Some observations on spot treating

[meaty-btz]

I've done it enough now to question the overall hardness of the end result and the need for annealing after the initial heating.

I use a mapp gas torch so it gets pretty damned hot. Quench in agitated water. However all 8 or so now receivers I have treated don't pass the backyard hardness test as being tool steel. The old "if a file does not skate easily on the surface it isn't tool steal hardness". All of the holes and the ejectors after being heated to the critical point, bright orange and quenched in agitated water end up something more like the ideal end hardness. Much harder than the starting fully annealed sheet metal but not "brittle hard". To test it I had a spare rail and gave it the critical temp, quench and then put it in the vice and hit it as hard as I could with a heavy hammer. It deformed instead of cracking though it was much tougher and bent little compared to the untreated material. So.. is spot treating annealing for 1 hour a 400F really needed to reduce the hardness from the hardening? I know that when you do a full heat treat it runs about 15minutes at the critical temp, which makes sense from what I know of metalurgy. Time is needed for the migration to occur and you just don't get that with spot treating unless you want to heat your part for 5+minutes per spot. I have not had any hole deformation so I know I am strong enough even after the annealing at 400 for an hour afterwards, still. I am kind of hoping those with more experience can step in here and state if I am way off base or not on the spot treating but I don't believe it gets hot, long enough, to reach tool steel hardness that would need the subsequent annealing step. I think if you tried you would warp the crap out of your receiver from long term uneven heat migration that you get with torches, like we used to get with the old oxy-acetle torch work on thin metal.

I was hoping to get KScott in here on this one.

[kcstott]

Here's a few things to think about.
1. You are not annealing after you harden the part, you are "drawing back" or "Tempering" it. Big difference.
2. You never stated at what color you stopped heating your part for the drawing back.

Now that said if you are following the proper procedure for spot treating what you are accomplishing is maybe a step up from a blacksmith at best.

I've always felt that the color that is recommended as a stoping point is to close to annealed to be of any good. I typically temper my holes to where they just turn blue whic is around 570 degrees F and should yeild a hardness of around 35Rc or a little more.
Your practice of back yard hardness testing is correct If the piece of steel was supposed to be 58Rc or higher. There are very few parts on a firearm that need to be that hard. So dragging a file across you part will not yeild the results you are after.

The other thing is 15 minutes at the critical temperature is incorrect. The minimum soak time for for any piece of tool steel is 1 hour per inch of thickness plus one hour.
My practice was if the part was less then an inch thick we placed it in a hot oven for no less then two hours then processed it from there. Drawing times are incorrect as well. One hour per inch again plus two hours soak time
This insured the whole part is properly soaked.

In reality what we are doing in terms of heat treating is adding just a touch of hardness to the material. The receivers would be much better off if they were fully heat treated but for what is needed spot treating works just fine.

Is it the best practice? Nope. Will it get the job done? Sure will.

[meaty-btz]

Here's a few things to think about.
1. You are not annealing after you harden the part, you are "drawing back" or "Tempering" it. Big difference.
2. You never stated at what color you stopped heating your part for the drawing back.

Now that said if you are following the proper procedure for spot treating what you are accomplishing is maybe a step up from a blacksmith at best.

I've always felt that the color that is recommended as a stoping point is to close to annealed to be of any good. I typically temper my holes to where they just turn blue whic is around 570 degrees F and should yeild a hardness of around 35Rc or a little more.
Your practice of back yard hardness testing is correct If the piece of steel was supposed to be 58Rc or higher. There are very few parts on a firearm that need to be that hard. So dragging a file across you part will not yeild the results you are after.

The other thing is 15 minutes at the critical temperature is incorrect. The minimum soak time for for any piece of tool steel is 1 hour per inch of thickness plus one hour.
My practice was if the part was less then an inch thick we placed it in a hot oven for no less then two hours then processed it from there. Drawing times are incorrect as well. One hour per inch again plus two hours soak time
This insured the whole part is properly soaked.

In reality what we are doing in terms of heat treating is adding just a touch of hardness to the material. The receivers would be much better off if they were fully heat treated but for what is needed spot treating works just fine.

Is it the best practice? Nope. Will it get the job done? Sure will.

Got it, so we are really just adding a gentle bit of hardening. I usually bring it to bright orange, hold it for a short time, dunk it in agitated water. I don't think we really have to worry about cracking even without drawing back down with 450F-570-ish. I mean what we are talking about hardness wise really isnt something that will crack. It remains quite soft even afterwards. I decided on the most recent one to not draw back. We will see how it performs vs the other ones. I seriously doubt I will see any cracking. It just isn't hard enough to be brittle. I know some people reported cracking as the reason for tempering it back down a little and some pre-treated ones were over hard. I just don't see us achieving those kinds of hardness.

One other test I do is the magnetic test, so I am reaching critical temp at least in the small area going non-magnetic. Again, I just don't see it getting hard enough to want to temper it back down some. If anything we are undoing what little we have done in the first place.

Thanks for the words though Kscott, I think I am clear for not tempering, drawing back, as long as I am finding the hardness to be below the range of brittle hard.

The funny thing is maybe it is the alloy we are using? I have hardened other materials in the past in a backyard forge and used critical temp as the guide but I could let it soak in the heat for a while and check magnetism and when quenched it was full tool hard and would shatter if I hit it with a sledge. So even the mapp gas torch is really inferior over all for hardening.

[kcstott]

One other thing I forgot to mention. You don't need to go to orange hot. Dull cherry red is 1500-1600 degrees F and that all you need. Over heating reduces the carbon in the steel and may cause issues.

As for the material and how hard it get gets compared to other materials.
You bet. The 4100 series of metals develop their full potential at 35Rc if you have a need for a harder part you can get it to 58Rc but you'll reduce a lot of it's qualities. and this is just the opposite for other materials like A2 and D2 which don't reach their full potential until 55Rc
The thing is 4100 series steel is extremely strong even annealed. After hardening it's around 150Kpsi of tensile strength and up to 180 Kpsi depending on the quench and tempering.

Personally I draw it back every time. Cracking is not just dependent on hardness it is also dependent on how well the grain structure is stabilized. not to mention just as we are doing very crude heat treating the level of control we have is also crude. there is a lot going on in heat treating. CArbides are dissolved, alloying elements precipitate out, grain structure changes. So to prevent brittleness and the possibility of cracking you should draw it back to at least dull straw color. The other thing is with heating in free air we are oxidizing the part and introducing contaminates. Just another reason to draw it back.
Also it's a stress relief as well.

But in reality we are talking about four small spots with very poor heat control. So it may just not make a damn bit of difference.

[meaty-btz]

One other thing I forgot to mention. You don't need to go to orange hot. Dull cherry red is 1500-1600 degrees F and that all you need. Over heating reduces the carbon in the steel and may cause issues.

As for the material and how hard it get gets compared to other materials.
You bet. The 4100 series of metals develop their full potential at 35Rc if you have a need for a harder part you can get it to 58Rc but you'll reduce a lot of it's qualities. and this is just the opposite for other materials like A2 and D2 which don't reach their full potential until 55Rc
The thing is 4100 series steel is extremely strong even annealed. After hardening it's around 150Kpsi of tensile strength and up to 180 Kpsi depending on the quench and tempering.

Personally I draw it back every time. Cracking is not just dependent on hardness it is also dependent on how well the grain structure is stabilized. not to mention just as we are doing very crude heat treating the level of control we have is also crude. there is a lot going on in heat treating. CArbides are dissolved, alloying elements precipitate out, grain structure changes. So to prevent brittleness and the possibility of cracking you should draw it back to at least dull straw color. The other thing is with heating in free air we are oxidizing the part and introducing contaminates. Just another reason to draw it back.
Also it's a stress relief as well.

But in reality we are talking about four small spots with very poor heat control. So it may just not make a damn bit of difference.

I guess it goes back to, this is an AK ;p it's good 'nuff. I mean we can get all anal and I was at first but as I built more AK's I just started to go... yeeeaah that's good nuff. Was helping my wife assemble her anniversary gift (every anniversary these past three years we have bought each other firearms.. gotta love her). And she was having trouble so I handed her a hammer, a rubber mallet, and a wooden mallet and said. These are the tools you need to complete the assembly. It's an AK. Hehe.

I will interject here that I am going to email Kurtis at AK-Builder. To save some time I got one of his barreled assembled kits for her build. The headspace is.. good, but if I gorilla it with my beefy self and slam it home it jams up but will mostly close on the no-go. I was very unhappy. On my builds I will sometimes spend hours tuning the headspacing, back and forth on the press till I want to scream but so even I can't gorilla it closed on a no-go but it will tight close on a go. I realize that there is more variable in the spec of the mil surplus I shoot than between a no-go and a go, still. I guess I am still anal that way.

As for color it is good to know that I am overheating, I was always told orange glow in the shadow or barely orange in full sun. I can dial that back.

[kcstott]

I think you are checking your headspace incorrectly as well.
The bolt should be removed from the carrier and you really should strip the bolt if you are just learning the feel of things. You want as little influence on the feel of the head space as possible.

Hours to play with headspace. Man If I did that I couldn't make any money doing it. I just populated five barrels fora gentleman and it took me all of an afternoon to do once the parts were demilled and degreased (that took some time). Thing is you start to develop a feel for when stuff is getting close. As for your frustration on the press next time try this. First off test fit your parts and if you have a means to accurately measure the ID of the trunnion and the OD of the barrel do it and get them to be .001" press fit by sanding the barrel journal. This will still be plenty of press fit and will not jump around when you are trying to get a fine touch with the movement. Then use some oil when you assemble the parts. Finally if you need to bump the head space tighter in the press just build up a little pressure the tap the ram with the jack handle this will cause a little vibration and allow that little bit of pressure to move the barrel without over shooting.

If you are a little tight don't be afraid to take a brass hammer to the trunnion and give it a generous whack. If you sanded the barrel journal to tha .001" press fit it will move. It will not come flying of nor will it move a mile but a couple of whacks will get it where it needs to be. You can even have the go head space gage in the chamber, bolt behind it and it not at full lock. Put pressure on the bolt with your thumb to try to lock the bolt as you tap the trunnion down in the direction of taking it off the barrel. as you tap the trunnion you will feel the bolt rotate into full locked position.
Use brass when you do this as it will not scare up the trunnion and the marks wipe off.

No the other thing is just because it closes on a No go doesn't mean it bad. the bolt has to close on a field gage before it need to be adjusted. But it's good practice and craftsmanship to get it as near perfect as possible and the barrel will last longer too. Headspace grows on every rifle out there and if set long to begin with well that just less shots it will have in it's life before it will close on that field gage.

[tujungatoes]

I think you are checking your headspace incorrectly as well.
The bolt should be removed from the carrier and you really should strip the bolt if you are just learning the feel of things. You want as little influence on the feel of the head space as possible.

^this. Also there is no need to "gorilla" the bolt/gauge into the chamber. That's not the way they work. Moderate finger pressure is all that is required. The bolt not fully closing on your no-go even if it's close is still well within tolerance.

[ar15barrels]

No the other thing is just because it closes on a No go doesn't mean it bad. the bolt has to close on a field gage before it need to be adjusted. But it's good practice and craftsmanship to get it as near perfect as possible and the barrel will last longer too. Headspace grows on every rifle out there and if set long to begin with well that just less shots it will have in it's life before it will close on that field gage.

How do you feel about the practice of using a 0.002" shim between the headspace gauge and the boltface and then just touching the barrel down against the gauge with the press?
The trick is having the barrel/trunion fit not-so-tight just like you mentioned so you can feel the barrel just stop moving.
My press has a large ram diameter with a remote pump and pressure gauge and a very good feel compared to the bottle-jack presses...

[ar15barrels]

In reality what we are doing in terms of heat treating is adding just a touch of hardness to the material. The receivers would be much better off if they were fully heat treated but for what is needed spot treating works just fine.

Is it the best practice? Nope. Will it get the job done? Sure will.

I have a programmable heat treating oven as I don't trust myself to do quality heat treating work with a torch.

On my personal AKs that I built years ago at a build party, we just spot treated the holes and ejector and they have survived just fine.

How many different alloys are being used for making flats?
I'm guessing that like all things, flats are not all created equal...

[kcstott]

How do you feel about the practice of using a 0.002" shim between the headspace gauge and the boltface and then just touching the barrel down against the gauge with the press?
The trick is having the barrel/trunion fit not-so-tight just like you mentioned so you can feel the barrel just stop moving.
My press has a large ram diameter with a remote pump and pressure gauge and a very good feel compared to the bottle-jack presses...

Feel is the critical thing. The old shop I use to work in has an enerpac press. It also had a gage on the ram so you could see any additional force building up immediately. I would not reccomended it with the harbor freight press unless you have really got the feel for it dialed in.

What I do now is I have a little gizmo made up that looks like the bolt face I had a bunch of barrels to do so I miced then and found a happy medium for the over all length of the gizmo. I place it in the trunnion rotate it like it was a bolt locked in position the seat the barrel on that gently. I'm usually within .006" of perfect headspace or I'm right on the money but never to short.
That little tool save a buch of time. I was actually thinking of making a run of them and selling them to builders

[meaty-btz]

Actually I have a cheap press (hense the lack of precision). I've gotten better at using it for precision motion but the first time was awful.

As for head-spacing, I do both with carrier and without carrier. Use the finger test with just the bolt and then test with carrier to verify everything. After disassembly and reassembly (usually for parking since my tank isn't long enough for doing a barreled receiver) I recheck.

Anyways the wife's gun ran fine for about 300 rounds for break in at the range.

[ar15barrels]

What I do now is I have a little gizmo made up that looks like the bolt face I had a bunch of barrels to do so I miced then and found a happy medium for the over all length of the gizmo. I place it in the trunnion rotate it like it was a bolt locked in position the seat the barrel on that gently. I'm usually within .006" of perfect headspace or I'm right on the money but never to short.
That little tool save a buch of time. I was actually thinking of making a run of them and selling them to builders

I'm assuming you measured lug length and boltface depth and then gauge protrusions from the breech face to find the happy mediums, then added the normal gap that exists between breech face and bolt nose to determine your final gizmo lug length?

What was your final length of the gizmo so I can make myself one?
What was the extreme ends of the size range?

[kcstott]

I'm assuming you measured lug length and boltface depth and then gauge protrusions from the breech face to find the happy mediums, then added the normal gap that exists between breech face and bolt nose to determine your final gizmo lug length?

Yep and took an average off of a hand full of bolts and trunnions to come up with a happy medium.

What was your final length of the gizmo so I can make myself one?
What was the extreme ends of the size range?

I believe the extreme spread was around .007"-.008" most were much closer then that. Thats why on some setups it's .006" long and on others it right on the money were the go gage locks up with just barley being able to feel the gage hit bottom.

I'm giving away my secrets here .629" For 47's I haven't had enough 74's to make an educated guess. Now that gage was made off of the dimensions of a bunch of Romy's i believe but it head spaced a bunch of Egyptians with no problems, and I've tryed it on Yugo's, Maadi's, polish, bulgarians all with no issues it's either a little long on headspace or within tolerance . You can come down fast know where to stop then tap it in from there

[ar15barrels]

I believe the extreme spread was around .007"-.008" most were much closer then that. Thats why on some setups it's .006" long and on others it right on the money were the go gage locks up with just barley being able to feel the gage hit bottom.

I'm giving away my secrets here...

Few people would even be able to make such a tool given only that one specific length which is why I only asked for that one specific length.
I know exactly how the rest of the tool would look when I make mine...
I owe you a second beer now.

[kcstott]

just get the difference between the two lugs and the rest is just clearances.

I think it took me all of an hour to make with adjusting the clearances. I will tell you this. do something to allow you to rotate the gizmo with a tool. like a screw driver slot or weld a socket head cap screw to it. once it's locked in and the barrel is up on it. It can be a little bit of work to remove.