Review Reports - Structural Characterization of Ion Nitrided 316L Austenitic Stainless Steel: Influence of Treatment Temperature and Time

Round 1

Reviewer 1 Report

The reviewed work „Structural characterization of ion nitrided 316L austenitic stainless steel: Influence of treatment temperature and time” refers to the determination of the effect of the time and temperature of the nitriding process on the properties and structure of the 316L steel. The work is well organized and the investigations results are described in detail. However, it requires a few minor comments and additions.

- First, the Authors of the work should add to the results schematic diagrams of nitrided layers for samples nitrided at temperatures of 450, 500 and 550^oC for the selected time of the proces (for a better understanding of the ongoing structural changes in the material).

- Secondly, with what step into the depth of the samples the microhardness measurements were made, especially at the surface some fluctuations in the microhardness of the investigated samples can be observed. What factors influence on these changes.

- Thirdly, the obtained nitrided layers have a relatively sharp limit of nitrogen penetration into the samples, which is visible on the microhardness profiles. Usually, the diffusion zone at the nitrided layer/steel substrate interface is not sharp (blurred). What is the reason for this behavior.

- Fourth, the surface composition of the samples in this type of process is usually different from the volume composition, which includes the subsurface and diffusion zones. It is related to two factors related to the gradient of nitrogen and chromium concentration into the samples and the presence of oxygen from the nitriding process. Oxynitrides may form in the near-surface zone because it is difficult to completely eliminate oxygen from the nitriding process and the sample. Does the appearance of oxynitrides may change the microhardness? The change in nitrogen concentration deep into the samples, starting from the surface, should be related to the observed structural changes depending on the temperature and time of nitriding. Therefore, some comment should be added to the description of the results.

Author Response

Reviewer#1

Response:

Thank you for your comments which helped us to improve the manuscript.

Comment-1

First, the Authors of the work should add to the results schematic diagrams of nitrided layers for samples nitrided at temperatures of 450, 500 and 550^oC for the selected time of the proces (for a better understanding of the ongoing structural changes in the material).

Response:

Thank you for suggesting the addition of a schematic of nitriding layer evaluation. The revised manuscript presents the schematic illustration of nitride layer formation and evaluation as Fig. 3 in the manuscript, which is shown below:

Figure 3. Schematic illustration of the microstructure of the nitride region which develops upon nitriding 316L stainless steel.

Regarding a new figure addition, the figure numbers are revised and the description of the Fig. 3 is added to the revised manuscript as follow:

“In this regard, a typical evaluation of nitride layer structure on 316L stainless steel is schematically proposed in Fig. 3.” Page 5, Line 206-207

Comment-2

Secondly, with what step into the depth of the samples the microhardness measurements were made, especially at the surface some fluctuations in the microhardness of the investigated samples can be observed. What factors influence on these changes.

Response:

The microhardness measurements were carried out with 3-5 µm steps until reaching a significant decrease in hardness values, subsequent measurements were performed with 10-15 µm to confirm that the measurements were from the matrix.

Therefore, the fluctuations in the near surface microhardness were able to be distinctly observed. The fluctuations were related to the distribution of ε and γ` phases in compound layer, as well nitride phase formation as described in the revised manuscript.

“The hardness of the matrix of untreated samples was about 330 HV. On the other hand, the hardness of the γN phase is able to reach over 1200 HV depending on the amount of diffused nitrogen in its structure [21]. Moreover, γ`, ε, and α phases as such CrN and Cr2N pre-cipitates can reach hardness around 2000 HV [18]. Therefore, the fluctuations in microhardness measurements can be attributed to the distribution of brittle phases (γ` and ε in compound layer) and nitrides (Cr_1-2N in diffusion layer) in the nitride layers.”

Comment-3

Thirdly, the obtained nitrided layers have a relatively sharp limit of nitrogen penetration into the samples, which is visible on the microhardness profiles. Usually, the diffusion zone at the nitrided layer/steel substrate interface is not sharp (blurred). What is the reason for this behavior.

Response:

Thank you for pointing out the seemingly sharp decrease in microhardness values. Since the scale of distance from surface in X axis was set to 200 µm in the related figures to present that the measurements reached to the matrix hardness (~300 HV), the decrease can be perceived sharp. In reality, the decreasing tendency in this study correlates with the reference studies in the manuscript.

Borgioli, F.; Fossati, A.; Galvanetto, E.; Bacci, T. Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment temperature. Surf. Coatings Technol. 2005, 200, 2474–2480, doi.org/10.1016/j.surfcoat.2004.07.110.
Li, G.Y.; Lei, M.K. Microstructure and properties of plasma source nitrided AISI 316 austenitic stainless steel. J. Mater. Eng. Perform. 2017, 26, 418–423, doi:10.1007/s11665-016-2435-3.

Comment-4

Fourth, the surface composition of the samples in this type of process is usually different from the volume composition, which includes the subsurface and diffusion zones. It is related to two factors related to the gradient of nitrogen and chromium concentration into the samples and the presence of oxygen from the nitriding process. Oxynitrides may form in the near-surface zone because it is difficult to completely eliminate oxygen from the nitriding process and the sample. Does the appearance of oxynitrides may change the microhardness? The change in nitrogen concentration deep into the samples, starting from the surface, should be related to the observed structural changes depending on the temperature and time of nitriding. Therefore, some comment should be added to the description of the results.

Response:

Thank you for mentioning the possibility of oxynitride formation. According to our literature survey, the oxynitride formation has been mostly associated to nitriding of Ti alloys rather than stainless steel.

Additionally, the XRD results in this study showed no clue for oxynitride formation, thus, the consideration of nitride layer formation and evaluation focused on ε-Fe_2-3N and γ`-Fe₄N phases in compound layer and chromium nitride (Cr_1-2N) and expanded austenite (γ_N) phase in diffusion layer.

Although the authors agreed on the possible diffusion of oxygen to the outermost layer of the nitride samples, the structural changes and consequent hardness profile were related to the nitrogen diffusion as the reviewer also highlighted in the comment-4.

Therefore, the results and discussion section in the manuscript was focused on the understanding the structural changes with different nitriding time and temperature regarding the nitrogen diffusion as the reviewer suggested.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper is focused on the structural characterization of ion nitrided 316L austenitic stain-less steel, describing the influence of treatment temperature and time. This is a very interesting work in which the authors manage to report for the first time that considering the phase-type and distribution with the consequent hardness characteristics in the nitride layer, an ion-nitriding temperature higher than 450ºC and time of 9h can be proposed as ideal processing parameters leading to an optimal phase composition, hardness distribution for 316L austenite stainless steels. These results are crucial for applications requiring a combination of both wear and corrosion resistance, so this work is extremely relevant not only for research but also for industrial purposes. The paper is well written, making a proper use of the English language. The Introduction provides a good background to the reader and the basic references to understand the context of the work. The manuscript includes high quality graphs and figures properly illustrating the key concepts of the work. The results are concise and potentially applicable in practical scenarios. Finally, the Conclusions are well supported by the obtained data. All in all, this is a very good work and an excellent match for Metals. Therefore, I recommend its acceptance and publication in its present form.

Author Response

We appreciate the comments of reviewer#2 suggesting this manuscript for publication in Metals. We hope that this study can attract citation in the field of nitriding and surface characterization of metals, particularly stainless steel.

Author Response File: Author Response.pdf