critical mass wrote:
This is correct. However, the plate can fail by cracking but still do it´s job (to keep the projectile out). The 2pdr AP impacts in this trial scooped off (no penetration), but also, at least once, caused a plate crack. But it´s the 2pdr capped A.P. performance which is so much off the chart.
Well, according to AT 37 there were five rounds fired during this test. The target consisted of the three Pz.III front hull plates still welded together - the glacis plate, the front nose plate, and the lower nose plate. The rounds were directed at the front nose plate at 20 deg.
Round 1 (AP) only hit the edge of the front nose plate, causing negligible damage by chipping. In essence, this round missed. (1502 fps)
Round 2 (AP) shattered on impact on the front nose plate, braking the plate in two longitudinally and causing three other severe cracks. (1497 fps)
At this point, the lower half of the broken plate was separated and used as the target. The attack was switched to APC as it was felt that further AP attack would only further shatter the plates.
Round 3 (APC) This holed the plate, with part of the shot lodging in the plate and generating a deep impression. It was assumed by the testers that the rest of the shot passed though the plate. (1502 fps)
Round 4 (APC) This struck only 3" from Round 3, and holed the plate cleanly, and caused the cracks already established to yield, breaking up the plate. (1496 fps)
With the shattering of the lower half of the original plate, the top half was then employed as the target.
Round 5 (APC) Shot shattered. Plate broke into four pieces. (1634 fps)
I have to say that this looks to me to have been a reasonably successful trial. It shows that either 2 pounder AP or APC could shatter the plate, and that 2 pounder APC could penetrate the plate. I suspect if more AP rounds had been fired they would also have penetrated the plate. It should be borne in mind that the APC rounds used in this test would not have been dedicated APC projectiles (as at this time no such design existed), but would most likely have been the standard AP round with an improvised cap.
One interesting but overlooked aspect is that I have seen British assessments that indicate the 30mm frontal armour of the earlier Panzer III's was NOT face hardened, so as well as the later increase in armour thickness, it may have been the case that at some point the Germans changed the armour specification for the Ausf. F and G. Maybe during the changeover between the F and G?
critical mass wrote:The temperature itselfe is one part of the issue with gas cutting. It´s an important component but not the most important one, the plate gets heated locally. If the temperature increases beyond 120°C it starts to temper (softening) the heat affected zone. But more important is how quickly the heated up section has been cooled down to prevent temper brittleness, which among other things, greatly reduced the local toughness and therefore increases the likelyhood of catastrophic plate failure (cracking, and more severe plate shatter).
Temper brittleness wasn´t yet properly understood in this period. If the plate was allowed to air cool after heat treatment (cutting or welding), You will have to reckon with a very high probability of serious embrittlement of the plate as the temperature slowly walks back through the embrittling temperature range. The effect depends on alloy state, impurities, section thickness and original hardness. Lean alloyed FH plates as manufactured in Germany are particularely problematic in this regard (they have smaller thermal ellbow spaces to prevent embrittling) and are referred to as thermo-mechanically treated armor. It was heat treated, hot rolled, heat treated again, then shaped and sized, then welded in place with another crew following the weld guys with high pressure water jet for rapid cooling of the heat affected zone. This process could be dispensed with by RHA of softer grades but FH was very sensitive, because the softening is occurring at fairly low temperatures here.
Well, the plate employed in this trial was 24" wide, and apart from Round 1, they hit towards the centre of the plate, so I' would think it likely that these would have been outside the heat affected zones from the gas cutting. With regard to the shaping and the sizing of the plate, I would presume that the carburising was undertaken on standard large plates, which were then cut down to produce the much smaller plates that would be used to build up the tank hulls. Due to the thickness of these plates (30mm+) I assume that these would have been gas cut, with the dressing of the edges of the plate for welding being undertaken by grinding. If I'm correct, then the plates would have been subjected to two secondary heating operations and one cold working operation prior to the British ever getting their hands on them.