1. Cooling curve of heat treatment
The heat treatment process generally includes heating, heat preservation, and cooling. The heating is to transform pearlite to austenite. The heat preservation is completely austenitized. The cooling method differs depending on the process, mainly to control the cooling rate. Different speeds transform into pearlite, bainite, martensite or mixed structure. Normally, martensite is desired during quenching, and tempered martensite (low temperature), tousite (troostite, medium temperature), and sorbite (high temperature) are obtained according to the different tempering temperatures during tempering after quenching. . The isothermal transformation curve of eutectoid steel is shown in Figure 1, which basically reflects the incubation time, transformation completion time and transformation product required for eutectoid steel transformation at different temperatures.
In actual heat treatment production, there are many cases of continuous cooling in addition to the graded isothermal quenching process. Quenching requires that the martensite structure velocity must be greater than the critical cooling rate, and the surface cooling rate of the part is generally greater than the core cooling rate. One of the selection principles of quenching oil: the quenching rate must be greater than the critical cooling rate in order to obtain martensite, and the reduction of deformation must be considered to prevent cracks. The cooling rate must be moderate and not too large. as shown in picture 2.
2. Ideal quenching method
According to the steel cooling transition law, it is hoped that the cooling rate will be large at the critical region temperature, and pass the nose area of the C curve as soon as possible to avoid transformation into pearlite or bainite structure. In the dangerous area where martensite transformation begins, the cooling rate must be slow In order to reduce the deformation caused by tissue transformation and even the cracks caused by tissue stress. This is the case for two-liquid quenching, which is first quenched in water and then transferred to oil for cooling. Single-liquid quenching requires that the cooling medium has such cooling characteristics: the cooling rate is fast at the critical zone temperature, and the cooling rate is slowed down at the dangerous zone temperature. The second selection principle of quenching oil: fast cooling at high temperature and slow cooling at low temperature, taking into account the requirements of hardness and deformation. As shown in Figure 3.
3. Quenching oil cooling curve
The heat treatment quenching oil needs to have the above-mentioned cooling performance. In the critical area, the temperature of the parts cools fast, and in the dangerous area, the temperature of the parts cools down. Figure 4 is the cooling characteristic curve of Houghton MT355 quenching oil at different oil temperatures, showing the relationship between the sample cooling time, cooling rate and temperature, indicating the cooling capacity of the quenching oil at different temperatures.
4. Hardness requirements and deformation effects
Different materials have different critical cooling rates, different parts have different hardness requirements, and parts have different accuracy and deformation control requirements. This requires appropriate cooling rate according to the actual conditions of hardness requirements and process technology requirements to control deformation Quenching oil. After confirming the selection of quenching oil, adjust the quenching temperature, oil temperature, stirring cooling speed and time, oil entry time and other control measures by controlling the quality and selection of raw materials (part materials and oil materials), combined with the tempering process after quenching , Tempering temperature, tempering time parameter test to meet the parts hardness requirements and metallographic structure requirements, through the adjustment and control of heat treatment process parameters and coordination with the cold and heat machining, control heat treatment deformation within a reasonable range.
5. Selection method of heat treatment quenching oil
The selection principle of quenching oil is mentioned from the heat treatment quenching process: the cooling performance of the quenching oil must be at the required hardness (fast cooling speed) and cracks must not be allowed, and the heat treatment deformation should be controlled within a reasonable minimum range (cooling speed requires slow ) Find an optimal combination between the two, and this contradictory cooling rate requirement is a requirement for different temperature ranges, which provides a direction for the development of heat treatment quenching oil. In view of the limitations of the testing conditions, in the heat treatment production practice of the factory in the past, we can only make a broad range of quenching medium selection based on general knowledge and experience and then use the test to verify. The extensive use of the cooling characteristic instrument is precisely to provide a means of analysis for the detection tool, to optimize the scientific choice of quenching oil for us, and to assist in the analysis and comparison. We can conduct conditional testing or outsourcing testing or require oil companies to provide quenching oil cooling characteristics testing data reports to provide a scientific basis for the selection of quenching oil.
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 cooling performance requirements should be compared with other technical indicators and comprehensive comparisons to select the most suitable. The stability and oxidation resistance of quenching oil, the repeatability and reproducibility of cooling performance, brightness, safety, and cost are all factors that need to be considered.