Ki Bong Kang

Effect of Microstructure on Hydrogen-Induced Cracking of Linepipe Steels

Abstract Hydrogen-induced cracking (HIC) has been studied phenomenologically. The effect of microstructure on HIC is discussed for steels having two different levels of nonmetallic inclusions. Steels with different microstructures were produced by a thermomechanically controlled process (TMCP) from two different heats, which had different levels of nonmetallic inclusions. Ferrite/pearlite (F/P), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) were three representative microstructures of the tested steels. For the steels with higher inclusion levels, the minimum permissible inclusion level for HIC not to develop varied according to the steel microstructure. On the other hand, HIC occurred in the martensite/austenite (M/A) constituents regardless of steel microstructure above a certain concentration. It was shown that M/A constituents were easily embrittled by hydrogen atoms. Steels having a F/AF were resistant to HIC in the tested service condition; they exhibited a wide range of diffusible hydr...

Study of Texture Development and Anisotropy of Mechanical Properties of API-X80 Line Pipe Steel for Spiral-Welded Pipe

This study has been conducted to analyze the effect of texture and microstructure on the anisotropy of yield strength and Charpy fracture toughness of an X80 line pipe steel. The texture and microstructure were investigated by X-ray diffractometer and electron backscattered diffraction (EBSD). The yield strength and impact energy were measured along 0o (longitudinal), 30o and 90o (transverse) to the rolling direction. It was found that the microstructure of the developed steel consisted of fine acicular and polygonal ferrite with small pearlite and martensite or retained austenite (MA constituents). The major components of textures were {332}<113> and {113}<110> orientations. In order to investigate the effect of both morphological and crystallographic texture on yield strength anisotropy, the prediction of the plastic property was carried out by using a viscoplastic self-consistent (VPSC) polycrystal model. The predicted anisotropy of yield strength with VPSC model assuming ellipsoidal grain shape was in a good agreement with experimental observation. EBSD results showed that the density of {001} cleavage planes of Charpy specimen, 30 degree to rolling direction, was the highest compared with that of other specimens. Therefore, the highest susceptibility to the cleavage fracture, i.e. increased ductile-brittle transition temperature, can be seen in the 30 degree direction.

Development of High Strength and High Performance Linepipe and Shipbuilding Steels

Advanced TMCP products, such as X80/X100 linepipe steels and YS 460MPa grade shipbuilding steel, have been developed by POSCO. In order to apply X80/X100 linepipe to strain-based design (SBD) concept, the main issue is to prevent strain aging after thermal coating. In case of YS 460MPa shipbuilding steel, challenging study is necessary to increase plate thickness up to 80 mm without deterioration of strength and toughness. It was found that solute carbon started to segregate at dislocation tangles in coarse ferrite during and after UOE pipe forming, resulting in strain aging. And it was shown that the optimum microstructure of X80 and X100 steels was the mixture of acicular ferrite, bainitic ferrite and fine ferrite. Those X80 and X100 steels revealed good strain hardening resistance as well as reasonable compressive strain capacity. The developed YS 460MPa grade steel plate for shipbuilding showed a good mechanical property at quarter and mid-thickness and also an excellent weldability. It was confirmed that the developed steel had a high CTOD value both HAZ and base metal to prevent an initiation of brittle crack and a good crack arrestablity to suppress long distance propagation of brittle crack.

Improvement of Charpy impact properties in heat affected zones of API X80 pipeline steels containing complex oxides

Abstract The present study aimed at investigating the improvement of Charpy impact properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of steels were fabricated by adding Ti, Al and Mg to form complex oxides and their microstructures, tensile properties and Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al and Mg was included in the steels. After the HAZ simulation test, the steel having a number of oxides contained about 72 vol.-% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and granular bainite, whereas the HAZ microstructures of the conventional steels consisted of bainitic ferrite and granular bainite with a small amount of acicular ferrite. The volume fraction of acicular ferrite increased as the amount of complex oxides increased and the heat input decreased, and thus, Charpy impact properties of the oxide containing steel HAZs were greatly improved over those of the conventional steel HAZs.

Effect of Thermomechanical Processing Parameters Upon Texture Development and Anisotropy of Mechanical Properties of API-X80 Line Pipe Steels for Spiral-Welded Pipes

This study has been conducted to analyze the effect of texture and microstructure on the anisotropy of yield strength and Charpy fracture toughness of an X80 line pipe steel. The texture and microstructure were investigated by X-ray diffractometer and electron backscattered diffraction (EBSD). The yield strength and impact energy were measured along 0° (longitudinal), 30° and 90° (transverse) to the rolling direction. It was found that the microstructure of the developed steel consisted of fine acicular and polygonal ferrite with small pearlite and MA constituents. The major components of textures were {332} and {113} orientations. In order to investigate the effect of both morphological and crystallographic texture on yield strength anisotropy, the prediction of the plastic property was carried out by using a visco-plastic self-consistent (VPSC) polycrystal model. The predicted anisotropy of yield strength with VPSC model assuming ellipsoidal grain shape was in a good agreement with experimental observation. EBSD results showed that the density of {100} cleavage planes of Charpy specimen, 30 degree to rolling direction, was the highest compared with that of other specimens. Therefore, the highest susceptibility to the cleavage fracture, i.e. increased ductile-brittle transition temperature, can be seen in the 30 degree direction.Copyright