If you have ever watched an episode of the popular show Forged in Fire or saw some blacksmiths in some public work, you may wonder why are they putting hot metal in a can of oil. All that smoke and fire coming out may seem unusual or even magical to you. In this article, we decided to explain the reason and magic behind that interesting process.
Blacksmiths quench their materials in oil to harden the particular metal. Oil is extremely popular in blacksmithing because of its ability to transfer heat slower and superior compared to water. In addition to that, oil quenching results in less distortion and metal cracking due to cooling the material evenly.
It’s important to note that not every material can or should be quenched in oil. This is mainly depending on the type and properties of a particular material.
Oil Quenching Explained
As far as we know, motor oil has been used for quenching since the 19th century. Since then, many advantages have been made in their advancement. Through proper formulation and blending, a wide range of quenching properties can be achieved. Emulsifiers are often added to allow easy cleaning of the metal after quenching. Quenching oil contains complex antioxidant packages in order to maintain its use for a long period of time, especially at high temperatures.
In this case, oil provides 2 primary functions.
Blacksmiths quench in oil because it promotes the hardening of steel by regulating heat. Second, it speeds up the wetting of steel to minimize the formation of unwanted soft spots which can lead to distortion and cracking.
As opposed to water or another suitable liquid, oil does not boil as quickly, so cooling is more even. As a result of that, it will reduce the chance of cracking.
Related to temperature, there are two opposing factors. As the temperature of oil rises, the cooling rate decreases because of the vapor. However, as the temperature of oil rises, it becomes more liquid and therefore increases the rate of heat conduction.
Important Quenching Factors
Quenching can be messed up in a lot of different ways, so it is very important to consider some of the major factors in quenching.
- Medium or quenchant
- Steel type
- Quenchant temperature
- Shape of steel
- Container size
The first is the quenchant or medium which refers to the thing you will use for quenching. Most commonly, water or oil is used for these purposes.
If you pick the wrong steel and plunge it in the wrong medium, you may get a low-quality blade, so choosing the proper steel type is crucial.
Except for the choice of the quenchant, another very important factor is the quenchant temperature. The hotter the quenchant, the slower the cooling process is. I know it sounds strange, but we will explain it later.
If you quench something that is narrow on one end, and wider on the other, a knife, for example, will greatly impact the cooling process and distortion.
Container size is of great importance for every blacksmith. The dimensions of the container highly determine the rate of cooling in the quenching process.
Phases of Quenching in Blacksmithing
Quenching is the process of cooling a heated metal by putting it mostly in some form of liquid or forced air in some cases. After the metal has been exposed to extremely high temperatures, the same metal must be reestablished to room temperature quickly to keep its structure and strength.
Although it not may seem, quenching actually has 3 different phases which happen at different times at the same location and these are:
- Vapor phase or Stage A
- Boiling phase or Stage B
- Convection phase or Stage C
The vapor phase is the beginning phase of quenching. Because of the big difference between liquid and metal, oil starts to vaporize. The heat transfer is very low in this phase, mainly because it is occurred mainly by radiation. Especially in this stage, some soft spots in the metal surface can occur in the process as tempering is required after the quenching process.
Boiling is the second quenching phase, also known as the fastest phase of quenching. Here, created vapor converts into boiling bubbles which allows the faster cooling of the metal. Because of the temperature drop, heat transfer is high. Since the whole purpose of the quenching is to make a transformation from austenite to martensite, the fast quenching rate is needed. Considering that, additives have been added to some quenchants for boosting the cooling process. This stage ends when the temperature of metal drops below the boiling temperature of the quenchant.
Stage C or convection phase is the final step of the quenching process. Further heat transfer occurs by convection and conduction. Convection removes the heat energy and among other things is controlled by the temperature difference between the material and liquid. Usually, this stage is where most distortion and cracking of the metal occurs. It is also the slowest phase of all 3.
Ok, ok, enough with the science part, let’s jump straight into practical stuff you will be actually doing.
Why Should You Quench a Blade in Oil?
As bladesmithing becomes more and more popular, maybe some of your plans are to make some kind of knife or even a sword. Although, it is worth mentioning that it is recommended to normalize your steel before quenching.
The process of normalizing is as it sounds, it is normalizing or resetting all of the microstructure inside the steel to a normalized state. It also reduces the stresses inside the steel caused by prior work.
It is done by simply heating the steel between 1500-1600°F (815-870°C), depending on the steel type. After that, just allow heated steel to cool in still air. You can repeat this process 2-3 times.
What Steels to Use as Beginner Bladesmith?
For beginners, we would recommend starting with steels like 1080, 1084, 1095, 5160(Amazon links). Of course, you have many other good-quality types of steels that are not mentioned, but these are simply more friendly for a beginner to use. Each of these knives has their ways of quenching, normalizing and hardening, so it is important to do your research before working with them.
For someone making a knife for the first time, 1080 and 1084 are excellent choices. When it comes to heat-treating, they are very similar. The major reason why I’m mentioning these two is that they can be heat treated adequately at home and you don’t need expansive equipment. Let’s move on.
The blade is out of the forge, now it is time to quickly plunge the blade in the can of oil or water. When inside of the oil, be sure to make a slicing movement, basically going back and forth with the blade. What that will do is it will reduce the formation of air bubbles around the steel. Don’t move side to side as it will increase the chance of warpage. After it is cooled, you should test the blade with a file. Your file should not be able to cut into the blade.
Do You Need to Heat the Oil Before Quenching?
Quenching is a process with a lot of variations. For example, some materials prefer pre-heated oil because it cools actually faster. I know it sounds counter-productive, so let me explain.
When you heat the oil, you actually lower its viscosity, which refers to the thickness of the oil. When you lower the thickness, oil becomes thin and therefore, it will cool faster.
Knowing that you could say the quenching is a balance between not going too fast and not going fast enough. In order to steel to harden, it requires cooling below 900 degrees in 2 seconds but that does not mean you should take the blade out then. It is generally recommended to stay somewhere about 10-15 seconds in the oil. Also, you can check is if are there any warps formed on your blade.
It is important to note that you should be using at least a gallon of oil for quenching. If you use too little, the whole system will get too hot too quickly, therefore, your blade won’t be hardened enough. Also, be sure to use a non-flammable container. Normally, you should use steel or metal container. Make sure to have a cover in case it catches fire and to prevent your oil from becoming contaminated with water.
After quenching, your blade is in a desired hardened state. If you leave it at that point, your blade has a great edge. The bad thing about that is that your blade is still very brittle and can easily crack if you apply some force. That’s where the process of tempering comes in, which we will discuss in another article.