Phase Transformations of Iron Nitrides during Annealing in Nitrogen and Hydrogen Atmosphere

Round 1

Reviewer 1 Report (Previous Reviewer 2)

Comments and Suggestions for Authors 1. What is the main question addressed by the research?  The authors compare the iron nitride annealing in nitrogen and hydrogen atmosphere. The decomposition of iron nitride to ε, γ’, and Feα in nitrogen annealing leads to the sample mass change (first decrease then increase); while the loss of sample mass in hydrogen annealing indicates the chemical impact of hydrogen in the annealing temperature.
2. Do you consider the topic original or relevant in the field? Does it address a specific gap in the field?  Yes, it is relevant to the field. This work is significant in the context of phase transformation under nitrogen and hydrogen. 4. What specific improvements should the authors consider regarding the methodology? What further controls should be considered?  The manuscript was improved after the first and second round of revision (please refer to coatings-2546277 to see my previous comments) 5. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?  Yes 6. Are the references appropriate?  Yes

 

 

Author Response

Dear reviewer,

The authors would like to thank you for your valuable comments and time.

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Please see the attached file

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

The authors would like to thank you for your valuable comments and time.

Below we respond to comments and inquiries

 

Ad 1

Balls made of AISI 52100 steel are used in rolling bearings operating under high loads, while balls made of AISI 1010 steel are used for low loads.

Heat treatment data was obtained from the distributor.

AISI 52100 balls- hardening in oil from a temperature of 870°C, tempering 200°C in air.

AISI 1010 balls - carburizing 930°C - hardening in water, tempering 150°C in air.

AISI 52100 steel is suitable for nitriding, as are most steels containing elements that can form nitrides.

AISI 1010 steel products undergo anti-corrosion nitriding processes

The choice of steel and ball shape was dictated by availability and price (the price of a thousand balls ranges from 3 to 16 euros). About 50 balls are used in one process.

From the ball, a metallographic cross-section is very easily ready.

A geometric enlargement is obtained on the metallographic specimen of the ball. Fig.1.

Fig.1. Diagram of the metallographic section of the ball-shaped sam-ple. Grey surfaces depict the iron nitride layer, g_mp—actual thickness of the iron nitride layer, measured on the diameter of the ball g_mp′—thickness of the iron nitride layer observed on the section, R—ball radius after the process with the iron nitride layer, R′—sample radius after subtraction of thickness of the iron nitrides layer, r—radius of the outer circle on the section of the ball, r′—radius of the inner circle on the section of the ball, h—distance of the centre of the nitride sample from the surface of the specimens.

 

Ad 2

In the first series of tests, the samples were annealed in a vacuum furnace under nitrogen flow maintaining a pressure of 200 Pa.

In the second series of tests, the samples were annealed in a tubular quartz reactor D 28 mm l = 300 mm.

An overpressure of 2-3 hPa above the current atmospheric pressure was maintained in the reactor.

The annealing processes used nitrogen of purity 99.99% N2, oxygen comes from 0.01% impurities.

Ad 3

Samples 2A (AISI 52100), 1A (AISI 1010) with a diameter of D = 3.9 mm) were nitrided in a single-stage process, sample 33A (AISI 1010) with a diameter of D = 2.5 mm were nitrided in a two-stage process (Table 2)

The first series of tests:

Samples 2A annealed for 2 h have code 7 and those annealed for 5 h have code 17

Samples 1A annealed for 2 h have code 6, and those annealed for 5 h have code 16

In the second series of tests, nitrided 1A samples annealed in nitrogen using a so-called thermoweight have the code 37N. An overpressure of 2-3 hPa above the current atmospheric pressure was maintained in the reactor

33A samples annealed in hydrogen using a thermobalance are coded 35H. Balls with a diameter of D = 2.5 mm were used in the experiment due to the limited load capacity of the so-called thermoweight

The reactor was maintained at an overpressure of 2-3 hPa above the current atmospheric pressure.

Ad 4

The description should be as follows:

¹ Thickness of the iron nitride layer

² Thickness of the porous zone in the iron nitride layer.

Ad 5.

The order of photos in Fig. 2 has been corrected.

Ad 6.

During the annealing process, pores are formed as a result of denitrification, and after the process, oxygen from the air may be trapped in the open pores.

Porosity decreases towards the substrate and the number of closed pores increases, which is accompanied by a decrease in oxygen concentration (Fig. 4c)

Ad 7.

The annealing processes used nitrogen of purity 99.99% N2, oxygen comes from 0.01% impurities. Porosity decreases towards the substrate and the number of closed pores

In the first stage of annealing to a temperature of 520C, the denitrification process takes place, after reaching the temperature of 520C, the iron oxides formed on the surface block the denitrification process, the increase in the thickness of the iron oxides is manifested by an increase in the mass of the samples.

Ad 8.

All concentrations in the figures are given in weight %

In Fig. 9, the incorrect description has been corrected.

Should - The photos show: nitrogen concentration (a) and oxygen concentration (b) on the surface of the samples

Ad 9.

Thank you very much for your attention. We have checked and to our knowledge, the summary is in the correct format. However, this will be the subject of further work with the editor.

Ad 10

The expression has been corrected in the text g’®Feα+N₂­

Ad 11

Recommended articles concern plasma processes. We carried out our processes without the use of plasma.

Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Abstract: What are the purposes of annealing of nitrided AISI 52100 and AISI 1010 steels?

Introduction: Why did you choose AISI 52100 steel and hydrogen atmosphere?  Please write the aim of the work meticulously. 

Materials and methods: What are the standard metallurgy tests? What are microsections? 

Results:  Figures 4, 5, 7, 10 and 13 (not 12) should be better.  Figure number should be correct.  Figures 3, 6 and 8(b): Two phases should not correspond to one peak.  Please elaborate on the effect of time on the iron nitride layer thickness and composition of AISI 52100 and AISI 1010 steels.   

Summary: Repetitive.  What are the reasons of largest weight loss of nitrided AISI 1010 steel in hydrogen?

Conclusions: 1 should be changed.  

Author Response

 

 

Author Response File: Author Response.pdf

Reviewer 4 Report (New Reviewer)

Comments and Suggestions for Authors

Dear authors,

Your research on steels denitrification is innovative and very stimulating for future scientific research and professional application. However, the organization of the text and the order of the information you provide to the readers should be improved.

Note 1.

After reading the introductory chapter of the paper, I remained confused about the purpose of the experiment you are conducting. Why spend energy, time and money to first nitride AISI 52100 bearing steel and then denitride it? Nitride formation processes, like other chemical processes for the formation of chemical compounds, are reversible under certain conditions, which is generally known. In your research, in the introduction, you announced the discovery of these special conditions in which nitride will decompose, but the reader still has an open question about the purpose and application of this method. Isn't it simpler to carry out nitriding in gas under controlled conditions and achieve the required combination of nitrides, instead of forming epsilon nitride and then its decomposition? I came to part of the answers to these questions in chapter 4. Summary. However, for the average reader, this approach to the organization of the text will not be interesting, and after the confusing introduction, they will reject further reading of the paper, regardless of the valuable results presented in the further part of the paper.

For a clearer and more logical presentation of the goal and assumptions of your research, I suggest that you supplement chapter 1. Introduction with the text found in chapter 4. Summary:

"During annealing of nitrided steel at a temperature of 520C, the percentage of the  phase.  in the +' iron nitride layer changes and, consequently, its magnetic properties change.

Obtaining iron nitride layers of the assumed composition in the nitriding process requires constant control of the nitrogen potential. It is much more convenient to create a layer of iron nitrides with a dominant share of the  phase, and then shape its required phase composition of the layer in the process of controlled annealing.

Controlled decomposition of the  phase can be used to produce a monophase ' layer, which is more plastic than the  phase. Obtaining a monophase layer ' in a standard nitriding process requires maintaining the value of the nitrogen potential within the stability of the ' phase, and the process time is almost twice as long as the method discussed in the article. As mentioned, the ' phase meets the requirements for soft magnetic materials well.

After the decomposition process of the  phase, a ' phase is formed with significant porosity. This layer, after impregnation with corrosion inhibitors, perfectly protects the steel substrate against corrosion. These layers can replace impregnated, brittle layers with a dominant share of the  phase

Registration of changes in sample mass during annealing enables continuous control of the degree of denitrification of iron nitrides forming the layer. For this reason, the use of a thermobalance in these processes is justified in the long run.

There are no reports in the available literature regarding studies of phase transformations in nitrided layers or the use of a thermobalance in these studies."

After this text, which introduces the reader to the issues of your research, comes the last paragraph from the current introductory chapter:

"The research conducted was aimed at examining the possibility of controlling iron nitride decomposition processes on AISI 52100 and AISI 1010 steels during annealing at 520 and 600°C in nitrogen and hydrogen."

Note 2.

Instead of the title 4. Summary, I suggest a more appropriate title 4. Discussion of results.

Note 3.

The conclusion of the work should be expanded and include the other findings you have reached in the research, for example, about the impact of denitrification on the topography of the surface and about the differences in the connection of denitrification that you observed in the tested steels. Also two paragraphs of the text from the current chapter 4. Summary would be more appropriate for the conclusion. Specifically, I mean the following passages:

"Our research results has shown that annealing in reduced pressure (200 Pa), nitrogen and hydrogen can be used for controlled decomposition of iron nitrides. During annealing of nitrided steel at a temperature of 520C, the percentage of the  phase.  in the +' iron nitride layer changes and, consequently, its magnetic properties change"

and 

"In the future, further studies of iron nitride decomposition processes are planned in order to investigate the difference between thermal reduction during annealing in a nitrogen atmosphere and in a vacuum and chemical reduction during annealing in hydrogen."

 

Comments on the Quality of English Language

Note 4.

The technical English should be improved. E.g. the title:

2.1 Materlials and parametres nitriding and annealing 

is not correct. I suggest eg.

 2.1. Nitriding and annealing materials and procedures

 

Note 5.

On figures 2, 5, 6 and 7 the sign for degrees Celsius should is not written correctly: e.g. see "...then annealed at 520oC for 2 h"

 

Note 6.

In several places in the text, the designation for AISI 52100 steel is written as AISI 52 100. match the way of marking this steel.

Author Response

Dear reviewer,

The authors would like to thank you for your valuable comments and time.

All comments and editorial suggestions have been implemented.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The presented paper by the authors demonstrates Phase Transformations of Iron Nitrides during crystallization under N and H ambiance. This work is significant in the context of phase transformation under nitrogen and hydrogen.

• Abstract must be written in a simple manner rather than making it confusing. It is too much confusing while reading. The first line of the Abstract itself makes more meaning rather than one.

• Fig 1 caption authors wrote, “The images show the nitrogen concentration values in the near-surface area of the iron nitride layer.” What does it mean? Can be rewritten in a simple manner. While reading it seems to be much ambiguity.

• In Fig 2, the Y axis is normal intensity, count per second why it is real.

• Most of the places authors stated that the loss of samples kindly provides the scientific reason behind the loss.

 

Overall, the manuscript requires major revisions to address the mentioned points and rewrite some portions very clearly. Once these revisions have been made and the manuscript has been thoroughly evaluated once, it may be considered for further proceedings.

Comments on the Quality of English Language

The English language is much more complex to read.

Meaning itself gives more confusing.

There is much scope to improve the English language.

Author Response

The authors thank you for your thorough review of our manuscript.

 

The general recommendations of the reviewer were taken into account by the authors and included in the manuscript.

 

Detailed comments

 

1. The abstract was revised as suggested by the Reviewer and included in the manuscript.
2. The captions under the images have been corrected as suggested by the Reviewer.
3. Axis descriptions in diffractograms have been corrected.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors compare the iron nitride annealing in nitrogen and hydrogen atmosphere. The decomposition of iron nitride to ε, γ’, and Feα in nitrogen annealing leads to the sample mass change (first decrease then increase); while the loss of sample mass in hydrogen annealing indicates the chemical impact of hydrogen in the annealing temperature. The work can be improved further by addressing the following comments. 

 

1. The authors used 200Pa nitrogen annealing and atmospheric pressure nitrogen/hydrogen annealing for comparison. Could the authors explain more on the annealing pressure/temperature selection? If possible, it’s good to test larger annealing window, e.g., more annealing temperature. 

2.  

2. For line 112, could authors explain why the porosity is affected by denitrification? 

3.  

3. For Figure 2, it is redundant to label peaks using symbols. It’s suggested to stack three plots to better compare the intensity/peak position change. 

4.  

4. The manuscript strength will enhance further if the authors can provide other physical properties characterization (e.g., anti-corrosion performance, surface mechanical properties) In other words, what’s the novelty/potential application of this work?

Comments on the Quality of English Language

Extensive grammar editing is suggested to improve the quality of the paper

Author Response

The authors thank you for your thorough review of our manuscript.

 

The general recommendations of the reviewer were taken into account by the authors and included in the manuscript.

 

1. Reduced pressure was used to avoid oxidation of the samples during annealing.
2. During denitrification, the e phase is transformed into the g' phase with a smaller volume than the e Since the solid is not compressible, pores are formed in the layer of iron nitrides. Therefore, the porosity increases with the degree of transformation.
3. The diffractograms are summarized in one figure.
4. The reviewer's suggestion was taken into account in the introduction.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Nitrides can improve various roperties of steels such as fatigue and corrosion resistance. The importance of nitriding treatments is thus well documented in the literature, which highlights the durability of nitrides at high temperatures (i.e., the tendency of iron nitrides to decompose during annealing) as one major shortcoming. The stability of nitrides is highly dependent on the microstructural configuration, i.e., whether a thin film, (Nano/micro) powders and so on. In this paper, Authors investigate the influence of the nature of the steel substrate on the decomposition behavior of the nitride layer, and attempt to describe both mechanisms and kinetics of aforementioned decomposition process. The article is interesting, but the decomposition behavior and phase transformation are insufficiently characterized and barely compared with existing results/state-of-the-art. I would strongly recommend to consider additional experimental work for a thorough characterization, which will strengthen current results and discussion. Please find some additional comments below:

1. The formation of iron nitrides in ammonia and hydrogen atmosphere has been documented for several decades, as highlighted by the Authors in the introduction. The stability and decomposition have similarly been studied earlier, in terms of structural characteristics and decomposition mechanisms. Hence, can the Author comment on the reason for pursuing such a well-documented phenomenon? Has not it been done already on this class of steels? Or are the Authors reporting this for the first time for these 2 specific compositions?

2. Can the authors provide more details on the fabrication process of the starting materials? The reader only knows that “samples have been previously nitrided”, which raises the interrogation on the “how”. Also, is there a reason for choosing spherical shaped particles? I doubt that in practical applications steel spheres are used. Also, as the Authors pointed out, nitrides stability and decomposition can be influenced by the microstructural configuration, hence I wonder to which extent spheres would be relevant in this specific case. Can the Authors justify their choice of these two specifics steel compositions and associated fabrication process?

3. What is the thickness of the observed nitride layer? Does the electron beam alter the nitride layer or the phase fraction/surface composition? Can the Authors comment on the stability of surface species under electron beam irradiation? (hydride species for example are quite prone to beam damage and tend to decompose during electron microscopy)

4. Is it possible to have a more quantitative characterization of Epsilon and gamma (‘) phases to better describe the phase transformation? Phase fractions and their evolution with annealing time, or Thermal analysis under controlled atmosphere for instance could bring deeper insights on the decomposition and benefit the overall discussion of results. Cross sectional TEM could also be envisaged to observe the surface region and characterize its layers, and their evolution as a function of annealing time.

5. The purpose and motivations of this study are not clearly stated, for instance, the reader has to reach the discussion section to understand the reasons for choosing the two specific annealing temperatures. Furthermore, the equation describing the decomposition is given in conclusions, while it should be provided much earlier since similar decomposition processes have been reported earlier. Besides. It is unclear how the knowledge acquired here will benefit the fabrication of steel grades that can perform better in the specified operating temperatures. The discussion does not bring sufficient insights, \thus needs to be revised. More comparisons with other articles are needed, and the literature review needs to be extended for the reader to have a better overview of the state of the art in the field (16 references are not enough to exhaustively describe one’s field of research).

 

Author Response

The authors thank you for your thorough review of our manuscript.

 

The general recommendations of the reviewer were taken into account by the authors and included in the manuscript.

 

1. In the field of research on phase transformations in iron nitride layers on steels, they are innovative research. The authors, apart from their own, did not find similar publications in the available literature.
2. The Materials and Methods chapter was supplemented with, among others, o information on nitriding parameters.
3. EDS analysis parameters are given in the Materials and Methods chapter. As can be seen from these data, the electron beam cannot introduce changes in the surface zone of the tested material.
4. In the chapter Results qualitatively and quantitatively characterized iron nitride layers in the initial state and after annealing.
5. The research conducted was aimed at examining the possibility of controlling iron nitride decomposition processes on AISI 52100 and AISI 1010 steels during annealing at 520 and 600C in nitrogen and hydrogen.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript was much improved and I agree to accept the manuscript in present form. 

Reviewer 3 Report

Comments and Suggestions for Authors

The revised manuscript shows thorough microstructural and structural characterization of the samples. It provides decent qualitative and quantitative description, and most previous concerns have been addressed. The nitride decomposition process during annealing treatment has indeed been examined, and observations have been well summarized in chapter 4.

However, there is no discussion of results, nor present or future perspectives: the reader does not know where do these results stand as per the state of the art, neither how these findings are relevant for future applications (see the previous revision report). Although the possibility of controlling nitride decomposition process is envisaged, no clear strategy is proposed in terms of future practical applications and future research perspectives. Finally, , beware of the multiplication of figures and tables (12 figure and 6 tables), since thorough characterization does not compensate for discussion.

Although the paper is well written and clear, for all the above reasons I do not recommend publication in Coatings.

 

Author Response

O

Author Response File: Author Response.pdf