6. Development of cryogenic metals technology

6.1 R&D Goals

6.1.1 Goals for Phase I

  1. 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.

  2. 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.

  3. To examine testing and evaluation methods for those characteristics that are not yet evaluated among the characteristics required for the structural materials from the other subtasks.

6.1.2 Goals for fiscal year 1997

  1. Evaluation of mechanical characteristics of the candidate materials in a liquid hydrogen environment using the new testing facilities

  2. Evaluation of extra low temperature characteristics of weld metal of the candidate materials, and development of new welding materials

  3. Evaluation of the susceptibility to hydrogen embrittlement of weld metal and of material characteristics in extra low temperature hydrogen gas environment

  4. Clarification of the mechanisms of low temperature brittleness and hydrogen embrittlement in austenitic stainless steels and aluminum alloys

6.2 Results in fiscal year 1997

In fiscal year 1993, two types of austenitic stainless steel (304L, 316L) and one type of aluminum alloy (A5083) that have been adequately proven at extra low temperature, were selected as candidate materials for structural use in liquid hydrogen (20 K) storage tanks and transportation tankers.

From fiscal year 1994, the existing low temperature (4K) testing facilities were used to evaluate mechanical characteristics (tensile strength, Charpy impact testing, fracture toughness testing) of base metal and weld metal of the candidate materials with attention to low temperature brittleness and hydrogen embrittlement. Evaluation of hydrogen embrittlement in hydrogen gas environment at low temperature was also carried out. The results revealed that no distinct degradation was caused in base metal through low temperature brittleness and hydrogen embrittlement but that susceptibility to brittleness was high in weld metal.

In fiscal year 1995, it was found that the d-ferrite content and whether treatment or non-treatment of hydrogen charging has great influence on tenacity and ductility of stainless steel weld metal at extra low temperatures. In the case of aluminum alloy, it was found that hydrogen embrittlement does not occur at room temperature or below, and investigation of the influence of welding method was carried out, by comparing the large current MIG welding with the ordinary MIG welding.

In fiscal year 1996, the effect of d ferrite content and whether treatment or non-treatment of hydrogen charging on stainless steel weld metal were determined in isolation from other factors. For 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 fiscal year 1997, material testing in a liquid hydrogen environment was started using the new testing facilities, and examinations were 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.

An outline of the results of in fiscal year 1997 is given below.

6.2.1 Stainless steel (304L, 316L)

Using the new testing facilities, tensile testing and fracture toughness testing in a liquid hydrogen environment (20 K) and fatigue testing at 20 K, 77 K and room temperature were started. As following the results of the previous year, weld metal (TIG welding) of d-ferrite = approximately 2% (midway between d= 0% and 4-5%) was made, and the effect of minute quantities of d-ferrite on material characteristics was investigated. In order to investigate the influence of the welding method, weld metals were made using SAW welding and MAG welding (d= 10% in both cases), and mechanical characteristics at low temperature as well as room temperature were compared with those in the case of TIG welding. (see Fig 6-2-1)

6.2.1.1 Tensile characteristics

It was found that tensile strength of 304L and 316L weld metals increases in line with fall in temperature and that it reaches a peak at 20 K. Elongation and reduction area were found to decrease as temperature falls, in particular, reduction area of 304L reached minimum levels at 20 K but increased again at 4 K. Dimpling could be seen on fractures at any test temperatures, but the diameter size of dimples on fractures at 20 K was excessively small. The tensile strength of weld metals formed by SAW welding and MAG welding showed similar values to that obtained in TIG welding. Dimpling could be seen on fractures of SAW and MAG weld metals at any test temperatures, and numerous spherical inclusions were observed at the bottoms of dimples. (see Fig 6-2-2)

6.2.1.2 Charpy impact characteristics

Charpy impact characteristics were found to be greatly reliant on the existence of d-ferrite in weld metals for both 304L and 316L; in the case where d-ferrite is contained, decreases in absorbed energy were observed at low temperature. This trend was found to be more pronounced in 316L than in 304L. Absorbed energy in weld metals of 304L formed by SAW welding and MAG welding was lower than that in TIG weld metal by around 1/2 and 1/4 respectively. A similar trend was observed in the case of 316L at room temperature, but levels similar to those in TIG weld metal at low temperatures. These findings are thought caused by differences in inclusions in weld metals depending on the welding method. (see Fig 6-2-3)

6.2.1.3 Fracture toughness

The effect of d-ferrite on fracture toughness increased sharply in line with decrease in the test temperature: the fracture toughness value at 4K fell by approximately 1/2 between d= 0% and d= 2% in both 304L and 316L. It was found that, even with the existence of small amount of d-ferrite, fracture toughness fell to a level equivalent to that seen when d= 5 or 10%. The fracture toughness value fell to approximately 1/3 of that seen in TIG weld metal for both 304L and 316L in the case of MAG weld metal, and approximately 2/3-1/2 in the case of SAW weld metal. Dimpling was observed in fractures of both of weld metals, but more inclusion was seen in fractures of MAG weld metal rather than SAW weld metal. The difference in fracture toughness according to the welding method decreased in line with fall in the test temperature, and no difference was observed at 4 K.(see Fig 6-2-4)

6.2.1.4 Fatigue testing

Upon determining the conditions for carrying out evaluation testing of fatigue characteristics at low temperatures, fatigue testing of 304L stainless steel was carried out at room temperature. Relationships in terms of fatigue strength were found as follows: base metal = weld metal (d= 5%) > weld metal (d= 0%), and it became clear that these relationships could be thought as a uniform trend through standardizing the maximum load by tensile strength.

6.2.2 Aluminum alloy (A5083)

In order to clarify the effect of Fe and Si reduction in weld metal on increase in tenacity at extra low temperatures, a high purity 5083 base metal was fabricated and weld metal was made using a welding wire with reduce Fe and Si. Moreover, with a view to raising fracture toughness through reducing crystal grain size of weld metal, weld metal was made using a welding wire with a high Ti content. The large current MIG welding method was adopted in each case. Tensile testing, Charpy impact testing and fracture toughness testing were carried out at room temperature, 4 K and 77 K. Tensile testing was also conducted in liquid hydrogen.

Concerning the effect of Fe and Si reduction in weld metal, improved characteristics were found at room temperature and 77 K but not so at 4 K. Upon using the welding wire containing a high Ti content, smaller crystal grains were obtained in the weld metal, however, no improvement was found in characteristics at low temperatures and indeed there was a slight deterioration in characteristics. It is thought that the compound of Al-Ti metals reduced the toughness value. Moreover, from the results of tensile testing in liquid hydrogen, it was found that strength and proof stress at 20 K lay somewhere between 77 K and 4 K. As for elongation and reduction of area, values slightly lower than those expected from 77 K and 4 K were shown.

6.2.3 Tensile characteristics in hydrogen gas

The influence of test temperature and distortion rate on brittleness in low temperature hydrogen environment was examined using 304L. Tensile testing was carried out in 1 MPa hydrogen gas environment and 1Mpa helium gas environment, over a temperature range of room temperature to 80 K, and at distortion rate ranging between 4.2 x 10-6 and 4.2 x 10-2 s-1. Susceptibility to hydrogen environment embrittlement increased in line with temperature fall before reaching a peak and dropping off again at a uniform distortion rate. For example, at a distortion rate of 4.2 x 10-5 s-1, susceptibility to hydrogen environment embrittlement increased as temperature fell from room temperature, reached a peak at close to 220 K, then decreased slightly as temperature was made even lower before suddenly disappearing at 165-180 K. The susceptibility to hydrogen environment embrittlement increased in line with decrease in the distortion rate. All the test pieces in the helium environment showed dimpled fractures, but all those in the hydrogen gas environment showed fractures that were composed of fractures running along martensite lath and fractures following twin crystals and some grain boundaries.

6.2.4 Basic examination relating to phase changes of stainless steel

Upon conducting plastic working on 304L and 316L welded metals in liquid helium, a-martensite increased in line with the degree of processing, and the rate of increase was found to be larger in 316L than in 304L. This martensite transformation was controlled slightly by hydrogen charging.

6.2.5 Basic examination relating to hydrogen diffusion and discharge

Test samples of 304L and 316L base metal and 5% cold-worked base metal, and weld metals of d= 0-10%, were kept for 1,292 hours in a hydrogen gas environment of 100 atmospheric pressure at temperatures of 77 K and 373 K (100°C). Hydrogen penetration was analyzed by SIMS and by the melting method. At 77 K, no hydrogen penetration at all was found in any of the samples. However, at 373 K, an increase in hydrogen content ranging between approximately 2.5-7.5 ppm was found in all the samples. Having said that, it was not sufficient to determine the clear effects of surface finish, cold-working and d-ferrite content on hydrogen penetration within the bounds of this experimentation.

Hydrogen discharge activity was studied on samples where hydrogen penetration occurred at 373 K. In the same way as was found in study of high temperature (573K) hydrogen charges in the previous year, two discharge peaks were found at approximately 800 K and 1,000 K in both 304L and 316L in both base metal and weld metal. However, the discharge peak of lower temperature was higher and showed possibility of the amount of diffusive hydrogen increases in the hydrogen penetrated materials. As for the effect of cold-working, the trend differed between 304L and 316L and it was not sufficient to obtain systematic results.

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 304L and 316L weld metals possessing 0%, 5% and 10% d-ferrite. Comparative evaluation was carried out on differences in fracture toughness in each portion, with respect to composition of materials and differences with values obtained in conventional CT test pieces. As a result, it became clear that this test method is only effective at low temperatures in the case of materials that possess low strength and high ductility at room temperature. Moreover, the fracture toughness value J-Rice obtained by this test method was higher than the value JIC obtained using CT test pieces. In addition, it became clear that fracture toughness is lower in weld metals than in base metals and decreases as the content of d-ferrite increases (in both 304L and 316L), and that fluctuations in the value of fracture toughness ranging between two and three times exist inside weld metals.

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

Through installing and starting the facilities for materials evaluation in liquid hydrogen environment, methods were established for safely and efficiently testing and for evaluating materials characteristics. At the same time, almost all the problem points that appeared during initial running of facilities were corrected, and safety issues requiring further attention were extracted. Tensile testing of stainless steel base metals was carried out in a series of environments consisting of liquid helium, liquid hydrogen, liquid nitrogen and room atmosphere.

6.2.8 Construction of a database on cryogenic materials

A prototype database was constructed. It was founded on the results between fiscal year 1994 and fiscal year 1997 and the findings of the survey of literature relating to cryogenic materials in the United States (a result in fiscal year 1995). The database was composed of a numerical database and a literature database. In the findings of the literature survey, 410 sources concerning 304L and 316L were referred to. However, for constructing the prototype database, only those sources including reference to mechanical characteristics of base metal at liquid hydrogen temperature (20 K) and those including reference to mechanical characteristics of weld metal at 4 K and 20 were targeted. As a result, approximately 110 sources were extracted, of which approximately 40 were input into the database.

6.3. Research plan for fiscal year 1998

The R & D shall be continued in fiscal year 1998 by placing importance on the following items:

(1) Evaluation of mechanical characteristics of the candidate materials in a liquid hydrogen environment using the new testing facilities
(2) Evaluation of extra low temperature characteristics of weld metal of the candidate materials, and development of new welding materials
(3) Evaluation of the susceptibility to hydrogen embrittlement of weld metal and of material characteristics in extra low temperature hydrogen gas environment
(4) Clarification of the mechanisms of low temperature brittleness and hydrogen embrittlement in austenitic stainless steels and aluminum alloys

Research shall be advanced centering on materials testing in liquid hydrogen, and the database shall be enlarged. Moreover, concerning hydrogen embrittlement and low temperature brittleness, joint research shall be continued with Chugoku National Industrial Research Institute and National Research Institute for Metals respectively. While conducting close exchange of information with the other subtasks, especially Subtask 3 (safety measures and assessment) and Subtask 5 (development of the liquid hydrogen transportation tanker and storage facility), development of materials and clarification of brittleness mechanisms shall be advanced with a view to achieving the R & D goals for Phase I.

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