Abstract
Invar alloy sheet was welded by resistance seam welding (RSW) with a constant electrode force and three different welding currents. Tensile properties were evaluated using instrumented indentation testing (IIT) with a spherical indenter and microstructure observations were obtained under an optical microscope. IIT performed on the base material at room temperature (RT) and −163 °C, a cryogenic temperature (CT), gave results in good agreement with those of tensile testing. The strength of each zone was higher in the order of heat-affected zone (HAZ) < weld nugget (WN) < base material (BM) because the amount of cold working was least in the BM, heavy metal elements and carbon vaporized during melting, and the WN was formed more tightly than the HAZ, effectively constraining the plastic zone generated by the indentation. As for the welding current, the nugget, which becomes larger and tighter as the current increases, more effectively constrained the plastic zone in the indentation, and this soon increased the strength. Generally, Invar is known to consist of single-phase austenite, and microstructure observations have confirmed that the average grain size is ordered as BM < HAZ < WN. Fan-like columnar grains developed in the direction of the temperature gradient, and equiaxed grains were observed near the BM. It was confirmed that the grain size in the WN also increases as the current is increased. Interestingly, the constraint effect with increasing nugget size was more important for strength than the grain size.
Highlights
The demand for liquefied natural gas (LNG) continues to rise with the recent sharp increase in crude oil prices, and the concomitant construction of LNG storage tanks is on the rise [1].Alloys such as stainless steel, nickel steel, aluminum, and Invar alloys are used for LNG tanks.Since these tanks are exposed to extremely low temperatures, it is essential to use cryogenic-temperature (CT) alloys with excellent mechanical properties at cryogenic temperature (CT), especially high strength [2].In the International Maritime Organization (IMO) classification, existing LNG carriers are classified as integrated or membrane-type (Mark-III and NO 96) and independent (Moss and SPB)
Since grain size and strength are known to be in inverse proportion (Hall–Petch equation) [28], the strength would be expected to decrease as the welding current increases, but Figures 7 and 8 show that the actual strength increases with the welding current
The results are since grain size and strength are known to be in inverse proportion (Hall–Petch equation) [28], the as follows: strength would be expected to decrease as the welding current increases, but Figures 7 and 8
Summary
The demand for liquefied natural gas (LNG) continues to rise with the recent sharp increase in crude oil prices, and the concomitant construction of LNG storage tanks is on the rise [1].Alloys such as stainless steel, nickel steel, aluminum, and Invar alloys are used for LNG tanks.Since these tanks are exposed to extremely low temperatures (as low as −163 ◦ C), it is essential to use cryogenic-temperature (CT) alloys with excellent mechanical properties at CT, especially high strength [2].In the International Maritime Organization (IMO) classification, existing LNG carriers are classified as integrated or membrane-type (Mark-III and NO 96) and independent (Moss and SPB). The demand for liquefied natural gas (LNG) continues to rise with the recent sharp increase in crude oil prices, and the concomitant construction of LNG storage tanks is on the rise [1]. Alloys such as stainless steel, nickel steel, aluminum, and Invar alloys are used for LNG tanks. Since these tanks are exposed to extremely low temperatures (as low as −163 ◦ C), it is essential to use cryogenic-temperature (CT) alloys with excellent mechanical properties at CT, especially high strength [2].
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