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David GaŠl




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PostPosted: Mon 23 Jan, 2012 1:54 pm    Post subject: Case-hardening         Reply with quote

Hello

I have read today The Antiquaries Journal volume XXXIII from 1953 it's title is The mail-makers technique by E. Martin Burgess, there is writen:

"The medieval armourer used a high quality soft iron to work with, which is almost unobtainable today, and when he had fashioned it to the shape he case-hardened it. Case hardening was done be surrounding the piece of iron with crushed charcoal packed tightly into an iron box. The box was then placed in the forge and kept at red heat for some time. The carbon turns the outside of the iron into steel, and this layer of steel gets deeper as the process continues. Eventually the iron is steel right through, but this is usually a disadvantage. Iron tools with steel faces are better than tools which are steel right through because these are liable to crack and split. Much plate armour is case-hardened, for the outside is diamond hard while the inside is as soft as the original iron. This case-hardened plate is much stronger and will resist much harder blows than if it were steel right through."

I have some questions about the text:
How big box is used for case hardening? How thick shall the iron box be?
With how much charcoal is the case-hardening made?
Do you have articles, metalographical studies about case-hardened armour plates?
And could you guess the frequency percent of the case hardened plates through centuries?

Waiting for any informations and hoping that you could post some good articles.

David
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Christopher Treichel




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PostPosted: Mon 23 Jan, 2012 6:34 pm    Post subject:         Reply with quote

Case hardening is still done... I do it for muzzleloader parts. The procedure I use for small parts say a frizzen is to take it and wrap it in leather bits and chuck it in a steel can and fill the rest of the can with bones, leather and charcoal. Crush the end of the can and chuck it in a roaring camp fire.... the can should be glowing red hot... let is sit in the fire for about 2 hours or so depending on size. Pull it out an let it cool. Now it will be dead soft anealed. Also having roasted in a oxygen low/free environment it also should not exhibit scale. You will now need to harden the item by rasing it to the critical point and then plunging it into water/oil. After this you will need to aneal it by either raising its temperature to appropriate color change or by allowing it to sit a medium heat in an oven for a few hours to allow the metal to relax a bit.
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Craig Johnson
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PostPosted: Mon 23 Jan, 2012 8:09 pm    Post subject: Re: Case-hardening         Reply with quote

David GaŠl wrote:
Hello

I have read today The Antiquaries Journal volume XXXIII from 1953 it's title is The mail-makers technique by E. Martin Burgess, there is writen:

"The medieval armourer used a high quality soft iron to work with, which is almost unobtainable today, and when he had fashioned it to the shape he case-hardened it. Case hardening was done be surrounding the piece of iron with crushed charcoal packed tightly into an iron box. The box was then placed in the forge and kept at red heat for some time. The carbon turns the outside of the iron into steel, and this layer of steel gets deeper as the process continues. Eventually the iron is steel right through, but this is usually a disadvantage. Iron tools with steel faces are better than tools which are steel right through because these are liable to crack and split. Much plate armour is case-hardened, for the outside is diamond hard while the inside is as soft as the original iron. This case-hardened plate is much stronger and will resist much harder blows than if it were steel right through."


Hi David

One needs to look at this article in the context of the period and the information available to young Burgess when he wrote this.

I would suggest looking at Williams's "Knight and the Blast Furnace" and the Monograph he wrote with another fellow who's name is escaping me at the moment on the "Greenwich Armours". They will address many of these issues. The case hardening appears to have been done less than Burgess is implying and it was far less hard than he indicates. The use of case hardening is part of the story of armor but it is one that is arguably a more after market option than something that seems to have been done as a solid production method.

As to the specifics of your questions there is not a lot of info from the period sources to answer the questions you have detailed but it is a process that is a bit more art than science in the context of medieval usage and you can see by Christopher's description that it can be a bit more of this works do it kind of operation than one that has a detailed set of specs.

Here are some quick stabs at answers.

How big box is used for case hardening? How thick shall the iron box be? Big enough to hold the object and have some charcoal and material around it. Thick enough to not fall apart but not so thick as to use a large amount of fuel to heat.

With how much charcoal is the case-hardening made? Probably about a 1/2 to 1 inch of stuff about the piece probably would work with less but would need to much fuel with more.

Do you have articles, metalographical studies about case-hardened armour plates? See items mentioned above. Also Tylecott would be the author to check.

And could you guess the frequency percent of the case hardened plates through centuries? No way to know. Not enough research and way to many samples lost to time. My guess is less than more sorry to be so vague but I doubt it can be more defined even in a shorter time period.

Best
Craig
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Joel Minturn





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PostPosted: Mon 23 Jan, 2012 9:04 pm    Post subject:         Reply with quote

Craig gave a better answer than I was going to but I was thinking along those same lines.

It sounds like a case of a little knowledge being a dangerous thing. The good author is over stating the effect and effectiveness of case hardening. It is true that carbon does migrate from high carbon area to low carbon area when the temperature is hot enough but its a slow process, maybe .010" to .020" per hour (can't find the actual rate now but it also varies by temperature and the Amount difference of carbon)

Typically a solid piece of high carbon, properly heat treated, is better than case hardened part but the case hardened part is usually cheaper or more convenient.

I'll try to find some details about how to case harden later. Its late and I have to run now.
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David GaŠl




Location: Hungary
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PostPosted: Tue 24 Jan, 2012 12:21 am    Post subject:         Reply with quote

Hello

The Knight and the Blast Furnace by Alan Williams is a good book and I have found it in the google books but unfortunately it's not the whole piece. And I haven't found a shop where I could buy it its out of print and no one wants to sell it, uhm and its not cheap.

I hoped that case-hardening would be a good method to make tools especially some kind of anvils cheaper and easier and with not bad quality. I haven't found yet such big compact steel which I would need in scrap. And this method could be a way to make better quality armour.

David
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Craig Johnson
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PostPosted: Tue 24 Jan, 2012 5:18 am    Post subject: Use of case hardening         Reply with quote

David GaŠl wrote:
Hello

The Knight and the Blast Furnace by Alan Williams is a good book and I have found it in the google books but unfortunately it's not the whole piece. And I haven't found a shop where I could buy it its out of print and no one wants to sell it, uhm and its not cheap.

I hoped that case-hardening would be a good method to make tools especially some kind of anvils cheaper and easier and with not bad quality. I haven't found yet such big compact steel which I would need in scrap. And this method could be a way to make better quality armour.

David


I think you have hit on the strong point of case hardening David. It is best used on the small parts and tools one needs, i.e. the frizzen example above. The larger parts and elements of armor and such are probably not the most efficient use of ones energies.

The comment by Burgess about the armor having a steely skin is probably not true. Not by ignorance on his part as at the time experts where of this mind as the testing method gave an incorrect result on hardness. This is no fault of Burgess as he was pretty young when he wrote this article, nineteen I think, and was working with some of the names in the armor world at the time.

In the period the armor that may have been most affected by the use of case hardening may have been mail as we have a description of how one can harden a shirt. But this would have been a pretty challenging piece to get a consistent hardness on for a shirt that is already made.

Best
Craig
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Gary Teuscher





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PostPosted: Tue 24 Jan, 2012 9:06 am    Post subject:         Reply with quote

Interesting, sounds like case-hardening was a somewhat inexact science, almost like cooking meat. Seems like there could be a great variety in the quality of this, some being "overcooked" and other not cooked enough, going for that perfect blend.
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David GaŠl




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PostPosted: Tue 24 Jan, 2012 9:55 am    Post subject:         Reply with quote

Hello

I have found a quite good table in Alan Williams book about case-hardening and in it stands that 2h on 870 Celsius causes 0,64 mm carburisation in iron. But how much carbon content does that mean in %? Does that mean that if I want to case-carburise a 10mm thick iron at 870C in 2h than 0,64 mm carbon would be in it?

David
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Aleksei Sosnovski





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PostPosted: Wed 25 Jan, 2012 12:14 am    Post subject:         Reply with quote

Usually carburization is used to improve wear resistance. There are also some modern steel that can be hardened only to some medium hardness and when carburized it will combine resilience and toughness of spring core and wear resistance of file-hard surface. Carburizing armor is a viable solution, I have made some carburized pieces and they are very tough. But using spring steel for armor is easier if you can work it hot or at least anneal.

AFAIK carburization speed is pretty much constant given constant temperature and carburizing agent. It is also a very slow process. That all allows this process to be very controllable.Experiment a little and you know exactly for how long you should keep your item hot to get the required depth of carburization.

Carbon content does not matter at all as long as carburization depth is considerably smaller than the thickness of the piece. Starting from about 0.45% carbon content has little effect on the final hardness. And considering that in order to be tough armor and most tools should be softer than the maximum possible hardnesss... In one word, you shouldn't care about carbon content. It is enough to know that depth of carburization is the thickness of the hardenable steel layer on the soft iron.

0.64mm in 2 hours... 0.064 would sound more correct to me. I must look up exact numbers in a book on the subject that I have, but as far as I remember carburization is a VERY slow process if simple carburizing agent such as charcoal is used.
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Johan Gemvik




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PostPosted: Wed 25 Jan, 2012 4:35 am    Post subject:         Reply with quote

A high tempered steel ring (if not made too brittle) will turn a blade edge instead of it biting into it. But then does that mean blades could actually cut soft iron rings, when worn not just draped over a hard stationary object? Putting munition riveted mild steel maille on a cutting block and appying a stout axe and some muscle you can cut it today, but that's not by any means the same as when worn so it can flex some. Perhaps it's not what really could happen, but just something medieval smiths and knights imagined it might (perhaps having done similar chopping block tests) and wanted to temper to be sure it couldn't.

Gary Teuscher wrote:
Interesting, sounds like case-hardening was a somewhat inexact science, almost like cooking meat. Seems like there could be a great variety in the quality of this, some being "overcooked" and other not cooked enough, going for that perfect blend.

Precisely.

To sucessfully box carburize a maille there are several factors to consider.

The wrought iron or mild steel used in making it has to be very pure, or at least to a high degree free from slag content, or there will be weak spots in the material after hardening where the maille will just crack under cuts and blows. Using soft iron it'd bend and save those spots far more, making it useable even with impurities.
A fine pure iron was possible to produce, at least in late medieval times, but would be more expensive than the regular faire. You can see in historic mailles that there were some iron mailles that were pure and low or even no slag content iron, and some that weren't even close to that.

- The box carburization needs to be done right, as in long enough. I don't think this is the kicker though.

- The tempering needs to be high enough to give a high hardness result. But for ayone who's handled just high temperd steel, it can be brittle as glass before you anneal it to spring steel. In this case lifting the maille would have to be done very carefully indeed.

- Annealing needs to hit the right spot. That's tricky with any blade but there you can polish and watch the "rainbow" colors change.
Perhaps it's easier with maille rings to untemper them a bit by just generally heating the whole piece, but you also don't want to lose all that precious hardening or the whole process becomes pointless. A large oven with it spread like a flat bread might work to heat it evenly, but you won't know if it's worked until you test bashing it with a stout stick or a sword afterward. Seems a bit risky with all the hard work already put into making a proper maille. We're talking possibly 6 months to a year of building it turned to shards if the heat treat is off just a little.

The fact remains though that Professor Williams did examine rings that were hard enough to take a chip out of cutting pliers when testing the mailles from the Wallace (as noted in The Knight and the Blast furnace). So someone was doing tempering of mailles back then. I suspect it was very uncommon though given all the extra work and resulting prohibitively expensive maille armour. A king or duke might have one like that though.

"The Dwarf sees farther than the Giant when he has the giant's shoulder to mount on" -Coleridge
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Bjorn Hagstrom




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PostPosted: Wed 25 Jan, 2012 6:38 am    Post subject:         Reply with quote

If you have alternating solid and riveted rings, why not just case harden the solid ones before weaving to save a lot of trouble?

Half the weave would be hard, half soft. Best of both worlds, since they would overlap at any given place?

Pure speculation without any backing, I know. But it seemed a lot less problematic than trying to case harden an entire hauberk in one go

There is nothing quite as sad as a one man conga-line...
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Chad Arnow
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PostPosted: Wed 25 Jan, 2012 7:15 am    Post subject:         Reply with quote

A 1981 Journal of the Arms & Armour Society article by Alan Williams looks at several pieces of late 14th/early 15th century armour including the Pembridge and Hawberk great helms. Metallurgical analysis of the pieces in the article show some to have been case carburised, where the iron is heated in carbon bearing material in an attempt to pull carbon into the iron for hardness purposes. Dr. William's research showed the results to be spotty and the process wasn't well understood. Some pieces achieved good hardness; others cooled too long or weren't heated enough and saw less benefit.

It should also be pointed out that many of these pieces are were owned (or likely owned) by people of high status and may have been of above-average quality.

Happy

ChadA

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Johan Gemvik




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PostPosted: Wed 25 Jan, 2012 8:38 am    Post subject:         Reply with quote

Bjorn Hagstrom wrote:
If you have alternating solid and riveted rings, why not just case harden the solid ones before weaving to save a lot of trouble?

Half the weave would be hard, half soft. Best of both worlds, since they would overlap at any given place?

Pure speculation without any backing, I know. But it seemed a lot less problematic than trying to case harden an entire hauberk in one go


Bra idť!
Yes, this could work really well actually. I haven't seen any mailles that have it yet going though various metallurgical research available out there today, but that doesn't say much really. Lots of maille haven't been properly researched, lots of papers are never published in the first place, and if you aren't lookng for this type of mixed assembly you might overlook it.

"The Dwarf sees farther than the Giant when he has the giant's shoulder to mount on" -Coleridge
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David GaŠl




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PostPosted: Wed 25 Jan, 2012 1:40 pm    Post subject:         Reply with quote

Aleksei Sosnovski wrote:

0.64mm in 2 hours... 0.064 would sound more correct to me. I must look up exact numbers in a book on the subject that I have, but as far as I remember carburization is a VERY slow process if simple carburizing agent such as charcoal is used.


Oh, please look it up for me if you can.

What do you others know about the carburization speed? And how long would you carburize a piece of iron rod with 3 or 4 mm diameter which could be hammered after it without deformation so all in all used as a shaped anvil?

Bjorn Hagstrom wrote:

If you have alternating solid and riveted rings, why not just case harden the solid ones before weaving to save a lot of trouble?

Half the weave would be hard, half soft. Best of both worlds, since they would overlap at any given place?

Pure speculation without any backing, I know. But it seemed a lot less problematic than trying to case harden an entire hauberk in one go


I think it's a really good idea. If I will have time I will let it a go. At first making only pieces to try the hardness of it.

David
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Hadrian Coffin
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PostPosted: Wed 25 Jan, 2012 2:49 pm    Post subject:         Reply with quote

Hello,

I have not finished reading this topic, but I did want to mention to those of you looking for The Knight in the Blast Furnace to try the library. The local library near me happens to own a copy, so other libraries may own it or be able to borrow it. Also if you happen to be connected to a university in any way I can almost guarantee there will be a way to get a hold of the book (through your school library, or via a service like WorldCat).

Cheers!

Hadrian

Historia magistra vitae est
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Bjorn Hagstrom




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PostPosted: Thu 26 Jan, 2012 2:03 am    Post subject:         Reply with quote

I have found a flaw in my idea for alternating hardened solids with soft riveted rings.
The hard solids might cause excessive wear on the soft rings, shortening the life-span of the maille
But if the hard ones are sand-barrelled smooth, that might mitigate the issues somewhat.

Would love if anyone tried it out for real. My own riveted maille project is still in the far future (need to find an anvil or anvil-like object for my workshop first)

And while I'm throwing around baseless theories into the pot: Would work hardening of rings or weave be feasible and/or worthwile?

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David GaŠl




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PostPosted: Mon 30 Jan, 2012 1:23 pm    Post subject:         Reply with quote

Case carburising thin iron plates and after it forge welding could cause more carbon contain than case carburising one piece of thick iron(same time spent on case carburising)?

After case hardening will the surface of iron have a homogenius carbon containt?( I guess not)

David
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Aleksei Sosnovski





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PostPosted: Fri 03 Feb, 2012 7:49 am    Post subject:         Reply with quote

I finally found my data on case hardening. For solid carburizer such as charcoal dust depending on the size of the "box" carburization depth would be:

3 hours: 0.1 - 0.25mm
5 hours: 0.2 - 0.7mm
7 hours: 0.6 - 1.1mm

The smaller the "box" the faster the carburization goes. This is probably because charcoal dust is a good thermal insulator and it takes quite a lot of time to heat it from "preheat" temperature (around 800C) to carburizing temperature (900-950C)

Gas carburizers work faster, because there is no "box" needed. Carburization time is reduced by 1 - 2 hours.

The beast carburizers are special pastes, these can carburize up to 1mm deep in just 1 hour.

Please note that this data is from year 1977 and modern carburizers may work even faster.
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David GaŠl




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PostPosted: Fri 03 Feb, 2012 2:04 pm    Post subject:         Reply with quote

Aleksei Sosnovski wrote:

3 hours: 0.1 - 0.25mm
5 hours: 0.2 - 0.7mm
7 hours: 0.6 - 1.1mm


Are these parameters related to one side of the piece of iron? I mean if I put a peace of iron in a box and I cover it from each side with charcoal dust for 3 hours than 0.1mm would be carburized from each side or is this an assumed number?

Aleksei Sosnovski wrote:

The smaller the "box" the faster the carburization goes. This is probably because charcoal dust is a good thermal insulator and it takes quite a lot of time to heat it from "preheat" temperature (around 800C) to carburizing temperature (900-950C).


Smaller box goes with less charcoal, or does charcoal not burn away by this process?
What if I use not only charcoal dust but animal skin(unworked leather), and bones? Will the process be faster?

Many thanks Aleksei that you have looked it up for me!

David
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Aleksei Sosnovski





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PostPosted: Sat 04 Feb, 2012 12:02 pm    Post subject:         Reply with quote

David GaŠl wrote:

Are these parameters related to one side of the piece of iron? I mean if I put a peace of iron in a box and I cover it from each side with charcoal dust for 3 hours than 0.1mm would be carburized from each side or is this an assumed number?


It will be carburized for 0.1 - 0.3 mm on each side.

David GaŠl wrote:

Smaller box goes with less charcoal, or does charcoal not burn away by this process?
What if I use not only charcoal dust but animal skin(unworked leather), and bones? Will the process be faster?

Many thanks Aleksei that you have looked it up for me!
David


If you calculate how much carbon the steel will absorb you will see that unless the "box" is not airtight and the carburizer is literally burning out there is not reason to take it into account. The more important thing is how many times you can use same carburizer before replacing it with the new one. Unfortunately I don't know the answer. Probably depends on the carburizer.

The book I have taken the data from lists several solid carburizers, all of the being mixtures of different but simple components, and carburization speed is the same. So probably it wouldn't matter much if you use charcoal dust or leather (it is only my guess though so I would advise you to dig for more info here). The more important thing is to have good contact between the carburizer and the metal.
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