformation region (below Ms) cooling speed should slow cooling, so as to ensure the work piece can be obtained after quenching quenching structure, and can reduce the the internal stress, with the cooling rate such as the ideal cooling speed. The traditional quenching medium such as tap water, salt water, salt water, water glass, mechanical oil, diesel oil, itself is not a special quenching medium in use, although it has certain advantages, but there are also many disadvantages, so quenching medium professional should have some characteristics as follows:
U has a good cooling performance, there should be enough cooling rate in the unstable region of supercooled austenite, and the cooling rate should be relatively slow in the low temperature martensitic transformation region;
U has a wide range of application; it has a stable composition, and it is not easy to deteriorate during use;
U has no corrosion of the workpiece, the workpiece after quenching surface smooth, easy to clean;
U is non-toxic, pollution-free and environment-friendly;
Quenching oil is the most common type of quenching medium, among which mineral oil is the main type. Mineral oils are generally classified according to aromatic, naphthenic and paraffinic bases, which are related to carbon and hydrogen atoms in oil molecules
