Papers by Author: Chong Sool Choi

Abstract: Effect of thermal cycling(γ↔ε) on γ→ε martensitic transformation kinetics and damping capacity of Fe-17mass%Mn alloy has been studied. The amount of ε martensite increases with thermal cycling in spite of decrease in Ms temperature. The increase in ε martensite content with thermal cycling is attributable to an increase in the density of martensite nucleation sites by introduction of dislocations during thermal cycling. The γ→ε martensitic transformation kinetics shows a burst mode in the non-cycled specimen, while the kinetics exhibits a sigmoidal mode in the cycled specimens. The damping capacity of the alloy increases with increasing the ε martensite content in the non-cycled specimen. On the contrary, the damping capacity of the alloy decreases with increasing the ε martensite content in the cycled specimens. The reason is that the dislocations introduced during thermal cycling, which obstruct the movement of the damping sources, become more with thermal cycling.

Abstract: Recrystallization behaviors have been investigated with respect to two different kinds of the initial structures, original austenite and martensite, in an Fe-32%Ni alloy. The recrystallized austenite grain size from the martensite is much smaller than that from the original austenite, and decreases linearly with increasing the initial hardness, independent of the initial structure. The recrystallization sequences are different between the two structures: only one step due to recrystallization appears in hardness-temperature curve of the original austenite, whereas two steps corresponding to reverse transformation of α’ to r’ and recrystallization are shown in that of the martensite.

Abstract: Effects of niobium content and cooling rate on ferrite and bainite start temperatures (Ar3, Bs) and microstructural features have been studied in niobium bearing ultralow carbon microalloyed steels. The Ar3 and Bs temperatures decrease as niobium content or cooling rate is increased. The dependence of Ar3 on cooling rate is greater than that of Bs in all niobium contents. The bainitic ferrite laths become longer and narrower with increasing niobium content and cooling rate, and niobium also shows a tendency to decrease polygonal ferrite grain size.