[ materials ]
thermal
pre-charging in hydrogen gas , and in situ high-pressure hydrogen gas charging . The testing was performed according to ASTM G129 . The test details are given in Table 3 . Reference testing was done in air , except for the cathodically charged specimens , where distilled water was used as the reference environment . The reference tests were made at the same temperatures as those in the specified environment . After SSRT , the fracture surfaces were investigated using SEM with secondary electrons .
Discussion of SSRT test results In general , the ranking of the three SSRT methods is the same . See Figure 1 and 2 . The results indicate that using the ratio for the reduction of area between hydrogen-charged specimen and inert is a meaningful parameter . On the other hand , the values of reduction of area differ between the different test methods . This indicates the need for further investigations , such as the observation of the fracture surface , to describe the material behavior . For example , a ductile fracture was observed for UNS S21900 with a reduction ratio of 0.79 , which emphasizes the importance of investigating the fracture surface when a decrease in ductility ratio is observed .
Examination of fracture surfaces
The fractured surfaces of the specimens were investigated using a Scanning Electron Microscope ( SEM ). Additional investigations were conducted through Electron Backscatter Diffraction ( EBSD ).
Investigation of fracture surfaces in SEM For the grades UNS S2190 ( Figure 3 ), UNS S31675 ( Figure 4 ), and EN 1.4435 ( Figure 5 ), the fracture surfaces showed no brittle features , and significant necking was observed , as well as dimples indicating ductile fracture .
The fracture surfaces for EN 1.4404 are shown in Figure 6 . From the overall fracture surfaces , it is evident that brittle fractures have occurred . The thermally pre-charged specimen in Figure 6 ( right ) shows less necking than the other two charging methods , as indicated by the lower ratio for the reduction of area given in Figure 2 .
Investigation of deformed microstructures with EBSD The following results are presented as phase maps , where the red color indicates the presence of martensite and the blue color indicates austenite .
Table 3 . Test conditions used in the SSRT experiments 7 Method Charging SSRT
Environment Temperature Strain rate
Thermal pre-charging
4 weeks at 13.9 MPa
H 2 at 300 ° C
Liquid hydrogen / air mixture
-40 ° C 5 x 10-5 s-1
Cathodic charging |
In situ |
0.5 M H 2
SO 4
|
4 ° C |
1 x 10-5 s-1 |
Hydrogen gas |
In situ |
87.5 MPa H 2 |
-40 ° C |
5 x 10-5 s-1 |
|
Hydrogen gas ,
UNS S31675
|
In situ |
85 MPa H 2 |
-40 ° C |
1 x 10-5 s-1 |
Fig . 1 . SSRT . Comparison of the ratio for tensile strength ( Ratio Rm ), the ratio for the reduction of area ( Ratio RA ), and the ratio for elongation ( Ratio A )
44 Hydrogen Tech World | Issue 17 | August 2024