Influence of composition on the constitutive flow behaviour of austenitic stainless steels under development for LH2 storage applications

Abstract

This paper presents an account of the hot deformation characteristics of three distinct compositional variants of austenitic stainless steels (ASSs) designed for developing high-strength liquid hydrogen (LH<inf>2</inf>) storage facility through thermomechanical processing, viz., SS, SS-Mn, and SS-Nb. Employing a Gleeble 3800 thermo-mechanical simulator, single-hit uniaxial hot-compression tests were carried out over a broad temperature and strain rate ranges of 850–1200 °C and 0.001–10 s⁻¹, respectively, for the three ASSs. The analysis of stress-strain curves provided an insight into the influence of chemical composition on the constitutive flow and dynamic recrystallization (DRX) behaviours under varying conditions of temperatures and strain rates. Through the analysis of experimental data, the peak stress/strain were observed, signifying the occurrence of DRX. Semi-empirical hyperbolic sinusoidal Arrhenius type expressions were developed for describing the flow stress behaviours of the three alloys. Also, an expression for the generalized Zener-Hollomon parameter as well as the activation energy of hot deformation (Q<inf>def</inf>) were determined for each alloy. The Q<inf>def</inf> were determined as 404, 456, 431.5 kJ/mol for alloys SS, SS-Mn, and SS-Nb, respectively. The highest Q<inf>def</inf> value exhibited by Alloy SS-Mn was due to the presence of high nitrogen and manganese contents, resulting in increased resistance to dislocation motion and broad peak strain behaviour causing delayed DRX. Electron backscatter diffraction (EBSD) was employed for ex situ investigation of the microstructure evolution following hot deformation. Necklace structures with fine dynamically recrystallized grains along prior grain boundaries were observed in all three alloys, indicating the activation of discontinuous dynamic recrystallization (DDRX) during hot deformation via the bulge mechanism. While Alloy SS-Mn (containing higher N and Mn) displayed higher flow stress and Q<inf>def</inf> compared to other alloys, Alloy SS-Nb (containing Nb and lower N) showed finer recrystallized grains, presumably as a result of high solute drag effect of Nb.