After initial casting of this steel it was subject to a process known as carburisation. The metal is heated to above the ferrite-austenite transition in a carbon atmosphere. This establishes a concentration gradient and hence carbon diffuses into the steel. Usually the steel is then hardened by quenching. This produces what is known as a case hardened steel - with a hard surface (case) surrounding a tough core. The carbon gradient can be seen in the changing shade of the sample from left to right, with high carbon concentration at the left (surface) and hence a martensitic phase, changing to the lighter shade consisting of mostly ferrite.
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