The Effects of Carbon Content on the Formation of Ti2CS and the Fracture Toughness of Ultra-High Strength Steels

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9712620 Garrison The objective of this research is to investigate the effects of carbon content on sulfide formation and void nucleation resistance in titanium-modified steels. It is hypothesized that increasing the carbon content of titanium-modified steels causes the precipitate particles of the Ti2CS sulfide to be formed at higher temperatures, thereby producing larger sulfide sizes. Since the void nucleation resistance of Ti2CS particles is expected to decrease with increasing particle size, tying up sulfur as Ti2CS should be less effective in improving fracture toughness as the carbon content increases. If it is confirmed that increasing the carbon content increases the sulfide size and diminishes the void nucleation resistance in the titanium-modified steels, then methods will be explored to: (1) refine the sulfide size when the sulfur is gettered as Ti2CS, or (2) create new methods of gettering the sulfur into void nucleation resistant particles that remain effective at higher carbon levels. One approach that will be considered is to dissolve the sulfides and later reprecipitate them at lower temperatures in the austenite. If the sulfide size is refined by this approach and the reprecipitated sulfides are Ti2CS, then the toughness of higher carbon steels which contain reprecipitated sulfides will be investigated as well as the void nucleation resistance behavior of these finer reprecipitated sulfides. Gettering sulfur as Nb2CS rather than as Ti2CS will also be considered. Recent results suggest that this is feasible and that such particles are resistant to void nucleation. %% This research explores microstructural factors that influence the fracture behavior of structural steels. It has potential impact on the design of fracture-resistant metal alloys that could be employed in structures with improved lifetimes and safety. ***