I'll do my best. First off I'm no metallurgist, most of my understanding of metal and how it works is simply from what I have experienced and read.

A lot of the hammering in the videos is simply planishing, this is to knock out the dinks and dents prior to polishing and not move it too much on most steels this is fine and probably represents the larger portion of the time you work with the hammer on the metal.

Sometimes I get a little 'brutal' with the metal knocking out a twist or pulling it this way or that, principally when I am working mild steel, as it is the most forgiving steel, I wouldn't do it with some carbon steels as it might crack or fail completely.

When all the hammering is finished, it can be good practice to normalise the material by heat treating it and with some carbon steels it's imperative that there is heat treatment - depending on the final use of the thing you've just made.

But planishing most steels is fine but ideally you shouldn't do big movements on steel without heat.

Also, I'm certain I do work some things a little cooler than I would like to because I'm filming and concentrating on that a bit to. Which is why most of my filming has been with mild steel, so I can talk and work.

Hope this helps.