Big Red here with a question from "Kevin Delaney"...
So, TAKE FIVE!
“Big Red, I recently bought a M-1 helmet on the internet. It has a rear seam, swivel bails, sewn straps but it doesn't pass the magnet test. What do I have? A fake, a movie prop? Should I return it?”
Hey Kevin, great to hear from you.
Don’t panic my friend, all M-1 helmets are magnetic to some extent.
To begin our discussion let me say I have no idea what the “magnet test” is, but I assume someone told you that real M-1 helmets are non-magnetic and that you can find fakes if a magnet sticks to them. That being the case, allow me run you through some of the truths about the steel used to make the M-1. Where the non-magnetic reference comes from, why a non-magnetic steel did not remain 100% non-magnetic and why it was chosen for use with the helmet.
When developing their list of characteristics for a new American helmet, the Ordnance Department noted that non-magnetic qualities were desired so that the helmets would not affect a compass held in the wearer’s hand.
Interestingly, every WWII compass reading manual you can find includes a passage that states the helmet should be removed prior to using the compass.
So, if you have to remove your helmet when using a compass why would it matter if the helmet was made of magnetic steel and if the M-1 helmet was not 100% non-magnetic why was it chosen for use with helmets?
Why Hadfield’s manganese?
Other potential steels were considered for use with the M-1 helmet until July 1942 when it was definitively decided that non-magnetic requirements for the American helmet would not be relaxed and that low alloy magnetic steels would not be considered for helmet application. Hadfield Manganese steel is a unique alloy steel containing an extraordinarily high manganese content mixed with high carbon. The principal advantage is that, unlike other steels that must be closely monitored through multiple heat treatments to become strong, it can be drawn into forms such as the M-1 helmet without annealing or heat treatment, it is non-magnetic, resists corrosion and will afford a high ballistic resistance. This steel has rapid hardening properties when cold worked and is capable of doubling in strength yet remain ductile, meaning it dented upon impact instead of fracturing, after shaping thereby meeting all Government requirements of helmet performance.
Magnet tests were the least fruitful of any testing the Ordnance Department performed on helmets during WWII. Originally, the magnet test was only intended to see if the steel affected the use of a compass. Later, magnet tests were conducted to see if there was a correlation with recorded magnetism to helmet stock and its deep draw ability or a helmet and its ballistic performance.
Testing revealed that the slight degree of magnetism in all M-1 helmets had little affect on the normal use of a compass. Magnetism testing to determine whether or not helmet stock had the potential to perform poorly during the initial draw proved ineffective and, as it turned out, magnetism testing on formed helmets only indicated the presence of outer layers of magnetic structures demonstrating no correlation to whether or not a helmet would perform badly in a ballistic test.
Why does a magnet stick to my non-magnetic helmet?
Answer: Martensite and Ferrite
The M-1 helmet has only ever been non-magnetic in specification and urban legend. In practice, all M-1 helmets will demonstrate at least a small level of magnetism resulting from cold-working the steel.
Fully austenitic manganese steel is non-magnetic but, putting aside for a moment that most helmet discs were not fully austenitic throughout to begin with, the cold-working of this steel will create a small displacement of carbon resulting in enough surface layer martensite to attract a magnet. In addition to displacing carbon, the severity of the cold-working operations upon the austenite will bring about the formation of ferrite.
Ferrites - A hard, brittle, iron-containing crystalline structures. They are made of iron oxide or rust mixed with another metal like manganese and are highly magnetic.
Although the magnetism of the helmet’s trimmed edge can be strong enough to attract a magnet through the edging, the stainless steel edging is actually the only absolutely non-magnetic portion of the M-1 helmet as even the loops, which are made of the same material, proved to possess a slight magnetism.
Big Red Says!
FIVE'S OVER - MOVE OUT!