thermodynamic reasoning for colossal n supersaturation in ...

by:DIgao     2020-05-30
Recently, during the treatment of low temperature gas nitriation, the huge N-saturation of ferrite and OHL stainless steel has obtained great technical significance.
However, the available thermodynamic model used to calculate the N-shun equilibrium solubility limit failed to explain the level of nitrogen-nitrogen oversaturation observed in several cases.
In this work, we show that considering the spin decomposition induced by N dissolution is essential for calculating the N-to-equilibrium solubility limit of ferrite and OHL stainless steel.
This modification in the thermodynamic model successfully explains the thermodynamic reasons for the huge N-oversaturation in ferrite and OHL stainless steel.
Experimental observations available in the literature support the occurrence of spin decomposition.
In order to improve the corrosion resistance and surface mechanical properties of stainless steel at the same time, the surface modification of stainless steel has important technical significance.
In this case, due to the strong chemical affinity of Cr to N, it has always been a challenging task to apply nitrogen treatment to stainless steel surface hardening, which, as we all know, will result in Cr-nitrides.
The formation of these Cr-
Nitrides deteriorated the corrosion resistance of the Nitrides stainless steel components due to the removal of Cr from solid solutions (
Necessary for anti-corrosion).
Because of this, it is traditionally impossible to apply nitriation treatment on stainless steel, although nitriation treatment is widely used to improve the mechanical and chemical properties of non-stainless steel surfaces
Stainless steel components.
The application of nitrogen treatment on stainless steel has become common in industry, only after the invention of low temperature nitrogen treatment, low temperature nitrogen treatment will lead to the development of nitrogen layer in the optimized application and dissolve in ferrite/ferrite without any Cr-
Nitrogen precipitation.
The implementation of this method has resulted in a great minimize of the problem of corrosion resistance deterioration of stainless steel after nitrogen, and in some cases corrosion resistance has even been enhanced.
Although solid solution hardening is not as effective as precipitation hardening, a large amount of N is dissolved in the ferrite/ferrite solid solution of low temperature nitrated stainless steel, the hardness level of the treated stainless steel surface is equivalent to the hardness level achieved by the usual precipitation hardening mechanism.
Therefore, while suppressing Cr-, significant progress has been made in generating huge N-saturation in stainless steel
\"Use \".
Due to the importance of understanding and optimizing the huge level of N dissolution, the researchers tried to calculate the maximum N-pair equilibrium solubility that can be achieved by the application chemical potential of N (
It depends on the T and nitrogen potential used)
At the same time of suppressing Cr-, different ferrite and Austrian stainless steel are treated in a nitrogen atmosphere
\"Use \".
This limit is defined by the point at which the thermodynamic driving force of absorbing nitrogen from the nitrogen atmosphere to the disappearance of the ferrite or OHL solid solution, I . E. e.
When the chemical potential of elemental nitrogen in ferritic/austenitic solid solution (
In the workpiece)
It is maintained in the atmosphere of nitrogen.
For the case of stainless steel controlled gas nitrogen, this calculation allows for precise control of the chemical potential of nitrogen in the gas phase, and it is expected that the measured nitrogen content can be correctly predicted.
Although such a thermodynamic model calculation seems to be correct in theory, this calculation has so far failed to explain the measurement level of N oversaturation achieved in stainless steel;
The calculated N-order equilibrium solubility limit of the AISI 316 stainless steel is far greater than the observed N content (see Fig. 7 in)
, And the calculated quasi-equilibrium N content of delta-ferrite of 17-
7 PH stainless steel is much smaller than observed.
This N absorption that is incompatible with the theoretical limit is not reasonable in thermodynamics.
These observations clearly show that the current available thermodynamic models are not sufficient in describing the low-temperature nitrogen-forming process.
A recent work has shown that spin decomposition may occur during Fe-n conversion of iron elements
Cr alloys, under the condition of limiting the diffusion of substituent elements, affect the phase transition path selected by the system during the nitrogen synthesis process.
In this work, we show that similar phenomena have occurred in the aging process of ferrite and OHL stainless steel.
We specifically show that, with this in mind, the spin decomposition phenomenon not found so far during the nitrid process of ferrite or OHL stainless steel, when calculating the limit of N-pair equilibrium solubility, the reported experimental observations were successfully explained.
Interestingly, there is a lot of evidence in the literature that delta-
Ferrite of stainless steel during thermal aging.
However, the role of spin-joint decomposition in determining the limit of N-position equilibrium solubility in nitrogen-free and ferrite stainless steel (
Nitrogen at temperatures similar to thermal aging treatment)
Not recognized until now.
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