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Periodically, someone will ask the gathered group of armourers, "what steel should I use?" or "what's the difference between these two metals?" If you are new to the craft, or just in the process of widening your knowledge, here is a brief dissertation on metals.
Now, metals can be broken down into two groups, ferrous and non-ferrous. That's basically iron based alloys, and everything else.
Iron is just that, iron ore. If carbon is added to iron, it becomes 'steel'. Steel doesn't actually become hardenable until about .4 of a percent carbon is added. That isn't that much, if you think about it, not quite 20 grams of carbon to every pound of steel. Chromium is added to make steel 'stainless', usually 12% to 20%. There are other things used to improve the workability of steel, such as Manganese, Vanadium, Molybdenum, Tungsten and Nickel. These are added in various quantities to increase certain characteristics of the steel such as strength and hardness.
If you are an armourer, the benefits that these alloys give you is minimal. If you are a knife or sword smith, then the alloy becomes important, because a knife doesn't need to be simply tough and dent resistant, but also hold an edge and resist violent shocks as well.
The primary steels that most armourers will use are of the 10xx series. This is plain unalloyed steel with a varying amount of carbon (the xx refers to the amount of carbon in the steel.) Occasionally, an armourer will use a 'stainless' steel. What this steel actually is, will depend on where they got the steel from. The only thing that is assured, is that the chromium content of the steel is fairly high, typically over 12%. This has two effects on the steel. First, it is rust resistant (not rust proof). Second, when chromium is added the steel becomes very resistant to working, becomes 'springy' and resistant to dishing and raising. It also has a much higher hot working temperature (forging temperature). From personal experience, I have found that stainless steels seem to react like a carbon steel that is 2 to 4 gauges thicker, i.e. a 16 gauge stainless steel elbow cop is as difficult to dish as a plain carbon steel elbow cop that was 12 to 14 gauge in thickness. (Using a heavy hammer becomes a blessing at this point.)
One point to bring up here is the difference between 'hot rolled steel' and 'cold rolled steel'. The hot or cold refers to the temperature of the steel during its final pass through the rollers at the steel mill. A 'hot' rolled steel will come out of the rollers "annealed" (soft), while a 'cold' rolled steel will come out "work hardened." Many armourers will advertise that they only use 'cold rolled steel'. This might make a difference on pieces that are not welded or heated in any way, but if the piece is heated or welded, it becomes annealed, and therefore no better than a hot rolled steel.
To understand this whole process, it's good to visualize the steel as a liquid in its solid form. The liquid (iron) has been mixed together with carbon and then frozen. What the carbon does, is bind together with the iron to make a mix of iron and iron carbide. At forging temperature, the mix is very even and is called 'Austenite.' When the steel is cooled rapidly (quenched) the iron and carbon freeze in place and make a coarse, uneven matrix of iron and large needle like crystals of iron carbide . This matrix is called 'Martensite.' The crystalline structure is fairly large and coarse, making the steel very hard, but also very fragile. When the steel is warmed to a medium temperature (400 to 650 degrees), the crystals are refined to a smaller size, and stresses within the metal are relieved. This is tempering and ends up in an even distribution of carbides with in the iron. Annealing (heating the steel, then letting it cool very slowly) removes all the strength from this careful matrix and make steel soft.
The other material that armourers use is aluminum, usually either 'street sign' aluminum, or T6061. This aluminum has the advantage of being very lightweight, and has been tempered to a fair hardness (compared to something like brass or bronze). Its down side is that it can be expensive and difficult to work. The thing that must be remembered when working with aluminum is that it work hardens very quickly, and will become brittle. One must be always on the lookout for cracking and chipping when working with aluminum.
For armour, remember that most armourers don't quench and temper their stuff, so the amount of hardenability of the steel comes more from work hardening (refining the crystal size and distribution through working the steel.)
For weapons, the criteria for choosing a steel is very different. A sword or knife needs strength, flexibility, shock resistance and edge holding ability. A 10xx series steel is an acceptable steel for most purposes, but it can be improved upon. Adding chromium to sword steels is not a good thing, as it reduces flexibility, and increases brittleness, but in a knife those things aren't as vital.
The most common steels in use at the moment for swords are probably 5160, W1 and L6. 5160 is a 'spring' steel that has .60 percent carbon, .8 percent chromium, and .8 percent manganese. This makes a tough, shock resistant steel. W1 is a like 5160, but with out chromium and manganese. L6 is essentially much like 5160 but with the addition of nickel and vanadium. Nickel and vanadium improves its hardenability and shock resistance. Which is better? Your choice, I don't know if it will make much of a difference for most people not involved in 'live steel combat' or japanese tameshigiri (cutting) practice. If you never use it as a sword, and only display it on a wall, it might as well be made from aluminum.
When deciding on a material, try to remember the application that the end product will be put through. Then choose whatever material will be best suited for that use.
I can recommend the following books to help you in your search for smithing expertise.
Unfortunately, there aren't any books that can similarly targeted to helping the novice armourer. You will have to do what every present-day armourer does, pore over thousands of photos of armour, visit museums and study the armour on display, and network with other armourers to share your knowledge.
The only book that attempts to cover the subject, "The Armourer and His Craft: From the XIth to the XVth Century by Charles Foulkes", is suspect and contains many fallacies. It was written by someone who, while he had great experience with armours in museums, had little to no experience in actually making the armour. It is worthwhile for its many details of measurement, and photos and engravings, but take the information held within with a grain of salt.
For armouring, you will find that the same techniques are used in silversmithing, blacksmithing and even auto body work (take a browse through Sheet Metal Handbook by Ron Fournier, HP Books.) If you take what you can from those methods and adapt it to armouring you'll be on the right track.
Frederich Von Teufel