Wednesday, December 11, 2019

Heat treating armour - A modern way

Many amourers and knife makers heat treat their own stuff, and while I have heat treated my own things before, I'm going to a professional company for this project. Having grown up around aerospace manufacturing, I learned years ago that there is more to heat treating than just making it hot and quenching it, and making it hot again. Modern heat treating has come a long way, and because of the complexity of this project, and the other advantages I'll address here in a bit,  I am using Certified Steel Treating (CST) Corporation in Los Angeles Ca.

Without going into too many technical details of what heat treating is, let me outline the process for those unfamiliar with what's involved in heat treating steel. However, first off, I am not an expert in heat treating, so forgive me if I get some technical bits wrong here.

The basics of Heat Treating steel.

 First: Heat the metal to the correct temperature, this varies with the type and alloy of metal being hardened, but with steel you are in 1500-1650 f / 815-870 c temp range. In addition; The amount of time at that specific temperature is important.
 Second: Rapidly cooling the piece, by quickly submersing it in a "quench" of water or oil or molten salt (see below)
 Third: Reheating the piece to a much lower temperature, in this case 500-750f / 260 - 400c to "temper" it so it is not brittle like a piece of glass.  This is where you can control how hard or flexible the piece can be. Either hard like a file, or flexible like a spring. The choice of metal alloy you use is a big factor of these properties.

For my Greenwich armour one of the factors in deciding to use a commercial heat treating company was I'm using a "modern" steel, in this case 1050, and they will know exactly how to process it to the desired hardness. If it's too hard, it will crack when struck, too soft and it will dent easily. 1050 is a close modern equivalent to what the medieval or renaissance armorer was using. This alloy of steel has just enough carbon (.50%) so it can be hardened, but is not an exotic modern alloy with lots of other types metals mixed in.

The big advantage to a commercial company, is the use of modern vacuum or atmospherically controlled furnaces, which eliminates oxygen around the heated part. This keeps the surface from oxidizing or "burning" while it's glowing orange, resulting in a "scale" on the surface.
This way, your piece can be surface finished to bright or polished, and the surface will be undamaged by the heat treating process. It will turn blue, but bringing it back to "white" is pretty easy.
Another advantage, is salt quenching. The advantage here is lowering the thermal shock to the piece being hardened. Going from 1500f to 80f  of your oil or water quench tank, is a severe drop and can cause a lot of distortion in the work. In order to harden the steel however, you do not need to drop the temperature by 1400 degrees, you only need a drop of  800 or so. One way to do this, is to heat salt to 600 degrees and quench the piece in that. The only down side to a salt quench is you can't have any plates riveted together or other places the salt can get trapped. Because after quenching, they will thoroughly clean the parts, and any place they can't get to, that traps the salt, will corrode.

Since this process is done on a lot of material in a commercial heat treating facility, it's pretty reasonable cost wise. A batch of around 20 ponds of material will cost in the neighborhood of $200 (This is a bit of a guess from memory, but when I get this next batch back, I'll update the cost)

I-Beam clamp from McMaster-Carr (PN-29915T81)
In order to keep the pieces from warping or distorting, you want to brace the parts with a fixture that is more rigid than the part, so it will hold the part in place. There are a number of ways to do this, but I like to clamp or pin the pieces, then weld a frame to these clamp points.
On the most recent pieces, in this case the pauldrons, fauld, and arms, I found some "beam clamps" which are used for securing things to metal I-beams. These can be bought on Amazon for around $1 each, but the only ones I found are galvanized, which would have to be stripped off before using them. Another source is the company McMaster-Carr, they have unplated ones for $2.30, so I used those.  These clamps come with a 3/8"-16 bolt and nut, so once clamped the nut can be used as a jam nut to lock the bolt in place. These are threaded on both sides, so another bolt can be threaded in from the other side giving you the option to pinch the part between the bolts.

The resulting clamp arrangement means you can now just "Connect the dots" with some steel bar, or in this case square tubing. The clamps can be pivoted to facilitate using a straight piece of tubing to connect multiple clamps like seen above on the fauld lames. Here is the finished bracing.

I try to leave a flat stable surface on one or more sides so the parts can sit nicely in the furnace. Anywhere there are rivet holes between plates, #5 screws are used to secure the plates to each other. While a #5 is an odd size, it is almost exactly an 1/8 inch in diameter, which fits perfectly in the rivet holes. Once the pieces come back from the heat treater, I'll cut the clamps off the frames, so I can reuse them. This also is a great way to use up the short scraps of steel tube our shop seems to generate.
The arms and pauldrons are braced in a similar manner.

For the cuirass, I used a technique I used before, where I make tabs from 1/8" strap stock with holes, that secure to the armour through rivet holes with the #5 screws, and then I connect these tabs with steel tube. In addition, I drill holes in the steel tube and weld nuts over the holes and thread a bolt through and pin the armour. In the case of the cuirass, this requires a frame on the outside as well. I try to design it so I don't have to cut any of the tubes to remove the armour.
 In addition, I have added tabs on the front and back of the bottom skirt to trap it against the inner steel frame.
Once these come back, I'll clean the surface back to white (shiny silver) and start the etching.


  1. Hi Chris

    I'll continue to be jealous of your work (and steal as many of your tips as I can - including second-hand from Ugo). This Greenwich harness is coming along beautifully. Thanks for the "cheat" suggestion on the clamp configuration: the only commercial heat treat facility in Calgary has very low vertical clearances, and I was despairing about the distortion of heat treating a cuirass "on it's back". I think that this bracing system will let me get "close enough" without needing to ship stuff down to Seattle for heat treating (increasing my cost by about an order of magnitude)

    Best regards!


  2. Scott,
    Thanks for the comments. I have many craftsmen I am jealous of their skills, but that is what keeps me striving to do new things. As a craftsman, I am always weighing at the amount of work in one step, against the work in the following ones. If step one is not sufficient, you can pay a big price in the following steps. In the case of heat treating, step 2 is out of your control, even if you heat treat your own stuff. Once that steel is orange, and get rapidly cooled, physics and geometry are in the drivers seat. I figure it's worth the extra "seat belts", after having suffered the ill effects, and had to correct them.