Heat treatment encompasses the heating and cooling operations that transform a cutlery steel, delivered in an annealed state, into a hard, stable, and sharpenable blade. The composition of a grade sets a potential; the thermal cycle — austenitizing, quenching, tempering, sometimes cryogenic treatment — largely determines what the blade will become. Two knives cut from the same bar can therefore behave differently if their treatments differ. For the user, the difference is concrete: a thread that holds or unravels, a sharpening that is simple or laborious, a blade that withstands a rib bone or chips on it. Understanding these steps helps to read a product sheet in a way other than just through the steel's name.
Quenching and heat treatment: two different things
In everyday language, people say that a blade is "quenched" and think they have said it all. Quenching does indeed exist: it is the rapid cooling that hardens the steel. But it is only one step in the middle of a complete sequence. Before it, precise heating prepares the structure of the metal. After it, tempering makes the blade usable. Without these operations, quenching alone produces a hard and brittle object — not a knife.
Quenching hardens the steel. Heat treatment makes it a blade.
Talking only about quenching is like summarizing a recipe by just mentioning the oven step. It's the spectacular step, not the entirety of the work.
The steps of heat treating a blade
The typical path of a blade follows this order:
- steel in its initial state, generally annealed;
- preparation: cutting, drilling, shaping;
- austenitization;
- quenching;
- cold treatment, depending on the grade;
- tempering, sometimes repeated;
- straightening if necessary;
- inspections;
- grinding and finishing, according to the chosen process.
This sequence provides the general framework. The exact order, temperatures, and durations vary depending on the steel grade, equipment, and desired outcome.
The initial state: still soft steel
The steel arrives at the workshop in the annealed state: a soft structure, easy to cut, drill, and shape. It is in this state that I prepare the blades — laser cutting, drilling, initial adjustments. Attempting these operations on already hardened steel would wear out the tools for a mediocre result.
Austenitization: the heating that transforms
Heating brings the steel into a temperature range where its structure changes: part of the carbon and some alloying elements go into solution in the metal. The temperature and holding time depend on the grade. Heating that is too high or too long causes the grain to grow and leaves too much austenite after quenching: the blade loses hardness and fineness. Heating that is too low does not dissolve enough carbon. The stage is played out within just a few tens of degrees.
Quenching: freezing the martensite
Rapid cooling freezes the structure and forms martensite, the hard phase that will hold the edge. The quenching medium — oil, air, gas, vacuum furnace — depends on the grade: some steels require a high speed, others harden in air. It is also the stage with risks: internal stresses, warping, cracks. A long, thin blade rarely comes out of quenching perfectly straight; it is then straightened within a limited time window.
Cold treatment
Some nuances, particularly stainless steels rich in alloying elements, retain a portion of untransformed austenite after quenching. Exposure to cold converts part of it into martensite and can provide a hardness gain of about one to two HRC points — this is what, for example, the manufacturer Alleima documents for its 12C27 and 14C28N steels. The benefit varies depending on the steel and the applied cycle.
Point of caution: the cold does not fix anything. Poorly adjusted austenitization or a failed quench remain so, cryogenics or not.
The income: making the blade usable
After quenching, martensite is hard but full of stresses: an untempered blade can break if it falls from the workbench. Tempering — one or more moderate heats — releases these stresses and trades a bit of hardness for toughness. This is where the compromise is made: a low temper retains hardness, a higher temper favors shock resistance. Some grades have their limits: for example, Alleima notes that for 14C28N, embrittlement and a loss of corrosion resistance occur beyond 450 °C tempering. Losing a point of hardness to gain a reliable blade is often a good trade.
Inspections
At the end of the cycle come the measurements: hardness on the Rockwell C scale, straightness, crack detection, surface condition. The blade then goes back for sharpening and finishing, and each finished knife undergoes a final examination before leaving the workshop: consistent hardness, stable edge, geometry conforming to the intended use.
| Step | What happens in the steel | What it changes for the blade | Risk in case of incorrect settings |
|---|---|---|---|
| Initial state (annealed) | Soft structure, carbon mostly bound in carbides | Steel easy to cut, drill, and shape | Difficult machining if the steel is not properly softened |
| Austenitization | Dissolution of part of the carbon and alloying elements | Prepares the final hardness and grain fineness | Coarse grain, excess residual austenite, low hardness |
| Hardening | Formation of martensite by rapid cooling | Hardens the blade, basis of the future cutting edge | Warping, cracks, insufficient hardness if cooling is too slow |
| Cryogenic treatment | Transformation of part of the residual austenite | Slight increase in hardness in certain grades | Marginal or no effect if the upstream cycle is poorly adjusted |
| Income | Relaxation of stresses, adjustment of hardness/toughness | Usable blade, reliable edge, reduced brittleness | Brittle blade if absent, too soft or weakened if excessive |
| Control | Hardness and straightness measurements, defect detection | Ensures consistency between the blade and its use | Undetected defect that becomes apparent to the user |
HRC hardness: a useful indicator, not a final grade
The Rockwell C scale measures the steel's resistance to penetration by a diamond cone under load. The number indicates the hardness of the blade — most knives are between about 54 and 64 HRC. It is a useful, but incomplete, piece of information. Two blades with the same hardness can behave differently: toughness, grain size, type of carbides, residual austenite, thickness behind the edge, and sharpening angle are not reflected in this number.
A high hardness helps the edge to hold, but brings closer the risk of micro-chipping if the geometry is fine and the use is harsh. A kitchen knife, a table knife, and an outdoor knife do not call for the same target. The HRC number gives an indication. It does not convey the whole knife. Fortunately, otherwise the profession would fit on a label.
Same steel, different knives
Take two knives in 14C28N, in N690 or in RWL34. Same shade, same composition numbers. Yet they can cut, sharpen, and age differently. The reasons lie in the treatment and the geometry: applied heat cycle, targeted hardness, amount of residual austenite, grain size, condition of the carbides, thickness behind the edge, sharpening angle, edge finish, intended use.
Alleima's documentation illustrates the extent of the possible variation in its own grades: overheating the austenitization of 14C28N produces a coarse structure and about 30% residual austenite — hardness and wear resistance drop — whereas a well-controlled cycle targets around 15% austenite and fine, well-distributed carbides. Comparing two product datasheets based only on the steel name is like comparing two dishes based on the ingredient list alone. The sandwich salami, on the other hand, will taste the difference.
Carbon, stainless steel, powder metallurgy: different logics
Not all steel families require the same cycle, nor the same trade-offs.
Carbon and low-alloy steels
Like the C130. The cycle must form a fine martensite with relatively low heating and rapid quenching, often in oil. In return: a very fine wire and easy sharpening, against a corrosion sensitivity that requires wiping the blade after handling tomatoes, not the next day.
Fine martensitic stainless steels
Like the Z50CDV13 or 14C28N. The cycle must retain enough chromium in solution for corrosion resistance while achieving the target hardness. The result: a balanced compromise between wire fineness, toughness, and reduced maintenance.
Stainless steels with higher carbide content
Like the N690. The chromium carbides provide cutting edge retention; the cycle must handle them without weakening the blade or depleting the chromium in the matrix. Sharpening becomes a bit more demanding.
Powder metallurgy steels
Like the RWL34. Their fine and homogeneous structure allows for high hardness and stable edges, provided a rigorous cycle, often conducted under vacuum. The Damascus steels, carbon or stainless, follow the logic of their constituent steels: the pattern does not change the metallurgy.
No family wins absolutely. Each one responds to different uses and priorities.
Heat treatment never works alone
The hardness obtained only makes sense when related to the geometry of the blade: overall thickness, thickness behind the edge, bevel, edge angle, length, and finesse. A long and thin kitchen knife, intended to slide into a vegetable or a piece of meat, favors the stability of a fine edge with sustained hardness. A knife meant to withstand more brutal forces accepts lower hardness in favor of toughness and a more open angle.
The sharpening angle also weighs heavily on the actual cutting performance — often more than moving from one grade to another. That is why I choose the target hardness and the geometry together, starting from the intended use. A knife is designed from the cutting board, not from the steel's technical datasheet.
Heat treatment: in the workshop or with a partner
Unique pieces and knives that are outside of series models are entirely made in the workshop, including heat treatment. For some series-produced models, the heat treatment or a preparatory grinding may be entrusted to a specialized partner: grouping these repetitive operations on dedicated equipment ensures consistent cycles across entire batches.
In both cases, the decisive work remains the same: I choose the shade, I define the targeted geometry and hardness, I set the expected specifications, and I check each part. Entrusting an operation does not exempt one from knowing how to define it or from knowing how to verify its result. The choice simply depends on the model and its production method.
What you can actually observe
No one sees the martensite or the grain size in their kitchen. What can be noticed, however, is seen in use: a stable edge over the course of meals, a reasonable sharpening frequency, a sharpness that can be restored in a few passes, the absence of micro-chips after a well-filled cutting board, a blade that slices through a dry sausage without flinching and withstands brief contact with acidic food without marking.
A consistent heat treatment is not visible; it is noticed by what the blade does not do. It does not roll, does not chip on the first rib bone, does not demand the stone every week. The discreet consistency is the true sign of a well-conducted cycle.
Some ideas to cool down
- "The harder, the better." No. Beyond a certain point, you gain in edge retention what you lose in toughness. The right hardness is the one that matches the blade and its use.
- "Stainless steel never rusts." It resists better, but can stain or pit if its protective layer is damaged or if acidic or salty residues remain on the blade. Rinsing and drying are still good habits.
- "Cryogenics necessarily improves all steels." It is mainly used for steels that retain a lot of residual austenite after quenching. Elsewhere, its effect is weak — and it never corrects a poorly set upstream cycle.
- "An expensive shade automatically gives a better knife." A reputed steel, poorly treated or poorly matched to its geometry, can produce a mediocre blade. A simpler steel, properly treated, can produce an excellent knife.
- "Entrusting a specialized operation removes all artisanal value from the knife." The value lies in the design, technical choices, and control of the result — not in the ownership of the furnace. Knife makers have always worked with steelmakers, suppliers, and specialists, in Thiers as elsewhere.
Steel proposes, treatment disposes
A grade sets a potential; heat treatment decides what the blade will do with it, and the geometry decides what you will do with it. These three elements are chosen together, starting from the use: kitchen, table, pocket, or outdoor. The name of the steel is therefore never enough to judge a knife, just as an isolated HRC number is not.
To situate the grades among themselves, their families, and their uses, consult the guide for understanding cutlery steels. To place the heat treatment cycle within the whole process, you can discover the steps of making a handcrafted knife, then see the finishing and final inspection that validate each piece before shipping.