"The other Thomas Gardner"
wrote in message ...
Thanks a bunch, Ed. Very, very clear and useful information.

'Glad to help, Thomas. You may be interested in one other point about
case-hardening versus through-hardening: Quenching low-carbon steel that has
a thin high-carbon case causes less warpage than quenching high-carbon steel
that hardens all the way through -- all else being equal. You can still warp
the heck out of a case-hardened part but you have to work harder at it. g

Warping that results from quenching steel can be a complex thing. Part of it
is due to thermal expansion and contraction of the steel, and uneven cooling
that leads to uneven stresses and strains; and part of it is due to the fact
that the hard phase of steel (martensite) is less dense than the soft phase
(ferrite), which leads to differential expansion upon quench-hardening,
unless the quenching is perfectly uniform.

So a thin case of martensite imparts less stress than thick layers of
martensite. More important for many applications, the soft steel underneath
winds up with fewer internal stresses because it doesn't convert to
martensite upon quenching. The case-hardened piece may be tougher and is
sure to be less brittle overall. That's an important property that makes
case-hardening a useful technique. It's used in the highest-quality,
high-speed gears, for example, while through-hardening is used in somewhat
cheaper gears. In between is flame- or induction-hardening, a
surface-hardening method that's used in many large industrial gears.

Ed Huntress