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Young-HyunNam,Seung-HoonNahm 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.6
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.
Effect of Highly Pressurized Hydrogen Gas Charging on the Hydrogen Embrittlement of API X70 Steel
Dong-Su Bae,Chi-Eun Sung1,Hyun-Ju Bang,Sang-Pill Lee,Jin-Kyung Lee,In-Soo Son,조영래,Un-Bong Baek,Seung-HoonNahm 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.4
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 toinvestigate the effects of the hydrogen content under high-pressure hydrogen gas conditions on the hydrogenembrittlement of air-cooled API X70 steel. The air-cooled API X70 steel was manufactured by hot rollingand was then air-cooled to room temperature. Tensile test specimens were held for 0 h, 1000 h, and 2000 hwithin a pressure vessel filled with 100% hydrogen gas at a gas pressure of 10 MPa, with the tensile teststhen performed at room temperature. The microstructure of the API X70 steel consists of coarse polygonalferrite, coarse pearlite, and fine acicular ferrite. The yield and tensile strength increased and elongationdecreased considerably after a holding time of 2000 h compared to those of 0 h and 1000 h within the pressurevessel. 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 wereformed on the surface of the tensile-tested specimen with a 0 h holding time; however, many external crackswere observed on the specimen surface subjected to hydrogen gas charging.