In most cases armour that thick would be face hardened armour and therefore not weldable. Face hardened armour such as used for belt armour on battleships is too hard and of too high of carbon and nickel content to be welded (The only use of homogenous, and therefore weldable, armour of such thickness was the American substitution of Class B for Class A for turrets. The Class A as originally called for, was of such poor quality that it was unacceptable, and so Class B was used instead.)
In the case of welding weldable homoguous armour the thickest the plates would likely be was 18cm, but preferably less than 15cm thick. For welding such heavy plates they would normally have used submerged arc welding whenever they could. SA is done by a machine, or is a type of automatic welding. (Yes even during the 30's and 40's) With SA the arc is protected from the atmosphere by pooring a protective powder over the arc. The machine makes several passes over and over until the bevel is finally filled. The cap may then be done manually to avoid undercutting the top of the parent metal. The root is sometimes done manually first and then the back of root is welded or "back -welded". Submerged arc until recently always has a higher rate of defects (all welds have defects but there's a tolerance to what is allowed- boiler tube welding has virtually no defects allowable though) than manual welds, but manually welding such wide and deep bevels over such distances are unpractical. Generally, submerged arc can make for amply acceptable welds-Unless the engineers get greedy and try too heavy of passes to make more time.
Welding armour plate during the 30's and 40's was usually done using stainless steel weld metals. For example, the German welding rod for armour plate was the Fox A7 which is a type of 307 stainless steel. The tensile strength is only about 90,000 psi compared to the tensile strength of about 130,000 psi for Wh. But SS is more ductile, than using a matching or over matching weld metal. Furthermore, austenic SS excludes hyrdogen from the weld metal and the fusion boundry (hydrogen enbrittlement being the lead cause of weld failure).The USN found a similar solution. The USN specified type 310 (25/20) SS for welding STS, and Type 309 SS for welding STS to carbon high tensile or mild steel.
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