Tailoring the mechanical properties of steel sheets using FeC films and diffusion annealing

In this work amorphous FeC films were deposited on thin sheets of interstitial free steel using physical vapor deposition. Annealing treatments were then carried out to diffuse C from the coating into the substrate at temperatures lower than those traditionally used in carburizing treatments. The yield stress was shown to significantly increase with annealing temperature from ~120MPa at 25°C up to a maximum of 300MPa at 630°C without any significant loss of ductility. At 710°C, a decrease in yield strength was related to the coarsening of carbides inside the IF steel (confirmed by atom probe tomography), and the associated reduction in the matrix solid solution carbon concentration (confirmed by thermoelectric power measurements). The through-thickness carbon diffusion profile was predicted using Fick's law and validated by Knoop hardness measurements. Yield strength predictions were accurate if the crystallization of the FeC film was taken into account as it controls the amount of carbon available to be diffused in the interstitial free steel substrate.