Effect of Nb on Pearlite Transformation Behavior of Eutectoid Steel

Effect of Nb on the microstructure and isothermal transformation kinetics of pearlite in eutectoid steels(C 0.75%,C 0.78%) was studied by metallographic,SEM and critical temperature of transformation and TTT(time temperature transformation).The results show that the amount of pro-eutectoid ferrite in the steels with micro-addition Nb(0.04%,0.064%) increases through comparing with steel without Nb addition.That is probably because that the carbon content at eutectoid point increases with addition of Nb,meanwhile the morphology of transformed pearlite changes obviously,namely the aspect ratio of cementite reduces markedly.In addition,the transformation temperatures of both pro-eutectoid ferrite and pearlite have been raised by addition of Nb,namely thermodynamic stability of overcooling austenite is reduced by Nb.The analysis of transformation kinetics shows that Nb 0.04% could raise the nose temperature of TTT graph by about 50 ℃ and retard the eutectoid transformation start time for about one order of magnitude,which imply that addition of Nb retard the nose temperature and the pearlite transformation at lower temperature,then the hardenability is improved.