Short version - I dropped a large histology microtome blade - these are about 500g wedges of steel, alarmingly sharp. I expected it to chip but the cutting edge dented. I know they're plain high carbon steel but what advantages does that give if it's not hardened?

Just because it bends or dents doesn't mean it's not hardened, it's just not brittle. Knives, machetes, brush knives, etc. edges generally, will roll before chipping if heat treated properly. You do NOT want swung blades to chip in use. Even if you sharpen or grind chips out there's too good a chance you didn't get the entire fracture and it WILL act as a stress riser snapping the blade under impact. Say whacking a limb. A microtome is an impact blade, slicing frozen samples isn't done with a slow push. 

High carbon steels are: more rigid, tougher and durable annealed, there are lots of uses for higher carbon steels, coat hangers come to mind. Didn't know the old wire coat hangers were typically about 40-50 pts. carbon?

Frosty The Lucky.

After tempering steel can be hardened and still bend/deform without chipping. 

However, to answer your question, the addition of carbon increases the yield and tensile strength of steel up to a point. 

Tougher and harder even when not hardened; but WAY harder when hardened.  (You can see this in ancient-early medieval blades where higher carbon blades had higher Vicker's hardness test results when unhardened; but when they were hardened they may have DOUBLE or more the test result of unhardened.)

Thanks all, good to know. We don't use them anymore (the disposables are even sharper and cheaper ) I always assumed they were very hardened because we used diamond paste on a lap to sharpen and they go through most body parts in very thin sections - I've done bone. I remember my training - if you drop one step back, do not try to catch, you could loose a digit. I've got a few to forge with at some point. 

I also use diamond blades - 100% carbon makes a great edge but the slightest inattention when your working and plink.... 

If they're being used in the medical field I wouldn't be surprised if they were some flavor of stainless steel. That's something to consider if you plan to forge weld them into a billet.

Diamond blades, eh? Interesting.. Sounds expensive.

They're not stainless - many warnings about oiling before storing, even the disposables aren't - trace elements and stuff. 

As for the diamonds, well my 2.5mm blade was 6k sterling and costs 1k to sharpen, it's one of the few bits of lab kit that's 'personal' - as in you use the communal blade unless I tell you in writing you can use mine - we get tetchy. Cuts to 70nm though, probably too thin for tomatoes. 

Microtomes are unbelievable pieces of kit, I’ve very little experience with them myself but I know they have been used to slice through steel powders, very hard steel powder at that too. I remember Jan Hoffman preparing TEM (transmission electron microscope) samples, <100nm in thickness, from mechanically alloyed oxide dispersion strengthened steel powder; some greater than 1000HV hardness.

As for the effect of carbon in non-hardened steel, if it sits on the lattice, or forms Cottrell atmospheres around dislocations, it can pin and inhibit dislocation movement, hence the steel has a greater resistance to deformation. But, as Thomas points out, the effects are insignificant when compared to the martensitic structure; they also diminish quickly if operating at temperature.

I really struggle to understand how a mecahnical mechanism produces movements on a near molecular scale, I've seen the schematics but brain says "what's the catch?". I've only done em on steel a couple of times - as the operator, I'm a biologist, I'd love to go back now I'm starting to grasp the very basics of steel metallurgy.

People have bee making machinery and devices to molecular tolerances for a long time, turn of the last century anyway. We've been working to atomic tolerances for at least 50 years. 

The first atomic weapons were machined to molecular tolerances. 

Father was a metal spinner and machinist who did a LOT of aerospace work. He has parts he made on the moon and leaving the Solar system. I grew up with silly tight tolerances.

A microtome is no mystery to me even if I don't know how they're made. 

Frosty The Lucky.