Big Red here with a question from "Bounty Hunter Boushh"...
So, TAKE FIVE!
"Big Red, Can you explain Sharon’s steel issues and how they impacted the helmet program?"
Wow, that question is a tall order. To make sure we cover everything were going to need a plan. Let’s start with the metallurgists at Watertown Arsenal Laboratories.
By late1943, Schlueter was suffering significantly from visor cracking problems to which the Office, Chief of Ordnance directed the metallurgists at Watertown Arsenal to investigate. Schlueter used Sharon helmet stock exclusively which lead to the analysis of their steel quality. Below is a paraphrased summary of Watertown’s findings regarding Sharon helmet steel:
To understand what our friends at Watertown are talking about, we need to define these new steel related defects before we can properly explain how they impacted the helmet program.
“Martensite” - An extremely hard but brittle steel. Martensite is formed when austenitic helmet steel is allowed to cool too fast resulting in a lack of carbon to make austenite.
“Decarburization” - Describes a low carbon condition in the steel mixture resulting from improper cooling time.
“Piping” - An ingot-forming defect that occurs when the ingot is not properly cooled. Improper cooling causes the ingot to shrink quickly forming small voids. These voids trap gas that cools the steel and creates pockets of low carbon content.
“Ingot Cropping” - A process for cutting away the brittle outer layers of newly formed ingots.
Now that we have a grip on the steel issues we can decipher Watertown’s metallurgical findings by reviewing what we have learned about making Hadfield manganese.
In order to make austenitic manganese steel correctly, the steel maker must maintain their furnace at the correct temperature for the correct period of time, to allow the carbon in the recipe to properly dissolve and mix into the formula.
Once this is done they are ready to make an ingot which is accomplished by pouring the properly heat treated steel into an ingot mould. The steel must be cooled slowly so the carbon in the mixture has time to settle into position within the manganese creating a fully austenitic structure.
Decarburization is a way of saying that an ingot cooled too fast which prevented the carbon from remaining fully mixed within the steel creating portions of the steel that now have a low carbon content. Martensite forms in all the portions of the steel that no longer have enough carbon mixed in to form austenite.
Because the exterior of an ingot is exposed to air, it is normal for the outside layer of an ingot to cool fast forming an outer skin of martensite. Ingot cropping is performed in order to crop or slice away and remove this brittle outer layer.
Once cropped, Martensitic structures only remain if the ingot experienced piping. Piping refers to the formation of small cavities inside an ingot. These cavities trap gasses that cause the surrounding steel to cool fast enough to prevent the carbon from settling into position, which results in pockets of martensite.
Martensite can be formed within austenitic manganese whenever the steel is heated to a temperature that the carbon inside the mixture becomes fluid and the steel is allowed to cool too quickly. Unfortunately this means that the formation of martensite was not restricted to the ingot phase but was possible at the rolling phase as well. If the steel maker did not properly control the heating and cooling of the steel during the rolling phase the resulting sheets would decarburize and form an outer skin of martensite.
Finally, regarding the impact on the helmet program,
Lessons learned from pressing Carnegie-Illinois helmet steel at McCord taught us that Hadfield’s manganese must be heated to the correct temperature for the correct amount of time or it retained undissolved carbides that made the steel brittle. Now we have learned from pressing Sharon helmet steel at Schlueter that Hadfield’s manganese must also be cooled at the correct temperature for the correct amount of time or the carbon doesn’t distribute evenly creating low carbon pockets that solidify into martensite that is rigid and brittle.
Although the factors for the brittleness of Sharon helmet steel were different than those of Carnegie-Illinois, the fact that Sharon helmet discs were brittle and were distributed to both fabricators only served to increase the total amount of breakage that was plaguing the fabrication of the helmet.
Big Red Says!
FIVE'S OVER - MOVE OUT!