Microstructural characterization of quench annealed duplex SAF 2507 utilizing EBSD

Abstract: Producing large bars of duplex stainless steel free from precipitated embrittling phases isdifficult due to the many factors that influence precipitation. Research shows that intermetallicslike sigma and chi deteriorate principal material properties for the main application areas.Constructing a process with minimized precipitates is made more leisurely by understanding theevolution of the microstructure and the effects on precipitation. This thesis investigates themechanisms accompanying annealing at elevated temperatures, mainly recrystallization, phasetransformation, and grain growth. Compression samples of duplex SAF 2507 were annealed at900 ◦C and strained in Gleeble. Afterward, the annealing process split in two, (1) quenching withair and (2) heating to 1100 ◦C and soaking for various durations before controlled air cooling.EDS and EBSD in SEM were used to analyze the microstructures. EBSD data shows that mostof the sigma phase was dissolved after 2.5 min at 1100 ◦C when cooling with 2.3 ◦C/s. With thesame conditions, the recrystallization of austenite was complete. However, ferriterecrystallization was mostly complete after annealing at 900 ◦C with subsequent air-quenching.Austenite and ferrite grains showed tendencies of considerable pinning effects that inhibit graingrowth in both phases, expected of a two-phase material. During strain annealing at 900 ◦C,with a strain rate of 1/s, a reduction in the true stress-strain curve was observed. The reductionvaried for the samples if at all present, and for one specimen, the decrease was 75 MPa.Establishing the cause needs further investigation, but dynamic recrystallization is conceivable.