What oil do you use to harden a knife?
What oil do you use to harden a knife?
The most common hardening technique for blacksmiths and bladesmiths is liquid hardening; the vast majority of steels that forge easily are oil or water hardening steels, most of which are oil hardening. This means that steel that taken above the critical temperature will harden when quenched. Oil is the more common of the two, as it is a more gentle and controllable process than water quenching, especially on a blade.
Water quenching is more violent due to the much faster cooling rate than oil. As a 1500°F blade is quenched in a liquid, the liquid converts into its gaseous state, forming a «vapor jacket» around the hot blade. The thinner the liquid, the faster the vapor jacket is shed from the blade, due to having less resistance to rise to the surface, and the quicker it cools the blade. A quicker quench usually results in a harder blade, though it will have more internal stress, which can manifest through warping, twisting, splitting, or cracking. This is one of the reasons why we recommend preheating most quenching oil to a specific temperature; the warmer the oil, the faster it cools the blade. Water will always be far thinner than oil, and as such, the vapor jacket (steam) sheds much quicker than in oil, resulting in a speedier quench.
A technique commonly used by bladesmiths and blacksmiths is to move the workpiece up and down or back and forth in the oil to help release the vapor jacket to cool the blade evenly. If the blade is moved side to side, it’s very likely that it will warp, as it doesn’t fully harden until it cools past 400°F (204°C), and the pressure of moving the oil side to side can bend it, as well as the vapor jacket shedding on one side and not the other, causing uneven cooling. The cooling from 1475°F (802°C) or critical temperature to 900°F (482°C) in less than 2 seconds is what causes the steels grain structure to form into martensite, the hardened grain structure of steel.
Each carbon steel has a specific temperature and soaking time for ideal hardness and toughness. This often isn’t possible to control without a digital controller. It can be estimated, but for exact control of temperature, a heat-treating oven, kiln, PID controlled forge, or salt/ sand pots are necessary. We prefer heat treating ovens since they’re precise and versatile, while still being very safe.
For the hobbyist blacksmith or bladesmith, a heat-treating oven generally isn’t a viable option due to cost and or space, so a forge is often the tool used for heat treating and thermal cycling. Most forges can be controlled very precisely by adjusting the fuel and oxygen intakes. By turning down the amount of air allowed in and adjusting the amount of fuel going in, it’s possible to run a forge in the 1500°F-1550°F (816°C-843°C) range (You can verify the temperature by using magnets to see how much hotter the steel is getting past the point of non-magnetism). As long as the steel gets just past that point, it will usually be close enough to harden a blade sufficiently.
The second most common hardening technique for bladesmiths is plate-quenching; this is the technique used to harden thin pieces of martensitic-stainless steels. Stainless steels are primarily air-hardening steels, meaning that they don’t need to submerge in a liquid, the shock would be too much and would likely crack most thin pieces.
Once a blade has gone through the proper thermal cycles and soak times, it is taken from the heat source, placed between two aluminum plates, and clamped. The aluminum sucks the heat from the blade and allows it to form martensite, and the straight aluminum plates help keep the blade straight as well. Compressed air is often blown onto the blade while it is between the plates, which allows for faster cooling. Stainless steels oxidize (form forge scale) differently than carbon steel, and need to be protected from oxygen while above critical-temperature.
There are two common ways of doing this; the most common way is to seal the blade in a packet of stainless steel foil, and the other is to purge the heat treating oven with argon. Plate-quenching isn’t generally feasible for those just working with a forge, as the steels that require plate quenching need a precise soak time at a specific temperature. An excellent example of the precise control required for the heat treatment of martensitic stainless steel is AEB-L, which needs to be taken to 1900°F (1038°C) and held for 10 minutes, then have a fast ramp-up to 1975°F (1079°F) for another 10-minute hold before being plate quenched. Custom makers commonly use a heat treating oven for hardening stainless steels, but a salt pot would work as well.
Most frequently forged steels are oil hardening, rather than air-hardening, except for some tool-steels such as H-13 and S-7.
The final type of heat treatment is a non-plate assisted air-hardening and is the least common type of heat treatment for blacksmiths and bladesmiths, but is more common in industrial settings. The process of air-hardening is essentially bringing the steel up to a specific temperature (for H-13 it’s in the 1800°F range) and letting it cool in room air or with a fan blowing on it, that cooling rate is enough to harden the steel.
Now that you’ve got some of the basic of heat treating, check out our article on how to work with and heat treat some of the most common steels you’ll use as a blacksmith or knifemaker.
What oil do you use to harden a knife?
First, know your steel. If you bought commercially available tool steel you should know precisely what it is. But if you are using something found, scavenged or of otherwise uncertain provenance you may have problems hardening it. The steel used in any given blade is not an easy thing to determine. A metallurgical lab charges a fair amount to test for alloy and there is no home test kit that I know of («Look, Honey, it turned blue!») And there is some risk in quenching, say, an oil hardening steel in water. It could fracture at worst or warp like crazy at least. The old-timers «sparked» steels to tell what was in them. The sparks generated from a grinder will burn with different visual characteristics depending on the alloying elements. (Like the different colorants in fireworks.) So you can grind a corner, observe the sparks, then grind a known steel and try to compare the little spark-flares for shape, brightness, complexity, etc. and attempt a match.
Mostly we’re talking oil vs. water hardening steels. The air hardening ones are the Cr-V and stuff that us Galoots don’t use too much and that weren’t used in old tools at all. It is safer to quench an unknown, perhaps water-hardening steel in oil than vice versa. The water-hardening steel may not harden in the oil and if that is the case, you can try again in water. I don’t mean to muddy the water with all this but, hey, if it were easy, everybody’d be doing it.
The first step is to get the metal to its critical temperature, which with good old O-1 (the oil hardening stuff) is 1450° — 1500°F. Got a good pyrometer? No problem. During the crystal transormation from ferrite to austenite steel ceases to be magnetic at that temp. This phenomenon is called the «Curie Point» after the discoverer, Pierre. So one can simply heat the metal till the magnet is no longer attracted to it then quench in oil. I like to use peanut oil because the flash point is very high which minimizes the risk of fire (the risk is still there, though; be prepared: use long tongs to handle the work to keep your hand out of the way, wear gloves and keep the fire extinguisher handy) and it smells nice(r) when it smokes. How to get the blade to the Curie point is probably the biggest problem for the DIYer. When the metal is glowing red, the carbon behaves as if it’s in a liquid and can therefore migrate around as it pleases. This is necessary for the hardening to occur but near the surface of the metal those unfaithful little carbon atoms would just as soon run off with any available oxygen-sluts it runs into (oxygen is soooo seductive) and they’re lost then forever. We hate that. We attempt to prevent this by: heating the metal in an inert (oxygen free atmosphere) and/or limit the time at red-heat (in air) to as little as possible. A torch makes both of those very difficult. It’s very hard to heat something as large as a Norris-type blade evenly with a small torch-generated spot of heat. A forge fire is better because of its uniformity and it can be starved for air a bit to decrease the oxygen in its immediate vicinity. A small lab-type test oven works quite well. (Also used for ceramic glaze tests.) Toss in a charcoal briquette to scavenge some of the oxygen.
Update: There are coatings that prevent oxidation and carbon loss at www.rosemill.com that promise to make home heat treating a more successful endeavor.
When it’s hit critical temp, remove it from the heat and quickly dunk it into a sufficient quantity of room temperature oil. Swish it around a bit until it’s cooled throughout to below 150°F. It should now be very hard and too brittle to use. (If you attempt to file it, the file should skid on the blade.)
Two ways to temper to a useable hardness/toughness: by colors or by temperature. If you have a very accurate oven in the kitchen, just heat it to 325°F and you’re done. An accurate deep-fryer will do the same but use a good thermometer to double check on the oven or deep fryer’s thermostat. Without accurate temperature control, you’ll have to use the surface oxide colors to know when enough is enough. First, clean some part of the blade (probably the flat area back from the bevel) till it’s bright metal again. When heated, that spot will change colors (you’ve seen the rainbow of colors on any overheated steel) starting with a very faint yellow (called light straw). Since we like our blades Good-n-Hard(tm), stop there (remove from the heat, quench if necessary to stop any further increase.) Any color beyond the faintest straw is too much. (The blade will still work, it just won’t hold the edge you want.) Be overly cautious with tempering. You can always re-temper a too-hard blade, but if you go too far and soften it too much, you have to re-harden it all over again. So if a blade seems too hard, just toss it back in the oven and go a little higher. The oven/deep fryer method is preferred, however because you can leave the part at tempering temperature long enough for true tempering to occur. The torch method, using the surface colors, may leave some of the transformation undone.
You’re done. If the blade looks awful, you can sandblast or grind it pretty but it should work well regardless. Before honing, be sure to grind back the bevel a bit . That thin section probably took more than its fair share of carbon burn-out abuse and you need to get to the good stuff. (it could take as much as .025″ to get through the de-carbed layer.) Same for the back. Doing a good job on the back is at least if not more important than the work on the bevel. A little extra elbow grease will remove the de-carbed layer and get to good metal. Don’t forget: the back IS the Cutting Edge. Think about it. If the back hasn’t been honed deeply enough, the blade will never work well.
Oil hardening steel
Oil hardening steel refers to steel that must be quenched by oil. Typically, this is the cold-work group of steels, including the O family of tool steels such as O1 and O2.
Pure steel often comes in a supply that is too soft for application. Therefore, quenching is the process needed to harden steel to its required strength and hardening properties. Although this article discusses hardening steel by oil, other methods are applicable depending on the grade of steel you have and if quenching has not already been performed before purchase. For example:
Oil is one of the most popular quenching methods worldwide and involves heating your steel to the recommended temperature before cooling it in oil to gather extra hardening properties. The oil contains properties that allow for slower cooling when analysing the results of water and brine. However, when looking at the analysis of air, oil cools much faster.
Properties of oil hardening steel
Oil hardening steel grades offer excellent wear resistance, and due to this, the O1 tool steel grade has grown to be one of the most popular worldwide. The ability to use oil for quenching compared to water or air adds extra properties that allow for tools to be produced that are less likely to crack.
How do oils harden steel?
Hardening tool steel in oil requires a quenching process to be used, which involves a range of steps. Once the steel has been heated, the toolmaker should place it into oil for quenching. When first submerged into oil, the heat from the treated steel and the quenchant form a layer of vaper, a process called firm boiling.
It’s essential to research the grade of steel you’re hardening and the properties of the oil it will be submerged, as the properties can massively change the time taken to cool and, more importantly, the ability to set.
Next, as the steel cools below the quenchant’s boiling point, the process moves into nucleate boiling. As mentioned before, the properties of the oil used to harden steel play a significant role in cooling speed. You can find a basic overview of quenching steel in oil below:
As the temperature continues to cool, it moves into the liquid stage, and cooling begins to become much slower than the previous stage (again, dependent on the oil and steel used). This process helps develop the properties within the steel to reach total hardness and reduce cracking and distortion of the finished tool. However, this is only true if the hardening of the steel has been performed correctly.
Benefits of oil quenching
Using oil to achieve the desired hardness of your steel after heat treatment has a range of benefits.
The main benefit is that there are many oils to choose from, which allows you, as the toolmaker, to have complete control over the quenching process. Each oil will achieve different outcomes, and having the ability to control this aspect can be more difficult with other hardening methods such as air and water. Furthermore, for even more control over the result, additives can be added to the oil.
Selecting the correct hardening oil
To select the correct oil for your quenching purposes, you first need to research your steel grade as different oils become more applicable depending on the steel type. For example, some oils are less likely to cause cracking to the steel as they maintain a consistent heat between the core and surface temperature – these are referred to as hot oils.
For further information relating to quenching oil and oil selection, please visit Engineering 360.
A popular choice for toolmakers is to use vegetable oils for their steel hardening needs. They offer extensive benefits such as cost and are relatively positive for the environment compared to some quenching oils.
Although vegetable oils and other cooking oils have significant benefits, they also contain a disadvantage. The disadvantage is that the level of hardness they can achieve compared to industrial lubricants and quenchants is much lower. So, when choosing if vegetable oil is the best quenchant for your project, you need to weigh up the cost, environmental impact, and hardness.
Industrial quenching oils
As the name suggests, industrial quenching oils have been designed with the sole purpose of oil hardening your steel. They offer significant benefits as you can find specific oil for the grade of steel you are quenching and even the properties that wish to be achieved.
On the other hand, these oils come with a high price tag and are usually much more challenging to find than other general oils. Don’t expect to see these in a general hardware store, but if you’re serious about the properties you wish to achieve, and the quenching speed needed, commercial quenching oils should be your first choice.
When quenching your steel in oil, you can use other quenchants such as motor oil. Although motor oil is a cheaper, more readily available option than commercial oils, the risk of toxins due to the added properties when quenching can cause health and safety risks. Furthermore, you risk damaging the steel or causing an unwanted finish when using this quenchant.