Identification of carbides in 28wt.%Cr high chromium cast irons by color etching and electron probe microanalysis

Abstract text (incl. figure legends and references)

Introduction

As-cast microstructures of high-chromium cast irons (HCCIs) with hypoeutectic composition in range of 27-30wt.%Cr and 2-3wt.%Cr irons consist of primary austenite dendrites with eutectic austenite, partially transformed to martensite, and eutectic M7C3 carbides. Normally, the hardness and wear resistance of these irons depend on the type of matrix and carbide formation. Mo can be added to HCCIs to modify the carbide and to increase the volume fraction of the carbide in their microstructures. The aim of this research is to identify the type of carbides and carbide transformation in 28wt.%Cr cast iron containing Mo additions using color etching techniques and electron probe microanalysis.

Materials and methods

Carbides in a 28wt.%Cr (R iron), a 28wt.%Cr-6wt.%Mo (Mo6) and a 28wt.%Cr-10wt.%Mo (Mo10) have been characterized by X-ray diffraction, light microscopy with color etching by Groesbeck"s reagent [1], and an Apreo S Thermo Fisher Scientific field emission scanning electron microscope. The chemical composition of the eutectic carbides was analyzed by electron probe microanalysis (EPMA).

Results

LM and SEM observations shown in Fig. 1 reveal the as-cast microstructures of 28wt.%Cr–2.6wt.%C iron (R iron), and the irons with 6wt.%Mo addition (Mo6) and 10wt.%Mo addition (Mo10). The microstructures consist of primary dendritic austenite (g), eutectic austenite which partly transformed to martensite during cooling and eutectic M7C3 carbides. Mo addition to HCCIs promoted the formation of M6C and M23C6 carbides rather than M7C3 carbides. In the Mo10 iron eutectic M7C3 carbides were not observed and the microstructure contained radiating colonies of M23C6 carbide and lamellar/fish-bone M6C carbide. After color etching, Fig. 1(a-c), M7C3 carbide was colored brown and blue, while austenite matrix remained lighter in contract. The M6C carbide with a lamellar structure/fish-bone had a dark-brown coloration, and the radiating colonies of M23C6 carbide were rainbow colored. At higher magnification, the SEM image in Fig. 1(d) showed that the M23C6 carbide was present as a transition zone between the eutectic M7C3 and M6C in the Mo6 iron, indicating a carbide transition of M7C3 → M23C6 → M6C [2]. SEM-BEI images (Fig. 1(d-f)) revealed that the eutectic M6C carbide in the Mo6 and Mo10 irons had the brightest contrast compared to the M23C6 and M7C3 carbides.Furthermore, Fig. 1(g-i) showing the phase mapping by EPMA, clearly demonstrates that M6C carbide had the highest Mo content, corresponding to the SEM-BEI displaying the brightest contrast due to the high atomic number of Mo. The M7C3 and M23C6 had higher Cr contents. The quantitative elemental analysis can be used to estimate the atomic formulae for M7C3, M23C6 and M6C in the Mo6 iron.

Conclusion

Color etching with Groesbeck"s reagent can be used to distinguish the M7C3, M23C6 and M6C carbides by brown or blue, rainbow, and dark brown coloration respectively. EPMA analysis suggested the stoichiometric formulae of these carbide in the 28wt.%Cr-6wt.%Mo (Mo6) as (Cr4.7Fe1.8Mo0.1)C3, (Cr14.1Fe6.9Mo1.3)C6 and (Cr1.5Fe3.2Mo1.0Si0.1)C, respectively.

References

E.C. Groesbeck, Solutions for Carbides, etc., in Alloy Steels, appendix to report of Committee E-4 on Metallography, Proc. ASTM, Vol 26 (Part I), 569 (1926). S. Imurai et al., Effects of Mo on microstructure of as-cast 28 wt.%Cr-2.6 wt.%C-(0-10) wt.% Mo irons. Mater. Charact., Vol. 90, 99-112 (2014).