STUDY OF CREEP CAVITATION IN A STAINLESS STEEL WELDMENT USING SMALL ANGLE NEUTRON SCATTERING AND SCANNING ELECTRON MICROSCOPY

Strain-relief cracking, also referred to as reheat cracking, is a generic creep failure mode that has been observed in many welded stainless steel structures operating at high temperatures in UK nuclear power plant. One of the challenges currently faced by nuclear utility EDF Energy is the need to predict, with high certainty, the life-time of ageing engineering plant operating in the creep regime. A study of creep cavities near a reheat crack, similar to 28 mm long, in an AISI Type 316H austenitic stainless steel header is reported. The cracked component was removed after 65,000 hours operation at a mean temperature of 525 degrees C in a nuclear power plant. Creep cavities in the vicinity of the crack are observed using scanning electron microscopy (SEM) and found to be mainly surrounding inter-granular carbides. Optimized metallographic and image analysis procedures are used to quantify the size and area fraction of creep cavities as a function of distance from the crack along a line normal to the crack face. Small angle neutron scattering (SANS) is used to quantify creep cavitation, averaged over a large gauge volume, similar to 20 mm(3), at similar positions normal to the crack line. A systematic increase in the true fractional size distribution of cavities is observed approaching the crack. The cavity size distribution peaks between 100-300 nm diameter and this correlates closely with the quantitative metallography results of cavities in this sample. SANS has the advantage of being non-destructive and also the examined volume is several mm(3), against an area of hundreds of mu m(2) in the case of SEM, therefore giving more statistically significant results. However the two techniques are complementary, as the detailed microscopic information from SEM is required to interpret the SANS data.