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Types and characteristics of cooling media
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Overview.
Cooling media include gases (air, nitrogen, etc.), water, oil, water-soluble coolants, and salt baths, with gases having the lowest cooling capacity and water the highest.
01 Type of cooling medium
Air and other gases are coolant of all cooling agents cooling capacity SKD61 and SKD11 have good hardenability and can be hardened by gas cooling.
Water is the quenching coolant with the greatest cooling capacity, but if the temperature of the liquid rises or if it is not stirred sufficiently during cooling, the cooling capacity will be extremely reduced, and it is easy for hardening to occur.
MINERAL OIL IS USUALLY USED AS QUENCHING OIL, AND JIS K 2242 SPECIFIES THREE TYPES OF HEAT TREATMENT OIL: ONE FOR GENERAL QUENCHING, TWO FOR HOT BATH QUENCHING, AND THREE FOR TEMPERING.
In addition, there are water-soluble coolants of polymeric compounds and nitrate salt baths, which have cooling capacities between those of water and oil.
| type | feature | |
| Gases [air, N2, Ar, etc.] | It has the lowest cooling capacity of all coolants. In vacuum furnaces, the cooling capacity is increased by pressurized cooling or forced cooling with a fan. | |
| water (esp. cool, fresh water, e.g. drinking water) | It has the greatest cooling capacity, but as the concentration increases, the cooling capacity immediately decreases, so it must be kept below 30°C. | |
| oil | １種 | There are two types, No. 1 and No. 2, the latter having a higher cooling capacity. |
| ２種 | There are two types of coolants: No. 1 for use at 120°C and No. 2 for use at 160°C. These coolants are effective in reducing quenching strain and are used for hot bath hardening at temperatures of 100°C or higher. | |
| water-soluble coolant | Aqueous solutions of polymeric compounds (a few % to 30%) with less cooling capacity than water and more cooling capacity than oil. Since the cooling rate is affected by concentration and liquid temperature, they must be well controlled. | |
| salt bath | Nitric acid salt baths are used at temperatures from 150 to 400°C. They do not have a vapor curtain stage like water or oil. It does not have a vapor curtain stage like water or oil, so it is less likely to cause uneven baking and has slightly greater cooling capacity than oil. It is often used as a coolant for circle temper and austempering. | |
02 Cooling behavior in coolant
When quenching and cooling with water or oil, cooling proceeds through three stages as shown in the figure above. That is, immediately after quenching, the treated material is covered with a vapor film ( vapor film phase ), so the cooling rate is slowest. However, when the characteristic temperature is reached, the vapor film is broken and bubbles are generated violently from the surface of the work piece ( Boiling Phase ) and the cooling rate becomes fastest. As the cooling progresses further, the cooling rate slows down and the convection phase The cooling rate slows down as the cooling progresses further, and a convection phase occurs. In order to increase the cooling capacity of water or oil, the vapor film must be broken down quickly, so the stronger the stirring, the better. In other words, the cooling method with the highest cooling capacity is jet cooling with water. In the case of salt baths, there is no vapor film phase as in the case of water or oil, so a relatively uniform cooling rate can be obtained.
03 Effect on hardening hardness
The effect of coolant on hardening hardness is not so pronounced in the case of alloy steels with excellent hardenability, in which case oil quenching is generally used. However, the hardness of carbon steel, which has poor hardenability, is extremely affected by the coolant. In other words, the highest values for both surface hardness and center hardness are obtained when water quenching is used.