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This study investigated the effect of the hydrogen charging time on the hydrogen embrittlement of hot-rolled and accelerated-cooling treated API X70 steel by using the electrochemical hydrogen charging method. Tensile and V-notch Charpy impact test were completed for the specimens of hydrogen charged for 0, 2, 4, and 10 h, and then microstructural observation were performed. The tensile and yield strength tended to decrease slightly and the total elongation started to decrease rapidly up to 2 h charging time and then these gradually decreased with the hydrogen charging time. The post-uniform elongation increased slightly after the holding time of 2 h due to the occurrence of two-step stress reduction phenomenon and its mechanism has suggested. The effect of the hydrogen embrittlement on impact absorption energy according to the hydrogen charging time were also investigated. The fracture surface of the hydrogen charged ones are generally composed of cleavage fracture surfaces. It was demonstrated that the external cracks were formed on the outside of tensile tested specimen and the external shape of the fracture portion was fractured without large elongation at hydrogen charged specimens.

The hydrogen-trap states in TiC and MoC that have coherent interfaces with ferrite were investigated using first-principles calculation. The trapping sites of TiC were the interfaces and interstitial sites of ferrite. On the other hand, the trapping sites of MoC were ferrite interstitial sites; the interface had a negative binding energy with H. Thermal desorption analysis confirms that the amounts of diffusible hydrogen were significantly reduced by addition of Mo in Ti-bearing steel.

During the use of API X70 steel as a pipeline structural material for the transportation of natural gas, hydrogen embrittlement can occur due to the hydrogen contained in natural gas. The aim of this study is to investigate the effects of the hydrogen content under high-pressure hydrogen gas conditions on the hydrogen embrittlement of air-cooled API X70 steel. The air-cooled API X70 steel was manufactured by hot rolling and was then air-cooled to room temperature. Tensile test specimens were held for 0 h, 1000 h, and 2000 h within a pressure vessel filled with 100% hydrogen gas at a gas pressure of 10 MPa, with the tensile tests then performed at room temperature. The microstructure of the API X70 steel consists of coarse polygonal ferrite, coarse pearlite, and fine acicular ferrite. The yield and tensile strength increased and elongation decreased considerably after a holding time of 2000 h compared to those of 0 h and 1000 h within the pressure vessel. The morphology of the fracture surface changed from ductile to brittle upon hydrogen gas charging. Secondary cracks were observed in both of the hydrogen-gas-charged specimens. No external cracks were formed on the surface of the tensile-tested specimen with a 0 h holding time; however, many external cracks were observed on the specimen surface subjected to hydrogen gas charging.

The magnetic, electrical and elastic properties of 3Cr-1Mo-0.25 V steel were investigated using heat treatment procedures (as-quenched→tempered→PWHT). The mechanical properties were determined for the respective specimens with three types of heat histories. The mechanical properties (yield stress, hardness) and magnetic properties (permeability, coercive force, remanence) were inversely proportional to the heat treatment procedures. The permeability decreased linearly with an increase in the grain size; however, the yield stress, hardness, coercive force and remanence did not vary greatly with grain size. The electrical resistivity and ultrasonic velocity of the as-quenched material differed from those of the tempered/PWHT material. This trend was due to the defects, precipitates and carbides in the grain and at the grain boundary caused by the heat treatment procedures. However, the electrical resistivity and ultrasonic velocity did not vary greatly with grain size.

Hydrogen-induced degradations have generally been investigated through the ex situ testing of cathodically hydrogen-charged specimens. However, the cathodic charging cannot realize damage accumulation by gaseous hydrogen in transportation pipelines. Thus, we designed an ampule specimen which enables an in situ tensile test containing gaseous hydrogen. Ampule specimens made of API (American Petroleum Institute) X65 pipeline steel showed significant reductions of 3.4 % and 4.1 %, respectively, in their ultimate tensile strength and fracture strain after exposure to 20 MPa of hydrogen gas. The resulting fracture surface showed quasi-cleavage perpendicular to the loading direction and a fracture thickness close to the initial wall thickness, significantly different from the complex dimples and highly reduced fracture thickness by shear deformations in nitrogen gas and air environments.