Heat Treating Knife Steel Explained Simply
Heat treating steel can get complicated in a hurry.
Start reading about it and before long you'll run into words like martensite, austenite, pearlite, carbides, grain structure, transformation temperatures, and a whole bunch of other metallurgy terms.
There's nothing wrong with learning all of that.
But maybe you don't want to become a metallurgist.
Maybe you just want to understand what happens to a piece of steel between the time I start making a knife and the time that knife is ready to carve.
That's what this article is about.
I'm going to explain heat treating the same way I would if you were standing in my shop.
In everyday language.
What Is Heat Treating?
Heat treating is the controlled heating and cooling of steel to change its physical properties.
For a knife maker, we're primarily trying to control things such as:
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Hardness
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Toughness
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Strength
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Flexibility
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Edge retention
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Ease of sharpening
We want the steel hard enough to take and hold a fine cutting edge.
But we don't want it so brittle that the blade chips or breaks during normal use.
Getting those properties where we want them is what heat treating is all about.
For a simple carbon-steel carving knife, you can think about the process in several basic steps:
Annealing
Normalizing
Hardening and quenching
Tempering
Each step does something different.
Heat Treatment Depends on the Steel
Before going any further, there's something important to understand.
There isn't one universal heat-treatment recipe for every knife steel.
Different steels require different:
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Temperatures
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Hold times
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Quenching methods
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Tempering temperatures
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Number of cycles
The intended use of the knife matters too.
I may want different characteristics from a carving knife than I would from a large chopping knife or a thin fillet knife.
I used to compare knife makers to chefs.
I still think that's a pretty good analogy.
Give several chefs the same basic ingredients and they may use somewhat different methods to arrive at the dish they want.
Knife makers are similar.
We develop processes that work with the steel we're using and the kind of knife we're making.
But unlike cooking, the steel manufacturer gives us a pretty important starting point.
Every steel has heat-treatment requirements that need to be respected.
What Is Critical Temperature?
You'll hear knife makers talk about critical temperature.
For this simplified explanation, think of it as reaching the temperature range where important changes begin taking place inside the steel.
We're heating the steel enough to change its internal structure so that the next part of the heat-treatment process can happen.
The exact temperature depends on the steel.
That's why I don't like giving one magic temperature and saying:
Heat every knife to this.
Steel doesn't work that way.
The alloy determines the process.
Annealing: Making the Steel Easier to Work
Annealing is used to put steel into a softer condition.
Why would a knife maker want soft steel?
Because hardened steel can be miserable to cut, drill, file, and grind.
Before heat treatment, we may need to:
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Shape the blade
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Drill holes
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File areas
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Grind bevels
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Straighten the steel
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Perform other machining operations
Those jobs are considerably easier when the steel is in a workable condition.
Depending on the steel, annealing involves heating it appropriately and then controlling the cooling process so the steel becomes softer and easier to machine.
The exact process varies with the alloy.
Think About a Bundle of Rubber Bands
In my original explanation of heat treating, I used a bundle of rubber bands.
It's not a perfect metallurgy model.
But it gives us something easy to picture.
Imagine holding a bundle of rubber bands.
If they're all stretched tight and under a lot of tension, the bundle behaves one way.
Let them relax and it behaves differently.
Heat treatment is obviously much more complicated than rubber bands.
But as we go through these steps, picture us changing what's happening inside that bundle.
During annealing, we're putting things into a more relaxed, workable condition.
Now we can more easily change the shape of our piece of steel.
Do All Knife Blades Need to Be Annealed?
Not necessarily as a separate step in every knife-making process.
If you purchase known knife steel in an annealed condition, the steel supplier has already provided material that's ready for machining and shaping.
If you're forging or working with steel in another condition, annealing or other thermal cycling may become part of the process.
This is another reason there's no single sequence that applies identically to every knife maker and every steel.
The starting condition matters.
Normalizing: Relieving the Effects of Working the Steel
When I forge steel, I'm heating it and physically changing its shape.
Hammering.
Drawing it out.
Changing the thickness.
Changing the profile.
That work affects the steel.
Normalizing is one of the thermal processes used after forging to help prepare the steel for hardening.
In simple terms, we're trying to get the steel into a more uniform condition after we've spent all that time moving it around.
With suitable steels and procedures, normalizing can help refine the grain structure and reduce stresses introduced during forging.
Back to the Rubber Bands
Think about our bundle of rubber bands again.
Imagine some are stretched tight.
Some are loose.
Some are pulling harder than others.
We'd like the bundle to be more uniform before moving on.
That's roughly the idea I'm trying to illustrate with normalizing.
Again, real steel metallurgy is considerably more complicated.
But you don't need a microscope to understand the practical reason for doing it:
We're preparing the steel for the next step.
Hardening: Creating a Hard Cutting Blade
Now we get to the part most people associate with heat treating.
Hardening.
For many knife steels, the blade is heated to a specified temperature or temperature range appropriate for that particular alloy.
At that temperature, changes occur inside the steel that prepare it for hardening.
Then we cool it rapidly enough to create a hardened structure.
That rapid cooling is called quenching.
This is the point where our relatively workable piece of steel becomes a hardened blade.
What Is Quenching?
Quenching is the controlled rapid cooling of the heated steel.
Depending on the alloy, the quenching medium might be:
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Oil
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Water or brine
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Air
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Gas
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Specialized quenchants
This is another place where knowing your steel matters.
You don't simply heat a piece of steel and throw it into whatever bucket happens to be nearby.
A steel designed for an oil quench may require a different process from an air-hardening steel.
Cooling too slowly may prevent the steel from developing the hardness you're trying to achieve.
Cooling too aggressively can create other problems, including distortion or cracking.
The quench needs to match the steel.
Why Does a Knife Need to Be Hard?
Because we're trying to create a very fine cutting edge.
Imagine the very apex of a carving knife.
That edge becomes extremely thin.
If the steel is too soft, that fine edge can deform or roll during use.
Proper hardening gives the steel the ability to support that fine cutting edge.
That's especially important on a carving knife.
A carving knife isn't just supposed to be "sharp."
It needs to maintain a fine edge while making controlled slicing cuts through wood.
Harder Isn't Always Better
This is one of the most important lessons in heat treating.
If hardness is good, it seems logical that more hardness must be better.
Not necessarily.
As hardness increases, we also have to think about toughness and brittleness.
A blade that's extremely hard but too brittle for its intended use isn't a good knife.
It may chip.
It may crack.
In an extreme case, it may break.
That's why hardening isn't the final step.
After hardening comes tempering.
Tempering: Finding the Useful Balance
After hardening, the blade is generally too brittle to use in that condition.
We need to temper it.
Tempering means reheating the hardened steel to a lower controlled temperature.
This reduces some of the hardness while increasing toughness and making the blade more suitable for actual use.
That's the tradeoff we're trying to manage.
Hard enough to hold an edge.
Tough enough to use as a knife.
The exact tempering temperature and process depend on the steel and the properties the knife maker wants.
One More Trip to the Rubber Bands
Go back to that bundle of rubber bands.
Imagine stretching every band as tight as you possibly can.
Now the bundle is under a tremendous amount of tension.
Push things too far and some of those bands are going to snap.
We want to relax them somewhat.
Not all the way back to where we started.
Just enough to give us a useful balance.
That's the simple idea behind my rubber-band explanation of tempering.
We've hardened the steel.
Now we're bringing it back to a condition that's better suited to doing actual work.
Can You Heat Treat Steel With a Torch?
People have been heat treating simple steels with relatively basic equipment for a very long time.
A forge or torch can provide enough heat for some steels and some small blades.
But there's a difference between getting steel hot and controlling a repeatable heat-treatment process.
As steel alloys become more demanding, accurate temperature control becomes increasingly important.
For repeatable knife making, knowing the steel and controlling the process gives you much more confidence in the finished blade.
Why I Prefer Known Steel
This connects directly with making carving tools from everyday objects.
You can make some surprisingly useful tools from old files, saw blades, springs, and other pieces of hardened steel.
I've done it.
It's fun.
But when I'm making a knife for a customer, I want to know what I'm working with.
Known steel gives me a known starting point.
I can use a heat-treatment process appropriate for that alloy.
I can aim for repeatable hardness.
And I can expect one knife to behave like the next.
For something I'm experimenting with in the shop, mystery steel can be interesting.
For a carving knife I'm sending out the door, I want repeatability.
Why Heat Treatment Matters to a Woodcarver
You may be thinking:
I'm a woodcarver. Why do I care about any of this?
You don't necessarily need to.
You certainly don't need to understand phase diagrams to carve a spoon.
But heat treatment explains some of what you're feeling when you use a knife.
A properly heat-treated carving knife should:
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Take a fine edge
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Hold that edge reasonably well
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Respond to stropping
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Resist unnecessary rolling or chipping
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Be practical to sharpen
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Have enough toughness for its intended use
If those things are happening, the knife maker probably did his job.
You don't need to know every detail of how he got there.
Heat Treatment Isn't Magic
There's a tendency in knife making to talk about heat treatment as though somebody has discovered a secret formula.
Sometimes you'll hear about:
"Secret heat treatment."
"Proprietary process."
"Special tempering."
Maybe there's something useful behind those descriptions.
But steel doesn't respond to marketing.
It responds to temperature, time, chemistry, and cooling.
A knife maker may develop a process that works particularly well for the steel and knives he makes.
That's valuable.
But ultimately, the proof is still sitting at the cutting edge.
Does the knife cut well?
Does it hold its edge?
Can you maintain it?
Does it behave the way the tool should behave?
That's what matters.
My Advice
If you're a woodcarver, you don't need to become a metallurgist to understand heat treatment.
Remember the basic sequence:
Annealing makes suitable steel easier to work.
Normalizing helps prepare appropriate steels after forging or thermal work.
Hardening develops the hardness needed to support a cutting edge.
Quenching is the rapid cooling used to create that hardened condition in steels that require it.
Tempering reduces brittleness and brings the hardened blade to a useful balance of hardness and toughness.
That's the simple version.
There's an entire world of metallurgy underneath those few sentences.
And if you enjoy knife making, it's worth learning.
But if you just want to understand why I heat a piece of steel, cool it, heat it again, and eventually turn it into a carving knife, that's really what we're doing.
We're changing the steel until it has the properties we need.
Not the hardest steel possible.
Not the most complicated heat treatment possible.
The right steel, at the right hardness, for the job the knife is supposed to do.
That's the goal.
Frequently Asked Questions
What is heat treating knife steel?
Heat treating is the controlled heating and cooling of steel to change properties such as hardness and toughness. For a knife, the goal is to produce steel capable of taking and holding a useful cutting edge without becoming unnecessarily brittle.
What is annealing?
Annealing is a heat-treatment process used to put suitable steel into a softer, more workable condition. This can make cutting, drilling, filing, and other machining operations easier.
What is normalizing knife steel?
Normalizing is a thermal process commonly associated with forged carbon steels. In simplified terms, it helps prepare the steel after forging and can help refine grain structure and reduce stresses before hardening.
What does hardening do to knife steel?
Hardening develops the properties that allow the steel to support a fine cutting edge. The exact hardening temperature and procedure depend on the particular steel alloy.
What does quenching mean?
Quenching is the controlled rapid cooling of heated steel during hardening. Different steels require different quenching methods, such as oil, water, air, or specialized quenchants.
Why do you temper a knife after hardening?
A freshly hardened blade can be too brittle for practical use. Tempering reheats the hardened steel to a controlled lower temperature to reduce brittleness and create a more useful balance between hardness and toughness.
Can you heat treat any steel?
Different steels respond differently to heat treatment, and some steels aren't suitable for making hardened cutting tools. When making a knife, knowing the steel allows the maker to use an appropriate and repeatable heat-treatment process.
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