Evolution of Microstructure in Two Austenitic Alloys at High Temperatures

Doctoral thesis, 2003

A niobium stabilised AISI 347 austenitic stainless steel was investigated after static ageing and creep deformation at temperatures between 500°C and 800°C for times up to 88 000 h. Coarse and fine NbX precipitates and also s-phase precipitates were formed in the temperature range 500-700°C. A new method for accurate measurement of σ-phase volume fraction using scanning electron microscopy and backscattered electrons (SEM/BSE) was developed. The volume fraction and precipitate size of coarse NbX precipitates and of s-phase particles were determined using different SEM techniques. Energy filtered transmission electron microscopy (EFTEM) was used to determine the volume fraction and precipitate size of secondary NbX precipitates. Atom probe field ion microscopy (APFIM) was used to follow the changes in matrix composition and to measure NbX composition. The material was affected by nitrogen uptake when aged at 800°C leading to the formation of Z-phase, Cr2N and M23C6. The iron-rich austenitic Alloy 800 was investigated for statically aged and creep deformed conditions between 600°C and 1000°C for up to 84 000 h. Coarse TiX precipitates were present in all investigated conditions. M23C6 and γ´-precipitates were found in material creep tested at 600, 650 and 700°C. M23C6 was also found at 800°C but not at 1000°C. The volume fraction and precipitate size of γ´ particles were determined using EFTEM at 600, 650 and at 700°C. At 1000°C AlN precipitated due to nitrogen uptake from the atmosphere. Thermodynamical simulations of growth and coarsening of precipitates have been performed using Thermo-Calc and DICTRA and the results were compared to the experimental results. The growth and coarsening of primary and secondary NbX particles in AISI 347 was modelled during manufacturing and creep at 700°C, and the results showed good agreement with experimental results. σ-phase growth at 700°C was also simulated, and it was shown that for good agreement with experimental results additions are needed to the thermodynamic description of σ-phase accounting for the stabilising effect of silicon. The evolution of the microstructure due to nitrogen uptake at 1000°C was also predicted.