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Swords, Cutting and Military History

Razor Edged 2: Steel hardness, structures and sharpness

Razor Edged 2:  Steel hardness, structures and sharpness

photo by Karstenphotos

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Another important determiner of blade sharpness is the quality of steel the blade is made of, as well as the hardness of the edge steel.  While going into full detail on steel production and sword smithing goes beyond the limits of this article, a quick review of steel elements which have an effect on edge sharpness is in order.   Any sword smiths or metallurgists reading this, note this article is supplying only very general background information, understandably imprecise in some area.

Speaking in a very general sense then, steel consists of refined iron — iron in which the majority of impurities have been removed — mixed with a variety of other metallic and organic elements;  these include carbon, nickle, and tungsten among many others.  These elements, when properly combined through the application of heat and/or forging, are spread evenly throughout the steel, undergo chemical reactions, bond together and transform to create crystalline-like structures.  The type of micro-crystalline structures the smith can create, and their density within the blade, determine the quality and hardness of the steel.

blacksmith_minThe art of smithing, then, partly depends on the smith’s ability to purify the iron/steel of chemical inclusions which might weaken it, to ensure an even distribution of the added elements throughout the steel, and to control heat and forging so the desired structures are created.   Depending on the amount and mix of elements, the steel being produced will have certain qualities of hardness;  carbon tends to be a reactive element in steel-making, so high-carbon steels contain the highest amount of the densest crystalline structures, creating harder steels.

As the best blades flex without breaking under stress, sword smiths often seek softer steels for the body of a blade.  Soft steels, unfortunately, cannot be sharpened to as thin an edge thickness as harder steels (see Razor Edged 1 for more information on this), and most certainly suffer from edge folding, breaking and wear much faster than hard steels.  Thus, smiths use various techniques to harden the steel.

In general, hardening refers to heating the blade to certain specific temperatures which causes the crystalline structures in the surface of the steel to undergo transformation into more dense (and therefore harder) structures. This process could include the blade being soaked or coated in high-carbon chemicals prior to heating,  to supply source elements for crystal formation.  Hardening creates a blade with a softer core, but a harder exterior metal

Depending on their level of technical sophistication or final goals, a smith could then use quenching (rapid cooling) which produces even more chemical and physical transformation in the surface of the steel, creating an even harder exterior skin over the previously hardened area.  Multiple quenchings can be used to create blades of a specific hardness. Unfortunately, with this hardness also comes brittleness.  Overly-dense collections of crystalline structures within steel are more likely to fracture along fault or stress lines.  Certainly a thin cutting edge of extremely hard steel is more likely to crack or chip than a softer steel would.

To balace this problem, the smith would  temper  the blade.  Tempering is the slow heating and cooling of the steel.  The blade is heated to a point where it causes a reaction which breaks down some of the crystalline structures, reducing the hardness of the steel;  the slow cooling then ensures no further dense structures would be created.   At times the smith would heat (temper) only part of a blade, or would use clay to cover parts of the blade before tempering, so that only certain areas would be hardened or softened, a process known as differential heat treatment which gave them some control over the qualities of the finished weapon.  The edge might be tempered and the body left unchanged, or vice-versa, depending the the level of steel-making technology a smith possessed.

The most common crystalline structures affecting steel hardness and therefore edge quality in swords, are:

  • Perlite:  one of the “softest” of carbon-based structures, produces a softer steel which flexes.
  • Bainite: the middle-ranking structure sought in sword smithing, these structures create a hard steel which make a decent cutting edge.  Steel of this quality was often found in period weapons
  • Martensite:  extremely dense/hard crystalline structures, these creates extremely hard steel which is also extremely brittle.  Civilizations which could produce high-density martensite blades had resort to tempering to break down some of the structures.

 

marteniste crystals

Martensite structures in quenched steel, under magnification

There is no doubt that some smiths, at various times and in various cultures, developed advanced techniques and were producing blades far more sophisticated than their countrymen.  Even today, there are some blades which modern swordsmiths and metallurgists cannot replicate by using the technology of the time.  Thus exceptional antique swords and other weapons can be found in all cultures and civilizations;  unfortunately, these examples are limited, as smiths either did not pass on their secret techniques to students, or the lineage of that smith died out over time.

Here’s a quick look at the forging of a pattern-welded blade, using modern forging techniques:

 

 

Measuring hardness

A harder steel can produce a thinner, more lasting edge, though too hard a steel would make the edge would prone to breaking during use.  Balancing the quality of the steel required great knowledge on the part of the smith.   Determing how hard a steel is would be an important skill.  In period times, smiths could gauge the quality / hardness of their steel by “eye”  (instinctive, experience-based knowledge) or by the use of “scratch tests”;  using minerals / metals of known relative hardness to scratch the surface of the blade.  If a piece of test material could scratch the surface, the smith would know their steels was softer than the test material.  More commonly, the quality of their blades, and steel, would be proven through practical use or test-cutting.

Today there are a variety of technological systems used to test for steel / material hardness;  some are only used within certain industries, while others are more common amongst geographic regions (much like the Celsius / Fahrenheit scales).   These include the:

The Rockwell scale tends to be in more common use, but is slightly less accurate than the Vickers scale.

Modern steel and blade manufacturers have the advantage of advanced mono-steel production (pure steel, often produced by sintering) , plus chemical and metallurgical analysis equipment to ensure steel quality and hardness.  Medieval smiths used their skills and best judgement in their steel making and sword smithing, using techniques such as folding the steel and pattern-welding to create better steels from poor sources. Unfortunately, folding and pattern-welding can easily cause dangerous inclusions and impurities to be folded into the metal, creating weak points.  Some modern proponents of these techniques argue that folding or pattern-welding allows for greater flexibility or other qualities in a blde;  this particular point has been a topic of much discussion amongst cutting practitioners.

When modern and ancient weapons are measured for carbon content, hardness and impurities, it shouldn’t be surprising that modern steels fall within narrow ranges and tolerances.  On the other hand, researchers have found ancient blades show a wide variety of quality and hardness, not only from blade to blade, but even on different points of the same blade. Thus, the potential sharpness and durability of cutting edges are extremely high today, while somewhat problematic in ancient blades.

The following graphic, from Sword Blade Hardness: A look at the current research shows the range of  Vickers and Rockwell C ratings of several types of modern high-carbon steel (types 6150, 5160, 1075), as well as those taken from a variety of ancient blades.

 

Comparison of period sword hardness ratings

 

In 2003, researchers tested a sword in the Wallace Collection for hardness in both the body of the steel and around the edges.  They found, using the Vickers scale, that while the body of the blade maintained a fairly steady hardness, the edge hardness varied wildly, as can be seen from the photo below, taken from the report: Some early medieval swords in the Wallace Collection and elsewhere
Edge hardness testing
In general then, we can say that blades manufactured today are much more capable of being given, and holding, a long-lasting cutting edge than the majority of antique weapons;  which not that there aren’t some exceptional weapons still in existance.

In the next part of this series, we’ll look at the manufacturing process for several famous classes of swords, their metallurgical analysis, and some comparisons between them.

 

Sources

  1. Folding vs. Pattern Welding – Sword Forum
  2. How Were Swords Really Made? – TheARMA.org
  3. Some early medieval swords in the Wallace Collection and Elsewhere
  4. Sword Blade Hardness: A look at the current research – MyArmoury.com

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