Precipitation of Secondary Phases in Lean Duplex Stainless Steel 2101 during Isothermal Ageing

Abstract

The DSS ingot was made by standard vacuum melting and casting, which possessed duplex ferritic-austenitic mi- 286© 2010 ISIJ ISIJ International, Vol. 50 (2010) In the present paper, the precipitation behavior of secondary phases in Lean Duplex Stainless Steel (LDX) 2101 had been studied. The alloy has a typical ferritic-austenitic duplex structure after solution treatment. Isothermal ageing treatments were carried out over the temperature range from 700 to 850°C for the period between 2 min and 1 440 min. Cr 2 N-type nitrides were formed as the main product of precipitates, together with small amount of Cr 23 C 6 -type carbides. Increase of N content and decrease of Cr and Mo contents in the steel had led to the absence of s-phase. Due to the solid solution strengthening by interstitial atoms, the hardness value of austenite was measured to be higher than that of ferrite. With increasing the isothermal ageing time, precipitates were observed to be preferentially nucleated at the d/g interface and grown toward the d phase. The d/g interfce migrated from the precipitate particles into d phase, leaving the precipitates behind along the original interfaces, which could increase the size of the secondary austenite (gЈ) phase and decrease the hardness of both austenite and ferrite. Inside gЈ phase, the hardness was measured to increase from about 3.20 GPa at the original interface to about 3.75 GPa at the newly formed interface of d/gЈ. The isothermal kinetics for the precipitation of secondary phases was measured to obey the JMAK-type law. The TTT (time-temperature-transformation) curve was constructed based on the measurements, which seems to have resulted from the overlap of two different precipitated products. The average impact energy at room temperature for the sample after solid solution treatment was measured to be 105 J. When 2 % secondary phases were precipitated during ageing, the impact energy decreased to about 43 J, reduced by about 59 % as compared with the as-annealed sample, indicating that formation of secondary phases can result in dramatic decrease of steel toughness.

Texture and Planar Anisotropy in Lean Duplex Stainless Steel Sheet

“…On the other hand, in Type 32101 of the present study, the value of nanohardness in the α and γ phases was measured to be 3.27-3.78 and 4.14-4.68 GPa, respectively. Fang et al 43) reported that the nanohardness of the α and γ phases were 4.03 GPa and 5.09 GPa, respectively, in Type 32101 after a solid solution treatment at 1 100°C for 30 min. Thus, although the values of our examination were lower than theirs, the γ phase was harder than the α phase in both studies.…”

Effect of the Temperature in the Mechanical Properties of Austenite, Ferrite and Sigma Phases of Duplex Stainless Steels Using Hardness, Microhardness and Nanoindentation Techniques

“…In addition, the experimental results also reveal that the ferrite phase percentage decreases in an opposite tendency to sigma phase and in a similar magnitude. This phenomenon is associated to the decomposition that ferrite experiments to form sigma phase and secondary austenite [16,44,45]. The objective of different percentages of sigma phase precipitated is to analyze the behavior of the phases in the first stages of sigma phase nucleation in α-α as well as α-ɣ boundaries, and in the subsequent stage, where the sigma phase grows towards the ferrite phase.…”

Effects of Temperature and Strain Rate on Tensile Properties of a Lean Duplex Stainless Steel

“…The fracture toughness, 3) precipitation of nitrides with heat treatment, 4) and improvement of mechanical properties using stressinduced martensitic transformation of the γ phase 5,6) have been reported for lean duplex stainless steels. Herrera et al 5) investigated duplex stainless steel with α and γ volume fractions of 37% and 63%, respectively, from a Fe-19.9Cr-0.42Ni-0.16N-4.79Mn-0.11C-0.46Cu-0.35Si steel.…”