Re: 2 more questions *LINK*
Andrew F in Australia
>Bob,
Once again, a quick off the top of my head reply here.
Grey cast iron is a matrix of steel with big flakes (think cornflakes but smaller) of graphite throughout the structure. As there's all the weak points due to the flakes of graphite, the cast iron is brittle.
Off the top of my head, I'd say that nodular cast iron and ductile cast iron are the same. The grey cast iron is heat treated to turn the flakes of graphite into nodules/spheres. The iron is therefore less brittle as the spheres are not as prone to inducing brittleness/weakness in the alloy.
As for the second question, it's beyond my time at the moment to get a clear concise answer together (was it Bill Clinton that said if you want me to give a 2 minute speech, I'll need a week to prepare but if you want an hour speech I'm ready now). Apologise but I'm running on about 4 hours sleep a night at the moment with everything I've got at home and work.
So, here goes an attempt that hopefully answers more questions than it raises:
Hardness: You want a chisel/iron that is hard enough to not wear away.
Modulus of Elasticity: This is a function of hardness too and measures how much force you need to bend the chisel tip a certain distance.
You want a chisel with a high modulus of elasticity. The higher, the better, providing that the steel is still tough and not brittle (usually, very brittle materials have a high MoE). THis is where the tie in between hardness and the chisel ratings comes in.
Having said this, steel tends to have similar (within 5%) values of the modulus of elasticity irregardless of the heat treatment history - what changes is how much force it takes to break it.
Toughness:
I don't know whether toughness is the main criteria for chisels, on reflection, as it indicates how much energy it absorbs as it bends/breaks. What use is a chisel if it's got a bent cutting edge.
However, toughness is an indicator of what you're looking for.
You want a steel that can take a bit of bending but spring back to the original shape (high yield strain) as well as take a fair bit of force to deform (high yield stress). This tends to be present in a tough material.
If you want to compare manufacturer's data, multiply the yield stress by the yield strain (or proof strain.) The highest number is the most resilient steel.
Anyway, must run. Sorry for the short reply - Ideally I'd like to give a more considered response.
Stellite would have made an excellent stone tool.
Cheers,
Andrew
Link showing stress-strain curve and other terms