Properties of knife steel

Steel Properties

When it comes to kitchen knives, the key properties of hardened steel are hardness, abrasion resistance, toughness, and corrosion resistance. However, these qualities don’t always work together. In fact, hardness and abrasion resistance are often at odds with toughness and corrosion resistance.

As steel becomes harder, it also becomes more brittle and more susceptible to corrosion. On the flip side, softer steels with lower carbon content and higher chromium levels can be highly resistant to rust, but they struggle to maintain a sharp edge over time.

The art of metallurgy lies in finding the perfect balance between these opposing factors, ensuring your knife performs at its best in every situation.

Hardness

Hardness refers to steel’s ability to resist permanent distortion at the microscopic level. In knife steel, this is a highly desirable trait because harder steel holds its edge longer than softer steel.

When cutting, a sharp edge (see Image A) is subjected to wear through abrasion (contact with food and cutting boards) and deformation (pressure against the board). Abrasion causes the edge to round off (see Image B), while deformation causes it to roll (see Image C). Think of it like this: if you push a piece of paper and a metal sheet of the same size onto a table, the paper will bend, but the metal will stay rigid.

Harder steel resists both abrasion and deformation, preserving the knife’s sharp edge (Image A) and allowing it to stay sharper, longer. This is why hardness directly impacts edge retention.

Harder steel can also be sharpened to a more acute angle, reducing the effort needed to cut through food. The sharper the angle, the easier the blade glides through ingredients—this is what we perceive as sharpness. Even if the edge is slightly worn, a narrow angle allows the knife to continue cutting effectively, greatly enhancing both sharpness and edge retention.

However, the trade-off is that harder materials are also more brittle, making them more prone to chipping or cracking under stress.

Graphics of cutting edges sharp edge rounded edge bent edge

Abrasion resistance

High abrasion resistance is crucial for maintaining a knife’s sharp edge. Most of the wear that dulls a knife in everyday kitchen use comes from the friction between the blade, food, and the cutting board. The greater the abrasion resistance, the less wear occurs—meaning your knife stays sharp for longer.

While hardness plays a significant role in abrasion resistance, there’s more to the story. Beyond hardness, the material composition, the types of carbides formed, and their size and volume all impact how well a knife resists wear.

Elements like vanadium and tungsten form extremely hard, tough carbides that enhance abrasion resistance, even at the same hardness level. Similarly, a higher volume of carbides in the steel also boosts resistance. The harder and more abundant the carbides, the longer it takes to wear them down.

While the benefits of high abrasion resistance are clear in terms of edge retention, there’s a trade-off. A knife with higher abrasion resistance is more difficult to sharpen, which makes sense—after all, sharpening works by abrading the steel.

Toughness

Toughness, or ductility, refers to a knife’s ability to withstand sudden impacts and forces that could otherwise cause cracks or chips. High toughness is essential in everyday kitchen use, especially when cutting through harder objects. However, toughness and hardness often work in opposition—improving toughness can lower hardness and vice versa. For example, tempering at higher temperatures increases toughness but reduces hardness.

The goal for any knife maker is to achieve the perfect balance: maximizing toughness without sacrificing too much hardness, or reaching optimal hardness while maintaining enough toughness. This balance is influenced by both the material chosen and the heat treatment process.

How tough a knife needs to be depends on its intended purpose. A Yanagiba knife for slicing sushi needs exceptional sharpness, but doesn’t require much toughness. On the other hand, a cleaver, designed for cutting through small bones, must prioritize toughness over sharpness.

Corrosion resistance

Corrosion resistance is a knife's ability to withstand contact with water and acids without undergoing a chemical reaction. Steel, which is primarily iron with added carbon, is naturally prone to corrosion. When cutting, your blade is exposed to moisture and acids, which can cause oxidation, leading to unsightly stains or, worse, pitting. The secret to preventing corrosion lies in the elements added to the steel. Generally, higher carbon content makes the steel more prone to corrosion, while higher chromium content reduces it. In modern knife steel, corrosion resistance is primarily achieved by adding chromium

It’s commonly stated that steel with over 11% or 12% chromium is considered "stainless," but that’s just a rule of thumb. When chromium is added to steel, it creates a passive layer of chromium oxide that protects against corrosion. A minimum amount of chromium is needed to form this layer, and increasing the chromium content beyond this strengthens the protective barrier. Elements like Molybdenum or Nitrogen don’t create a passive film on their own, but adding them to the steel helps to strengthen the existing Chromium oxide passive layer.

But there is one more thing to it – the amount of Carbon in the steel. With increasing Carbon content, Chromium in knife steel is tied up in Chromium carbides formed during hardening, so that only a certain share of Chromium goes into solution and forms the oxide layer. E.g. steel with 12% Cr, but 1.5% C, in a hardened state doesn’t have enough Cr in solution to be called stainless. Subsequently, steel with a larger amount of Carbon requires more than 12% Chromium to have a reasonable corrosion resistance.

On the market, there are knives with varying degrees of corrosion resistance:

pure Carbon steel, with hardly any Chromium
“semi stainless” knives, with less than 11% Chromium in solution
truly stainless knives, with more than 11% Chromium in solution.

In daily kitchen use, a knife's corrosion resistance determines how long it takes before stains or pitting appear, especially if the knife is left wet or uncleaned—like when it's left in the sink or, worse, put in the dishwasher. Many so-called "semi-stainless" knives won’t show signs of corrosion with normal use but will if subjected to harsh conditions like dishwashing.

Ultimately, the level of corrosion resistance you need depends on how you use and care for your knives. If you’re diligent about wiping down your blade immediately after use, a high-performance carbon steel knife could be a great fit. But if your knives tend to end up in the dishwasher, it’s best to opt for a knife made from softer, low-carbon steel with high chromium content to avoid corrosion.