Volume 57, Issue 10

Over the years, the majority of the studies that addressed the study of the solidification of free graphite cast irons have focused primarily on explaining the influence of graphite morphology on the solidification mechanism, but instead, have dedicated much less attention to the influence of the metallic phase that grows together with graphite, which is the austenite. However, the combined use of the direct autempering after solidification technique and color etching makes it possible today to extend the study of the solidification of these materials by considering the growth of the austenite. The objective of the present study is to demonstrate how these two techniques can be exploited to understand the solidification macro and microstructure and the microshrinkage cavities formation in free graphite cast irons. The results show that these techniques find potential application in studies aimed at developing procedures to refine both the macroscopic and microscopic solidification structures, such as those investigating the use of inoculants to refine the austenitic grain structure, and its consequences on the microsegregation pattern, the eutectic solidification and the tendency to form microshrinkage.

The optimization of the etching process parameters in high chromium cast irons (HCCI) for general microstructural observation, eutectic and secondary carbide quantification and 3D analysis of the carbide network is indispensable for further microstructure-properties correlation. The main aim of the current work is to evaluate the efficacy of certain etchants to aid in ‘objective specific' microstructural characterization of a 26 % Cr HCCI alloy in the as-cast (AC) and heat treated (HT) state. Samples were etched with various solutions, including Vilella's reagent, Nital, Murakami's reagent and its modification, and two other etchants. It was seen that specific etchants are needed depending upon the final objective and the results are reliant on their selection and optimal usage. Etching the AC and HT samples with Vilella's reagent for 7 s was sufficient to develop a contrast between the phases which was observed both in confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM). Murakami's reagent and a modification of the same, were used in successfully quantifying the eutectic and secondary carbide volume fraction (CVF) in the AC and HT samples, respectively. Consequently, the knowledge gained in this study will be used as a basis for further microstructural analyses of these alloys and be applied to other alloys in the family.

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 M 7 C 3 carbides (5.3 vol. %) and the eutectics based on M 7 C 3 carbide (rosette-shaped cells) and M 3 C carbide (coarse-net-shaped cells or Ledeburite-shaped cells). The total amount of carbide phases was 51.8 vol.% whereas the volume fractions of M 7 C 3 -eutectic cells and M 3 C-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 (Fe 3.88 Cr 2.76 Mo 0.22 Mn 0.12 Si 0.02 )C 3 . Substantial part of carbide precipitates (primary and eutectic both) exhibited the duplex nature presenting Cr-rich/Mo-depleted core (M 7 C 3 carbide) and Cr-depleted/Mo-rich shell (M 3 C carbide). Duplex carbides were closely adjusted to austenite + M 3 C 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 M 6 C 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.