API 571 Certification (Corrosion & Materials) Practice Test

Which materials are susceptible to graphitization?

1. C-0.5Mo steels

2. Stainless steels

3. Aluminum alloys

4. High-strength carbon steels

The correct answer is: C-0.5Mo steels

Graphitization is a form of material deterioration where carbon in a steel alloy transforms into graphite. This typically occurs at elevated temperatures and can significantly affect the mechanical properties of the material, leading to loss of strength and ductility. C-0.5Mo steels, which contain a small amount of molybdenum for enhancing creep resistance, are particularly susceptible to graphitization when subjected to high temperatures, especially in service conditions of pressure vessels, piping, and other critical components. The presence of carbon in these steels allows for the transformation to occur, particularly if the material is exposed to a reducing atmosphere or prolonged heating. Other materials listed, such as stainless steels, aluminum alloys, and high-strength carbon steels, are less prone to graphitization under the usual conditions they experience. Stainless steels generally resist graphitization due to their alloying elements, which enhance corrosion resistance and stability at high temperatures. Aluminum alloys do not contain carbon in a structure that can graphitize, while high-strength carbon steels, despite containing carbon, do not typically exhibit this failure mode in the same way as the C-0.5Mo steels do. Therefore, C-0.5Mo steels are identified as the material most susceptible to graphitization.