Beginners Guide to Knife Making - Introduction to Knife Steels
Beginners Guide to Knife Making - Introduction
One of the questions I often get is what knife steel should I use?
This journal entry, a part of the Beginners Guide to Knife Making series, is aimed at absolute beginners to knife steels. It introduces the basic principles in simple and easy to understand language so that the beginner knife maker can understand some of the industry terminology and make informed choices.
For the purposes of this journal entry, there are 4 main types of steel that you need to know:
Plain carbon steels. Good old iron and carbon heat treated to make it hard.
Alloy steels, often referred to as tool steels, are plain carbon steels with other elements added in to modify there properties.
Stainless steels (a subset of alloy steels) defined as a steel with greater than 11% chromium and usually around 12 to 13%.
‘Super Steels’ are created through the use of highly specialised manufacturing processes that give the steel outstanding properties. Many of these Super Steels are also stainless.
This is a subject I could really nerd out about! I have endeavoured to keep it simple.
Beginners Guide to Knife Making - What is steel?
Steel is the fusion of iron (ferrite), a relatively soft metal with Carbon. These steels are referred to as plain carbon steel and generally only vary in the quantity of carbon added to any particular grade of steel.
When the carbon content gets to around 0.35%, the steel gains a magic quality. When it is heated above a certain point and cooled rapidly, the steel transforms from soft to hard. This point is known as the steels eutectoid point and the process is generally known as hardening. The hardening process transforms the elemental iron and carbon in the steel into iron carbide (Cementite).
Steels below 0.35% carbon, that cannot readily be hardened, are known as Mild Steels.
The eutectoid point and the subsequent properties of the steel can be altered by adding other elements to the basic iron/carbon content. These additions form an alloy.
Before we can have a meaningful conversation about alloys, we need to get a few scientific terms defined.
There are four main material properties that we need to understand:
Hardness
Brittleness
Toughness
Ductility
These properties are all interrelated with a change in one affecting the others often in negative ways, and are controlled in the manufacturing process, usually when the blade is heat treated.
Hardness - this is the material’s ability to resist wear or indentation - so in the context of knives this translates to how long it retains its edge (stays sharp) for. Everyone wants their bushcraft knife to be hard but many don’t really understand that comes with some downsides.
Brittleness - is the biggest potential downside of hardness and not a desirable property in a bushcraft knife. In engineering terms, brittleness is defined as an inability to undertake plastic flow or deformation i.e. the material cannot easily move when subject to a force like being bent or impacting something (think levering and chopping and no, your knife is NOT a substitute pry bar!). Lack of plastic flow in this context could result in the blade snapping.
Toughness - is the material’s ability to withstand impact. This a really desirable trait in a working tool but has to be just right. As the Assyrians used to say, a bent sword can be repaired, a broken sword is no use to anyone. The two downsides of toughness are brittleness and ductility. Counterintuitively, as a tough material gets colder it starts to behave as if it’s hard and brittle. A consideration if we are using our knives in the boreal forest for example.
Ductility - defined as the percentage elongation at failure, it is the material’s ability to stretch under load. Unfortunately, if a material is too ductile this stretch becomes permanent. A ductile material will be prone to bending and denting, and will not hold an edge for long.
There are many other material properties but these are the main ones that concern us when trying to understand the properties of knife steels.
Beginners Guide to knife Making - what are the common alloying elements?
Picture credit - https://periodictable.com/Items/041.13/index.htm
There are many elements that are added to steels to alter its properties. The interaction of these elements within an alloy is complex. Any one element can provide secondary effects in conjunction with another. The effects listed below are the main affect associated with the addition of that element.
The elements most commonly used to make steels that are suitable for making knife blades are:
Carbon - already mentioned, the addition of carbon allows elemental iron to form iron carbides and thus be hardened. Too much carbon can lead to brittleness.
Silicon - present in almost all steels, Silicon is used as a deoxidising agent, bonding with oxygen and impurities in the molten steel to form a slag which is removed resulting in a reduced oxygen content in the finished material.
Chromium - probably the most recognised alloying element in knife steels, chromium is added to improve the corrosion resistance of steel, chromium when present in large quantities will contribute to increased hardness of the steel through the generation of chromium carbides.
Manganese - is added to the steel to improve the hardenability. What does this mean? The technical description is that it lowers the critical cooling rate of the steel. In the real world this translates to whether the steel needs to be quenched in oil or in water. A water quench is relatively violent and can result in a blade cracking. Generally, lowering the critical cooling rate is a good thing as it allows the steel to experience a gentler transition during the quench.
Molybdenum - primarily added to improve the toughness and strength of the steel, it can also contribute to corrosion resistance.
Vanadium - commonly added to improve the wear resistance of the steel.
Tungsten - is added to knife steels to improve the hardness and wear resistance of the steel.
Nickel - when used as an alloying element rather than as a decorative coating, improves the toughness of the steel, especially at low temperatures. Knife steels containing Nickel are sought after for pattern welded (Damascus) steels. When used in conjunction with non Nickel containing steels, it provides high contrast in the resulting pattern.
Niobium - improves hardness whilst improving the grain structure.
Alloy steels make up the vast majority of common knife steels.
Beginners Guide to Knife making - Can I use any stainless steel to make a knife?
Short answer is no. Any stainless steel can be used to make a knife shaped object but only very specific stainless steels can be hardened to take a lasting sharp edge.
As previously mentioned, stainless steels are a subset of alloy steels that contain a large percentage of chromium, usually around 13%.
From an engineering materials perspective there are four main types or classes of stainless steels:
Ferritic - this stainless steel tends to have greater than 17% chromium and thus has very good corrosion resistance but is relatively weak. It is mainly used in kitchen fittings and ornamental or architectural works where corrosion resistance is more important than strength. Ferritic stainless steels cannot be hardened so are not suitable for making high quality knife blades.
Austenitic - these stainless steels typically have between 8 to 20% Nickel added to them and form the largest group of engineering stainless steels, being widely available and used in many applications. They are weldable and non magnetic, and having a relatively low carbon content, they cannot be hardened through heat treatment. They are therefore not suitable for making quality knife blades.
Duplex stainless steels - these steels combine the properties of both the Ferritic and Austenitic stainlesses to from a class of materials that that remains weldable but has increased strength. Again this class of stainless steels is not hardenable through heat treatment.
Martensitic - this class of stainless steel has up to 2% carbon added and can be hardened sufficiently through heat treatment to produce quality knife blades. Martensitic stainless steels can be magnetic but they are not weldable. This is the only class of stainless steels that are suitable for high quality knife blades.
These classifications are based on differences in the microstructures of each class of steel, a detailed description of which is beyond the scope of this journal entry. If you wish to understand these microstructures in more detail, this link and the associated video provides an excellent entry level explanation:
If you are looking for something a little more detailed with an engineering or scientific background its worth starting here:
The main reason for choosing a stainless steel for a knife blade is the increased corrosion resistance offered. It’s important to note that this is a resistance to corrosion, not a corrosion free material. Stainless steels will still corrode if exposed to the right conditions and thus still need to be cared for in these conditions.
Beginners guide to knife making - Super steels
Picture credit - Böhler Edelstahl GmbH & Co KG
I hesitated to put this section into this journal entry as the term ‘super steels’ is really a misnomer even though the term has been in use since at least the 1970’s. There remains no actual consensus as to what ‘super steels’ actually means or refers to. I was worried that it would cause confusion BUT as the term is in common use some explanation is warranted.
The label ‘Super Steel’ seems to be applied to the latest or newest steel or steel technology that promises to be the panacea for the knife maker. As new developments arise, this label gets applied to different steels and drops out of use for the now old or commonly accepted steel it was previously used for.
At the time of writing, ‘super steels’ is most often used to describe a class of alloy steels manufactured using a highly specialised process called Crucible Particle Metallurgy (CPM).
Let's start with the normal steel manufacturing processes to set the scene.
In this process, a molten batch of the steel alloy is cast into an ingot mould and allowed to cool until it has all turned into a solid. As a liquid, the molten alloy is homogeneous (evenly mixed with all parts being the same uniform composition). As the liquid cools into a solid, the different alloying elements solidify at different rates and start to settle in the remaining pool of liquid through a process known as precipitation. This leads to a solid material that is no longer homogeneous but has layers of different elements spread throughout it. This process is known as stratification and reduces the overall benefits of the homogeneous material.
In a knife steel stratification is undesirable as it results in the carbide forming constituents being unevenly distributed. In simple terms, at a microscopic level the steel can have hard bits and soft bits randomly distributed throughout the blade that affect the strength, toughness and edge holding ability of your knife.
The only just solid, but still red hot, Ingot is passed through a rolling mill to reduce the size of the material into the bar or rectangular section stock material we might purchase from our suppliers.
This crushing and lengthening process helps to break up the stratification and reduce some of its negative affects but, also tends to stretch and layer the grains within the finished material, which can present its own challenges. The resultant material is never as good as the promise embodied within the homogeneous molten material.
In the CPM process, the same homogeneous material is poured through a nozzle at the top of a tall tower where a high pressure inert gas is blown into the stream forcing the liquid material to break up into tiny droplets. As these droplets now have very little thermal mass (compared to an ingot) they cool very rapidly forming tiny spheres of almost completely homogeneous material with no stratification. The solidified droplets are collected at the bottom of the tower as power particles.
This powder is passed through a series of graduated sieves to separate it into batches of virtually identical sized particles.
The graded powder is packed into a robust steel mould that is sealed. The moulds are heated up to temperatures just below the melting point of the material and subjected to intense pressure, simultaneously from all directions, in a process known as Hot Isostatic Pressing (HIP). The combination of heat and intense pressure causes the individual particles to bond together forming a dense compacted solid material. This material has a very uniform grain structure with evenly distributed carbide forming elements. Depending on the application, the resulting material can be used in this form or can undertake further milling operations similar to the conventional process.
The uniform distribution of very fine carbides within a HIP material helps prevent grain growth during heat treatment, that results in very fine grained microstructures post heat treatment (that’s a good thing in a knife blade).
This all leads to a material that has good toughness, good edge retention and provides consistent heat treatment results.
The down sides to CPM materials are that they are relatively expensive and, depending on the alloy mix, may require specialised heat treatment to get the best result from the material.
Beginners Guide to Knife Making - Recommendations
So that’s all well and good, but as a beginner all you really want to know is what steel should you be using to make your first knife?
Before we recommend specific steels let’s ask some basic questions.
What do you want to do with it?
The steel that is suitable for a razor sharp scalpel dedicated to eye surgery, may not be tough or flexible enough to make a long lasting, large bladed, chopping knife like a Khukri. Conversely, the tough flexible steel that is good for a chopping knife might not hold a sharp enough edge to make a good scalpel.
How do you plan on making it?
Do you plan to forge your knife blade to shape or is stock removal more your thing? Different steels, section sizes and shapes all have pros and cons when considering the best way of going from a piece of raw material to a finished blade.
How are you going to heat treat it?
Do you already have specialist heat treatment equipment? Are you just dipping your toe into the water of knife making and want to make your knife on a shoe string? Or are you planning on sending your knife out for professional heat treatment? All of these factors affect your steel choice.
Availability and cost
If you are making your first knife there is little point in ordering a CPM steel or a piece of pattern welded steel from an artisan maker that’s been shipped half way around the world. Knife sized pieces of these steels can easily cost 10 times more than an equivalent sized piece of plain carbon or alloy steel. All to be thrown away if you make a mistake shaping or heat treating it.
The following specific steels are recommended on the premise that you are a beginner at knife making, you do not own a lot of specialised tools or heat treatment equipment and you plan to do all the work yourself at home.
General purpose steel for stock removal.
My recommendation here is for O1 tool steel. This steel is also known as ground flat stock as it is used in the gauge making industry and comes precision ground to specific thicknesses for that purpose. This is very convenient for the knife maker who is filing bits away to form the shape of the knife. The main work, getting the thickness consistent, is already done for you.
O1 tool steel is also very forgiving in the heat treat and can be adequately hardened in commonly available oils (vegetable oil, rape seed or canola oil or sunflower oil). It can also be tempered to a sensible knife hardness in a domestic oven.
You will read on the internet that using alloy steels at home is a waste of good money as you cannot get the full potential from the steel without specialised heat treatment. This is true. It is difficult to get full and consistent carbide conversion using home heat treatment methods. However, O1 being very forgiving of the heat treatment, what results is a more than adequate heat treatment for most purposes with a lot lower risk of a failure when you quench it. The slight increase in cost of O1 tool steel over a more generic plain carbon steel is in my opinion worth it for all of the benefits it brings.
Lots of really good knives have been made out of O1 tool steel without professional level heat treatment.
General purpose steel for Forging
My recommendation here is one of the 1050 grade carbon steels (EN9, 070M55, 1.0535). They have good malleability across a wide temperature range allowing more time to move it with a hammer between reheats. Other more complex steels have a tendency to ‘toughen up’ significantly once the temperature has dropped even a little bit. This results in a lot of effort hitting a solid lump for very little movement of the metal and the potential of introducing micro fractures that may weaken the blade and cause a failure during heat treat.
1050 can be a little tricky during the quench requiring a fast quench medium like water. This can be a little nerve wracking as the violent transition from hot too cold can cause cracking. Unfortunately oil quenches have a habit of not fully hardening this grade of steel without spending some money on a ‘fast quench oil’ like Parks 50.
Stainless steels
My recommendation here is don’t bother.
Have fun with plain carbon or alloy steels recognising that you can get good results at home with minimal financial outlay. To get good results from stainless steels really requires specialist heat treatment equipment as they tend to have a narrow temperature band for hardening and, tempering temperatures that are above those in a domestic oven.
If you really do want to use stainless either plan (and cost) for getting it professionally heat treated or, if you absolutely must do it yourself, use good old 440C. This has just about the broadest hardening range of the modern stainless steels. Be warned though, this temperature is a lot higher than the ‘just above non magnetic’ temperatures of plain carbon steels and is quite difficult to judge accurately by eye.
‘Super’ steels and pattern welded steels.
Whilst looking at steel specs and fantasising about everlasting razor sharp edges makes super steels seem attractive, they come with all of the downsides of the stainless steels alongside a hefty price tag. Leave these steels until you have some experience and are prepared to invest in the equipment necessary to get the best from the steel and hence justify the cost.
Similarly high quality pattern welded steels can be very expensive and difficult to heat treat. Whilst they look beautiful, save your money until you are confident that your skills in shaping and handling the blade will match the beauty of the material.
A future journal entry will look at heat treatment in more detail.
