PSI - Issue 68

Ahmed W. Abdelghany et al. / Procedia Structural Integrity 68 (2025) 520–526 Abdelghany et al. / Structural Integrity Procedia 00 (2025) 000–000

521

2

1. Introduction Austenitic stainless steels (ASS) are promising grades of steels for cryogenic applications, particularly as storage tanks for liquefied hydrogen (LH2) and liquefied natural gas (LNG). ASS possess good corrosion resistance, weldability, and high toughness at cryogenic temperatures (Gardner, 2005). Despite these benefits, a notable limitation of ASS is in respect of its low yield strength (Anoop et al., 2021). In large storage structures like LH2 and LNG tanks, where structural integrity under high loads is critical, this lower yield strength can pose a challenge. With low yield strength, ASS storage structures may require thicker walls or additional reinforcement to meet safety standards, which can increase both the material usage as well as overall costs. One effective approach to enhance the overall combination of mechanical properties in steel is by tailoring the microstructure, which is influenced by both the thermo-mechanical processing history (Irani et al., 2001) as well as chemical composition of the alloy (Yamamoto et al., 1987). Particularly, thermo-mechanical controlled processing (TMCP) is an effective technique for producing fine-grained, pancaked structures with high dislocation densities that can improve the yield and tensile strengths without significantly compromising on ductility, toughness and corrosion resistance (Maki, 1997; Mallick et al., 2018). During TMCP, the steel undergoes several metallurgical processes, including work hardening, dynamic recovery, and dynamic recrystallization (Abdelghany et al., 2022; Mirzadeh, 2015). These processes are critical for optimizing the balance between strength and ductility through controlled microstructural reconstitution and subsequent pancaking. In particular, the strength of austenitic stainless steels produced via TMCP is influenced by both grain refinement and substructure strengthening. While grain refinement contributes to a limited strength increase (up to 100 MPa), further strengthening is achieved through rolling in the no-recrystallization regime (Yamamoto et al., 1993). The degree of reduction in this temperature range is a key factor in achieving the desired strength level on an industrial scale with a concomitant enhancement in toughness. Additionally, strain hardening in ASS is highly efficient for improving yield and tensile strengths without significantly impairing the ductility, especially due to the low stacking fault energy of alloyed austenite (Karjalainen et al., 2008). This makes TMCP a valuable tool in optimizing both the strength and toughness of ASS, particularly for cryogenic applications. This study aims to evaluate the processing route for a 201LN stainless steel using small-scale, two-hit uniaxial compression tests employed on a Gleeble thermo-mechanical simulator and optimize the TMCP parameters. In all these tests, the second compression hit was performed in the no-recrystallization temperature (T nr ) to achieve a pancaked structure for substructure strengthening. Six distinct TMCP schedules were attempted to comprehensively understand the effects of reduction (pancaking), deformation temperature, and cooling conditions on the microstructural evolution. Detailed microstructural analysis and hardness measurements were employed to evaluate the relationship between processing parameters and resulting material characteristics. 2. Experimental Methods 2.1. Alloy composition Table 1 shows the composition of the ASS alloy utilized for the current study. The steel adhering to the composition limits of 201LN alloy with relatively low Ni (5.4 wt.%) and high Mn (7 wt.%) contents, as specified in EN 10028-7 (DIN EN 10028-7), was prepared through vacuum induction melting and cast as 80 kg ingots. Additionally, the alloy contains a high nitrogen content (0.2 wt.%).

Table 1. Chemical compositions of the 201LN ASS alloy produced as 80 kg ingots. Element Fe C Si Cr Ni Mn Mo Cu

N

201LN

Bal.

0.025

0.40

16.7

5.4

7.0

0.3

0.8

0.20

2.2. TMCP testing schedule The 201LN alloy containing 5.4 wt.% Ni and 7.0 wt.% Mn was initially cast into 80 kg ingots, and subsequently hot rolled to produce thick plates with a final thickness of 60 mm. Cylindrical samples of dimensions f 8 x 10 mm,