Fatigue crack growth(FCG)rate tests were conducted for main pump casing material at room temperature to investigate the effect of long-term aging at 400 ℃.It was found that the FCG rate increases with advancing aging time.However,there were similar △Kth values for both the virgin material and aged material under the same R.At room temperature,△Kth was observed to decrease with R increasing.An equation incorporating the effects of the Kmax and △K,is proposed to describe FCG rates in the Paris regimes for all of the loading ratio condition.Additionally,fatigue-fractured appearance near crack initiation sites reveals quasi-cleavage fracture in thermal aged specimens.