In terms of steel alloying elements, generally, nickel is used as an austenite stabilizing element to increase the toughness of steel. The low temperature materials, such as nickel alloy steels with a nickel content of 3.5% to 9%, stainless steel and Invar, show excellent toughness at low (173 K) and cryogenic (108 K) temperatures. In particular, in the shipbuilding industry, it is mainly used for liquefied ethane and Liquefied Natural Gas (LNG) carriers, and research on low-temperature steels are attracting attention again as regulations on environmental issues are strengthened in recent years. Therefore, in this study, fatigue and fracture performances of nickel alloy steel containing 9% or less among nickel alloy steels are evaluated. Moreover, we assess the Fatigue Ductile to Brittle Transition (FDBT) of nickel alloy steels based on crack tip opening displacement (CTOD). In order to discuss the fatigue and fracture performances of nickel alloy steels, microstructure analysis carried out. As a result, CTOD and Fatigue Crack Growth Rate (FCGR) of nickel alloy steels increases as nickel contents increase. In addition, FDBT of 9% nickel alloy steel is the lowest compared to other nickel alloy steels.

High-strength steel is widely used in hot forging products for application to the oil and gas industry because it has good mechanical properties under severe environment. In order to apply to the extreme environment industry requiring high temperature and high pressure, heat treatments such as austenitizing, quenching and tempering are required. The microstructure of high-strength steel after heat treatment has various microstructures such as Granular Bainite (GB), Acicular Ferrite (AF), Bainitic Ferrite (BF), and Martensite (M) depending on the heat treatment conditions and cooling rate. Especially in large forged products, the difference in microstructure occurs due to the difference in the forging ratio depending on the location and the temperature gradient according to the thickness during post-heat treatment. Therefore, this study attempted to quantitatively analyze various phases of F70 high-strength steel according to the austenitizing temperature and hot forging ratio using the existing EBSD analysis method. In addition, the correlation between microstructure and mechanical properties was investigated through various phase analysis and fracture behavior of high-strength steel. We found that various microstructures of strength steel depend on the austenitizing temperature and hot forging ratio, and influence the mechanical properties and fracture behavior.

This study examines the mechanical behavior and deformation mechanisms of hot-forged 304L stainless steel for cryogenic applications such as LNG storage and low-temperature structural systems. Tensile testing revealed a significant strength increase from 618 MPa at room temperature to 1432 MPa at cryogenic temperatures, with elongation decreasing from 83.7% to 23.3%. Charpy impact testing showed a 28% reduction in absorbed energy at cryogenic temperatures due to enhanced strain-induced martensitic transformation (SIMT). The observed mechanical responses are attributed to reduced stacking fault energy (SFE) at lower temperatures, which promotes SIMT, deformation twinning, and dislocation interactions, affecting material strength and toughness. SEM and EBSD analysis confirmed extensive martensitic transformation, increased deformation twinning, and reduced remaining austenite, indicating a γ → ε → α’ transformation pathway that governs strain hardening. The high strain rate during Charpy impact testing induced localized adiabatic heating, partially suppressing SIMT and modifying fracture behavior by enhancing localized plasticity. These findings emphasize the interplay between SFE, strain rate, and phase transformation in governing the cryogenic mechanical performance of 304L stainless steel.

Functionally graded materials (FGMs) have a characteristic whereby the composition and structure are gradually changed according to the location, and the mechanical properties or chemical properties are gradually changed accordingly. In this study, using a multi-hopper direct energy deposition 3D printer, an FGM material whose composition changes gradually from P21 ferritic steel to stainless steel 316L austenitic steel was fabricated. From optical microscope, scanning electron microscope, and X-ray diffraction analysis, columnar, cell, and point type solidified micro-structure and precipitations were observed depending on the deposited compositions. Electron probe microanalysis and electron backscatter diffraction analysis confirmed the component segregation, ferrite austenite volume fraction and phase distribution behavior according to compositions. In the FGM specimen test, the ultimate tensile strength of STS316L, which was the most fragile, was measured, and the toughness was measured for the notch area, which did not represent the FGM characteristics. Hardness showed changes according to FGM position and was suitable for FGM analysis. The maximum hardness was measured in the FGM duplex area, which was caused by grain refinement, precipitate strengthening, and solid solution strengthening. In nuclear power plant welds high strength can cause adverse effects on stress corrosion cracking, and caution is needed in applying FGM.