AmetastableaggregateofferriteandcementiteresultingfromthetransformationofausteniteattemperaturesbelowthepearliterangebutaboveMs,themartensitestarttemperature.Upperbainiteisanaggregatethatcontainsparallellath-shapeunitsofferrite,producestheso-called"feathery"appearanceinopticalmicroscopy,andisformedaboveapproximately350°C(660°F).Lowerbainite,whichhasanacicularappearancesimilartotemperedmartensite,isformedbelowapproximately350°C(660°F).