WE-NET Home Page/Summary of Annual Reports 1998

6. Subtask 6: Development of cryogenic material technology

6.1 R&D Goals

6.1.1 Goals for Phase I

To clarify the material characteristics suitable for individual applications by performing characteristics tests on candidate materials in liquid hydrogen temperature environment and by constructing the database on these cryogenic materials.

To be concrete, the following items have been set:

To grasp the limits on various properties of existing materials (both base metal and weld metal) by establishing the techniques to evaluate the mechanical characteristics (tensile strength, fracture toughness, fatigue strength, etc.) in the temperature area of liquid hydrogen and by constructing database. And then clarify whether it is necessary to improve existing materials and/or develop new materials.
To grasp the crack sensitivity of existing materials (both base metal and weld metal) by establishing the techniques to evaluate the susceptibility to hydrogen embrittlement in the room temperature area and the temperature area of liquid hydrogen and by constructing database. And then clarify whether it is necessary to improve existing materials and/or develop new materials.
To examine testing and evaluation methods for those characteristics that are not yet evaluated among the characteristics required for the structural materials from other subtasks.

6.1.2 Goals for FY 1998

Evaluation of mechanical characteristics of the candidate materials in a liquid hydrogen environment using the new testing facilities
Evaluation of extra low temperature characteristics of weld metals of the candidate materials, and development of new welding materials
Evaluation of the susceptibility to hydrogen embrittlement of weld metal and of material characteristics in extra low temperature hydrogen gas environment
Clarification of the mechanisms of low temperature brittleness and hydrogen embrittlement in austenitic stainless steel and aluminum alloys.

6.2 Results in FY 1998

Two types of austenitic stainless steel (304L, 316L) and one type of aluminum alloy (A5083), those have been adequately proven at extra low temperature, were selected as candidate materials for structural use in liquid hydrogen transportation tankers and storage tanks. From FY 1994, evaluations of mechanical characteristics (tensile strength, Charpy impact testing fracture toughness testing) have been performed on base metal and weld metals of the candidate materials. Those evaluations have been performed with attention to low temperature brittleness and hydrogen embrittlement. Evaluations of susceptibility to hydrogen embrittlement in hydrogen gas environment at extra low temperature were also carried out. In FY 1994, it was revealed that no distinct degradation was cause in base metal through low temperature brittleness and hydrogen embrittlement but that susceptibility to brittleness was high in weld metal. In FY 1995, it was found that d ferrite content and treatment of hydrogen charging have great influence on tenacity and ductility of stainless steel weld metal at extra low temperatures. On the other hand, it was found that hydrogen embrittlement does not occur on the aluminum alloy at room temperature or below. In FY 1996, the effect of d ferrite content and treatment of hydrogen charging on stainless steel weld metal were further clarified. For the aluminum alloy, examinations were conducted on improving the low temperature characteristics of weld metal using new welding materials. Also, a new facilities allowing materials testing in liquid hydrogen environment was fabricated and installed. In FY 1997, material testing in liquid hydrogen was conducted with respect to new welding materials and welding methods. Works were also started on study of hydrogen penetration activity in materials and construction of a cryogenic materials database.

In FY 1998, research and development activities mainly consisting of characteristic tests on candidate materials in liquid hydrogen were performed to accumulate data in the database on cryogenic materials. Further, taking in consideration the actual state of use of materials, studies were performed on the influences of plastic working on materials characteristics and that of hydrogen invasion activity into materials and also investigation of material quality was performed on the materials disassembled from a liquid hydrogen lorry tank. Moreover, studies were performed in cooperation with Subtask 5 (subcommittee on liquid hydrogen transportation and storage) for setting the target values of development of materials for each application. An outline of results in FY 1998 is given below.

6.2.1 Stainless steel (SUS304L, SUS316L)

While material characteristics of base metal and TIG weld metals were evaluated in liquid hydrogen (20K) and they were compared with the values in liquid helium (4K) and in liquid nitrogen (7K) in succession to the previous year. The influences of hydrogen charge on the materials characteristics of SAW weld metal and influence of pre-distortion working (20% of tensile working) on base metal were investigated. However, SAW weld metal already contain more than 10 ppm of hydrogen in as welded state and the influence of hydrogen charge was not shown clearly. Further, by summarizing the results of evaluation for 6 years since FY 1994, comprehensive evaluations was performed on testing temperature, hydrogen charging process, welding method, and the influences on various mechanical characteristics such as the volume of d ferrite.

6.2.1.1 Tensile characteristics

As tensile characteristics of base metals, both 304L and 316L showed high elongation and reduction area at extra low temperatures with few influences of hydrogen charge. On the other hand, in the case of 304L and 316L base metals that have been subjected to pre-distortion showed twice as large as 0.2% durability and a high elongation of 30% in the extra low temperature area though their ductility lowered. On the tensile strength of TIG weld metals except 304L material containing 10% d ferrite, no influence of the volume of dferrite or the hydrogen charging process was observed. In the case of 304L material containing 10% dferrite, tensile strength lowered at 20K showing the influence of hydrogen charging process. (See Fig. 6-2-1). Although no influence of hydrogen charge on the tensile strength of SAW weld metals was observed, influence of hydrogen on ductility was observed. Fractures after the tensile test were ductile fractures with dimpling on both materials of base metal and weld metal and a trend that the diameter of dimples become smaller as the temperature lowers.

6.2.1.2 Charpy impact characteristics

In the case of SAW weld metal for both 304L and 316L, the value of Charpy absorbed energy of hydrogen charged material was equal to that of non of hydrogen charged material (materials as welded). Further, by applying pre-distortion on base metal, decrease in absorbed energy of approx. 30J in 304L and approx. 100J in 316L was observed but still sufficiently high value of absorbed energy at 4K such as 130J or more of 304L, and 200J or more of 316L.

6.2.1.3 Fracture toughness

The values of fracture toughness of base metal and TIG weld metal at 20K were equal to or slightly lower than the values of fracture toughness at 20K both in 304L and 316L. (See Fig. 6-2-2). Few influences of hydrogen charge was observed on the values of fracture toughness of SAW weld metal at 4K and 77K. Although decrease in the values of fracture toughness of 10% to 50% of 304L and 5% to 30% of 316L was observed when base metal was subjected to pre-distortion, the values of fracture toughness at 20K were maintained on a sufficiently high level such as 176 MPa (m)^1/2 in 304L and 356 MPa (m)^1/2 in 314L.

6.2.1.4 Fatigue testing

In the case of base metal, though the fatigue strength increased as the temperature lowers within the rage of 10⁴ to 10⁶ cycles, the values at 20K and 77K were almost equal. On the other hand, no remarkable difference was observed between 304L and 316L. The gradient of the S-N curve of 316L base metal varied around 10⁵ cycles (See Fig. 6-2-3). As is the case of base metal, weld metal also showed the trend of increasing fatigue strength of as the temperature lowers within the rage of 10⁴ to 10⁶ cycles. Further, while the influence of the volume of d ferrite was observed on the fatigue characteristics of 304L, no influence was observed in 316L.

6.2.2 Aluminum alloy (A5083)

Evaluation of mechanical characteristics including fatigue characteristics was performed on base metal and weld metal. Further, to investigate the changes in material characteristics caused by fabrication during the manufacture of tanks, characteristics of the material made by applying pre-distortion (20% elongation) to base metal were also evaluated. Among base metals, pre-distorted materials, and weld metals, those with higher tensile strength showed higher fatigue strength at any of the room temperature, 77K, and 4K. Further, from the S-N curve normalized by tensile strength, though some scattering was observed, a good interrelation was observed between the normalized stress and the number of cycles until the fracture.

Although the tensile strength and durability at of the room temperature, 77K, and 4K were largely improves by applying pre-distortion to base metal, elongation and reduction area lowered. Further, Charpy absorbed energy and fracture toughness showed equal and very low values at different temperatures. From these results, it has been revealed that the toughness of base metal lowers extremely lowers after being subjected to 20% elongation.

As a result of a fracture toughness test on base metal and weld metal at 20K, though the fracture toughness of base metal showed a value similar to the value at 77K, the fracture toughness of weld metal showed a value lower than the value at 4K (See Fig. 6-2-4). However, as a result of observation of fractures, no large difference was observed between the fractur4es at 4K and 20K.

6.2.3 Tensile characteristics in hydrogen gas

Hydrogen environment embrittlement characteristics of 304L, 304, and 316 austenitic stainless steel were investigated within the temperature range of room temperature to 80K and were found to increase as temperature fell from room temperature. With regard to the lower limit temperature where the susceptibility showed no more increase after it had reached the maximum value around 200K, dependency on material, distorting speed, and heat treatment was examined. Susceptibility to hydrogen environment embrittlement increased as distorting speed decreased and the lower limit temperature shifted to the lower temperature side as distorting speed decreased. The lower limit temperature was affected by heat treatment, where the lower limit temperature in sensitized material was lower by approx. 20K than that in solution heat treated material (See Fig. 6-2-5).

In 316LN, which has more stable austenite, in spite that distortion inducing martensite was generated, no hydrogen environment embrittlement over the whole temperature range from room temperature to 80K.

6.2.4 Basic examination relating to phase changes of stainless steel

The phase at the tip of cracks was identified on SUS304L stainless steel using a micro X-ray equipment with 100-mm probes. As a result, it was found that phase changes at the tip of cracks were occurring in either of base metal and weld metal and the martensite phase also increases as the plastic area changes at the tip of cracks increased. Further, there was a trend that the changes at 20K were smaller than those at 4K.

6.2.5 Examination relating to hydrogen invasion

Investigation on the possibility of hydrogen invasion was performed through a hydrogen charging process under two different conditions of 30°C, 100 atmospheric pressure and 100°C, 10 atmospheric pressure, which are nearly the severest conditions that the vessel is actually exposed to. Under the conditions of 30°C, 100 atmospheric pressure and 100°C, 10 atmospheric pressure, no hydrogen invasion into stainless steel was observed. On the other hand, no influence of a long time hydrogen charging process on Charpy absorbed energy was not observed at this time.

6.2.6 Fracture toughness testing using small test pieces

Using small round-cut tensile testing pieces, testing was carried out to examine the fracture toughness in different portions of weld joints made of 304L and 316L stainless steel possessing 0%, 5%, and 10% d ferrite. Comparative evaluation was carried out on differences in fracture toughness in each portion of weld joints, with respect to composition of materials, influence of long time hydrogen charging on hydrogen environment embrittlement characteristics, and difference with values obtained in conventional CT test pieces. As a result, it became clear that the fracture toughness value J-Rice obtained by this test methods was higher than the value JIC obtained using CT test pieces, fracture toughness is lower in weld metals than in base metals and decreases as the content of d ferrite increases (in both of 304L and 316L). Further, as the influence of long time hydrogen charging on hydrogen environment embrittlement characteristics, it became clear that toughness decreased in the304L materials with 5%, and 10% d ferrite with remarkable steps on fractures and so on.

6.2.7 Establishment of evaluation methods using the facilities for materials evaluation in liquid hydrogen

While implementing and completing enhancement measures against safety issues extracted in the previous fiscal year, various defects and troubles were drastically solved to achieve the smooth operation of testing equipment. In connection with it, tensile tests, fracture toughness tests, and fatigue tests are carried out vigorously to accumulate material characteristics data in liquid hydrogen, which are precious on the world level. Particularly from the tensile tests stainless steel, it was clarified that SUS304L base metal shows a quite characteristic S-S curve, where micro serration and abrupt decrease in deformation stress appear during the stagnating period of the first working and hardening when it is subjected to hydrogen charging. Further, while facilities were improved, by devising the sample replacement work, the efficiency of the fracture toughness test, in particular, was improved drastically, where seven or more sample can be tested within a day as compared with the conventional pace of about one sample per 2 days.

6.2.8 Investigation of materials disassembled from the tank for a liquid hydrogen lorry

Sample materials disassembled from the tank for a liquid hydrogen lorry that has been used for 10 years for transportation of liquid hydrogen (SUS304, SUS304L with a thickness of 4 and 9 mm) were obtained and materials investigation was performed on them. As a result, no defect such as cracks in weld metal was observed by the dye penetrate test and no particular deterioration of material was observed in the evaluation by the tensile test and Charpy impact test. Further, no particular increase in residual hydrogen component in the material was observed. Sensitization was observed in weld metal exposed to thermal effects, the portion subjected to strong fabrication was extremely hardened, and it was found that it contained a large volume of ferrite.

6.2.9 Reports on trips overseas

At the 12th World Hydrogen Energy Conference held in Buenos Aires, while presenting four cases of research results, trends of research and development on structural materials for use at extra low temperatures were investigated. There were few presentations relating to structural materials and, therefore, the presentation made by Subtask 6 evoked strong responses. Further, the delegate visited Canada, the United States, Germany, and the United Kingdom to perform investigation of low-temperature structural materials and technologies for using and working them.

6.2.10 Application of state-of-the-art welding and jointing techniques to liquid hydrogen storage and transportation vessels

As an international joint research project for prevention of global warming implemented by the NEDO, a joint research was performed with the TWI of the United Kingdom. In the TWI, friction and stirring jointing of aluminum alloy A5083 with a thickness of 30 mm and CO₂ laser welding of stainless steel SUS304L and SUS316L were performed. Then, evaluation of material characteristics at extra low temperatures will be performed on welded and jointed metals.

6.2.11 Examination of material target values for each application

With regard to low-temperature structural materials used for large liquid hydrogen tankers and storage tanks, examinations for the purpose of setting individual development target values of materials characteristics required for each application were carried out in cooperation with Subtask 5. However, there is no particular necessity of appropriately using materials for each application from the viewpoint of designing liquid hydrogen tankers and storage tanks and examinations of the safety evaluation based on characteristics of hydrogen are not yet started, so it has become clear that setting of target values is difficult under present conditions where no design guides are provided. On the other hand, it is possible that the wall temperature of the liquid hydrogen storage tank above the liquid level exceeds 150K and from the viewpoint of the hydrogen environment embrittlement characteristics of materials, it is considered that appropriate selection of material according to the temperature area of the portion of application will become necessary.

6.2.12 Construction of the database on cryogenic materials

Additional data input was performed to the numerical database for the prototype that was configured in FY 1997. In succession to the previous fiscal year, material characteristic data as the results of researches during FY 1996 and 97 and further data was read from approx. 80 charts extracted from foreign literatures and input to the database. Further, to accommodate the increase in the number of items, data tables, etc. were added.

6.3. Summary of results and future prospects

While summarizing the results achieved in FY 1998, results of the six years of first phase research and development were summarized to examine the subjects for the second phase and succeeding research and development. As the subjects for the second phase, it is considered necessary to evaluate the suitableness of materials for liquid hydrogen transportation and storage applications and develop element technologies relating to optimum welding materials and optimum welding methods while continuing material characteristics tests on candidate materials and accumulating material data.