Study of phase transformation and mechanical properties evolution of duplex stainless steels after long term thermal ageing (>20 years)

The ferrite of thermally aged CF3M duplex stainless steels has been studied at the atomic scale. Accelerated ageing of ingots has been performed at 350 °C in laboratory up to 200,000 h (>20 years). Spatial and chemical evolution of the microstructure of the ferrite has been characterised by 3D atom probe. In addition, micro-hardness has been performed on the same samples. The results obtained have been compared to the microstructural and mechanical characteristics of ferrite of the same ingots aged at 325 °C (close to service temperature) and to the ferrite of an elbow steel aged on-site. This work has shown that: (i) accelerated ageing at 350 °C anticipates the on-site ageing at 323 °C, (ii) the linear relationship found between micro-hardness measurements and the variation V (defined as the integral of the difference between the Cr frequency distribution of the aged sample and the corresponding binomial distribution characteristic of a random solution with the same concentration) is still valid after 200,000 h of ageing at 350 °C, (iii) the activation energy is the same for both spinodal decomposition and G-phase precipitation and finally (iv) the coarsening of G-phase particles has no influence on the relationship between ferrite micro-hardness and the V parameter.