"Sebapop" ha scritto
Post by SebapopPost by ADPUFMa come hai detto tu solo negli ultimi secoli la produzione
potè avvenire per fusione (prima solo forgiatura, credo)
Credo. Ma non me ne intendo.
Riposto una discussione apparsa in altro gruppo
Post by SebapopPost by ADPUFhttp://www.signainferre.it/modules.php?name=News&file=article&sid=693
http://www.signainferre.it/modules.php?name=News&file=article&sid=694
In un passo si dice che le spade dal V al X secolo erano generalmente
saldate a disegno (nucleo di ferro dolce ricoperto da vari strati di
acciaio a diverso contenuto di carbonio), mentre già nell'undicesimo
secolo erano quasi tutte di acciaio omogeneo (si dice proprio così).
Questo mi suona strano.
Già verso il 1742 Benjamin Huntsman (l704 1776), nei pressi di
Sheffield, in Gran Bretagna, riuscì ad ottenere temperature
sufficienti per produrre acciaio liquido adoperando dei crogiuoli
chiusi dargilla che inseriva dentro l'altoforno: nei crogiuoli
inseriva il ferro battuto che faceva fondere insieme a del carbonio.
Per prima volta in Europa, si otteneva cosi dell'acciaio fuso, anche
se non in grande quantità e si aveva la possibilità di fare oggetti
omogenei in acciaio e non con la sola superficie cementata o
carburata.
http://www.cronologia.it/mondo28l.htm
Ripeto, la materia mi è oscura, ma mi sembra che le due versioni
collidano :)
Aggiungo questo interessante link sulla storia dell'acciaio.
http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/advanced/t5_1_4.html
L'apparente collisione è dovuta al fatto che nell'antichità si otteneva il
ferro, e anche l'acciaio (ma credo principalmente si trattasse di ghisa) in
forma solida.
Per arrivare a fondere il ferro e l'acciaio era necessario che qualcuno
inventasse un altoforno che sarebbe giunto solo molto più tardi.
We may safely assume that the old materials scientists tried
everything to smelt iron from suitable stones. They did have
tricks to raise the temperature of a fire - in a 4500 old mastaba
in Egypt, I took a picture of a relief showing six gold smiths
(probably rather their Ph.D. students) blowing into the fire with
hollow reeds. But just blowing with lung power will not do -
maybe you get 1200 °C, but that's it.
Tuttavia era possibile ottenere il ferro battuto e fondere una lega fragile,
quella che si usa per i modellini delle auto (ti si sono mai rotte le
portierine?)
So you do not get liquid iron - but you do get solid iron because
reduction does take place - in a solid state reaction. What you get
is an iron bloom ("Eisenblüte" in German), a mixture of fine iron
particles, unreacted iron oxide, slag and charcoal residue.
The iron in the bloom was rather pure (and thus comparatively
soft) because a solid state reaction produces only iron - carbon
or other impurities - have to diffuse in from the outside (if the
iron would be liquid, it would just dissolve the dirt up to the
solubility limit)."
The early iron smiths (probably being Hethites of some form)
could "wring" the iron from this bloom by separating the iron
from the rest mechanically and repeatedly hammering together
what was left at high temperatures (about 800 °C; some of the
slag then is liquid and gets squeezed out) with, no doubt, proper
prayers to the respective gods and many (magical) tricks.
What they finally obtained was "wrought iron" ("Schmiedeeisen"),
i.e. a lump of rather pure iron consisting of welded together
small pieces with plenty of small inclusions (small, because of
the hammering that breaks up large pieces of slag).
Extreme care was necessary - from the selection of the iron ore,
the reduction process and the hammering business. If you were
careless, the iron oxidized again (it really "burns" at temperatures
in excess of about 800 °C), and if you kept your reduction
process going too long, carbon diffuses in and you may end up
with cast iron (C content about 3% - 4%; melting point as low
as 1130 °C).
Then you actually got it liquid - "casting" was possible - but cast
iron is brittle and useless (for weapons, that is).
Senza altoforno al limite riesci a imporre una trasformazione solo sugli
strati superficiali del metallo. E questo bastava per gli oggetti piatti,
come le spade e, immagino, le armature.
Nevertheless - the early smiths, [...] sometimes succeeded in
making good steel. What was their secret?
It is rather simple - looking at it retrospectively: You need
the proper concentration of C in the Fe bcc lattice at room
temperature [...]
Not being able too melt iron [...] the only way to get carbon
into the Fe lattice was diffusion via the surface. What you
needed to do was to "roast" you iron (possibly the whole sword)
for the right time at the right temperature in a charcoal fire.
E in questo la magia aveva il suo ruolo.
Magic and praying helped - it did indeed: How do you keep
track of the time without a watch? You utter a long prayer
that you learned from your master - the right ones "worked"!
The rest of the magical ritual was helpful in providing
reproducible conditions. Of course the old practitioners had no
idea of what the really were doing; if they thought about it, t
hey felt that were purifying the iron in the (more or less holy) fire.
This erroneous believe (like so many others) goes back to the
(from a materials science point of view somewhat questionable)
philosopher Aristoteles who certainly asked the right questions
about life the universe and so on, and is righteously famous
for that.
His answers, however, were invariably wrong - even in the few
instances where he could have known better.
Bei tempi per i maghi, i tempi in cui l'ignoranza imperava. : )
saluti,
Peltio
il resto sta al link indicato. Ho tagliato, per non fare salti temporali
questa parte importante:
Somewhat later, with larger furnaces and increased experience,
the bloom obtained may have contained some high-carbon melted
parts on its top layer. It then consisted of a whole range of
iron-carbon alloys - from rather pure wrought iron to cast iron
with good steel - say 0,5 % - 1,5% carbon - in between.
The art of the smith than included to pick the right pieces.
E ho completamente saltato il ruolo della tempra e della battitura.