I've noticed this before, and shrugged it off. I did some powerhammering tenon-type operations this week and it happened again. Using 3/4 inch round, and laying off a bit of the end to make 3/8 inch round tenon, occasionally a hollow void will show up in the center of the bar.. about the size of a pencil lead. I assume it's from taking a few extra blows at a fading heat, but I'm pretty careful about putting back in the forge when it's still red. Anybody else notice this?

Try beveling the very end first, either grinding it or hammering it if you can.

The force of the blow has to be sufficient to make it to the middle of the bar so the outside surface is flowing faster than the inside and a tube or seam is formed. Normally it's a classic symptom of not enough mass and although you are using a power hammer (you didn't say what size), it may still not be enough force to penetrate to the center of the bar. Another possibility is that the material is folding axially so a tube is formed along the centerline - if this is happening, you should see a seam or cold shut along one side.

Pictures are worth a thousand words and may show something you think inconsequential that may give a useful clue.

Forgive me for asking if it seems I'm teaching my grandmother to such eggs, but are you giving the iron adequate soak time? It may be that the outside of the bar is moving faster than the inside because it's significantly hotter.

It very well may be a soak time issue. I'm using an 80# mechanical hammer with flat dies, hitting hard to rough square, and then softer to octagon, and round. I took a picture of the first one of these I discovered, it's 3/8 round, forged from 3/4 stock.. It's an interesting phenomena.

Yeah, that is interesting. I thought you were talking about a cold shut on the end of the bar.

Not a great idea but have you sliced off a piece of the stock before forgeing?

Starting with a flat cut end is probably one culprit in this, not enough of a soak is probably another, and if you are just cutting this with the edge of the flat die that would be the other... 3/4 to 3/8 is a good sized drop, and you probably end up moving the steel more than you realize at below a safe forging temp.

Using a spring die cutter to point the end, and a spring butcher to cut your shoulder, and then work your 3/8 tenon. Taking a bit of extra time to set up the job like that would probably fix the problem.

Normally you notice this kind of thing worse on square stock, but you can get it in round too. Think of it as axial fishlips;-) I think pointing the end will fix a bunch of this, then butchering in the shoulder, sets things up so they squirt the right direction... Just a guess, from listening to Clifton Ralph (haven't made the tooling either, always in a hurry and just rough it out on the dies too ;-)

Well, let me say this about that. What you have is a pipe. Pretty rare to see that in mild with so little working. I'd check a fresh bar real close too, it can happen in the rolling process. They do it on purpose to start seamless pipe. Usually caused by working the piece "round and round", but it sounds like you did a proper SOR (square, octagon, round).

Through heating: The idea that a bar can be hot on the outside and colder on the inside is a TOTAL MYTH! Even with my induction I can show you just how fast the heat conducts to the center. With fuel heating it is impossible! Carpenter Technology (Carpenter Tool Steel) studied it quite thoroughly. From their book “It has always been assumed that a piece of steel heats much more rapidly on the surface than at the center, and that outside might be fully up to temperature while the center was still “black”. This is not true. Regardless of the mass of the piece, when the entire outside surface is up to heat, the center is up to heat also."

They put a pyrometer on the surface of a 3” Diameter bar and buried one in the exact center. Throughout the entire heating cycle the center never lagged behind by more that 30 degrees! Now this was a 3” round where you might reasonably expect to see the center heat more slowly. Steel conducts heat very rapidly, especially compared to the relatively slow input of heat from a fuel forge.

In reviewing your original post, as you've seen this occur before, it must be your forging technique. Are you forging it all the way down to 3/8 square then 3/8 octagon? Are you trying to get it in one heat and letting it get too cold? I've abused a lot of steel and rarely seen a pipe, and never in mild steel..

Through heating: The idea that a bar can be hot on the outside and colder on the inside is a TOTAL MYTH! Even with my induction I can show you just how fast the heat conducts to the center. With fuel heating it is impossible! Carpenter Technology (Carpenter Tool Steel) studied it quite thoroughly. From their book

Through heating: The idea that a bar can be hot on the outside and colder on the inside is a TOTAL MYTH! Even with my induction I can show you just how fast the heat conducts to the center. With fuel heating it is impossible! Carpenter Technology (Carpenter Tool Steel) studied it quite thoroughly. From their book

Are you forging the bar down to the 3/8 before turning it and forging it square? If so that could be your problem.

Thanks Grant, I'd never heard that about soaking in heat either.. I've talked to you a couple times in the past few years at hammer-ins, you remind me of a human alka-seltzer tablet, effervescing little bubbles of information... Anyway, I usually guillotine fuller my bar to square dimension plus a 64th inch or so, heat to bright red going to yellow, and Phammer to the square fuller. When it gets to blood red, it goes back in the fire. I've found when forging to dimension, things work better for me to count blows, and I learned from Mark Aspery, forge all appropriate facets. I set my hammer up for about 260 beats/minute. The typical scenario goes: pull from forge, hit 3 times, spin 180, hit 3 times, spin 90, hit 3 times, spin the last 180, hit 3 times, check with calipers, back in the fire. Hitting 3 times per side is probably the only weird thing I do, I need to get pretty coffee'd up to spin the piece after every blow. When the calipers say I'm to the square dimension, I hit to octagon, same regimen, but softer, and maybe 2 blows per cycle. When i'm happy with the octagon, I spin to round, indexing a little bit after each blow, and helixing out of the dies. The piece in the picture was maybe 8 inches of 3/4 hot roll round, I was doing some ball and stick coat hangers. I forged the ball in a homemade trough clapper die, both ends, forged the remainder to 3/8, and then cut in half to get the right length. That's how I discovered the pipe effect. Last week when it happened again, I was making some reproduction horse-drawn wagon U-bolts for the leaf springs up front. I started with 3/8 x 1-1/4 HR flat, guillotined an inch and a half from each end, and forged those out to 1/2 inch round, so I could thread the ends. The pipe effect only happened in one out of four tenons, I found it when cutting the tenons to length. It must be something in my moves that caused it.. I think it's something to do with caffiene deficit early in the morning. I also wonder if this happens to other folks, but they don't have the occasion to cut stuff in half when it's done.

The defect shown, as Grant noted is forge pipe. In this case, it is almost certainly caused by the rounding up process. If the corners of the square/octagon are not perfectly in line under the dies, a shearing action is created at the center of the bar and, depending on the grade and type, this will result in the centers being torn apart. This can be minimized by forging corners at a high heat and then rounding up in a V spring swage.

Grant, I must disagree with the idea that the surface and center of a bar will not have significant variation in temperature. This may be true on a relatively small size like 3", but it is abosultly NOT true for large scale forgings. A surface temperature of 2300 F, does not mean you have a core temp any where close to that. It is common practice to allow large billet and ingot to soak at 2300 F for 10-30 minutes per inch to insure uniformity of temperature. This has been verified experimentally and can be shown mathematically by looking at the thermal conductivity of a given material. If steel is not heated evenly, you can get both bending and piping conditions. I have seen both, but I am talking about large scale stuff. On the small scale I'm sure you are correct.

I'm sure you're right, Patrick. The fact that the cited experiment was done on three inch material does not mean it scales UP, it does mean it scales down though, which applies to most of these folks. Yeah, you're working with REALLY BIG STUFF, often measuring the diameter in feet. If they had a 30 degree lag in three inch bar, you could easily have ten times that (300 degrees) on a 30 inch bar. Probably not linear either, so the difference could be even greater.

Hey guys, notice he says a relatively small size like 3"? Just shows ya gotta be more clear when you say things like big or small. All thing are relative, aren't they?

Edited July 14, 200916 yr by nakedanvil

Patrick has sent me pictures of them working 40" dia stock at Scott Forge; gotta love a set of tongs you *drive* around....;so 3" is small stock to them...

This has been extremely informative. Thank you all. I'd like to learn more about the effect of using a V-block or half-round bottom fuller, as compared to flat dies and the SOR method. Maybe Mr. Asbury can help out.

Mike: You're probably right about it happening a lot more than people think, they just never see it. Caused by that incremental turning you're doing. You should be finishing with a swage (OCP part # swrd-016). Rarely a problem except in structural work.

You knew Grant was going to try to sell you something didn't you? He even said learning and making money are his two favorite things. He didn't appear to be learning much from this thread, so he might as well be making some money!!! No offense Grant, I've met you too and agree with Mike that you're like a human alka seltzer.

Update to my earlier comment disagreeing with Grant on differneces in temp accross large cross sections:

I was just shown data this morinng froma 10" square that agreed with Grant's statements. The differneces on heating were not greater than about 40-50 degrees F. I still think that that differences will be more pronounced in larger sections.

Patrick

Patrick: Thanks for the update! I don't know, 10" is getting up there. There are thing that just seem like they should be one way, but good data is hard to argue with. Large billets DO take a long time to heat, but they stay black on the outside for a long time too. Where is all that heat going? Well, I guess it's being conducted to the interior of the bar faster than the surface can be heated. The conduction rate of steel must be greater than the ability of the furnace to transfer heat to the surface of the bar. Hmm, I can believe that.

Update to my earlier comment disagreeing with Grant on differneces in temp accross large cross sections:

I was just shown data this morinng froma 10" square that agreed with Grant's statements. The differneces on heating were not greater than about 40-50 degrees F. I still think that that differences will be more pronounced in larger sections.

Patrick

That's pretty interesting. I would have assumed there would be a greater difference also. I don't think you will find much of a difference with a larger section going by information. 1", 10", 100", 1000"... What's the difference when you put it in something sufficient enough to bring either up to temp?