1. Threshold stress intensity factor for hydrogen assisted cracking of Cr-Mo steel used as stationary storage buffer of a hydrogen refuelling station
Takuya Matsumoto1, Masanobu Kubota2, Saburo Matsuoka2, 3, Patrick Ginet1, Jader Furtado4, Francoise Barbier4
1 K.K. Air Liquide Laboratories
2 International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University
3 Research Center of Hydrogen Industrial Use and Storage (HYDROGENIUS)
4 Air Liquide R&D, Paris-Saclay Research Center
International Conference on Hydrogen Safety ICHS 2015
Yokohama, Japan
October 21st 2015

2. Background and objectives
Leak-before-break (LBB) assessment for steel storage buffers
How to determine the threshold stress intensity factor for hydrogen assisted cracking, KIH?
Constant displacement test
Rising load test
LBB satisfied (KIH, KIC > Kmax)
LBB not satisfied (KIH, KIC < Kmax)

3. Background and objectives
KIH strongly depends on the standards and the test methods
Constant displacement test
Crack arrest threshold
Rising load test
Crack initiation threshold
KIH
KIC
Fig. General form of environment-induced crack growth as a function of applied stress intensity1)
Objective
To clarify what is the most suitable test method in order to determine KIH appropriate for the LBB assessment
(1) Howard G. Nelson et al. NASA TM X-62, 253 (1973)

4. Experimental procedures

5. Material Low alloy steel of JIS-SCM435 (Cr-Mo steel) Dimensions
OD: mm, T: 40.5 mm
Tensile strengths in longitudinal direction
σ0.2 = 772 MPa
σUTS = 948 MPa
Plane strain fracture toughness in air at RT (JSME S )
KIC = 116 MPa·m1/2

6. Constant displacement test (ASTM E1681-03)
Test specimen
Wedge-opening-load (WOL) specimen (W = 70.0 mm, B = 35 mm)
Test procedures
Introduction of fatigue pre-cracks in air
Tightening bolts in air
Exposure to high-pressure H2 at the pressures of 35 MPa and 115 MPa at RT for 1000 h
Measurement of crack arrest thresholds

7. Rising load test (ISO 11114-4 method B)
Test specimen
Compact-Tension (CT) specimen (W = 50.8 mm, B = 14.9 mm)
Test procedures
Two types of rising load tests in H2 at the pressures of 35 MPa and 115 MPa at RT
Stepwise rising load test
Monotonic rising load test

8. Results and discussion

9. Microstructures and Vickers hardness
W microstructure
B microstructure
Heterogeneous microstructure due to the difficulty of heat treatment
T of the buffer was 40.5 mm
Average hardness
HV270 at W microstructure
HV331 at B microstructure

10. Crack arrest thresholds in 115 MPa H2 gas
Variation in BFS during 115 MPa H2 exposure
BFS: Back Face Strain obtained by strain gauges
The crack initiation and the crack arrest were clearly determined from the change in BFS

11. Crack arrest thresholds in 115 MPa H2 gas
Initial applied stress-intensity factor (Kapp) vs. incubation time
The incubation time had a tendency to increase as Kapp decreased
The crack initiation threshold for the constant displacement test was 65 MPa·m1/2

12. Crack arrest thresholds in 115 MPa H2 gas
Small-scale yielding (SSY) and plane-strain validity criteria in ASTM E
B, a, a – W ≥ (K/σ0.2)2
The crack arrest thresholds were 44.3 MPa·m1/2 and 44.5 MPa·m1/2
The crack arrest thresholds were much smaller than the crack initiation threshold under constant displacement test (= 65 MPa·m1/2)

13. Crack initiation thresholds in 115 MPa H2 gas
Load vs. time duration (P-t curves)
Stepwise rising load test
Monotonic rising load test
The crack initiation thresholds
35.9 MPa·m1/2 in the stepwise rising load test
35.3 MPa·m1/2 in the monotonic rising load test

14. Crack arrest thresholds vs. crack initiation thresholds
The crack arrest thresholds were roughly coincident with the crack initiation thresholds
Nibur et al.2) showed there was big difference between the thresholds
Effect of specimen sizes, i.e., SSY and plane-strain criteria
Different test methods for the rising load tests
ISO method B
ASTM E
The heterogeneous microstructure
HV270 at W microstructure
HV331 at B microstructure
However…
The reasons;
(2) Nibur et al., Metall. Mater. Trans. A (2013)

15. Conclusions
Threshold stress intensity factor for hydrogen-assisted cracking, KIH, of SCM435 low alloy steel (Cr-Mo steel) has been examined in high-pressure H2 gas
Crack growth monitoring technique using back face strain gauges was established to detect crack initiations and crack arrest during high-pressure H2 exposure
The crack initiation thresholds in the constant displacement tests were much higher than the crack arrest thresholds. The crack arrest threshold shall be used to determine appropriate value for KIH
The crack arrest thresholds from the constant displacement tests were roughly coincident with the crack initiation thresholds from the rising load tests. Both test methods could be suitable to determine appropriate value for KIH

16. THANK YOU FOR YOUR ATTENTION