A Comprehensive Guide to Japanese Knife Steels

A Comprehensive Guide to Japanese Knife Steels

                                                                                                                                                                                                                                    

TLDR:

  • Rust resistance (RR) - Higher RR, knife is less prone to rusting when exposed to moisture and salt.
  • Toughness (TN) - Higher TN, knife doesn’t chip as easily when damaged.
  • Edge Retention (ER) - Higher ER, knife stays sharp for longer when used.
  • Ease of Sharpening (EOS) - Higher EOS, knife easily takes a sharp edge, and takes less time to sharpen.
In general:
  • Higher RR - Lower ER, TN and EOS
  • Higher TN - Lower ER
  • Higher ER - Lower RR, TN and EOS
  • Higher EOS - Lower ER

 

Recommended Stainless Steels (& Our Ratings):

  Rust Resistance Toughness Edge Retention Ease of Sharpening
SG2 8/10 4/10 9/10 3/10
VG10 / AUS10 7/10 4/10 7/10 8/10
AUS8 7/10 6/10 6/10 9/10

                                                                                                                                                                                                                                    

 

There are many factors that go into selecting a knife steel that is ideal for you. Unfortunately, there isn't a single "best" knife steel. Instead, it comes down to understanding the different properties of each steel, prioritizing what is most important to you and figuring out what trade-offs you're comfortable making.

Prepare yourselves, we've got a lot to share and it's going to be a lengthy post - so we're diving right into it.
 
In no particular order, here are the 4 key factors you should know:


1. Rust Resistance / Stain Resistance

A steel that is very rust resistant will not rust at all when exposed to moisture, air, salt and acidity. A rust resistant knife will be easy to care for. Knife steels can broadly be categorized into two groups - stainless steel and carbon steel. Stainless steels contain more than 13% chromium and can exhibit varying levels of rust resistance. Carbon steels on the other hand contain less than 5% chromium, and rust easily if not kept away from moisture.

While it may seem purely beneficial to choose a knife steel that has the highest rust resistance possible, this is not without its drawbacks. A high level of rust resistance usually requires a high composition of chromium in the steel.

The reasons are complicated, but a high composition of chromium might make the knife steel:

  • Have a lower edge retention
  • Difficult to sharpen
  • Brittle


Basically, having an extremely high rust resistance comes at the cost of every other aspect that makes a knife steel good, so choose wisely. A good stainless steel for knives would usually not be immune to rusting, so don’t leave them in a wet sink overnight. However, it should be stainless enough to be left wet for an hour or so without causing a fuss.

 

2. Toughness / Brittleness

The opposite of a brittle steel is a tough steel. When damaged, brittle steels tend to chip, while tough steels tend to dent. All things being equal, a tougher steel would be desirable: as a dent would be easier to fix compared to a chip.
 
When we are looking at the structure of the steel, you can think of the steel like a concrete mix. Concrete is made up of cement which is the binder, sand which are the fine aggregates, and gravel which are the coarse aggregates. A concrete mix with different ratios of binders and aggregates will exhibit different properties. And the size of the aggregates may impact how strong or weak a concrete mix will be.
 
In the case of steel, the binder is iron and the aggregates are the carbides which are formed between carbon and other alloys present in the steel. Similar to concrete, adjusting the mix of iron to carbon to alloys will influence the properties of the steel greatly. Whether the carbides are fine or coarse depend on the size of the carbide crystals, which comes down to how the steel is heated and cooled in the heat treatment process, as well as the process of making the steel.
 
One key drawback of having large carbide crystals in the steel is that it creates opportunity for fault lines to develop within the steel structure. Think about a concrete mix with only gravel inside, cracks will easily develop between the large pieces of gravel in this concrete mix. This will eventually lead to major structural failure in the concrete mix. Similarly, cracks would more easily develop in a steel with a larger carbide structure and these cracks eventually lead to chips or even worse, a hairline crack down the width of the blade.
 
In short, to avoid these issues we want a steel to have a fine grain structure with small carbides and there are generally two ways to achieve this:


Low Alloy, Low Carbide Steel

First, is a low alloy and hence low carbide steel. In other words a simple steel with iron, carbon and little else in the way of alloys. These steels will have a very regular structure with a low density of small carbide crystals. When under stress, a fault line is unlikely to form in the fine grain structure of the steel. Hence, it is less likely to chip. An example is the White #1 (Shirogami #1) carbon steel, an extremely simple and pure steel with a mix of iron and carbon with only a small amount of other alloys to improve steel hardness. Even when heat treated to a relatively high HRC 62-63 hardness, the steel retains a fairly high toughness.


Heat Treatment 

Second, is appropriate heat treatment. The size of carbides can be minimized by heating and cooling the steel in a specific way that reduces the size of the carbide crystals that form. This specific heat treatment process would vary from steel to steel, but from experimental data we have seen, generally heat treating to a higher HRC hardness tends to reduce toughness.


With everything we have mentioned so far, there are still trade offs to choosing a higher toughness steel. For low alloy, simple steels, they generally suffer worse edge retention due to their low carbide concentration. For a lower HRC heat treatment, edge retention is also lowered as the steel is softer and deforms (rolls) more easily.

 

3. Edge Retention

A knife that stays sharp even after it has been used for a long time is said to have a high edge retention. The knife steel can affect edge retention greatly. (There are other factors that affect edge retention, we will discuss those another time.)
 
With regard to the steel, there are three factors that affect edge retention:
  1. Hardness (measured with a HRC number)
  2. Carbide Hardness
  3. Carbide Concentration

 

Hardness

Steel hardness is determined by two things: the composition of the steel and the heat treatment. Generally, steels with a higher carbon content have a greater potential to achieve a higher hardness with the appropriate heat treatment. Heat treatment is the process of heating and cooling steel in multiple stages so that the molecular structure within the steel arranges themselves in a desirable way. Different heat treatment temperatures can give different results in terms of hardness, brittleness and rust resistance. Generally, higher hardness gives the steel a better edge retention, but makes it more brittle and less rust resistant (as usual, there are trade offs).


Carbide Hardness & Concentration

Carbides are crystalline compounds formed in the steel when metals interact with carbon. They are extremely hard compounds and very abrasion resistant, which contribute greatly to a steel’s edge retention. Common carbides include:

  • Iron Carbide (Cementite): approx. HRC 70
  • Chromium Carbide (Cr23C6): approx. HRC 77
  • Chromium Carbide (Cr7C3): approx. HRC 83
  • Tungsten Carbide (WC): approx. HRC 84
  • Vanadium Carbide (VC): approx. HRC 89


It stands to reason, given the hardness of these carbides, that a steel with a higher concentration of high hardness carbides will have a better edge retention.

An interesting thing to note is that the HRC hardness of the steel and its carbide concentration are not correlated. For example, a simple low-carbide carbon steel like White #1 can achieve a high HRC of 65, but its edge retention is less than that of a softer SG2 steel with a HRC of 62. This is due to the high carbide hardness and concentration in SG2. So HRC is simply one indicator of a steel’s properties and cannot be used to fully assess the edge retention of the steel.

While having a higher edge retention is generally good, it comes with some drawbacks too:

  • If a steel is hardened to a higher HRC hardness, it can become brittle and less rust resistant.
  • If a steel has extremely high hardness carbides, it can become difficult to sharpen.
  • If a steel has a high concentration of carbides, it can become both brittle and difficult to sharpen.

 

4. Ease of Sharpening

This is something a lot of beginners don’t consider but it can absolutely ruin the experience of owning a knife if the steel is difficult to sharpen. All knives go dull with enough use, and eventually you’ll need to bring your knife to a sharpening stone and get that sharp edge back again. Some steels sharpen up extremely easily, some require special stones, and others simply refuse to sharpen up despite a good sharpening setup and technique.
 
Here are a two factors that affect the ease of sharpening:


Carbide Concentration

Having a high carbide concentration makes a steel extremely difficult to sharpen, especially if you have a lower end sharpening stone. The carbides themselves are extremely abrasion resistant, which is great for edge retention, but makes it a pain in the butt to sharpen. It takes a good stone, consistent angles and a significantly amount of time to sharpen a high carbide steel. But the pay off is that you don’t have to sharpen as often due to its great edge retention.

 
Grain Structure

A steel with a coarse grain structure (large carbide crystals) can make sharpening a lot tougher. This is because the large carbide crystals can dislodge or even chip off during the sharpening process, creating microscopic chips on the blade’s edge. Some steels suffer from this so badly that it is almost impossible to create a consistent edge on the blade that is sharp enough to push cut paper.

 
Notably, the hardness of the steel doesn’t affect the difficulty of sharpening that much. Even high hardness, low carbide steels can be easily sharpened. Hence why carbon steels like White #1 (that can achieve extremely high HRC of 65) are still famously considered one of the easiest steels to sharpen. On the other hand, low hardness, high carbide (chromium carbide) stainless steels like you find in most super market knives can be downright impossible to sharpen properly.

 

4 Notable Stainless Steels with a Good Balance of Attributes:

SG2

SG2 is made by the Japanese company Takefu. It is a direct successor to the VG10 steel, however the SG2 steel is made using a process known as powdered metallurgy (PM). The PM process uses micronized powders of iron, carbon and other alloys and binds them together under great pressure and controlled heat to create an extremely fine grained steel. This process allows a much higher carbide content to be produced while still retaining a fine grain structure which improves toughness. The resulting SG2 steel has four times the edge retention of VG10, while maintaining the same toughness and rust resistance. The only downside is that due to the carbide content, it can be quite challenging to sharpen without the right tools.


VG10 / AUS10

VG10 and AUS10 are made by the Japanese company Takefu and Aichi respectively. By composition, they are almost the same with very slightly different ratios of alloys in each steel. For this recommendation, we will treat them as identical steels and refer to them as VG10 (since it’s more popular) because in practice they are not really distinguishable.

The great thing about VG10 is that it balances a lot of great properties without sacrificing too much. The edge retention is great, it is rust resistant enough to be considered a stainless steel and is actually surprisingly easy to sharpen. The only downside is that if the heat treatment is not done correctly, it can be prone to chipping. But since this steel is so popular in Japanese knife making, most manufacturers get it right and chipping really shouldn’t be much of a concern.


AUS8

AUS8 is a Japanese steel made by Aichi. Like VG10, it’s a great balance of properties, but skews more towards a higher toughness and lower edge retention. It is also extremely easy to sharpen, making it a great steel for those who want easy maintenance but good performance.

 

You may have noticed that our recommendations purely consist of Stainless Steels. That’s not to say that Carbon Steels have no place - they provide a cutting experience like no other and sharpen easily too. The major downside is its maintenance and ease of rusting, which for the most part, is a deal breaker for many. We have thus kept our recommendations fairly generic and suitable for the majority of you.

 

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