Effects of the Hydrogen-to-Nitrogen Ratio in Plasma Nitriding on the Surface Properties of Cold Work Tool Steels

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The Introduction section is overly brief. The authors are encouraged to expand it by including a concise overview of recent studies in which plasma nitriding has been applied for the surface modification of cold-work tool steels. Highlighting relevant developments and achievements by other researchers would help contextualize the present work and clarify its novelty.

 

2. In lines 120–122, please provide the approximate indentation depth for each sample when measuring hardness using the Vickers hardness tester. Considering that plasma-nitrided coatings often have a limited thickness, the indentation depth plays a crucial role in ensuring measurement accuracy and in avoiding substrate influence.

 

3. The content in lines 136–138 appears to be template text and should be removed to maintain the professionalism of the manuscript.

 

4. Since the study investigates plasma nitriding of cold-work tool steel under different H₂/N₂ ratios, it would be highly informative to include surface morphology images taken before and after nitriding, as well as under different hydrogen–nitrogen ratios. Corresponding analyses and discussions should be added to clarify how the gas composition affects surface evolution.

 

5. For Figure 4, higher-resolution images of the plasma-nitrided surfaces are recommended. If possible, the inclusion of cross-sectional EDS elemental mapping would provide valuable insight into nitrogen diffusion behavior. The authors should also discuss any observed transformations of carbides/nitrides in the matrix under different processing conditions to help readers better understand the heterogeneous diffusion phenomena.

 

6. In Figure 7, it is suggested that the authors label key wear features such as deep grooves, microcracks, and metallic debris. Alternatively, adding magnified sub-images with clear annotations would improve the interpretability of the wear track analysis.

 

7. In the wear test results, the data corresponding to samples H300 and H400 are missing. The authors should clarify the reason for omitting these results—whether due to experimental limitations, data inconsistency, or other factors. This clarification would improve the completeness and transparency of the study.

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the editor and reviewers for taking the time to review this article. All comments have been very helpful in improving the manuscript, and we have revised it in response to these comments. All changes in the revised manuscript are highlighted in yellow. The major changes in the revised manuscript are as follows:

-The Introduction section was revised and expanded, including additional references.

-The Materials and Methods section has also been revised regarding the SEM, GD-OES, and the calculation of the specific wear rate.

-The Results and Discussion section has been improved with more detail. A surface morphology subsection was added; therefore, Figures 2 and 3 were inserted. The figure numbers were updated accordingly. Figures 5–8 in the submitted version were rearranged for greater clarity. More discussion and references have also been added to this section.

We look forward to hearing from you soon.

Sincerely,

A. Chingsungnoen

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

While the manuscript contains several strengths, it presents some major defects that preclude publication. First, the references are insufficient and literature review is short. Second, methods are not completely explained in section 2 for example what are the characteristics of GD-OES test? Third, some additional tests like XPS and EBSD can show better results. Moreover, the results are not complete for example there is not wear rate data. Finally, the discussion is relatively brief, which weakens the manuscript. 

Author Response

The authors would like to thank the editor and reviewers for taking the time to review this article. All comments have been very helpful in improving the manuscript, and we have revised it in response to these comments. All changes in the revised manuscript are highlighted in yellow. The major changes in the revised manuscript are as follows:

-The Introduction section was revised and expanded, including additional references.

-The Materials and Methods section has also been revised regarding the SEM, GD-OES, and the calculation of the specific wear rate.

-The Results and Discussion section has been improved with more detail. A surface morphology subsection was added; therefore, Figures 2 and 3 were inserted. The figure numbers were updated accordingly. Figures 5–8 in the submitted version were rearranged for greater clarity. More discussion and references have also been added to this section.

We look forward to hearing from you soon.

Sincerely,

A. Chingsungnoen

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

General Review

The paper presents a systematic and relatively well-structured experimental study on the effect of the hydrogen-to-nitrogen ratio during plasma nitriding on the microstructure, hardness, and tribological properties of DC53 tool steel. The topic is current and relevant to the field of surface engineering and heat treatment of steels, considering the increasing demand for environmentally friendly and highly controlled surface modification techniques.Only minor revisions are required.

General Comments

The paper includes only a few samples and does not employ any statistical or regression models to quantify the relationship between hydrogen flow rate and steel properties. Therefore, the term “optimization” may be too strong or even inappropriate.

Instead of “optimization”, you could use: determination of the most suitable hydrogen flow rate.

In the conclusions, you may emphasize that, “A hydrogen flow rate of 200 sccm in this experiment produced the best results among the investigated conditions.”

In other words, it is necessary to avoid using the terms optimal and optimization. You may discuss observed trends and the most favorable experimental conditions, but without claiming that this represents a statistical “optimum.”

The manuscript should also avoid overly strong expressions such as dramatically, remarkable, superior, high-performance, etc., since the presented results do not statistically justify such emphatic wording.

The Abstract is well written and detailed.
The Introduction should be expanded with more emphasis on the DC53 substrate material and the applied plasma nitriding method.

The Materials and Methods section is detailed and clearly written. It would be helpful to include a schematic diagram showing all the equipment used in the study, following the sequence of experimental steps.

How many samples were tested? Based on the text, it appears there were five — one untreated base material and four samples with different hydrogen flow rates. Were any replicates performed?

It would be useful to show images of the sample surface before and after plasma nitriding at different hydrogen flow rates.

What is the thickness of the nitrided layer?

In the sentence “To prevent thermal shock, the substrate temperature was ramped gradually to the final treatment temperature over a period of 20 minutes,” what exactly was the final temperature?

It would also be good to emphasize that only the hydrogen flow rate was varied, while the nitrogen flow rate remained constant at 1000 sccm.

Why were the hydrogen flow rates of 100, 200, 300, and 400 sccm chosen instead of smaller intervals, such as every 50 sccm?

Why was the plasma nitriding time set to 4 hours (240 minutes)? In the text, you state “4 hours,” while Table 1 shows “240 min.” Please use one consistent unit (either hours or minutes).

Ensure consistency throughout the paper in terminology — either use H₂ gas flow rate or hydrogen flow rate, but not both.

Results and Discussion

In Figure 2, instead of “control,” use the material name “DC53 tool steel (substrate).”
Replace “H100” with “H₂ 100 sccm” - the same applies to all samples.

For the hardness measurements, were repeated measurements performed? Do the reported values represent the arithmetic mean of all measurements? For better clarity, the data should be presented in a table. What does 564.5 ± 28 HV0.2 represent — are these standard deviations, and how were they calculated?

In the caption of Figure 3, specify surface hardness and bulk hardness instead of just “control DC53 tool steel.”

Figure 4 is of poor quality and visibility; also, replace “H100” with “H₂ 100.”

For the wear test, only the DC53 tool steel and samples H₂ 100 and H₂ 200 sccm were tested. Although it is clear why H₂ 300 and H₂ 400 were omitted, this should be explicitly stated. Since all previous analyses included all four hydrogen flow rates, it would be logical to maintain the same scope here as well.

In Figure 5, it would be better to replace “control” with the material name “DC53 tool steel (substrate material)”. Instead of “H100”, it should read “H₂ 100 sccm”, which applies to all samples.

Conclusion

As previously mentioned, avoid using overly strong expressions and the word optimal. The conclusion should also include a brief discussion of the potential application of this research and a proposal for future work.

Author Response

The authors would like to thank the editor and reviewers for taking the time to review this article. All comments have been very helpful in improving the manuscript, and we have revised it in response to these comments. All changes in the revised manuscript are highlighted in yellow. The major changes in the revised manuscript are as follows:

-The Introduction section was revised and expanded, including additional references.

-The Materials and Methods section has also been revised regarding the SEM, GD-OES, and the calculation of the specific wear rate.

-The Results and Discussion section has been improved with more detail. A surface morphology subsection was added; therefore, Figures 2 and 3 were inserted. The figure numbers were updated accordingly. Figures 5–8 in the submitted version were rearranged for greater clarity. More discussion and references have also been added to this section.

We look forward to hearing from you soon.

Sincerely,

A. Chingsungnoen

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Now, its better than before.