Microstructural Evolution and Mechanical Properties of FCAW Joints in 9% Ni Steel Prepared with Two Types of Ni-Based Weld Metals

Abstract

The microstructural and mechanical evaluation of 9% Ni steel with Flux-Cored Arc Welding was performed with two different Nibased weld metals: Inconel 625 and Hastelloy 609. Weld metals showed the microstructural change depending on the temperature gradient and crystal growth rate for each region during the cooling after welding. At the bottom of the weld metal, which is rapidly cooled in contact with the cold base metal, a cellular/planar growth was exhibited due to a large temperature gradient and low crystal growth rate. While, columnar dendrites were exhibited in the central region cooled relatively slowly and precipitates were observed in the interdendritic region. In the low-temperature toughness test, the absorbed impact energies were 89 and 55 J for Inconel 625 and Hastelloy 609, respectively. When Inconel 625 is used as the weld metal compared to Hastelloy 609, the high content of the γ stabilizer and martensite start temperature decreasing elements leads to the formation of a thicker γ-phase layer and thinner martensite layer in the transition region. In addition, high content of these elements suppresses the martensite transformation and maintains the stability of the weld joint interface even at low temperatures, resulting in the higher absorbed impact energy.

The Effects of Post-Weld Heat Treatment on the Microstructure and Mechanical Properties of 9% Ni Steel Weld Joint with Fe-Based Filler Material

“…Ni강은 조질처리 (quenching and tempering)를 통해 미세 조직이 제어된 합금으로, 용접시 주로 모재인 9% Ni강과 이종재인 Ni계 합금을 용가재로 적용한다 [6]. 9% Ni강의 용가재로 IN625 [15][16][17], ERNiCrMo-3 [18], DW-N70S [19], Hastelloy C-276 [20], Hastelloy 609 [17]등의 다 양한 Ni계 합금이 활용되며, 가스 텅스텐 아크 용접(gas tungsten arc welding, GTAW) [15,16,18], 플럭스 코어드 아크 용접(flux cored arc welding, FCAW) [17,19], 서 브 머지드 아크 용접 (submerged arc welding) [20], 피…”

Fe‐Based Filler Material Forming Austenite‐Covered Packet Lath Martensite for Gas Tungsten Arc Welding of 9% Ni Steel

“…Ni-based filler materials, including Inconel 625 and Hastelloy C-276, are used for welding 9% Ni steel through various welding processes such as gas tungsten arc welding (GTAW), shielded metal arc welding, and flux-cored arc welding. [9][10][11][12] A dendritic austenitic microstructure has been obtained in the weld metal (WM) of weld joints. [9][10][11][12] Weld joints using Ni-based filler materials exhibit comparable crack resistance to that of 9% Ni steel.…”

“…[9] These weld joints show excellent strength and toughness, making them suitable for fabricating the inner walls of LNG storage tanks at 25 °C as well as cryogenic temperatures. [9,10] In general, 9% Ni steel is welded with Ni-based filler materials that are primarily composed of expensive elements such as Ni, Mo, and Cr. [9][10][11][12][13] According to the marine emission regulations of the International Maritime Organization, the production volume of LNG storage tanks is expected to increase drastically after 2020 [1] ; however, the extensive usage of expensive elements such as Ni and Mo is not economically suitable for the industrial-scale development of LNG tanks.…”

“…[9][10][11][12] A dendritic austenitic microstructure has been obtained in the weld metal (WM) of weld joints. [9][10][11][12] Weld joints using Ni-based filler materials exhibit comparable crack resistance to that of 9% Ni steel. [9] These weld joints show excellent strength and toughness, making them suitable for fabricating the inner walls of LNG storage tanks at 25 °C as well as cryogenic temperatures.…”

“…[9,10] In general, 9% Ni steel is welded with Ni-based filler materials that are primarily composed of expensive elements such as Ni, Mo, and Cr. [9][10][11][12][13] According to the marine emission regulations of the International Maritime Organization, the production volume of LNG storage tanks is expected to increase drastically after 2020 [1] ; however, the extensive usage of expensive elements such as Ni and Mo is not economically suitable for the industrial-scale development of LNG tanks. Consequently, major shipbuilders, who are responsible for the maintenance of most existing LNG tanker fleets, have started searching for inexpensive filler metals to meet their economic requirements.…”