[ Hydrogen Embrittlement ] diffusion pathways. However, elevated temperatures enhance hydrogen mobility, allowing it to surmount lattice energy barriers more easily. Impurities, dislocations, and grain boundaries in the metal serve as potential traps or pathways for hydrogen atoms, affecting their diffusion behaviour. These sites can lead to localised hydrogen accumulation, which, under certain conditions, may contribute to embrittlement. A thorough understanding of these diffusion mechanisms and influencing factors is crucial for designing stainless steel components that can reliably perform in hydrogen-rich environments where material integrity is paramount. The theoretical foundations have now been outlined, detailing the prerequisites necessary for hydrogen embrittlement of steel, along with an explanation of the mechanism by how hydrogen enters the material. Building on this, the following illustrates how the hydrogen resistance of austenitic stainless steels can be influenced by their chemical composition, where the so-called nickel equivalent value in particular plays a significant role.
[ Hydrogen Embrittlement ] diffusion pathways. However, elevated temperatures enhance hydrogen mobility, allowing it to surmount lattice energy barriers more easily. Impurities, dislocations, and grain boundaries in the metal serve as potential traps or pathways for hydrogen atoms, affecting their diffusion behaviour. These sites can lead to localised hydrogen accumulation, which, under certain conditions, may contribute to embrittlement. A thorough understanding of these diffusion mechanisms and influencing factors is crucial for designing stainless steel components that can reliably perform in hydrogen-rich environments where material integrity is paramount. The theoretical foundations have now been outlined, detailing the prerequisites necessary for hydrogen embrittlement of steel, along with an explanation of the mechanism by how hydrogen enters the material. Building on this, the following illustrates how the hydrogen resistance of austenitic stainless steels can be influenced by their chemical composition, where the so-called nickel equivalent value in particular plays a significant role.
Influence of the nickel equivalent Nickel equivalent is a measure of the amount of nickel and nickel-like elements required to stabilise the austenitic phase in a stainless steel. This phase is known for its high ductility and toughness, properties that are particularly important in preventing hydrogen embrittlement. Nickel equivalent, defined by a combination of alloying elements such as nickel( Ni), chromium( Cr), manganese( Mn) and others. The nickel equivalent has a decisive influence on the stability
Figure 2. Mechanism of hydrogen diffusion 4
of the austenitic microstructure and thus on the ability of the material to withstand the damaging effects of hydrogen. Several empirical formulas have been established for its calculation. The most commonly used formulas in the hydrogen industry are Schäeffler’ s formula( 1) and Sanga’ s formula( 2), which, in contrast to formula 1, includes several alloying elements in the calculation. It is possible that the customer specifies a minimum value for the nickel equivalent, on the basis of which a material selection can be made 5.
( 1)
Nieq = Ni + 30C + 0.5Mn
( 2) Nieq = Ni + 0.72Cr + 0.88Mo + 1.11Mn − 0.27Si + 0.53Cu + 12.93C + 7.55N
In the Table 1, the DMV 316L and DMV 316LMoS materials were calculated using formulas( 1) and( 2). As the individual compositions of the alloying elements of the DMV materials each consist of a minimum and a maximum value, this was also taken into account in the calculation. As the results in the table show, the values differ greatly depending on the calculation formula, and the minimum and maximum values also show differences that need to be considered. Therefore, if a material is to be selected for use in a hydrogen application based on the size of the nickel equivalent, the formula to be used must also be discussed. In addition, it should be determined whether the minimum, maximum or average values of the alloy element proportions should be used for the calculation. Slow strain rate tests( SSRT) are generally used to prove the influence of the nickel equivalent on the hydrogen resistance of stainless steel. SSTR tests are tensile tests with a very slow strain rate, allowing the hydrogen resistance of materials, especially austenitic stainless steels, to be evaluated. A sample is loaded at a very low strain rate under the influence of hydrogen.
Table 1. Calculation of the nickel equivalents for DMV 316 L and DMV 316 LMoS |
|
Ni |
Cr |
Mo |
Mn |
Si |
Cu |
C |
N |
Nieq( 2) |
Nieq( 1) |
|
|
|
|
|
DMV 316L |
|
|
|
|
|
Min |
11 |
16,5 |
2 |
1 |
0,2 |
0 |
0 |
0,04 |
26 |
11,5 |
Max |
12 |
17,5 |
2,4 |
2 |
0,7 |
0,4 |
0,03 |
0,08 |
29,95 |
13,9 |
Avarage |
11,5 |
17 |
2,2 |
1,5 |
0,45 |
0,2 |
0,02 |
0,06 |
27,97 |
12,7 |
|
|
|
|
|
DMV 316LMoS |
|
|
|
|
|
Min |
13 |
17 |
2,5 |
1 |
0,2 |
0 |
0 |
0,04 |
28,8 |
13,5 |
Max |
14 |
18 |
2,75 |
2 |
0,7 |
0,4 |
0,03 |
0,08 |
32,61 |
15,9 |
Avarage |
13,5 |
17,5 |
2,63 |
1,5 |
0,45 |
0,2 |
0,02 |
0,06 |
30,71 |
14,7 |
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