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Tamahagane tanto wip


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Posted (edited)

As some of you have seen in another thread, I find myself preparing to move back to France, and have started selling my shop. The truth is, it will take a long time for me to rebuild a shop there, likely more than a year. In the meantime I'll have to work on smaller projects and will probably end up focusing on engraving and irogane.

 

In spite of being quite busy trying to empty the house these days, I'll be doing my best to complete the forging on this piece before I get rid of the tools that make it possible for me to do so efficiently (mainly the power hammer at this point).

 

I'm at weld #8 and the billet now feels very good.

 

bsTvUzE.jpg

 

mdRcZ5F.jpg

 

821g (~500g loss in 8 folds, confirming a bit over 5% loss per weld)

 

I also consolidated the 3rd stack, the last of my raw tamahagane. This was the first time I did so with a press and let me just say... oh my god, how incredibly easier this was! 

 

uT9omHJ.jpg

 

Obviously my experience with all this is quite limited, but if I had any advice to give to anyone wanting to work with bloom more than once, it would be to invest in a press. It was night and day, the initial consolidation took literally 10s with no loss... it made this very tricky stage entirely trivial.

 

Edited by Francis Gastellu
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I really struggled today after developing cracks on the corners of the billet after weld #9. I know for sure that I wasn't forging too cold, so it's not entirely clear to me what I was doing wrong... As mentioned before, I've been resetting the grain after hot cutting for most of the welds by quenching the billet in water. This always leads to some light cracking in the corners, but that normally heals on the next weld with a light pass on the bias at welding temp. I suspect that this time I did not do a good job at this and the cracks continued to propagate while drawing out. 

 

I did my best trying to fix this for next to an hour. At some point I had to stop fighting because it was clearly getting worse. I let the billet cool down and ground out the cracks. I lost a lot of material, I'm at 644g (20% loss! ouch)

 

I squared the billet, let it cool again, cleaned up the sides for inspection, and decided that was enough folding: I like what I'm seeing and I don't have much time left before my move, so next session will be kobuse.

 

iE5DmyM.jpg

 

z3q0nPE.jpg

 

FqKPJ7M.jpg

 

I am very happy with where I'm at with carbon content. Although I've been very careful to control the atmosphere of my gas forge, nearly all the credit goes to the power hammer for allowing me to get to this point in spite of my many mistakes and struggles.

 

8aqSpDN.jpg

 

 

 

 

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Your material is looking good! You definitely have cleaned it much more meticulously than I have for most of mine. I hadn't heard of quenching during the folding process, definitely an interesting idea, though not without risks as you have seen. Your engraving and inlay for past projects has been super impressive, I'm glad to hear you have plans to keep going with that after the move. It is definitely much more portable than bladesmithing, I managed to put together a setup to practice it that fits in a suitcase and weighs less than 50 lbs for when I was away from my shop for long periods. I have had a lot more shop time lately than I originally thought I would, and admittedly my kinko journey has stalled somewhat as a result!

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Posted (edited)
On 3/23/2022 at 9:08 AM, Aiden CC said:

Your material is looking good! You definitely have cleaned it much more meticulously than I have for most of mine. I hadn't heard of quenching during the folding process, definitely an interesting idea, though not without risks as you have seen. Your engraving and inlay for past projects has been super impressive, I'm glad to hear you have plans to keep going with that after the move. It is definitely much more portable than bladesmithing, I managed to put together a setup to practice it that fits in a suitcase and weighs less than 50 lbs for when I was away from my shop for long periods. I have had a lot more shop time lately than I originally thought I would, and admittedly my kinko journey has stalled somewhat as a result!

 

Thank you for the kind words, Aiden. I do love working on engraving and inlays, I find it extremely zen, and it is something I am very much looking forward to focusing more on (though if I'm honest, looking at the prices of power hammers in europe is starting to make me rethink the idea of selling & rebuying, versus increasing the size of the container I ship...).

 

Quenching during folding was advice given to me by a smith with seemingly good experience forging tamahagane, I'm not sure if this is something traditional smiths do. I have not seen it mentioned before either, but for sure plenty of details are left out of the process as described in the various resources I've been using (mainly The Craft of Japanese Swords, and all the videos and threads I can find online), so it's not out of the question I suppose. His reason for it is that traditional steel grain grows like crazy and needs to be reset often, and this doubles as a means for impurities to escape on the next weld (he does it after right after hot cutting and before the subsequent fold/weld). YMMV and agreed, it does have its risk, though I'll be a lot more vigilant now that I've had this issue.

 

On 3/23/2022 at 1:28 PM, Kreg Whitehead said:

For some reason I can see any of the pics.....maybe its my security settings here at work.

 

Sorry about that Kreg, and you're probably right about the reason, they are hosted on imgur, which is a public image hosting service, and I could see a business not wanting to take the risk!

Edited by Francis Gastellu
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This piece of steel looks extremely good at this point. Rock on!

 

On 3/23/2022 at 10:25 PM, Francis Gastellu said:

rethink the idea of selling & rebuying, versus increasing the size of the container I ship...).

Yeah, that's a good point. You also need a way to move and store the equipment when it arrives.

Edited by Joshua States
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“So I'm lightin' out for the territory, ahead of the scared and the weak and the mean spirited, because Aunt Sally is fixin’ to adopt me and civilize me, and I can't stand it. I've been there before.”

The only bad experience is the one from which you learn nothing.  

 

Josh

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Posted (edited)

Making a hot dog.

 

y7rsJlX.jpg

 

Yes, I am using a rebar as my handle. Sacrilegious! Ultimately none of my handles have been tamahagane, because I'm just not that tamahagane-rich. The nice thing about crappy mig welds is that they quickly break off clean. The not so nice thing is that you have to reweld handles often.

 

dQvFUGS.jpg

 

jz7V5kN.jpg

 

I thought I was clever and made my core steel a bit long so I could hold on to it more easily with pickup tongs while joining the two pieces.

 

The excess got pushed to the side of the handle and messed up my geometry. The soft spine ended up moving off axis, and I'll need to correct by grinding before sunobe. More material loss...

 

I'm thinking I might have my handle on the core steel next time, though it'll be a bit trickier to fold the edge steel. Or I could try having one on both, but since I enjoy having fingers, I'd want to bend the core's handle, up or to one side. Or I could just keep doing it this way but trim the core to size... I'd love to hear what other people do!

 

oSVWJkp.jpg

 

5Mnyk6E.jpg

 

You can see the right side edge steel is a bit thinner because of that.

 

There's a bit of a questionable spot at the bottom left, but I'm choosing to believe that this doesn't go very deep, and/or that it's ok since this is nakago-side.

 

XefuEzK.jpg

 

The converse kissaki-side: more edge steel on the right than the left. I'll be thinning the right side by the spine and re-square to compensate.

 

KDnNV1i.jpg

 

With those corrections, I think I should be able to work with that....

 

 

...

 

wait..

 

...

 

well, I'll be damned.

 

R8u0ZGd.jpg

 

xe6XxUO.jpg

 

Those look eerily familiar... :(

 

So, given that I was never *anywhere near* forging too cold, I think my first failure wasn't that either. I might have a problem.

 

A couple theories:

 

1) Since those didn't show up at all until I etched, I think they are decarb shadows of prior cracks, likely from quenching. Assuming I grind them out (I have plenty of meat to do so), I'm hoping this will not recur. Since I had not ground the sides before after kobuse and before sunobe on the first blade, similar cracks might have propagated until they finally became obvious by the time I was done beveling. Recall that on the first blade, I quenched *after kobuse* because I wasn't able to fix a blister any other way... I think I just did not get away with that.

 

2) Or much less likely, could my steel be *too high carbon*? I'm still sparking fairly high. It's a bit lower than in my previous post, but still high. It's similar to 1095 as far as I can tell, but of course this is not a reliable way to measure carbon content. I'm having difficulties believing that this is the problem though, after 9 folds and kobuse, all in a gas forge with plenty of soak at each step.

 

My money is on #1. I do feel like those quenches are helping for the first 4 or 5 folds or so, but in the future that's as far as I'll take them (though I don't know how I would deal with a late blister like I had in the first blade... after all ,this was a hail mary to try and save it... :/ )

 

Comments and advice are welcome!

Edited by Francis Gastellu
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Posted (edited)

I reached out to a former member of this forum whose work with tamahagane we all respect, and discussed my recurring issue. His opinion was that my steel is likely high in either sulfur (red short) or phosphorus (cold short) depending on when the cracks form. Given that, it's likely sulfur since the cracks seem to show up during forging rather than while cooling (the smell of burnt match while forging seems to confirm that too). He also doesn't buy the quenching thing, which at this point I'm assuming just made things worse.

 

He also thinks 9 folds isn't enough for a homogeneous steel, and that's my bad, no one else's.

 

We'll see if I can save this one. If it were monosteel I would simply forge it really thick and grind out any cracks that form during final forging, but given the soft core's thickness in my kobuse billet, that's not going to be much of an option.

 

Learning can be painful sometimes, but that's fine. It only bothers me that I won't be able to try again for a while because of my move. This project has seen my first and possibly second blade failure in 3 years, and I hate to leave it at that for so long.

 

I'll do my best.

 

---

The Río Tinto (Spanish pronunciation: [ˈri.o ˈtinto], red river or Tinto River) is a river in southwestern Spain [...] with very high levels of iron and heavy metals. [...] This area has large amounts of ore and sulfide deposits.

Edited by Francis Gastellu
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I think you might have touched on the issue with the sulfide deposits part of your most recent post, I've never had this issue with my steel even at 1% of above with a finished piece! You could try oroshigane with the steel, though it could be that the process won't remove the sulfur. 

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I never thought about sulfur, but yeah, that'd do it.  :(  I don't think I would buy more from that source unless it comes with a guarantee.  

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It would be a bummer if it's a composition issue, hopefully you can still make something out of this! If the problem is S hot shortness, maybe you could try forging below the melting point of FeS to reduce the cracking? Forge welding may need to happen above that temperature (which looks to be around 1200 C), but keeping the low-melting phase from melting might be able to help. 

 

You might be able to do a bit of failure analysis to see if hot shortness is the failure. A typical "break test" of a brittle metal will produce a fracture where the fracture travels through grains, essentially making a tiny flat mirror out of each one, which appears shiny for large grained fracture surfaces. If I understand correctly, hot shortness from sulfur happens from incipient melting at grain boundaries. This would lead to failure between instead of through grains, which makes a "rock candy" like surface. It may not do this for a cold surface though. For coarse grains you may be able to see the difference without magnification, though I'm not entirely sure.

 

Best of luck, and I hope you can still get something out of this!

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11 hours ago, Emiliano Carrillo said:

I think you might have touched on the issue with the sulfide deposits part of your most recent post, I've never had this issue with my steel even at 1% of above with a finished piece! You could try oroshigane with the steel, though it could be that the process won't remove the sulfur. 

 

Thank you for the suggestion. I looked into it briefly (i admit most of what I read was way over my head!) but unfortunately it seems that sulfur is only marginally soluble until steel reaches a molten state.

 

11 hours ago, Alan Longmire said:

I never thought about sulfur, but yeah, that'd do it.  :(  I don't think I would buy more from that source unless it comes with a guarantee.  

 

Well, since I'll be finding myself in the countryside soon-ish 1) I won't have any more excuse not to make my own tamahagane, and 2) once I do, I'll have only myself to blame if it doesn't work out ;) 

 

8 hours ago, Aiden CC said:

It would be a bummer if it's a composition issue, hopefully you can still make something out of this! If the problem is S hot shortness, maybe you could try forging below the melting point of FeS to reduce the cracking? Forge welding may need to happen above that temperature (which looks to be around 1200 C), but keeping the low-melting phase from melting might be able to help. 

 

I'm after anything that'll give me a chance to make this work, so thank you for this suggestion, that's helpful.

 

8 hours ago, Aiden CC said:

You might be able to do a bit of failure analysis to see if hot shortness is the failure. [...] This would lead to failure between instead of through grains, which makes a "rock candy" like surface. It may not do this for a cold surface though. For coarse grains you may be able to see the difference without magnification, though I'm not entirely sure.

 

I actually cut up a bit of excess on the previous billet right after kobuse. I could draw that out to thickness, let it grow the grain, and break it while hot. If it's brittle, that shouldn't be an issue!

 

8 hours ago, Aiden CC said:

Best of luck, and I hope you can still get something out of this!

 

Thank you! I think catching it at this stage this time around might have given me a chance. What got me the previous time was the depth of the cracks, as some went all the way. I might be able to minimize that somewhat now and grind my way around what's left. We'll see :rolleyes:

 

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Posted (edited)

I did several rounds of grind/etch/grind/etch until all the cracks were gone, then fired up the forge and squared the billet.

 

XzRUQiT.jpg

 

As an aside, I would normally have stopped there to do some volume calculations for sunobe and I would have made one of these:

 

tXxcR9W.jpgMYj08BT.jpg

 

I started making those templates for thickness and width a while back because I was tired of messing with calipers while forging; they're convenient and would potentially allow me to make the same blade geometries again.

 

I wasn't really sure where I was going on this one because of the cracking issue, so I decided to reuse the machi and kissaki gauges (first and last notches) of a wakizashi template to shoot for a significantly thicker/wider than usual sunobe for a tanto, and I let the length be what it needed to be accordingly.

 

Following Aiden's suggestion, I kept the forge temperature well  under the melting point of iron sulfide, hoping that this would prevent new cracks from forming. I could not see any by the time sunobe was completed (other than by the handle, but that's just my nasty mig weld). Thank you Aiden!

 

ddEz5mt.jpg1roVreA.jpg

 

I cut the tip and flipped it so it'd flow with the grain, and proceeded to bevel the blade. This is near the end:

 

NrjBzjA.jpg

 

You can barely tell but some cracks are developing on the tip. My probe is out of the way of the burner, and I was careful to keep the blade out of the forge's hot spot, but I'm guessing the thinner cross section went a bit hot anyway. At that point I straightened everything as quickly as I could and called it good.

 

It's definitely a lot thicker than I would have wanted, and that makes for a significantly shorter blade than the previous one, but I think I'm out of the woods. Here's a quick etch before I start final profiling.

 

1rqAkoK.jpg

 

cF38bQm.jpg

 

Look Ma, no cracks! :D

 

I am now terrified of yaki ire :ph34r: Then again the edge is currently very thick (about 4mm!) so I'll keep it on the thick side and hope the bladesmithing gods are kind to me. Anybody know a good prayer?

 

Edited by Francis Gastellu
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13 hours ago, Francis Gastellu said:

Following Aiden's suggestion, I kept the forge temperature well  under the melting point of iron sulfide, hoping that this would prevent new cracks from forming. I could not see any by the time sunobe was completed (other than by the handle, but that's just my nasty mig weld). Thank you Aiden!

I'm glad the temperature control helped! It's odd that you had so much of an issue with it, especially given Emiliano's experience with high S material. I guess it could also technically be carbon hot-shortness if the composition was close to or north of 2%, something I have experienced with my own material. With uneven carbon distribution and fast solidification you might see some ledebubrite (Fe/Fe3C eutectic) forming even below 2% C, which has a similar melting point to FeS. It seems like time at high temperature would homogenize things enough for that to not be an issue though. It might be hard to know exactly what's going on without knowing the chemistry.

 

I'm excited to see where this goes. Good luck with yake ire, I don't have any prayers, I usually just take a single deep breath before the blade goes into the water.

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2 hours ago, Aiden CC said:

I guess it could also technically be carbon hot-shortness if the composition was close to or north of 2%, something I have experienced with my own material.

 

It was one of the theories but it is hard to believe that's the case. The bloom was originally estimated by its maker to be between 1.2 and 1.5% C. I did all my folds in a gas forge and could see a visible reduction in sparks along the way, which ended somewhere similar to the 1095 and W2 I have lying around (my W2 actually has more explosions than either of my billets, and I do have an assay for it that says exactly 1.0%). It's Certainly not a foolproof test but certainly consistent with what I would expect. I'm discussing and sharing photos in PM with the person who made and is familiar with it, we'll see what they say.

 

The smell of burnt matches definitely betrays quite a bit of sulfur though, but I couldn't venture a guess as to how much. At this point I suspect that it would have gone a lot better had I not follow that quenching advice from IG, which truth be told, no one else seems to really agree with. I'm sure it works for that person, perhaps it's simply not something you can get away with at the same time as with the amount of sulfur I have.

 

I don't know if your temperature advice is what did it for me but it's interesting that i started seeing cracks again as the billet got thinner and heated much faster. Maybe I just left some tiny cracks on the tip and they just took a long time to propagate, but for now I'll chose to credit you. I might still take the time to break a leftover piece of the first billet at temperature to see what I get.

 

I'll forge my 3rd stack without any high temp quenching. That should tell me a lot, but it'll be a long while before that happens.

 

2 hours ago, Aiden CC said:

I'm excited to see where this goes. Good luck with yake ire, I don't have any prayers, I usually just take a single deep breath before the blade goes into the water.

 

:) thank you!

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Well, it seems that this project is intent on kicking my behind at every opportunity :P 

 

U02zdWn.jpg

Vgcq0sK.jpg

 

That nie is showing up just enough to tease me! I was aiming for about 1500F but I must have been on the cold side. I'll try again tomorrow and wait for nighttime, daylight made it hard to see much and I had to rely on my pyrometer more than I'd like to admit. I know, I know, another beginner mistake :rolleyes:

 

If anything, this project is giving me a renewed respect for the modern steel alloys I've been using until now, and how forgiving they can be.

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I decided to forego ashi lines this time and aim for a more humble suguha.

 

It almost looks like it'll be alright...

 

ciMY9GC.jpg

 

But nope. Notice how there's no sori at all? Yet this was a full brine quench.

 

nEeCYIm.jpg

 

That black tip is the only bit that's really hard, it skates a 65RC file. The rest of the edge only skates 40, which I take it would have been above the hamon line if there was one. I was in denial for a bit hoping that decarb was messing me up but I don't think so at this point, the tip was probably just hotter than the rest of the blade.

 

I will give it another go before putting this project on ice but I really need to pack the forge very soon, and I'm clearly not very good at doing this by eye (and yes, I have read many a post about decalescence, i swear...!). At this point I feel I had better results with the HT oven, but I realize that's been allowing me to miss out on the fundamentals for a few years now. That was fine with known steels and a definite temperature target, but this is something else and I'm beginning to suspect that I'm trying to bite a little more than I can currently chew.

 

Then again, nothing worth doing ever comes easy.

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On 4/14/2022 at 1:30 AM, Francis Gastellu said:

Then again, nothing worth doing ever comes easy.

This has definitely been my experience with home made steel. Off-eutectoid steels generally need higher temperatures to fully austenitize and with bloom steel etc it’s definitely hard to tell exactly where you are carbon wise. Without Mn etc. you also have lower hardenability and can expect shallower hardening for the same thermal history. Thinning the edge may help with this, but increases

the risk of cracks in the quench, so definitely a trade off and it depends on how much risk is worth it to you (the same goes for higher austenitizing temperatures, more aggressive quench media, etc).  
 

For carbon “hot shortness” (I think it would probably just be called  “incipient melting” now that I think about it), I did a back of the envelope model based on some of my previous work, and with fast solidification/sluggish diffusion, you would expect a decent amount non-equilibrium eutectic at 1.2-1.5 wt %, though hopefully that would be gone with time and deformation at temperature. Not sure if this is the right place place for that figure, but I could post it if there is interest. I have forged steel with this problem and it fails pretty dramatically (sparks and cast-iron droplets spraying out everywhere). 
 

I am no expert in the area, but I like the look of the hada! Hopefully your next yaki ire goes well. For what it’s worth, I think suguha hamon have an unrivaled subtlety and elegance, harkening back to an age of swords with a purpose, so if that’s the way you go I would say it still has significant artistic merit. Best of luck. 

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13 hours ago, Aiden CC said:

I have forged steel with this problem and it fails pretty dramatically (sparks and cast-iron droplets spraying out everywhere)

 

I've had (some, not a lot of) sparking on the initial stacks, as I wanted them well hot for consolidation, but little to none after a fold or two. I've had copious spraying but have no reason to suspect it was cast iron droplets rather than flux and impurities, like I normally get when forge welding wrought iron to high carbon.

 

For the most part those billets have forged fine, other than the cracking, which I expected during folding from the repeated quenches, but did not expect during final forging (right now I think the quenches may have been the biggest culprit, but I should know soon if something else also contributed).

 

13 hours ago, Aiden CC said:

I am no expert in the area, but I like the look of the hada! Hopefully your next yaki ire goes well

 

Thank you, I do wish that big noticeable weld line across the length of the blade was a little more subtle, definitely the result of my struggle at weld #9. It's now part of that steel's history so as long as it still stays together on the third quench, I'll be happy. Well as long as it's hard too <_<

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Posted (edited)

Phew.

 

mfqhgkD.jpg

 

KMy7dmV.jpg

 

Definitely closer to the edge than I'd like, but considering... I'll take it. I see hints of bright nie waiting for a good polish.

 

Quench was in brine, 3 Mississippis in, 2 Mississippis out and repeat. Sori is very slight, a little above 1/16". I also got a slight warp that I'll correct on the second tempering cycle, the blade is currently in the oven at 400F.

 

Clay was for suguha but the tip chose otherwise. My guess is that the carbon content there is a bit more heterogeneous, as it made a very similar pattern on the previous attempt. If I had more time and more guts, I'd go for a fourth yaki-ire a tad hotter on the edge. As is, I'll call it hoso-midare and move on :rolleyes:

 

I actually had to pack the forge on Friday so that I could finally start moving the heavy equipment this weekend, so I made a makeshift one with refractory bricks and a mapp torch. This is actually not too far from what my first forge looked like 4 years ago ^_^

 

1MlOeLv.jpg

 

Other than some preliminary polishing, this project is now on pause until I find myself in France and I've received my shipping container. There won't be space for a shop initially, as we'll be at a temporary location for up to a year while we look for the house we want to live in for the long term (a large barn is on the list of non-negotiables!). I'm hopeful that I'll be able to set up a small space where I can work on fittings.

 

Edited by Francis Gastellu
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Nope.  It is the copper.  See this:

 

The presence of more than 0.2 % of Cu in steel produces a characteristic checking on forging surfaces. The degree of this effect increases with the increase in the contents of Cu and C. The effect also increases with the increase in preheat time and temperature if heating is done in an oxidizing atmosphere. This happens because of the preferential oxidation of iron near the steel surface which leaves a Cu enriched zone containing the low melting ? phase on grain boundaries. In severe cases, the steel has hot shortness and is unworkable. Three solutions available to this well known problem are namely (i) preheating of steel in protective or non oxidizing atmosphere, (ii) carrying out the hot work only below 1090 deg C which is the melting point of the Cu rich phase, and (iii) addition of nickel (Ni) or cobalt (Co) in amounts which is equal to around one third to one half of the Cu content in order to increase the melting point of the Cu phase. The third solution is applied more frequently. For this reason some Ni is added to the high Cu steels.  As the Ni content rises, so does the allowable forging/rolling temperature, although a practical limit in Ni content is signaled by the formation of a protective glaze at temperatures which is more than 1280 deg C.

 

I got that here: https://www.ispatguru.com/copper-in-steels/

 

And this:

 

Concentrations of copper over 0.1 wt % cause hot shortness, a phenomenon leading to surface cracking in hot rolling and forming.   This is from https://pubs.acs.org/doi/10.1021/acs.est.7b00997#.

 

I never suspected your ore would have that much copper, but I suppose if Rio Tinto is gossan ore, it will.  The Ducktown ore body in Tennessee had the same problem.  The surface ore was weathered to really nice iron, but once they got below the weathered surface, the amount of copper prevented use of the iron.  people were bummed until they realized that right under the iron ore was a very large body of copper sulfide.

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1 hour ago, Alan Longmire said:

The presence of more than 0.2 % of Cu in steel produces a characteristic checking on forging surfaces.

 

Yep, looks familiar.

 

image.png

 

It's interesting that this is only the case in oxidizing atmospheres. This could explain why it's not an issue for the smith who produces it, as he works with a traditional japanese charcoal forge, while I use a propane forge.

 

Thank you Alan. Between that and the excessive quenching, I believe I have my answers. It doesn't matter for this blade anymore but it is hard to correct mistakes with only ambiguous data, so this feels like a resolution I can learn from. I'll forge the third stack accordingly.

 

I'll also likely look at setting up a charcoal forge in the new shop, as it's an all around better match for this kind of forging.

 

 

 

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I'm just glad we finally figured out what the problem was!  And yes, charcoal is just all-around better for tamahagane and other homemade steels.  The higher surface area when consolidating them leads to all kinds of contamination issues when using coal or coke, and as we just discovered, higher oxidation in propane when adjusted to welding heat.  The few times I've consolidated bloom steel into barstock I've built a quick-and-dirty side blast charcoal forge to do so.  Literally some bricks and clay on a steel drum lid set atop the coal forge.  

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  • 4 weeks later...
On 4/19/2022 at 7:11 PM, Alan Longmire said:

Nope.  It is the copper.  See this:

 

The presence of more than 0.2 % of Cu in steel produces a characteristic checking on forging surfaces.

And that just answered a question I had since I started forging. I had the same problem every time I use mild steel. Apparently S235 can have up to 0.55 % copper content. 

Jeroen Zuiderwijk

Facebook page: https://www.facebook.com/barbarianmetalworking

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