On 2018-08-08 14:48:54
Heat treatment quenching oil selection principle and method
1. Heat treatment cooling curve
Heat treatment process generally includes three heating, heat preservation, cooling process, the heating is for pearlite to austenite transformation, heat preservation is completely austenitizing, cooling method for process, main is to control the cooling speed, with different cooling rate respectively into pearlite, bainite and martensite or hybrid organization. Generally, martensite is expected to be obtained when quenching, and tempering is conducted after quenching according to the tempering temperature to obtain tempering martensite (low temperature), trostenite (trostenite, medium temperature), sostenite (high temperature).
In the actual heat treatment production, continuous cooling is more common except the graded isothermal quenching process. The speed of martensite structure must be greater than the critical cooling speed. One of the selection principles of quenching oil: quenching both want to martensite, cooling speed must be greater than the critical cooling speed, and to consider the reduction of deformation, to prevent cracks, cooling speed must be moderate, not too large.
2. Ideal quenching method
According to the law of steel cooling transformation, it is hoped that the cooling speed will be high when the critical region temperature is reached, and the nose area of the C curve will be passed as soon as possible, so as to avoid the transformation into pearlite or bainite tissues. In the dangerous region where the martensitic transformation begins, the cooling speed must be slowed down to reduce the deformation or even crack caused by the tissue stress caused by the tissue transformation. The double - liquid quenching is quenched first in water and then transferred to oil cooling is the reason. Single liquid quenching requires the cooling medium to have such cooling characteristics: the cooling speed is fast when the critical zone temperature, the cooling speed to slow down when the dangerous zone temperature. The choice of quenching oil principle two: high temperature fast cooling low temperature slow cooling, both hardness and deformation requirements.
3. Quenching oil cooling curve
Heat treatment quenching oil needs to have the above cooling properties, the cooling speed of parts in the critical region temperature is fast, the cooling speed of parts in the dangerous region temperature is slow. FIG. 4 shows the cooling characteristics of haofton MT355 quenching oil at different oil temperatures, showing the relationship between the cooling time, cooling speed and temperature of the sample, and the cooling capacity of the quenching oil at different temperatures.
4. Hardness requirements and deformation effects
The critical cooling speed of different materials is not the same, the hardness requirements of the parts are not the same, the precision of the parts, the control of deformation requirements are also different, this needs to according to the hardness requirements and the actual situation of the control of deformation process technology requirements, choose the cooling speed of the appropriate quenching oil. Quenching oil after the selection, by controlling the raw material quality of material and product (parts) and choose to adjust the quenching temperature, cooling speed and oil temperature, stirring time, into the oil time, etc. Various control measures, combining the tempering after quenching process, tempering temperature and tempering time parameter to satisfy the requirement of the parts hardness test and metallographic organization requirements, through the process of heat treatment process parameters adjustment and control and coordinated with the cold and hot machining, control heat treatment deformation within a reasonable range.
5. Selection method of heat treatment quenching oil
Embarks from the heat treatment quenching process mentioned in front of the quenching oil selection principles: quenching oil cooling performance must be meet the requirements of hardness (quick cooling speed requirements), and will never allow to produce cracks, control heat treatment deformation within the scope of reasonable minimum (slow cooling speed requirements) to find a best combination between, and the cooling speed requirement of this contradiction is put forward in the temperature range for different requirement, it is developed for heat treatment of quenching oil provides a direction. In view of the limitation of testing conditions, we can only select a rough range of quenching media according to the general knowledge and experience in the production practice of heat treatment in the factory and then verify it by experiments. The extensive use of the cooling characteristic instrument is to provide a detection tool analysis means for us to optimize the scientific selection of quenching oil, analysis and comparison to help. We can have the condition to detect or the outside inspection or request the oil company to provide the quenching oil cooling characteristic test data report to provide the scientific basis for the quenching oil selection.
According to the above methods, there are many kinds of quenching oils that meet the requirements for use. According to the actual production situation and the company's quality policy, the quenching oil that meets the requirements of cooling performance should be compared and compared with other technical indexes to select the most suitable one. The stability and oxidation resistance of quenching oil, the repeatability and reproducibility of cooling performance, brightness, safety and cost are all factors to be considered.