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Thermal ageing and short-range ordering of Alloy 690 between 350 and 550 °C
Mouginot, Roman,Sarikka, Teemu,Heikkilä,, Mikko,Ivanchenko, Mykola,Ehrnsté,n, Ulla,Kim, Young Suk,Kim, Sung Soo,Hä,nninen, Hannu Elsevier 2017 Journal of nuclear materials Vol.485 No.-
<P><B>Abstract</B></P> <P>Thermal ageing of Alloy 690 triggers an intergranular (IG) carbide precipitation and is known to promote an ordering reaction causing lattice contraction. It may affect the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor (PWR) components. Four conditions of Alloy 690 (solution annealed, cold-rolled and/or heat-treated) were aged between 350 and 550 °C for 10 000 h and characterized. Although no direct observation of ordering was made, variations in hardness and lattice parameter were attributed to the formation of short-range ordering (SRO) in all conditions with a peak level at 420 °C, consistent with the literature. Prior heat treatment induced ordering before thermal ageing. At higher temperatures, stress relaxation, recrystallization and α-Cr precipitation were observed in the cold-worked samples, while a disordering reaction was inferred in all samples based on a decrease in hardness. IG precipitation of M<SUB>23</SUB>C<SUB>6</SUB> carbides increased with increasing ageing temperature in all conditions, as well as diffusion-induced grain boundary migration (DIGM).</P> <P><B>Highlights</B></P> <P> <UL> <LI> SRO was suggested in Alloy 690 with 9.18 wt% Fe after thermal ageing at 350, 420 and 475 °C. </LI> <LI> Prior thermal treatment promoted SRO before ageing. </LI> <LI> Cold work led to recrystallization and precipitation of α-Cr upon ageing at 550 °C. </LI> <LI> Thermal ageing promoted IG precipitation of Cr-rich M<SUB>23</SUB>C<SUB>6</SUB> carbides and DIGM. </LI> </UL> </P>