[en] As part of the helium environment creep test programme at FIAT, fifteen specimens from a single cast were tested in the same stringer at 700°C and 77 N/mm². Ten of the samples had much higher creep strengths than the other five. This report describes structural investigations into this discrepancy. Two ''strong'' and two ''weak'' samples were examined by chemical analysis, optical and electron microscopy. As shown in the Appendix, the ''strong'' samples contained a fine intragranular precipitate of M₂₃C₆ which was absent in the weaker samples. This precipitation was associated with a relatively coarse grain size. It is thought that the differences probably arose from inhomogeneous hot working of the bar before the final solution treatment. Average carbon levels increased by up to 0.007 wt.% during the 5543 hours of exposure of the samples to simulated HTR helium. (author)