Effects of Initial Microstructure on the Low-Temperature Plasma Nitriding of Ferritic Stainless Steel

Round 1

Reviewer 1 Report

The authors attempted to improve the wear and corrosion resistance of AISI 430 ferritic steel substrates having different microstructures using plasma nitriding. The component's microstructure was investigated using XRD, SEM, and optical microscopy. The coating's properties are evaluated using a hardness tester, a tribometer, and an electrochemical workstation. According to the authors, plasma nitriding improves the component's wear and corrosion resistance. The authors used their microstructure to solve existing engineering problems in the low temperature nitride layer on ferritic steel substrates. This work is appropriate for publication in the Coatings Journal. But in its current form, it is not recommended, and the following comments must be answered before it can be published in Coatings Journal.

1.      Keywords – what is meant by initial microstructure? The word “initial” may be removed.

2.      Introduction – please mention where Ferritic steel AISI-430 is being used in industries or commercial sectors (insert after line 32).

3.      Table 1 – Cite a source or explain how the elemental compositions were determined.

4.      Figure 3 – please mention the percentage (weight or atomic) of C, Fe and Cr content.

5.      From Fig.4 it is noticed that the thickness of the plasma nitride layer around 20 micron (also line 229). But the figure 6 says the hardness is higher beyond 20 microns in the case of DN & SN. Why?

6.      The authors claim that the microstructure & phase of plasma nitride layer provide better corrosion resistance and hardness improvement. Hence, it is recommended to have FESEM cross-sectional image to see growth pattern of plasma nitride layer.

7.      The trend in potentiodynamic polarization and impedance values seems mismatching. Why does the author uses two circuit model to fit and compare the same set of samples. The author requested to try with simple circuit model even for plasma nitrided samples and see the fitting. If the fitting comes good, it is requested to use a single circuit to compare the data.

Author Response

Dear Editor:

We would like to thank you and the reviewer’s comments. Based on the comments, we have revised the manuscript. The followings are our detailed responses to the reviewer’s comments:

1. Keywords – what is meant by initial microstructure? The word “initial” may be removed.

Response: Thanks for the comments.

The initial microstructure refers to the initial matrix microstructure of the stainless steel before plasma nitriding. According to your suggestion, the word “initial” was removed from the title.

2. Introduction – please mention where Ferritic steel AISI-430 is being used in industries or commercial sectors (insert after line 32).

Response: Thanks for the comments.

The industries or commercial sectors for using of AISI-430 ferritic stainless steel was added in the introduction part:

Ferritic stainless steel possesses the characteristics of high thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and so on [1,2]. AISI 430 ferritic stainless steel is preferred in many applications, including automotive, chemical and food industries, such as fuel burners, screw nuts, household appliances and appliance part [3].

3. Table 1 – Cite a source or explain how the elemental compositions were determined.

Response: Thanks for the comments.

The elemental composition of the material was tested by EDS.

4. Figure 3 – please mention the percentage (weight or atomic) of C, Fe and Cr content.

Response: Thanks for the comments.

In the present investigation, carbon (C) element mainly comes from the contamination during the tests, no need to consider it. The percentage (weight and atomic) of N, Fe and Cr content are given in Table 3.

5. From Fig.4 it is noticed that the thickness of the plasma nitride layer around 20 micron (also line 229). But the figure 6 says the hardness is higher beyond 20 microns in the case of DN & SN. Why?

Response: Thanks for the comments.

The reason for the difference should mainly because that the microstructure showed in Figure 4 depicts the compound layer clearly but not very clear for the diffusion layer boundary. The layer depths were determined by choosing 5 positions along the cross-section of the nitrided layer, then the average of layer thicknesses at 5 positions were chosen as the layer thickness and using the white dashed lines to divide the nitrided layer and the matrix, which can only give a rough value of the layer thickness. However, the hardness profiles in Figure 6 show the hardening effects of the nitrogen penetration into the surface, so the layer thickness obtained by which will be more accurate and meaningful.

6. The authors claim that the microstructure & phase of plasma nitride layer provide better corrosion resistance and hardness improvement. Hence, it is recommended to have FESEM cross-sectional image to see growth pattern of plasma nitride layer.

Response: Thanks for the comments.

Using FESEM to observe the cross-section of the nitrided layer is a good idea. However, considering the present test condition, FESEM results are not available at present stage. In the present investigation, SEM and EDS were used to character the surface and the cross-section of the nitride layer, which can give us enough information to determine the growth pattern of plasma nitrided layer. Therefore, No FESEM cross-sectional images do not have evident influence on the conclusion of this paper.

7. The trend in potentiodynamic polarization and impedance values seems mismatching. Why does the author uses two circuit model to fit and compare the same set of samples. The author requested to try with simple circuit model even for plasma nitrided samples and see the fitting. If the fitting comes good, it is requested to use a single circuit to compare the data.

Response: Thanks for the comments.

For the delivered, S1160 and A850 samples, they can use the same simple circuit model, there is no mismatching between the potentiodynamic polarization and impedance values. For the AN450-8, DN450-8 and SN450-8 samples, the simple circuit model is not suitable, because the error value of the fitting parameters exceeds 100 %. For the AN450-8, DN450-8 and SN450-8 samples, using the circuit model in Fig.10 (b), the errors in fitting parameters are reasonable and acceptable, so it is the right model to model these samples. For the DN450-8, SN450-8 and AN450-8 sample, there are also no mismatching between the potentiodynamic polarization and impedance values. Though there is a high corrosion potential for SN450-8 sample, the evident high corrosion current density indicates its bad corrosion resistance. As for the DN450-8, SN450-8 sample, the deterioration of the corrosion resistances shown in potentiodynamic polarization values are corresponding to the low film resistance (R/R_ct2). The low film resistance should mainly be due to microcracks and less nitride phases, which result in not uniform and dense passivation films formed on DN450-8 and SN450-8 samples.

Moreover, some other errors found during revision also corrected and marked with red font words.

R.L. Liu

Harbin Engineering University

Author Response File: Author Response.docx

Reviewer 2 Report

In article AISI 430 ferritic stainless steel with different initial microstructure were low temperature plasma nitrided for improving its hardness and wear resistance.

The work is interesting and topical. Reading the article raised a few comments and questions:

·        Obviously, in Figures 3a and 4a, the highlight circle has gone to the wrong place.

·        The Introduction section is broad enough. The justification for the need for the study is adequately presented.

·        Materials and methods: This is worth revisiting and clarifying.

o   The solid solution treatments were conducted at five temperatures. OK

o   Then the solid solution treated AISI430 ferritic stainless steel was annealed at five temperatures.  Was this done for all five solid solution treated samples, only for a specific one?  This information (S1160) will come later but it would be good to explain the process here.

o   The delivered, solid solution treated, and solid 128 solution treated + annealed samples after low temperature plasma nitriding are marked 129 as D450-8, S450-8, and A450-8, respectively. It's the same here. It is common that several samples are taken at the beginning and the best one is selected for further processing. However, it is worth mentioning this in the Materials and methods section.

·        Results and discussion: The presentation is clear, as are the figures and tables, except for those mentioned at the beginning.  

Author Response

Dear Editor:

We would like to thank you and the reviewer’s comments. Based on the comments, we have revised the manuscript. The followings are our detailed responses to the reviewer’s comments:

1. Obviously, in Figures 3a and 4a, the highlight circle has gone to the wrong place.

Response: Thanks for the comments.

The highlight circles were set at the right place in Figure 3 (a) and Figure 4 (a).

2. The Introduction section is broad enough. The justification for the need for the study is adequately presented.

Response: Thanks for the comments.

3. Materials and methods: This is worth revisiting and clarifying.

The solid solution treatments were conducted at five temperatures. OK

Response: Thanks for the comments.

Then the solid solution treated AISI430 ferritic stainless steel was annealed at five temperatures.  Was this done for all five solid solution treated samples, only for a specific one?  This information (S1160) will come later but it would be good to explain the process here.

Response: Thanks for the comments.

In the present investigation, the optimal solution treated temperature was chosen firstly. Then, using the optimal solution treated sample (S1160), the annealing treatments were done under five annealing temperatures.

The delivered, solid solution treated, and solid 128 solution treated + annealed samples after low temperature plasma nitriding are marked as D450-8, S450-8, and A450-8, respectively. It's the same here. It is common that several samples are taken at the beginning and the best one is selected for further processing. However, it is worth mentioning this in the Materials and methods section.

Response: Thanks for the comments.

Yes, we took the same sample for different solid solution treatment at the beginning and obtained the best solid solution treatment temperature sample (S1160). Then the samples after solid solution treatment (S1160) were annealed at different temperatures to obtain the best annealing treatment sample (A850). The delivered sample, solid solution treated 1160℃ sample (S1160), and solid solution treated 1160℃ + annealed 850 ℃ sample (A850) after low temperature plasma nitriding are marked as DN450-8, SN450-8, and AN450-8, respectively.

4. Results and discussion: The presentation is clear, as are the figures and tables, except for those mentioned at the beginning.  

Response: Thanks for the comments.

The questions mentioned at the beginning have been revised.

Moreover, some other errors found during revision also corrected and marked with red font words.

R.L. Liu

Harbin Engineering University

Author Response File: Author Response.docx

Reviewer 3 Report

The review on “Effects of initial microstructure on low temperature plasma nitriding of ferritic stainless steel” by Li et al. Please find the reviewers' comments on each section to improve the quality of the paper.

Introduction: (Line 36, Lines 50-54) Try to avoid citations like [4-8] or [9-15]. One reference is enough to support the statement. If the authors would like to refer to all the papers cited in their work, then each reference should be discussed separately to clearly indicate the value of each one for the manuscript.

Materials and methods: (Table 1) Please clarify how the chemical composition was measured.

Material and methods: (Table 2) Please explain how the process parameters were determined. Were they optimized somehow?

Material and methods: There is no description of the surface condition (preparation method) of the base material before nitriding.

Results: (Section 3.1) Each Figure (a, b, c etc.) from Fig.1 and Fig.2 should be described or at least referred in the text.

Results: (Line 177) Please specify the desired size of ferrite in the matrix.

Results: (Figures 1 – 2) It would be beneficial to name each phase in Figures for example by using arrows.

Results: (EDS measurements) The authors investigated the chemical composition of each surface using the EDS method. However, such a method is not that accurate for nitrogen/nitrides detection. Moreover, the intensity was measured up to 1000 counts which is relatively low. Please comment.

Results: (EDS measurements) Maybe it would be better to replace b, d and f from Figure 3 with the table? The authors should comment on how (qualitatively / quantitatively) the EDS method was used in their studies.

Results: (Line 212) “The top surfaces of low temperature nitrided layers are uniform and evenly covered by lots of dispersive particles.” – could the authors indicate these particles in Figure 3 by using arrows?

Results: (Line 214) “there appears some dark holes, as shown in the red circle in Fig.3” – please make sure that the circle is showing the right area.

Results: (Figure 4) How did you measure the depth of nitriding? Which criteria have you applied?

Results: (Figure 4) Could the authors indicate the thickness of each layer in Figure? It would be beneficial for the paper. Furthermore, each a, b, and c should be described separately or at least referred in the text.

Results: (Line 232-233) “The main reason for this should be…” – please support with the proper reference

Results: (Line 248-249) “The weakness of the peak intensity…” – please support with the proper reference

Results: (Figure 6): Was the hardness measurement at depth 0 measured in cross-section or on the surface? There also could be some chromium nitrides on the surface, and the hardness would also need to be measured just below the surface below the zone of continuous nitrides.

Results: (Figure 6) The addition of the error bars would be beneficial.

Results: (Table 3) Please clearly indicate in the materials and methods section how the quality before and after grinding was measured. What machine was used for that and what was its accuracy? Since the changes are relatively small, the accuracy of the testing equipment will surely affect the results. How the measurement correctness measurement was ensured?

Results: (Line 349 – 352) Are there any particular mechanisms standing behind?

Conclusions: Please quantify where possible. Try to avoid that something was “smaller/lower/higher” etc. Use numbers/percentages to demonstrate your findings.

Editing issues: Please be consistent when using steel grade name. AISI 430 ferritic stainless steel (in introduction), 430 ferritic stainless steel (Line 170) in results and methods.

Whilst the reviewer appreciates the author's efforts that have gone into the research and the preparation of the publication, it should be improved to consider it for publication. Therefore, the reviewer will reconsider the work after major revision.

Author Response

Dear Editor:

We would like to thank you and the reviewer’s comments. Based on the comments, we have revised the manuscript. The followings are our detailed responses to the reviewer’s comments:

1. Introduction: (Line 36, Lines 50-54) Try to avoid citations like [4-8] or [9-15]. One reference is enough to support the statement. If the authors would like to refer to all the papers cited in their work, then each reference should be discussed separately to clearly indicate the value of each one for the manuscript.

Response: Thanks for the comments.

Only one or two references are cited in the revision and ach cited reference was discussed separately. Other references were removed.

2. Materials and methods: (Table 1) Please clarify how the chemical composition was measured.

Response: Thanks for the comments.

The elemental composition of the material was tested by EDS.

3. Material and methods: (Table 2) Please explain how the process parameters were determined. Were they optimized somehow?

Response: Thanks for the comments.

The process parameters were tested firstly and the optimal one was used in the present paper.

4. Material and methods: There is no description of the surface condition (preparation method) of the base material before nitriding.

Response: Thanks for the comments.

The surface treatment before nitriding are described in section 2.2. After the heat treatments, all the stainless steel samples were cut into dimension of Ф25 mm x 5 mm. Then the samples were ground up to 800# SiC paper with Ra 0.05～0.1 μm, and cleaned by ultrasonic wave with acetone, ethanol, and the deionized water in turn, and then blow-dried for using.

5. Results: (Section 3.1) Each Figure (a, b, c etc.) from Fig.1 and Fig.2 should be described or at least referred in the text.

Response: Thanks for the comments.

Each Figure (a, b, c etc.) from Figure 1 and Figure 2 were described or referred in the text: Figure 1 shows microstructure of AISI 430 ferritic stainless steel after solid solution treatment at different temperatures. Microstructure of the delivered, S1080, S1160, S1200, and S1240 samples are shown in Fig.1 (a)-Fig.1 (f). Figure 2 shows microstructure of AISI 430 ferritic stainless steel solid solution treated at 1160 ^oC with annealing at different temperatures, microstructure of A750, A800, A850, A900 and A950 samples are displayed in Fig.2 (a)-Fig.2 (e).

6. Results: (Line 177) Please specify the desired size of ferrite in the matrix.

Response: Thanks for the comments.

After solution treatment, the precipitation phases were dissolved, and the rolling orientation of ferrite disappeared gradually. There is no desired grain size of ferrite, the desired microstructure is uniform, and the grain size of ferrite is almost the same without abnormal big grain size.

7. Results: (Figures 1 – 2) It would be beneficial to name each phase in Figures for example by using arrows.

Response: Thanks for the comments.

Some arrows are added to name phases in Figure 1 and Figure 2.

8. Results: (EDS measurements) The authors investigated the chemical composition of each surface using the EDS method. However, such a method is not that accurate for nitrogen/nitrides detection. Moreover, the intensity was measured up to 1000 counts which is relatively low. Please comment.

Response: Thanks for the comments.

Indeed, EDS method is not an accurate method to get the nitrogen content quantitatively. Since the higher accuracy methods are not available at present condition, we use EDS to verify the presence of nitrogen on the surface and the change trend of nitrogen content with different initial microstructure. However, the contents of typical elements are also shown in Table 3. As for the low intensity counts, which should be due to the test parameters used in the present EDS tests.

9. Results: (EDS measurements) Maybe it would be better to replace b, d and f from Figure 3 with the table? The authors should comment on how (qualitatively / quantitatively) the EDS method was used in their studies.

Response: Thanks for the comments.

We keep the figures in Figure 3 and added the quantitative values a new Table 3.

Considering the higher accuracy methods are not available at present condition, we use EDS to verify the presence of nitrogen on the surface and the change trend of nitrogen content with different initial microstructure, though EDS method is not an accurate method to get the nitrogen content quantitatively.

10. Results: (Line 212) “The top surfaces of low temperature nitrided layers are uniform and evenly covered by lots of dispersive particles.” – could the authors indicate these particles in Figure 3 by using arrows?

Response: Thanks for the comments.

Some typical particles on the top surfaces of low temperature nitrided layers are referred by using arrows in Figure 3.

11. Results: (Line 214) “there appears some dark holes, as shown in the red circle in Fig.3” – please make sure that the circle is showing the right area.

Response: Thanks for the comments.

The red circle is shown in the right area in Figure 3 in the revision.

12. Results: (Figure 4) How did you measure the depth of nitriding? Which criteria have you applied?

Response: Thanks for the comments.

The microstructure showed in Figure 4 depicts the compound layer clearly but not very uniform and clear for the diffusion layer boundary. In the present investigation, the layer depths were determined by choosing 5 typical positions along the cross-section of the nitrided layer, then the average of layer thicknesses at 5 positions were chosen as the layer thickness and using the white dashed lines to divide the nitrided layer and the matrix, which can give a rough value of the layer thickness.

13. Results: (Figure 4) Could the authors indicate the thickness of each layer in Figure? It would be beneficial for the paper. Furthermore, each a, b, and c should be described separately or at least referred in the text.

Response: Thanks for the comments.

The thickness of each layer was indicated in Figure 4. Moreover, Fig.4 (a), Fig.4 (b) and Fig.4 (c) are described separately and referred in the text.

14. Results: (Line 232-233) “The main reason for this should be…” – please support with the proper reference

Response: Thanks for the comments.

Reference(s) have been added to the corresponding section: The main reason for this should be the uneven microstructure (including precipitation phases, rolling orientation) of the delivered stainless steel, which will also result in the poor performance of the compound layer [22].

15. Results: (Line 248-249) “The weakness of the peak intensity…” – please support with the proper reference

Response: Thanks for the comments.

Reference(s) have been added to the corresponding section: The weakness of the peak intensity after low temperature nitriding should be mainly due to the high stress of the nitrided layer after nitrogen supersaturation [26].

16. Results: (Figure 6): Was the hardness measurement at depth 0 measured in cross-section or on the surface? There also could be some chromium nitrides on the surface, and the hardness would also need to be measured just below the surface below the zone of continuous nitrides.

Response: Thanks for the comments.

In Figure 6, the hardness at 0 μm depth are measured on the surface with 6 test positions and averaged. The hardness values at 0 μm depth obtained from cross-section will be difficult or result in the big error.

Low temperature nitriding can promise that there are no chromium nitrides formed. In the present investigation. XRD analysis also shows that there are no obvious chromium nitrides on the surface layer. The hardness of the layer below the surface were tested and shown in Fig. 6.

17. Results: (Figure 6) The addition of the error bars would be beneficial.

Response: Thanks for the comments.

Error bars were added in Figure 6.

18. Results: (Table 3) Please clearly indicate in the materials and methods section how the quality before and after grinding was measured. What machine was used for that and what was its accuracy? Since the changes are relatively small, the accuracy of the testing equipment will surely affect the results. How the measurement correctness measurement was ensured?

Response: Thanks for the comments.

Measuring instruments and methods of the quality before and after grinding have been added in Section 2.5: The analytical balance（Secura224-1CN the accuracy up to 0.0001 mg）is used to measure the quality before and after wear tests. For ensuring the correctness of the measurements, three samples treated under the same condition were ground and the quality were tested, the quality of each sample was measured for 3 times.

19. Results: (Line 349 – 352) Are there any particular mechanisms standing behind?

Response: Thanks for the comments.

The reasons for the samples with different initial microstructure have different corrosion resistance after the same plasma nitriding should be due to the different microstructure of the nitrided layers and the quantities of iron nitrides in the nitrided layers. That is, different initial microstructure results in different microstructure of the nitrided layers and the quantities of iron nitrides in the nitrided layers. The dense microstructures with no microcracks and high volume of nitrides in the nitrided layers are corresponding to the better corrosion resistance.

20. Conclusions: Please quantify where possible. Try to avoid that something was “smaller/lower/higher” Use numbers/percentages to demonstrate your findings.

Response: Thanks for the comments.

The 4^th and 5^th conclusions were changed as following:

(4) The stable friction coefficient of each sample after low temperature plasma nitriding decreased 24.0%, 16.7% and 34.3%, respectively. And the mass wear rate is 0.00367 of the sample before plasma nitriding, indicating their wear resistance are significantly improved.

(5) The corrosion resistances of DN450-8 and SN450-8 samples are worse than that of before low temperature nitriding. However, The polarization potential of AN450-8 sample reached 0.027 V, and the corrosion current density was as low as 6.73 × 10^-8 A/cm². and its corrosion resistance was improved.

21. Editing issues: Please be consistent when using steel grade name. AISI 430 ferritic stainless steel (in introduction), 430 ferritic stainless steel (Line 170) in results and methods.

Response: Thanks for the comments.

All the steel grade name were changed into the same one.

Moreover, some other errors found during revision also corrected and marked with red font words.

R.L. Liu

Harbin Engineering University

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thanks for revising the whole manuscript; now the quality of the manuscript has improved.

Reviewer 3 Report

The paper has been revised and all comments were included. I recommend accepting this paper for publication.