I’m familiar with the use of rollers to make glass panes but surely it’s not that?
In: Engineering
Rolling is a form of cold working which aligns the grain structure of the steel to add strength.
Steal has a grain to it similar but not as extreme as how wood has a grain to it. If you forge or roll steel you can control that, and you can make a product that is a higher quality materials. If you cast a metal you just pour the molten metal into a cast and you get whatever you get. You could heat and cool after casting but there’s no way to reshape the metal.
You might think that steel is something nearly indestructible. It’s not. With sufficient amount of forces, even metal can be warped or pressed like clay. This requires infrastructure, but its worth it
Cold rolling is using rollers to compress the metal at a certain temperature, which caused it to have its molecular structure changed, making it harder, less plastic, and stronger.
So yes. In a sense it is like the glass roller.
Most solid metals are poly crystalline; if you cut, polish and etch them and put them under a microscope you will see they are made up of many single crystals packed together called grains. Glass has no such crystals and is called amorphous.
In cast steel, molten metal is poured into a mold, and as the liquid metal cools, grains are formed according to how heat is extracted from the mold ie as temperature drops liquid changes to solid. Grains crystallise from the contact area of the mold to its interior metal in arrangements called dendrites or columns, depending on the speed of heat loss. If very slow, they are planar.
In rolled steel, hot solid metal is squeezed through rollers and the grains are elongated and stretched against each other in a single direction, which strengthens the steel plate on the surface where it is being rolled.
Comparing the surfaces of the cast and rolled steel, the grains are arranged more strongly in rolled metal than cast metal to compressive and tensile forces, which makes it better for resisting projectiles.
Steel has to be rolled at the correct temperature so the desirable grain shapes are maintained and no new grains are formed(recrystalisation), and just like rolling glass then is a limit to how much it can be deformed in one go, or it will crack.
It has to be hot enough to be worked and formed, but not so hot that the elongated grains shrink and form new compact grain shapes.
> I’m familiar with the use of rollers to make glass panes but surely it’s not that?
It is exactly that. There are two types of steel rolling: cold rolling and hot rolling, which work exactly as you’d imagine. Both realign the crystal structure of the steel to increase strength. Cold rolling produces a more polished finish and is a bit stronger, but the force needed is much larger than hot rolling, so very thick metal pieces (like rolled armor) were typically hot rolled.
Video of hot rolling in action: https://www.youtube.com/watch?v=6xnKmt_gsLs
Wikipedia on rolled armor that mentions hot rolling: https://en.wikipedia.org/wiki/Rolled_homogeneous_armour
Vendor discussing hot vs cold rolled products: https://www.metalsupermarkets.com/difference-between-hot-rolled-and-cold-rolled-steel/
Just keep in mind that it’s not a hard and fast rule that “rolled armor” was better than “cast armor.”
Each technique has specific advantages and disadvantages. For example, while rolled steel may be “tougher” than a cast steel plate of similar thickness and manufacturing quality, it would be harder to work it into more complex shapes. And armor shape has a great deal of influence on effectiveness of said armor.
Balancing between protection, weight, ease and scale of manufacture, ease of quality control, etc. was always a challenge, and each tank represents a specific selection among all those factors.
Rolling isn’t the specific point to worry about. Rolling works just like you think. For steel it can be either hot or cold.
if it is done hot you get rolled homogenous armor (Rha)
the important part is the homogenous part. That means that all the grains are oriented in any and all directions.
this leaves it soft enough not to shatter when hit by a shell.
from what I understand Rra will be about 10% thinner than cast armor
If you want to roll steel plate for battleships, you need something like this:
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