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

Primary and secondary intragranular austeine precipitation and its relationship with chromium nirade (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 ferritelaustenite (alpha/gamma) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr2N) and intragranular secondary austenite suggests the heterogenous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr2N/gamma) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.