Effect of the Shielding Gas and Heat Treatment in Inconel 625 Coatings Deposited by GMAW Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In order to evaluate the effect of shielding gas and heat treatment on the wear of coatings of Inconel 625 deposited by the GMAW 18 process, a series of tests were carried out in this paper. However, the following problems need to be solved before publication:

1.     Page 1, lines 22-23. There is a grammar error in the sentence “To evaluate the coatings, Vickers hardness tests (HV), scanning electron microscopy (SEM), energy dispersion spectrometer (EDS), and wavelength dispersion spectrometry (WDS)”. It is recommended to carefully check and correct it.

2.     Page 1, lines 30-31. “In coastal and offshore installations, deterioration of materials due to corrosion can reduce reliability and increase the cost of maintaining engineered components.” is the same as the first sentence in the abstract. It is best not to have duplicate sentences throughout the article to improve the professionalism and scientificity.

3.     Page 3, lines 109 and 116. There are two interpretations of EDS in the article, please clarify the correct and appropriate statement.

4.     Please unify the entire article: (1) the font is " Times New Roman"; (2) is there a space between numbers and units (such as 25 cm/min and 16 1/min in Table 1); (3) Numbers (such as whether the numbers in Table 3 are 62.90 or 62,90).

5.     It is recommended that the EDS mapping in Figure 4 be reorganized to make it nice and neat.

6.     Page 10, lines 26-265. If possible, please quantify the “the tendency to reduce segregation” to increase the reliability of the data.

7.     Conclusion: the conclusion of this paper is divided into 8 points. Appropriate consolidation is recommended.

Comments on the Quality of English Language

Extensive editing of English language required

Author Response

Reviewer 01:

In order to evaluate the effect of shielding gas and heat treatment on the wear of coatings of Inconel 625 deposited by the GMAW 18 process, a series of tests were carried out in this paper. However, the following problems need to be solved before publication:

 

1. Page 1, lines 22-23. There is a grammar error in the sentence “To evaluate the coatings, Vickers hardness tests (HV), scanning electron microscopy (SEM), energy dispersion spectrometer (EDS), and wavelength dispersion spectrometry (WDS)”. It is recommended to carefully check and correct it.

We thank the reviewer for carefully checking our manuscript. We have in fact fully revised the language in the manuscript with the help of an experienced writer and scientist who did her PhD in England. Moreover, she was supported by a native speaker.

2. Page 1, lines 30-31. “In coastal and offshore installations, deterioration of materials due to corrosion can reduce reliability and increase the cost of maintaining engineered components.” is the same as the first sentence in the abstract. It is best not to have duplicate sentences throughout the article to improve the professionalism and scientificity.

We agree with the reviewer and have changed the abstract accordingly.

 

3. Page 3, lines 109 and 116. There are two interpretations of EDS in the article, please clarify the correct and appropriate statement.

Sorry, we have corrected the ambiguity.

 

4. Please unify the entire article: (1) the font is " Times New Roman"; (2) is there a space between numbers and units (such as 25 cm/min and 16 1/min in Table 1); (3) Numbers (such as whether the numbers in Table 3 are 62.90 or 62,90).

Regarding the font type, we used the font recommended in the template for the journal Coatings (Palatino Linotype). Spacing between numbers and units and decimal points have been corrected.

 

5. It is recommended that the EDS mapping in Figure 4 be reorganized to make it nice and neat.

Thanks for the nice suggestion, both Figures 4 and 5 have been reorganized.

 

6. Page 10, lines 26-265. If possible, please quantify the “the tendency to reduce segregation” to increase the reliability of the data.

We appreciate the suggestion, but it was not possible to quantify the reduction of segregation.

 

7. Conclusion: the conclusion of this paper is divided into 8 points. Appropriate consolidation is recommended.

Thanks, the consolidation was done.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

First, a request to verify the assumptions for the experiment and their purposefulness, especially in terms of the number of layers and the value of the heat treatment temperature. Next, you need to verify the method of etching the samples, because unfortunately you did not reveal the structure, but you examined (observed) cracked oxide layers. EDS and XRD penetrate oxide layers, but the results may be distorted due to their presence.

Introduction - should be completed, in particular indicating the applications where nickel coatings are used on unalloyed steels and what requirements are imposed on these coatings, and in particular discussing the problem of single- and multi-layer coatings and measurements of iron content on the surface (in last bead). Please also complete the heat treatment conditions, which will indicate the appropriateness of using a temperature of 1000 C deg., and also the cooling conditions. Are there other methods of applying coatings than GMAW - if so, what gases are used and explain why. 2.1.

Please complete the table with the chemical compositions of steel and filler metal and then analyse what happens in the GMAW coating process between this two alloys. 

Line 90 – what heat treatment at 1000 Cdeg causes in non-alloy steel. And why it is done. I point out the base steel and the nickel alloy, which are completely different.

Table 3. How to explain the effect of shielding gas on iron content.

Fig. 3. These are not cracks, just an improperly etched sample and oxide layers formed on the surface. This means that you could not observe anything due to improper sample preparation. The entire microstructure section is incorrectly described and needs to be corrected. Regarding hardness measurements, please pay attention to the iron content and mixing area, which is not shown in the results at all. This is where you should look for answers regarding changes in hardness.

Unfortunately, the presented test results are incorrect and result from incorrect preparation of test samples and microscopic observations and interpretation of the results. Moreover, the assumptions of the experiment itself are incorrect, because at a temperature of 1000 C deg. the steel structure of the substrate is destroyed.

Author Response

Reviewer 02:

First, a request to verify the assumptions for the experiment and their purposefulness, especially in terms of the number of layers and the value of the heat treatment temperature. We thank the reviewer for pointing out these points, which we have addressed both in the Introduction and in the Materials and methods sections.

Next, you need to verify the method of etching the samples, because unfortunately you did not reveal the structure, but you examined (observed) cracked oxide layers. EDS and XRD penetrate oxide layers, but the results may be distorted due to their presence.

We understand and value the reviewer´s concerns regarding the microstructural preparation of the specimens. We had in fact spent a good amount of time to etch the specimens properly, including a careful literature review to find the best routes for the metallographic preparation of Inconel 625. Despite this, we agree with the reviewer that the excessive etching of our specimens led to the cracking observed in some specimens. Since the SEM and EDS analyses do not necessarily require a topographical contrast since they rely on the interaction of electrons with the superficial and sub-superficial atoms, we decided to prepare new specimens for the revised manuscript without any etching. In fact, considering that some very small precipitates may be removed during etching due to loss of mechanical support when the matrix around them is etched, this may be preferable for a more reliable analysis. Obviously, in the case of the SEM images, they had to be restricted to BSE images, which are generated by chemical contrast, since SE images require topographical contract, which can only be achieved via etching.   We have also obtained new EDS maps for the specimens only polished, without etching. We believe that the new images are adequate to characterize the microstructure of the coatings. With the changes in the images, part of the description and discussion of the microstructures was changed accordingly.

Introduction - should be completed, in particular indicating the applications where nickel coatings are used on unalloyed steels and what requirements are imposed on these coatings, and in particular discussing the problem of single- and multi-layer coatings and measurements of iron content on the surface (in last bead). Please also complete the heat treatment conditions, which will indicate the appropriateness of using a temperature of 1000 C deg., and also the cooling conditions.

We have added to the Introduction considerations regarding the single layer x multilayer issue, as well the heat treatment of the coatings.

Are there other methods of applying coatings than GMAW - if so, what gases are used and explain why. 2.1.

These points have been explored and discussed in the Introduction.

Please complete the table with the chemical compositions of steel and filler metal and then analyse what happens in the GMAW coating process between this two alloys.

This information has been added in Table 1.

Line 90 – what heat treatment at 1000 Cdeg causes in non-alloy steel. And why it is done. I point out the base steel and the nickel alloy, which are completely different.

The use of thick weld overlays is based on the premise that only the properties are relevant for the surface phenomena they aim to protect against, such as corrosion and wear. This is particularly relevant for multi-layer coatings, where the influence of the substrate is substantially reduced. The aim of the heat treatment was to affect the microstructure of the Inconel overlay. Obviously, the microstructure of the substrate will change as well, but it was not in the scope of our work because in thick coatings the influence of the susbtrate on phenomena such as friction, wear and corrosion is negligible. However, we have added a comment about possible changes in the microstructure of the substrate in the revised manuscript.

Table 3. How to explain the effect of shielding gas on iron content.

The shielding gas affects the plasma temperature, which in turn affects the amount of melting in the substrate, and thus the dilution of the substrate into the coating. This was explained in the revised manuscript.

Fig. 3. These are not cracks, just an improperly etched sample and oxide layers formed on the surface. This means that you could not observe anything due to improper sample preparation. The entire microstructure section is incorrectly described and needs to be corrected. Regarding hardness measurements, please pay attention to the iron content and mixing area, which is not shown in the results at all. This is where you should look for answers regarding changes in hardness.

The reviewer is absolutely right and we thank you for pointing this out. As explained before, we have prepared new images for unetched specimens and changed the discussion of our results accordingly. We fully agree with the reviewer that dilution of the substrate into the coating is key for the changes in microstructure, which was discussed in the manuscript.

Unfortunately, the presented test results are incorrect and result from incorrect preparation of test samples and microscopic observations and interpretation of the results. Moreover, the assumptions of the experiment itself are incorrect, because at a temperature of 1000 C deg. the steel structure of the substrate is destroyed.

The changes in the images and discussion have corrected the issue correctly pointed out by the reviewer. However, we disagree with the reviewer regarding the heat treatment, because the substrate should have very little effect on the surface properties of thick coatings. For that reason, heat treatment of thick coatings has been widely used in the literature [1-3] to improve their performance. But we agree with the reviewer that, in case of a thin coating, the microstructural changes of both coating and substrate should be addressed

1. Lourenço, M.J.C., et al., Microstructure evolution, hardness response, and corrosion resistance of Inconel 625 weld overlay obtained by the ESSC process after post-welding heat treatments. The International Journal of Advanced Manufacturing Technology, 2023. 127(7): p. 3357-3369.
2. Dai, T., et al., Precipitation behavior and hardness response of Alloy 625 weld overlay under different aging conditions. Welding in the World, 2019. 63(4): p. 1087-1100.
3. Ban, S.-d., et al., Corrosion Resistance of Inconel 625 Overlay Welded Inside Pipes as a Function of Heat Treatment Temperature. International Journal of Electrochemical Science, 2016. 11(9): p. 7764-7774.

Reviewer 3 Report

Comments and Suggestions for Authors

In the summary section, more results should be given.

The first sentences of the summary and the introduction are almost precisely the same. It is recommended that the first sentence of the introduction be changed.

No reference was given for the first paragraph of the introduction.

The language of the work needs to be checked. There are significant language errors. For example, “his” is used when talking about nickel-based alloys. Introduction section, second paragraph, sentence 2.

The introduction part of the study needs to be rewritten. The literature is not sufficiently presented. Additionally, only four references were used in the introduction section.

In the materials section, it is explained why the gases used in the study were chosen. However, this information should be given with the help of literature in the introduction section.

The text of the SEM images in Figure 1 is not readable.

The change in the Fe ratio is highlighted in Table 3. This change should be discussed in more detail with the support of the literature.

In the first sentence of Section 3.2, it was not understood why the initials were written in capital letters in the phrase "Section Figures". There are many similar spelling and grammatical errors in the study.

Figure 2 shows SEM images, but the details are unclear because the samples were not etched well. A better-etched microstructure and higher magnification images should be given.

Intensity should be used for the y-axis in XRD analyses. Also, there is no need to write Cu on the X axes.

Comments on the Quality of English Language

The language of the work needs to be checked. There are significant language errors.

Author Response

Reviewer 03:

In the summary section, more results should be given.

We agree with the reviewer. The abstract has been extended,  incorporating more of the results.

The first sentences of the summary and the introduction are almost precisely the same. It is recommended that the first sentence of the introduction be changed.

We agree with the reviewer and have changed the abstract accordingly.

No reference was given for the first paragraph of the introduction.

The paragraph was referenced in the revised manuscript.

The language of the work needs to be checked. There are significant language errors. For example, “his” is used when talking about nickel-based alloys. Introduction section, second paragraph, sentence 2.

We have in fact fully revised the language in the manuscript with the help of an experienced writer and scientist who did her PhD in England. Moreover, she was supported by a native speaker.

The introduction part of the study needs to be rewritten. The literature is not sufficiently presented. Additionally, only four references were used in the introduction section.

The Introduction has been substantially improved and new references have been added to the revised manuscript.

In the materials section, it is explained why the gases used in the study were chosen. However, this information should be given with the help of literature in the introduction section.

We have added information regarding the choice of gases in the materials section, which was justified in the Introduction.

The text of the SEM images in Figure 1 is not readable.

Figures 1 to 3 have been edited to improve readability.

The change in the Fe ratio is highlighted in Table 3. This change should be discussed in more detail with the support of the literature.

The dilution of iron from the substrate into the deposits due to the GMAW process was analysed in the revised manuscript.

In the first sentence of Section 3.2, it was not understood why the initials were written in capital letters in the phrase "Section Figures". There are many similar spelling and grammatical errors in the study.

This has been corrected here and in other parts of the manuscript.

 

Figure 2 shows SEM images, but the details are unclear because the samples were not etched well. A better-etched microstructure and higher magnification images should be given.

SEM and EDS analyses do not necessarily require a topographical contrast since they rely on the interaction of electrons with the superficial and sub-superficial atoms. In fact, since etching was leading to surface oxidation and cracking, we decided to prepare new specimens for the revised manuscript without any etching. In fact, considering that some very small precipitates may be removed during etching due to loss of mechanical support when the matrix around them is etched, this may be preferable for a more reliable analysis. Obviously, in the case of the SEM images, they had to be restricted to BSE images, which are generated by chemical contrast, since SE images require topographical contract, which can only be achieved via etching.

Intensity should be used for the y-axis in XRD analyses. Also, there is no need to write Cu on the X axes.

Thanks, the XRD spectra were corrected.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript was improved after revision. However, some minor revisions are needed.

1. “acceleration voltage of 15 keV” should be revised to “acceleration voltage of 15 kV”;

2. The full names of “LDE1, LDE2, TAP, PET/L-H, LIF-L/H” should be supplied.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Reviewer 1

The manuscript was improved after revision. However, some minor revisions are needed.

1. “acceleration voltage of 15 keV” should be revised to “acceleration voltage of 15 kV”;

Thanks, that was corrected.

2. The full names of “LDE1, LDE2, TAP, PET/L-H, LIF-L/H” should be supplied.

All acronyms were defined in the revised manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you very much for correcting the article. Assuming that the substrate material is not analyzed and is only a source of iron introduced into the coating, the heat treatment temperature can be any.

There are still some minor shortcomings:

Figure 11. - please round the hardness values to whole numbers, which results from the test method. Additionally, please complete the indenter load - HV10.

Figure 8. What does the map show in terms of O content? Is EDS the right method to analyze the oxygen content in the solid solution of alloys? On the other hand, XRD revealed NbO – are you sure about this phase? You haven't found this phase anywhere before, even if you refer to the literature. Moreover, NbO upon heating to 1000 Cdeg. will it remain the same but disappear in your the analyses. What happens with this phase? In my opinion, you only revealed M23C6 carbides and Laves phases.

Fig. 10 - please standardize the phase descriptions in individual drawings.

Why don't you analyze the iron content in individual coatings, which is crucial for corrosion resistance and which gets into the coating from the substrate during surfacing?

Line 360-366 kindly ask you to re-analyze the test results of the structure and the effect of hardness decrease as a result of heat treatment. The diffusion mechanism was not indicated in the article in any way. As a result of heating to 1000, all precipitates in the post-surfacing state were dissolved. At this temperature, only M7C can occur, and during cooling, M23C6 carbides and Laves phases were formed. You did not analyze segregation and diffusion in your research, or at least due to the presented results, they were not observable.

Author Response

Reviewer 2

Thank you very much for correcting the article. Assuming that the substrate material is not analyzed and is only a source of iron introduced into the coating, the heat treatment temperature can be any.

Thanks for your positive feedback

There are still some minor shortcomings:

Figure 11. - please round the hardness values to whole numbers, which results from the test method. Additionally, please complete the indenter load - HV10.

Done.

Figure 8. What does the map show in terms of O content? Is EDS the right method to analyze the oxygen content in the solid solution of alloys? On the other hand, XRD revealed NbO – are you sure about this phase? You haven't found this phase anywhere before, even if you refer to the literature. Moreover, NbO upon heating to 1000 Cdeg. will it remain the same but disappear in your the analyses. What happens with this phase? In my opinion, you only revealed M23C6 carbides and Laves phases.

We have improved the analysis of the EDS maps, including the analysis of the oxygen map. We agree that EDS does not detect oxygen in solid solution. In fact, we believe that it is mainly associated with the surface oxidation of the alloys, since it is uniformly distributed. One exception was when it was associated with locations rich in Ti, showing that when welding with CO2 a small amount of TiO2 precipitates was formed.

Regarding NbO, we agree with the reviewer that, considering how faint the possible peak in DRX is, and taking into account that it was not detected by the other techniques, it was not NbO. This was corrected in the revised manuscript.

Fig. 10 - please standardize the phase descriptions in individual drawings.

We apologize for the poor description and standardization of the phases in the DRX spectra. We have in fact reanalyzed the peaks and completely corrected Figure 10, making sure that the phases are described in the same way in all individual drawings. The text was corrected accordingly.

Why don't you analyze the iron content in individual coatings, which is crucial for corrosion resistance and which gets into the coating from the substrate during surfacing?

Since the coating is deposited in two layers, the amount of iron on the top layer is small, but we have emphasized the importance of iron dilution in the revised manuscript.

Line 360-366 kindly ask you to re-analyze the test results of the structure and the effect of hardness decrease as a result of heat treatment. The diffusion mechanism was not indicated in the article in any way. As a result of heating to 1000, all precipitates in the post-surfacing state were dissolved. At this temperature, only M7C can occur, and during cooling, M23C6 carbides and Laves phases were formed. You did not analyze segregation and diffusion in your research, or at least due to the presented results, they were not observable.

We have completely revised the analysis of the microstructure. Although the dilution of the precipitates at 1000oC involved diffusion, we agree that we have not analyze that specifically in the manuscript. We hope that this final polishing of the microstructures and hardness results are now satisfactory.

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for the serious corrections.

Author Response

Reviewer 3

Thank you for the serious corrections.

Thank you for your positive feedback.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Did you use all WDS detectors or just one? I think only one of them, or maybe two. It is worth indicating which ones.

Thank you for your improvments. In my opinion the paper is much better than at beginning.

Author Response

Did you use all WDS detectors or just one? I think only one of them, or maybe two. It is worth indicating which ones.

Si was detected with TAP, Cr, Ti, and Mo with PET/L, Ni and Fe with LIF-H and Nb with PET/H. This information was added on the methodology.

Thank you for your improvments. In my opinion the paper is much better than at beginning.

Thank you for your positive feedback.