Abstract
Near-threshold fatigue crack growth behaviour is investigated for four stainless steels of ferritic 17Cr, martensitic 12Cr, and austenitic 18Cr-8Ni and 25Cr-20Ni in laboratory and dry air at room temperature. The stainless steels are free from oxide-induced closure because large amount of chromium in the stainless steels assures formation of the passive film on the fracture surface and thus, prevents oxide thickening within the crack. In laboratory air, therefore, δKth-values for stainless steels are lower when compared with those for carbon or low alloy steels with similar strength levels. Dry air does not influence the near-threshold crack growth behaviors for stainless steels, while it eliminates the excess oxide debris from fracture surface of carbon or low alloy steels and reduces their δKth-values to the level of stainless steels. In austenitic stainless steels, furthermore, δKth- values are significantly reduced and fracture surfaces near δKth are markedly crystallographic at lower load ratios, compared to ferritic and martensitic stainless steels. Such phenomena are related to strain-induced martensitic transformation peculiar to the austenitic stainless steel.
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