The rooling contact fatigue strength tests were performed on austempered spheroidal graphite cast irons with graphite nodules ranging 120 to 800/mm
2, by using Nishihara-type wear machine under a light oil lubricant. The irons were austenitized at 1148 K for 3600 s and then austempered at 648 K or 508 K for 7200 s. The relative sliding rate was fixed at 9 %. Rolling contact fatigue limit (F
∗) of upper bainite iron decreased from 1400 MPa to 1200 MPa as graphite nodules increased from 120/mm
2 to about 300∼350/mm
2, and F
∗ was almost unchanged with nodule count above 350/mm
2. On the other hand, F
∗ of lower bainite iron took the maximum value, 1700 MPa, at about 300∼350/mm
2. The more graphite nodules, that is, less inter-nodule distance should shorten the fatigue limit, since fatigue crack tended to propagate through graphite nodules. However, the graphite nodules could make crack blunt and retard the crack propagation, especially in lower bainite iron. These functions of graphite nodules should influenece the fatigue limit.
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