The study of the deterioration of Fe-Cr-Ni alloys by Mössbauer spectroscopy

Abstract

Duplex stainless steels (alloys) with chromium content in the ferritic phase greater than 13% are subject to embrittlement due to hardening of the ferrite phase. Some components of the primary loops of Pressurised Water reactors (PWRs) are made of cast duplex stainless steels ( austenite and ferrite). This kind of steel may age even at relatively low temperatures ( under 400°C i.e. in the temperature range of PWR service conditions) due to a micro structural evolution of the ferritic phase to a hard and brittle chromium-rich cx'-phase, and the ageing results in degradation of mechanical properties. In spite of this, the cast stainless steels are frequently used in primary coolant piping because of their high resistance to corrosion. To prevent hot cracking in the cast materials, the presence of some delta (o) ferrite is required. An important consequence of this ageing process is the hardening or the embrittlement of the ferritic phase, caused by a chromium rich ex ' -prime phase, possibly produced by either nucleation-and-growth or spinodal decomposition. In the present research, the samples from the Hot leg and Crossover Leg elbows in the primary loop of the PWRs from the Koeberg Nuclear Power Plant were investigated for mechanical degradation. The impact energy measurements were performed on these samples at the ESKOM TRI Centre by means of the Charpy V-notch measurements at room temperature. From these measurements, it was found that the samples that were not aged have high impact energy strength and this strength decreases with increasing ageing time. The 6-ferrite and they-phase (austenite) content have been determined by means of Mossbauer spectroscopy and electron microscopy. The Mossbauer spectra obtained for these samples consisted of a paramagnetic singlet arising from the austenite and the magnetically split sextet resulting from 6-ferrite phase in the sample. From both the Mossbauer and the electron microscopy, the amount of 6-ferrite was found to be about 10 to 20%. The amount of the ferrite and they-phase present in the samples is thought to be in direct relation to the impact energy. The amount of 6-ferrite obtained from Mossbauer spectroscopy was compared with charpy impact energy values. No clear correlation existed between the 6-ferrite content and the Charpy impact energy values.