Quenching is the most critical process in the heat treatment process, which ultimately determines the intrinsic quality and distortion of the quenched parts. Quenching and cooling technology has progressed along with the development of heat treatment technology. However, due to the complexity of the cooling process and the instantaneous nature of the process, coupled with the limitations of observation and measurement, quenching and cooling is somewhat of a mystery. ROCHO technical team shares the knowledge and selection of quenching and cooling media with our long experience in heat treatment industry.
What are the three major types of quenching media used to quench metals?
Here are the main types of quenching media:
Air.
Oil.
Water.
Brine.
What are the six basic properties that quenching and cooling media should have?
(1) The Ideal Cooling Rate
The ideal cooling rate, refers to a certain material and the specific quenching parts, hope that it is in the cooling of the cooling rate is greater than the critical cooling rate (to ensure that the workpiece can complete the martensitic transformation of the minimum cooling rate). In other temperature ranges, especially in the martensite transformation temperature range (Ms → Mf) should be slow cooling.
Different steels and different workpieces have different requirements for the above "fast" and "slow", and there is a theoretical concept of "ideal quenching cooling medium". Different steel curve is not the same, to get a variety of steel and different sizes of workpiece quenching the so-called "ideal quenching and cooling medium" is impossible, we can only choose the right quenching and cooling medium according to the specific circumstances, so that the quenched parts quenched hard, small distortion, good metallurgy.
(2) Good Stability
Quenching and cooling medium in use should remain relatively stable, not easy to decomposition, deterioration and aging. A variety of quenching oils and organic solvents, such as the existence of varying degrees of aging phenomenon, should be replaced and maintained in time.
(3) The Uniformity Of Cooling
Different parts of the workpiece and the surface as evenly cooled as possible, to avoid quenching soft spots, soft blocks.
(4) Non-corrosive
After quenching process, to keep clean, no corrosive effect on the workpiece.
(5) Environmentally friendly
Quenching process should not produce a lot of smoke, toxic and irritating gases, and the waste liquid from the quenched parts should not cause pollution to the environment.
(6) Safety
Quenching and cooling medium must be non-flammable, non-explosive, safe to use.
What are the 9 factors that affect the cooling performance of quenching media?
(1) Temperature
Quenching and cooling medium temperature is different, its cooling capacity is also different. Water medium with the temperature of the cooling capacity decreases, while oil and salt bath are the opposite, with the temperature increases, with good mobility, easy to heat dissipation, while cooling capacity increases, so it is important to grasp the specific workpiece cooling temperature.
(2) Surface Tension
The size of the surface tension directly affects the cooling rate, generally speaking, the surface tension of the quenching and cooling medium and quenching parts of the surface contact intimate, and therefore fast heat dissipation, cooling capacity to improve.
(3) Stirring
Stirring will increase the quenching and cooling medium heat transfer coefficient, as early as possible to destroy the vapor film, improve the cooling rate, and make the quenching parts cooling uniform.
(4) Thermal Conductivity
Thermal conductivity, also known as the coefficient of thermal conductivity, characterizes the physical properties of material heat transfer, the greater the thermal conductivity, the stronger the cooling capacity.
(5) Specific Heat Capacity
The larger the specific heat capacity of the quenching and cooling medium, the faster the cooling rate.
(6) Viscosity
Viscosity characterizes the liquid flow, the internal friction between the liquid molecules presented. Viscosity of the quenching and cooling medium is poor mobility, is not conducive to convection cooling, its cooling capacity is poor, conversely, the viscosity of small cooling effect is good.
(7) Heat Of Vaporization
Heat of vaporization refers to the unit mass of liquid completely into the same temperature as the heat required for the gas. The chemical stability of water at room temperature is 8 times that of steel, the heat capacity of water is large, the boiling point of water is low, the heat of vaporization with the increase in temperature and decrease, water with the increase in temperature, the cooling capacity drops sharply, but the water temperature rises to 80 ℃ after the cooling capacity of little change, the quenching intensity is maintained at about 0.72.
(8) Additives
Additives are added to change the cooling performance, such as the addition of a small amount of salt or alkali in the water, its cooling capacity will increase several times. The addition of polyvinyl alcohol in the water will form an extremely thin plastic film on the surface of the quenched parts, which has poor thermal conductivity and reduces the cooling rate. Water with oil, soap and other suspensions or emulsions formed, will accelerate the formation of steam film, increasing the stability of the steam film, so that the cooling capacity is reduced. In addition, in order to change the quenching and cooling medium of other properties, we often consciously add oxidizing agents, brighteners, rust inhibitors, preservatives, etc. It will affect the cooling performance to varying degrees. The addition of additives will generally receive a multiplier effect.
(9) Environment
Environmental impact on the cooling capacity is often not noticed, the cooling effect varies between winter and summer, and the quenching effect varies between day and night.
What are the 3 basic principles for choosing quenching and cooling media?
A large number of reality tells us that many heat treatment quality accidents with quenching and cooling media, if improperly selected or operating errors, will cause quenching parts scrap. Therefore, the quenching and cooling medium is the basic condition to ensure product quality. Regardless of the choice of quenching and cooling medium, a uniform quenching effect should be obtained.
1, obtain high and uniform surface hardness and sufficient hardening depth.
2, can not be quenched and cracked.
3, quenching distortion is small.
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