The effect of increasing carbon level on titanium carbosulfides and their influence on toughness in ultrahigh-strength steels

Titanium additions to HY180 and AF1410 steels, which have carbon levels of 0.10 and 0.16 wt.% respectively, have resulted in a doubling of the fracture toughnesses of these steels. This improvement in toughness is due to the superior void nucleation resistance of the titanium carbosulfide particles which are formed when a titanium addition is made in the absence of manganese or rare earth elements. Titanium additions to AF1410 heats which have been modified to carbon levels of 0.20 and 0.25 wt.% lead to less marked toughness improvements.;The objective of this work was to determine the causes of the sharp decrease in toughness with increasing carbon content. Two sets of AF1410-type materials were made at carbon levels of 0.16, 0.20 and 0.25 wt.%. A titanium addition of 0.02 wt.% was made to one chromium sulfide.;Characterization of the titanium-modified heats has shown that increased carbon levels lead to increased numbers of larger titanium carbosulfide particles. Increasing the carbon content leads to increasingly earlier void nucleation in these heats. Titanium carbosulfide particles greater than 2 mum in dimension nucleate voids by particle fracture at low strains while void nucleation at the finer titanium carbosulfide occurs both by particle fracture and particle/matrix interface decohesion at plastic strains as high as 0.9. Void nucleation at both titanium carbosulfide and chromium sulfide particles does not appear to be effected by changing matrix strength.;Experimental results suggest that the drop in Charpy impact energy associated with increased carbon contents is not due to the changing titanium carbosulfide particle distributions or increasing strength level alone. No evidence was found to suggest that cementite, precipitated upon tempering, plays a direct role in fracture.;Oxide particle distributions were found to be of importance in optimizing toughness. Heats containing magnesium oxide appear less tough than those in which the oxygen is predominantly gettered as MgO.Al2O3. Differences in thermal expansion coefficients between the inclusions and matrix may be an explanation for this behavior.