Big Red here with a question from "Slim Pickens"...
So, TAKE FIVE!
"Big Red, What were the differences between the Sharon Steel and Carnegie-Illinois steel making processes and how did the differences affect helmet cracking?"
We have discussed the issues encountered by the fabricators, the various types of cracks, why cracks formed, and, in general, the metallurgy of the steel defects that contributed to the breakage of helmets during fabrication. With this knowledge we can now discuss the differences between the steel making methods used by Carnegie-Illinois and those of Sharon Steel, which will put the origin of all the steel defects we have discussed into perspective.
Carnegie-Illinois used an open-hearth furnace to mix steel. This method makes it difficult to maintain a specific temperature for a set time whereas the electric arc furnace used by Sharon made this relatively easy. This is why Carnegie-Illinois helmet stock often suffered from undissolved carbides while Sharon’s did not.
The open-hearth process, by its nature, created an environment of radiant heat, which allowed the necessary slow cooling Hadfield steel needs to become fully austenitic whereas the process that utilizes an electric arc furnace does not. The cooler environment of Sharon’s process allowed the outer skin of their ingots to harden quickly while the core is still hot. The core then contracts as it cools and forms little pockets of trapped gas called piping. These small cavities of gas cause the steel in the cavity to harden quickly forming areas of low carbon. This is why Sharon helmet stock suffered from brittle pockets and runners of martensite while Carnegie-Illinois sheets generally avoided this condition.
Further, Carnegie-Illinois heat rolled their steel creating an environment that allowed sheets to cool slowly and remain austenitic. Sharon cold rolled their sheets followed by an annealing process, which if time and temperature were not managed correctly caused martensite to form. The result of Carnegie-Illinois hot rolling process was that only one side of the final sheet had a risk of forming a layer of martensite while the cold-rolling process followed by heat treating performed by Sharon Steel often resulted in a loss of carbon and the formation of martensitic layers on both sides of their sheets. This fact is important because if a helmet disc with martensite on one side is purposefully or accidentally pressed to the inside of the helmet it is less likely to cause cracking issues but if it is on the outside it will. This means that Sharon discs, with martensite on both sides, had a 100% potential for breakage whereas a Carnegie-Illinois sheet only had a 50% potential.
Ultimately, M-1 helmets cracked due to “residual stress” in the helmet resulting from severe cold-working combined with poor-quality steel and poor manufacturing habits. In the beginning, the addition of Schlueter and Sharon to the helmet field basically turned out to be doubling down on these issues. In truth, it didn’t matter if the root cause was undissolved carbides or martensitic structures because the result was brittle steel and brittle steel cracked. Even though Carnegie-Illinois had better control over their steel quality by this time, Sharon Steel provided defective helmet stock to both fabricators causing a significant increase in production breakage that in total, raised serious questions about the viability of the M-1 helmet’s future.
Big Red Says!
FIVE'S OVER - MOVE OUT!