Abstract
The rate of creep crack growth å in the intercritical layer of HAZ for parent steel P91 was measured for a range of loads at 600 °C. The TDCB samples were used, the test times ranging from 19 to 6603 h. The crack tip parameter C^* (t) was calculated using line displacements due to creep, the latter obtained from permanent deflections as corrected on instant plasticity.
For long term tests, relatively high growth rates appear so that no unique asymptotic power law å ~ C^*Φ exists for all load regimes. Furthermore, an anomalous range of load dependence on crack initiation times was revealed. An attempt is made to correlate these effects with changes in ductility due to possible re – precipitation during thermomechanical exposition. This suggestion is documented by scanning electron microscopy of fracture surfaces and transmission electron microscopy of the exposed intercritical zone ahead of the crack tip.

Key words
steel P91, heat affected zone, creep crack growth, growth initiation times, re - precipitation

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