Crucible steel blades

[Jens Nordlunde]

Robert Hoyland and Brian Gilmour quotes in the book ‘Medieval Islamic Swords and Swordmaking’ al-Biruni who lived in the 10th century. On page 168 the swords of the Rus or Vikings are described. “…. These swords are described as composite blades with edges of hard, or male, iron (shaburqan), which we know from Kindi’s definition means, at least in this case, directly smelted or bloomery steel. Biruni reports that these swords had a central pattern welded part forming a wide channel or fuller, running down the blade, made of soft iron (narmahan). This composite design ensured that the blades would be tough and able to withstand being struck, unlike those made of crucible steel (fuladh), which he says were prone to snap in the extreme cold of the Russian winters. The very high carbon content of the crucible steel used to make the blades was no doubt the reason. The problem of steel being prone to brittle stress failure in cold weather has been encountered more recently, and the effect is now known to increase with carbon content.
(note: See Reed-Hill, Physical Metallurgy Principles, 783-786).

Can anyone explain why more carbon in the steel will make it more brittle in cold weather, and how far north did they use crucible steel for blades?

[Gt Obach]

Hi Jens

i didn't find any brittleness with my crucible steel.... and in the winter, back home, it would reach -43 cel on occasion... ( fun time to be in the forest

- i do agree that "toughness" in steel goes down with temperature... but i also think that you should have a servicable blade at those temps... just not a bullet proof one..

-- lower temps may trip off some retained austenite in the blade.... and this can definitely make a blade brittle.. ( i can explain it... but it'll be a wordy post, if your patient)
-- higher alloy blades are definitely prone to retained austenite..

-- they also say sulphur in steel is problematic at low temp.. ... as some undesirable elements will sneek in when you do a crucible melt... and this could be a bad thing at cold temps....but ok at warmer climates..

-- prior stress's in the blade.... if the steel is in a stress condition ... i believe that cold will definitely act on this area and be much more brittle..

i'm sure i'll think of more

Greg

[Jens Nordlunde]

Thanks Greg,

I seem to remember that it, in some places in Siberia, can be as cold as –70 C during winter, but this is in special places, not everywhere. I have never tried –40 C, only –25 C, and I found that pretty cold, as it always blows where I lived at the time.

From the text it seems as if the Rus’s/Vikings swords had a bigger chance to ‘survive’ than the blades made of crucible steel. I know the description says a softer core and hard edges, but somewhere else I have seen that they also used soft and hard steel forged together.

On page 169 he writes: “Phosphorus affects the hardness as well as giving the iron very pale etching optical properties”. The two authors don’t agree much with the last statement, and earlier in the book they state that al-Biruni is less reliable than al-Kindi, but what about the Phosphorous affect?

If you don’t have the book, maybe you should try to have a look at it in a library – I think toy will like it.

Jens

[Jeff Pringle]

Carbon makes steel brittle in general, not just at cold temperatures. Over 1 % C, the amount of carbide increases and the carbides form at grain boundaries, reducing the strength of the grain boundary. Wootz blades are heat treated in such a way as to minimize the bad effects of having a lot of excess carbide, to take advantage of their excellent edge enhancing properties.
Sulfur makes the steel more prone to cracking while it is being worked ‘hot’, phosphorus makes it more brittle when ‘cold’ – hot and cold meaning ~1500 F + and room temp, respectively. Phosphorus also collects at grain boundaries, and produces brittleness under shock – not good for a sword blade.
I think phosphorus is the culprit in this case, or the combination of high carbon and phosphorus. Verhoeven said in one of his JOM articles that he thought that Phosphorus was needed for good pattern development in wootz. Which one comes into play more as the temperature goes from +60 F to -20 F, or if it’s the synergy of both - I don’t know if that’s been determined.
Thanks for the heads-up on that book, ‘Medieval Islamic Swords and Swordmaking’ – I think Gilmore hinted that would be on the way in ‘Persian Steel’, but I hadn’t noticed it was out.


Greg - the low temps would get the retained austenite to go to untempered martensite? That would increase breakage, for sure! I think you should go ahead with that wordy post.

[Rivkin]

I am by far no specialist, and know absolutely 0 about the field, but I would see it this way:

Imagine that you have a metall at a very high temperature. Atoms move randomly with large amplitudes. Now we introduce a defect - a few atoms are moved away from the equilibrium configuration. Because of random motion the rest of the atoms is likely to adjust to the dislocation - kind of like water does not fall into pieces if you stir it - randomly moving molecus fill the empty space. So is the metall it is plastic - it is actually softer than at small temperatures, but more crack resistant. Now at low temperatures if you move an atom so far away it effectively looses contact with one of its neighbours (the lattice "breaks"), the neighboring atoms can not readjust to maintain the stability, the crack forms and propagates in the material (again, the material can not adjust).

This process obviously depends on how much energy one has to spent to adjust/change the configuration of atoms, so I would expect the "brittleness" being given by energy structure calculations. It is going to greatly depend on the material and its composition, it is quite possible that wootz was more brittle than other steels, however I think it especially relates to the wootz that Biruni was talking about, not some other wootz - I would expect a significant variations depending both on chemistry and on geometry of the wootz (since wootz is essentially two types of lattices in one material).

[Battara]

At risk of repeating what has been said and in hopes of adding to it, carbon stiffens the steel in other ways by stiffening the crystaline structure. Brittleness implies that something is not as flexible and as temperatures drop, the molecules vibrate less and "stiffen" more. Flexibilty drops and the piece, or any metal for that matter becomes more rigid, thus being more prone to snap off. IN warmer temperatures, metal molecules vibrate more, become more "fluid" and thus a piece can bend more, including bending upon impact.

[Jens Nordlunde]

Well Rivkin, you may not be a specialist - but neither am I - you do however know far more about this than I do
.
Hi Jeff,
You write: ‘Carbon makes steel brittle in general, not just at cold temperatures. Over 1 % C, the amount of carbide increases and the carbides form at grain boundaries, reducing the strength of the grain boundary. Wootz blades are heat treated in such a way as to minimize the bad effects of having a lot of excess carbide, to take advantage of their excellent edge enhancing properties.’

Now it is easy to test, but how did they test it in the early days, the carbon, and all the other things being in the ore? Besides the quality must have differed from where the iron was mined – or from where the iron ore was imported from.

[Rivkin]

Thank you Jens.

As far as I remember a lot of steels become brittle at temperatures around 0 centigrade; I doubt that one can fight or even run at minus 40 degrees, so most likely we must look at higher temperatures.

Concerning phase transitions - I have no idea about the topic, but 20-30 degrees change in temperature - can it force a fast phase transition ? Or it will take 200 years for the transition to occur ?
I think the cause here is a simple brittleness.

[Gt Obach]

in terms of retained austenite..... .. to make it simple...

1- you bring steel up to a red temperature... which will be about 1475 to 1550 F
(say for 1095 steel ).. you approach a temperature where all the carbon in the steel goes into solution ... ( much like table salt goes into solution when you stir it into a glass of water..)
-- this state is called austenite .....

2- the steel is quenched ..... and if cooled quick enough ....it will become untempered martensite ..... this is a very hard but brittle state.... (much like a metal file.... it is very hard to file steel but if you smack it on the side of a table... it may shatter like glass)

3- the steel is put in an oven and tempered to remove some of the hardness and give the steel toughness in return..

now... the higher the alloy steel... the more its tendency to retain austenite... that means.... when the steel is cooled ...some of it will get confused and not transform to untempered martensite... .... now... if you later trip off this retained austenite... it will turn to untempered martensite......

you could end up with an area or a percentage of steel with a very brittle nature do to the heat treatment....

the austenite can be set off by the first tempering... (thats why its good to temper more than once )
-- but more importantly..... it can be set off by undercooling the steel...
-- so the cold climate could simple be doing this...

-- part of my heat treat cycle is to put my blades in the freezer inbetween oven temperings to reduce this possibility....

heat treaters that work with lots of stainless ....sometimes will use liquid nitrogen to undercool the steel and force the retained austenite to convert..
-- then tempering it to give the steel toughness

so far this is the basics of how retained austenite can make things brittle if not taken care of...

alloying elements like S and P are some more factors.... as steel is very complex and there can be so many ways it can be altered...
-- i heard the S in the titanic steel, when cool was overly brittle.. ?


wootz was a very good sword steel.... but all materials have their limits... ...even more importantly.... not all wootz is heat treated the same... some blades were oil quenched..... others edge quenched... others not...
-- all this would definitely affect toughness

take care
Greg

[Rich]

The Japanese had the same problem with their blades are cold temps during
the Manchurian invasion/occupation in the 1930's-40's. That's why they
developed the Koa Isshin Mantetsu blades, to withstand the cold Manchurian
winters without breaking.

Rich