What causes carbide precipitation between 800-1500 F?

What causes carbide precipitation between 800-1500 F?

• A. Chromium and carbon combine, depleting chromium available to bond with oxygen to form the surface oxide layer
• B. Excess nickel content
• C. Overheating beyond melting point
• D. Atmospheric contamination

Answer: (A)

The idea being tested is why chromium carbide forms in stainless steels at elevated temperatures and how that affects corrosion resistance. At roughly the 800–1500 °F range, carbon diffuses and readily reacts with chromium to form chromium carbides (Cr23C6) at grain boundaries. As these carbides form, chromium is tied up in the carbides and is no longer available to maintain the chromium-rich surface oxide that protects the metal. The result is chromium depletion in the nearby areas and a loss of corrosion resistance along grain boundaries, which is the sensitization phenomenon. The other factors don’t cause this same precipitation mechanism. Excess nickel changes the alloy’s austenitic structure but isn’t what drives carbide formation. Overheating beyond the melting point would melt the metal rather than precipitate carbides in a controlled manner. Atmospheric contamination leads to oxidation or other surface issues, not the internal chromium-carbide precipitation that depletes the protective chromium at the surface.

The idea being tested is why chromium carbide forms in stainless steels at elevated temperatures and how that affects corrosion resistance. At roughly the 800–1500 °F range, carbon diffuses and readily reacts with chromium to form chromium carbides (Cr23C6) at grain boundaries. As these carbides form, chromium is tied up in the carbides and is no longer available to maintain the chromium-rich surface oxide that protects the metal. The result is chromium depletion in the nearby areas and a loss of corrosion resistance along grain boundaries, which is the sensitization phenomenon.

The other factors don’t cause this same precipitation mechanism. Excess nickel changes the alloy’s austenitic structure but isn’t what drives carbide formation. Overheating beyond the melting point would melt the metal rather than precipitate carbides in a controlled manner. Atmospheric contamination leads to oxidation or other surface issues, not the internal chromium-carbide precipitation that depletes the protective chromium at the surface.