Thermal Aging of Duplex Stainless Steels

Thermal ageing of duplex stainless steels entails prolonged exposure at moderate elevated temperatures (typically 300–500 °C), driving microstructural transformations predominantly within the δ-ferrite phase. Principal mechanisms include spinodal decomposition of ferrite into Cr-enriched and Fe-enriched regions, precipitation of intermetallic G-phase particles, and coarsening of secondary phases. These nanoscale modifications impart increased strength and hardness but concomitantly reduce toughness and corrosion resistance, a phenomenon often termed 475 °C embrittlement. Duplex alloys are extensively employed in chemical processing, oil and gas, power generation and marine applications, where service reliability under thermal stress is critical. Current research integrates advanced characterisation techniques—atom probe tomography, transmission electron microscopy, small-angle neutron scattering—and electrochemical diagnostics to map the kinetics of phase separation, model compositional fluctuations and devise remedial heat treatments such as solution anneals or reversion programmes. Establishing time–temperature transformation diagrams and non-destructive monitoring strategies underpins efforts to predict service lifetimes and to optimise alloy compositions and processing routes for enhanced durability.

Research from Nature Portfolio

Recent studies have employed electrochemical potentiokinetic reactivation analysis to quantify the progression of spinodal decomposition and G-phase precipitation in δ-ferrite regions of duplex steel welds aged at around 400 °C for up to 20 000 h. Formation of Cr-depleted zones within ferrite was shown to correlate directly with increases in reactivation current, reflecting diminished corrosion resistance. A reversion treatment at 550 °C effectively dissolved compositional modulations, restoring the initial electrochemical signature of δ-ferrite despite retention of some intermetallic particles. These insights demonstrate that nanoscale spinodal structures, rather than gross precipitate morphology, govern the degradation of corrosion behaviour during long-term thermal exposure.

Thermal Aging of Duplex Stainless Steels publication trend

The graph below shows the total number of articles in thermal aging of duplex stainless steels across all publications each year (not limited to Nature Index journals).

Technical terms

Duplex stainless steel: Alloys comprising roughly equal proportions of ferritic and austenitic phases to achieve a balance of strength, toughness and corrosion resistance.

Spinodal decomposition: A spontaneous phase separation process in which small compositional fluctuations grow continuously within a miscibility gap, producing fine-scale enrichment and depletion regions.

G-phase precipitates: Intermetallic particles rich in nickel, silicon and other elements that nucleate within ferrite during thermal ageing and influence mechanical and electrochemical properties.

δ-ferrite: The body-centred cubic ferritic phase in duplex stainless steels, prone to phase separation under moderate thermal ageing.

Reversion heat treatment: An annealing procedure at elevated temperature intended to dissolve ageing-induced compositional or precipitate structures and restore initial microstructural attributes.

Electrochemical potentiokinetic reactivation: A corrosion sensing technique that measures the ease of repassivation by recording current–potential responses to detect local compositional heterogeneities.

Small-angle neutron scattering: A method for probing nanoscale structural inhomogeneities by analysing neutron scattering intensity at low angles, sensitive to phase separation in alloys.

References

1. Evaluation of the thermal aging of δ-ferrite in austenitic stainless steel welds by electrochemical analysis. Scientific Reports (2018).
2. 748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior. Metallurgical and Materials Transactions A (2017).
3. Fe and Cr phase separation in super and hyper duplex stainless steel plates and welds after very short aging times. Materials & Design (2021).
4. Small-angle neutron scattering quantification of phase separation and the corresponding embrittlement of a super duplex stainless steel after long-term aging at 300°C. Materialia (2020).