Investigation of Microstructure, Nanohardness and Corrosion Resistance for Oxi-Nitrocarburized Low Carbon Steel
Round 1
Reviewer 1 Report
Line 72: Was the steel plate just cold rolled or cold rolled and annealed?
Table 2: Explain why have you applied different oxidation times (5 and 30 min) with air and steam
Line 143: Can you explain why the compound layer is larger when steam instead of air is used? In both cases the carbonitriding treatment was the same.
Fig. 6: According to Fig. 6 c), indent is not visible in the oxide layer (how could you make any hardness measurement?). An explanation of this fact is needed.
Line 256: As magnetite layer is denser and form a quite continuous layer when air oxidation is applied, shouldn´t be expected a better corrosion behaviour in this case?
Conclusión: Some conclusion is expected in relation with the use of air or steam in the final oxidation treatment
Author Response
Response of Authors to Reviewer #1
Thank you very much for your thoughtful considerations on our paper. We revised our manuscript according to the comments of reviewer. The key points we revised parts are as follows:
Comments and Suggestions for Authors;
1. Line 72: Was the steel plate just cold rolled or cold rolled and annealed?
[Answer] Thank you very much for your comment. The steel plate was just cold rolled state. The steel plate cold commercial (SPCC) is widely used in rolling or drawing process for plastic deformation.
2. Table 2: Explain why have you applied different oxidation times (5 and 30 min) with air and steam.
[Answer] Your comment is reasonable. We have to say the reason why we selected different oxidation times (5 and 30 min). Originally, we wanted to design three different states, such as untreated, nitrocarburizng, and oxi-nitrocarburizing with steam. Because of the limitation of our equipment, we could not inflow rapidly the air into the process chamber and cool down the specimens suddenly. Therefore, we selected an oxi-nitrocarburizing with air for 5 min instead of just nitrocarburizing.
3. Line 143: Can you explain why the compound layer is larger when steam instead of air is used? In both cases the carbonitriding treatment was the same.
[Answer] We added the reason for different compound layer in the part of exploration in Table 3. In the case of the steam-specimen, the ratio of oxygen in the injected gas was 33.3%, which is higher than the oxygen ratio of 21% in air. In the oxidation process for the steam-specimen, a longer oxidation time and a higher oxygen content were processed. Therefore, the steam-specimen could have a characteristic of thick oxide layer and porous microstructure in the ε-phase. Also, compared to the air-specimen, the steam-specimen showed a thin ε-phase in the compound layer and a thick compound layer of 11 μm. This was attributed to the more phase decomposition of the ε-phase to the γ'-phase and enhanced growth of the γ'-phase during the oxidation step for steam-specimen.
4. Fig. 6: According to Fig. 6 c), indent is not visible in the oxide layer (how could you make any hardness measurement?). An explanation of this fact is needed.
[Answer] Your comment is reasonable. As you can see in Fig. 5B, the thickness of oxide layer is very thin and is almost 2 um. Therefore, it is impossible to measure the microhardness by using conventional micro-Vickers hardness.
However, we measured the nanohardness by using nanoindentation method with very small load of 10 mN (1.0 g) even though the indenter is very close to the surface/edge.
Because calculation principle of nanohardness by nanoindentaion is not observation of indentation area but depth of indenter under load. Therefore, we can measure the nanohardness even though the indentation is barely seen for nanoindentation method.
5. Line 256: As magnetite layer is denser and form a quite continuous layer when air oxidation is applied, shouldn´t be expected a better corrosion behaviour in this case?
[Answer] You comment is reasonable. In the oxidation process for the steam-specimen, a longer oxidation time and a higher oxygen content were processed. Therefore, the steam-specimen could have a characteristic of thick oxide layer and porous microstructure in the ε-phase. In this paper, we found that thicker oxide layer with porous microstructure is better than thin denser microstructure in view point of anti-corrosion properties.
6. Conclusion: Some conclusion is expected in relation with the use of air or steam in the final oxidation treatment.
[Answer] You comment is reasonable. Because of different oxidation time in oxidation process, however, we avoided the direct comparison of the specimens between air-specimen and steam-specimen. Instead of this, we highlighted the potential of analysis by EBSD, EPMA, and nanoindentation for characterization of oxi-nitrocarburized low carbon steel.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this study, characterization of surface layers formed by oxi-nitrocarburizing on a ultra-low carbon steel was carried out. The data reveal the characters of the surface layers, which have not investigated systematically in the past. The reviewer feels that this article could be published after minor revisions as suggested below: 1) Fig. 4: Some points with various colors are found in the gamma’ layer. Please confirm they represent small particles in the layer or just noise in the EBSD measurement. The size of measured pixels in the EBSD analysis would be mentioned in the “Materials and methods” section. 2) Page 7 Line 223: It is hard to understand the expression “the hardness of the oxide layer decreases with the formation of the oxide layer“. Please confirm and correct if necessary.
Author Response
Response of Authors to Reviewer #2
Thank you very much for your thoughtful considerations on our paper. We revised our manuscript according to the comments of reviewer. The key points we revised parts are as follows:
Comments and Suggestions for Authors;
In this study, characterization of surface layers formed by oxi-nitrocarburizing on a ultra-low carbon steel was carried out. The data reveal the characters of the surface layers, which have not investigated systematically in the past.
The reviewer feels that this article could be published after minor revisions as suggested below:
1. Fig. 4: Some points with various colors are found in the gamma’ layer. Please confirm they represent small particles in the layer or just noise in the EBSD measurement. The size of measured pixels in the EBSD analysis would be mentioned in the “Materials and methods” section.
[Answer] Thank you very much for your comment. According to your comment, we added detailed information on EBSDS analysis in the “Materials and methods” section: For the EBSD analysis, an equipment of AMETEX [Model: Hikari XP EBSD Camera] was used and a step size for phase analysis was fixed to 60 nm.
2. Page 7 Line 223: It is hard to understand the expression “the hardness of the oxide layer decreases with the formation of the oxide layer“. Please confirm and correct if necessary.
[Answer] Thank you very much for your comment. According to your comment, we revised the sentence as follow: In general, the hardness of the surface hardened layer decreases with the formation of the oxide layer due to the low hardness of the oxide compared to the nitride [4,13].
Author Response File: Author Response.pdf
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
Page 2, Line 73: Give the thickness of the steel plate
Page 2, line 78: Give the dimensions of the samples (full thickness?)
Page 3, line 84: Why have you marked (1) and (2) in Fig. 1? Shouldn´t be deleted?
Page 4, line 123: Remember here which compounds are epsilon and gamma prima phases.
Page 7, line 193: Indenter mark in fig. 6 b) is barely seen. How have you measured magnetite hardness in this sample?
Page 7, line 206: You are comparing here the magnetite nanohardness of the steam sample with the original hardness of the steel (it is assumed to be fully ferritic). Which is the reason for such comparison? The quite low hardness of the magnetite in this sample seems be due to its porosity.
Page 8, line 242: Anyway, as the surface hardness of the sample treated in air is higher, it should be also mentioned that a better wear behaviour should be expected in this case (you are mentioning wear in the Conclusions section).
Page 2, Line 73: Give the thickness of the steel plate
Page 2, line 78: Give the dimensions of the samples (full thickness?)
Page 3, line 84: Why have you marked (1) and (2) in Fig. 1? Shouldn´t be deleted?
Page 4, line 123: Remember here which compounds are epsilon and gamma prima phases.
Page 7, line 193: Indenter mark in fig. 6 b) is barely seen. How have you measured magnetite hardness in this sample?
Page 7, line 206: You are comparing here the magnetite nanohardness of the steam sample with the original hardness of the steel (it is assumed to be fully ferritic). Which is the reason for such comparison? The quite low hardness of the magnetite in this sample seems be due to its porosity.
Page 8, line 242: Anyway, as the surface hardness of the sample treated in air is higher, it should be also mentioned that a better wear behaviour should be expected in this case (you are mentioning wear in the Conclusions section). It should be also recommended to compare the wear behaviour of both treatments in order to finish your work.
Page 9, line 252: I think it is better to say untreated specimen instead of normal specimen.
Author Response
Author response for reviewer 1:
Thank you very much for your considerable comments and suggestions on our paper. We carefully revised our manuscript according to your comments point by point.
[Value of this paper] The things we want to focus in this paper are just three: measurement of nanohardness by nanoindentation with 10 mN, element mapping by EPMA and phase analysis by EBSD for surface hardened layers such as magnetite, e-phase and gamma g’-phase for oxi-nitrocarburized low carbon steel. We think that these three things have originality for publication of this paper.
[Comments and Suggestions for authors]
Page 2, Line 73: Give the thickness of the steel plate
[Answer] As you commented, we added the thickness of the sample in the text.
Page 2, line 78: Give the dimensions of the samples (full thickness?)
[Answer] We added the thickness of steel plate in the text [t = 0.8 mm].
Page 3, line 84: Why have you marked (1) and (2) in Fig. 1? Shouldn´t be deleted?
[Answer] Thank you very much. We deleted the (1) and (2) mark in the Fig. 1.
Page 4, line 123: Remember here which compounds are epsilon and gamma prima phases.
[Answer] As you commented, we added chemical formula of epsilon and gamma prima phases.
Page 7, line 193: Indenter mark in fig. 6 b) is barely seen. How have you measured magnetite hardness in this sample?
[Answer] You are right. As you can see in Fig. 5B, the thickness of oxide layer is very thin and is almost 2 um. Therefore, it is impossible to measure the microhardness by using conventional micro-Vickers hardness.
However, we measured the nanohardness by using nanoindentation method with very small load of 10 mN (1.0 g) even though the indenter is very close to the surface/edge.
Because calculation principle of nanohardness by nanoindentaion is not observation of indentation area but depth of indenter under load. Therefore, we can measure the nanohardness even though the indentation is barely seen for nanoindentation method.
Page 7, line 206: You are comparing here the magnetite nanohardness of the steam sample with the original hardness of the steel (it is assumed to be fully ferritic). Which is the reason for such comparison? The quite low hardness of the magnetite in this sample seems be due to its porosity.
[Answer] As you mentioned, we revised the sentence in the view point of porosity. We just wanted to emphasize the hardness of magnetite for the steam-specimen is very low because of high porosity.
Page 8, line 242: Anyway, as the surface hardness of the sample treated in air is higher, it should be also mentioned that a better wear behaviour should be expected in this case (you are mentioning wear in the Conclusions section).
[Answer] According to your comment, we deleted the word of wear in the conclusions section. Because the focus in this paper is the corrosion properties of oxi-nitrocarburized specimen.
Page 9, line 252: I think it is better to say untreated specimen instead of normal specimen.
[Answer] Thank you very much. We revised the term according to your comments.
Thank you very much.
Author Response File: Author Response.pdf
Reviewer 2 Report
Review report ”Effect of oxidation condition on the microstructure, nanohardness and corrosion characteristics of low carbon steel in oxi-nitrocarburizing”
The manuscript addresses the effect of air and steam oxidation of nitrocarburised steel. In principle this topic is interesting and deserves systematic investigation. Unfortunately, the manuscript does not convey new insights. It has to be underlined that the process is performed in industry on a daily basis. Moreover, a comprehensive review of the effect of oxidation and nitriding/nitrocarburising, including the effect on corrosion properties, is available in “Thermochemical Surface Engineering of Steels” by Mittemeijer and Somers; chapter 6 by H-.J. Spies.
Major points:
· It is difficult to compare the effect of different oxidizing atmospheres when the duration of the treatment differs (5 versus 30 minutes). In this respect it is important to consider the thermal impact without the presence of the nitriding atmosphere; this causes rapid development of porosity in the iron based (carbo)-nitrides. At the point where the nitrocarburising is terminated, the effective nitriding potential becomes nil and the iron based nitrides will decompose into Fe and nitrogen gas (porosity). This happens in combination with the “annealing effect” where nitrogen is pushed inwards by the formation of oxides (there is no mixed solubility of nitrogen and oxygen). Hence a change in thermal exposure (oxidizing) will dramatically change the morphology of the (carbo)-nitride compound layer. See also comment below.
· The oxidizing temperature seems very high – the authors should explain why this temperature was chosen. Normally post oxidizing is carried out around 400C where porosity formation by N2 development is suppressed.
· The authors should include a nitrocarburised sample without oxidation to see the effect of the last step.
Minor points
· Page 4, bottom (“During oxidation….Specimen”). This section does not make much sense.
· Fig. 5B. the indent is very close to the surface/edge; this violates good practice and the result might be flawed.
· Fig. 8. What is “Normal”? is this the untreated material or the nitrocarburised materials without post-oxidation?
Other comments:
Figure 2. The EPMA mapping results are interesting; this technique would be interesting to combine with systematic variations of the process parameters. Can carbon be included here?
It could also be relevant to make (average) depth-composition profiles over the extent of the hardened case.
It would be logical to present Fig.4 first to provide an overview of the hardened case
Author Response
Thank you very much for your considerable comments and suggestions on our paper. We carefully revised our manuscript according to your comments point by point.
[Comments and suggestions for authors]
The manuscript addresses the effect of air and steam oxidation of nitrocarburised steel. In principle this topic is interesting and deserves systematic investigation. Unfortunately, the manuscript does not convey new insights. It has to be underlined that the process is performed in industry on a daily basis. Moreover, a comprehensive review of the effect of oxidation and nitriding/nitrocarburising, including the effect on corrosion properties, is available in “Thermochemical Surface Engineering of Steels” by Mittemeijer and Somers; chapter 6 by H-.J. Spies.
[Answer] Of course you comment is right. The things we want to focus in this paper are just three: measurement of nanohardness by nanoindentation with 10 mN, element mapping by EPMA and phase analysis by EBSD for surface hardened layers such as magnetite, e-phase and gamma g’-phase for oxi-nitrocarburized low carbon steel. We think that these three things have originality for publication of this paper.
[Major points]
· It is difficult to compare the effect of different oxidizing atmospheres when the duration of the treatment differs (5 versus 30 minutes). In this respect it is important to consider the thermal impact without the presence of the nitriding atmosphere; this causes rapid development of porosity in the iron based (carbo)-nitrides. At the point where the nitrocarburising is terminated, the effective nitriding potential becomes nil and the iron based nitrides will decompose into Fe and nitrogen gas (porosity). This happens in combination with the “annealing effect” where nitrogen is pushed inwards by the formation of oxides (there is no mixed solubility of nitrogen and oxygen). Hence a change in thermal exposure (oxidizing) will dramatically change the morphology of the (carbo)-nitride compound layer. See also comment below.
[Answer] As you mentioned, the process of nitrocarburizing and oxi-nitrocarburizing has been studied long times ago. However, this paper has the some originality such as measurement of nanohardness for each phase. The nanoindentation with very small load of 10 mN make possible to measure this nanohardness. Also, we clearly observed distribution of each element like nitrogen and oxygen in the surface hardened layer by using EPMA element mapping. Furthermore, we analyze phase of surface hardened layer by using EBSD. These three experimental results make possible clear description of microstructure like magnetite, e-phase and gamma g’-phase for oxi-nitrocarburized low carbon steel.
· The oxidizing temperature seems very high – the authors should explain why this temperature was chosen. Normally post oxidizing is carried out around 400C where porosity formation by N2 development is suppressed.
[Answer] The temperature for nitrocarburizing process could be reached about 600 ºC for reducing the treatment time. [Ref. Shi Li et al. Surface and Coatings Technology 71 (1995) 112-120].
As the same manner, we processed to shorten the oxidation process time as short as possible because time is money in the industry. This high oxidation temperature was asked by industry.
· The authors should include a nitrocarburised sample without oxidation to see the effect of the last step.
[Answer] You are right. Actually, we wanted to compare the results for the samples come from nitrocarburizing without oxidation and oxi-nitrocarburizing. In this work, the air-specimen is very similar to the nitrocarburized specimen.
We wanted to make nitrocarburized specimen using our gas furnace. For the safety of gas furnace, we processed two steps: first is oxide inflow into chamber for 5 min, and second is open the furnace. Namely, the air-specimen in this work is a kind of modified process for nitrocarburizing.
[Minor points]
· Page 4, bottom (“During oxidation….Specimen”). This section does not make much sense.
[Answer] We carefully revised this section according to your comments. Especially, we added the sentence “the iron based nitrides will decompose into Fe and nitrogen gas”. In addition, we cited two paper written by Mittemeijer and Sommers et.at.[11,12].
· Fig. 5B. the indent is very close to the surface/edge; this violates good practice and the result might be flawed.
[Answer] You are right. As you can see in Fig. 5B, the thickness of oxide layer is very thin and is almost 2 um. Therefore, it is impossible to measure the microhardness by using conventional micro-Vickers hardness.
However, we measured the nanohardness by using nanoindentation method with very small load of 10 mN (1.0 g) even though the indenter is very close to the surface/edge. This result of nanohardness for oxide layer is thought to have originality in this paper.
· Fig. 8. What is “Normal”? is this the untreated material or the nitrocarburised materials without post-oxidation?
[Answer] As you mentioned, normal is not clear. Therefore, we changed the normal to untreated specimen.
[Other comments]
Figure 2. The EPMA mapping results are interesting; this technique would be interesting to combine with systematic variations of the process parameters. Can carbon be included here?
[Answer] As you can see here, element mapping image of C for steam-specimen was obtained. We think that the element mapping of C is not much necessary for the explanation of our paper.
[Answer] Your comment is right. We try to explain the relationship between depth-composition profiles over the extent of the hardened case in the text.
It would be logical to present Fig.4 first to provide an overview of the hardened case
[Answer] Thank you very much. As you suggested, we relocated the Fig.4 first. Namely, changed Fig. 4 to Fig. 2 in the revised paper.
Thank you very much
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have made only minor changes addressing the serious criticism in the first review report. Most importantly, the design of experiment should be different. The two treatments carried out are difficult to compare directly and obtain information about the effectiveness of air versus water vapor for oxidation. Perhaps the authors should change the scope of the article to a description/characterization of two different types of nitrocarburizing+oxidation treatments and avoid comparing “apples and pears”. They could emphasize/highlight the use of EBSD, EPMA and nanoindention.
My point of referring to a recent textbook on thermochemical surface engineering is that this type of process is well described (e.g. impact on corrosion properties). Even though new techniques are applied in the manuscript there are not new insights when it comes to microstructural development and performance.
However, if the scope of the paper is changed, the use of new techniques could merit publication.
Author Response
Author response for reviewer 2
Thank you very much for your thoughtful comments and suggestions on our paper. We carefully revised our manuscript according to your comments.
[Comments and suggestions for authors]
The authors have made only minor changes addressing the serious criticism in the first review report. Most importantly, the design of experiment should be different. The two treatments carried out are difficult to compare directly and obtain information about the effectiveness of air versus water vapor for oxidation. Perhaps the authors should change the scope of the article to a description/characterization of two different types of nitrocarburizing+oxidation treatments and avoid comparing “apples and pears”. They could emphasize/highlight the use of EBSD, EPMA and nanoindention.
My point of referring to a recent textbook on thermochemical surface engineering is that this type of process is well described (e.g. impact on corrosion properties). Even though new techniques are applied in the manuscript there are not new insights when it comes to microstructural development and performance.
However, if the scope of the paper is changed, the use of new techniques could merit publication.
[Answer]
According to your suggestion, we changed the scope of our paper into the characterization of two different types of oxi-nitrocarburizing treatments. For this purpose, direct comparison about effectiveness of air-specimen and steam-specimen was avoided. Also, the changed scope was emphasized and highlighted in the introduction and conclusion. As you know, this paper has some originalities in view point of EBSD, EPMA and nanoindentation. Therefore, we expect that this paper would be published in Metals.
Thank you very much.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
no comments
