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Hearth Steel Tanto WIP


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A bit over a year and a half ago I decided I wanted to try making some knives out of home made steel. A bit after that I started making hearth steel and after a number of failed attempts, I finally got together about 6 lbs of good material. I have a thread in Bloomers and Buttons for anyone interested in that process, but this thread will be about the process of turning it into a tanto.

 

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The left and right in the first picture are my two best pucks, pretty much 100% of the mass was high carbon steel. The middle turned out to be mostly cast iron and pretty much unforgeable (it actually started melting in the forge, so maybe it would be possible at a lower temperature). The second picture shows all of the material I got up to six folds, the level of refinement where it seems like there is a good sense of how a piece of material is going to turn out and consolidate the most promising pieces. I had a single piece with excellent consolidation and the highest carbon content, but decided to save it until I have more material of that quality. I moved forward with two of the lower quality high carbon pieces and a medium carbon piece.

 

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The last fold on the high carbon material! That makes 11 folds. I was pretty relieved to get to that point. That's flux on top in the second picture, I didn't brush it off in time for the picture.

 

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Making the kobuse construction with six fold medium C in the middle and the "good stuff" on the outside. The tip is cut in reversed so the hardenable material flows around to the spine.

 

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Here is where it stands now! It lost some carbon, so I'm a bit worried for heat treat, but we'll see. This is just a practice for a larger future blade, but still represents countless hours of hearth runs and hand forging. Thanks for looking, I hope to get some more done on this soon.

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Posted (edited)
5 hours ago, Alan Longmire said:

You know I'm gonna pin this once you get more posted, right? ;)

We may have to see if it gets that far, it’s possible I have a problem. 
 

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The tip looks ok hardening wise, but the rest, not so much:

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It’s hard to tell with this preliminary etch, but I don’t know if I got enough hard steel at the edge (there seems to be more along the spine <_<). It’s a shame because barring a few flaws the hada promises to be decent. This is the best of a series of quenches, I think the carbon content may be sub 0.4 which would be fine in a modern steel but is a challenge here. I went to the literature for martensitic sheet steels, which tend to be about this carbon level (lower, actually but they get to cheat with Mn, Cr, B, etc), and picked an austenizing temperature of 1700 F (lower temperatures I tried got less hardening) and quenched it soapy brine with no clay. 

 

Unfortunately I won’t have much time this week, but my plan going forward is to do a higher quality prep followed by a deeper etch and polish to get a diagnostic of the heat treatment. If it’s good enough, great! Otherwise, I’ll grind thinner maybe do a grain-coarsening treatment which would feel pretty weird, but at that point enough I would be grasping at straws anyways. 
 

On a related note, does phosphorus accelerate decarburization? I used globe elevator nails but I had fine uptake. However, the steel seemed to lose carbon pretty quick, though that might just be because I did all of this by hand which meant smaller pieces and slow work. I have some other starting material if that might be the problem, and if it’s just time working I may try to enlist some help striking and/or settle for a lower layer count.   

Edited by Aiden CC
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Such is the nature of homemade steel.  I don't know about the phosphorus. It does things for sure, but I've not played with it enough to get a handle on it. 

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It looks like there may be a hamon in there, but it’s very weak, likely shallow, and IMO bot quite right:

 

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This is after four or five etch and polish cycles with ferric chloride. The hamon isn’t really visible unless the light is just right. It’s possible I over etched it and hid the hamon with the hada, I may have time tonight to grind deeper on the other side to see if it gets any cleaner with some more depth. I have enough material to make another low layer count blade, and I don’t think I’m out of tricks yet for this one, especially if it turns out to be scrap. 
 

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This is my first etch which was likely too deep. It was following a 1200 grit polish. 

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On 3/13/2022 at 12:08 PM, Aiden CC said:

This is just a practice for a larger future blade, but still represents countless hours of hearth runs and hand forging

 

On 3/13/2022 at 6:21 PM, Aiden CC said:

We may have to see if it gets that far, it’s possible I have a problem. 

 

I can honestly say I genuinely feel the sting of it, and you have put a lot more sweat and hours into this than I have on my own kindred project by producing the steel yourself and working it all by hand... 

 

18 hours ago, Aiden CC said:

I have enough material to make another low layer count blade, and I don’t think I’m out of tricks yet for this one, especially if it turns out to be scrap. 

 

I'm rooting for you. The ignorant and possibly crazy part of me is wondering if Japanese smiths ever re-carburized a blade after it had been forged... :unsure:

 

Out of curiosity and if I may ask, where are you located? 

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4 hours ago, Francis Gastellu said:

I can honestly say I genuinely feel the sting of it, and you have put a lot more sweat and hours into this than I have on my own kindred project by producing the steel yourself and working it all by hand... 

 

 

I'm rooting for you. The ignorant and possibly crazy part of me is wondering if Japanese smiths ever re-carburized a blade after it had been forged... :unsure:

 

Out of curiosity and if I may ask, where are you located? 

 Thank you, seeing your project is part of what convinced me to pick this back up. As for location, I'm currently located in Colorado. You read my mind about the carburizing. I haven't heard about Japanese smiths doing it, but it's my next option. I thought about it, and a grain growth heat treatment to increase hardenability would be quite time intensive for something unlikely to work. I looked at some hardenability-grain size plots and I just don't think I have the alloy content to have a chance. My operating theory was that I had reduced grain size with the four quenches and was still getting some hardening, but I did a more thorough polish and I'm pretty sure what I thought was a ghostly hamon may have been just parts of the hada highlighted by over etching.

 

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The second image shows my best carburizing results, granted the carburizing medium was Pecorino Romano cheese (it was a fun project). I know, they should have scale bars but this is an old project and I couldn't find the image, the frame is about 1mm of material and for the record, I didn't polish the one on the left. The right is the source 1018. It may be "cheating," but I think I'll be able to get something going.

 

I have some plans moving forward involving my cast iron (though some may not technically be that). I'm hoping some of the lower C pieces are hot short not because they have more than 2% C, but because of incipient melting of non-equilibrium eutectic. I haven't run the numbers on it, but in solidification, you often get some eutectic below compositions where you think you wouldn't due to the rate of diffusion in the solid. I had some pieces that I worked "low and slow" which didn't crumble and I'm hoping I can be careful and make a super high C stack. A homogenization treatment may remove some non equilibrium eutectic as well. I may also try grinding some of the iron up as a carburizing flux additive.

 

 

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It lives!!

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No need for squinting or a nice polish, you can see the hamon straight off of a sharp 60 grit belt.

 

How did it end up working? I cheated, sort of. I had the space and money for either a press or an electric furnace and I chose the furnace. Not having the former likely got me into this predicament, so maybe it's alright I used the latter to get out of it :P.

 

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I'm no welder (and my tiny flux core machine makes the lights go out every so often), so I folded up up some stainless foil. I've done this before and it works great. You get a flexible gas envelope that you can make in 5 minutes and open with scissors. I filled it with crushed charcoal, left a little large to leave some oxygen to make carbon monoxide, and added a piece of leather because that apparently can help. Antacid tablets would have probably been better (calcium carbonate), but a back of the envelope said the evolved gas might have popped my steel balloon. As luck would have it, I have a colleague researching the differential hardening of carburized parts so I got some advice and light reading. I settled on 1700 F for 150 minutes to shoot for about 0.6% on average in the piece, possibly much higher at the surface. I also wanted to stay below the temperature where grain growth really starts to sky rocket for low-medium carbon steels. The numbers I found were for industrial gas carburizing, and I assumed that the CO content of the atmosphere would be a bit higher in my packet than the amount typically used in industry.

 

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I put it in the furnace cold and carefully watched over it until it was up to temp to make sure I didn't make a mess. You can see home much it expanded from the combustion of the charcoal and expansion of the gas inside. It let out a tiny bit of smoke in the beginning, but otherwise was fine. You can see that even after two and a half hours that the foil remained remarkably intact. I can't remember if it's 304 or 316, it's whichever one is sold as the "high temp" heat treating wrap.

 

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The clay was a mixture of Satanite and anti-scale compound. The ATP covered up some of the clay and made it look straighter, but I actually set it up for somewhat of a midare hamon, as you can see in the actual result in the first picture. Some ashi as well, this should be incredibly shallow hardening and I would love some activity after all of this work.

 

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A fair bit of sori for a small blade! Definitely managed to put the fight back in this one. On the right you can see the reason you don't normally do this; uneven absorption of carbon leads to differential hardenability which caused a decent warp. I'll be careful with it, but differentially hardened blades tend to be forgiving with straightening.

 

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The first photo is my trying to show the post-carburization sparks of the knife, but it didn't come out too great. I also tossed a piece of six-fold hearth steel I had forged into the wedge for a welded kitchen knife blade (more on that project another day). You can see the sparks for it in the second photo much more clearly since I pushed it into the belt a bit harder.

 

I hope to get the blade finish-ground next and then finish the habaki soon so I can do a final polish. I have plans to make simple kaiken mounts for this one, in a material meaningful for the eventual recipient, so if the blade doesn't look wonky from the carburizing I may actually be able to finish it out soon. Thanks for looking!

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On 3/16/2022 at 8:17 PM, Aiden CC said:

It lives!!

[...]

I settled on 1700 F for 150 minutes to shoot for about 0.6% on average in the piece, possibly much higher at the surface. I also wanted to stay below the temperature where grain growth really starts to sky rocket for low-medium carbon steels

 

This is excellent! It may not be what you intended to do originally, but the fact that you were able to save the blade at this stage is, in a way, even more impressive to me than the already staggering amount of manual work that you've poured into it. It is also incredibly valuable information for anyone who someday finds themselves in a similar predicament. Great job, sir, I am looking forward to seeing this blade in its polished beauty!

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6 hours ago, Francis Gastellu said:

 

This is excellent! It may not be what you intended to do originally, but the fact that you were able to save the blade at this stage is, in a way, even more impressive to me than the already staggering amount of manual work that you've poured into it. It is also incredibly valuable information for anyone who someday finds themselves in a similar predicament. Great job, sir, I am looking forward to seeing this blade in its polished beauty!

Thank you! I would like to eventually get a finished blade without a "boost" at the end, be it starting with the almost-cast iron, making shear steel from wrought iron and folding it to a very high layer count, etc. That being said, I'm very glad this worked! I guess it's a pretty niche situation to be in, but if anyone else has a decarburized blade and an electric furnace, maybe this can help them out!

 

Also, thank you for the pin, Alan! I have plan for a second blade from lower layer count, higher C, material, if that welds up ok I'll post the progress here as well. Without further ado, here is the blade post-polish:

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I tried all sorts of different combinations of lighting and backgrounds, but the best shots of this hamon came when I was carrying the blade around and just happened to see it pop out to me. A gentle midare with some light activity, it's a bit shy but holding it in hand you can see more the longer you look. Many nihonto have higher contrast, but there are a significant number (especially those without a hadori polish) that have a more ethereal hamon than this. There are some bright patches of hada, my theory is that they are regions with more or less carbon than the bulk inside the edge region and hardened to a dual phase microstructure (either martensite + ferrite or martensite + cementite) due to incomplete austenite transition, making them come out bright in the polish. I quenched at 1550 F, higher might have alleviated this. 

 

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Here it is out in the sun. With modern steel I have struggled to get a good "turnaround" at the tip, but this material did it quite naturally. 

 

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I realized I mostly took pictures of one side, but here is what the other looks like. You can see the bright layers of low/high C hada in the hamon.

 

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I actually started the habaki before final polish, but here is the process. I used 0.125" copper for it, substantially oversized for some margin. The tool in the picture on the right is a super nice one to have for this and a ton of other things. It is a harden steel punch, 0.19" thick with sharp edges and a gentle radius in the profile. I'm not sure if this is traditional, but contact on two points is much more reliable than trying to get two faces to meet. This tool forces a concavity to the habaki spine side which makes fitup much more reliable.

 

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I didn't get more pictures of the machigane (little wedge that goes on the edge side of the habaki), it definitely is a pain to make. I like to hammer the machigane stock up to the limit of work hardening then finish shaping, and anneal the habaki before the final bend. This means you can really hammer on it and get the jacket to flow around the wedge, as in the second picture. This is maybe a bit of an amateurish way to do this, but it gets reasonably tight joint. 

 

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The habaki is soldered with hard silver solder and then ground down to shape. I also drilled the mekugi ana which is quite off center (it was supposed to only be a little off center) due to the drill wandering down the taper of the tang, despite the use of a center drill to spot the hole. The actually mekugi will be smaller than this hole, so it should be ok on the final handle.

 

That's what I've got for now, thank you for looking!

 

 

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that was good thinking to carburize and re-heat treat. Nice move. The blade is small enough that you pulled it off. The blade had some great hada, too. nice.

please visit my website http://www.professorsforge.com/

 

“Years ago I recognized my kinship with all living things, and I made up my mind that I was not one bit better than the meanest on the earth. I said then and I say now, that while there is a lower class, I am in it; while there is a criminal element, I am of it; while there is a soul in prison, I am not free.” E. V. Debs

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Thanks for the kind words! Hopefully this will be done shortly. I have been deliberating about how to finish it out, but I think I have my material and design. Admittedly, I may have let myself get distracted, but I swear I'll finish the carburized blade first! I decided to forge another tanto blade, this time using my highest quality 6 fold material with a medium C 6 fold bar for the core in an effort to see what I can get just from hearth carburized material.

 

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Based on my experience with the last one, I forger the sunobe longer and narrower. This was also generally more steel, shooting for an 8-9" blade, more akin to a "real" tanto blade length"

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Here it is after forging and then with the clay added. Since the hada will be very "crunchy" (approx 240 layers vs 2-4k for the first one) I decided to go for a suguha hamon to not make something too busy. The blades that @DaveJ makes use this type of hamon to wonderful effect, and I'm hoping that maybe this lower layer material will look somewhat like the antique shear steel he uses.

 

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I ended up getting a lot more sori than I planned for! The profile is based off of a kata of an old tanto, I may need to adjust my design a bit. In the second image you can see the hamon, or at least I think you can. There is some worrying stuff up by the spine as well. I guess we'll see how this cleans up :blink:. The plan is to practice making tanto koshirae, I have had mateial prepped for a fuchi-kashira-kozuka set for a long time but haven't formed any of it yet due to not knowing what size blade they might go with and had gotten stalled out in banging out some useable hearth steel. I guess I need to forge a kogatana too if I choose to go for the full ensemble. This one could end up being a long haul.

 

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I haven't forgotten about mounting the first one! It will be a gift for the person who often lent a hand collecting and processing fallen wood when I was in New England, so I thought it would be fitting if her knife used some of that material. This is is piece of spalted crab apple collected last spring. The activity is concentrated around the exterior, but it really is stunning. The plan is to cut it so that the omote side showcases the spalting, which should also let me hide the seam in the more homogenous region of the wood. Normally I would split the material for a construction like this, but because the grain isn't consistent and I'm so close to the center of the piece, I'm a bit worried about it splitting in a funny way so I will probably saw and plane it. The plan is not quite a shirasaya, more of a kaiken concept; "practical" mounts (i.e. not too big to hold comfortably) but slim and simple. If I can get away with it, no seppa interrupting the flow of the wood.

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I haven't chimed in on this project, but I have been following along. Quite frankly, it has simply boggled my brain watching you work through the complexities of the materials and processes (and I get confused easily with all the Japanese terminology). You have been extraordinarily persistent in showing your ability to overcome. Control of the process is where mastery begins. Amazing work sir.

Edited by Joshua States
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