Characterization of Nb-Microsegregation and Eutectic Carbide in As-Cast Nb-Microalloyed Al-Free Case Hardening Steel

Abstract

The current work involves systematic experimental examination of the as-cast microstructure of a Nb-microalloyed Al-free case hardening steel, 18CrNiMo7-6, with 0.078 wt pct Nb. The aim is a thorough quantification and characterization of Nb-microsegregation and eutectic NbC. A combination of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis with energy dispersive and wavelength dispersive X-ray spectroscopy (EPMA, EDS, and WDS) are used for this purpose. Eutectic NbC is characterized for its location, shape, size, and amount. Differential scanning calorimetry (DSC) measurements indicate that γ-Fe − NbC eutectic forms in the temperature range of 1399–1390 °C, while Scheil simulation predicts this range to be ≈1418–1402 °C. The average eutectic composition is measured to be 10.91 wt pct Nb using EDS. γ-Fe − NbC eutectic in steel can be classified as an irregular eutectic with faceted (NbC) and non-faceted (γ-Fe) constituent phases. Scheil solidification simulation predicts that about 30 mole pct of the total NbC in this steel can be present in the form of eutectic. These results, thus, point out the need for modification of the eutectic via alteration of cooling rate or using modifiers.