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The key to understanding edge steel is in its anatomy. For the needs of the woodworker, three characteristics define steel’s anatomy—grain, structure, and hardness.

GRAIN

For woodworking hand tools, the grain of the steel is the most important characteristic of a blade. Ordered, repetitive arrangements of iron and alloy atoms in a crystalline structure comprise steel. The crystals can be small and fine or large and coarse. They can be consistent in size (evenly grained) or vary widely, with odd shapes and outsized clusters in among the rest. The steel’s grain affects how finely the blade sharpens and how quickly it dulls. Generally, the finer and more consistent the grain, the more finely it sharpens, the slower it dulls, and the better the blade performs.

Grain is a function of the initial quality of the steel used, the alloys added, and how the steel is worked or formed. In addition to the average size of the crystals, the initial quality of the steel may include impurities, called inclusions, which may persist throughout refining. Inclusions add large irregularities to the grain. Irregularities sometimes are used to good effect in swords and perhaps axes, but except for the backing steel on laminated blades, impurities are a detriment to a plane blade. Sharpening impurities out to the edge causes them to break off easily, causing chipping and rapid dulling of the edge.

Alloys change the texture of the grain. They may be part of the steel’s original composition (though usually in small amounts), or added in a recipe to increase the steel’s resistance to shock and heat. Alloys often coarsen the grain, so there is a trade off. While the edge of an alloy blade may be more durable, especially under adverse working conditions, it may not sharpen as finely as an unalloyed blade. To shear wood cleanly, no other attribute of an edge is more important than fineness of the edge.

STRUCTURE

Structure, the second most important aspect of a woodworking blade, results from changes in the original composition of the steel because of heating it and changing its shape with a hammer (or rollers), often called hot work. Heat causes the crystals of the steel to grow. Hammering steel when it is hot causes its crystalline structures to fracture and impedes growth as the grains fracture into smaller crystals. Before being hot-worked, the crystals of steel are randomly oriented, and frequently inconsistent in size. Through forging (repeatedly re-shaping with a hammer while the steel is hot), the grain aligns and knits together in the direction of the metal flow. Proper forging increases grain structure consistency. When exposed at the edge through sharpening, crystals consistent in size and orientation break off one at a time and dull the blade, rather than breaking off randomly in big clumps. The consistency of the crystals allows for a sharper blade that stays sharp longer.

The techniques used in preparing steel for woodworking tools are hammer forging, drop forging, and no forging. Hammer forging, where repeated hammer blows shape the steel, is the most desirable because it aligns the grain particles (or crystals) of the steel. It is a time-consuming, skillful process and therefore expensive. If improperly done, hammer forging stresses the steel, reducing, rather than increasing reliability. With the general decline in hand-woodworking skills during the last century, and the increased reliance on power tools, the discriminating market that would appreciate the difference forging makes has shrunk considerably. As a result, hand-forged tools are not commonly manufactured or available in the United States, but are still the prevalent method of producing plane blades, chisels, and other edge tools in Japan.

Drop forging verges on die cutting. A large mechanized hammer called the punch drops on the heated blank, smashing it into a die (mold), giving the tool blade its rough shape, often in just one blow. Drop forging imparts a marginally more consistent structure than a blade cut or ground from stock, because the steel often elongates in the process resulting in some improvement in the crystalline structure alignment. Drop forging is a common way of producing chisels in the West.

Drop forging is preferable to no forging at all. No forging is an over-simplification because all tool steel receives some hot work during reshaping. Bar stock is hot-formed by rolling or extruding the ingot into lengths of consistent cross section. The process rearranges the crystalline structure and the crystals tend to align in the direction of the flow as the steel lengthens. However, the arrangement is not very refined compared with the structure resulting when steel is hot-worked further at the forge. Modern Western plane blades, even many after-market premium blades, are usually ground from unworked, rolled stock.

HARDNESS

Hardness is a major selling point in the advertising of woodworking tools made from various types of steel. However, as explained earlier, grain and structure are the most important factors in the performance of a blade.

Hardness must be in balance with the intended use of the tool. High-impact hand tools, such as axes, should be softer than plane blades. Otherwise, the edge fractures quickly under the pounding an ax takes. The blades of fine tools for fine work can be very hard, but if their hardness exceeds the ability of the steel to flex without breaking at the microscopic edge, the tool will be next to worthless. It is the task of the bladesmith to produce blades that are in balance.

The Rockwell C (Rc) scale measures the hardness of woodworking blades. This is a unit of measurement determined by the impact of a ball-shaped point into the steel measured in terms of the depth of the resulting impression. Decent plane blades are in the range of 60–66 Rc range. Only some finely wrought steels work effectively in the upper-half of this range, principally high-quality hand-forged Japanese blades, and some high-alloy steels. In carbon steels, Rc 66 seems to be a limit above which the edge breaks down too rapidly in use, though I have heard of a Japanese master blacksmith who has made it a personal goal to develop steel that will hold an edge at about Rc 68. However, his experiments have not been available commercially.

The tungsten makes the blue steel harder to forge but increases its wear resistance when cutting difficult woods. On the other hand, adding tungsten widens the critical temperature range needed for hardening the steel, and makes this step a little easier for the blacksmith. In contrast, some white steels are fussy about their hardening temperatures. White steel is easier to sharpen, and takes the keen edge necessary for soft woods.

The difference between white and blue steel is subtle. A Japanese woodworker I know makes an enlightened distinction between the two. He describes white steel as having a sharp, angular grain structure, and blue steel as having smaller, rounded grains. This allows the white steel to be sharpened a nuance sharper, but under harsh conditions or with difficult woods white steel’s grain structure breaks off a little quicker and in slightly larger clumps. For that reason, dealers often recommend blue steel for working hard, abrasive, or difficult tropical woods.

Both white and blue steel are too hard to use for the whole blade; it is too susceptible to shock and prone to cracking during use (it is also too expensive). It would also be too difficult to sharpen a blade made entirely of such hard steel. Instead, a thin layer of it is forge-welded (laminated) to a back of softer steel that has more tensile strength. The combination is better able to absorb shock without breaking.

The backing steel used for these blades is a low-carbon, softer, and more flexible steel.

Figure 1-3. Fine Japanese Blade and Chipbreaker by Blacksmith Miyamoto Masao. If you look closely at the edge of the chipbreaker, you can see the color difference that distinguishes the edge steel from the backing steel.

It is basically wrought iron with impurities: before the mid-nineteenth century, smelting techniques allowed the inclusion of impurities, which in the grain structure appear as strands, somewhat similar to the glass in fiberglass. The impurity increases the steel’s flexibility and resistance to breaking—both desirable qualities. However, because the smelting process improved after the 1850s, steel produced since then lacks these impurities. Scrap iron produced earlier is highly coveted by Japanese blacksmiths who stockpile these treasures, such as pre-1850 anchor chain, for future use as backing steel in laminated blades.