The relationship between chromium nitride and secondary austenite precipitation in duplex stainless steels

Abstract

A detailed investigation of the early stages of secondary austenite precipitation in five duplex stainless steel (DSS) commercial alloys (UNS S32304, S32205, S32550, S32750, and S32760) has been conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on this study, a model is proposed that describes the interaction between Cr 2 N and austenite (intergranular and intragranular) precipitation in these alloys. Depending on nitrogen availability and interface mobility, Cr 2 N precipitation along existing ferrire/austenite interfaces precedes intergranular secondary austenite growth. The low-energy interfaces formed between the Cr 2 N, the ferrite, and the austenite, along with the coupled diffusion processes, are the factors controlling this phase transformation. Finally, in the case of the intragranular nitrides, a mechanism is proposed whereby the nitrides serve as sites for heterogeneous nucleation of intragranular secondary austenite.

Cyclic Polarization Study of Thick Welded Joints of Lean Duplex Stainless Steel for Application in Biodiesel Industry

“…These precipitates resulted in depletion in Cr in the adjacent matrix region. The chromium-depleted zones provide a favorable site for pitting corrosion nucleation, as reported by several authors 6,8,9,14,20,23,[28][29][30][31][32] .…”

“…It is noteworthy that this type of austenite formed in the temperature range 800-1000°C and it is derived by dissolving Cr 2 N precipitates, which provides nitrogen to the formation of γ 2 23 , and its nucleation originates in inclusions and dislocations 23 . The HAZ exhibited chromium nitrides mainly in the interior of the ferrite grains.…”

Characterization of duplex stainless steel weld metals obtained by hybrid plasma-gas metal arc welding

“…δ and γ grains in fusion zones of all weldments were analyzed using EDX to examine the solute distribution in aswelded structure. High-cooling-rate caused suppression of solute redistribution in FZ and partially melted zone (PMZ) of GMA weldment [22], where Cr-rich precipitates were observed either in δ grains or at δ/δ or δ/γ grain boundaries (Figure 7). Nilsson and Wilson [2] and Kokawa et al [23] characterized these precipitates as Cr 2 N and reported that the hydrogeninduced cracking susceptibility increased as the density of these precipitates increased in the material.…”

Influence of Deleterious Phases Precipitation in the Corrosion Resistance Measured by Double Loop Electrochemical Polarization Reactivation Test in Superduplex Stainless Steel

“…Secondary austenite (γ 2 ) can also be formed during thermal treatments. The precipitation can occur at ferrite/ferrite grain boundaries, as chromium nitrides particles, in association to sigma phase as divorced eutectoid (δ→γ 2 +σ), or even due to the growing of primary austenite 7,8 . Besides γ 2 and σ, the chi phase (χ) may also be formed in Mo containing steels heated in the 700 -900 °C range 9,10 .…”