Deformation mechanisms in meta-stable and nitrogen-stabilized austenitic stainless steel during severe surface deformation

Bo Wang,Chuanshi Hong,Grethe Winther,Thomas L Christiansen,Marcel A.J Somers

doi:10.1016/j.mtla.2020.100751

Abstract

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.

Highlights

Metastable austenitic stainless steels are materials of high interest for a plethora of engineering applications due to their intrinsically high corrosion resistance and good formability [1]
Iso activity lines as referred to by the given N2 pressures are superimposed on the isopleth, so the equilibrium nitrogen content is given as a function of temperature and N2 pressure
After 2 hours of high-temperature solution nitriding (HTSN) at 1150 °C, the absorbed nitrogen atoms have diffused to a depth of ~ 450 μm and the nitrogen content averaged over the first 2 μm is 0.40 wt %, in good agreement with the nitrogen content predicted by assuming thermodynamic equilibrium at the surface (Fig. 1a)

Summary

Introduction

Metastable austenitic stainless steels are materials of high interest for a plethora of engineering applications due to their intrinsically high corrosion resistance and good formability [1]. Nitrogen can be introduced to stainless steels in the liquid state to manufacture a so-called high nitrogen steel or in the solid state at elevated temperature by thermochemical treatment in a nitrogencontaining gaseous atmosphere. Both procedures are viable means to stabilize austenite from transforming into martensite on deformation and both are applied in industrial practice. HTSN was utilized for suppressing the martensitic transformation during subsequent cold drawing [10] or rolling [18,19] of type AISI 304 steel These investigations confirmed that effective stabilization of austenite can be achieved by introducing nitrogen in solid solution. This suggests the possibility of severe deformation of austenite that has been stabilized by prior HTSN, and thereby potentially the development of a nano-crystalline surface layer in austenite

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Discussion

Conclusion