Role of Ultrasonic Shot Peening in Environmental Hydrogen Embrittlement Behavior of 7075-T6 Alloy

The effect of ultrasonic shot peening on the environmental hydrogen embrittlement behavior of the 7075-T6 aluminum alloy is investigated. The 7075-T6 tensile specimens were treated by ultrasonic shot peening for 50 s. Surface residual stress and the depth of residual stress under the surface were evaluated using an X-ray diffractometer. Then, the specimens were tensile tested in humid air and dry nitrogen gas by the slow strain rate technique. The results showed that the ultrasonic shot-peened specimen has a superior hydrogen embrittlement resistance. Further, the ultrasonic shot peening changes the fracture mode from an intergranular fracture mode to the transgranular one. It was suggested that ultrasonic shot-peening has two effects on hydrogen embrittlement behavior; the distribution of hydrogen inside the surface layer by introducing dislocations/vacancies as hydrogen traps and reducing the normalized amount of hydrogen trapped per unit length of the grain boundary.

Evaluation of Hydrogen Embrittlement of Cr-Mo Low Alloy Steel by Slow Strain Rate Technique With Cathodically Charged Specimen

As an alternative method to slow strain rate technique (SSRT) under high-pressure hydrogen gas evaluation, SSRT was performed with a cathodically charged specimen. Cr-Mo low alloy steel with a tensile strength of 1000 MPa grade was selected as a test material. Cathodic charging was performed in 3% NaCl solution and at a current density in the range of 50–600 A/m2. The effect of specimen size on the hydrogen embrittlement properties was evaluated. Relative reduction of area (RRA) values obtained by tests at a cathode current density of 400 A/m2 were equivalent to those performed in hydrogen gas at pressures of 10 to 35 MPa. Fracture surface observations were also performed using scanning electron microscopy (SEM). The quasi-cleavage fracture surface was observed only after rupture of small specimens that were subjected to hydrogen charged tests. It was also necessary for the diameter of the specimen to be small to form the quasi-cleavage fracture surface. The results indicated that to simulate the high-pressure hydrogen gas test, a specimen with a smaller parallel section diameter that is continuously charged until rupture is preferable.

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.