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 betwee Cr2N and austenite (intergranular and intragranular) precipitation in these alloys. Depending on nitrogen availability and interface mobility, Cr2N precipitation along existing ferrire/austenite interfaces precedes intergranular secondary austenite growth. The low-energy interfaces formed between the Cr2N, 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.

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

Effect of welding processes and conditions on the microstructure, mechanical properties and corrosion resistance of duplex stainless steel weldments—A review

Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr2N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr2N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr2N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr2N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is p...

Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels

Study on microstructure and tensile properties of high nitrogen Cr-Mn steel processed by CMT wire and arc additive manufacturing

Investigation of the sensitization and intergranular corrosion of tube-to-tubesheet welds of hyper duplex stainless steel using an electrochemical reactivation method

Duplex stainless steel base material was welded using gas tungsten arc welding with an Ar-10%H2 shielding gas and laboratory-made filler wires were employed to deposit duplex and fully ferritic weld metals having different nitrogen contents. Weld metal slow extension rate tensile (WM-SERT) testing was used to examine the hydrogen-induced cracking susceptibility and fractography of the weld metals. An increase in nitrogen content in fully ferritic stainless steel weld metal increased the density of precipitates and the hydrogen-induced cracking susceptibility. The facets on the quasi-cleavage fracture surfaces of broken WM-SERT test specimens were parallel to the {100} plane in ferrite. Scanning and transmission electron microscope observations revealed the crystallographic features and morphology of the precipitates. The precipitates were rod-like Cr2N nitrides. Many of them had directions and were parallel to the cleavage {100} plane in ferrite. An orientation relationship shown between Cr2N precipitates and ferrite suggested that the axes of the Cr2N precipitates were parallel to direction in ferrite and that they were more coherent along their long faces than at their tips. As a result, the tip of these Cr2N precipitates could act as sinks for hydrogen and may be preferential sites for initiation of hydrogen cracking; this could promote crack propagation on {001} cleavage planes in ferrite on which Cr2N precipitates are located. Higher densities of Cr2N precipitates were nucleated at solidification boundaries and at oxide inclusions in ferrite.

E. Tsory

Papers

Nitride Precipitation in Duplex Stainless Steel Weld Metal