Once you've completed an AK receiver flat or bend blank, it should be heat treated in the high wear areas to harden the steel so it will not wear out. Where do we heat treat on the receiver? Well, ideally we would have a heat treat furnace oven and would heat, and therefore harden the whole thing. But there's a problem with that. The receivers are mostly 1 mm thick so if you heat treat the whole thing, the sides tend to warp out of shape. I'm sure there's a method used by the overseas AK production factories to accomplish this but there are 2 problems. 1st, including myself, most guys don't have a $100,000 heat treat oven in the garage, and 2nd, most guys don't know the proper procedure for heat treating the whole thing. What can we do? The answer is simple. Just like most commonly available 100% complete receivers on the market, we will heat treat the high wear areas only.
   Where do we heat treat? The fire control group pin areas in the receiver will need some hardening. (This is the area where the hammer and trigger pins go thru the receiver) Also the bolt guide rails must be hardenened, and the top folds of the receiver. (where the bolt carrier actually rides on the top of the receiver)
    How to heat treat:

STEP 1. Now we're going to do this outside of course. You'll need a torch (a regular shop torch, propane torch or a map gas torch will all work fine for this), a bucket of motor oil SAE 30 or SAE 40 works great, (used motor oil will work if you're on a budget but remember it CANNOT be contaminated with gasoline or any other flammable liquid, at the least you'd lose some eyebrows and arm hair.), a pair of leather welder's gloves and a fire extinguisher for safety.

STEP 2. Clean the receiver and note the areas you want to heat treat. You can use a marker to circle the areas if you wish. It'll burn away after you add the heat. Start with the first FCG pin hole. Apply heat evenly around the hole and heat until it just starts to turn red. You don't want to flow the metal, just heat it until it just turns red. Quickly quench the red hot receiver hole in oil to cool it quickly. Remove from oil, wipe away the oil from the receiver with a shop rag and now you'll repeat the process. Heat the area again, and when red, quench in oil just like the first time. Now I've received an email from a well known author on heat treating metals and he reminded me that one cycle is enough and the second cycle doesn't really do anything. He's right, if the metal was red all the way around then dunked in the oil while it was still red of course there's no need to repeat the cycle again. Unless you're using an acetylene torch, I still say do it the second time in case you didn't get it exactly right the first cycle. I say this only because keeping in mind that some guys will have propane or map torches and while one side is red, the other side may not be as hot. Use your own judgement of course. 

  Ok, you've heated red once or twice and quenched in oil once or twice. Now the third step

STEP 3. Once the FCG pin holes have all been heat treated, you'll need to heat the top folds of the receiver. (the area where the bolt carrier rides)Do this the same way as you heated the FCG pin holes. Only heat about 3 or 4 inches at a time of the top rails. It'll take you longer to finish, but that's better than warping the top. Heating the entire top at one time could cause the receiver to become wavy and warped so remember, only work with a few inches at a time. Heat each area once or twice, quench in oil once or twice.

STEP 5. Now it's time to heat treat the bolt guide rails on the inside of the receiver. Do these the same way as the other areas. Heat twice, quench in oil twice. The most important area on the guide rails is the left rail extractor tip. TIP: Remember after you heat treat, if you assemble and need to grind off a bit of the extractor on the left rail for the bolt to clear, heat treat the tip of the rail again where you did the grinding. The part you ground off will more than likely expose soft metal again that needs to be hardened in order to last.

   That's it. You can heat treat your own AK receiver easily. Assuming you didn't get sick from the oil smoke smell or set yourself on fire, it wasn't that bad was it? Remember this method is for the Normalized steel AK-47 and AK-74 bend blanks and AK-47 AK-74 receiver flats we stock. If you use 8620 or something else, watch to make sure you are not thinning the metal. As always , this is for informational purposes only. This is the way I do it. If you feel you cannot do this, don't try. There are many gunsmiths out there that would be tickled to have the work. Above all be safe and know your limitations. 
Good Luck, Chris B.