Abstract
Several experiments have been made in an effort to reveal some of the fundamental factors which play a role in the graphitization of pure high-carbon steels deoxidized with aluminum and silicon. The authors suggested following mechanism for graphitization of high carbon steel.
(1) It is considered that the graphitization proceeds by the ‘nucleation and growth’ reaction. In order for cementite to decompose into graphite and ferrite, nuclei must be formed. The growth of graphite nodules takes place by the diffusion of carbon atoms through the ferrite lattice.
(2) It is considered that these nuclei may be formed by decomposition of unstable cementite, and the rate of nucleation depends on the form and composition of cementite. Nitrogen-and carbide-forming elements such as chromium, manganese, tungsten and molybdenum make cementite stable. Aluminum, silicon, titanium, zirconium and boron tend to make cementite unstable and promote graphitization through its role as a scavenger for nitrogen. A small amounts of tin and arsenic make a steel highly resistant to graphitization.
(3) Graphite forms more rapidly in steels after quenching to martensite of cold working than after normalizing or annealing. It is considered that the strain added to cementite particles by prequenching to martensite and tempering or cold working would tend to decrease the stability of cementite.
