Cutting Technology for Knife Blade Metals
An article was recently published by the World Steel Organisation about a master Japanese blacksmith who said: “The quality of a Japanese knife lies in its hardness and strength. Ideally, it’s unbreakable, unbendable, and unchippable, but these qualities conflict with one another.”
The article talks about how the secret to knife success involves the master-quality craftsmanship and reverence for perfection in even the smallest of details, including the microstructure of the metal itself. It was explained that different metals, including steel, have different properties and uses, and craftsmen apply their expertise to bring out the best in their chosen source material.
For blade crafting, one Japanese knife company uses locally-sourced high-carbon steel that has been specifically formulated for making sharp blades. The blacksmiths there believe the ability of steel to adapt to multiple use cases and its ability to be optimised for the qualities needed, such as rust-resistance, is fascinating. He also believes that there are scientific reasons why a super sharp Japanese blade makes a difference when chefs prepare foods, especially sushi.
So how can that combination of hardness, strength, and rust-resistance needed for the perfect Japanese knife be attained? It’s all about the alloying – the combining of two or more metallic elements to enhance metal properties – and working the metal properly.
A previous post explained about the difference between metal strength and stiffness. A material can have high strength and low stiffness. If a metal cracks easily, it has low strength, but if it has low stiffness, it can deflect a high load. All steel has approximately the same stiffness, but comes in many different strengths depending on the alloying metals used. Stainless steel comes in more than 100 grades which are created by adding alloys such as chromium, silicon, nickel, carbon, nitrogen, and manganese to impart properties such as heat resistance, strength, flexibility, and ductility.
Also, a metal’s properties such as weldability and heat or corrosion resistance depend on the carbon content in steel. Carbon works as a hardening agent. Increasing carbon content increases hardness and strength, but increases brittleness as well.
Of course one must get the alloy recipe right, mixing together the precise amount of each element. If not, the quality will suffer, and the product can fail. A steel blade that breaks during use can possibly hurt the user if it snaps off and cuts someone. And a blade that rusts would certainly tarnish a manufacturer’s reputation.
Looking at a collection there’s a representation of the most common knife steels, and it’s noted the typical C% for reference:
440C – 0.95-1.20%
1095 – 0.95%
154CM ‒ 1.05%
420 ‒ 0.15% min
N690 ‒ 1.0%
AUS-8 ‒ 0.75%
To help ensure quality and product integrity, XRF handheld analysers are used to confirm the elements in the metal. X-ray fluorescence (XRF) is a proven technology for the elemental analysis of speciality alloys to ensure the correct alloys are combined in the right percentages and the finished material meets precise manufacturing specifications. This is critical for quality control and quality assurance (QA/QC) of incoming materials and outgoing finished goods.
There is one caveat though. An XRF analyser with light element feature is a great tool for measuring all alloying elements with exception of carbon. But carbon is what drives hardenability and ultimately edge retention and ease of sharpening so for exacting chemistry or verifying a knife blade prior to shaping, LIBS would be the best option.
Laser Induced Breakdown Spectroscopy (LIBS) is the analytical technique using a high-focused laser to determine the chemical composition of materials. The technique is available in a portable handheld analyser and is capable of measuring elements, including carbon, in the field for material identification. However, since the LIBS technique utilises a high-focused laser that interacts with the surface of a material, it forms a plasma in which the material is broken down into single elements. A laser pulse is produced by the analyser and pointed at the sample surface. The surface is ablated and enters the plasma. Most, if not all blade manufacturers, would want to avoid the sample prep and arc strike on a finished blade. So if carbon is important to the finished product, LIBS should be used prior to forming the blade itself.
In the article, the blacksmith said that it is important to constantly make adjustments during the forging process of the knife. “Knowing how to bring these qualities out of the steel and adjust them at a fine level is something that can only be gained through first-hand experience.”
But before getting to the forging stage, it’s important that they are working with the right materials. And according to the article, the quality of your sushi could depend upon it.