Hydrogen Embrittlement Behavior of Ultra-high Strength Dual Phase Steel Sheet under Sustained Tensile-loading Test

Abstract

The hydrogen embrittlement behavior of an ultra-high strength (1180 MPa grade) dual phase steel sheet composed of ferrite and tempered martensite, as compared with that of a single phase steel sheet composed of tempered martensite, has been investigated by a sustained tensile-loading test. No fracture of the dual phase steel occurs under the low hydrogen-charging current density of 5 A/m 2 except under high applied stress substantially larger than the yield stress. With the high current density of 50 A/m 2 , the time to fracture of the dual phase steel varies widely, but is almost the same as that of the single phase steel. The critical applied stress for fracture of the dual phase steel is higher than that of the single phase steel. Under the high applied stress, however, the time to fracture of the dual phase steel is shorter than that of the single phase steel, and a unique intergranular-like morphology is observed at the crack initiation area on the fracture surface. Upon plastic deformation before the sustained tensile-loading test under the high applied stress, the time to fracture of the dual phase steel increases and the initiation area on the fracture surface exhibits typical quasi-cleavage features. The results of the present study indicate that the hydrogen embrittlement of the dual phase steel displays some anomalous behavior.KEY WORDS: hydrogen embrittlement; delayed fracture; high strength steel; dual phase steel; martensite; ferrite.

Hydrogen Embrittlement Induced by Hydrogen Charging during Deformation of Ultra-high Strength Steel Sheet Consisting of Ferrite and Nanometer-sized Precipitates

“…The amount of desorbed hydrogen, i.e., hydrogen absorbed during hydrogen charging, for the present ferritic steel after hydrogen charging even for 30 min (4.13 mass ppm) was larger than that for the martensitic steel after hydrogen charging for 24 h (1.13 mass ppm). 8) Under hydrogen charging for 24 h, the amount of desorbed hydrogen for the ferritic steel (8.00 mass ppm) was approximately 7 times larger than that of the martensitic steel. 8) The amount of desorbed hydrogen after hydrogen charging for 48 h was 7.19 mass ppm.…”

“…8) Under hydrogen charging for 24 h, the amount of desorbed hydrogen for the ferritic steel (8.00 mass ppm) was approximately 7 times larger than that of the martensitic steel. 8) The amount of desorbed hydrogen after hydrogen charging for 48 h was 7.19 mass ppm. This indicates that hydrogen absorption saturates after 24 h. Upon aging in air for 24 h after hydrogen charging for 24 h, the amount of desorbed hydrogen was 3.61 mass ppm; thus, the amount of hydrogen decreased by approximately one-half.…”

“…21) For the tensile test, the specimens were machined parallel to the rolling direction with a thickness of 1.2 mm, gauge length of 15 mm and gauge width of 6 mm, as reported previously. 8,10) The tensile tests were carried out at a strain rate of 8.33 × 10 − 4 s − 1 at room temperature (25 ± 2°C). The tensile properties of the specimens are summarized in Table 2.…”

“…Owing to the single ferrite phase, total elongation (17 ± 1%) was larger than that of the martensitic steel of the same strength grade in the previous report. 8) Hydrogen embrittlement behavior was evaluated by tensile tests at strain rates of 8.33 × 10 − 4 s − 1 or 8.33 × 10 − 6 s − 1 after and during hydrogen charging, and delayed fracture properties were investigated by the sustained tensile-loading test and tensile-straining test during hydrogen charging using specimens of the same size as the tensile test specimen. Hydrogen charging was performed by cathodic electrolysis using a 3% NaCl + 0.3% NH 4 SCN aqueous solution at room temperature with a current density of 50 A/m 2 .…”

“…[1][2][3][4][5][6][7] Even in dual phase steel sheets including almost no prior austenite grain boundary, crack nucleation is dominated by the characteristics of martensite itself. [8][9][10] It is likely that the presence of martensite substantially increases the susceptibility to hydrogen embrittlement of ultra-high strength steel sheets.…”