The medium used for quenching and cooling the workpiece is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress. This requires slow cooling at the temperature above the “nose” of the C curve to reduce the thermal stress generated by the rapid cooling; the cooling rate at the “nose” should be greater than the critical cooling rate to ensure that the supercooled austenite does not occur non-Martensitic Body transformation; under the “nose”, especially when Ms points to a temperature, the cooling rate should be as small as possible to reduce the stress of tissue transformation.
Commonly used quenching media are water, aqueous solution, mineral oil, molten salt, molten alkali, etc.
Water is a quenching medium with strong cooling capacity. Wide source, low price, stable ingredients and not easy to deteriorate. The disadvantage is that in the “nose” area of the C curve (about 500-600°C), the water is in the vapor film stage, and the cooling is not fast enough, and a “soft spot” will be formed; while in the martensitic transformation temperature zone (300-100°C), The water is in the boiling stage, and the cooling is too fast, which is easy to make the martensite transformation speed too fast and generate a large internal stress, resulting in deformation or even cracking of the workpiece. When the water temperature rises, the water contains more gas or insoluble impurities (such as oil, soap, mud, etc.) are mixed in the water, which will significantly reduce its cooling capacity. Therefore, water is suitable for quenching and cooling of carbon steel workpieces with small cross-sectional dimensions and simple shapes.
● Brine and alkaline water
Add an appropriate amount of salt and alkali to the water, after the high-temperature workpiece is immersed in the cooling medium, the salt and alkali crystals are precipitated in the steam film stage and burst immediately, the steam film is destroyed, and the oxide skin on the surface of the workpiece is also blasted. The cooling capacity of the medium in the high temperature area. The disadvantage is that the medium is highly corrosive.
In general, the concentration of brine is 10%, and the concentration of caustic soda aqueous solution is 10% to 15%. It can be used as a quenching medium for carbon steel and low-alloy structural steel workpieces. The operating temperature should not exceed 60 °C. After quenching, it should be cleaned and rust-proofed in time.
The cooling medium generally uses mineral oil (mineral oil). Such as engine oil, transformer oil and diesel oil. No. 10, No. 20, and No. 30 oil are generally used for engine oil. The larger the oil number, the higher the viscosity, the higher the flash point, the lower the cooling capacity, and the correspondingly higher operating temperature.
The new quenching oils currently used mainly include high-speed quenching oil, bright quenching oil and vacuum quenching oil.
High-speed quenching oil is a quenching oil whose cooling speed is increased in high temperature area. There are two basic ways to obtain high-speed quenching oil. One is to select mineral oils of different types and different viscosities, mix them with each other in an appropriate ratio, and improve the cooling capacity in the high temperature area by increasing the characteristic temperature; the other is to use ordinary quenching. Additives are added to the oil to form ash-like floats in the oil. Additive barium salt, sodium salt, calcium salt, phosphate, stearate, etc. of sulfonic acid. Production practice shows that the cooling rate of high-speed quenching oil in the unstable region of supercooled austenite is significantly higher than that of ordinary quenching oil, and the cooling rate of high-speed quenching oil is close to that of ordinary quenching oil in the low-temperature martensitic transformation region. In this way, high hardenability and hardenability can be obtained, and the deformation is greatly reduced, which is suitable for the quenching of alloy steel workpieces with complex shapes.
Bright quenching oil can keep the workpiece bright surface after quenching. By adding polymer additives of different properties to mineral oil, bright quenching oils with different cooling rates can be obtained. The main component of these additives is the brightener, whose role is to suspend the aging products that are insoluble in the oil, preventing accumulation and precipitation on the workpiece. In addition, bright quenching oil additives also contain antioxidants, surfactants and refrigerants.
Vacuum quenching oil is a cooling medium for vacuum heat treatment quenching. Vacuum quenching oil must have low saturated vapor pressure, high and stable cooling capacity, good brightness and thermal stability, otherwise it will affect the effect of vacuum heat treatment.
Salt bath and alkali bath quenching medium are generally used in graded quenching and isothermal quenching.
● New quenching agent
There are polyvinyl alcohol aqueous solution and trinitrate aqueous solution.
Polyvinyl alcohol is commonly used in an aqueous solution with a mass fraction of 0.1% to 0.3%, and the total cooling capacity is between water and oil. When the workpiece is quenched into the solution, a vapor film and a gel film are formed on the surface of the workpiece, and the two films cool the heated workpiece. After entering the boiling stage, the film is broken and the cooling of the workpiece is accelerated. When the low temperature is reached, the polyvinyl alcohol gel film is formed again, and the cooling rate of the workpiece decreases again. Therefore, the cooling capacity of this solution is low in the high and low temperature regions, and it is cooled in the medium temperature region. High capacity and good cooling characteristics.
The trinitrate aqueous solution is composed of 25% sodium nitrate + 20% sodium nitrite + 20% potassium nitrate + 35% water. At high temperature (650 ~ 500 ℃), due to the precipitation of salt crystals, the steam film is broken and the cooling capacity is close to that of water. At low temperature (300 ~ 200 ℃), due to the extremely high concentration, poor fluidity, and cooling capacity close to oil, it can replace water-oil dual-medium quenching.
The most widely used quenching classification in production practice is the different division of cooling methods. There are mainly single-liquid quenching, double-liquid quenching, graded quenching and isothermal quenching.
●Single liquid quenching
It is a quenching operation method in which austenitized workpiece is immersed in a certain quenching medium and cooled to room temperature. The single-liquid quenching medium includes water, brine, alkaline water, oil and specially formulated quenching agent. In general, carbon steel is quenched, and alloy steel is quenched with oil.
The operation of single-liquid quenching is simple, which is conducive to the realization of mechanization and automation. The disadvantage is that the cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality. For carbon steel, single-fluid quenching is only suitable for workpieces with simpler shapes.
● Double liquid quenching
It is to immerse the austenitized workpiece in a medium with strong cooling ability, take it out before the steel reaches the temperature of the quenching medium, and immediately immerse it in another medium with weak cooling ability to cool, such as water first and then oil , water first, then air, etc. Double-liquid quenching reduces the tendency of deformation and cracking, the operation is not easy to master, and there are certain limitations in application.
● Martensitic graded quenching
It is to first immerse the austenitized workpiece in a liquid medium (salt bath or alkali bath) with a temperature slightly higher or lower than the martensitic point of the steel, and keep it for an appropriate time until the inner and outer layers of the steel workpiece reach the medium. After the temperature is taken out, it is taken out for air cooling to obtain a quenching process of martensitic structure, also known as graded quenching.
Since the grading quenching stays at the grading temperature to the same temperature inside and outside the workpiece and then air-cooled, it can effectively reduce the phase transformation stress and thermal stress, and reduce the quenching deformation and cracking tendency. Graded quenching is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes.
● Bainite isothermal quenching
It is a quenching process that austenitizes the steel, makes it quickly cool to the bainite transformation temperature range (260-400℃), and maintains it isothermally, so that the austenite is transformed into bainite. It is sometimes called isothermal quenching. The general holding time is 30 to 60 minutes.
● Compound quenching
The workpiece is quenched to below Ms to obtain 10% to 20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method can obtain the M+B structure of the workpiece with a larger cross-section. The martensite formed during prequenching can promote the bainite transformation, and then temper the martensite at the isothermal time. Compound quenching is used for alloy tool steel workpieces, which can avoid the first type of temper brittleness and reduce the amount of retained austenite, that is, the tendency of deformation and cracking.
1. What problems should be paid attention to when using new oil in the whole tank?
Before pouring new oil, you must carefully check and clean the quenching oil tank and cooling system. Residual water, sludge and other dregs should be cleaned up. When adding new oil to the old oil tank system, the oil stains on the tank wall and various frames above the oil surface in the quenching oil tank should also be removed and cleaned up. If the old oil residue and sludge are mixed into the new oil, the cooling characteristics of the oil may be changed. Therefore, cleaning should be done more thoroughly than using a new oil tank.
After the new tank is filled with new oil, it should not be used for quenching production immediately. Quenching oils always carry a small amount of air during production, transportation and dumping. Metalworking Hot Working Journal. The presence of gas reduces the cooling rate of the quench oil in the high temperature stage and should be removed. The solubility of gas in oil decreases with the increase of oil temperature. Raising the oil temperature can reduce the viscosity of the oil and facilitate the floating of the air bubbles. Therefore, the gas in the new oil can be removed by raising the oil temperature.
2. Why should the quenching oil be circulated and stirred?
A good cooling cycle can prevent the local oil temperature from being too high, so that the oil temperature in each part of the tank tends to be uniform. The cooling cycle of the oil can increase the relative flow rate between the workpiece and the quenching oil, thereby improving the cooling capacity of the oil and avoiding soft spots on the surface of the workpiece.
When the oil temperature is too high, the oil will instantly lead to a higher local oil temperature, which may cause a fire hazard.
3. How to reduce oil pollution?
The pollution sources of quenching oil include external pollution and self-contamination.
External pollution: oxide skin brought in during the quenching process of the workpiece, water leaked from the cooling system and other substances from outside.
Self-contamination: oxidative deterioration products that cannot be automatically discharged during use and remain in the oil; plus the residual products after the reaction of foreign pollutants with quenching oil and its pollutants. The accumulation of internal and external contaminants will gradually change the color, transparency, viscosity, flash point, carbon residue and acid value of the oil. This change process is the deterioration process of quenching oil. Among the influences caused by the deterioration, the changes in the oil cooling characteristics and the deterioration of the brightness of the workpiece after quenching are most related to the heat treatment effect of the workpiece. Changes in cooling characteristics often affect the quench hardness, hardened layer depth and deformation of the workpiece. Excerpted from Heat Treatment Ecosphere.
Preventing and reducing foreign pollution, rational use and management of quenching oil, and regular cleaning can slow down the deterioration of quenching oil and prolong the service life of quenching oil.
4. What is the oil change index of quenching oil?
The replacement of quenching oil is determined by the specific aging degree, not by how long it has been used. Regular sampling and analysis are required during the use process. The aging degree of quenching oil mainly depends on the base oil, additives and on-site use conditions.
General oil change index of quenching oil:
(1) The characteristic temperature drops more than 40℃;
(2) Under normal use, the moisture content exceeds 0.1% (volume fraction).
(3) The viscosity rises by more than 15%;
(4) The brightness is obviously reduced.
5. What should I do after the quenching oil enters the water?
Water entering the quenching oil will affect the cooling performance, especially the low temperature cooling rate, and the workpiece may cause excessive deformation or cracking after quenching.
The quenching oil after entering the water should be subjected to high-temperature dehydration treatment, and the oil temperature should be heated to 80-130 ℃ and stirred for a certain period of time. The specific time depends on the amount of water. After dehydration, the cooling performance should be tested, and it can be used normally only when it meets the factory requirements. Otherwise, additives should be added for adjustment.
6. How to use water-based quenching fluid?
The water-based quenching fluid is mixed with water. The water-added ratio depends on the actual workpiece and material. The operating temperature cannot exceed 50 °C. If the temperature is too high, the cooling performance will be seriously reduced, resulting in the hardness of the workpiece not meeting the requirements.