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THE EFFECTS OF d-FERRITE PHASE IN WELD METAL OF AUSTENITIC STAINLESS STEELS ON THE MECHANICAL PRORERTIES AT CRYOGENIC TEMPERATURE
(STUDY ON LOW TEMPERATURE MATERIALS USEN IN WE-NET3)

M.Saito
Materials & Welding Research Lab. Nagasaki R&D Center, MHI
5-717-1 Fukahori-machi, Nagasaki-city, 851-03, Japan
T.Horiya, A.Yamamoto, T.Iida and T.Ogata
Members of Subtask 6 in WE-NET Project
17 Mori Bldg.6F, 1-26-5, Toranomon, Minato-ku, Tokyo 105, Japan

ABSTRACT

The austenitic stainless steels such as SUS304L are one of the likeliest candidate material for liquid hydrogen storage system and transportation system (ex.liquid hydrogen tanker). In order to keep the reliability of these large scale structures, it is very important to comprehend the mechanical properties and the hydrogen embrittlement behavior of materials between room temperature and cryogenic temperature. For the evaluation of low temperature embrittlement and hydrogen embrittlement , two commercial austenitic stainless steels, SUS304L and SUS316L, are selected and a series of tensile tests have been conducted. Test specimens were taken from the base metals and the weld metals which contained different levels of d-ferrite phase. Hydrogen charging was carried out under the condition of different levels of hydrogen gas pressure at 400degree. In this report, the effects of d-ferrite phase in the weld metals and hydrogen charging on the mechanical properties , especially on ductility at cryogenic temperature, are described.

1. INTRODUCTION

In WE-NET project, hydrogen energy is focused on as future energy due to its cleanliness and inexhaustibility. Hydrogen energy is assumed to be transported in the world network system as liquid hydrogen; therefore, a liquid hydrogen tanker should become very important means of transportation.
In order to design the system for liquid hydrogen services, it is essential to know the mechanical properties of materials at cryogenic temperature; however, there have been few material data of structural materials, in particular the data of weld metals of these materials, at cryogenic temperature. Moreover, the effect of hydrogen embrittlement on mechanical properties of materials at cryogenic temperature is still not clear, though hydrogen embrittlement is considered to be the problem mainly at ambient temperature.
For determining suitable structural materials used for liquid hydrogen storage and transportation system, it is necessary at first to know mechanical properties and hydrogen embrittlement behavior of materials at service temperature. The purpose of this investigation is to evaluate low temperature embrittlement and hydrogen embrittlement of commercial materials at cryogenic temperature.

2.EXPERIMENTAL PROCEDURE

Two commercial austenitic stainless steels, SUS304L and SUS316L (t 25mm), were selected as the test materials. Welded joints of each stainless steel were made by Tungsten Inert Gas arc welding (TIG).
The chemical composition and the mechanical properties of base metals and weld metals are shown in Table1. To make the different levels of d-ferrite phase content in weld metals, 308L type, 308LN type and 310 type welding wire were used for joint A, joint B and joint C respectively on the welding of SUS304L and 316L type, 316LN type and 316L Hi Mn type welding wire were used for joint D, joint E and joint F respectively on the welding of SUS316L. The d-ferrite phase content of the base metals and the weld metals were as follows.

Base metal SUS 304L: 1%
SUS 316L: 1%
Weld metal Joint A(308L): 12%
Joint B(308LN): 5%
Joint C(310): 0%
Joint D(316L): 11%
Joint E(316LN): 4%
Joint F(316L Hi Mn): 0%

The dimension and sampling position of test specimen for tensile test is shown in Fig.1. A round bar type tensile test specimen of 7mm in diameter was cut from the base metal in C direction and from the weld metal of welded joint. Hydrogen charging was carried out in the autoclave under hydrogen gas with the pressure between 1.1MPa and 9.8MPa at 400degree to make the level of hydrogen content in the specimens 10ppm and 30ppm. A series of tensile tests were carried out in liquid helium (4K) and at room temperature by using test specimens with and without hydrogen charging.

3.TEST RESULTS

3.1 Base Metal

Fig.2 shows the effects of hydrogen charging on the mechanical properties of SUS304L and SUS316L base metal. 0.2% proof stress and ultimate tensile strength at 4K of both SUS304L and SUS316L increased by 1.5乣2.5 times those at room temperature. This is because the martensitic transformation accompanied with deformation of these alloys increased remarkably with decreasing of temperature.
There is no significant effect of hydrogen charging on the strength observed on SUS304L and SUS316L. Though the values of reduction of area and elongation of SUS304L decreased gradually with hydrogen charging at both RT and 4K, they were still more than 40% and all fracture surface showed ductile manner even on the specimen with hydrogen content of more than 30ppm. On the other hand, the values of reduction area and elongation of SUS316L were not affected by hydrogen charging at both RT and 4K.
Judging from these test results, it is considered that the mechanical properties, especially ductility which is the most important characteristic for structural materials used at cryogenic temperature, of SUS304L and SUS316L base metal are sufficient as structural materials even at 4K.

3.2 Weld Metal

The effect of d-ferrite phase content on the relative strength of weld metal specimens is shown in Fig.3. Relative strength means the ratio of the strength to the strength of hydrogen uncharged specimen at RT.
The relative ultimate tensile strength of 304L weld metals (A,B,C) at 4K increased with d-ferrite phase content. In case of 316L weld metals (D,E,F), there was little effect of d-ferrite phase content on relative ultimate tensile strength. On the other hand, the relative 0.2% proof stress of both 304Lweld metals and 316Lweld metals at 4K increased with decreasing of d-ferrite phase content. Regarding to the effect of hydrogen charging, the relative strength of hydrogen charged specimens tended to increased by about 10% that of uncharged specimens. The effect of hydrogen charging on the reduction area of weld metal specimens is shown in Fig.4.
The values of reduction area at RT of both 304Lweld metals and 316L weld metals decreased with increasing of hydrogen content; however, they were still more than 40% even on the specimen with hydrogen content of about 30ppm. On the other hand, the values of reduction area at 4K decreased to less than the half of those at RT. In particular, when the weld metals except F were hydrogen charged, the values of reduction area of them dropped to 20% or less and some quasi-cleavage fracture was observed on the fracture surfaces of these specimens. The value of reduction area of weld metal F (316L Hi Mn) was higher than 30% and there was no effect of hydrogen charging observed. On the tensile tests of this time, the improvement of ductility of weld metals by reducing d-ferrite phase content to about 5% was not recognized. As for the weld metals which have no d-ferrite content (complete austenitic weld metal), weld metal F showed relatively good ductility even at 4K; however, there was no improvement of ductility on 310 type weld metal.

4. CONCLUSIONS

For the evaluation of low temperature embrittlement and hydrogen embrittlement of the commercial austenitic stainless steels, SUS304L and SUS316L, and the weld metals which have different levels of d-ferrite phase , a series of tensile tests were carried out at liquid helium temperature (4K) and room temperature. The test results were summarized as follows.

1) Though the effect of hydrogen charging was recognized on the ductility and the elasticity of SUS304L base metal, the mechanical properties of SUS304L and SUS316L base metal are considered to be sufficient as a structural material at 4K.
2) The ductility of both 304L type and 316L type weld metals decreased remarkably at 4K and dropped to less than the half of that at RT. The hydrogen charging also reduced the ductility of the weld metals. Some quasi-cleavage fracture surfaces were observed on the specimens with hydrogen charging.
3) The improvement of ductility of weld metals by reducing d-ferrite content to about 5% was not recognized on the tensile tests. As for the complete austenitic weld metals, weld metal F (316L Hi Mn) showed relatively good ductility even at 4K; however, there was no improvement of ductility on 310 type weld metal.
4) The investigation on the effect of chemical composition and d-ferrite content of weld metal on the ductility at cryogenic temperature will be continued hereafter.

ACKNOWLEDGEMENT

This study was conducted as a part of research activities of Subtask 6 in WE-NET project which was supported by NEDO (New Energy and Industrial Technology Development Organization) and MITI (Ministry of International Trade and Industry of Japanese Government).

THE EFFECTS OF d-FERRITE PHASE IN WELD METAL OF AUSTENITIC STAINLESS STEELS ON THE MECHANICAL PRORERTIES AT CRYOGENIC TEMPERATURE (STUDY ON LOW TEMPERATURE MATERIALS USEN IN WE-NET3)