sumitomo1996

HYDROGEN EMBRITTLEMENT OF AUSTENITIC STAINLESS STEELS AT EXTRA-LOW TEMPERATURE
(Study on low temperature materials used in WE-NET 2)

S.Okaguchi
Corporate Research & Development Labs., Sumitomo Metal Industries, Ltd.
1-8 Fuso-cho, Amagasaki 660, Japan
T.Horiya, T.Iida, M.Saito, N.Yaegashi, A.Yamamoto,
N.Yamamoto and T.Ogata
Executive members of Subtask 6 in the WE-NET project
17 Mori Bldg. 6F, 1-26-5, Toranomon, Minato-ku. Tokyo 105, Japan

ABSTRACT

For the evaluation of susceptibility of two commercial grade austenitic stainless steels (304L and 316L) to hydrogen embrittlement, Charpy impact test and fracture toughness test on both base metal and welded joints were carried out at temperatures of 4K and 298K. Hydrogen charging was achieved by exposing the steels to high pressure hydrogen gas in an autoclave at 673K. The test results demonstrated that the hydrogen charging (up to 30ppm in weight) remarkably decreased the absorbed energy of the weld joints in both steels, whereas it had little effect on the absorbed energy of the base metal. In the fracture toughness test of 316L steel, K_ＩＣ values of the weld metal was decreased by the charging. From the observation of microstructure and fracture surface, it is assumed that the embrittlement in the weld metal was related to the formation of d-ferrite which become the preferential path of the crack by the hydrogen charging.

1. INTRODUCTION

The austenitic stainless steels such as SUS304L and SUS316L are generally more reliable on mechanical properties at cyrogenic temperatures and less susceptible to hydrogen embrittlement than ferritic steels, therefore, they have been expected as candidate materials to be used for the liquid hydrogen storage and transportation system in International Clean Energy Network Using Hydrogen Conversion (WE-NET) Project. Some investigations, however, have pointed out that metastable austenitic stainless steels were embrittled by hydrogen charging even at room temperature^{１），２）}. The presence of α'-martensite transformed during deformation induces the hydrogen embrittlement at austenite grain boundary^３）. However few study concerned with the susceptibility of hydrogen embrittlement in weld metals which contained d-ferrite at extra low temperature was reported. For designing the system of liquid hydrogen services, it is necessary to know the effect of hydrogen on the mechanical properties, especially on fracture toughness at service conditions. The purpose of this study is to evaluate the susceptibility of commercial grade austenitic stainless steels (SUS304L and 316L) and their weld joints to hydrogen embrittlement in Charpy impact tests and fracture toughness tests in liquid helium (4K) and at room temperature. The effect of hydrogen charging on mechanical properties is discussed focussing the testing temperature and d-ferrite.

2. EXPERIMENTAL PROCEDURE

Two commercial grade austenitic stainless steels (SUS 304L and 316L) melted in electric furnace and refined by Argon Oxygen Decarburization (AOD) process, were prepared as the test materials. The chemical composition of the steels are shown in .Table 1. After hot rolling, plates of the steels (25 mm thickness) were solution treated at 1040degree for 304L and at 1100degree for 316L, followed by water quenching. Welded joints of each steels were made by Tungsten Inert-gas arc welding (TIG). The volume fraction of d-ferrite phase in the weld metals of 304L and 316L were 12% and 11%, respectively. The chemical composition of the weld metals are also shown in Table 1.
After preparing of specimens, hydrogen charging was achieved by exposing the specimens of the steels to high pressure hydrogen gas in an autoclave maintained at 400゜C. Hydrogen content in the steels was controlled by the condition of different level of hydrogen gas pressure and the exposing time. The hydrogen contents of the materials with and without charging are shown in Table 2. Charpy impact test and fracture toughness test were performed both in liquid helium (4K) and at room temperature (RT) by using test specimens with and without the hydrogen charging. For the evaluation of fracture toughness of the materials, K_ＩＣ values of the steels were evaluated from J_ＩＣ value in J-integral characterization tests with using half-inch CT specimens.

3. RESULTS AND DISCUSSION

3.1 Charpy Impact properties

Figure 1 shows the effect of the hydrogen charging on the absorbed energies of base metal and weld metal of Type 304L steel. At room temperature (RT), the hydrogen charging had little effect on decreasing the absorbed energies of both base metal and weld metal, whereas the absorbed energies of both base metal and weld metal were decreased by the hydrogen charging at liquid helium temperature (4K). The effect of hydrogen charging on the Charpy impact testing properties in Type 316L steel is shown in Fig.2. In 316L steel, hydrogen charging had little effect on the absorbed energy of base metal at RT. Even at liquid helium temperature (4K), it reduces the absorbed energy only 15% of that of the uncharged one. On the other hand, the absorbed energy of the weld metal was reduced by the hydrogen charging about 50% of that of uncharged weld metal at both RT and 4K.

3.2 Fracture Toughness

The effect of hydrogen charging on the K_ＩＣ values of SUS304L and 316L steels are summarized in Figs.3 and 4, respectively. In the case of the base metal, hydrogen charging significantly decreased the K_ＩＣ values of 304L steels at both liquid helium temperature (4K) and room temperature (RT), whereas the value of the base metal of 316L steels did not change significantly at both 4K and RT. The K_ＩＣ　values of the base metal of 304L steels were reduced to about 100MPa√m at both 4k and RT. It is well known that strain-induced martensite causes the hydrogen embrittlement in metastable austenitic stainless steels４）. Since the stability of austenite in 304L steel is lower than that of 316L steel, the reduction in the K_ＩＣ values of the base metal is thought to be more pronounced in 304L steels. In the case of weld metal, the hydrogen charging reduced the K_ＩＣ value of both 304L and 316L at RT as well as at 4K. The K_ＩＣ value of the weld metal of both 304L and 316L were about 100MPa√m at 4K. Figures 5 show the fracture surface of the weld metal of 316L steels with and without hydrogen charging. The fracture surface of the charged specimen includes a brittle fracture mode with decohesion pattern of d-ferrite/austenite morphology, while the fracture surface of uncharged specimen shows a ductile fracture mode.d-ferrite/ austenite interface in the weld metal was found to become the preferential path of the charged weld metal specimen in 316L steel.

4. CONCLUSIONS

For the evaluation of hydrogen embrittlement of the commercial grade austenitic stainless steels, SUS304L and SUS316L, and their weld metals, Charpy impact tests and fracture toughness tests were carried out at liquid helium temperature (4K) and room temperature (RT). The test results were summarized as follows.

(1) The hydrogen charging decreased the absorbed energy of the weld joints in both steels, whereas it had little effect on the absorbed energy of the base metal.
(2) In 304L steel, the K_ＩＣ values of both the base and weld metal was decreased by the hydrogen charging. The loss of the K_ＩＣ values by the hydrogen charging was pronounced in the weld metals of 316L steels, while the charging had little effect on the K_ＩＣ values of the base metal of 316L steels.
(3) From the observation of microstructure and fracture surface, it is assumed that the embrittlement in the weld metal of austenitic stainless steels was related to the formation of -ferrite which become the preferential path of the crack by the hydrogen charging.

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

References
1) A.W.Thompson : "Hydrogen in metals", ed.by I.M.Bernstein and A.W.Thompson [ASM],Metals Park, (1974), p.91
2) C.L.Briant : "Hydrogen Effect in Metals",ed.by I.M.Bernstein and A.W.Thompson [AIME],(1981), p.527
3) D.Hardie and J.J.Butler : Mater. Sci. Technol., 6 (1990), p.441
4) C.L.Briant : Met. Trans., 10A (1979), p.181

HYDROGEN EMBRITTLEMENT OF AUSTENITIC STAINLESS STEELS AT EXTRA-LOW TEMPERATURE(Study on low temperature materials used in WE-NET 2)