There are a few basic behaviors of iron that fundamentally control what blacksmiths are going to do with it in this stage. To begin with, we need to introduce some terminology to avoid this coming confusing: a given piece of metal can be hard (resistant to deformation) or soft; it can also be ductile (able to deform significantly before breaking) or brittle (likely to break without deformation). This is easiest to understand at the extremes: a soft, brittle material (like a thin wooden dowel) takes very little energy and breaks immediately without bending, while a hard, ductile material (the same dowel, made of spring-steel) bends more easily under stress but resists breaking. But it is also possible to have hard brittle materials (pottery being a classic example) which fiercely resist deforming but break catastrophically the moment they exceed their tolerances or a soft, ductile material (think wet-noodle) which bends very easily.
(I should note that all of these factors are, in fact, very complex – far more complex than we are going to discuss. In particular, as I understand it, some of what I am using “hardness” to describe also falls under the related category of yield strength. Hopefully you will all pardon the necessary simplification; if it makes you feel any better, ancient blacksmiths didn’t understand how any of this worked either, only that it worked.)
Of course these are not binaries but a spectrum. Materials have a degree of hardness or ductility; as we’ll see, these are not quite opposed, but changing one does change the other – increasing hardness often reduces ductility.
The sort of things that pre-modern people are going to want to be made in iron are going to have fairly tight tolerances for these sorts of things. Objects that had wide tolerances (that is, things which could be weak or a little bendy or didn’t have to take much force) got made out of other cheaper, easier materials like ceramics, stone or wood; metals were really only used for things that had to be both strong and relatively light for precisely the reasons we’ve seen: they were too expensive for anything else. That means that a blacksmith doesn’t merely need to bring the metal to the right shape but also to the right characteristics. Some tools would need to finish up being quite hard (like the tip of a pick, or the edge of a blade), while others needed to be able to bend to absorb strain (like the core of a blade or the back of a saw).
Bret Devereaux, “Collections: Iron, How Did They Make It, Part III: Hammer-time”, A Collection of Unmitigated Pedantry, 2020-10-02.
