Big Red here with a question from "Blondie"...
So, TAKE FIVE!
Blondie asks,
"Big Red, Why did M-1 helmets crack?”
Well Blondie, that is a simple question with a complex answer,
The short of it is “residual stress” in the helmet resulting from a combination of poor-quality steel, poor manufacturing habits and the design of the helmet which required several cold working operations to shape it.
A quick rundown to put this into perspective is that no company within American industry really had much experience working with Hadfield’s Manganese since it was used for armor and helmets during WWI. Manganese is used for helmet steel because it is exceptionally strong and gets stronger when it is cold worked, meaning hammered into shapes without heating it first.
The problem is if the steel sheets you are trying to make something out of, in this case a helmet, weren’t perfectly heated during the ingot and sheet rolling phase, the steel would have brittle pockets that would break under the stress of forming it. This problem existed during WWI for the manufacturers of the M1917 helmet and still existed at McCord when they manufactured the M1917A-1 helmet in 1940.
Let’s start by defining the steel related issues, followed by manufacturing habits and finally the design of the helmet as they relate to cracking.
Hadfield’s Manganese is austenitic steel, which is a fancy way of saying it must be heat treated just right to make it super strong. Basically, Hadfield Manganese is a mixture of carbides and manganese which, if correctly treated with heat when you mix it, the steel forms into a nice even crystalline structure called austenite. If the steel isn’t heat treated just right when mixed and then rolled into sheets, the resulting steel will have small pockets of undissolved carbides which are brittle and will crack when cold worked into shapes.
“Hadfield Manganese” (Helmet Steel), This steel is made from a formula invented by Sir Robert Hadfield for mixing carbon and manganese together with heat to form a super strong steel.
SIr Robert Hadfield
“Austenite” Refers to microscopic crystalline structures formed in steel when carbon and manganese are blended correctly with heat.
“Fully Austenitic” means that the steel is super strong and without any defect because it was perfectly mixed and heated both when they made the steel and when they rolled it into sheets.
“Undissolved Carbides” Refers to helmet steel that wasn’t heated at the correct temperature or long enough. This means that some of the carbon didn’t dissolve and mix into the manganese to form austenite.
“Cold Working” Refers to hammering or working the steel into the shape of a helmet without using heat.
“Residual Stress” Refers to the stress left in the steel created by the pressing and spanking operations used to bend and shape the steel disc into a helmet.
After the steel disc was pressed into the shape of the helmet on the forming press and trimmed, the helmet underwent a spanking operation where the front of the helmet was bent back in the opposite direction of the initial draw to form the helmet’s visor. All three of these operations added significant stress to the steel and if the steel was of poor-quality, cracks would appear in the areas of high stress.
Poor manufacturing habits compounded this issue. McCord was experienced in pressing automotive radiators from softer steels that retained nowhere near the level of stress that helmet steel did. Initially, not understanding that trimming die maintenance would be critically important, small nicks from use were allowed in the knives of the trimming dies. Nicked knives left small notches on the edge of the helmet and these notches created a point of weakness where it didn’t matter if the steel was good or bad because the stress alone could cause an edge crack to form on a notch.
Finally, the shape of the M-1 helmet’s design from the initial draw into a pot shape combined with the spanking operations retained an incredible amount of residual stress. The cold working of the steel would more than double its hardness from sheet form to that of the helmet however, Arsenal tests recorded that M-1 helmets would also retain from 80,000 to 90,000 psi of stress in the areas that were cold worked. Ultimately, the design of the helmet’s shape created stress from the manufacturing process that came close to exceeding the stress limit of the steel formula even if all conditions were perfect.
So to sum up, the residual stress left behind in the steel from drawing it into a pot and spanking it into shape would crack the helmet apart at any point where the steel was weak or the cutting dies left a little notch.
Thanks Blondie!
and remember,
Big Red Says!
FIVE'S OVER - MOVE OUT!
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