Masaaki Imade

Development of New Material Testing Apparatus in High-Pressure Hydrogen and Evaluation of Hydrogen Gas Embrittlement of Metals

A new type of apparatus for material testing in high-pressure gas of up to 100 MPa was developed. The apparatus consists of a pressure vessel and a high-pressure control system that applies the controlled pressure to the pressure vessel. A piston is installed inside a cylinder in the pressure vessel, and a specimen is connected to the lower part of the piston. The load is caused by the pressure difference between the upper room and the lower room separated by the piston, which can be controlled to a loading mode by the pressure valves of the high-pressure system supplying gas to the vessel. Hydrogen gas embrittlement (HGE) and internal reversible hydrogen embrittlement (IRHE) of austenitic stainless steels and iron- and nickel-based superalloys used for high-pressure hydrogen storage of fuel cell vehicle were evaluated by conducting tensile tests in 70 MPa hydrogen. Although the HGE of these metals depended on modified Ni equivalent, the IRHE did not. The HGE of austenitic stainless steels was larger than their IRHE; however, the HGE of superalloys was not always larger than their IRHE. The effects of the chemical composition and metallic structure of these materials on the HGE and IRHE were discussed. The HGE of austenitic stainless steels was examined in 105 MPa hydrogen. The following were identified; SUS304: HGE in stage II, solution-annealed SUS316: HGE in stage III, sensitized SUS316: HGE in stage II, SUS316L: HGE in FS, SUS316LN: HGE in stage III and SUS310S: no HGE.Copyright

Hydrogen Embrittlement of Metals in 70 MPa Hydrogen at Room Temperature

Hydrogen embrittlement (HE) of metals used in the system of fuel-cell vehicles, i.e., high-pressure hydrogen storage tanks and vessels, compressors, valves and pipes, is investigated in 70 MPa hydrogen at room temperature. The materials tested are austenitic stainless steels (i.e., SUS304; in the Japanese Industrial Standard (JIS), SUS316, SUS316L, and SUS316LN), a low-alloy steel (i.e., SCM440), carbon steels (i.e., SUY, S15C, S35C, S55C and S80C), a Ni-based superalloy (i.e., Inconel 718), and an aluminum alloy (i.e., A6061). Tensile tests were conducted at room temperature using a specially designed equipment developed by our laboratory, which was designed to measure the actual load on the specimen with an external load cell irrespective of the axial load caused by the high pressure and friction at sliding seals. SUS304 and SUS316 showed severe HE, while SUS316L and SUS316LN showed slight HE. Fracture occurred on strain-induced martensite of the austenitic stainless steels in hydrogen. SCM440 showed extreme HE depending on heat-treatment; in particular, quenched materials showed marked HE. The carbon steels showed extreme and severe HE depending on carbon content. Inconel 718 also showed severe HE, while A6061 showed negligible HE. These results and other HE testing results which AIST has done previously are summarized in the AIST HE data table. HE behavior of the material in high-pressure hydrogen is discussed in this paper.Copyright

Atomic structure of interface between monolayer Pd film and Ni(111) determined by low-energy electron diffraction and scanning tunneling microscopy

The atomic structure of Pd ultrathin films grown on Ni(111) at 300 K is investigated by low-energy electron diffraction and scanning tunneling microscopy. It is determined atomically that the growth of monolayer Pd films leads to a periodic arrangement of triangular misfit dislocation loops in the underlying Ni(111) surface, resulting in a triangular superstructure on the monolayer Pd surface. The triangular dislocation loops tend to align at an angle of about 5° from the Ni atom row, owing to a slight rotation of the Pd films with respect to the Ni substrate, and appear as a moirelike superstructure on the multilayer Pd surfaces. Atomistic simulations indicate that the slight rotation of monolayer Pd films and the formation of misfit dislocation loops in the Ni surface minimize the Pd–Ni interface energy.

Apparatus for material tests using an internal loading system in high-pressure gas at room temperature

A new type of apparatus for material tests using an internal loading system in high-pressure gas up to 100 MPa at room temperature without conventional material testing equipment was developed. The apparatus consists of a high-pressure control system and a pressure vessel, in which a piston is installed in the cylinder of the pressure vessel. The load caused by the pressure difference between spaces separated by the piston in the vessel cylinder is applied on the specimen connected to the piston in the vessel cylinder. The actual load on the specimen is directly measured by an external load cell and the displacement of the specimen is also measured by an external extensometer. As an example of the application of the apparatus, a tensile test on SUS316 stainless steel the Japanese Industrial Standard (JIS) G4303, which is comparable to the type 316 stainless steel ASTM A276, was conducted in 90 MPa hydrogen and argon. Hydrogen showed a marked effect on the tensile property of the material. The hydrogen gas embrittlement of the material was briefly discussed.

Hydrogen Embrittlement of Austenitic Stainless Steels at Low Temperatures

Stainless steels suffer from hydrogen embrittlement (HE) at low temperatures, thus the improvement of the resistance to HE has been expected by alloying. The main elements of Ni and Cr improve resistance to HE by stabilizing austenite phase with respect to martensitic transformations and by increasing the stacking fault energy (SFE) to promote cross-slip. The addition of N also stabilizes austenite phase and improves resistance to HE. Although N does not change the SFE, N promotes planar-slip by inducing a short range order in the matrix [1,2]. Thus, N and Ni play distinct roles in improving resistance to HE. It provides a way to distinguish HE due to strain-induced alpha’ martensite from HE due to planar-slip.

Effect of Nitrogen on Hydrogen Embrittlement of Austenitic Stainless Steels Based on Type 316LN

The effect of nitrogen on hydrogen gas embrittlement (HGE) in 1 and 70 MPa hydrogen and internal reversible hydrogen embrittlement (IRHE) of austenitic stainless steels of 17Cr11Ni2Mo(0.4 in max.)N alloys, based on type 316LN, was investigated by slow strain rate technique tests at room temperature in comparison with the effect of Ni on HGE and IRHE of Ni-added type 316 stainless-steel-alloys. For the nitrogen-added alloys, HGE and IRHE decreased with increasing nitrogen content, where α′ martensitic transformation occurred. HGE was not observed but IRHE was observed above the nitrogen content, where austenite is completely stabilized by nitrogen. Hydrogen-induced fracture related to the strain-induced α′ martensite structure was observed in HGE specimens and that together with brittle transgranular fracture was observed in IRHE specimens. HGE of the nitrogen-added alloys is larger than that of the Ni-added alloys in the Nieq range, where α′ martensitic transformation occurred. No HGE was observed in both the nitrogen-added alloys and the Ni-added alloys, but IRHE was observed in not the Ni-added alloys but the nitrogen-added alloys above the Nieq , where no martensite is identified in both alloys. It is discussed that the α′ martensite and the austenite of the nitrogen-added alloys were more sensitive to HGE or IRHE than those of the Ni-added alloys.Copyright

Internal Reversible Hydrogen Embrittlement and Hydrogen Gas Embrittlement of Austenitic Stainless Steels Based on Type 316

The internal reversible hydrogen embrittlement (IRHE) of austenitic Fe(10–20)Ni17Cr2Mo alloys based on type 316 stainless steel was investigated by tests using the slow strain rate technique from 80 to 300 K in comparison with its effect on the hydrogen gas embrittlement (HGE) of the alloys in hydrogen at a pressure of 1 MPa. The IRHE and HGE of the alloys in 70 MPa hydrogen at room temperature was also investigated. At low temperatures, IRHE occurred below a Ni content of 15% (Ni equivalent (Nieq ):29%), increased with decreasing temperature, reached a maximum at 200 K, and decreased with further decreasing temperature, similarly to the temperature dependence of HGE. At room temperature, IRHE and HGE were observed below a Ni content of 14% (Nieq :28%) and decreased with increasing Ni content (Nieq ). The dependence of HGE on hydrogen pressure increased with decreasing Ni content (Nieq ). Hydrogen-induced fracture closely related to the strain-induced α′ martensite structure and twin boundaries mainly occurred for both IRHE and HGE. Dimple ruptures caused by hydrogen segregation occurred in only IRHE at 150 K. The content of strain-induced α′ martensite increased with decreasing temperature and Ni content (Nieq ). Thus, the susceptibility to IRHE and HGE depended on Ni content (Nieq ). It was concluded that both IRHE and HGE were controlled by the amount of strain-induced α′ martensite above 200 K, whereas they were controlled by the hydrogen transport below 200 K.Copyright