Microstructural evolution and carbides in quenched ultra-low carbon (Fe-C) Alloys

Abstract

Binary Fe-C alloys are basic for various carbon steels in view of chemical compositions, thus, understanding the microstructural evolution in Fe-C alloys is fundamentally necessary. Three Fe-xC (x = 0.02, 0.05 and 0.1 (wt.%)) alloys were selected and the microstructure changes during water-quenching process has been studied using transmission electron microscopy (TEM). Fine carbides with a size of tens of nanometer, mainlθ-Fe3C cementite, are observed in quenched Fe-0.02C and Fe-0.05C samples. Lath martensite has been commonly observed in all the quenched alloys. Body-centered cubic (bcc) {112}<111>-type twinning structure with fine particles on the twinning boundaries has been frequently observed in the quenched Fe-0.1wt.%C samples; while the fine cementite are absent in the twinned region. A novel formation mechanism for the quenched microstructure has been proposed as follows: Austenite →twinned martensite (bcc + on twinning boundaries) →lath martensite + carbides on lath boundaries +carbides + ferrite (α-Fe). The first transition corresponds to the martensitic transformation; while the second and third transitions are the auto-tempering results of the first one. In the ultra-low carbon sample (Fe-0.02C), the third transition will be the observed microstructure; Slight high carbon (Fe-0.05C) will result in the second transition, not to the third one. The product of the first transition will be the twinned martensite (Fe-0.1C), and the final microstructure will not experience the second and third transitions. Of course, the quenching condition can also affect the final microstructure.