Sigma phase precipitation and growth in solution annealed 321 stainless steel during thermal aging: Effects on toughness & intergranular corrosion

Sigma (σ) phase is considered as one of the most deleterious phases in stainless steels (SS) due to its detrimental effects on mechanical and corrosion properties. In this work, σ phase formation and growth in annealed SS 321 (without δ-ferrite) and its effects on impact toughness and intergranular corrosion (IGC) were investigated, following thermal aging at 750 °C up to 5500 h. Microstructural characterization using X-ray diffraction and electron microscopy techniques revealed (a) precipitation of intergranular Cr-rich M23C6 and fine Ti-rich intra & intergranular carbides after short aging time of 27 h, (b) σ phase nucleation at the Cr-rich M23C6 - austenite interfaces after 100 h of aging and (c) σ phase at the austenite grain boundaries (GBs) beyond 700 h of aging. The experimental results, combined with thermodynamic calculations (ThermoCalc) showed that the meta stable Cr-rich carbide is the source of σ phase nucleation. σ phase started to grow at the expense of Cr-rich M23C6, and further, its size increased preferentially along the GB and into one of the austenite grains with increase of aging duration. Hardness remained in the range of 190–200 HV for all the specimens aged beyond 100 h and is shown to be due to stable Ti-rich intragranular carbides of 7–20 nm size. Impact toughness values decreased with the growth of σ phase, and up to 50% reduction in toughness was found by about 1% σ phase, with a ductile-brittle cleavage fracture. The normalized values of σ phase fraction, determined from its size and the toughness values, with aging duration followed Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and both yielded the same empirical rate constant (k) of 2.7 × 10−7/s and the n value of ∼1. The lower k value and the n value of 1 suggest slower kinetics and one-dimensional growth of σ phase, which is in well agreement with microstructural observations. Susceptibility to IGC was seen in specimens with sub-microscopic σ phase (aged up to 100 h). However, aging beyond 700 h decreased resistance to uniform corrosion due to σ phase growth.