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Posted: 1/19/2005 9:12:52 AM EDT
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After reading everything on AR15.com and GUNCO.net, plus my machinery's handbook, the applicable heat treating sections in two material science books and a metallurgy book, AISI specifications for alloy steels, and talking to two other government test engineers, I have satisfied my curiosity into quencing and tempering AK receivers after bending. You can buy 4130 chrome moly alloy steel flats from TAPCO or you can buy 8620 nickel chrome moly flats from DPH. You simply heat the rails red hot (1550-1600F)moving the MAPP (MAPP can only be used to get that temp) torch over the rails (pay particular attention to the ejector) and quench (drop) them in synthetic motor oil (as to not smoke as much with regular oil). If you want to, you can quench the ejector into KASENIT you get at Brownell's to carborize for extra hardness then reheat and quench in oil. But just the heat and quench in oil is sufficient. You can also use 1600F Tempilaq, which is a coating that runs at given temperatures, to tell when you get to exactly 1600F but not necessary. You then heat the four FCG holes one (with a circular motion around the holes) at a time and quench in oil one at a time, same as rails. After you spot weld the rails, you have to temper (not normalize nor anneal)the receiver. Now it is recommended to go from 750F to 1050F for 4130 steels and 400F to 600F for 8620 steel, but the highest setting that you can comfortably set your oven on is completely fine. I have heard the talk otherwise but can't find any indication that you HAVE to get over 500F, it is simply a heat to deserved harness or toughness deal. (There is actually a big drop of in toughness between about 550 and 700F and should be avoided - and over 750 you change the microstructure, so 500 is best anyway in my opinion) There is no magic transition temperature you are looking for. The higher the temp the more hardness you loose but gain toughness. A regular oven is fine for 500F and is adequate to reduce stresses and give a low but acceptable toughness. Pop in the receiver, heat to 500 for half hour, cut off the oven, and open the door and let sit until completely cools down. I'd set the receiver on some scrap sheet metal on the rack though. You don't harden the rivet holes. |
| Sounds about like what I was going to try but I will check the Rockwell on it when done....Also, I was going to use an oven rather than a torch and do the entire thing...you plan sounds good to me for what I've been digging around and reading.....seems the ones that use the spec 750 to 900 to anneal wind up with a too soft of receiver...... |
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Great description 1Gunner. The process you describe has pretty much been my take as well after doing a lot of reading on the subject and having done a lot of "flame" hardening and tempering in the past. I've got a question however. When you say;
are you refering only to the two short internal lower rails or are you also talking about treating the full length upper rails as well? Some accounts that I've read seem to imply that the upper rails need to be heat treated as well and, obviously, if you were going to heat-treat the entire receiver (as some have recommended) then the upper rails would also get treated. I think you're on the right track, but I'm just curious. All good . . . . . . Doug |
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FYI- I just finished up one build on a laser cut receiver flat made of 4130 steel. I heat treated the entire receiver after I was finished machining it. With a oil quench from the proper temperature the material did not reach the hardness specified. I know this because I checked it on a rockwell hardness tester. Rather than do the entire receiver again I tried the spot heat treating the pin holes,rivet holes and rails & ejector. Using a oxy/acy. torch and heating and water quench the material did get hard. The water quench did the trick in my case. I did the rails 1/2 length at a time. After everything hardened I put the entire receiver back in the oven at 900 Deg.F to draw everything back to Rc38. Specs for 4130 heat treatment- AISI 4130 Steel, water quenched 855°C (1570°F), 480°C (900°F) temper Material Notes: As quenched hardness: 51 HRC surface Draw @ 900Deg.F Hardness, Rockwell C 36 http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=M4130F |
That's a good question, I thought that only the part of the internal lower rails that protrude toward the inside of the receiver get heat treated. I don't know about the upper receiver bend. I am just trying to read up before a Buildfest on 20 March, I haven't done an actual heat treat of a receiver yet. |
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Some more heat treatment specs for 4130 and 8620: www.suppliersonline.com/propertypages/4130.asp www.suppliersonline.com/propertypages/8620.asp I just bought a new metallurgy book at UVA, still digesting the heat treatment stuff. |
| Hmm...With proper care and lube, the upper rails in my opinion wouldn't need hardened. Now if one were used on full auto to the point the lube would dry out and seize the bolt to the upper rails ?h.gif |
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