question

[Lazyme]

...

[Richard Erichsen]

I have a question about heat treating the ejector and axis pin holes on my AK's. All the reading I do says the basic heat treating process goes something like this:

1. Heat until metal is visibly red if you are in the shade.
2. Quench metal in oil ASAP after red is obtained.
3. Heat until metal is grey/blue-ish.
4. Let sit until cool.

I told a few old school guys who have been working with metal for 45+ years (each) this method and they said it would leave the metal about the same hardness as before it started. They recommended a simplified version that goes something like this:

1. Heat metal until it is a deep brown color just before turning red.
2. Quench metal in oil ASAP after color is obtained.
3. Keep metal fully submerged and moving while cooling. (keeps expanding gasses around metal to a minimum and helps keep the final product strong)

What is the basic science going on between these 2 methods? I am thinking the first method makes the metal so hard it is brittle and needs some relaxing after the serious heat that was applied to it.

Would the second method just cut straight to the point and get the strength and hardness while using fewer steps?

Unless you are very experienced, using color to determine temperature is very error prone.

The temperature you need to reach, regardless of steel alloy used is called the "critical" temperature, this is the point at which steel loses it's magnetic properties and is the point at which undissolved carbon is absorbed into the the iron. An old blacksmith trick is to have a suitably small magnet handy on a section of rod and when the temperature is reached the magnet will not attract. When you quench you should swish it around in the oil with tongs or a suitable makeshift handle to make sure the surface is always in contact with the cooling medium and that there are no bubbles clinging to the surface.

Temperature control is the key to preventing warping. Despite this, you'll want to use all-thread rod or very long screws and nuts to support the receiver through the rivet, support and axis pin holes. Doing this kind of work without an oven makes doing more than the small areas of the axis holes and ejector tip very difficult to reach the correct critical temperature (without going too far or not far enough) and holding it within a tight temperature range across the whole part before quenching. Zone hardening is the better option, since the size of the parts is small and reaching critical is a whole lot easier and you really don't need to provide extra hardness for wear resistance to the majority of the receiver.

Good luck,

R

[tujungatoes]

....Individually heat each hole to 1600 degrees(red/orange), and Immediately quench in oil or brine solution.. A magnet is a very good way to check if you're not sure on the color.

Here's a color chart


For the second step make sure you've removed all the scale from quenching(makes it easier to see the color change). Then you're heating to 750-800 degrees and letting the part air cool. Just watch the colors radiating out from the heat source you'll see the blue ring expand and as soon as you see the following grey stop. With mapp gas it will probably take 4-5 seconds to reach the correct temprature. This will relieve the stress from the previous step which made the part hard but brittle.

Whomever you spoke to was mistaken. These receiver flats are made from annealed 4130 they're actually quite soft compared to the critical areas after being heat treated.

*ETA* do your ejector first. Then weld the rails into the receiver. Drill/ream axis pin holes. Then heat treat the axis pin holes.

[oriansbelt]

The 4130 steel that these receivers are made out of is very heat treatable. In the annealed state the steel should be around 14RC, about as soft as steel gets. To harden this steel it should be heated to 1600F and quenched as tujungatoes mentioned. In this state the steel will be very hard, in the 50-55RC range. This is a bit too hard and brittle. The ejector is the only thing that needs to be really hard and that should be in the 40-45RC range. I would shoot for 35-40RC for the pin holes. To do this you want to get the metal to 750-800F. If you go over that temperature you risk softening the metal too much so be careful when you are heat treating by hand. It takes time and patients to get it just right. For the ejector you only want to bring it to about 560F so be very careful to only heat it to the point where it just turns blue. For the pin holes you will want to go just past that to the point when the metal turns gray.

There are only three colors that are fairly easy to get when tempering, gold, blue and gray. For 4130 gold should be around 46RC, blue 43, gray 36. That being said, color is a fairly crude way of heat treating so results are bound to vary quite a bit. A computer controlled heat treating furnace would be the way to go .

[jamesob]

If you do it the way the other guys said it will most likely crack or break the ejector.

[jamesob]

Yep parents can be hard headed, I have one of them.

[tujungatoes]

Well, the other guy is my dad and arguing with him isn't easy.

A small review of information:

A. If the first temperature range of 1600ish is not reached then there is still carbon deposits left in the metal creating weakness.

B. The hotter the application during the second process the softer the metal will become after the air cool.

C. Practice makes perfect.

One more question. Where can I go to have these professionally heat treated ?

1. I know full well that you cannot(nor should you try to) argue with your pops. Don't even bother. Realistically the only problem he "might" have is his ejector deforming. In that case just re do the heat treat and don't rub his face in it. He's still your dad and and he'll probably kick your ***.

A. I have no idea. I'm far from the king of metallurgy. I just know you get the f**ker hot and quench it which makes it hard as a mofo.

B. Yes. This is my understanding.

C. Always

Lastly. I have no idea, but I know it's not a simple process. Nor is it really necessary for a rifle that's just for giggles. The are people with multiple thousands of rounds through flat built rifles with no problems. If you insist on a fully heat treated receiver I recommend NoDak Spud 100% receivers. They are the best complete receiver on the market.

[69powerwagon]

No, not carbon deposits left in the metal. the only time there is free interstitial carbon is when you get above o.85% (or eutectic steel). 4130 is way below that. ill explain very briefly what happens when you heat treat. you bring the metal up to 1600 and turn the steel crystals austenitic, or from a body centered cubic to a face centered cubic, causing it to rearrange the carbon locations in the crystaline lattice. when you quench you create large amounts of martensite from the austenite. martensite is very strong and hard, but way to brittle. you then need to temper by increasing temperature to break down much of the martensite into stabler/softer crystals to stress relieve and bring down to a useful strength. 1600 is a critical temperature