I've read a few metallographic reports on Republican-era pila, and they were made from a very low carbon ferritic iron. Ferrite (iron with few hardening impurities) is both ductile (soft) and relatively tough, which means it can be deformed easily without breaking it. This means one could, without much effort, bend a bent pilum straight again (especially if you had someone with a forge, hammer, and ancil handy, as the Roman army always would have). Even if a pilum snapped, you could still recycle the metal into a new pilum head. Given the intensive effort needed to make iron in the ancient world, you would probably make an effort to avoid simply abandoning the metal.

The modification of the pilum to make it unusable by the enemy was one of the reform of Marius.

Here is what Plutarch says about it:

"And it is said that it was in preparation for this battle [with the Cimbri] that Marius introduced an innovation in the structure of the javelin [pilum]. Up to this time, it seems, that part of the shaft which was let into the iron head was fastened there by two iron nails; but now, leaving one of these as it was, Marius removed the other, and put in its place a wooden pin that could easily be broken. 2 His design was that the javelin, after striking the enemy's shield, should not stand straight out, but that the wooden peg should break, thus allowing the shaft to bend in the iron head and trail along the ground, being held fast by the twist at the point of the weapon."

So the head or the shaft of the pilum were not normally be deformed, they were just partly disassembled. Plutarch does not say it explicitly, but it would have been a simple matter for the legions armorers to recycle most of used pilums by just straightening them and inserting a new wooden peg.