Logo from Scott Cunningham.
Logo from Scott Cunningham.
ast armour is made by pouring molten metal into moulds which are shaped as the required vehicle component. After removing from the moulds the rough spots, risers and gate marks are ground off and the component is heat treated. Heat treating consists of heating, quenching (rapid cooling in water or oil) and tempering as described for RHA. Cast armor is never worked or squeezed down into thinner form as rolled armor is, therefore it has an inferior grain structure and lower ballistic resistance.1
Cast armor is always homogeneous and was never face hardened (apart from a few experiments). The difference in hardness between the outer surface and inner surface found on some cast armour is more a result of poor heat treatment or insufficient alloy content than any intentional effect intended to increase ballistic resistance.1
An M4A1(76)W Sherman showing the rounded faces and edges characteristic of cast armour. From Armour in Focus.
Homogeneous armour works best when it is the same hardness throughout, as changes in hardness form stress concentration boundaries which significantly degrade ballistic resistance. It was not until the Sherman got well into production that the USA learnt to control the hardness of varying thickness castings by hitting the thick parts with water jets during the quenching process. This cooled the thick parts so they were better quenched, and consequently were harder than they would be if they had simply immersed the whole casting into still water.1
The chemical composition of cast armour is thus much the same as RHA but its quality is inherently more variable and the thickness of the casting cannot be controlled as accurately. Cast armour is also at a disadvantage because it is generally of complex shape and varying thickness and cannot be given uniform heat treatment, which has an adverse effect on its ballistic properties.3
On the other hand, cast armour is more readily fabricated into complex shapes than RHA plates. It facilitates an efficient distribution of armour as well as allowing a reduction in the number of welded joints and the preparation that goes with them. The ultimate demonstration of this is provided by turrets and hulls that are cast in one piece, but the size of such castings is apt to tax industrial resources.3
Cast armour resists less well than RHA…
Cast armour resists less well than rolled armour of the same hardness and thickness. USA tests of production quality armour in 1942 and 1943 clearly shows that cast armour resists less well than RHA of the same hardness and thickness. Cast armour 51mm thick showed a 15% to 20% inferiority compared to 51mm RHA plates when hit by 75mm projectiles. The tests also demonstrated that RHA can be raised to higher hardness levels than cast armour without losing ductility, and therefore ballistic resistance.1
The USA accepted the lower ballistic quality of cast armour when manufacturing the M4A1 Sherman, relying on a little extra thickness and the rounded corners to make up for the essential weakness of the armour material. The net effect was that the later versions of M4A1 Sherman were less well protected than the later versions of RHA Shermans. One unit which had both, the 743rd Battalion, kept their cast hull Shermans out of combat, a lesson apparently learned the hard way.1
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