Role of nitrogen in the cyclic deformation behavior of duplex stainless steels

Abstract

The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.

The Fatigue Properties of Duplex Stainless Steels: Role of Microstructure

“…Nitrogen in solid solution also modifies the behaviour of dislocations by changing the slip mode from wavy to planar slip. Previous investigations have attributed to DSS a typical behaviour of softening or hardening/softening especially when alloyed with nitrogen [6,15]. By another way, it has been shown that development of planar arrangements of dislocations is most of the time associated with a softening of the material.…”

Cyclic deformation of superduplex stainless steels at intermediate temperatures

“…[27][28][29][30][31][32] To summarize their conclusions, it may be said that plastic deformation occurs initially in austenite, but the rapid strain hardening of this phase leads to an early plastic transfer to ferrite. Afterward, cyclic softening results mainly as a consequence of strain localization in ferrite.…”

“…This fact is in agreement with the high nitrogen content of the studied duplex steel (0.24 pct) and also the strong partitioning of this element to austenite that elevates this value to around 0.60 pct. [32] It is well known that nitrogen reduces the stacking-fault energy and therefore enhances planar slip character. [37,38] On the other hand, a trend toward wavier features was observed at intermediate temperatures, as explained in Section III-B.…”

Improved Microstructure and Mechanical Properties of Fe-Cr-Mo-V-N Steel by Controlling the Quenching Temperature

“…In general, the excellent properties of nitrogen-alloyed steels are attributed to the ability of nitrogen to stabilize austenite and refine the second phase. 1,2 For nitrogenalloyed Fe-Cr-N series steels, recent investigations mainly focus on the following aspects. Hertzman et al 3 carried out thermodynamic analysis on Fe-Cr-C-N and Fe-Cr-Ni-N systems in the temperature range of 1273-1473 K. Uebing et al 4 studied the surface segregation of Cr and N in Fe-15%Cr-N single crystal at 490-750 °C.…”