The Shards of Narsil
November 2, 2011 3:17 AM   Subscribe

What would happen if you reforged a particular weapon/set of weapons repeatedly?

I'm reading a story in which the families of these heroes hand down their weapons from father to son, each time melting the steel and reforging it into a completely new weapon. (For example, a broadsword becomes two short swords, a heavy saber, and a dagger in one scene.)

Ignoring for the moment the question of the relative amount of steel for that to happen, it was previously my understanding that the forging process - at least in a medieval context, as this is - blends carbon with the iron to make the metal stronger and yet also more brittle. Wouldn't repeated reforgings over several generations result in weapons that snapped like twigs? Or am I missing something fundamental in the metallurgy? (I am by no means an expert in the field, nor even in chemistry.)
posted by Scattercat to Science & Nature (7 answers total) 7 users marked this as a favorite
 
The brittleness of steel is largely determined by the speed of cooling (should be slow), rather than the carbon content. Similarly, the strength of steel is normally determined by the impurities - for example, adding big fat atoms that are chemically similar to iron like molybdenum or tungsten will make the steel much, much stronger.

Assuming you're not introducing too many additional impurities in the reforging process (and a bit of extra carbon probably wouldn't hurt too much), the reforged steel should be pretty much the same as before as long as it's cooled slowly.

Having said all this, if the weapons were at any point allowed to go rusty, this would give you a problem when trying to reforge the weapons - a blacksmith's forge wouldn't be at a sufficient temperature to go from iron oxide back to iron and having a weapon forged from rust is not going to be very strong.
posted by BigCalm at 3:54 AM on November 2, 2011


I haven't looked into medieval methods, but I have read a bit on more modern toolmaking and made a few knives, chisels and other small tools from commercial tool steels. The carbon in the steel doesn't make the steel hard or directly determine its hardness; the carbon makes it possible to harden the steel by heating and quenching. Think of the presence of a certain amount of carbon as a sort of binary switch (though this is oversimplifcation). Is there a certain percentage of carbon in this steel? Yes? Great, it can be hardened. No? Oh, then we can heat and quench it all day long and it will still be too soft.

I'm sure there is a point at which the blacksmith could add too much carbon to the steel, but I don't know where than line is. In any case, they wouldn't have to keep adding more every time they reshaped the metal.
posted by jon1270 at 3:55 AM on November 2, 2011 [2 favorites]


Oh, and I forgot to mention tempering. Heating the steel cherry red and then quenching it in oil or water is not the last step. After quenching it is too hard for most purposes - difficult to sharpen, brittle, etc. So you temper it by gently heating to a much lower temperature. Blacksmiths judge that temperature by looking at the colors of oxides that appear on the surface. If you've ever heated a piece of polished steel and seen that the surface first turns a yellowish straw color which gets deeper and more orangey before swinging into purples, then blues and finally back into gray, that's what the smith is looking for. Each color corresponds to a particular temperature and hardness.
posted by jon1270 at 4:10 AM on November 2, 2011 [4 favorites]


The brittleness has to do with the way it cooled. Once you melt it down, you hit the reset button in it.
posted by empath at 5:56 AM on November 2, 2011


If you're reforging an extant blade, then the iron already has the right carbon content: it's "just" a question of reshaping it and using the right heat treatments to achieve the desired level of hardness / toughness etc etc. These processes don't in themselves add any more carbon to the steel.
posted by pharm at 6:00 AM on November 2, 2011


It would very much depend on the blade and the process the smiths used.

There are three forms of iron/carbon alloys that matter to this discussion: wrought iron, a low-carbon, soft metal which will not hold an edge well, but will bend rather than break; cast iron, a high-carbon alloy which will hold an edge well, but is brittle; and steel, a magical mixture that is both tough and hard enough to take an edge.

Swords could be made many ways: from homogeneous eutectic steel or from a composite of the iron/carbon alloys. Both were used in historical times.

A sword of homogenous steel would work as described by the posters above. It could be reforged (not melted, but softened and reshaped), if the smith were careful. Hot iron decarburizes in air, transforms to wrought iron, so the smith would have to be careful not to change the alloy as they worked it. Quenching and tempering, as described above, would determine the mechanical characteristics of the blade. So, maybe.

A more hi-tech way to make swords was to weld together high and low carbon alloys, to use the suppleness of low-carbon steel to strengthen the blade and high-carbon steels in areas where the blade needed to be sharp. Damascus steel (wootz) and pattern welding were two such processes. The Japanese sword-making also employed composite steels, for example.

In this case, it would take a lot more smiting to get the metal back into a shape that could be used to make a similar kind of weapon. The metal would have to decarburized, cooked at high temperatures for a few days to remove the excess, unknown, amount of carbon. The now-wrought iron would have to be split to form the high- and low-carbon portions and re-welded into the sword blank. Decarburization was a technique not widely used in medieval Europe---it was a Chinese invention---and would have been considered very exotic in that sort of setting. This would be an enormous amount of work. Essentially, the smith would have to treat the sword like ore and not only reforge, but resmelt the iron. Easier to start from scratch.

Finally, in both cases, the newly forged weapon would be shaped by grinding to put an edge on it. This could remove a lot of metal, a significant loss each time. Metal would have to be added at each reforging to compensate for this loss anyway. After a few times through the forge, the amount of original metal would be fairly dilute.
posted by bonehead at 7:09 AM on November 2, 2011 [4 favorites]


Having said all this, if the weapons were at any point allowed to go rusty, this would give you a problem when trying to reforge the weapons - a blacksmith's forge wouldn't be at a sufficient temperature to go from iron oxide back to iron and having a weapon forged from rust is not going to be very strong.

BigCalm, a blacksmith who didn't know this, and/or didn't clean the rust off before forging, shouldn't be allowed to make knives, period. Let's assume a competent blacksmith.


Finally, in both cases, the newly forged weapon would be shaped by grinding to put an edge on it. This could remove a lot of metal, a significant loss each time. Metal would have to be added at each reforging to compensate for this loss anyway. After a few times through the forge, the amount of original metal would be fairly dilute.


bonehead, I disagree. Shaping to edge the weapon will remove far less than 1% of its mass. Thus, it would take dozens of generations before the mass was reduced by even one-half.

Now, if it's not kept rust-free (BAD LEGACY INHERITOR!), or if it's used frequently, requiring lots of resharpenings, it could reduce in size by several % in one lifetime - see old professional chef's knives for examples.
posted by IAmBroom at 12:49 PM on November 2, 2011


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