Big Red here with a question from "Captain Stillman"...
So, TAKE FIVE!
"Big Red, If the steel in the M-1 helmet gets stronger when cold-worked and weaker if heat treated, what happens to the steel if you use your helmet as a cooking pot?"
Excellent question Sir,
If you have done your “M-1 Helmet Production Breakage” homework, you now understand that Hadfield manganese steel was chosen for use with the helmet because, in its fully austenitic condition, the steel is extremely hard yet ductile and has a high ballistic limit. We also know, when making this type of steel, that strict control over temperatures during both heating and cooling operations is critical. You may recall that during our discussion on "Sharon Steel Corp. and Helmet Steel Defects” how Sharon struggled controlling the annealing and cooling of their steel during the sheet rolling phase resulting in outer layers of brittle martensite. The martensite was a result of carbide precipitation along the grain boundaries of the austenitic microstructure which left portions of the steel decarburized.
Well Sir, you see…..
Decarburization is a way of saying that, heating a helmet to a temperature that the structure of the steel becomes fluid and is then allowed to cool too fast prevents the carbon from remaining fully mixed within the steel. This creates portions of the steel that now have a low carbon content and all portions of the steel that no longer have enough carbon mixed in to form austenite, form martensite instead.
Simply put when the fully austenitic microstructure of Hadfield manganese steel is exposed to a temperature of 550°F decomposition from austenite to martensite begins. At a temperature of 600°F and above, the austenitic structure becomes fluid and unstable. The carbon in the austenitic structure begins to move around inside the microstructure of the steel and then gets trapped in flux when the helmet is removed from the heat source and cools resulting in the formation of martensite in the burn zone.
Yes Sir, sorry Sir….. So,
A small campfire will burn at temperatures, on average, between 600°F and 930°F which is more than enough heat to alter the microstructure of helmet steel. From our discussions on “why helmets crack”, we know that martensite, although very strong, is extremely brittle and will crack or fracture under the impact of a projectile.
In fact, if a helmet was heated over a fire, like a cooking pot, the heat could relax the residual stress in the burn zone sufficiently to allow the stress in the unaffected areas to crack the helmet.
Tests were performed on burnt helmets at the Watertown Arsenal Laboratories in late 1945, under the direction of Abreham Hurlich, in order to determine if there was any value in attempting to salvage helmets that had sustained burns. Hurlich and his team determined that due to the nature of the changes that occurred in the burn zone that a helmet exposed to heat sufficiently intense enough to discolor the metal also altered the microstructure of the steel resulting in a total loss of ductility and effectively eliminating the helmet’s ballistic resistance rendering it useless for combat.
Therefore, it was recommended that no attempt should be made to salvage any helmet that showed burn discoloration.
Your sergeant my not have a clue what the heck a precipitating carbide is but he darn sure knows that cooking in your helmet will turn it into scrap metal so remember.....
It is however, ok to use your helmet as a bathtub….
Big Red Says!
FIVE'S OVER - MOVE OUT!