Allied tank armor, quality control

[critical mass]

For whats worth, German data in general attribute 15% loss of resistence to CHA if compared to similar thickness RHA. Generally, the difference was attributed to lack of cross rolling, which helps orienting the crystal structure of the metal matrix. This lead to considerable differences in reference nominal resistence of plate (=G(s) in german parlance, with only a smooth bulge allowed on the back side but no cracks or spall emitted form the backside of the plate) when attacked obliquiely by APC.

As a further notice, mastering thick RHA or CHA armor was not a trivial achievement. It was extremely difficult to translate lab knowledge into applicable mass production techniques. Large section thoicknesses don´t cool quickly internally and this can cause temper brittleness. In order to retain ductility, all manufacturers added relatively high% of Nickel, which was an effective toughening agent in thick sections. However, large amounts of Ni also increase the chance of temper embrittlement from secondary and tertiary reheating exposure such as welding. That was even moreso if the heat had elevated impurities (sulpur and phosphorus) or other hardening but also embrittling agents (like manganese and silicium, among others).
The Germans largely resorted back to Krupps expensive Ni/Cr steel for the thickest section plates to adress these issues and developed very clean electrosteel (low impurities) with very complex multi stage thermo-mechanical treatment to create weldable medium section thickness plate. Even then, the welding had to be done interlocked to provide constructive resistence if the weld seam broke.

[Don Juan]

Chapter and verse on IT 70 cemented armour cracking on Humber armoured cars here:

https://heritage.canadiana.ca/view/ooci ... c5774/1666

[Peasant]

This document might be of relevance to this topic: https://apps.dtic.mil/sti/tr/pdf/ADA953337.pdf
Interesting that they seem to be testing the size of the exit hole when the armour is perforated at velocity much above the BL of the plate.

[Peasant]

...A report from beginning of 1943 says: "Armour of the new American tanks [talking here about M4A2] despite having large thickness (55-60mm) is not very resistant. For instance, during testing at Gorohovetz firing range, it was perforated by a 45mm sharp tipped shell already from 400m distance, while the PTP [safety] limit for a normal 45mm shell is about 150-200m.
Also, ballistic testing of side armour with PTRD using tungsten carbide bullet shows that it can be defeated from 500-100m(?).

I believe the last part says that 1,5in(38mm) hull side armour it can be defeated at up to 500m distance, which is indeed a bit farther away than would be expected from good quality RHA.

Source: "Stalin's Steel Fist: History of the Soviet Tank (1943 - 1955)" by M.Svirin.

Edit: Another one:

The hull of the Mk.II tank is made of cast and rolled steel parts with high chrome, nickel and molybdenum content, characterized by good homogeneity, hardenability and toughness. Brinell hardness measurements show that it belongs to the class of homogeneous, medium hardness armor. It's chemical composition is close to that of domestic chrome-nickel-molybdenum steel FD-7924.
Thickness of the armour of Mk.II "Matilda" tank is 70-78mm its protection is overall similar to that of a KV tank. Tempering of the armour is good. Spalling of the armour with hits close to limits of penetration was not observed.

[Peasant]

Might prove useful in this discussion: