Atom Probe Tomography of Carbides in Fe-Cr-(W)-C Steels

In this study, Fe-Cr-C and Fe-Cr-W-C alloys are characterized using atom probe tomography. The alloys have been heat treated at 1070 degrees C for 30 min and subsequently at 780 degrees C for various time periods. Carbide formation is observed at each state. Cr-C precipitates smaller than 5 nm in radius for short heat treatment times and larger than 50 nm for heat-treatment times greater than 1000 s are observed. It is found that the phase interface moves during the first time period at an almost constant speed. Later on the velocity of the phase interface decreases. Furthermore, kinetic assumptions for carbide growth from a previous theoretical study have been verified. As expected, a decrease of the microhardness with increasing aging time is detected which is caused by martensite tempering. The aim of this study is to measure the change in chemical compositions across phase interfaces between matrix and precipitates to obtain a better understanding of the precipitation process.