Microstructures of Pearlite and Martensite Transformed from Ultrafine-Grained Austenite Fabricated through Cyclic Heat Treatment in Medium Carbon Steels

This study investigated the microstructures of pearlite and martensite transformed from ultrafine-grained austenite in 0.45C steel and V-added 0.45C steel. The mean prior austenite grain sizes were refined to be 4.5 mu m in the 0.45C specimen and 2.4 mu m in the V-added 0.45C specimen through cyclic heat treatment of austenitizing and water quenching, i.e., repetition of martensite <-> austenite transformations. The austenite with various grain sizes was either air-cooled or water-quenched to cause pearlitic or martensitic transformation, respectively. The fragmented lamellar structure and isolated cementite particles having particulate morphology were observed in the pearlite transformed from the ultrafine-grained austenite. Results indicated that the ferrite and cementite formed during eutectoid transformation from the ultrafine-grained austenite were not necessary to grow cooperatively maintaining lamellar shape. The packet size in lath martensite was refined significantly accompanied by decreasing of the austenite grain size, since several different variants of martensite were formed from different segments of austenite grain boundaries even when the austenite grain size was fine. In contrast, the block size did not depend on the austenite grain size.