1999 Volume 71 Issue 10 Pages 698-703
Fatigue tests were carried out on austempered spheroidal graphite cast iron (ADI) and ferritic spheroidal graphite cast iron (ferritic SG iron) to clarify the influence of exposure to water on fatigue strength. In wet condition, i. e. the test condition of exposure to water during testing, ADI and ferritic SG iron showed the same fatigue strength as that of irons tested at stress amplituds above the fatigue limits in air. While at the value of stress values below the fatigue limit in air, specimens of ADI and ferritic SG iron had no fatigue limit. Even though the number of cycles exceeded 107 cycles, the fatigue strength decreases continuously. The decrease of the fatigue strength of ADI results largely from the preferential dissolution of acicular ferrite, which is induced by corrosion fatigue. Therefore, the fatigue strength of 107 cycles for ADI exposed to water was reduced to 50 % of the fatigue limit 310 MPa in air. However, the fatigue strength of 107 cycles for ferritic SG iron was decreased only 10 % from the fatigue limit 205 MPa in air, because there is no preferential dissolution of ferrite. On the other hand, specimens of ADI and ferritic SG iron dried after dipping in water for 480 hours, showed obvious fatigue limits, and the fatigue limits were reduced by 32 % for ADI and 25 % for ferritic ferritic SG iron than that tested in air. The reduction of fatigue limits results from the corrosion pits, which act like a notch.
