AND ITS WELD METAL AT LOW TEMPERATURE
(Study on low temperature materials used in WE-NET 6)
M.Yabumoto, H.Fujii, T.Iida and H.Eguchi
1. INTRODUCTIONFrom the viewpoint of global environmental protection, it is necessary to reduce CO2 gas exhaust. It is focused to use hydrogen for new energy source because of its cleanness and inexhaustibility. Study of using hydrogen as energy source is carried forward in many countries. In Sub-task 6 of WE-NET project, we investigate about mechanical properties of structural materials at low temperature and AA5083 is one of candidate materials for liquid hydrogen tank.
We reported about the little influence of hydrogen charging on mechanical properties of 5083 base metal and weld metal. However, it was recognized that the mechanical properties of weld metal became brittle at 4K . So it is necessary to improve the mechanical properties of weld metal at low temperature.
In this study, tensile test, Charpy impact test and fracture toughness test are carried out to understand the mechanical properties of both 5083 base metal and weld metal at 293K,77K,20K and 4K. And also five kinds of weld wire are prepared to investigate the influence of chemical composition on mechanical properties of weld metal.
2. EXPERIMENTAL PROCEDUREAA5083-O with final gauge 30 mm was used for base metal. Five kinds of filler were prepared to investigate the influence of chemical composition on mechanical properties of weld metal. Weld metals were made by high current MIG (Metal Inert Gas welding) with 2 passes. High current MIG method is usually used for LNG tank.
The chemical composition of filler is shown in Table I. Filler 'a' is used for standard filler. 'b' contains Zr for the purpose of refining grain size of weld metal. 'c' and 'd' are prepared for high purity to reduce intermetallic compounds and 'c' contains also Zr. 'e' contains high Mg for the purpose of high strength.
For evaluation of mechanical properties, tensile test, Charpy impact test and fracture toughness test were carried out at 293K, 77K and 4K. And tensile test was also performed at 20K.
Tensile test specimen was prepared with 7 mm in diameter. It was cut from long transverse direction (LT) for base metal. And for weld metal, it was cut from the center of weld metal of the 2nd pass as tensile direction parallel to welding direction. Test was carried out at initial strain rate 8x10-4/s.
Charpy impact test specimen was prepared with V notch. It was cut from T-L orientation for base metal and from the center of the welding metal of the 2nd pass for weld metal as crack orientation parallel to welding direction.
Fracture toughness test specimen was prepared with 1 inch compact specimen. It was cut from T-L orientation for base metal. And for weld metal, it was cut from center of weld metal as crack orientation parallel to welding direction. Fracture toughness test was performed by JIC test using pre-cracking and side-grooved specimen. KIC value was calculated from JIC value.
3. RESULT AND DISCUSSION3.1. Microstructure and chemical composition
Table II shows chemical composition of both base metal and weld metal. It is shown the influence of filler on chemical composition for weld metal become small because of the mixture with base metal. From this result it is recognized that weld metal consists of about 50% filler and 50% base metal.
Figure 1 shows microstructure of base metal and weld metal. It is seen that weld metal 'D'(used high purity filler 'd') and 'E'(used high Mg filler 'e') have almost equal grain size to 'A' (used standard filler 'a'). And it is seen that grain size becomes slightly large in both 'B' and 'C' (used Zr added filler 'b' and 'c') than 'A'.
3.2 Tensile Properties
Table III and figure 2 show the result of tensile test both base metal and weld metal. It is seen that UTS and YTS increase as decreasing temperature in both base metal and weld metals. In the case of elongation, it is shown that elongation, especially uniform elongation, becomes large at 77K in both base metal and weld metals. This is caused by increase of work harden-ability.
Elongation of weld metals at 4K is recognized small value in comparison with base metal. Reduction of area (R.A.) of both base metal and weld metals decrease as lowering temperature.
About the influence of filler composition, there is no effect on UTS and YTS in the range of 293K-4K. As for the elongation and R.A., 'C' and 'D' show better properties than 'A' at 293K and 77K. 'E' shows smaller elongation and R.A. than 'A' at 77K. But little influence of filler composition among weld metals is seen at 4K.
Tensile test about base metal and weld metal ('A' and 'D') at 20K,under liquid hydrogen, is carried out at Nippon Steel Corporation . These result are also plotted in figure 2. UTS and YTS of both base metal and weld metal are indicated between 77K and 4K. Elongation and R.A. are slightly lower than expected but this reason may be considered the influence of scratch for elongation measurement. More details are still under investigation.
3.3 Charpy Impact Properties
Table IV shows the result of Charpy impact test. Figure 3 shows the effect of temperature on absorbed energy and relative absorbed energy against to base metal. It is shown that absorbed energy decreases both base metal and weld metals mono-tonously with the decline of temperature. 'C' and 'D' shows higher absorbed energy than base metal at 293K. But absorbed energy of all weld metals are shown lower value than that of base metal at 77K and 4K.
For the influence of filler composition, 'C' and 'D', used high purity filler, show higher absorbed energy than 'A', used standard filler, at 293K and 77K. But little difference is seen at 4K. 'E', high Mg filler, shows lower absorbed energy than 'A' in the range of 293K-4K.
3.4 Fracture Toughness
Table IV shows the result of JIC test and figure 4 shows the effect of temperature on KIC and relative KIC against to base metal. In the case of base metal it is shown that KIC increases at 77K, but decreases remarkably at 4K. In the case of weld metals KIC at 77K is almost equal to KIC at 293K but decreases at 4K as well as base metal.
As compared with each weld metals, 'C' and 'D'(used high purity filler) show higher KIC value than that of 'A'(used standard filler) at 293K. But the effect of purity becomes small as decreasing temperature and little effect is seen at 4K. 'E'(used high Mg filler) shows no improvement in comparison with 'A' in the range of 293-4K.
Figure 5 and 6 show SEM observation of fracture surface after JIC test at 293K and 4K. Dimples are observed in both base metal and weld metals at 293K but intergranular crackings are observed at 4K. Decline of fracture toughness value corresponded to the occurrence of grain boundary fracture. This result was similar to tensile test and Charpy impact test.
4.CONCLUSIONFor the evaluation of low temperature embrittlement and hydrogen embrittlement, AA5083 base metal and weld metals were investigated. To investigate the influence of chemical composition on mechanical properties of weld metals, tensile test, Charpy impact test and fracture toughness test were carried out at 293K, 77K, 20K and 4K. Test results were summarized as follows.
AcknowledgementThe studies are administrated through the New Energy and Industrial Technology Development Organization (NEDO) as a part of the International Clean Energy Network Using Hydrogen Conversion (so-called WE-NET) Program with funding from the Agency of Industrial Science and Technology (AIST) in Ministry of International Trade and Industry (MITI) of Japan.