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

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7/8/03

9:52 PM

Page 1575

The Relationship between Chromium Nitride and Secondary Austenite Precipitation in Duplex Stainless Steels A.J. RAMIREZ, J.C. LIPPOLD, and S.D. BRANDI 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 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.

I. INTRODUCTION

BY controlling chemical composition and thermomechanical treatments, duplex stainless steels (DSSs) generally exhibit a balanced microstructure, having approximately equal amounts of ferrite and austenite in the wrought form. This balanced microstructure confers exceptional mechanical and corrosion properties to these alloys.[1] Therefore, it is a major concern for the engineers who deal with DSSs in industrial applications to maintain the balanced microstructure during the fabrication of components with these steels and during service. One of the most critical and widely used fabrication processes is welding, in which the material is subjected to extremely high heating and cooling rates, depending on the welding process used and the weld region. When DSSs are heated above 1100 °C, austenite () starts to dissolve, and its fraction may become very low, or zero, near the solidus temperature, depending on the chemical composition of the alloy.[2,3] When the steel undergoes rapid cooling from elevated temperatures, such as in the heat-affected zone (HAZ) of a weld, the austenite-to-ferrite transformation is partially suppressed.[4,5] The DSSs are intentionally alloyed with nitrogen, with some alloys containing up to 0.3 wt pct N. In the solutionannealed condition, these alloys retain almost all the nitrogen in solid solution within the austenite due to the high solubility of N in the austenite relative to the ferrite. This solubility difference is particularly large at low temperatures.[1] Hence, as the partially or totally ferritized microstructure cools rapidly from temperatures above 1100 °C, the ferrite becomes supersaturated in nitrogen, resulting in a competition between chromium nitride and austenite precipitation. The chemical composition, cooling rate, and ferrite grain size are the factors A.J. RAMIREZ, Research Associate, and J.C. LIPPOLD, Profe

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