Abstract text (incl. figure legends and references)

Introduction

The as-cast microstructures of the 22-30wt.%Cr and 2.0-2.7wt.%C high chromium cast irons (HCCIs) consist of primary austenite dendrites and eutectic M₇C₃ carbide. The wear resistance of these irons can be improved by alloying and heat treatments. In some applications, annealing treatment is necessary for machining operations [1-2]. There are few reports on the morphology of coarse and fine secondary carbides forming during annealing.

Objectives

The purpose of this research is to use TEM for studying the secondary carbide in the annealed structure. A preliminary EELS examination of the carbides will also be presented.

Materials and methods

As-cast specimens of the 28wt.%Cr (R) and the 28wt.%Cr-1wt.%Mo (Mo1) irons were annealed at 800^oC for 4 h, followed by slow cooling to 500^oC and then furnace cooling. A JEOL JEM-2010 TEM equipped with an Oxford® Inca detector was used for conventional TEM and EDS microanalysis. A JEOL, ARM200F TEM equipped with a Gatan Quantum ERS electron energy loss spectrometer was used for EELS. Both microscopes were operated at an accelerating voltage of 200 kV.

Results

BF-TEM images in Figs. 1 (a & b) show that the microstructures after annealing of the reference (R) and the Mo1 irons, consisted of eutectic carbide with faulting contrast and precipitated coarse secondary carbides in areas close to eutectic carbides and fine secondary carbides in the centres of the dendritic matrix. The Mo1 iron contains finer and denser secondary carbides. The selected area diffraction patterns (SADPs) confirmed that the eutectic carbides are M₇C₃, the secondary carbides are M₂₃C₆, and the matrix is ferrite (α). The coarse secondary carbides may be nucleated along preferred directions during the early stages of annealing. The finer secondary carbide precipitation may have occurred later during longer holding time or during slow cooling. TEM-EDS in Fig. 2(a) revealed that the Fe/Cr wt.% ratio of eutectic M₇C₃ carbides was 0.4, while that of coarse and fine secondary M₂₃C₆ carbides were in range of 0.52-0.69. Preliminary EELS results are given in Fig. 2(b). The differences in the spectra of ELNES of the C K-edge of M₇C₃ and M₂₃C₆ were (i) the peak at 289 eV (marked by arrows) has been observed only in the case of M₇C₃, and (ii) the relative heights of peaks 1 and 2 were higher in M₇C₃.

Conclusion

TEM revealed that coarse and fine secondary carbides formed by annealing of the 28wt.%Cr without and with 1wt.%Mo addition are M₂₃C₆ carbides. The finger print of the C K-edge, including the Fe/Cr wt.% ratio can be used to distinguish M₇C₃ from M₂₃C₆ carbides.

References

1. K.A. Kibble and J.T.H. Pearce, Cast Metal, 1995, 8, 123-127.

2. Y.G. Song et. al., Metals 2021, 11, 1690.