The Blade - Smyth Knives
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Steel

The blade of a knife can be made from a variety of materials, the most common being carbon steel, stainless steel, tool steel and alloy steel. Other less common materials used in knife blades include: cobalt and titanium alloys, ceramics, obsidian, and plastic. The hardness of steel is usually stated as a number on the Rockwell C scale (HRC). The Rockwell scale is a hardness scale based on the resistance to indentation of a material, as opposed to other scales such as the Mohs scale (scratch resistance) testing used in mineralogy. As hardness increases, the blade becomes capable of taking and holding a better edge, but is more difficult to sharpen and more brittle (commonly called less “tough”). Laminating a harder steel between a softer one is an expensive process that to some extent gives the benefits of both types

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Carbon Steel

Carbon steel is a popular choice for rough use knives. Carbon steel tends to be much tougher and much more durable, and easier to sharpen than stainless steel. They lack the chromium content of stainless steel, making them susceptible to corrosion.
Carbon steels have less carbon than typical stainless steels do, but it is the main alloy element. They are more homogeneous than stainless and other high alloy steels, having carbide only in very small inclusions in the iron. The bulk material is harder than stainless, allowing them to hold a sharper and more acute edge without bending over in contact with hard materials. But they dull by abrasion quicker because they lack hard inclusions to take the friction. This also makes them quicker to sharpen. Carbon steel is well known to take a sharper edge than stainless

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Tool Steel

O-1 TOOL STEEL, oil hardening, general purpose tool steel is perfect for knife blades due to the excellent abrasion resistance, toughness and machinability characteristics. Typical chemistry C .95, Mn 1.20, Si .30, CR .50, Va.20, Mo .50.

440-C is high-carbon chromium stainless that resists corrosion from fresh water, steam, crude oil, gasoline, stains from food acids and fruit. The excellent wear and resistant qualities of 440-C steel make it the natural choice of knife makers. Hot rolled and annealed. Typical chemistry C 1.00, Mn .45, Si .30, CR 17.00, Mo .50.

A-2 TOOL STEEL

A fine grain, electric furnace air-hardening, 5% chrome oversize precision ground tool steel. It is a superior quality steel which has excellent wear and abrasion resistance properties. Typical chemistry C 1.00, Mn .60, Si .30, CR 5.20, Va .30, Mo 1.10

Stainless steel is a popular class of material for knife blades because it resists corrosion and is easy to maintain. However, it is not impervious to corrosion or rust. In order for a steel to be considered stainless it must have a chromium content of at least 13%

The principle of stainless steel is that in an oxidizing chemical environment the oxide (chromium and sometimes nickel and other metal oxides) is stable, and when in a reducing (shortage of oxygen) environment at least one metal is stable. This usually works, except in an acid environment. In order to be hardenable, knife steel can contain limited chromium and very little nickel. So, even though stainless, hard knife steel has limited resistance to corrosion.

Austenitic stainless retains its non-magnetic crystal structure at room temperature, usually because it has high nickel content. It is therefore not hardenable by heat treating as typical hard steels are. So as knife steel it depends on other hardening methods such as alloying elements and cold working. It is highly corrosion resistant

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Heat Treatment of Steel

Many changes occur when steel is subjected to heat. There are different heat treatment processes which are listed below:

Normalising

Heating to a suitable temperature, between 800-900 degrees Celsius, according to analysis, holding at temperature followed by cooling in still air. Relieves internal stresses, refines the grain size and improves mechanical properties.

Annealing

Heating and holding at a suitable temperature and cooling slowly in the furnace with the object of softening the steel, improving machinability and cold working properties.

Stress Relieving

Frequently carried out after rough machining or cold work to remove stresses. It is usually carried out at a temperature range of 600-650 degrees Celsius.

Hardening

Heating to a temperature slightly above the critical range, soaking for sufficient time at that temperature followed by quenching in a suitable medium such as water, oil, or air.

Tempering

Carried out immediately after hardening to relieve stresses remove brittleness and reduce hardness to the required range. Usually carried out between 150 – 650 degrees Celsius. Cool in still air or quench.

Nitrating

A process for producing a very hard case by the absorption of nitrogen into the surface of the steel. Depending on the specification hardness figures up to 1100 VPN can be attained.

Carburising

The diffusion of carbon into the surface of a steel that is low in carbon by heating in a solid, liquid, or gaseous medium, containing carbon at a temperature around 900 degrees Celsius.

Induction Hardening

A surface hardening process where a component is heated by high frequency induction followed by immediate quenching. The surface hardness will depend on the carbon content of the steel. For ideal results this is usually in the range 0.40%-0.45%C.