Study On B/M Microstructure And Properties Of Ultra-High Carbon Steels

The four kinds of high carbon experimental steels with different component wereprepaired by vacuum induction furnace. On this bases, the microstructure and mechanicalproperties of UHCS(Al 0%) were studied after the quench transformation. In the process theaustenitizing temperature range from 880℃to 920℃while austempering temperaturerange from 250℃to 350℃.The effects of Al and C addition to the UHCS on microstructureand properties were examined respectively.The results showed that, the aluminum addition, austenitizing temperature,austempering temperature are the three factors which affected the bainitic transformation inUHCS significantly. With the raising austenitizing temperature, the incubation period ofbainite transformation was extended significantly,and bainite transformation rate becamesslow. The amount of undissolved carbide increases with the decrease of austenitizingtemperature. Meanwhile, the morphology of bainite in UHCS exhibited finer characterasitcs.At the higher austenitizing temperature, bainite became thicker. The transformation rate ofbainite became higher and the bainite is thinner with the shorter austenitizing time.Coarsening austenite grain and much more carbide solveing at the extended austenitizingtime will cause the bainite incubation time extende and bainite morphology coarsened.Austempering process is another factor of bainite transformation. With the raisingisothermal temperature, the incubation period of bainite transformation was shortensignificantly, the transformation period of bainite was shorten, length of bainite plate wasshorten, and the thickness was increased. The bainite microstructure morphology wascharacterized in lateral grown up remarkably. The bainite transformation volume fractionvaries with isothermal time in"s"curves. The bainite microstructure transformation rate wasgrown up remarkably since the start of transformation with the raising isothermaltemperature.The different contents of Al addition to UHCs lead to the different effects. When smallAl addition to ultra-high carbon steel Al (such as 13 # test steel, 0.5% Al), bainitetransformation was promoted, and the complete bainite transformation time shorten; while Al content further increases, such as 8 # Al content in steel high as 1.5%, bainitetransformation was not speeded up further, the dynamics of bainite transformation wasdelayed otherwise, opposite to 13 # with Al content (0.5%). It is noticed that adding Al to theultra-high carbon steel more or less, the comparison with and without added Al, havecontributed to bainite. For the higher carbon content, the incubation period of bainitetransformation was longer.Under the same austnitizing condition, the toughness of UHCS with bainite complexmicrostructure processed by 300℃isothermal quenching improved significantly than 250℃. Impact toughness of 1# steel processed by austenite 900℃×30min and 300℃isothermal 180min in bath was up to 96J. The impact toughness of UHCS became improvedwith aluminum addition. The wear resistance of UHCs without aluminum was not affectedby heat treatment.