I had another post here but after looking at the order and confusion that is in that original post I decided that instead of updating the old post I would make a new thread and simply lock down the old post....
Background:
When building an AK-47 from a receiver flat the flat and the ejector rails normally require hardening. This hardening is necessary as the metal is in a soft state that allows these pieces to be bent, formed, and trimmed as necessary to build a functional AKM receiver. If a person was to bend and form a receiver without hardening specific points (Hammer and Trigger pin holes as well as the ejector) then these holes and the ejector would fail VERY QUICKLY. Heat treating these flats is a critical component of building an AK from a flat.
Method:
Following is the procedure that is the most generally accepted method to heat treating an AKM receiver. Remember when doing this to use proper safety precautions as you will be heating your receiver to 1600-deg and dropping the receiver into water. Gloves and eye protection are more then just a good idea.
Following are the tools that are needed:
1. Some type of tool to hold the receiver (vice grips work).
2. Heavy insulated gloves (longer gloves like welders gloves are best)
3. MAPP gas torch with the wide nozzle (Not the pinpoint one like you would use to braze)
4. Plastic Bucket with water
Here is the Theory behind heat treating:
(Credit goes to dropdbombnow of http://www.ak47.net for the following as his text and description is very good)
In order to harden steel, you first heat it, red-hot, and then rapidly quench it in cold water. This leaves the metal in a state that is "file-hard", somewhere in the Rockwell hardness range of 60-70, depending on the type of steel. In this hardness range, the holes would wear forever, but the steel will also have become brittle (just like a file) and will be subject to break-out under impact loading. The steel around the holes has also much become much harder than the pins which are passing through them. This excessive "hardness" would cause the pins themselves to wear much quicker than they normaly should.
The process of removing this "excess" hardness is called "tempering" and it is intended to restore "flexability" to the metal while still leaving it in a semi-hardened state. Heating the metal around the holes, untill it turns a blueish/purple color, and then allowing it to slowly air cool will reduce the Rockwell hardness of the steel down into the 40-50 range (about the hardness of a good knife blade) and will greatly reduce the "brittleness" factor of the metal.
Polishing the metal around the holes, to a mirror finish, prior to the tempering stage, will allow you to more accurately see the delicate color changes in the steel.
Here is the Steps to heat treat a receiver:
1. Take a Mapp torch and heat one hole (trigger or hammer hole) to "cherry read" and keep it red for about one minute.
2. Drop the receiver into the bucket of water to quench.
3. Repeat steps 1 & 2 for the other three holes
4. Heat the ejector (with rails in the receiver is how I do them) to cherry red and quench as you had done with the holes/
5. Wipe the receiver clean and remove any slag or residue from around the holes and ejector.
6. Heat the hammer and trigger holes one at a time and when all four are heated allow the receiver to cool at room temperature.
NOTE ON #6 - I have seen some posts on the web were some guys do not temper the ejector but instead leave the ejector as hard as possible. I have not seen an example of an ejector breaking because it was too hard or brittle but I it seems most people do temper the ejector. I don't really know which way is better hence the reason I am putting this note here so you can decide what you want to do for yourself.
7. If you are tempering the ejector, heat the ejector to a blue color and allow that to cool at room temperature (Can be done with #6 and allowed to cool at one time)
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