> Dear all,
>
> For several month, I'm analysing ferrous artefacts coming from direct
> process and that are made from an ore containing high levels of Manganese (a
> 14th c. forge in the French Pyrenees). The slag inclusions contain up to
> 30mass% of MnO and Mn is not detectable in the metal (<100ppm). Indeed, the
> metal is highly carburised (over eutectoid).
>
> Several authors talk about the carburising role of Mn and, indeed, several
> historical zones of producing steel were associated with high Mn ores
> (Italian alps, Pyrenees...)
Dear Philippe,
I have seen this effect in many case of bloomery steel. The Roman
Carmarthen billet, I don't know if you saw the poster at the BM
conference, had very few slag inclusions but those it did have were Mn
rich. The 5-6th century AD site in West Somerset which produced every
thing from low carbon iron, through eutectoid and hyper-eutectoid steel,
to grey and white cast iron, again one of the ores they were using was
relatively manganese rich.
> Nevertheless, I have several questions about the exact metallurgical role of
> this element in the carburising process during bloomery operating chain.
> Indeed, in the thermodynamic conditions of the shaft furnace, the MnO oxide
> can not be reduced into metallic Mn.
I am not sure that this is necessarily true. In one experiment of Peter
Crew's there was some reduction of Mn in the steely areas (I admit it
was slight around the 0.1% wt, sorry not yet published).
>Thus, this element can not play a role
> in the metal.
Clearly, the element does play a role, although that role may only be at
the gas-metal interface.
>Moreover, even if this element were in the iron, it seems that
> the influence of Mn on the diffusion coefficient of carbon is not proved.
>
I am not sure what the effect of Mn on the diffusion of C in Fe is but
that can be looked up in the tables, but the more important effect is
that Mn is a powerful deoxidiser especially in acid slags (silica rich)
at high temperatures. The slag inclusion compositions I have analysed
definitely fall into the acid class.
The oxygen content of the metal needs to be kept low for carbon to enter
into the metal, or remain in the metal. So once the system gets into the
temperature and reducing conditions for high C content, these slag
reactions start so that the presence of high MnO will promote C in metal
compared with a low Mn slag.
> So, I ask the questions: does anyone study the exact role of manganese on
> the carburisation of iron in the direct process. Does Mn play a role indeed?
> Can you help me?
The trouble is we do not know exactly what the conditions are within the
bloomery furnace or how they vary with position and time during the run.
from what I have seen is that most bloomery steels do have a tendency to
have high Mn slag inclusions (Mn > 8%) - that does not necessarily imply
a high Mn ore, but one that includes low levels of non-volatile elements
other than Fe,Mn, and Silica.
Unfortunately most work has been done on liquid steel at 1600C.
> Best regards
>
> Philippe Dillmann
>
>
--
Chris Salter,
Oxford Materials Characterisation Service,
&
Electron Microscopy Research Support Group,
&
Material Science-based Archaeology Group,
Oxford University Begbroke Science Park,
Sandy Lane, Yarnton, Oxford, OX5 1PF
Tel 01865 283722, EPMA 283741, Mobile 07776031608
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