General information about knife steel

When to choose steel As a knife maker or blacksmith, there are a lot of considerations that need to be made, and there is no one steel that is right for everyone. In this article, we will give a quick introduction to steel, how to navigate the world of steel and introduce some of the popular types of steel. This way you will have the opportunity to choose for yourself what is right for you and what your knives are used for. If you would like a more detailed introduction, please contact us and we can help you.

How are the properties of steel described?
Sometimes it is difficult to understand or make oneself understood, so it is important that we have the same understanding of what lies behind the words. For example, toughness may have negative associations for some, while for others it is a very important characteristic.

Hardness
Defined as the ability to resist deformation. Hardness is measured on the Rockwell C scale (HRC)

Toughness
The opposite of brittleness, toughness is the ability to resist chipping in the edge, cracking and breaking. This is important for having a knife blade that can withstand use, and is often underestimated in comparison to hardness, rust resistance and edge retention.

Egg retention (Retention)
How long the blade will stay sharp during use. There is no widely standardized testing for edge retention, so it is often a subjective assessment based on tests such as cutting a given amount of rope, cardboard boxes, etc.

Rust resistance
The ability to resist oxidation/rust. Comes from a high content (over 10.5%) of chromium, which leads to the sacrifice of other properties, often the hardest to overcome is toughness. It is worth noting the words rustfree and stainless. It is not "free from rust", as all steels rust eventually, they just "stain less".

Popular knife steels:

80CrV2
Carbon steel known for its toughness, forgiving hardening, easy sharpening and edge retention, but like other carbon steels it will rust if exposed to moisture. A true golden mean.

1075 also known as C75S
A very underrated steel that provides good performance for most purposes without being picky about the hardening process. In other words, it is easy to shape, easy to harden, gives a very sharp edge that is easy to maintain.

ELMAX
Powder metallurgical steel with somewhat more demanding hardening than simpler stainless steels, provides good edge retention and good rust resistance, but at some expense of toughness and is therefore best suited for smaller knives.

Sleipner
A high-alloy tool steel that is noted for its good performance and relatively easy hardening, but requires an oxygen-free environment. It is often described as an improvement on the well-known D2 tool steel that is widely used by knifemakers. Sleipner has improved toughness, and is therefore a much better suited steel for edge tools.

M4
Made to hold an egg for as long as possible, while maintaining good toughness. The downside is that it is demanding and hard to set up a new egg.

S30V
Very good edge retention and rust resistance, often used in slightly more expensive kitchen knives and folding knives.

S35VN
"Upgraded" version of S30V that makes it easier to work with, increases toughness and makes it easier to set up a new egg.

AEB-L
Stainless steel often used in Gillette razors. Widely used in custom chef's knives, hunting knives and also actually in Norwegian customs knives with stainless blades, but somewhat demanding to harden compared to carbon steel, but easier than Elmax.

1095
Sought after for its high carbon content and ability to make a hardening line/hamon. 1095 has a high carbon content, is easy to grind and relatively easy to harden.

The structure of steel

The material consists of a mixture of carbon and iron with additional additives such as manganese, vanadium, silicon, chromium, etc. It is mainly the composition of the steel and the work of the steel manufacturer that determines how suitable the steel is when it arrives at the blacksmith, who in turn determines the properties of the steel within these limits through the hardening process.

Alloy is the word we use to describe the composition of steel. We often talk about unalloyed, low-alloyed and high-alloyed. Unalloyed will be just or almost just iron and carbon (e.g. 15-steel and 1095), low-alloy will have a few additives or small amounts that give desired effects (e.g. 80CrV2 and D2), high-alloy steels are advanced mixtures often specifically designed to perform a specific purpose (e.g. S35VN, ELMAX and AEB-L).

Carbon steel and stainless
Steels are often divided between high carbon and stainless, where you find extremes at each end and a wide range in between. The rust-free nature comes from a high chromium content. In many cases, this compromises important properties such as toughness and edge maintenance. This means that stainless is not the holy grail that many believe, but if you know the knife's intended use, the steel can be selected and hardened to obtain optimal properties for the intended use.

Know the steel!
Not all steels can be hardened, and it is therefore important to use steel whose content you know and can thus harden correctly.

If you are going to use found/recycled steel, you must know that it can be hardened or start by investigating whether the steel can be hardened. The investigation is primarily done using two techniques.

One is called the spark test and is based on the fact that different ferrous materials produce different spark patterns when sharpened. Identifying general steel types requires experience and/or research. We won't go into that in this article, but the main point is that high carbon will produce more sparks (longer and more branched) than low carbon. Low carbon, also called "mild steel", cannot be hardened into knife blades.

The second technique is to heat the steel until it is no longer magnetic, and cool in rapeseed oil. If it is not well hardened, you can heat the steel again and cool in water. Some steels will also harden from being cooled in air. This way you can find out how the steel hardens, and further testing with temperatures and holding times can give even better results.

Hardening of steel
Steel hardening is practically a world of unlimited possibilities, but roughly speaking it involves heating the steel above a given temperature for the steel in question where the steel changes structure, and then quenching the steel below a given temperature which causes the steel's crystals to settle down in martensitic form. This gives the steel its hardness. Then you will sacrifice some hardness in exchange for a stronger/tougher material by tempering (baking at a lower heat e.g. 200C), as in most cases the steel would be very brittle and unsuitable for use in a knife.

Example of hardening processes:
Carbon steel is often the easiest to harden, with the lightest unalloyed and low-alloyed steels it will often give a good result to heat the steel until it is no longer magnetic, and then cool quickly with an oil. Old motor oil works, but rapeseed oil has good properties such as high ignition temperature, less toxic gases, smoke and odor, plus it didn't cost more than 20kr per liter last time I checked! Remember to keep the oil in a vessel that can withstand heat. The oil can get so hot that plastic buckets etc. can be deformed and/or melted.

Below are examples for hardening knife steel. These are not necessarily ideal recipes, but will give an insight into how the different hardening processes behave. The hardening processes can vary significantly with temperature and holding time, and one should therefore investigate the ideal hardening for the steel in question and the knife's area of use.

15-steel and other unalloyed:

1. 810 degrees (non-magnetic)
2. Cool in water for higher hardness (and higher risk of cracking) or rapeseed oil for lower hardness (and lower risk).
3. Tempering 200C (oven) for 2 hours
4. Cool to room temperature in air.
5. Tempering 200C (oven) for 2 hours

80crv2 (low alloy)

1. 850C - the steel has changed structure and is held for 5-15 min to ensure even distribution of the alloying elements
2. Refrigerated in rapeseed oil
3. Tempering 200C (oven) for 2 hours
4. Cool to room temperature in air.
5. Tempering 200C (oven) for 2 hours

AEB-L (stainless)

1. Packed in steel heat-sealed foil to prevent oxygen ingress
2. Heat to 1040C (possibly 1080C*)
3. Cool between 2 thick aluminum plates (min. 20mm)
4. (* at higher temperatures, deep cooling should be performed to prevent retained austenite, done using a dry ice alcohol bath or liquid nitrogen)
5. Tempering 200C (oven) for 2 hours
6. Cool to room temperature in air.
7. Tempering 200C (oven) for 2 hours

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