Graphitization

Graphitization can take place in ferritic steels after exposure to high temperature for extended time, owing to reversion of the cementite in the pearlite to the more stable graphite phase. It is a particular form of microstructural degradation that was formerly observed relatively frequently in petrochemical components. With the development of more stable Cr Mo steels, it is not often seen today, but occurs from time to time both in petrochemical plant and in steam generators in which the temperature is high and the material is not entirely stable.

Graphitization of carbon and carbon-molybdenum steel piping during service at temperatures above 425 C (800 F) has caused numerous failures in steam power plants and refineries.  The graphitization tendency of carbon and carbon-molybdenum steels is increased when the aluminum content exceeds about 0.025%. Steels deoxidized with silicon may also be susceptible to graphitization. Deoxidation with titanium will usually produce good resistance to graphitization.

Carbon-molybdenum steels exhibit greater resistance to graphitization than do carbon steels.
The degree of embrittlement depends on the distribution, size, and shape of the graphite. The severity of graphitization is frequently evaluated by bend testing. If graphitization is detected in its early stages, the material can often be rehabilitated by normalizing and tempering just below the lower critical temperature. Steel that has undergone more severe graphitization cannot be salvaged in this manner; the defective region must be cut out and re welded, or the section must be replaced. Carbon and carbon-molybdenum steels can be rendered less susceptible to graphitization by tempering just below the lower critical temperature.