Inthiswork,theevolutionofretainedausteniteduringisothermalbainiticheattreatmentinSAE52100steel,1.01C–1.36Cr–0.32Mn–0.25Si(wt.%),isinvestigatedwithopticalmicroscopy,X-raydiffractionandthermo-magneticmeasurements.Asignificantamountofausteniteisretainedinthematerial.Thedependenceofthevolumefractionofretainedausteniteonbainiticholdingtimeat503Kshowsamaximumat45min.Ithasbeendemonstratedthattheincreaseofvolume