[JamesKelly]

Axes. In the late 1960's I did some work with the old Kelly Axe factory, Charleston, West Virginia, USA. Axes at that time were made of AISI 1086 carbon steel, meaning steel with a nominal carbon content about 0.86% (sorry, I do not know corresponding W.Nr. or EN #). They heat treated both axe and hammer heads in a lead bath. As I recall, tempered hardness of an axe was about Rockwell C55. The heaviest hammer they had made was 50 pounds, or 22.7 kilogram. That is the origin of the American expression "putting the hammer down" (i.e. go as fast as you can). When two steamboats chose to race on the Mississippi river one way to get some extra speed out of that old steam engine was to set a 50 pound hammer down on the steam safety valve. If the boiler was strong enough, you might win the race. If not, well, it no longer mattered.

What is the best steel & how hard should it be? Depends upon what you want the knife to do. If you make your living skinning bears or cutting ropes then a tool steel called D2 in the US, about 12% chromium 2% carbon, maybe Rockwell C 58 to 62, is appropriate for good edge holding.
If your life depends upon that blade not breaking, then a lower carbon & softer temper is appropriate. I.e., you want a tougher steel, less likely to break when abused. In the USA power lawnmower blades, all of them, were formerly made of AISI 1070 carbon steel (nominal 0.7% carbon), quenched in molten salt ("austempered" maybe 300C/600F), to end up with a hardness about Rockwell C40. This hardness is about the best combination of hardness and toughness one can get. No, if will not hold an edge as long as your favorite 440C blade at Rockwell C58, but neither is it like to break when you swing it at a large heavy or hard object. Now in the USA they use a lower carbon steel, 10B35, which has boron (B) for hardenability but only 0.35% carbon, for better toughness. These things do need to remain intact after striking the odd rock (or foot . . .).
Steel heat treatment is well described in various publications, some used to be free from the steel companies. It helps learn it from some knowledgeable metallurgical source. E.g., if you like to forge your blades, then do anneal them before hardening, to refine the grain a bit. Makes them a LOT tougher at the same hardness. Water with about 10% common salt in it quenches plain carbon steel faster, with less chance of distortion and cracking. &c, &c, &c.

I am a metallurgist & just edited this post as I am in a Grumpy Mood & this is a polite site. Apologies to all who read the un-edited version. Just put my less grumpy comments above.