[ materials ] gas . 4 , 5 Equation 1 provides an updated Niequivalent including nitrogen . 4 , 5 , 6
Equation 1
Ni eq
= Ni + 12.93 C + 1.11 Mn + 0.72 Cr + 0.88 Mo − 0.27 Si + 7.55 N ( wt- %)
In the presented study , testing with three SSRT methods was performed on four stainless steels with varying nickel content and Ni eq
, as well as tensile properties . Two UNS S31603 grades , EN 1.4404 and EN 1.4435 , with different Ni content , were tested , as well as UNS S31675 , which has higher mechanical strength due to its alloying with niobium and formation of small Z-phase particles . Additionally , cold-worked UNS S21900 was included to investigate the effect of cold working . The aim was to compare the different SSRT methods as well as the influence of chemical composition and strength levels . Furthermore , a study regarding the difference in fracture mechanism from the different testing methods was conducted by investigating the fracture surfaces using Scanning Electron Microscopy ( SEM ) and Electron Backscatter Diffraction ( EBSD ) to study the formation of martensite and the deformation mode .
UNS S31603 but with different Ni content were tested . These two UNS S31603 grades have different designations in the European standards , which will be used further in this article : EN 1.4404 and 1.4435 . These were tested as hot-rolled and cold-rolled material in the solution annealed condition . A high-strength austenitic stainless steel , UNS S31675 , was also investigated . Those specimens were obtained from a hot-rolled and solution annealed bar . The UNS S21900 specimens were obtained from a cold-worked seamless tube . Table 1 lists the chemical compositions of the materials . Table 2 shows the mechanical properties of the tested material .
All materials were free from deleterious intermetallic phases and precipitates . UNS S31675 has a Z-phase present due to the alloying with niobium . As shown in Table 2 , EN 1.4404 and EN 1.4435 had a grain size around ASTM no . 6 , and UNS S31675 had a finer grain size smaller than ASTM no . 10 . Both the cold work operation for UNS S21900 and the Z-phase present in UNS S31675 result in finer grain size and , thus , higher mechanical strength .
Materials
The testing was performed on four stainless steels . Two standard austenitic grades fulfilling
Slow strain rate testing
SSRT was performed with three different hydrogen charging methods : in situ with cathodic charging ,
Table 1 . Chemical composition ( nominal %) of tested materials and Ni eq according to Equation 1
UNS |
EN |
C |
Si |
Mn |
Cr |
Ni |
Mo |
Cu |
Nb |
N |
Ni eq |
S31603 |
1.4404 |
≤0.03 |
0.4 |
1.8 |
16.6 |
11.4 |
2.0 |
0.3 |
|
0.06 |
27.4 |
S31603 |
1.4435 |
≤0.03 |
0.4 |
1.5 |
17.2 |
13.0 |
2.6 |
0.3 |
|
0.05 |
29.6 |
S31675 |
|
≤0.06 |
0.5 |
4.1 |
20.6 |
9.6 |
2.3 |
0.1 |
0.3 |
0.39 |
33.9 |
S21900 |
|
≤0.04 |
0.4 |
8.9 |
19.5 |
7.2 |
0.3 |
0.3 |
|
0.29 |
33.6 |
Table 2 . Mechanical properties and grain size |
Alloy |
Grain size |
Yield strength |
Tensile strength |
Elongation |
|
ASTM no . |
MPa |
MPa |
% |
1.4404 |
5−6 |
248 ± 4 |
591 ± 2 |
57 ± 1 |
1.4435 |
5−6 |
239 ± 3 |
566 ± 1 |
57 ± 1 |
S31675 |
> 10 |
584 ± 1 |
922 ± 1 |
41 ± 1 |
S21900 |
6.5−7.5 |
996 |
1071 |
28 |
Hydrogen Tech World | Issue 17 | August 2024 43