How the Nitrogen affect 316LN stainless steel?

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316LN is the Nitrogen addition version based on 316L steel (0.06% ~ 0.08%), so that it has the same characteristics as 316L,  has been used in the manufacture of high-temperature structural components in fast breeder reactor (FBRS). Reducing the carbon content greatly reduces the susceptibility to stress corrosion cracking due to welding in subsequent corrosive environments. The creep, low cycle fatigue and creep-fatigue interaction are the most important considerations for FBRS components. The high-temperature strength of 316L stainless steel can be improved to 316 stainless steel by alloying 0.06% ~ 0.08% N. The influence of nitrogen content higher than 0.08% on mechanical properties of 316L stainless steel at high temperature will be discussed in this paper.

 

Chemical composition of 316LN stainless steel

Furnace N C Mn Cr Mo Ni Si S P Fe
Standards 0.06-0.22 0.02-0.03 1.6-2.0 17-18 2.3-2.5 12.0-12.5 ≤0.5 ≤0.01 ≤0.03 –
1 0.07 0.027 1,7 17.53 2.49 12.2 0.22 0.0055 0.013 –
2 0.11 0.033 1.78 17.63 2.51 12.27 0.21 0.0055 0.015 –
3 0.14 0.025 1.74 17.57 2.53 12.15 0.20 0.0041 0.017 –
4 0.22 0.028 1.70 17.57 2.54 12.36 0.20 0.0055 0.018 –

Thses four batches of 316LN stainless steel with a nitrogen content of 0.07%, 0.11%, 0.14% and 0.22%, and carbon content of 0.03%, were tested to study the effects of nitrogen on tensile, creep, low-cycle fatigue and creep-fatigue properties of 316LN stainless steel. The aim of this experiment is to find the optimum nitrogen content to obtain the best combination of tensile, creep and low cycle fatigue properties. The experimental results show that nitrogen can improve the tensile strength, creep and fatigue strength of austenitic stainless steels. The reasons for the increase in strength include solution enhancement, reduced stacking fault energy (SFE), precipitation hardening, formation of composites (interstitial solutes), atomic segregation, and ordered hardening. Due to their different electron exchange properties, the dissolved nitrogen in austenitic stainless steel has a larger expansion volume than carbon.

In addition to the elastic interaction between nitrogen and dislocation, the electrostatic interstitial dislocation interaction also influences the strength. Dislocation nuclei are characterized by the lack of free electrons, which means they have a positive charge. The nitrogen atoms in austenitic stainless steels are negatively charged due to the position of free electrons near the nitrogen atoms and the electrostatic interaction between the dislocations and the nitrogen atoms.

The effective binding energy between the nitrogen atom and the dislocation increases with the increase of the nitrogen content in Austenitic steel, but the correlation is not obvious for carbon. In Austenitic steels, interstitial nitrogen interacts with substituent elements and tends to form interstitial substituent atomic compositions. The compound binds easily to elements to the left of Fe in the periodic table, such as Mn, Cr, Ti and V. There is a strong correlation between the properties of interatomic bonding (that is, orientation versus unorientation) and the proximity of adjacent atoms in a multicomponent alloy system. Bonding between metal atoms facilitates short-range ordering, which is the bonding of atoms of different elements. Interatomic polarization facilitates the exchange of covalent electrons, the bonding between atoms of the same element. Carbon promotes the aggregation of substitution atoms in the iron-based solid solution, while nitrogen facilitates short-range ordering.

In general, the yield strength(YS) and ultimate tensile strength(UTS) of 316L stainless steel are significantly improved by the alloying of 0.07% ~ 0.22% nitrogen. The increase in strength was observed in all tests in the temperature range of 300 ~ 1123K. Dynamic strain aging was observed within a limited temperature range. The temperature range of dynamic strain aging (DSA) decreases with the increase of nitrogen content.