An essential phase in the thermal treatment of metals is quenching. It involves quickly cooling a metal object to obtain or change qualities like hardness, strength, or toughness.
Rapid cooling reduces metal exposure time to high temperatures and shields it from flaws. Moreover, metal can undergo alterations depending on the application method and media.
Air, oil, water, and brine are a few typical quenching agents.
Oil is widely used for quenching because it rapidly transfers heat without significantly distorting the metal. Even while water-based caustic quenchants are faster, the force with which they work can cause some materials to shatter or distort.
The difference between oil and water is the main point to be discussed in the article.
What Is The Quenching Process?
Quenching is a rapid cooling process that results in the hardening of materials. The quenching rate depends upon the respective material’s grade, application, and composition of alloying components. Additionally, several properties of the quenching medium also affect it.
Theoretically, before quenching, a metal or glass material undergoes heating beyond its standard temperature. After that, it is put into quick cooling to remove heat immediately. It helps to modify those properties in the crystalline structure of a material that is lost during heating.
To make metal or glass harder and stiffer as an item, we often quench them. An object’s quenching temperature should always be above its recrystallization temperature but below its melting temperature.
Stages of the Quenching Process
There are typically three stages of quenching that occur when a hot piece comes closer to the liquid quenchant. These stages define the change in the characteristics of the quenchant and the material. The three steps are:
- Vapor Stage
- Nucleate Boiling Stage
- Convection Stage
Now, let’s review them in depth.
Vapor Stage
The vaporization stage comes into play when the hot surface of the component makes initial contact with the liquid quenchant. It results in a vaporous shield formation around the element. Conduction occurs to some extent during the vapor phase.
However, this stage’s primary heat transport method is radiation through the vapor blanket. The blanket formed is relatively stable.
The only way to speed its removal is by agitation or adding different additives. Moreover, it is preferable to make this stage as brief as possible
The reason is that it contributes significantly to the soft areas that develop during quenching. Therefore, unwanted micro-constituents may develop if they are allowed to continue.
Nucleate Boiling Stage
It is the second stage after the vaporous phase. It begins when fluid closer to the material surface begins to boil, and the vapor stage begins to collapse. It is the fastest stage of cooling the given component.
Due to heat transmission from the heated surface and subsequent absorption into the liquid quenchant, substantial heat extraction rates are possible. It allows cooled liquid to take its place at the surface.
Several quenchants have included additives to boost a fluid’s maximum cooling rates. The boiling ends whenever the surface temperature of the component falls below the liquid’s boiling point.
For those components prone to distortion, mediums like high-temperature oils and salts provide good results. Otherwise, the materials might become brittle and quickly damage during the desired applications.
Convective Stage
Convection is the final stage of the process. It happens when the material reaches a temperature lower than the quenchant’s boiling point. The convection stage involves heat transfer through the bulk fluid, and its starting point is conduction.
It is the slowest stage because heat transfer takes a long time to reach all the molecules within the bulk. Controlling heat evacuation through convection involves many variables, including the specific heat of the quenchant and its thermal conductivity.
The temperature difference between the quenchant and the material may affect the convection process. Usually, most of the distortion happens at this point.
The above three quenching steps take place in order at a particular location. Nevertheless, depending on the part’s geometry and agitation, different areas will start the various phases at various times.
Quenching Mediums
Quenching occurs through any medium, and the following is the list of 4 different media. Each has pros and cons, depending on its properties, contacting elements, time, heat transfer laws, and relations.
- Air: Utilization of a regular ambient temperature to cool the heated material
- Brine: A solution of salt and water is the quickest cooling medium when quenching.
- Oil: A reliable and quicker quenching alternative to air.
- Water: Quicker than air or oil at quenching liquids.
Even though literature has vast information on the above mediums, let’s explore the two major ones, oil and water.
Water Quenching
Water has the property of cooling the material down quicker than oil and air. So, quenching through water is a fast-paced process.
- The brine quenching procedure has a significantly harsher reaction when cooling than any other, sousing water is the most effective method.
- Before this process, the water has to be at room or the desired temperature. After that, when the heated material is put into cooling water, it changes its phases according to the stages.
- The results come faster in water quenching. Another advantage is that it’s a rapid cooling method. Therefore it is the least expensive in terms of both money and time. However, of course, the fast result comes with significant drawbacks too.
- The disadvantage of stiff, brittle, and readily breakable end products comes with this quick or instant speed. The quenched material might be labeled as having either sound quality or bad quality.
- Water quenching is a viable option in the case of steel hardening. The reason is that steel has a unique way of cooling which can be achieved through water. The carbonized steel heats above its re-crystallization temperature.
- By immediately cooling the steel, water quenching prevents the steel from melting at this stage when it would otherwise melt if not stopped. Therefore, water quenching is more suitable for steel than the other mediums.
Oil Quenching
One of the most popular quenching techniques in the metal quenching sector is oil quenching. The optimal method for hardening metal alloys gives them the necessary hardness and power without causing them to become stiff and brittle during the process.
Going with oil quenching has several pros, but the main one is that it warms up more slowly than other quenching mediums and cools for a longer duration, giving the heated material greater stability and hardening time.
Additionally, this guarantees that the quenched material won’t be overly brittle and will hold perfectly well. Therefore, it is preferable over water, air, or brine methods because it lowers the possibility of quenched metal’s body distorting or cracking.
Difference Between Water and Oil Quenching
Water and oil are two different types of media. Both are distinguishable in some aspects and behave differently in quenching. The table below summarizes an overview of disparities between the two media.
| Characteristics | Water Quenching | Oil Quenching |
| Thermal Conductivity | The thermal conductivity of water is higher, which in turn leads to faster cooling and higher hardening. | The thermal conductivity of oil is lower than water. Therefore the process of cooling and hardening is slower than the water. |
| Specific Heat | The specific heat of water is higher than oil. It means the water takes more energy to raise and lower its temperature. | The specific heat of the oil is about 50% of that of water. To cool by the same amount, it must lose less heat. |
| Viscosity | Water is less viscous than oil. It undergoes a slight change in viscosity with the temperature difference. | Oil is more viscous than water. They are adjustable, and additives can very well modify their properties. |
| Density | The density of water is higher than oil. | Oil is less dense than water. |
| Quenching Rate | Water quenching is the way to go if you want to quench something more quickly. | Oil rapidly transfers heat without significantly distorting the metal. |
| End Product | Although the water quenching procedure is quicker, the final product is somewhat brittle. | The oil quenching process takes a little longer; it often yields a superior product. |
Conclusion
- Quenching quickly cools metals to change their properties.
- Various quenching agents like air, oil, water, and brine affect metals differently.
- Water cools fast but may make metals brittle.
- Oil cools slower but stabilizes metals and prevents excessive brittleness.
- Water conducts heat and holds heat better than oil. It cools faster but makes metals more brittle.
- Oil efficiently transfers heat without distorting metals much.
- Knowing the differences helps choose the right method for specific metals and needs.
- Water and oil quenching are both important in metalwork. Both have their advantages.