Discontinuous precipitation in a nickel-free high nitrogen austenitic stainless steel on solution nitriding

Abstract

Chromium-rich nitride precipitates in production of nickel-free austenitic stainless steel plates via pressurised solution nitriding of Fe–22.7Cr–2.4Mo ferritic stainless steel at 1473 K (1200 °C) under a nitrogen gas atmosphere was investigated. The microstructure, chemical and phase composition, morphology and crystallographic orientation between the resulted austenite and precipitates were investigated using optical microscopy, X-ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (TEM) and Electron Back Scatter Diffraction (EBSD). On prolonged nitriding, Chromium-rich nitride precipitates were formed firstly close to the surface and later throughout the sample with austenitic structure. Chromium-rich nitride precipitates with a rod or strip-like morphology was developed by a discontinuous cellular precipitation mechanism. STEM-EDS analysis demonstrated partitioning of metallic elements between austenite and nitrides, with chromium contents of about 80 wt.% in the precipitates. XRD analysis indicated that the Chromium-rich nitride precipitates are hexagonal (Cr, Mo)₂N. Based on the TEM studies, (Cr, Mo)₂N precipitates presented a (1 1 1)_γ//(0 0 2)_{(Cr, Mo)2N}, _γ//_{(Cr, Mo)2N} orientation relationship with respect to the austenite matrix. EBSD studies revealed that the austenite in the regions that have transformed into austenite and (Cr, Mo)₂N have no orientation relation to the untransformed austenite.

Keywords:

• Discontinuous precipitation
• solution nitriding
• nickel-free austenitic stainless steel
• EBSD
• TEM

Acknowledgements

The authors would like to thank Thomas L. Christiansen, H. Alimedadi and Steffen S. Munch, Department of Mechanical Engineering and Center for Electron Nanoscopy of Denmark Technical University, for access to the laboratories and simulating discussion and their fruitful comments.