Construction Types

Various zones of a blade meet different mechanical demands. It is advisable to select the steels for the different functional areas according to their specific properties and construct a blade in this way. With an adequate construction, the mechanical loading capacity can be significantly increased. The cutting performance can also be increased by greater hardness of the cutting edge or layer, relative to the rest of the blade.

Ideally, compressive stress is present in the cutting edge. Compulsorily this leads to tensile stress in the sides or back of the blade. Compressive stress inhibits crack propagation completely. Such compressive stress is created by the larger volume of martensite relative to perlite/ferrite. Combining steels of different hardening characteristics will generate such stress; another possibility is differentially hardening the blade.

Techniques to create compression stress in the cutting edge are commonly used in Scandinavian knives as well as in Japanese swords. Many historical blades show a differential construction; surface hardening was most likely a common practice. Due to the poor condition of archaeological findings, it is often difficult to impossible to identify this technique. Such composite blades can be made from Damascene- as well as from mono-steels. In this context it becomes obvious that pattern-welded steels are an ornamental element in such blades. Only the technique of micro-interlocked zones requires a center of multi-layered steel.

Layer Constructions

One of the simplest methods to create an improved mechanical behavior is the layered construction. The blade is composed of different, flat layers of steel, typically two or three. By welding the layers together the properties of the different steels can be used more purposefully. The layer forming the cutting edge should be made from steel that adopts hardness very easily. The adjacent layers stabilize the blade, hence they should be made of a spring-steel.

Two Layer Construction

Japanese kitchenknives are a well-known example of blades consisting of 2 layers. They normally consist of steel higher in carbon, holding the cutting edge, and a stabilization layer made from mild steel. The latter is most commonly made from a random pattern non-hardening Damascene steel and is normally thicker than the cutting layer.

Due to the asymmetric construction there are blades made for right- and left-handed users. The cutting edge is always on the side facing the free hand. The layer made of hardening steel is ground flat from the back of the blade till the cutting edge.

Two-layered blades warp during hardening due to the volume-change while forming martensite in the cutting layer. The stabilizing layer produces compressive stress in the cutting layer by counteracting the deformation. To have a straight blade after hardening, the blade can be curved before heat treatment. But in most cases it will still be necessary to straighten the blade after hardening. To do so, small dents are hammered into the surface of the stabilizing layer, provoking an expansion of the material close to the surface, causing the blade to straighten. The straightening has to be done before tempering the blade. Minor curvature can be straightened by manually bending the blade and heating to about 240F. This has to be done immediately after hardening. When the blade is straightened, all correction-marks are removed by grinding and finishing the surfaces. The thinner the cutting layer is, the less warpage will occur during hardening; yet the compressive stress will be higher.

Three Layer Construction

A symmetrical blade composed of three layers will warp during hardening minimally, if at all. With relatively little welding effort, a high-quality blade can be produced. Flux can easily escape from the welds due to the free edges on all sides.

Norwegian-Style Blade

This construction is similar to the three layered blades. The outer layers are made from one piece by bending mild to non-hardening steel to a U-shape. The steel forming the cutting edge is then inserted in this U. First, the pieces must be welded in the bend of the U. Next the sides are welded. The flux can only be pressed out of the weld in one direction, creating the risk of inclusion. These blades have a soft back and therefore can be driven by a hard object without being destroyed. The compressive stresses in the cutting edge can be raised against the simple three layered technique, but only slightly.

Since martensite corrodes easier than ferrite or perlite the three layered blades will need less care than fully hardened mono- or Damascene blades of comparable materials.

Attached Cutting-Edge-Strip

Another effective method to produce compressive stress in the cutting edge is welding a band of hardening steel to a blade-core. This strip can be fitted to the blade by a straight weld or by an interlocking joint. The interlocking geometry can be produced by filing or stamping, even by twisting the edge-strip. A technique developed by Ulrich Gerfin generates a micro-interlock.

Differential Hardening

By differential hardening of a blade, compressive stress is created in a similar way to attaching a strip of easily hardening steel. Blades of Japanese swords are be made from a mantle-piece with inserted iron core or other differential constructions, like an attached cutting-strip, normally in combination with stabilizing layers on the sides. Independent of their construction, Japanese swords are hardened differentially, so that a hamon (hardening line) is created. Often the hamon does not form a straight line–wave- or tooth-like shapes are believed to be less sensitive to crack propagation. A form of macro-interlocking is created this way.

The possibilities caused by differential construction and hardening of a blade are commonly neglected or ignored, even if these techniques provide an effective method to increase the performance and to use steels according to their special properties.

© 2005 G.v.Tardy