Steel quench places work as well as aluminum.
You can see this by the time it takes to cool a blade enough to handle it. Try it. They work the same.
The limiting factors in the effectiveness of quench plates tends to be how well they contact your work (air gap is a great insulator) and if they're already hot. If you can't handle your blade with bare hands after one minute something needs work. Quench rate is very important with some steels so you need to get this right.
Heat transfer out of your work is a function of the quality of the contact and the difference in temperature between your work and the quench plate. This temperature difference at the point of contact is helped by the improved thermal conductivity of aluminum, but harmed by its reduced heat capacity.
Pound for pound, aluminum is probably a little better. Volume for volume steel is probably a little better. The difference doesn't matter if your plates are dinged up, warped or filthy.
The real advantage of aluminum is you can machine it flat without it warping and it doesn't rust when you dip it in water. It is also more flexible so it's possible the surface may conform a little better to your work giving you better contact.
None of this matters if your plates aren't flat. As-extruded aluminum might be .015" out across 6". That won't work well. if it isn't flat you need to deck it flat in a mill or buy ground plate.
You just hit on pretty much everything I was going to say.
Not that it seems to be an issue with these air-hardening steels, but the thermal conductivity of the plate changes how it pulls heat AWAY from the
point of contact with the hot blade and into the mass of the plate. An aluminum plate pulls the heat
away from the hottest point of contact much more quickly than a metal with less thermal conductivity. The area right next to the hot blade is going to rapidly heat up if the plate can't pull the heat away from that spot. Copper is even way better than aluminum in this regard, and as Stacy said, silver is even better. Titanium, having low rate of thermal conductivity, would make for relatively slow-quenching plates, but of course still plenty fast enough for what's being described.
This is why titanium is said to feel "warm," because it doesn't suck the heat from the skin very quickly.
It can also be seen when TIG welding these metals. Titanium puddles instantly under low amperage because the heat of the plasma arc column is unable to flow away from the puddle quickly, while TIG welding copper requires way more amperage, and one may not see a puddle until the entire piece of copper has absorbed a huge amount of heat and may in fact be glowing dull red before it puddles.
I need to come up with quench plates to harden thin titanium that warps horribly in the quench, and will be using 1/4" copper thermal-pasted to bricks of aluminum, chilled in ice water or with ice water flowing through the aluminum bricks, because the quench must be very fast.
Apparently diamond has the highest thermal conductivity, so how about some diamond quench plates!
