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I just dropped off a #250 block of my 4340 at a pro heat treat shop to have one face hardened for a buyer. The guy said "we will be "flame hardening on Friday".  Oh, I said how does that work? He explained they have a torch that is rigged with a water hose. He seemed in a hurry so I didn't try to get more info. Wondering if it might be robotic. 

 

I hadn't thought about this being a professional process. In fact after several years of being around I know of only one fellow who ever played with this. Some years back Alan did a long blade with something like this. At the time I just thought it was a brilliant makeshift solution. Started doing a little research, finding that flame hardening is a widely used industrial process. 

 

So the question is: Is this something we could do in our own shops? Not necessarily thinking blades but tooling etc. that is too big for normal ht.  In my case thinking of 6.5" X 10.5" inch blocks weighing well over #100. To harden one face.  Anyone have any experience or ideas?

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You know those really big excavators like this one?  Hitachi Excavator.  I used to work at a foundry that made some tracks shoes for things like that.  The kind where each track segment (track shoe) weighed a few hundred pounds.  The section between the lugs was flame hardened to increase the wear resistance there.  It took two specially build 3-rosebud heads (so 6 total rosebuds).  Each torch had to be hooked to an array of 9 acetylene bottles (a 3x3 grid on a pallet for mobility) otherwise the draw on the tanks would be too much and it wouldn't work.  This was done over a water thank that the track was then lowered into.  If I remember correctly, the alloy there was either 4330 or 4340.  The better option for this is induction hardening, but that is not a cheap system to set up and takes a bit of floor space, and it just didn't seem worth it for the quantity produced.  I have seen it done with an induction coil and was 100% automated, including the coolant hose following right behind the induction coil.  That was done in Australia, and was very cool.  My plant tour guide didn't seem to be that interested so I only watched 2-3 parts done before we moved on, but it was awesome.  

 

All that said, it really depends on what kind of depth of hardening you really want to shoot for, and how many oxy/acetylene torches you want to run.  If the alloy is right, quenching is easy, but the heating can be a challenge still.  All said and told, I would imagine it is still cheaper to pay a professional shop to do it.  Unless you had enough to do to justify some equipment upgrades.  

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Thanks Jerrod, We value your experience. And yes that is a huge machine.

I saw a couple of videos of the induction system on smaller parts, and yeah it looks expensive.

 

The pro shop is doing one face for $75. I understood 1/8" deep.  I guessed if I tried it with my biggest rosebud (3/4") it would burn most of that in gas.   But I was thinking heat and quench. I had no idea how the pro planed to do it when I got the quote on the phone. I hope on pick up I can get more info and maybe get to see their set up. Then I may experiment. The torch and water at the same time in a connected unit was a new idea to me until I remembered Alan's experience. But I do realize when he did an edge it was a very different application of the process.

 

I hope you're safe from the fires out there!

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I'm glad Jerrod chimed in, because I have no experience of how the big things are done.  I knew it was a bunch of rosebuds ganged on a sled with a water spray follower, and I know that with the three or four torches you and me can wrangle on short notice that's still not enough to do one of those blocks of 4140...;)  especially if AirGas won't give us bigger cylinders.

 

The piece I did is on here somewhere, it was a big single-edged sword.  I rigged up a mister that blew water on both sides of the blade, and a guide block to keep the blade level.  Heated a couple of inches of the edge to critical and then started slowly running it into the mist to quench it.  Finding the right speed was not easy.  I tried it later with a thin double-edged blade and found out just how bad an idea that was.  The thick single edge survived only because it was thick and single-edged.  I got the idea from Brian VanSpeybroek, the only person I know to have ever gotten an actual hamon on 5160 using the same idea.

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OK, I think I may be getting the picture. Are you guys saying the whole face needs to be done in one pass? (bunch of rosebuds ganged)  From the brief description I got from the guy (including hand motions) while he was on the running tow-motor. I kinda pictured covering the face in overlapping parallel lines, with one torch closely followed by a water flood. I even pictured this being hand operated, sounds like my imagination is way off. 

 

I may have to take doughnuts and/or beer when I go pick that piece up to get to see the operation! Another thing I saw there that was interesting. The shop seemed about the same size as mine but there were multiple transformers and the electric lines serving the little building looked about the size of my leg.

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On a face like that I would think you would be better off heating it all up in one go with flame, then quenching it all at once.  If you were doing induction it would be easier to do it in one progressive pass, but I suppose it is possible to do with flame too.  Doing multiple passes would be less than ideal as each subsequent pass would mess with the heat treat of the previous pass.  This would be very hard to do hand-held.  If you had a method to control the torch and quenchant line travel that would be best.  

 

And doing the 2 torches with 3 rosebuds each was the most efficient we could come up with.  The process involved moving the torches 3 times to get the whole area needed (heat area, rotate to new area, go back to the first area, revisit second area, quench).  Total torch time was only 3-4 minutes.  We designed a torch head that would have done it all at once with 8 rosebuds, having 3 different sizes, but they couldn't come up with a way to get us enough gas without a major investment.  

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Thanks Jerrod,

It's becoming clear why in over 30 years I've never heard of a blacksmith doing this or seen any articles in any blacksmith publications. It's just too expensive for a small operation.  And way too technical for a backyard operator. But it is an interesting thing to learn about.

 

I'll do my best to see the set-up when I go back!

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Just picked up! No testing yet, still in my truck. I did see the tool. A by hand operation, not even a rosebud, good sized welding tip followed by a water flood. 

 

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Well I banged on it a little. It is noticeably harder! Is it $75 harder is yet to be determined.

Edited by Matt Walker
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It would be interesting to find out if there are some sections that are harder than others.  I would suspect there are some softer sections in between the obvious paths there.  Probably/hopefully not enough to worry about, but possibly noticeable.  

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I suspect you may be correct Jerrod. The problem is I don't have a hardness test system other than a center punch and I kinda hate to ding up this piece because it was done for a friend. After seeing the simplicity of the set up, I'll have to do some experimenting. I can work with a test piece and regrind as necessary until I figure out if if is worth the effort. Any suggestions for a low tech/cheap hardness test method? I have a friend who owns a ball drop scleroscope but I don't think he would sell it and I searched for one a while back and failed to find a simple one.

 

They had already unwired the copper water tube from the torch when I got there. But I saw it! This shop has been in business current location since the 1980s. Not sure how long before then. The guy said their father taught them this technique. They have other modern high tech equipment in the shop. 

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I'd bounce the ball of a ball-peen hammer around on it.  If you can't find a difference that way anywhere on it, it is probably good enough.  If that surface was going to get constant wear, like if it was a chute liner for aggregate, any soft spot could be hugely problematic, because any uneven wear leads to a lot more wear in that location which causes early failure.  For an anvil like this I would think a hammer test would be sufficient.  

And when I say bounce a hammer over it, I mean start lightly and increase in force as you feel confident/necessary.  Watch for any marring of the surface as well as rebound.  

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Thanks Jerrod,

All good points. I really can't find much difference with a hammer. A hard ball (1.25") drop beside a crudely calibrated 10 " stick is showing some spots a little harder maybe. 

 

Here is what I'm seeing: As cut and cleaned face 30% rebound with minor denting.  If I work over an area the size of a quarter with a hammer for a minute or two I get +40%. On the treated piece I'm getting 50% to 60% and no deformation on the treated piece. So yeah, I think it's probably is worth doing for a guy with a torch already or a person with a few extra bucks.  On the other hand I think this stuff is very serviceable with a little time spent work hardening the surface. 

 

 

 

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