Turning off the hardening blade in oil or water?
The shutdown phase after the temper is a key moment for a successful hardening!
There are several types of tempering treatments:
- hard or ordinary,
- vacuum one and
- induction temper
are among the most used.
Today it is also very fashionable among knife makers cryogenic hardening but we will make a specific post for this type of heat treatment.
In common they have the abrupt cooling that follows the heating and maintenance phase that takes place inside the ovens that can take place in:
the shutdown medium is tasked with absorbing as fast as possible the energy of the part to be tempered to ensure that we are inside the top critical speed (the red line with fast cool indication).
Ps. Personally I have never performed a shutdown in the air but the procedure consists of lowering the temperature with compressed air the piece to be hardened that is located inside two thick aluminum plates.
Consider that today there are many companies specialized in tempering and that have over the years optimized the process both as a thermal cycle,as a cooling and as a finding as well as of course to certify the hardness achieved.
So even if we use hardening furs suitable for knives and increasingly performing with regard to the possibility of choosing the correct thermal cycle according to the type of steel, all phases are still home made!
But how to harden at home by managing to have professional results?
Because in addition to achieving the correct hardness according to the steel used there are many other features to be respected.
Today many makers are hardened at home but in recent years it is also easy to see images of knives used in survival but not only where for example where they use techniques that stress the knife such as batoning and find yourself with a broken blade.
Knives should be made to be used for testing as big companies do!
Now in addition to choosing a suitable steel for the right uses you must also consider that the blades are still hardened home made!
That’s why it’s important to do everything to the fullest of what you can do with home-made equipment, including the founding.
Today many makers no longer use kitchen ovens for the founding because even the founding has different temperatures depending on the type of steel.
They then use two hardening furnips to be able to make the find immediately after cooling or they wait for the oven used for the thermal cycle to reach the temperature for the find.
Here are already here as you can note also for the find there are two philosophies:
- Who makes the redo immediately after cooling
- and who believes that even doing it after hours or the next day does not change anything.
It is not easy to answer this question!
But going back to the theme of today’s post is better to turn off the blade in hardened oil or water?
Oil is preferred for a talk of viscosity.
The extinguishing in oil is less drastic because being denser dissipates the heat of the blade more gently than water and having a higher boiling point than water and not evaporating is also more “homogeneous”.
It is important to heat the oil first to 70 degrees Celsius because by becoming more fluid it dissipates heat faster but with less heat shock.
This is important because this greatly reduces the risk of cracks, blade distortions, etc.
The temperature drop is gentler than in the water.
If the shutdown takes place in oil it is better to set or aim at a temperature at the maximum range dictated by the steel producer, while if you want to turn off in the water it is better to stay at the lower limit of the austenization temperature.
As an example, if we have a steel whose aaustenization temperature recommended by the manufacturer of 790 degrees Celsius up to 820 degrees Celsius, we will choose a temperature of just over 790 degrees Celsius if it is turned off in the water and at an upper limit of 820 degrees Celsius if the shutdown is made in oil.
But as you can guess it’s not easy to check these temperatures!
To further avoid blade distortions and cracks, it is appropriate after completing the roughing and shaping of the blade perform a recoatingtreatment.
The recoating process consists of bringing the piece in addition to the austenizationtemperature, a stay adequate to the thickness and size and subsequent very slow cooling, in the graph of the CCT at the top corresponds to the Slow Cool (right red line).
Since austenite is a metastable crystalline form, it cannot exist at room temperature; Austenitisation is therefore the first stage of the treatment of:
- Temper, in which case it is followed by a rapid cooling, aimed at freezing the austenitic structure and precipitating the iron-carbon alloy in the form of martensite.
- respray, when it is followed by a more or less slow cooling, tended to solve the austenite in ferrite and cemented.
It is not easy because it needs certain very mild temperature descents, if you do not have an oven with ramp controllers this turns out to be a difficult process.
Alternatively, those who work with the forge leave the piece in the middle of the embers to cool it slowly, or do a normalization process that is a kind of hardening but without turning off where the piece should be cooled in calm air.
The use of water as a means of extinguishing should be avoided because it is too drastic a means of cooling and it is easy to find the crooked blade (twist the knife) or crack it.
The means of turning off to ambient temperature water if as shown in the graph previously must be inside the curve of the higher critical speed, in this case the slope of the cooling speed is too drastic.
The problem is generated by water because of its characteristics:
- Thermal water capacity. The water has a large thermal capacity (physical magnitude linked to the specific heat) that precisely because of this characteristic manages to “steal” from our blade a lot of energy in a short time.
- Temperature evaporation water, which is known to be 100 degrees Celsius. The problem arises when water particles in contact with the blade evaporate. In this swirl of liquid and gaseous state, we have discontinuity of the temperature descent and heterogeneous zones. That’s why we have a number of areas where energy is dispersed differently than others.
This can cause cracks, especially in the weak parts of theblade,such as bisellation and the collection area. Changing geometry, switching speed, and non-homogeneity of temperature descent at various points in the blade can cause distortions and cracks, if not even breakages.
As I said the choice of oil as a means of extinguishing has a thermal capacity of the oil that is half that of water and its boiling temperature is over twice as high.
This allows for a less drastic decrease in temperature and a greater homogeneity of temperature distribution on the blade that theoretically avoids distortions and crettas.
A second factor that could lead to problems during shutdown is the too high temperature of austenization.
In this case, in addition to the problem of finding a large crystalline grain, we also have more energy to disperse.
To have an objective test would require an instrumentation to be able to measure their size but you can do a visual test where usually when you find homogeneity and a “severe” appearance this guarantees a certain quality of the result.
What oil to use?
It’s also fine seed oil or discarded olive oil but I recommend you buy it again if you want to avoid the smell of fried.
A fairly well-known hardening oil in the industrial sector is the Catrol Iloquench 77 and is an oil used industrially for industrial tempering but the problem is that these oils are sold in large quantities (200 litre stem of tempering oil).
It has the characteristic of:
- Being low viscosity… so very liquid and tend to slip away from the hardened piece very quickly.
- Each type of tempering oil then has special uses, some have low sulfur presence so as not to spoil certain steels etc…
- They run out (lose their characteristics) after hundreds of tempers.
- They have very high fire triggers.
Even if it’s hobby use are still features that are important but being sold in barrels you have to look for someone who sells you a few liters.
If you go to a specialist company I don’t think they have much trouble selling you a few liters.
I’ve never used it so I don’t give to tell you if it fundamentally changes the temper compared to a more common oil but their characteristics are more suitable for industries with large numbers and are designed for this.
Ps. My advice is to look for the technical features (data sheet) of the top temper oil and then look for similar features in some oil that you find more easily on the market.
Some use amber oil 68 sold in 5-litre drums that leaves little patina of dirt,although it’s not really for tempering use but oil for pressure plumbing.
Let’s start with the basics.
The temper is obtained by thermal shock between the red-hot steel and a “refrigerant” that can be oil water or air.
The colder the coolant, the greater the thermal overhang.
Heated oil is preferred because the thermal overhang is lower and decreases the risk of steel fractures and blade distortions.
The oil has the risk of catching fire so a specific oil has a higher combustion temperature than a kitchen one (it is always advisable to keep a fire blanket at hand)
the contact fluids of the blade in turn overheat, dissipating the heat less quickly.
A more fluid oil (or “mixing” with the blade) allows the blade to be in contact with unheated liquid.
There are steels that tempt well even with forced air and some that you could harden them in freezing water (as in the movie The Hunted!) but they are techniques that makers usually do not practice.
Experiments with steel bars
If you want to have fun and really understand the differences do a little experiment.
Take 4 bars of steel equal to the same size, bring them to the same temperature and try to turn them off in:
- icy water,
- normal water,
- oil and
- compressed air.
Ps. You can also use different oil scan to see the differences also because using small plachettes you do not have to use large amounts of oil, etc.
Then make a found in the same oven and the same duration and then measure with the hard meter the hardness obtained or do the file test.
The moment of the founding of the knife is also an important operation that eliminates the internal tensions in the steel caused by the thermal shock of the shutdown.
The procedure changes the structure of the Martensitic structure by cancelling, as far as possible, tensions and fragility.
To understand, in the process of remediation in the hardened state the steel has the maximum hardness that can be obtained, but it has a low toughness.
On the other hand, the increase in toughness decreases the hardness, so depending on the destination of use of the knife you have to decide the characteristics that must have a blade.
You can do it in a normal kitchen oven: take it to 200 C and I leave the knife for 90 minutes, then, off the oven, I wait for it to cool naturally.
But in reality each steel has its own optimal temperature of the founding and therefore should be made with a hardening oven brought to the temperature indicated on the data sheets for the indicated time.
The only concrete way to understand the differences is to test with samples of the same size under different conditions and measure the hardness obtained.
Some general guidance
In general, air cooling is valid for all stainless liquids:
The goal is to get down quickly below 200 degrees Celsius within 120 seconds of the draw.
The faster you are and the better you have, but generally you take advantage of the cooling in the air because, with the use of aluminum plates, you can contain the deformations and partially recover them while the piece is still warm, the remaining recoveries by forcing the piece during the founding.
Forced air without plates can still introduce deformations that could still be reduced with a détente before working the blade.
Also for inserting the knife into the oven
The theories about baking are the most varied:
The ramp time is critical, so if the oven does not raise the temperature at a sufficient speed the thing is not good for the material, if it is too fast it can be critical because of the geometries of the pieces it bakes and introduce cracks and/or tensions…
The time to stay at austenization temperature for steels tied “speaks” of 0.8 s/mm thick, usually takes into account 60 s/mm, so you have to calculate your specific case and paradoxically in 60″ the heart temperature is already reached, but it is also true that at the wire you have 0.5 mm so you have the choice!
Leave time to make sure the transformations are complete and off!
5 minutes is usually more than enough for 5 mm thicknesses.
In domestic hardening products such as anti-scale products are used or lead to avoid decarbonisation, and in the latter case for example the blade must be inserted in Temperature (T) not less than 600 degrees C so the oven is conditioned.
The immersion in the liquid of the knife
At the end of this operation the knife is quite dirty!
My advice is to do tests and talk to people who are expressed about thermal treatments because you have to find the best solution for the type of object.
There are many methods but metallurgy is an empirical science based on tables and practice.
Testing can help you both understand the differences but also to get the best result for your blades because knife hardening is a fundamental step that heavily affects the quality of the knife and its performance.
Are you experience?
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