Tohru Awane

Pressure Cycle Testing of Cr–Mo Steel Pressure Vessels Subjected to Gaseous Hydrogen

Pressure cycle tests were performed on two types of Cr–Mo steel pressure vessels with notches machined on their inside under hydrogen-gas pressures, between 0.6 and 45 MPa at room temperature. Fatigue crack growth (FCG) and fracture toughness tests of the Cr–Mo steels samples from the vessels were also carried out in gaseous hydrogen. The Cr–Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air. The fracture toughness of the Cr–Mo steels in gaseous hydrogen was significantly smaller than that in ambient air. Four pressure vessels were tested with gaseous hydrogen. All pressure vessels failed by leak-before-break (LBB). The LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen KIC,H; accordingly, the LBB assessment based on KIC,H is conservative and there is a possibility that KIC,H does not provide a reasonable assessment of LBB. In contrast, the fatigue lives of all pressure vessels could be estimated by using the accelerated FCG rates in gaseous hydrogen.

Highly sensitive detection of net hydrogen charged into austenitic stainless steel with secondary ion mass spectrometry.

Secondary ion mass spectrometry (SIMS) is used to detect local distributions of hydrogen in various materials. However, it has been well-known that it is extremely difficult to analyze net hydrogen (H(N)) in metals with SIMS. This was because hydrogen, which is originated from moisture (H(2)O), hydrocarbon (C(x)H(y)) or other organic materials (C(x)H(y)O(z)) existing on a sample surface or in the SIMS chamber, is simultaneously detected in the SIMS measurement of the H(N), and the H(N) and the background-originated hydrogen (H(BG)) cannot be distinguished in a SIMS profile. The effective method for reductions and determinations of the H(BG) in hydrogen measurements of metallic materials with the SIMS method has not been established. The present paper shows an effective method for reduction and estimation of H(BG) in SIMS analyses of hydrogen charged into type 316 L austenitic stainless steel, and an accurate estimation method of the net charged hydrogen. In this research, a silicon wafer is sputtered by a primary ion beam of a SIMS near an analyzed area (silicon sputtering method) to reduce H(BG). An uncharged type 316 L sample was prepared for estimation of H(BG) in SIMS measurements of the hydrogen-charged sample. The gross intensities of hydrogen between the hydrogen-charged sample and the uncharged sample were compared. The gross intensities of hydrogen of the uncharged sample (26.8-74.5 cps) were much lower than the minimal gross intensities of hydrogen of the hydrogen-charged sample (462-1140 cps). Thus, we could reduce the H(BG) enough to estimate the hydrogen charged into the type 316 L sample. Moreover, we developed a method to determine intensities of H(BG) in the measurement of the hydrogen-charged sample by estimating the time-variation of hydrogen intensities in the measurements of the uncharged sample. The intensities of the charged hydrogen can be obtained by subtracting the estimated intensities of the H(BG) from the gross intensities of hydrogen of the hydrogen-charged sample. The silicon sputtering method used to reduce H(BG) and the determination method for H(BG) in this research can be applied to the accurate hydrogen analysis for other various metallic materials.

Fatigue-life and leak-before-break assessments of CR-MO steel pressure vessels with high-pressure gaseous hydrogen

Pressure cycle tests were performed on two types of Cr-Mo steel pressure vessels with inner diameters of 306 mm and 210 mm and notches machined on their inside under hydrogen-gas pressures, varied between 0.6 and 45 MPa at room temperature. One of the Cr-Mo steels had a fine microstructure with tensile strength of 828 MPa, while the other had a coarse microstructure with tensile strength of 947 MPa. Fatigue-crack growth (FCG) and fracture-toughness tests of the Cr-Mo steels were also carried out in gaseous hydrogen. The Cr-Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air with an upper bound corresponding to an approximately 30-times higher FCG rate. Furthermore, in gaseous hydrogen, the fracture toughness of the Cr-Mo steel with coarse microstructure was significantly smaller than that of the steel with fine microstructure. Four pressure vessels were tested; then, all of the pressure vessels failed by leak-before-break (LBB). Based on the fracture-mechanics approach, the LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen. The fatigue lives could be estimated by using the upper bound of the accelerated FCG rates in gaseous hydrogen.Copyright