[CharlesM]

I've done some checking, and what I said earlier about the cats is not quite right (see below). Face-hardened armour was used on the 4, 6 and 8-wheeled German armoured cars, i.e from SdKfz 221 through to 234 inclusive.

Pzkfw III Ausf G onwards and IV Ausf F1 onwards had face-hardened frontal armour, except for the glacis which was homogeneous, presumably because the slope was so shallow that the extra hardness was not needed, except for the glacis on the Pzkfw IV Ausf D, which had only this plate hardened, but throughout, rather than just face-hardened. PzKfw III Ausf G onwards and IV Ausf F1 onwards had face-hardened upper and lower hull sides, but not turret sides or on any rear armour plates. The last 50% of PzKfw IV Ausf J had only homogeneous plates.

For Panthers it is a bit more complicated, and made worse by the fact that the armour on many Panthers (about 50%) was of poor quality dut to bad heating and quenching processes. Bear in mind that all notionally on PzKfw IV and Panther were often actually 82-85mm thickness. The notional 60mm plates on the Panther nose were usually 65mm, later up to 75mm.

Panther Ausf D had glacis and nose armour face-hardened, aslo upper and lower hull sides. Nowhere else. Panther Ausf A had a face-hardened glacis (poor quality) and a face-hardened nose plate, and face-hardened upper and lower hull sides, but from 9/43 only homogeneous plate was used. From the sdummer of 1944 about half of the Panthers produced had flawed glacis armour. One other reason apart from poor heat treatment may have been the reliance on carbon as a hardener as non-ferrous metals like nickel and chrome dried up. The Panther Ausf G therefore had only homogeneous plate, and again with a glacis plate that often cracked under impact.

Tiger I armour was of better quality, as it was produced earlier in the war, and appears to have always been homogeneous. Tiger II armour was also only homogeneous, but of inferior quality for the reasons already given. Jagdpanther armour was softer that Panther armour, and so less prone to cracking.

Let me know if you want sources.