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.

The effect of martensite–austenite (M–A) constituents and simulated microstructure on low-temperature toughness was investigated in YS 500 MPa grade structural steel welds. The specimens were fabricated using a direct quenching and tempering process. After simulated weld thermal cycles, the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) were produced using a Gleeble tester and real welded joint to support the simulation results. The largest low-temperature toughness was observed in the fine-grained heat-affected zone (FGHAZ) owing to the fine-ferrite microstructure. However, the toughness decreased in the IRCGHAZ because of the slender morphology of the M–A constituents that formed primarily along the prior austenite grain boundaries in the IRCGHAZ.

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.

Bipolar plate is one of the most important parts in polymer electrolyte membrane (PEM) fuel cells. However, bipolar plate of stack in fuel cell is expensive and heavy weight. In this study, low carbon steel was used as the base metal of bipolar plate for the lowering the cost than stainless steels, which are widely researched as bipolar plate. To improve the corrosion resistance and electrolytic conductivity, low carbon steel needs to be surface modified. Chromium electroplated layer was deposited on the surface of low carbon steel, and thermally nitrided in a vacuum furnace under nitrogen gas pressure. The microstructures of the surface treated low carbon steel were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Crystal structures are also observed by X-ray Diffraction (XRD). Interfacial contact resistance (ICR) and corrosion test were considered to simulate the internal operating conditions of PEM fuel cell stack.