Effect of heat treatment and cold work on the ordening of alloy 690 at 475 °C

Alloy 690 was developed to improve the corrosion resistance of components in nuclear power systems since the 90s. Its composition is close to the Ni2Cr stoichiometry which promotes an ordering reaction upon ageing at temperatures below 550 °C with a maximum effect at 475 °C. Atomic ordering promotes lattice contraction and brittle intergranular fracture and reduces the mechanical properties. It is a concern for the structural integrity of Alloy 690 in modern pressurized water reactors (PWR), especially considering the primary water stress corrosion cracking (PWSCC) behavior. To assess the effect of ageing time, cold work and thermal treatment on the level of ordering of Alloy 690, four original conditions (solution annealed, solution annealed and cold rolled and/or heat-treated) were aged for up to 10 000 h at 475 °C. The microstructures were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), and the ordering levels were assessed by hardness and lattice parameter measurements. All conditions showed evidence of short-range ordering. As compared to the solution annealed condition, thermal treatment decreased the level of ordering. Cold worked samples showed contradictory results with reduced hardness increase but stronger lattice contraction.