Characterization of Microstructure and Phase Elemental Composition of 15 wt.% Cr – 2 wt.% Mo Cast Iron with Boron Addition

Abstract

Abstract The article is aimed at the study of structural features and phase chemical composition in 3.6 wt.% C-15 wt.% Cr-2 wt.% Mo cast iron prepared with adding 0.50 wt.% boron. The work was fulfilled using optical and electron scanning microscopy, EDS, XRD, hardness measurement, and thermodynamic modelling. It was found that due to boron effect the cast iron acquired hypereutectic structure consisting of the primary M7C3 carbides (5.3 vol. %) and the eutectics based on M7C3 carbide (rosette-shaped cells) and M3C carbide (coarse-net-shaped cells or Ledeburite-shaped cells). The total amount of carbide phases was 51.8 vol.% whereas the volume fractions of M7C3-eutectic cells and M3C-eutectic cells were 21.0 vol.% and 74.7 vol.% respectively. The matrix comprised austenite, martensite and pearlite whereas austenite volume fraction was measured as 32 vol.%. Chromium and molybdenum were unevenly partitioned in phases performing the opposite character of a distribution. The primary carbides contained about 37 wt.% Cr and 1.6 wt.% Mo with the stoichiometry of (Fe3.88Cr2.76Mo0.22Mn0.12Si0.02)C3. Substantial part of carbide precipitates (primary and eutectic both) exhibited the duplex nature presenting Cr-rich/Mo-depleted core (M7C3 carbide) and Cr-depleted/Mo-rich shell (M3C carbide). Duplex carbides were closely adjusted to austenite + M3C eutectic cells to be the starting interface for their solidification. A minor amount of micron-sized areas with 67.5 wt.% Mo and 4.6 wt.% Si were observed to be presumed as M6C carbide. The cast iron was subjected to holding at 950 °C for 2 hours with subsequent oil-quenching. This treatment caused the precipitation of fine secondary carbides followed by the martensite transformation which resulted in significant hardness increase from 672 HV (as-cast) to 1038 HV. The solidification sequence is discussed in terms of structure observation and Thermo-Calc modelling.