- 1 Turning off the hardening blade in oil or water?
- 2 But going back to the theme of today’s post is better to turn off the blade in hardened oil or water?
- 3 Let’s start with the basics
- 4 Experiments with steel bars
- 5 Some general guidance
- 6 The immersion in the liquid of the knife
- 7 Conclusions
Turning off the hardening blade in oil or water?
The choice between oil or water cooling for blade extinguishing in a hardening process depends mainly on the type of steel used and the specifications required for the blade.
Knowledge of the blade extinguishing process during hardening is essential to understand the choices and considerations involved in the production of knives and cutting tools.
The selection between using oil or water as a coolant is a critical decision that affects the final properties of the blade and requires careful assessment of the specific needs of the material and application.
In this post we will explore the reasons and criteria behind the choice between oil and water for quenching during quenching, as well as the implications of this choice on the desired results but also advice on how to perform hardening done by professional makers.
Both methods have their own features and advantages:
- Oil cooling:
- Advantages: Oil cooling is slower than water, which allows for smoother cooling and less likelihood of deformation or cracking due to thermal shock. It is particularly useful for thicker blades or high-carbon steels.
- Detriments: Oil cooling can take longer, and it is possible that the material does not reach the maximum possible hardness compared to the use of water. In addition, the oil must be carefully handled and may require additional safety measures due to the risk of fire.
- Cooling in water:
- Advantages: Cooling in water is much faster than oil, which can lead to increased surface hardness. It is suitable for thin blades or steels with low carbon content.
- Detriments: Water can cause much more drastic and rapid cooling, which can lead to deformation, cracking or even blade breakage if not handled properly. This method is less suitable for thick blades or for some steel alloys.
The choice depends on the specific application and the desired results. In many cases, it is advisable to carry out hardening tests on test samples to determine which cooling method produces the desired properties for the blade in question.
In addition, it is important to follow the appropriate safety procedures when working with oils or hot water for quenching, as both methods carry potential risks.
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
Each with its own specific characteristics and applications:
- Ordinary hardness hardening: This is the most common method of hardening and is often used to improve the hardness and wear resistance of materials such as steel. In this process, the material is heated to a critical temperature (generally above the austenite line) and subsequently cooled rapidly in water or oil. This rapid cooling “locks” the crystalline structure of the material into a harder form, increasing its hardness.
- Vacuum quenching: This type of hardening takes place in an air-free or partial vacuum environment. Vacuum hardening is often used for oxidation-sensitive materials or for workpieces of complex shape. The main advantage of this method is the ability to precisely control the cooling rate, which can produce specific results in terms of hardness and microstructure.
- Induction hardening: Induction hardening involves heating the material by means of alternating magnetic fields. This method is known for its speed and efficiency in localized heating of specific parts of an object, such as gears or shafts. Induction hardening can be an effective solution to improve the surface hardness of a component without affecting its inner part.
Each type of hardening has its own specific applications and is chosen according to the requirements of the material and application.
The selection of the appropriate hardening method is essential to obtain the desired properties in the treated material.
These are among the most used but clearly if you are a knife maker or you harden the knife in specialized companies or you will perform the Hardening of ordinary hardness with shutdown in oil or between aluminum plates with compressed air.
Today it is also very fashionable among knife makers cryogenic hardening but we will make a specific post for this type of heat treatment.
Cryogenic quenching is an additional step in the quenching process that involves cooling the material to extremely low temperatures, generally below -100 degrees Celsius (-148 degrees Fahrenheit) using cryogenic agents such as liquid nitrogen or helium.
This cooling step at low temperatures follows the initial hardening in oil or water and is often used to further improve material properties, such as hardness, strength and dimensional stability.
Cryogenic hardening offers several benefits, including:
- Increased hardness: Cryogenic cooling can further increase the hardness of the material, making the final product more resistant to wear and abrasion.
- Improved dimensional stability: Cryogenic hardening can minimize material deformation or distortion, improving dimensional stability and maintaining specified tolerances.
- Reduction of internal stresses: Cryogenic cooling can help reduce internal stresses that can develop during initial quenching, improving resistance to cracks and fracture.
- Improvement of mechanical properties: Cryogenic hardening can improve the mechanical properties of the material, such as fatigue resistance and toughness.
This process is often used for knife blades, cutting tools, high-performance metal parts and stainless steel components.
Cryogenic hardening is a critical step in the production of high-quality materials that require high performance and durability.
However, it is important to note that cryogenic hardening requires specialized equipment and accurate temperature control to achieve the desired results.
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).
I personally use both the extinguishing in high performance cooling oil such as DURIXOL and the method with air using the procedure of inserting the blade extracted from the hardening furnace inside two thick aluminum plates and lowering the temperature quickly with compressed air.
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!
Both approaches have their reasons and advantages, and the choice often depends on the specific needs of the application and the characteristics of the material.
Here is a brief description of both approaches:
- Immediate tempering after cooling: This approach involves tempering the material immediately after cooling from quenching. The main reasons for doing this are:
- Control of residual voltages: By relieving immediately after quenching, residual stresses in the material can be reduced, which can help prevent deformation or breakage of the workpiece.
- Improvement of mechanical properties: Immediate tempering can help improve the toughness and strength of the material, providing an optimal balance between hardness and resilience.
- Postponed tempering (after hours or days): Some professionals prefer to temper at a later time, even after several hours or days after quenching. The main reasons for doing this are:
- Increased dimensional stability: After quenching, the material may undergo dimensional changes. Postponed tempering can allow the material to stabilize completely before further processing or assembly.
- Better uniformity: Postponed tempering can help achieve a more even distribution of mechanical properties in the material, avoiding potential stress points or more fragile regions.
Both approaches are valid and may be appropriate depending on the specific needs of the project.
The choice between immediate and postponed tempering depends on factors such as the type of material, tolerances required, part geometry and end use.
In many cases, testing and experimentation are necessary to determine which approach produces the desired properties for the treated material.
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.
My advice is to use high-performance cooling oils DURIXOL, with very fast cooling effect, they are characterized by a particularly intense cooling behavior.
They are mainly used for cooling in continuous cycle plants and well furnaces with open cooling systems, as well as for hardening in submerged baths after flame heating and induction.
A particular advantage is the high process reliability due to the superior stability of the product.
The oils have a practically unlimited shelf life and require minimal effort for monitoring and care of the bath.
Thanks to its high evaporation resistance, low viscosity level, low specific consumption and excellent cleanliness, unparalleled profitability is also achieved even in the most difficult applications, to be used also to harden tool steels designated as water hardeners.
I have never used olive oil but always specialized products so I do not have to tell you if the hardening changes substantially 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 igniting 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 liquids in contact with the blade overheat in turn dissipating 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.
The choice of coolant and hardening conditions really depends on several factors, including the type of steel, the shape of the part, the desired mechanical properties and the skills of the operator.
Here are some key summary points:
- Temperature shock: The intensity of the thermal shock, i.e. the temperature difference between the red-hot steel and the coolant, affects the final properties of the material. More intense temperature changes tend to produce greater surface hardness, but can increase the risk of deformation or cracking, especially in sensitive steels.
- Oil risks: Heated oil is often preferred for its moderate intensity of thermal shock, which can help reduce the risk of fractures and deformations. However, it is important to pay attention to the risk of fire, and therefore the use of specific oils for quenching with higher combustion temperatures is a common practice to reduce this risk.
- Forced air: As you pointed out, in some cases it is possible to use forced air as a coolant, but it is a less common technique and requires more advanced knowledge of steel properties and quenching conditions. The use of air can result in higher temperature changes than oil, which can affect the properties of the material.
- Steel Specifications: Different types of steel have different hardening behaviors. Some steels can be hardened in water, while others require more moderate methods such as oil. Knowledge of steel specifications is essential to achieve the desired results.
Finally, it is important to emphasize that hardening is a critical process in the production of blades and cutting tools, and requires a combination of technical skills, experience and appropriate equipment.
Safety is always a key consideration when working with high temperatures and flammable substances, so it is essential to follow appropriate safety practices, such as the use of specific oils for quenching, the presence of fire extinguishers and fire blankets, and careful supervision during the process.
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 and steel plates.
Ps. You can also use different oils to see the differences also because using small plates you do not have to use large quantities of oil, etc. but important always work safely!
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:
Baking, the process of inserting the knife into the furnace before quenching, is another critical step in the production of knives and cutting tools. Again, there are different opinions and approaches among makers and knife makers. The two main theories are:
- Bake cold: This approach involves inserting the knife into the oven when the oven is still cold or at a very low temperature. In this way, the knife gradually heats up along with the oven as the temperature rises. Cold baking is often preferred by those who want to minimize the risk of knife deformation due to thermal shock and by those who work with temperature-sensitive steels.
- Bake hot: Some makers and manufacturers prefer to bake the knife when the oven is already pre-heated to the desired temperature for quenching. This method can be used when faster cooling and greater thermal shock are needed to achieve certain material properties.
The choice between cold baking and hot baking depends on various factors, including the type of steel, knife geometry, desired properties and operator preferences.
Some considerations to consider include:
- High-carbon or temperature-sensitive steels could benefit from cold baking to reduce the risk of deformation.
- High-alloy steels or those that require faster hardening may require hot baking.
- Cold baking may require a longer residence time in the oven to reach the desired temperature, which can affect the overall duty cycle.
Ultimately, the choice between cold or hot baking depends on the specific needs of the project and the experience of the craftsman or knife maker.
It is important to experiment and adopt the approach that produces the desired results in terms of hardness, strength and knife shape.
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.
You are right, when performing home quenching or handcrafted manufacture of knives, products such as antiscale and condursal can be used to protect the blade from oxidation and decarburization during the heating and quenching process.
Here’s how these products work:
1. Antiscale: Stair heaters are chemicals or coatings applied to the surface of the blade to protect it from oxidation during heating. These products form a protective layer on the blade that prevents oxygen from the air from reacting with the metal while at high temperatures. Ladders help keep the chemical composition of the blade intact and prevent the formation of stairs or surface crusts.
2. Condersal: Condursal is a product used to prevent decarburization during heating. Decarburization is the loss of carbon from the surface of the metal due to high temperature. The condusal creates a carbon-rich environment around the blade, preventing carbon loss during heating. Usually, the condursal is applied to the blade before heating, and the blade is then heated to temperatures above 600°C to ensure that the conducsal has the desired effect.
The use of ladder and conducsal is common in the manufacture of handmade knives, especially when working with high-carbon steels.
These products help ensure that the blade retains its chemical composition and properties during the quenching process.
It is important to follow the manufacturer’s instructions for the correct application and use of these products to achieve the best possible results.
However, it is crucial to pay attention to safety during the heating and hardening process, as it involves very high temperatures.
Proper handling of materials and temperature management are essential to ensure operator safety and the quality of the finished knife.
The immersion in the liquid of the knife
At the end of this operation the knife is quite dirty!
Inconel “candy” is a term that refers to a quenching technique used in the manufacture of knives, particularly when working with stainless steels such as Inconel.
This technique can be used to improve the properties of the knife, especially corrosion resistance and hardness.
The “candy” with Inconel involves heating the knife in an oven at high temperatures, often exceeding 1000°C, and then wrapping the blade in a thin foil of Inconel.
The Inconel acts as a protective coating for the blade during quenching.
This coating prevents oxygen from coming into contact with the surface of the knife during cooling, thus reducing the formation of oxides and improving corrosion resistance.
After the blade is heated and covered with Inconel, it is subsequently hardened by cooling it quickly in water or oil, depending on the specifics of the process.
This rapid cooling gives the blade the desired hardness.
The “candy” with Inconel is an advanced technique that requires specialized equipment and specific skills.
It is mainly used when working with high-performance stainless steels such as Inconel to obtain knives that have excellent anti-corrosion properties and at the same time are very hard and sharp.
In conclusion, knife-making is an art that involves a number of critical processes, including hardening, that help define the properties and performance of the finished knife.
The choice of hardening methods, coolants, protective coatings such as Inconel and tempering times depends on the specific needs of the application, the type of steel and the preferences of the craftsman or manufacturer.
Safety is paramount throughout the entire process, given the high temperatures involved.
Knowledge and experience in knife hardening are essential to obtain high quality blades that are sharp, durable and corrosion resistant.
The continuous experimentation and adaptation of hardening techniques according to specific needs are essential characteristics of master cutlers and craftsmen involved in this fascinating art.
In summary, knife hardening is a crucial part of the manufacturing process that combines materials science with the art of craftsmanship to create sharp, functional tools that meet the needs of a wide range of applications.
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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