Transition welds have been used in some Advanced Gas-Cooled Reactors to attach a 2¼Cr1Mo spool piece to a hot reheat header, which is fabricated from 316L material. These welds were made using Inconel-82 filler and were post-weld heat-treated (PWHT) at around 705°C. In this paper, a creep-fatigue crack growth assessment has been carried out on the 2¼Cr1Mo ferritic side of the weld using the R5 procedure.

As the welds have been PWHT rather than solution heat treated, therefore there are residual welding stresses present. On the 2¼Cr1Mo ferritic side of the weld, a 60MPa through-wall bending stress has been applied in the axial direction and a 60MPa membrane stress in the hoop direction. These are consistent with the recommendations of R5 Volume 7.

Due to the differing thermal expansion coefficients of Inconel and ferritic materials, thermal mismatch stresses induced from changes in temperatures have been considered. It has been assumed that at the PWHT temperature of 705°C there are no thermal mismatch stresses. Any reduction from this temperature generates thermal mismatch stresses which are accounted for by using secondary Stress Intensification Factor values in the calculation.

On the 2¼Cr1Mo ferritic side of the weld, a 5mm deep by 50mm long circumferential defect (semi-elliptical surface-breaking surface defect) postulated to exist on both internal and external surfaces has been used. Limiting defect sizes have been calculated following the R6 procedure.

Four cases have been investigated. After presenting the investigation results, conclusions are drawn.

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