EVALUATION OF MECHANICAL PROPERTIES OF
AUSTENITIC STAINLESS STEELS AND
ALUMINUM ALLOY IN LIQUID HYDROGEN
(Study on low temperature materials used in WE-NET 4)

H.Fujii, A.Yamamoto
Steel Research Labs., Nippon Steel Corporation
20-1 Shintomi Futtsu, 293-8511, Japan

M.Yabumoto, T.Ogata, M.Hayashi, M.Saito,
S.Okaguchi, Y.Wada and H.Nakagawa
Executive Members of Subtask 6 in WE-NET Project
17 Mori Bldg. 6F, 1-26-5 Toranomon Minato-ku Tokyo, 105-1111, Japan

1. INTRODUCTION

Liquid hydrogen is believed to be an effective energy carrier and the development programs for its future usage are now ongoing in several countries. Japan is also conducting the project called "WE-NET" (World Energy Network)[1], in which the materials selection for the storage and transportation vessels for liquid hydrogen is one of the principal subjects. To properly evaluate the materials properties required for the vessels, the evaluation should be conducted in the circumstances which are as close as possible to the ones in which the vessels are actually exposed. In this respect, the establishment of the test methods in liquid hydrogen as well as in gaseous hydrogen is of great importance in the whole of the project. In particular, the accumulation of mechanical data in liquid hydrogen (20K) are strongly demanded because the available data in this circumstance are very limited. Although relatively abundant data in liquid helium (4K) may be utilized in some cases, some difference caused by the temperature difference between 4K and 20K should exist. In fact, there are some disputes over the temperature dependence of the strength at cryogenic temperatures[2-5] and the occurrence of the serrated deformation[6-7], both of which might exert an influence on the selection of the materials and the design of the vessels. In addition, it is interesting to examine whether the materials properties are affected by the directly adsorbed hydrogen on the newly created crack surfaces. Here in the present paper, newly installed materials testing facilities are introduced at first, by which various kinds of mechanical and fracture mechanics tests can be conducted in the circumstances of air, liquid nitrogen, liquid hydrogen and liquid helium. These tests are now being conducted using the facilities and some of the obtained data for austenitic stainless steels (SUS304L and SUS316L) and an aluminum alloy (A5083) are presented next in the paper. Both materials are considered to be some of the promising candidate ones for the liquid hydrogen storage and transportation vessels.

2. MATERIALS TESTING FACILITIES IN LIQUID HYDROGEN

The facilities were installed in 1997 at Research and Engineering Center, Nippon Steel Corporation in Chiba prefecture, Japan. The outline is schematically represented in FIGURE 1. The facilities are basically a combination of a mechanical testing system and a cryogenic system, and each apparatus or device in both systems is effectively allocated in three rooms. As a mechanical testing system, Shimadzu Full-Digital Servohydraulic Testing System was installed with some modification, by which various kinds of mechanical and fracture mechanics tests, such as tensile, fatigue(S-N), KIC, JIC, COD, fatigue crack growth, are available. Maximum piston stroke and static load of the testing machine are +-75mm and 300kN, respectively.

In the "Test machine room" in FIGURE 1, apparatuses and devises which directly contact or may contact with liquid or vaporized hydrogen are installed. Walls of the room are explosion proof and every electric apparatus is explosion protected including a load cell and electric lights. Two cryostats made of SUS304 (FIGURE 1 b.) are used depending on the kind of the mechanical tests : a larger cryostat, 600mm in inner diameter and 1300mm in depth, is used for tensile tests and a smaller cryostat, 300mm in inner diameter and 1300mm in depth, is used for the other tests. The larger cryostat is a multi-specimens type[8] quipped with a turret disc on which six tensile test specimens can be mounted. The procedure of tensile tests is as follows. Firstly, one specimen is placed under the pull-rod and subject to the test. Once the test is finished, the specimen is rotated by a handle on the top flange of the cryostat and the next specimen is placed under the pull-rod. By repeating this procedure, as many as six tensile tests can be conducted at one chance of immersion into liquid. Both cryostats are highly heat insulated by four shielding plates inserted in the upper portion and multi-layer vacuum insulated walls, which enable mechanical tests to be conducted at 4K (in liquid helium) as well as at 20K(in liquid hydrogen), 77K(in liquid nitrogen) and at ambient temperature in air. When the tests in liquid hydrogen are completed, liquid hydrogen is pushed out directly to the vent stack outside the building by applying the pressure of helium gas into the cryostat. Liquid hydrogen evaporates in the vent stack and is released into the atmosphere.

Both mechanical testing and cryogenic systems are controlled in the "Control room" separated from the test machine room with explosion proof walls. The mechanical test machine is controlled by a full-digital servo controller (FIGURE 1 i.) and a personal computer (FIGURE 1 h.). The computer is also used for collecting and analyzing the test data. Hydraulic power supply (FIGURE 1 k.) is installed in the "Hydraulic power room" together with the other apparatuses which are not explosion protected. Temperature in the cryostat and surface heights of liquid in both cryostat and hydrogen reservoir (Figure 1 d.) are watched in the cryogenic system monitor (FIGURE 1 j). Gases used for pressuring the reservoir and the cryostat and purging the cryostat are also controlled in this monitor, in which opening and closing of the valves are operated by air pressure generated by a compressor (FIGURE 1 l.) in the hydraulic power room.

As explained above, the facilities are basically remote-controlled both in mechanical testing and cryogenic systems and are equipped with various safety devices. Moreover, supplementary liquid is automatically supplied if necessary even when no operator are in the facilities such as at night. Consequently, various kinds of mechanical tests can be carried out quite effectively and safely even in liquid hydrogen.

3. TENSILE PROPERTIES OF AUSTENITIC STAINLESS STEELS AND ALUMINUM ALLOY IN LIQUID HYDROGEN

Using the above mentioned facilities, tensile tests were carried out in liquid hydrogen for two austenitic stainless steels (SUS304L and SUS316L) and an aluminum alloy (A5083).

Experimental Procedures
Materials used were commercial SUS304L, SUS316L and A5038 thick plates of 28mm, 28mm and 30mm, respectively, in thickness. All of three materials were produced by the conventional hot-rolling and heat treatment process, that is, two stainless steels were finally solution treated and the aluminum alloy was annealed. Chemical compositions are given in TABLE I. For the stainless steels, the properties of TIG weld metals were also examined. As filler wires, SUS308L and SUS316L wires were used for the welding of SUS04L and SUS316L, respectively. In addition to the tests in liquid hydrogen, tensile properties of SUS304L and SUS316L plates were also evaluated in liquid helium, liquid nitrogen and in air (278K) for comparison. Round specimens having the parallel portion of 7mm in diameter and the gauge length of 25mm were subject to the tests with the following strain rates.

At ambient temperature in air ; 1.67x10-4/s (up to 0.2% proof stress)
and 1.67x10-3/s (afterwards)

In liquid helium, hydrogen and nitrogen; 2.38x10-4/s (up to 0.2% proof stress)
and 9.52x10-4/s (afterwards) for stainless steels,
7.9x10-4/s for A5083

As described above, tensile properties in liquid hydrogen are specially focused here in this paper. More extensive investigations on the mechanical properties of the welds at cryogenic temperatures are reported and discussed in reference [9] and [10].

Results and Discussions
Stress-strain curves of the aluminum alloy in liquid helium and hydrogen are presented in FIGURE 2. As widely recognized, a curve in liquid helium was serrated from the very early stage of the plastic deformation. However, no serrated deformation was observed in liquid hydrogen. Stress-strain curves of SUS304L and SUS316L thick plates in liquid helium and hydrogen are shown in FIGUREs 3 and 4, respectively, together with those in liquid nitrogen and in air at ambient temperature. Unlike the aluminum alloy, curves in liquid hydrogen were heavily serrated after the deformation stress was exceeded 1600 MPa in SUS304L and 1400MPa in SUS316L although curves in liquid helium were serrated from the very early stage of deformation just like the aluminum alloy. As described in Introduction, there is some disagreement about the serrated deformation at 20K. Warren et al.[6] reported that Type304L stainless steel does not exhibit the serrated deformation in liquid hydrogen and other austenitic stainless steels show the serrated deformation only at the beginning of the deformation. Deimel et al.[7] showed a sporadically serrated stress-strain curve of a austenitic stainless steel having a similar composition to SUS316LN. Since the occurrence of the serrated deformation is strongly affected by compositions, geometry of specimens, strain rates, moduli of test machines, etc.[11-12], it is difficult to directly compare the curves with each other. However, the tendency of the serrated stress-strain curves obtained in the present study are considered to be reasonable because of the following reasons. Higher temperature of liquid hydrogen than liquid helium prevents from being serrated in the early stage of the deformation. As the deformation hardening proceeds and the strength level increases to around 1400-1600MPa, the serrated deformation starts as a result of the larger heat buildup, which also leads to the heavily serrated deformation from the very beginning.

In the welds of both SUS304L and SUS316L, similar stress-strain curves were observed in liquid hydrogen, as shown in FIGURE 5. However, the initiation stress of the serrated deformation was smaller than that of the base metals (FIGUREs 3 and 4), especially for SUS304L. Probably, this is partly caused by the coarse and hetero-geneous solidified microstructures in the weld metals, which are considered to enhance the localized deformation.

Temperature dependence of tensile properties of SUS304L and SUS316L plates is shown in FIGUREs 6 and 7, respectively. 0.2% proof stress and tensile strength of both steels at 20K in liquid hydrogen were almost the same as those at 4K in liquid helium. Or those at 4K were rather smaller than those at 20K. From 20K to ambient temperature, they monotonously decreased as test temperatures increased except 0.2% proof stress of SUS304L, which was almost constant or exhibited the minimum at 77K in liquid nitrogen. These obtained results indicate the possibility of the anomalous temperature dependence of the strength as previously pointed out by Verkin[3-4] and Ogawa[2]. As for ductility, an anomaly was found in reduction of area of SUS304L at 20K, which was by approximately 10% lower than that at 4K. Because some scatter was recognized in elongation and reduction of area of SUS316L at 4 and 20K, the lower value of reduction of area in SUS304L may also be attributed to the scatter. However, it is rather reasonable to think that the anomaly actually exists because all four specimens in the present investigation exhibited the lower values of reduction of area at 20K while elongation was slightly higher at 20K than at 4K. The tendency is also recognized in the stress-strain curves in FIGURE 3, in which the stagnation of the deformation hardening only appeared in liquid hydrogen at the strain from 30 (start of the serration) to 37%. The deformation hardening after the stagnation and the decrease of the deformation stress after ultimate tensile strength were also smaller at 20K than at 4K. Those characteristics in the stress-strain curves are considered to lead to the rather uniform deformation with less necking, resulting in the lower values of reduction of area without losing elongation. It is one of the assignments to be made clear in the future whether these characteristic stress-strain curve and reduction of area in liquid hydrogen are caused by the existence of hydrogen or only by temperature. Although similar stagnation was also observed in the stress-strain curve of SUS316L in liquid hydrogen, the clear drop of reduction of area was not recognized. This is probably due to the small deformation hardening behavior of SUS316L compared with SUS304L.

4. SUMMARY

To evaluate the materials properties used for the storage and transportation vessels for liquid hydrogen, new mechanical testing facilities were installed. By the facilities, various kinds of mechanical and fracture mechanics tests, such as tensile, fatigue(S-N), KIC, JIC, COD, fatigue crack growth, can be fully carried out in liquid helium, hydrogen and nitrogen as well as at ambient temperature in air. Materials data are now extensively being accumulated and will be used for the materials selection and the design for the structures. In the latter part of the paper, tensile properties of SUS304L, SUS316L and A5083 in liquid hydrogen are focused, which were actually evaluated using the facilities. Major obtained results are as follows : 1. Serrated deformation occurs in SUS304L and SUS316L in liquid hydrogen while A5083 does not exhibit any serrated deformation in the same circumstance. 2. Initiation stress of the serrated deformation is smaller in the weld metals than that in the base metals of the two stainless steels. 3. Temperature dependence of the strength of the two stainless steels is not monotonous over the temperature range from 4K to 278K. 4. Anomalous drop of reduction of area is found at 20K in liquid hydrogen.

Acknowledgment

The studies are administrated through the New Energy and Industrial Technology Development Organization as a part of the International Clean Energy Network Using Hydrogen Conversion(so-called WE-NET) Program with funding from Agency of Industrial Science and Technology(AIST) in Ministry of International Trade and Industry (MITI) of Japan.

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