An Introduction to Making Hardened Steel Armour
By: Dave Wise, aka Sir Alexis LaBouche
The use of hardened steel armour is probably the single biggest difference between what we do in most recreation groups and those who actually lived and fought in armour in the past. For as long as armour has been made there has been a balancing between protection and encumbrance. As a rule the easiest way to improve the ability of armour to resist penetration or crushing is to increase the thickness, however this increases the weight proportionately. There is a natural limit in terms of how heavy armour can be before the average fighter will abandon the armour. Therefore, the goal is to achieve both strength and durability while minimizing weight. If increasing the thickness is not a viable option then the alternative is to increase the quality of the steel. This can be achieved through the use of a steel that is responsive to heat treating, whereby the steel is made both significantly harder and tougher than a traditional mild steel.
The reason that steel can be hardened is that steel has different crystalline structures, each with different mechanical properties. The structures that can be formed are a function of the carbon content, the presence of alloying elements and the pattern of heating and cooling that the material is subjected to. By carefully controlling the temperature and the rate of cooling the same piece of steel can be very hard, but brittle (almost glass like), very soft, but tough and best of all both hard and tough. It is this last state that we are striving for in the production of armour. As a quick note steels are generally classified as low carbon (or mild, carbon content less than .04), high carbon (carbon content between .04 and .15), or alloy (iron and carbon with significant amounts of additional elements, stainless is a type of alloy with additional elements to resist corrosion). As a rule mild steel is not responsive to heat treating, therefore no matter how hot you get the metal or how quickly you quench it there will not be an appreciable increase in the hardness. A high carbon (or straight carbon steel) will respond to heat treatment. An alloy steel may respond to heat treatment depending on the percentages of carbon and alloying elements. 4130 is a type of chrome-moly steel that generally would not be responsive to heat treatment since its carbon content is only .03, however due to the presence of the chrome and the molybdenum the steel will respond to heat treatment.
The process of making hardened steel armor begins using the same techniques as with mild steel. The high carbon sheet should be purchased in the fully annealed condition. When annealed the material moves like mild steel, therefore the traditional techniques of dishing, raising and planishing are used to shape the material. One difference is that all steps should be done cold to keep the metal fully annealed. This includes grinding and polishing to the desired finish. Once the material has been hardened it will be much more difficult to correct problems without re-annealing the piece.
In order to harden the piece it will be necessary to raise it to its critical temperature. For carbon steel this will be about 1400 degrees Fahrenheit. For some stainless steels this can go up to about 2100 degrees F. The approximate temperature of the metal can be determined according to the following chart:
The Heat Source
There are generally two options for your heat source. The first is a coal forge and the second is a gas forge. In order to get a uniform hardening the entire piece must be at or above the critical temperature prior to the quench. For the armorer of old this was done over a charcoal forge powered by bellows. The bellows could be powered by hand or by water. The principal fuel was charcoal. This is not the same as your barbecue briquettes, but was reduced from wood, preferably a hardwood. The advantage is that charcoal will burn clean and hot. The drawback is that charcoal is hard to find and time consuming to make. Alternatively, the most commonly used fuel today is a soft bituminous coal that is readily available through farrier supply houses. This fuel is burned in a coal forge.
A simple forge can be constructed from a brake drum as illustrated below. In this case the drum is serving as the fire pot and the air source is piped in to raise the heat. The size of the drum will determine your effective working area and if a larger forge is desired then you can build your own firepot to your specifications and use a similar air setup. The drum can be obtained inexpensively from a parts yard and the rest of the parts can be obtained at most hardware stores. The airflow can be supplied by use of a bellows, a squirrel cage or even an ordinary hair dryer. I recommend setting the air supply on a rheostat, i.e. a dimmer switch, so you can adjust the airflow. Alternatively you can regulate the airflow through the use of a choke valve.
Building your fire
Once you have built your coal forge you will need to get a good fire going in order to reduce the coal to coke. The soft coal that is usually available has quite a few impurities in it and if you were to forge over a coal fire these impurities would end up in your metal. The solution is to heat the coal and burn off the impurities leaving you with coke, which will then continue to burn clean. In order to get the fire going it is usually easiest to build an igloo by laying a bed of coals and then using a piece of wood over which more coals are applied. Once the fire begins burning there will be a large amount of greenish yellow smoke. For this reason you will want to either build a hood for the forge to direct the smoke outside or work in a very well ventilated area. Once the fire is burning hot the smoke will subside. As a note, when you need to add additional fuel while working, place it on the outside edge of the fire and let it reduce to coke before working it into the center. As the fire gets hot you can raise the temperature by increasing the airflow. Pay attention to the fire since it is much easier to maintain a fire than it is to get one going. This simple coal fire will produce more than enough heat to harden your pieces.
The Gas Forge
An alternative to the coal forge is to build a gas forge. While this may sound difficult, it isn't. Gas has the advantage of burning cleaner and being much easier to bring up to heat. An easy way to build your own forge is with firebricks or an old kiln if you can get your hands on one. A simple burner can be built based on a design by Ron Reil. The plans can be found at http://netnow.micron.net/~rreil/Forge.htm; this page also has tons of information on forges. A copy of his design is included in the appendix. I would recommend a modification suggested by Robert Bordeax where he drills ½" holes in the top of the bell housing to mount the jet pipe which is then held in place by set screws (shown on the inset on the design page). I used a ¼" pipe since that was available at Home Depot and I used a 1/16" drill for the jet hole instead of tracking down a fractional drill bit. Neither of these modifications seemed to make a crucial difference and that way I was able to use tools I already had. For controlling the gas flow rate and connecting to the propane tank the preferred way is to get a gas regulator. Alternatively for regulating the flow a simple ball valve will work as well. For the connection to the propane tank most home improvement stores sell a hose and fitting designed to fit into a standard 20# propane tank used for outdoor barbecues. If you do not have a regulator I would recommend obtaining an inexpensive gauge for the propane tank to let you know when you are running low. There are few things more frustrating than running out of gas halfway through a project. With either heat source follow basic safety precautions and be sure to keep all combustibles away from your working area. Be sure to set your quench tank nearby and keep a fire extinguisher and first aid kit handy.
Hardening the Piece
Now we have the opportunity to make or break all of the work we have put into the piece until now. When you are ready to harden your first piece of steel be sure not to rush it. Let your heat source get nice and hot. If you have an enclosed forge the inside should be glowing. There are several reasons for this. First it provides a more even heating of the piece since your piece will be able to absorb heat from all sides and not just directly from the heat source. Second, if your forge is hot enough you can reduce your direct heat source in order to minimize scaling on your piece. When you are ready to heat a piece, grasp it in your tongs. Be sure to have a good set of gloves on when you open the door to an enclosed forge since there will be plenty of heat coming out. On an open forge you may end up holding the piece until it is hot enough. Try not to set it down more often than necessary to minimize scaling on the piece, however it is often a good idea to shift the location of the tongs since they will act as a heat sink on the piece. In an enclosed forge you will probably set it down due to the heat coming out of the forge. You can use a piece of firebrick to position the piece. Be sure to set it down in such a way as to make it easy to pick up with the tongs. Once the piece is up to a sufficient heat, at least a cherry red for a carbon steel, higher for most alloys, then it is ready for the quench. Be sure to handle the piece carefully since it will be more plastic at the higher heat and you can inadvertently put a twist in the metal if you handle it roughly. On this note if you are working with thinner stock it might be good idea to put in temporary bracing to preserve the shape for the heating and quenching. There are examples of holes in period breastplates that have no obvious function that are considered to be such ‘constructional aids’.
The next step is to quench the piece. This ‘freezes’ the crystal structure of the metal that results in the increased hardness. As a rule, the faster the material is cooled the harder the piece will get. Attempting to maximize the hardness sometimes causes more problems than it is worth. First, the piece is going to be tempered, so some of the hardness is exchanged for toughness. Second is a problem with warpage. When a piece is cooled rapidly, bubbles will form on the surface. These bubbles will then act as an insulator slowing down the rate of cooling in the affected area. This allows the surrounding areas to cool at a different rate. Since metal expands as it heats and contracts as it cools then these differential rates will result in warpage of the piece. One way to minimize this is to gently move the piece in the quench so as to allow a more even cooling. An alternative is through the selection of an appropriate quench medium. Over the years, almost every viscous liquid has been tried and there are period ‘recipes’ that call for urine and goat’s blood and nearly everything else you wouldn't want in your soup. What all of these recipes seek to do is to establish a correct degree of thermal conductivity to control the cooling rate. Without going into the exotic recipes, generally water serves as a fast quench, and oil will serve as a slower quench. For armoring purposes I recommend an oil quench since the piece will still come out very hard with the slightly slower quench and I want to avoid ever losing a piece to warpage, especially on something like gauntlets where the fitting is crucial to the piece working properly. My favorite is to use transmission fluid because it is a medium weight oil that is cheap and is self extinguishing. When a very hot piece is put in the quench you will occasionally get a flash fire at the surface, but once the piece is fully submersed the fire will go out.
Now that you have successfully hardened the piece the next step is to temper it. The steel is now very hard and generally very brittle, although that will depend on the steel used. Great caution is now required because the piece of steel that we formed may now be almost as brittle as glass. Especially with high carbon steel the steel can be snapped like crackers at this point so it pays to be careful in handling it. One advantage of an alloy is that it is often tougher at this point, so there is less likelihood of breakage. What you will do next depends on how you intend to temper the piece. One method is to carefully remove any scaling, including the bluing that the quenching will produce, so that you have a bright surface. Now you can temper the piece with a torch. As you apply the heat the metal will change colors according to the following chart:
You have now joined a select few armorers who have recognized the benefits of hardening the steel. In terms of preferred thicknesses, I have been using pieces in the .025" to .040" range (approx 23 ga. to 17 ga.) and have been amazed at the results. On one piece I noticed a small crease that had not been taken out prior to hardening so I grabbed a planishing hammer, set the piece on the anvil and gave it a couple of hits to take out the crease. When the first hits had no effect I continued hitting until I broke the hammer, all without taking out the crease! Let this be caution that you should be done with the forming operations before you harden. Now I can go back and re-anneal the piece, but that will involve a lot of work so to paraphrase the old carpenter's saying 'check twice, harden once'.
Following are some sources for high carbon steel. If you want to go with a straight carbon steel, then Admiral Steel is the only source in the country that I have found for it in sheet gauges. The drawback to Admiral is that they have a high up front cost because of the packaging cost and the cut fee. It definitely pays to place larger orders because the same fees apply to a large or small order. If you are looking for an alloy, then there are a number of suppliers of alloy steels. The alloy I have been using is 4130 because it is readily available and less expensive to get (it is commonly used for custom car work). Any search on the web will turn up a list of distributors. The cost will run about $2.50 to $3.25/#, or about $3.15/ sq. ft.
4152 West 123rd Street
Alsip, Illinois 60803
(800) 323-7055 tel
(708) 388-9317 fax
About every kind of steel
5373 W. 86th Street
Indianapolis, Indiana 46268
Good source for annealed 4130
Here is a short list of some additional reading materials and sources of the above information: