Watching a heated blade ignite after being placed in a liquid-filled container may have prompted a few of you to get started in bladesmithing. If you ever watched Forged In Fire, you know what I am talking about. Since quenching is a crucial part of the heat-treating process of every blade, it is important to know how to do it properly.
That is why I decided to write this in-depth guide that will answer some of the most common questions about quenching. By the end, I will answer the following:
- What are quenching and its purpose?
- How to properly quench a blade?
- What quenchant medium should you use for quenching a blade?
- How many times can you quench a blade?
- Water vs Oil for quenching
- What oil to use?
- Should you heat the medium before quenching?
And now, let’s finally dive in.
What Is Quenching?
Before explaining how to quench a blade, we must establish what quenching really is. Everything you do in the workshop must have a purpose. The same thing is with quenching. Since some people probably don’t know what quenching is, I will give my best to answer it simply so everyone can easily understand it.
Quenching is part of the metal heat-treatment process. The main purpose of quenching is to harden the blade. It is done by rapidly cooling a metal piece by placing it into some kind of liquid or forced air. Oil or water is typically used for quenching a blade, depending on the type of steel.
For steel to say it is hardened, it must be harder than HRC50, and its structure has to be martensite. That is why quenching is vital. It prevents metal from breaking down from austenite into cementite and ferrite.
Note that quenching consists of 3 phases in the same location but at different times. These three include the vapor phase, boiling phase, and convection phase.
The vapor phase is the starting phase of this interesting process. Due to the large difference between the liquid and the blade, the liquid vaporizes. The heat transfer here is low due to the radiation. The soft spots on the blade’s surface are typical in this phase. That is why tempering is necessary after quenching.
The boiling phase is the second and fastest phase of the whole process. A created vapor transforms into boiling bubbles that allow faster cooling of the blade. The heat transfer is very high due to the temperature drop. The boiling phase ends when the blade temperature drops below the quenching medium.
The convection phase is the last quenching phase, where further heat transfer is caused by conduction and convection. In this stage, most of the cracking and distortion occurs. It is also the slowest of all.
Since this guide is all about quenching, steel blades deserve a special mention due to their sensitive mechanical properties to quenching. Namely, rapid cooling changes the microstructure of the steel compared with slow cooling.
Keep in mind that the steel blade is very hard but also brittle after quenching. The hardness primarily depends on the carbon content of the particular steel.
It is easy to mess up the quenching process, so it is important to understand some of the main quenching factors:
Medium or quenchant – refers to the thing you use for quenching. It is typically water, oil, or brine.
Steel type – not every steel needs the same quenching process. For instance, some steels should be quenched in oil while others in the water. Choosing the wrong medium can produce a low-quality blade.
Quenchant temperature – the hotter the quenchant temperature is, the slower the cooling
Container size – a general rule of thumb is one pound of parts to one gallon of quenchant.
Read This Before Quenching
If safety is not a number one concern to you, make it so. Otherwise, you can get yourself into big problems. So, before you quench the blade, consider the following:
- Wear safety glasses
- Wear appropriate clothes
- Make sure your quenching tank is nonflammable
- Make sure you have something near you to put off the potential fire
- Heat the medium before quenching takes place
Wearing safety glasses is one of the most important safety principles you should do in the workshop. As far as I know, no blind bladesmiths are walking around and forging knives. To prevent any possible eye injuries, wear safety glasses. That should be one of the first things you do when entering the shop. Make it a habit. You can only regret not doing so.
You’ve probably never heard about someone who injured their eyes doing something like blacksmithing, and I haven’t as well. Until it happened to me, more precisely my left eye got stung by a tiny piece of hot metal that broke off.
Since then I have always kept safety glasses on my apron and made a habit of not touching any tools until I’m wearing both. I use NoCry Safety Glasses because they offer me a good bang for the buck.
Whichever you choose, make sure they have at least some form of scratch resistance and comfort, unless you want to throw them away after each working day.
Wearing appropriate clothing is another important factor you should seriously consider.
Having a quality real leather apron, like QeeLink Leather Work Shop Apron, is the most important in my opinion. Real leather is highly resistant to high temperatures and it won’t catch fire easily. In fact, it’s almost impossible unless you throw it into the fire, and unlike your clothes, you can take the apron off very quickly in case of an accident.
Avoid wearing synthetic clothing as much as possible, especially during quenching. Synthetic clothing is much more prone to catching fire than natural fibers like cotton, for example. Especially vital when you are placing the blade in the quenching tank when the fire occurs.
I would recommend wearing a long sleeve shirt for better protection as well. The typical recommendation is long pants made from natural fibers when it comes to pants.
Don’t ever use a plastic container for quenching for obvious reasons. Get yourself a metal container to avoid possible fire. Always have a fire extinguisher or bucket of water near the container so you can react quickly and efficiently in case of a fire. Never put your face near the quenching tank.
Another important thing I should note is that you should ALWAYS preheat the quenching medium before quenching any blade. The reason why I will explain later in the guide.
How Do You Quench a Blade?
Now when we understand the fundamentals of this interesting process, let’s learn how to bring it into practice. Before answering, note that the quenching process primarily depends on the steel type, and therefore there is no only one way of doing that. For example, you must not quench some types of stainless steel in either water or oil; instead, they are air-cooled.
To quench a blade, you must first reach the “non-magnetic” temperatures in the forge. After that, the heated blade is normally dipped in the can of water or oil for 10-20 seconds, depending on the steel. That should be done very fast and precisely. During the quenching, the “slicing” movement of the blade is recommended to reduce the formation of air bubbles and increase the cooling rate. In the end, check for possible cracks.
Ok, that would be a nice, short, and direct answer on how to quench a blade, but I think we should get more specific. So here we go.
Step 1) Heating the blade
To fully comprehend the quenching process, you must understand what happens before this. Typically, the blade is annealed or normalized. The important thing to note here is that the normalized blade is in the softened state while the quenched blade is in the hardened state. After normalizing, the blade is placed back in the forge or furnace and heated to non-magnetic temperatures, which vary depending on the steel type.
Make sure that the blade heats evenly in the forge. Hot spots in the forge often heat the blade unevenly and therefore cause differences in grain growth at different parts of the blade. Suppose you don’t have a heat-treating furnace, the easiest way of determining that temperature is by using a magnet. You simply place the magnet on the blade and observe whether it sticks onto it or not. If it does, it is ready for quenching. If not, put it back in the forge.
Step 2) Quenching
After heating the blade to the right temperature, remove it from the forge and plunge the blade into oil or water. This part must be done quickly and precisely. Otherwise, the blade will cool down and potentially ruin the rest of the process.
Once the heated blade is in the quenching medium, expect a lot of vaporing. As I have mentioned earlier, move the blade back and forth (the slicing movement) to prevent the formation of air bubbles and increase the cooling rate.
Although the temperature will drop tremendously in just two seconds, it doesn’t mean you should take the blade then. The blade should be quenched somewhere between 10-15 seconds if you are using oil. Don’t forget about the flare-ups, which are much more likely to happen during this part. Always have something to cover the container.
Step 3) Testing the blade
And now the moment of truth. The blade is taken out from the liquid, ready for analysis. Before that, please take a deep breath and pray everything went well. Search for any possible warps on the blade. If there are none, wonderful, and if there are some, reheat the blade to the critical temperature and lightly hammer it out.
If everything is fine, the blade is ready for a file test. The purpose of this is to test the hardness of the hardened blade. To do that, simply hold the blade and grind it using the file. If the file doesn’t bite the edges, the blade reaches a sufficient hardness. Contrarily, if the file bites the blade’s edges, it is still too soft.
Right now, your blade is hardened but also brittle. The next part of the heat treatment is crucial. The process is called tempering. To learn more, check How to Temper a Knife – The Ultimate Guide (2021).
Is It Better to Quench the Blade in Oil or Water?
Deciding which quenching medium to use is one of the most important factors in this process. There are a lot of debates online about whether bladesmiths should use water or oil for quenching. Most of these debates don’t have any sense because of one reason. Not all steel is the same; therefore, not a single method will work for every steel. However, there are some advantages and disadvantages to both options.
Quenching the blade is typically done using oil, although some bladesmiths use water, depending on the steel type and experience. The reason why using oil is used more is because it provides good hardness but also a less stressed blade in the end. That means there are fewer chances of cracking. However, some steels must be quenched in water, W1, for example.
In the end, there is not such a thing as one is better than another. Both have pros and cons and should be used according to the working material. For example, water quenching provides faster cooling and a higher hardness of the blade. On the other side, water quenching also produces a much more stressed blade which means greater possibilities of distortion and cracking.
On the other hand, oil cools the blade slower, generating moderate hardness. Although the blade is slightly softer, it is less stressed. As you can see, the difference lies in the rate of the cooling process. Usually, the higher the carbon content, the slower you need to quench.
Since most steels for knife making are also high-carbon steels, oil is often used for quenching them. Remember, we are talking about blades, not tools. The blades are so thin that we have to minimize the chances of cracking as much as possible. Since oil typically allows that, it is logical to use it.
However, some bladesmiths use water, but these are usually those with a lot of experience who know what they are doing. For beginners and intermediate bladesmiths, I strongly recommend using oil instead of water for quenching unless the water is specifically required.
My personal experience also thought me that oil is a more “forgiving” quenchant. I have managed to produce more quality knives using oil instead of water. However, that doesn’t mean the water is a bad choice.
As I have already said, some use water without any problems, but the majority use oil for the previously mentioned reasons.
What Is the Best Oil to Quench a Knife In?
Ok, we have learned how to quench the blade properly and when we should use oil or water. Now let’s find out what quenching oil you should use.
Before I answer this, remember that sometimes, “the best” quenching oil is the available one. There is no use in beating your head just because you cant order it or it is too expensive. However, if these superior quenching oils are available and you can afford them, by all means, do it.
Generally, the best oils for quenching a knife are commercial oils like Parks AAA and Parks 50. These two are precisely made with special properties of being either slow or fast quenchant. Another great and commonly used option is canola oil which offers great oxidation resistance. Other possible solutions are peanut oil, mineral oil, and motor oil.
Now let’s break down each and analyze its pros and cons.
Commercial quenching oils
First, you have a Parks AAA, one of the most popular quenching oils. Parks AAA is considered to be medium to medium-fast quenching oil. Its Nickle Ball test is 9-11 seconds. The usual steels that get along with this oil are 01, 1084, 1080, and 5160. The usual sweet-spot temperature is around 120°F.
Right next to Parks AAA, you have Parks 50, used for “water hardening” steel. Namely, at the beginning (vaporing) phase, Parks 50 acts similarly to water. Then, as the cooling ends, this oil starts to slow down to prevent the possibility of cracks.
Due to its low viscosity, temperatures between 50°F and 120°F are typically fine.
That is great news for someone who doesn’t have a quenching tank with a heater. So, as long as the temperature doesn’t go over 120°F, you are good. Parks 50 is classified as a fast quenching oil. Typical steels combined with this quenching oil are W1, W2, and 1095.
Advantages of commercial quenching oils
- Designed specifically for this process
- Properties for optimal quenching
- Apposite for various speeds
Disadvantages of commercial quenching oils
- High cost
Due to its lower price and availability, canola oil became the extremely popular quenching oil for many bladesmiths. So, if you are a beginner, this is a great option. Canola oil is rich in oleic fat, which gives you great oxidation resistance of all vegetable oils. It is an oilseed primarily found in Canada. Canola oil is also readily biodegradable. But of the biggest advantages of this oil is that it doesn’t produce any smell, which means more comfortable quenching.
Keep in mind that Canola oil does require preheating. The ideal temperature is nearly around 130°F, although it may work slightly higher or lower. I should mention that it has a high flash point which is highly desired for quenching. Also, the boiling temperature is much higher than most petroleum oils. All these properties make canola oil a great oil for quenching.
Canola oil is my favorite cheap alternative to quenching oil. I still used it today for the majority of my simpler knifemaking work. I have also tried both Parks AAA and Parks 50. In my opinion, they are still a superior option, but unfortunately.
Advantages of canola oil
- Good biodegradability
- Low smell
- Ideal for indoor bladesmithing shops
- High boiling temperature
- High flashpoint
Disadvantages of canola oil
- Not specifically made for quenching
Although not so commonly used, peanut oil is another quenching oil that you can use for this purpose. The benefits of using peanut oil are that it is non-toxic and biodegradable. If you work indoors, peanut oil is a great choice because it is not toxic. The only disadvantage is that it is slightly more expensive than canola oil, sometimes even twice as much.
You can get peanut oil here on Amazon for a few bucks more than canola oil.
Advantages of peanut oil
- Good biodegradability
- Ideal for indoor workshops
- High boiling and flashpoint
Disadvantages of Peanut oil
- Higher price than canola oil
Another option for quenching medium is mineral oil. It is known for its good cooling properties, but it is more expensive than canola oil. Unfortunately, the buildup of toxic chemicals occurs during high temperatures due to oxidation. Note that the extended inhalation of these toxic chemicals can cause negative health effects. Except that, the buildup of these chemicals negatively impairs the quality of the quenching process.
Advantages of mineral oils
- Good cooling process
Disadvantages of mineral oils
- Lower biodegradability,
- Not ideal for the indoor workshop
- The high amount of toxic chemicals
Motor oil is commonly used for quenching blades. If I have to state its advantages for quenching, that would be its price and availability. The chances are that you have some in your garage. The downsides are that motor oil contains toxic chemicals and additives, which is not ideal for indoor workshops. It also has a bad smell that can even continue during the tempering.
Advantage of motor oil
- Low price
- Easy available
Disadvantages of motor oil
- Contains additives and toxic chemicals
- Not ideal for indoor spaces
- Bad smell
Why Do You Preheat Quenching Oil?
If you are still waiting for an answer on preheating the oil before quenching takes place, your waiting is over. Namely, quenching is the balance between going too fast and too slow. To harden the steel blade, it will cool down below 900 degrees in just a couple of seconds, but that doesn’t mean you should take it out then.
Preheating the oil before quenching is done because you lower its thickness, also called viscosity. So, when you lower the thickness of the oil, it becomes thinner much sooner, and therefore it cools the blade much faster. Recommended temperature ranges somewhere between 110℉ and 140℉, depending on the oil.
Preheating the quenching oil becomes especially important when temperatures are low in the wintertime. You can heat the oil by placing the metal container on the gas oven or by placing the heated steel bar in the container.
A third way includes placing a floating heater in the tank, which should be at least 250W, although I would recommend 500W.
However, note that you should not heat some quenching oils before quenching. Such an example is the commercial quenching oil Parks 50, which you should preheat only if it’s cold in the workshop.