What are the quenching mediums commonly used in forgings?

The medium used for quenching and cooling of forgings is called quenching cooling medium (or quenching medium). The ideal quenching medium should be equipped with the condition that the forgings can be quenched into martensite without causing too much quenching stress. This requires that the "nose" of the C curve above the temperature slow cold, in order to reduce the thermal stress caused by the cold; at the "nose" the cooling speed is greater than the critical cooling rate to ensure that the supercooled austenite does not occur non-martensite transformation; under the "nose", especially when Ms Point temperature, the cooling speed should be as small as possible to reduce the stress of tissue transformation.

Commonly used quenching medium has water, aqueous solution, mineral oil, molten salt, alkali and so on.

Water

Water is a quenching medium with strong cooling capacity. The source is wide, the price is low, the composition stability is not easy to deteriorate. The disadvantage is in the "nose" area of the C curve (500~600℃ or so), water in the vapor film stage, cooling is not fast enough, will form a "soft point", and in the Martensite transition temperature Zone (300~100℃), water in the boiling stage, cooling too fast, easy to make martensite transition speed too fast and produce a lot of internal stress, resulting in deformation and even cracking of forgings. When the water temperature rises, it contains more gas or water mixed with insoluble impurities (such as oil, soap, mud, etc.), will significantly reduce its cooling capacity. Therefore, the water is suitable for quenching and cooling of carbon steel forgings with small sectional size and simple shape.

Brine and alkali water

Add the appropriate amount of salt and alkali in the water, so that high-temperature forgings immersed in the cooling medium, in the vapor film phase of salt and alkali crystals and immediately burst, the steam membrane damage, forging surface of the oxide skin is also fried, so as to improve the medium in the High-temperature zone cooling capacity. The disadvantage is that the medium has a large corrosive behavior. In general, the concentration of salt water is 10%, the concentration of caustic soda solution is 10%~15%. Can be used as carbon steel and low alloy structural steel workpiece quenching medium, the use of temperature should not exceed 60 ℃, after quenching should be timely cleaning and anti-rust treatment.

Oil

The cooling medium generally uses mineral oils (mineral oil). such as oil, transformer oil and diesel. The general use of oil 10th, 20th, 30th Oil, the number of oils, the greater the viscosity, the higher the flash point, cooling capacity, the lower the use of temperature to improve accordingly.

At present, the new quenching oil is mainly used in three kinds of high speed quenching oil, bright quenching oil and vacuum quenching oil.

High-speed quenching oil is the quenching oil which is improved in the high temperature zone. There are two basic ways to obtain high speed quenching oil. One is to select different types and different viscosity of mineral oil, with the appropriate proportion of each other mixed, by increasing the characteristic temperature to improve the cooling capacity of high-temperature zone; another is to add additives to the common quenching oil to form a powder ash-like float in the oil. Additives travel sulfonic acid barium salt, sodium salt, calcium salt, as well as phosphate, hard fatty acid salt and so on. The production practice shows that the cooling rate of high speed quenching oil in supercooled austenite is higher than that of common quenching oil, but the cold speed in the low temperature martensite transition zone is close to that of common quenching oil. This can not only get higher hardenability and quenching rigidity, but also greatly reduce the deformation, applicable to the shape of the complex alloy steel forgings quenching.

The bright quenching oil can make the forgings keep the bright surface after quenching. In the mineral oil to add different properties of polymer additives, can obtain different cooling rate of light quenching oil. The main ingredient of these additives is Brightener, which is to suspend the aging products which are insoluble in oil and to prevent accumulation and precipitation on forgings. In addition, the bright quenching oil additive also contains antioxidants, surfactants and refrigerant and so on.

Vacuum quenching oil is used for quenching the vacuum heat treatment of the cooling medium. Vacuum quenching oil must have low saturated vapor pressure, high and stable cooling capacity and 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 three nitrate aqueous solution.

The common quality fraction of polyvinyl alcohol is the aqueous solution between 0.1%~0.3%, and the total cooling capacity is between water and oil. When the forging is quenched into the solution, a vapor film and a layer of gel film are formed on the surface of the forging, and two layers of film make the heating forgings cool. After entering the boiling stage, the film rupture, forging cooling speed up, when reaching low temperature, the PVA gel film is formed, the forging cooling rate is reduced, so this solution in the high and low temperature zone cooling capacity, high cooling capacity in the middle temperature zone, has good cooling characteristics.

The three nitrate solution is composed of 25% sodium nitrate +20% sodium nitrite +20% potassium nitrate +35% water. In the high temperature (650~500℃), because of the precipitation of salt crystals, breaking the vapor film formation, cooling capacity close to water. At low temperature (300~200℃) due to high concentration, poor fluidity, cooling capacity close to the oil, it can replace the water-oil dual medium quenching.