Isothermal transformation of austenite to bainite in high carbon steels

Abstract

Bainitic transformation occurs in a temperature range between the high temperature transformations such as ferrite and pearlite and the low<br><br>temperature transformations such as martensite. Bainite consists of aggregates of platelets or laths of ferrite separated by regions of phases such<br><br>as austenite or martensite. The aim of this research is determination of the kinetics of the austenite transformation over the temperature range<br><br>250–500 ◦C. For this purpose, several specimens from a plain-high carbon steel were isothermally heat treated at different temperatures from<br><br>250 to 500 ◦C for different holding times. The specimens then were studied using a transmission electron microscope. The results showed that<br><br>austenite, in the high carbon steel, transformed to bainite over the temperature range 250–475 ◦C, according to separate C-curves on the isothermal<br><br>transformation diagram. Itwas shown that above∼325 ◦C, austenite transformation starts with formation of upper bainite and belowthat temperature<br><br>the transformation starts with formation of lower bainite. Also, in some regions in the temperature range 250–375 ◦C both upper and lower bainite<br><br>could be seen, simultaneously.