Synergistic influence of alloy grain size and Si content on the oxidation behavior of 9Cr-1Mo steel

The synergistic influence of prior-austenite grain size and silicon content of 9Cr-1Mo steel on the resistance to scale spallation has been studied in air at 773 K (for 500 hr) and 973 K (12 hr). Two steels, irrespective of their grain size and Si content, did not show spallation during oxidation at 773 K. Spallation occurred at 973 K, and fine-grain steels exhibited less spallation resistance than coarse-grain ones (in low-as well as high-Si steels). Among the four possible combinations of grain size ans Si content, the steel with low Si and fine grains showed least resistance to spallation, while the steel with high Si and coarse grains showed the best resistance. Spallation was found to initiate in the areas adjoining the oxide ridges formed at the alloy grain boundaries. Oxide scales at the ridges and within the grains were analyzed by scanning electron microscopy (SEM/EDX) and secondary-ion mass spectrometry (SIMS). These analyses suggest depletion of silicon from the areas adjoining grain boundaries, resulting in thicker scaling that triggers spallation in such areas. For similar grain-size materials, the necessary thickness for spallation was attained earlier with low-Si steel rather than in high-Si steel.