[XorNot]

Uh what? This is exactly the inverse of what happens. Quenching leads to stronger, more flexible steel. Slow cooling leads to harder, but more brittle steel. The reason is grain structure - slow cooling allows larger grains, which don't allow the movement of dislocations as freely, but the loss of flexibility costs in ultimate tensile strength.

[elif]

Please cite your correction or delete it if you find you are in error. I have provided my 2 sources which are in addition to my peer and personal experience in blacksmithing.

[CriticalCathed]

I don't know about his general claim, but a detail within his comment is right.

Smaller grain size leading to stronger material -- this is a result of more grain boundaries as the scale of the grain goes down. Hall-Petch strengthening.[0] A secondary effect is that with smaller grains oriented in random directions the metal is more resistant in general from stresses in all directions; whereas with larger grains you tend to get weakness in a particular direction (along the slip planes.)

This is common knowledge, at least it's basic material physics that I learned in college.

[0] https://en.wikipedia.org/wiki/Grain_boundary_strengthening

[kwk1]

The GP comment is claiming the opposite--that larger grain size results in a stronger material.

[elif]

Actually I said strong is too ambiguous of a term to use. For instance, do you want it "strong" enough to not bend under x force at y temperature, or do you want it "strong" enough to bend rather than shear at z force?

It is more practical to discuss the hardness and ductility at specific temperatures, as well as its ability to keep its carbon content under those temperature conditions.

[XorNot]

What I get for combining some rusty mat-sci with layman's understanding of terms. You're right regarding the specific processes