As you become more skilled and proficient as a beginner blacksmith, you’re going to find out that there are a lot of different techniques out there. One thing you’re going to need to learn how to do is quenching your metal. Usually, this is down with water, with a traditional blacksmith simply dipping their hot metal into a bucket of water to cool it down.
However, now that we’re in the 21st century, this isn’t necessary anymore. In fact, blacksmiths around the world have come up with a much better techcnique, otherwise known as oil quenching.
Just as is the case with water quenching, oil quenching has a couple of main purposes;
1. It rapidly cools down your metal, which can help bring out certain qualities in the metal. IT also lets your metal achieve a level of strength and hardness that helps prevent cracking in the long-run.
2. In minimizes the development of undesirable thermal and transformational gradients that pop up rather frequently when you are working with steel.
That’s great and all, but you’re probably wondering, “why is quenching with oil any better than just using water? Isn’t is much simpler that way?”
Well, the answer is a bit more complicated than it seems.
Why Oil Quenching for Blacksmithing?
For one, the first difference comes down to the boiling temperatures. Water turns to steam very quickly in comparison to oil, which leads to a number of different problems. In comparison, oil has a much higher boiling point.
This creates a vapor blanket around the metal that allows it to absorb more initial heat. This isn’t possible when you use water instead of oil. The water cools down the extreme temperature of the metal fast. It doesn’t play any role in the strengthening and hardening process. This is the reason why water quenching isn’t considered valuable or safe for many industries and products anymore. You’re local, small-town blacksmith of 50 years ago might have still down water quenching, but in today’s day and age, oil is the way to go for many different types of projects.
What’s more, we even have many professional companies now that include certain additives in the oils or change the heat treatment process a little to customize the results.
For instance, for reduced residual stress or cracking from the thermal stress, they use specialized hot oil baths that are often in the range of 220 to 280 degrees Fahrenheit. This lessens metal part distortion and gives it a more uniform transformation during hardening. Sometimes, they use salt-bath quenching as well after the actual oil quenching method. It reduces gas absorption and thereby prevents bubbling. It also keeps the steel parts flat and smooth.
Quenching oil for blacksmithing comes in many forms also.
The conventional quenching oil type is good for those steel types that have low critical cooling rates. On the other hand, fast quenching oils are great for steels with high critical rates.
So, opting for the right oil type is necessary.
You have to learn all the details about them, such as which steel type can’t be hardened by the conventional oil, which additive is used for fast quenching abilities, or which additive specifically prevents sludging and oxidation.
When you coat the metal in the oil, it undergoes three phases of cooling.
This might get a little technical, I understand, but its important that these things are mentioned
Vapor Blanket Phase… This is the stage-1 in which the hot metal is immersed in a cooling medium (oil). The high heat of the metal vaporizes the oil and forms an insulating cloud that envelopes it. That’s why they call it the ‘Vapor Blanket Stage’. As there is no water, the heat escapes slowly. The overall cooling rate is very slow. Eventually, the vapor blanket washes off by the surrounding layers of the liquid.
Nucleate Boiling… This phase is known as the ‘Intermittent Contact Stage’ or ‘Vapor-Transport Cooling Stage’ also. As the surface of the metal or steel cools down after the instability or collapse of the vapor blanket or film, the heat starts to remove very rapidly. It causes violent boiling of quenching medium. You notice a lot of vapor bubbles that are carried away by convection currents. The heat transfers to the oil from the metal. In other words, due to the rapid cooling process, the surface of the metal comes below the boiling point of the quenching medium; it causes the oil to boil vigorously.
Liquid-Cooling… The third and the final phase is the ‘Direct Contact Stage’ or ‘Liquid-Cooling Stage’. The vapors stop forming and the temperature of the metal reduces more. This liquid cooling process happens by conduction and convection into the liquid.
If you want to have optimal quench process control in your hands, it is pertinent to know the physical properties of the quench oil.
This can provide some insights into the meaning of the results.
- Viscosity: It refers to how easily oil pours at a specific temperature. For that reason, the oil quenching process depends on the viscosity of the oil. Experts monitor the viscosity variation in their tanks to see what kind of results it can produce.
- Water Content: If you are using water-contaminated oil, it will cause soft spots and uneven hardness. Blacksmiths who complain that their final product is uneven or produce a crackling sound should change their oil as it is probably not pure.
- Flash Point: Flashpoint is actually the temperature of the oil that produces ignitable gas, however, it doesn’t cross its limits, therefore, it is not possible to burn it. The flashpoints can be different in the oils and tested in both closed-cups or open-cups.
- Sludge Formation: It is a sedimentary deposit that reduces heat efficiency and causes scale formations. If you notice sludge formation in the oil after using it, it is a sign that it isn’t good for reuse because it can cause non-uniform heat transfer and distortion.
- Cooling Rate: The cooling rate is another factor that matters the most in oil quenching. You have to perceive the cooling rate of the oil you are using. It is better to activate the timer as it can show you the real results that are associated with the type of material you are using.
These are just a few important properties.
Blacksmithing industries that work with the oil quenching process all the time concentrate on all the additional features also that play their part in adding versatility and functionality to their product.
If they are working on high volume operations, it becomes important to know whether the oil is biodegradable since the disposal cost for degraded oils can be very high. Furthermore, they mostly opt for those fluids that are low-foaming. Quenching oils that produce a high amount of foam can pump damage due to cavitations; they also affect the cooling ability of the fluid.
So, there you have it, exactly what is quenching oil, and why blacksmiths are using it more and more for more serious, professional quality projects. Of course, if you’re still a beginner just looking get familiarized with blacksmithing as a hobby, you wont’ really need to go all out on a quenching oil.
However, once you have a fair bit of experience and skill under your belt, I’d strongly encourage you to consider investing in some quality quenching oil. It can make a surprising difference in the end result of your projects.
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