10. Task10 Development of Cryogenic Materials Technology

10.1 Research and Development Goals

10.1.1 Goals of Phase II

The goals of Phase II are to test material properties under liquid hydrogen environments and to develop elemental technology related to optimized welding material and welding method. Moreover, the material characteristic database will be enhanced.

10.1.2 Goals for FY1999

(1) Research on the material properties in the temperature area of liquid hydrogen for candidate materials (base metal, weld metal)

Fatigue strength tests will be implemented in liquid hydrogen for the base metals and weld metals that selected as candidate materials in Phase I and material characteristic data will be enhanced. Moreover, studies will be performed on new candidate materials and evaluation factors.

(2) Research on elemental technology related to optimized welding material and welding method

Elemental technology will be developed with respect to new welding materials and welding methods such as laser welding, reduced pressure electron beam welding and friction stir welding.

(3) Research on the extension of the low temperature material characteristic database

With respect to the database of material properties produced in Phase I, additional data will be input and the system will be improved to achieve greater utility.

(4) Analytical research into the mechanism of low temperature embrittlement and hydrogen embrittlement

By analyzing the mechanism of low temperature embrittlement and hydrogen embrittlement, research will be conducted on methods of preventing material embrittlement.

10.2 Results in FY1999

In Phase I (FY1993 to FY1998), research and development on structural materials for extremely low temperature applications involving storage tanks and transport tanks for liquid hydrogen (20K) were implemented as Subtask 6. Selected as candidate materials were 2 types of austenitic stainless steel (SUS304L and SUS316L) and an aluminum alloy (A5083) with track records in extremely low temperature applications. Various mechanical properties were evaluated with respect to the base metal and weld metal in temperatures ranging from extremely low (4K) to room temperature. With respect to material evaluation in liquid hydrogen environments that had been difficult to perform in the past, new equipment was designed and installed and the evaluation was implemented having established evaluation test technology. As a result, it was found that while the base metal maintained sufficiently high tenacity in liquid hydrogen environments, the weld metal showed high embrittlement susceptibility with respect to low temperature embrittlement and hydrogen embrittlement in liquid hydrogen environments thus indicating need for improvement. On the other hand, with respect to hydrogen environment embrittlement in low temperature hydrogen gas atmosphere (150K to room temperature), SUS304L showed high embrittlement susceptibility. A cryogenic material database was formed utilizing material data accumulated in WE-NET and overseas reference data.

The following are the results of research and development in FY1999 that is the first year in Phase II.

10.2.1 Research on the material properties of candidate materials (base metal, weld metal) in liquid hydrogen temperature range

Fatigue strength tests and other material evaluation were continued with respect to the candidate materials in Phase I (SUS304L, SUS316L and A5083) to collect data that had been missing from tests made up to FY1998 and to increase fatigue characteristic data. The fatigue properties of base metals SUS304L and SUS316L showed extremely high fatigue strength in tests conducted at room temperature, 77K and 20K experiencing no breakage through 10⁶ repetitions with a load equivalent to the 0.2% proof stress (Fig.10.2.1-1).

From knowledge accumulated in Phase I, one type of stainless steel (SUS316LN) and two types of aluminum alloy (A5086, A5454) were selected as new candidate materials with superior tenacity at extremely low temperatures and material characteristic evaluation was initiated. In the case of both the base metal SUS316LN and TIG weld metal, tensile strength increased as the temperature dropped and the 0.2% proof stress was about double that of base metal SUS316L and TIG weld metal. However, though the base metal SUS316LN showed high tenacity throughout, that showed lower tenacity in liquid hydrogen environment. The TIG weld metal showed unstable extension of cracking and low tenacity at low temperatures below 77K.

A study of the method of evaluating such hitherto unevaluated factors that are required of structural materials as fatigue crack growth rate, notch susceptibility and material test on thin materials was conducted. Moreover, in order to separate the influence of temperature and hydrogen in liquid hydrogen environments, a study of material evaluation in helium gas atmosphere at 20K was jointly discussed with the German research organization MPA (National Institute for Material Testing; Amtliche Materialprufungsanstalt).

10.2.2 Research on elemental technology related to optimized welding material and welding method

Towards drastic improvement of tenacity of weld metals of the candidate materials in liquid hydrogen environments, laser welding of stainless steel and friction stir welding (FSW) of aluminum alloy were produced at the UK institute TWI (The Welding Institute) and evaluation of material properties of the welding metal was conducted.

In the case of the laser weld SUS316L, while almost 10% δ-ferrite, somewhat numerous porosity and numerous minute inclusions were found, low temperature tenacity superior to TIG weld metal was achieved (Fig.10.2.2-1). However, it showed deterioration of ductility was noted and it is believed that defects such as large porosity act as notching.

The FSW of aluminum alloy is a innovative welding method developed by TWI in the 1990s (Fig.10.2.2-2). From observation of the histology, the weld metal shows a unique ring formation and the interior histology is extremely minute. FSW shows superior tensile strength at extremely low temperatures compared to the large current MIG welding and from the results of the Charpy impact test and fracture toughness test, it was confirmed that the tenacity of FSW material was significantly higher than in the case of the large current MIG welding (Fig.10.2.2-3). The reason for this is believed to be improvement of tenacity through increased minuteness of the histology and prevention of crack growth through metal flow. From these results, it was found that FSW is extremely useful as a method of improving tenacity of aluminum alloy at extremely low temperatures.

In the case of stainless steel, MIG weld joints were produced and properties evaluation was performed. With the MIG weld metal, while the quantity of δ-ferrite was high for SUS316L, higher Charpy energy absorption was achieved than for the TIG weld metal.

In the case of the large current MIG welding for aluminum alloy, evaluation at extremely low temperatures was performed on weld joints that had been homogeneity processed to eradicate segregation. However, very little improvement was seen through the homogeneity processing at extremely low temperatures.

10.2.3 Research on extending the extremely low temperature material database

Additional data were input to the database produced in Phase I and modifications were made in certain data tables to facilitate access. Moreover, analytical tools were introduced in order to allow use of figures and tables. The analytical tools used were comprised of Microsoft's Access and Excel. Functionally, Access is used to identify data and Excel for graph display such as distribution graphs and for analysis. The tables and forms in the database have been entirely translated into English.

10.2.4 Analytical research into the mechanism of low temperature embrittlement and hydrogen embrittlement

The optimum welding materials have been almost selected for candidate base metals. The influence of welding conditions on characteristic was investigated more precisely. Using SUS316L as the base metal and making 4 types of TIG weld metal using the same electrode, the specimens were subjected to Charpy impact test at the temperature of liquid nitrogen 77K, where the convenient test apparatus and condition was adapted rather than liquid hydrogen environment. As a result, it was confirmed that even using the same base metal and electrode, the absorbed energies differ by welding condition and that even within the same weld metal, the absorbed energies differ by the part. It is believed that the deterioration of low temperature tenacity of the weld metal is caused by the influence of repeated heating in weld passes. Moreover, it was confirmed that absorbed energies may be improved by performing solution treatment of the weld metal.

It was found through the research in Phase I that some austenitic stainless steels show the embrittlement at around 220K under hydrogen gas environment. Using SUS316LN and SUS316, tensile tests were performed at 220K, 1.1Mpa in hydrogen gas and helium gas to study the influence of the nitrogen and carbon content on low temperature hydrogen environment embrittlement. As a result, while SUS316NG that contains nitrogen did not show hydrogen environment embrittlement, SUS316LN showed hydrogen environment embrittlement. It is believed that the influence of Ni content is larger than that of nitrogen content. Moreover, SUS316 that had been adjusted to carbon content in the range of 0.01% and 0.06% showed high hydrogen environment embrittlement but no influence depending on the content of carbon.

In order the grasp to what extent hydrogen invades the material and how it is distributed in the material after extended use of liquid hydrogen storage and transport tanks, a long term hydrogen charging at low temperature (327K, 9.8Mpa, 1,128h) was performed on the base metals of SUS304L and SUS316L as well as the TIG weld metal. As a result of hydrogen analysis and hydrogen discharging test during reheating, hydrogen invasion of almost equilibrium concentration (about 5ppm) was confirmed. In the case of TIG weld metal of SUS316L, a tendency for hydrogen invasion into the metal to increase as the quantity of δ-ferrite increased was determined. Moreover, through destructive tenacity test on tensile test specimen comprised of a small circumference with notches, it was confirmed that with weld metal of SUS304L with high ferrite content, tenacity decreases through hydrogen invasion and significant stages were found in the section.

10.3 Future research and development issues

(1) Fatigue strength tests will be implemented in liquid hydrogen for the base metals and weld metals that constitute the candidate material and material characteristic data will be enhanced. Moreover, material evaluation at 20K will be jointly conducted with the German research organization MPA (National Institute for Material Testing; Amtliche Materialprufungsanstalt).

(2) Research into laser welding and FSW will be continued and reduced pressure electron beam welding (RPEB) will be produced.

(3) Addition data will be input to the low temperature material characteristic database.

(4) Studies of the possibility hydrogen invasion of structural materials in the real environment and its influence will be implemented to make clear the mechanism of hydrogen embrittlement. Studies of the influence of re-heating in multiple layer welding, residual stress in the weld metal and various other factors to the low temperature embrittlement will be implemented.