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홍현욱,최호진,이종봉 대한금속·재료학회 2009 METALS AND MATERIALS International Vol.15 No.1
The optimization of electric resistance welding (ERW) conditions was studied to improve the resistance to hydrogen induced cracking (HIC) at the bondline in small diameter API X60 ERW pipes fabricated with slit coils. The results show that HIC is initiated preferentially at the elongated Si, Mn and Al-rich oxide inclusions, normally known as a penetrator on the bondline. However, no evidence was found of any centerline segregation effect. The HIC ratio increases with the fraction of penetrators at the bondline, regardless of the degrees of center segregation. Furthermore, for a satisfactory level of HIC resistance, the fraction of penetrators must be less than 0.03 % and most of the penetrators should be circular-shaped. The design of experimental (DOE) method was used to determine the optimum ERW condition for minimization of the penetrator ratio. Finally, guideline is suggested for the optimum ERW condition for achieving excellent HIC resistance. The optimization of electric resistance welding (ERW) conditions was studied to improve the resistance to hydrogen induced cracking (HIC) at the bondline in small diameter API X60 ERW pipes fabricated with slit coils. The results show that HIC is initiated preferentially at the elongated Si, Mn and Al-rich oxide inclusions, normally known as a penetrator on the bondline. However, no evidence was found of any centerline segregation effect. The HIC ratio increases with the fraction of penetrators at the bondline, regardless of the degrees of center segregation. Furthermore, for a satisfactory level of HIC resistance, the fraction of penetrators must be less than 0.03 % and most of the penetrators should be circular-shaped. The design of experimental (DOE) method was used to determine the optimum ERW condition for minimization of the penetrator ratio. Finally, guideline is suggested for the optimum ERW condition for achieving excellent HIC resistance.
홍현욱 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
High-frequency electric resistance welded (ERW) pipes have been used increasingly in natural gas and petroleum services due to remarkable progress in coil manufacturing, forming, welding and other pipe-making techniques. The quality standards of ERW pipes have become more stringent to be suitable for harsh environments such as the artic region and areas that contain sour H2S gas. Hence, ERW pipes should have low temperature toughness, a high level of strength, and excellent resistance to hydrogen induced cracking (HIC). With regard to the improvement of HIC resistance at the bondline, the essential factors are the base metal, the welding operations, and the subsequent online heat treatment of the ERW area. In the present study, the optimization of ERW conditions was studied to improve the resistance to HIC at the bondline in small diameter API X60 ERW pipes fabricated with slit coils. The design of experimental (DOE) method was used. It is evident that HIC is initiated preferentially at the elongated Si, Mn and Al-rich oxide inclusions, normally known as a penetrator on the bondline. No evidence was found of any centerline segregation effect. To meet the requirement to the HIC resistance, the fraction of penetrators must be less than 0.03 % and most of the penetrators should be circular-shaped. The guideline for an optimum ERW condition highlight the special relationship of the power input, the welding speed, the upset force and the strip thickness.
홍현욱,김태영,김태훈,문준오,이창훈,정준호 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
Recently, there has been a tendency for the buildings to be constructed in the form of high-rise and multipurposes. In addition, to secure the safety design of structures from natural disasters such as earthquakes and fire, the building standards have become more stringent. The present study aimed to design low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb for both seismic and fire-resistant qualifications. The hot-rolled steel containing 0.3 wt% Mo-0.02 wt% Nb (‘A’ hereinafter) was primarily strengthened by bainitic transformation, whilst the steel with 0.2 wt% Mo-0.13 wt% Ti (‘B’ hereinafter) consisted of ferrite with a high density of nano-sized (Ti,Mo)-rich MX precipitates. The bainitic microstructure (‘A’ steel) was quite favorable to high-temperature strength and thermal stability. The yield strength of ‘A’ steel at both room and 600 °C temperatures increased consistently with increasing thermal exposure time (600 °C/200-1000 h), since the precipitation of NbC particles occurred while maintaining bainitic ferrite platelets with a high density of dislocations during exposure. During low cycle fatigue tests at room temperature, the main different feature between the two steels is that the ‘A’ steel showed cyclic softening while cyclic hardening was evident in the ‘B’ steel. The bainitic microstructure showed a better fatigue life due to increased ductility manifested by cyclic softening, by which dislocation cell was developed.