Secondary ion mass spectrometry and scanning electron microscopy analysis of grain boundary oxides in 9Cr–1Mo steel and influence of grain size on scale spalling

Abstract

When oxidised at 973 K, annealed 9Cr–1Mo steel forms convex oxide ridges at the boundaries of prior austenite grains. In addition to this morphological difference these grain boundary oxides show compositional dissimilarities with the oxides formed within the grains, as suggested by the results of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), and substantiated by secondary ion mass spectrometry (SIMS) results. Both EDX and secondary ion imaging confirm the high Si content of the oxides in the ridges. However, it is the synergistic influence of the variations in the chemical composition and morphological features of scales that governs the stability of the oxides growing within the alloy grains. These variations have their origin in the oxidation process at the alloy grain boundaries. Hence, to achieve a more complete understanding of this phenomenon, oxide ridges and the scales within the grains have been extensively characterised by obtaining depth profiles using SIMS and observing the morphological evolution using SEM and surface profilometry. The oxide ridges act as discontinuous interfaces, so that the oxides growing in the adjoining areas experience sufficiently high stresses to produce spalling. The finer the alloy grains, the greater the number of discontinuous interfaces (i.e., grain boundary ridges) and the stressed area, and therefore the earlier spalling occurs.

MST/1824