Drew;
I served my apprenticeship in an Air Force installation (AEDC, Tullahoma TN) which had their own metallurgical lab. During my time there I made quite a few of those little tensile pieces to be pulled apart.
Craig;
Note in the figures I gave above the % of the yield to the ultimate does tend to decrease as the steel gets harder. If one went to still higher carbon %age the tendency would be more pronounced. In the case of the case hardened 1020 the yield vs tensile would be much closer together in the case than the core.
Note that Drew's chart shows 1020 as having a tensile strength of 60K psi @ Rockwell B68. On the case hardened 1020 a tensile of 80K psi is shown. No hardness given, only that it was drawn to 400°F. This would give a quite hard case. Rockwell B68 is about the equivalent of Brinell 121 when tested with the standard 10mm diameter ball @ 3,000 KG load. This is quite soft & is basically off the Rockwell C scale which is normally used for heat treated steel. From these two figures we see the case hardening gives an increase in tensile strength of around 33%. This I believe is due primarily to the case as there would be little change in the strength of the core at the 20 point level of carbon due to heat treating it.
