P. R. Gangasani, Dura Bar, Woodstock, IL
Continuous cast gray iron bar (pearlitic grade) was selected for evaluation of fatigue strengths using rotating beam test machine. Twenty to 25 samples (prepared from mid-radius or a region away from edge of the bar) in each case were tested at various stresses to establish fatigue limits and low cycle fatigue strengths of the plain and nitrided gray iron samples (test samples were machined from the same continuously cast bar and then nitrided). The results in the form of S-N plots are presented.
S-N plot illustrates the fact that nitrided surfaces clearly increased fatigue stress cycles of gray iron and at low cycle fatigue tests (with higher stresses) the samples of nitrided surfaces lasted for longer number of stress cycles. Nitrided surface is expected to have compressive stresses imposed by the surface treatment process. This may be the most probable cause of the improved performance.
Key words: Cast-iron 1, gray-irons 2, pearlitic-gray-iron 3, nitriding 4. Fatigue 5.
Summary: 1. The fatigue lives of the nitrided samples at the elevated stresses were significantly higher than the plain gray iron samples.
2. There were more failures of plain gray iron samples just above the fatigue limits than the nirided gray iron samples.
3. There are no significant differences in the microstructures and fractures of plain gray iron samples and the nitrided gray iron samples except for the white layer that could be seen after etching the microstructure with 5 % natal.
4. The most probable cause for improved fatigue lives of the nitrided gray irons could be due to the compressive residual stresses set up during the nitriding process as reported by many investigators on nitrided steels.
