Investigation of Changes in the Structural-Phase State and the Efficiency of Hardening of 30CrMnSiA Steel by the Method of Electrolytic Plasma Thermocyclic Surface Treatment

Round 1

Reviewer 1 Report

The article has a scientific character. The article concerns the research on the mechanical and tribological properties of 30CrMnSiA steels after heat treatment of the EPTCH type. The Authors applied correct research methods and used the appropriate measuring equipment. The content of the work is logically written. The manuscript contains 9 figures and 3 tables. Figures and tables aren’t properly prepared. Authors cited 48 literature sources. The authors presented an interesting work, but it requires numerous improvements to be of satisfactory quality.

The general disadvantage of the work is the lack of a practical comparison of the 30CrMnSiA steel after EPTCH treatment with the 30CrMnSiA steel in the quenched and tempered state, after e.g. laser, induction and electron hardening. Therefore, the results cannot be compared with the properties of the technologies already used. The authors should also demonstrate the applicability of the described EPTCH technology. Other comments are given below.

General remarks

1. The novelty of the work in comparison with the state of the art or science (highlights) should be indicated. They also need to show that their work brings an aspect of novelty compared to already existing solutions.

2. Specify the data of the apparatus in the article in the following order: device designation, manufacturer's name, city, country.

3. In general, improve the discussion of the results obtained

4. The work also requires careful text editing

Detailed comments

1. Line 108 For 30CrMnSiA steels, enter the designation in the 1.XXXX system; also provide information on how to determine the chemical composition; note according to the GOST standard, this steel has the designation 25ChGSA

2. Row 111, this steel is rather rarely used for gears and devices, more often for responsible shafts and screws

3. Line 152 ball-disk - More commonly referred to as ball-on-disk

4. Enter what was the heat treatment before hardening (normalizing?)

5. Fig. 2a - scale is not visible; correct the drawing

6. Figure 3 - invisible scale; photos not sharp - need to be corrected; indicate the individual phases in the drawing

7. Fig. 4 - it should be shown in the description that the main peak (about 45 degrees) is martensite; coincides with ferrite but with advanced techniques it can be concluded that it is martensite

8. Figure 5 estimate the content of martensite for detailed treatment variants; use the martensite content in the discussion of the results

9. Figure 6 - indicate the individual phases in the figure

10. Lines 264 and 270 - do not provide references to literature when marking the equation

11. Table 2 and 3 - give the measurement uncertainty and write the result in the form Y = X ± δ;

12. Fig. 8 the drawing is hardly legible; enlarge the font; specify the thickness of the hardened layer under the drawing in the text

13. In the text, explain the reasons for the large differences in CoF shown in Fig. 9a

14. Lines 336 and 337 - delete Cyrillic text

15. Table 3 also gives the maximum wipe depth

16. Line 352 - 355 Not very revealing conclusion - it is known that the ferritic-pearlitic structure has worse properties than the martensitic structure.

Author Response

We thank the reviewer for his in-depth analysis of our manuscript and his comments and suggestions, which helped us to significantly improve the quality of our data analysis and presentation. In the revised version of this paper, we have tried, as much as possible, to take into account the comments and suggestions presented in the review.

Below are the responses to the comments

Comments for Author:

1. The novelty of the work in comparison with the state of the art or science (highlights) should be indicated. They also need to show that their work brings an aspect of novelty compared to already existing solutions.

Response: In an industry where steel is used in the manufacture of each component, surface improvement plays a vital role in ensuring the desired mechanical properties of steel. Workshops for the production of press tools, automobile companies, manufacturers of press parts, the aerospace industry, the weapons industry - these are some of the areas where steel is a common metal, and therefore improving the properties of steel by various methods becomes important. Electrolyte-plasma thermocyclic surface hardening is an attractive solution for chemical and thermal treatment used to improve the properties of the steel surface through structural and phase transformations.  This technology is economical, and the process itself is harmless to the environment. However, although this process is performed for both different types of steel, to date there is no standardized heat treatment cycle in terms of material.

2. Specify the data of the apparatus in the article in the following order: device designation, manufacturer's name, city, country.

Response: We agree with the reviewer. A change has been made to the article.

3. In general, improve the discussion of the results obtained

Response: We agree with the reviewer. A change has been made to the article.

4. The work also requires careful text editing

Response: We agree with the reviewer. A change has been made to the article.

Detailed comments

1. Line 108 For 30CrMnSiA steels, enter the designation in the 1.XXXX system; also provide information on how to determine the chemical composition; note according to the GOST standard, this steel has the designation 25ChGSA

Response: We agree with the reviewer. A change has been made to the article.

2. Row 111, this steel is rather rarely used for gears and devices, more often for responsible shafts and screws

Response: 30CrMnSiA steel is used in the production of parts with increased wear resistance requirements: piston mechanisms operating in engines, pushrods, gear shafts, levers, axles, blades, flanges, steel wheels, responsible load-bearing structures designed for alternating loads.

3. Line 152 ball-disk - More commonly referred to as ball-on-disk

Response: We agree with the reviewer. A change has been made to the article.

4. Enter what was the heat treatment before hardening (normalizing?)

Response: Quenching 880 °C, oil. Vacation 560 °C.

5. Fig. 2a - scale is not visible; correct the drawing

Response: We agree with the reviewer. A change has been made to the article.

6.  Figure 3 - invisible scale; photos not sharp - need to be corrected; indicate the individual phases in the drawing

Response: Other reviewers demanded that the picture be removed.

7. Fig. 4 - it should be shown in the description that the main peak (about 45 degrees) is martensite; coincides with ferrite but with advanced techniques it can be concluded that it is martensite

Response: We agree with the reviewer. In the future, we will study the structures of martensite using translucent electron microscopy.

8. Figure 5 estimate the content of martensite for detailed treatment variants; use the martensite content in the discussion of the results.

Response: We agree with the reviewer.

9. Figure 6 - indicate the individual phases in the figure

Response: We agree with the reviewer. A change has been made to the article.

10. Lines 264 and 270 - do not provide references to literature when marking the equation

Response: We agree with the reviewer. A change has been made to the article.

11. Table 2 and 3 - give the measurement uncertainty and write the result in the form Y = X ± δ;

Response: We agree with the reviewer. Table 2 has been changed.

12. Fig. 8 the drawing is hardly legible; enlarge the font; specify the thickness of the hardened layer under the drawing in the text

Response: We agree with the reviewer. Periodically turning on the high voltage of the electric potential 320 V and low (pause) 50, 250 V, the heating speed increased and decreased, which then allowed to increase the time and get a thicker heated layer. During the demonstrated voltage mode, the pulse mode heated the samples, and the pause mode in the experiment cooled the samples or maintained a constant temperature. The periodic connection of high (320 V) and low (pause 50, 250 V) electric potential leads to a periodic increase and stabilizes or slows down the increase in the heating rate, which then allows you to increase the time and get a thicker heated layer without melting the surface. 

13. In the text, explain the reasons for the large differences in CoF shown in Fig. 9a

Response: We agree with the reviewer. A change has been made to the article.

14. Lines 336 and 337 - delete Cyrillic text

Response: We agree with the reviewer. A change has been made to the article.

15. Table 3 also gives the maximum wipe depth

Response: This makes it difficult for readers.

16. Line 352 - 355 Not very revealing conclusion - it is known that the ferritic-pearlitic structure has worse properties than the martensitic structure.

Response: We agree with the reviewer. A change has been made to the article.

Reviewer 2 Report

The authors addressed the modifications in the structural-phase state and the hardening efficiency of the 30CrMnSiA steel by employing the method of electrolyte-plasma treatment. Several modifications have to be made before the manuscript publication, as follows:

1. The influence of the treatment is understandable, but the reasons are not sufficiently discussed, in such a way that the connection between the mechanism (cause) and the result (effect) to be clearly understood.

2. L14: Thermal treatment has to be replaced with heat treatment.

3. Table 1: The measure unit abbreviation for time is [s], not [sec].

4. Fig. 6: “Zone of Thermal Influence” should be replaced with Heat Affected Zone. “Unaffected” has to be replaced with “Unaffected material”

5. Several figures resolution is poor and has to be improved (Figs. 2, 3, 4, 5, 7 8, 9).

6. Conclusions: A sentence in a Slavic language is untranslated.

Author Response

We thank the reviewer for his in-depth analysis of our manuscript and his comments and suggestions, which helped us to significantly improve the quality of our data analysis and presentation. In the revised version of this paper, we have tried, as much as possible, to take into account the comments and suggestions presented in the review.

Below are the responses to the comments

Comments for Author:

1. The influence of the treatment is understandable, but the reasons are not sufficiently discussed, in such a way that the connection between the mechanism (cause) and the result (effect) to be clearly understood.

Response: The electrolyte-plasma technology used for heating and quenching the surfaces of parts has been known for 50 years. This technology is unique due to its ability to change the properties of surfaces. In EPT, electrical energy is usually transferred from the metal anode to the workpiece itself through a layer of electrolyte and plasma. The plasma layer is formed from the electrolyte material in the gap between the liquid electrode and the conductive surface of the workpiece. A water-based electrolyte was used as a liquid electrode. The results obtained are briefly presented in this article. Tribological properties of martensitic structures were also studied. The samples were heated to a temperature within the austenitic phase region and the sample was completely austenized, then cooled to room temperature in the electrolyte. The final microstructure is martensite. Both SEM and optical microscope drawings show a strong formation of martensite. A change has been made to the article.

2. L14: Thermal treatment has to be replaced with heat treatment.

Response: We agree with the reviewer. A change has been made to the article.

3. Table 1: The measure unit abbreviation for time is [s], not [sec].

Response: We agree with the reviewer. A change has been made to the article.

4. 6: “Zone of Thermal Influence” should be replaced with Heat Affected Zone. “Unaffected” has to be replaced with “Unaffected material”

Response: We agree with the reviewer. A change has been made to the article.

5. Several figures resolution is poor and has to be improved (Figs. 2, 3, 4, 5, 7 8, 9).

Response: We agree with the reviewer. A change has been made to the article.

6. Conclusions: A sentence in a Slavic language is untranslated.

Response: We agree with the reviewer. A change has been made to the article.

Reviewer 3 Report

The reviewed article presents the results dedicated to the effect of the electrolyte-plasma thermocyclic surface treatment on the surface structure/properties of 0.3wt%C-low alloy steel. The structural and hardness gradients after this treatment are described as well as sliding wear behaviour. However, the scientific soundness of the paper is doubtful. The paper contains many drawbacks which must be revised according to the comments below.

1. English grammar and style are not appropriate. There are a lot of mistakes and typos. Also, there are many too-long sentences that should be divided (for example, 292-295, 296-300, 202-206

2. According to Fig. 2 the initial structure of the steel consisted of ferrite only although pearlite was present as well. However, after the EPTCH the authors found the peaks of cementite (Fig. 4, lines 200-201). How the appearance of cementite can be explained if the authors claim that ferrite and cementite are dissolved in austenite (Lines 181-182) and  “The high cooling rate of the electrolyte does not allow carbon to be properly distributed to obtain the phases of perlite or bainite, which leads to the formation of martensite (Lines 187-189)?

3. Figure 3 was just mentioned but the presented microstructure was not discussed. What depth is shown in Fig.3? The images are too poor (dark and blurred) to illustrate the microstructure clearly. I suggest removing these images. 

4. The captions to the Figs. 3 and 7 are not correct: they should start from “Microstructure of the samples processed ……” and “P−h-diagrams curves of…”.

5. Some specific terms were inappropriate presented such as: “perlite” instead “pearlite”, “sorbitol” instead “sorbite”,  “Fe-C state diagram” instead “Fe-C phase diagram”, “zone of thermal influence” and “thermal impact zone” instead “heat-affected zone”, “shooting parameters” instead “scanning parameters”, “fine-fine (???) needle martensite”. 

6. The article contains parts with general “text-book” information which do not give any new results (for example, the last paragraph on page 5, lines 195-198, lines 250-255, lines 137-138).

7. Lines 250-270 should be moved to the Experimental section.

8. Lines 159-160: “Figure 2 a-b shows that the interplate distance of ferrite (light) and perlite (dark) looks different depending on the angle to the surface”. Actually, the interplate distance can not be revealed on these Figures because of their low magnification.

9. Table 2 should be mentioned before discussing the results of nanohardness testing (lines 273-275).

10. Fig. 8 presents three images of the microstructure. What lines do they refer to? What do the inclined dashed lines mean?

11. Tables 2 and 3 should be provided by confident intervals.  

12. The sense of the sentence “This is due to the rapid quenching process and the reduction of thermal cycling in depth” is not understood.

13. What was the sliding distance at the sliding test? How the “Wear volume” and “Wear intensity” were found? The description of their calculation with the equipment names should be added to the Experimental section.

14. Lines 279-282: “This technique consists in selecting the parameters of a power function describing the experimental dependence of the depth of immersion of the indenter and the contact area on the applied force, and calculating the hardness and modulus of elasticity according to these data.” The word “consists” should be changed by “implies” or “requires”. 

15. Table 1 presents seven regimes while other Figures and Tables contain the results for four regimes. Where are the others? 

16. Line 53: “Electrolyte-plasma hardening (heating-quenching) is a special thermomechanical process”. Why “thermomechanical”? Is there any surface deformation during EPTCH?

17. The main task of the paper was to study how the number of EPTCH's cycle affects the surface structure and hardening depth. Unfortunately, the authors did not provide any scientific-based explanation of the results obtained. Why the hardening depth is maximal at 3 cycles and minimal at 1 cycle? 

18. The paper lacks a deep understanding of the mechanisms and kinetics of the structural evolution in steel under EPTCH treatment.

Author Response

We thank the reviewer for his in-depth analysis of our manuscript and his comments and suggestions, which helped us to significantly improve the quality of our data analysis and presentation. In the revised version of this paper, we have tried, as much as possible, to take into account the comments and suggestions presented in the review.

Below are the responses to the comments

Comments for Author:

1. English grammar and style are not appropriate. There are a lot of mistakes and typos. Also, there are many too-long sentences that should be divided (for example, 292-295, 296-300, 202-206

Response: We agree with the reviewer. A change has been made to the article.

2. According to Fig. 2 the initial structure of the steel consisted of ferrite only although pearlite was present as well. However, after the EPTCH the authors found the peaks of cementite (Fig. 4, lines 200-201). How the appearance of cementite can be explained if the authors claim that ferrite and cementite are dissolved in austenite (Lines 181-182) and “The high cooling rate of the electrolyte does not allow carbon to be properly distributed to obtain the phases of perlite or bainite, which leads to the formation of martensite (Lines 187-189)?

Response: We agree with the reviewer The microstructures prior to electroplasma hardening of 30CrMnSiA steel are shown in Figure 2(a). As shown in the optical micrographs, the main phase component of the initial state of 30CrMnSiA steel is ferrite (white part) and pearlite (dark part).

3. Figure 3 was just mentioned but the presented microstructure was not discussed. What depth is shown in Fig.3? The images are too poor (dark and blurred) to illustrate the microstructure clearly. I suggest removing these images.

Response: We agree with the reviewer. A change has been made to the article. Figure 3 has been deleted.

4. The captions to the Figs. 3 and 7 are not correct: they should start from “Microstructure of the samples processed ……” and “P−h-diagrams curves of…”.

Response: We agree with the reviewer. A change has been made to the article.

5. Some specific terms were inappropriate presented such as: “perlite” instead “pearlite”, “sorbitol” instead “sorbite”, “Fe-C state diagram” instead “Fe-C phase diagram”, “zone of thermal influence” and “thermal impact zone” instead “heat-affected zone”, “shooting parameters” instead “scanning parameters”, “fine-fine (???) needle martensite”.

Response: We agree with the reviewer. A change has been made to the article.

6.  The article contains parts with general “text-book” information which do not give any new results (for example, the last paragraph on page 5, lines 195-198, lines 250-255, lines 137-138).

Response: We agree with the reviewer. A change has been made to the article.

7. Lines 250-270 should be moved to the Experimental section.

Response: We agree with the reviewer. A change has been made to the article.

8. Lines 159-160: “Figure 2 a-b shows that the interplate distance of ferrite (light) and perlite (dark) looks different depending on the angle to the surface”. Actually, the interplate distance can not be revealed on these Figures because of their low magnification.

Response: We agree with the reviewer. A change has been made to the article.

9. Table 2 should be mentioned before discussing the results of nanohardness testing (lines 273-275).

Response: We agree with the reviewer. A change has been made to the article.

10. Fig. 8 presents three images of the microstructure. What lines do they refer to? What do the inclined dashed lines mean?

Response: We agree with the reviewer. A change has been made to the article.

11. Tables 2 and 3 should be provided by confident intervals.

Response: We agree with the reviewer. Table 2 has been changed.

12. The sense of the sentence “This is due to the rapid quenching process and the reduction of thermal cycling in depth” is not understood.

Response: We agree with the reviewer. Table 2 has been changed. 

13. What was the sliding distance at the sliding test? How the “Wear volume” and “Wear intensity” were found? The description of their calculation with the equipment names should be added to the Experimental section.

Response: The total sliding distance reached 60 m, that’s enough for wear testing of the hardened layer. Wear tracks were investigated using a noncontact 3D profilometer MICROMEASURE 3D station.

14. Lines 279-282: “This technique consists in selecting the parameters of a power function describing the experimental dependence of the depth of immersion of the indenter and the contact area on the applied force, and calculating the hardness and modulus of elasticity according to these data.” The word “consists” should be changed by “implies” or “requires”.

Response: We agree with the reviewer. A change has been made to the article.

15. Table 1 presents seven regimes while other Figures and Tables contain the results for four regimes. Where are the others?

Response: We have 4 samples. A cyclic voltage between 320 and 250, 50 was performed during the stipulated period. The process of plasmoelectrolytic treatment of sample No. 1 was carried out in the following mode: applied voltage 320 V, processing time 2 s. All data are given in Table 1. 3 processing cycles were carried out for sample No. 2. And for samples No. 3 and No. 4, there were 7 processing cycles.

16. Line 53: “Electrolyte-plasma hardening (heating-quenching) is a special thermomechanical process”. Why “thermomechanical”? Is there any surface deformation during EPTCH?

Response: Electrolyte-plasma hardening (heating-quenching) is a complex process that combines physical metallurgy and electrochemical processes, such as heating the workpiece in the cathode mode (gas release, spark ignition, continuous plasma shell and arc welding mode), where phase transformations and deformation occur simultaneously. Microplastic deformations strengthen steels, and recrystallization increases their plasticity.

17. The main task of the paper was to study how the number of EPTCH's cycle affects the surface structure and hardening depth. Unfortunately, the authors did not provide any scientific-based explanation of the results obtained. Why the hardening depth is maximal at 3 cycles and minimal at 1 cycle?

Response: We agree with the reviewer. A change has been made to the article. We agree with the reviewer. Periodically turning on the high voltage of the electric potential 320 V and low (pause) 50, 250 V, the heating speed increased and decreased, which then allowed to increase the time and get a thicker heated layer. During the demonstrated voltage mode, the pulse mode heated the samples, and the pause mode in the experiment cooled the samples or maintained a constant temperature. The periodic connection of high (320 V) and low (pause 50, 250 V) electric potential leads to a periodic increase and stabilizes or slows down the increase in the heating rate, which then allows you to increase the time and get a thicker heated layer without melting the surface. The depth of the hardened layer can be 0.5-3.5 mm, depending on the EPTT conditions.

18. The paper lacks a deep understanding of the mechanisms and kinetics of the structural evolution in steel under EPTCH treatment.

Response: We agree with the reviewer. A change has been made to the article.

Reviewer 4 Report

The paper is well written, I reccomend publication in its present form.

Author Response

We thank the reviewer for his in-depth analysis of our manuscript and his comments and suggestions, which helped us to significantly improve the quality of our data analysis and presentation.

Round 2

Reviewer 1 Report

The authors did not make all the suggested changes. I cannot recommend further publication of the work. Details in the file.

Comments for author File: Comments.pdf

Author Response

 We thank the reviewer for his in-depth analysis of our manuscript and his comments and suggestions, which helped us to significantly improve the quality of our data analysis and presentation. In the revised version of this paper, we have tried, as much as possible, to take into account the comments and suggestions presented in the review.

Below are the responses to the comments

Comments for Author:

1. The novelty of the work in comparison with the state of the art or science (highlights) should be indicated. They also need to show that their work brings an aspect of novelty compared to already existing solutions.

Response: In an industry where steel is used in the manufacture of each component, surface improvement plays a vital role in ensuring the desired mechanical properties of steel. Workshops for the production of press tools, automobile companies, manufacturers of press parts, the aerospace industry, the weapons industry - these are some of the areas where steel is a common metal, and therefore improving the properties of steel by various methods becomes important. Electrolyte-plasma thermocyclic surface hardening is an attractive solution for chemical and thermal treatment used to improve the properties of the steel surface through structural and phase transformations.  This technology is economical, and the process itself is harmless to the environment. However, although this process is performed for both different types of steel, to date there is no standardized heat treatment cycle in terms of material.

Reviewer's note: the text still lacks a clear description of the novelty of the work compared to the state of the art.

Response: To increase the hardness and wear resistance of steel surfaces, various methods of surface modification are used, in particular chemical-thermal treatment; this increases the reliability, durability and service life of machine parts and tools. Electrolytic plasma quenching (heating-quenching) is a special chemical-thermal process in which electrolysis in an aqueous solution is used under certain conditions, for example, voltage, current, electrolyte, duration and speed of heating-quenching. It is known that electrolytic plasma thermocyclic hardening is a repetitive process of breakdown voltage and arc discharge (at the cathode), and that intense arc discharge occurs at a late stage of gas release of electrolytic plasma thermocyclic hardening. Heating or hardening of medium-carbon steels can change their microstructure, which causes changes in mechanical and physical properties and affects the behavior of steels under operating conditions. However, many experimental studies were carried out in laboratory conditions and on equipment. In addition, many electrolytic plasma processes were performed in small containers in the form of anodes and small samples immersed in an aqueous electrolyte. This article discusses the improvement of the technology of electrolyte-plasma thermocyclic treatment, focused on industry. The microstructure and microhardness of 30CrMnSiA steel samples treated with EPTT were determined, and the parameters of the EPTT process and the temperature rate (heating–cooling) were investigated.

3. In general, improve the discussion of the results obtained

Response: We agree with the reviewer. A change has been made to the article. Reviewer's note: the text still lacks a broader discussion of the results

Response: We agree with the reviewer. A change has been made to the article.

1. Line 108 For 30CrMnSiA steels, enter the designation in the 1.XXXX system; also provide information on how to determine the chemical composition; note according to the GOST standard, this steel has the designation 25ChGSA

Response: We agree with the reviewer. A change has been made to the article.

Reviewer's note: The authors did not introduce the suggested changes to the text

Response: As the research material, samples of 30CrMnSiA steel in the state of delivery were used (quenching at 880 °C and tempering at 540 ° C in oil in accordance with GOST 4543-71). Chemical composition of steel: 0.28-0.35% C; 0.8-1.1% Sg; 0.8-1.1% Mp; 0.9-1, % Si; 0.025% P; 0.025% S, the rest Fe according to GOST 4543-71.

2. Row 111, this steel is rather rarely used for gears and devices, more often for responsible shafts and screws

Response: 30CrMnSiA steel is used in the production of parts with increased wear resistance requirements: piston mechanisms operating in engines, pushrods, gear shafts, levers, axles, blades, flanges, steel wheels, responsible load-bearing structures designed for alternating loads.

Reviewer's note: The authors did not provide detailed information about the tested steel in the text

Response: We have removed the following suggestions according to your recommendations.

4. Enter what was the heat treatment before hardening (normalizing?) Response: Quenching 880 ° C, oil. Vacation 560 ° C.

Reviewer's note: The authors did not introduce the suggested changes to the text

Response: As the research material, samples of 30CrMnSiA steel in the state of delivery were used (quenching at 880 °C and tempering at 540 ° C in oil in accordance with GOST 4543-71).

8. Figure 5 estimate the content of martensite for detailed treatment variants; use the martensite content in the discussion of the results.

Response: We agree with the reviewer.

Reviewer's note: The authors did not introduce the suggested changes to the text

Response: It was found that the dominant phases formed on 30CrMnSiA steel are a mixture of martensite, Fe₃C and BCC-iron, confirmed by radiography. We cannot determine the exact quantitative analysis of martensite, since the phase of BCC-iron and martensite intersects the same peaks.

9. Figure 6 - indicate the individual phases in the figure

Response: We agree with the reviewer. A change has been made to the article. Reviewer's note: The authors did not introduce the suggested changes to the text

Response: In Figure 6 (Figure 5), changes were made.

10. Lines 264 and 270 - do not provide references to literature when marking the equation Response: We agree with the reviewer. A change has been made to the article.

Reviewer's note: The authors did not introduce the suggested changes in the text (reference is in line 168)

Response: We agree with the reviewer. A change has been made to the article.

11. Table 2 and 3 - give the measurement uncertainty and write the result in the form Y = X ± δ;

Response: We agree with the reviewer. Table 2 has been changed. Reviewer's Note: The authors did not make the suggested changes to Table 3

Response: Changes have been made to table 3.

12. Fig. 8 the drawing is hardly legible; enlarge the font; specify the thickness of the hardened layer under the drawing in the text

Response: We agree with the reviewer. Periodically turning on the high voltage of the electric potential 320 V and low (pause) 50, 250 V, the heating speed increased and decreased, which then allowed to increase the time and get a thicker heated layer. During the demonstrated voltage mode, the pulse mode heated the samples, and the pause mode in the experiment cooled the samples or maintained a constant temperature. The periodic connection of high (320 V) and low (pause 50, 250 V) electric potential leads to a periodic increase and stabilizes or slows down the increase in the heating rate, which then allows you to increase the time and get a thicker heated layer without melting the surface.

Reviewer's note: ???

Response: With periodic switching on of high voltage electric potential (320 V), medium (200 V) and low (50 V), there is a periodic increase and decrease in the heating rate, which allows you to increase the time and get a thicker heated layer. The periodic change in the electric field strength between the surfaces of the liquid electrode and the product changes the surface heating power density, which ensures the control of electrolyte-plasma heating and the creation of the necessary thermal regimes for the formation of quenching structures. Periodically changing the heating power density, it is possible to obtain hardened layers with a thickness of 0.6 mm, 2.2 mm and 3.4 mm. A 15% aqueous solution of sodium carbonate was used as the electrolyte. The hardness of the hardened layer was measured by a hardness tester.

13. In the text, explain the reasons for the large differences in CoF shown in Fig. 9a

Response: We agree with the reviewer. A change has been made to the article. Reviewer's note: The authors did not introduce the suggested changes to the text.

Response: A change has been made to the article.

15. Table 3 also gives the maximum wipe depth

Response: This makes it difficult for readers.

Reviewer's note: The authors did not introduce the suggested changes to the text

Response:

The area of the groove obtained using a 3D profilometer (30CrMnSiA - initial).

Using a profilometer, we obtained the groove area of each sample. Using the following formulas, we calculated the wear volume of the sample and the wear intensity.

V=S•Ɩ

V – wear volume, mm³;

Ɩ - groove length, mm;

S – cross-sectional area of the wear groove, mm²;

I – Wear intensity, mm³/m•Н;

N – mileage value, m;

P – applied load, Н;

Reviewer 3 Report

The authors performed some revisions however, some of my comments were not addressed properly.

1. Comment 16. The authors insist that electrolyte-plasma hardening is a “thermomechanical process” since it is accompanied by strengthening caused by microplastic deformation. According to this approach, each heat treatment (especially ordinary quenching) should be recognized as “thermomechanical process” because each heat treatment results in a microstrain caused by heating/cooling or structure transformation.  This is wrong. “Thermomechanical” treatment is a standardized term that means “method consists of rolling reductions and cooling rate controls that result in mechanical properties in the finished plate that are equivalent to those attained using conventional rolling and heat treatment processes, which entail reheating after rolling” (ASTM A1066/A1066M-22). This is not the case for the EPTCH treatment. The standard terms should be used correctly.

 2. The authors ignored comment 2 concerning the presence of cementite peaks on the XRD pattern of the EPTCH-treated sample. Cementite should not appear in the structure because of the high cooling rate. Thus, the cementite peak must be explained.

3. Comments 4, 8, 10, 17, and 18 are not addressed and corrections (explanations) were not performed (added) although the authors claim that they made the corrections. When answering the comments the authors should specify (with line number) where the exact changes were made.

Author Response

Below are the responses to the comments

Comments for Author:

1. Comment 16. The authors insist that electrolyte-plasma hardening is a “thermomechanical process” since it is accompanied by strengthening caused by microplastic deformation. According to this approach, each heat treatment (especially ordinary quenching) should be recognized as “thermomechanical process” because each heat treatment results in a microstrain caused by heating/cooling or structure transformation. This is wrong. “Thermomechanical” treatment is a standardized term that means “method consists of rolling reductions and cooling rate controls that result in mechanical properties in the finished plate that are equivalent to those attained using conventional rolling and heat treatment processes, which entail reheating after rolling” (ASTM A1066/A1066M-22). This is not the case for the EPTCH treatment. The standard terms should be used correctly.

Response: Microplastic deformation is subjected to the material from the local action of concentrated heat, since, with rapid heating of 500-550 ° C / s and subsequent cooling due to heat removal from the material, and due to flow cooling by the action of an electrolyte in circulatory modes. With such local impacts by concentration energy flows, such as healing from a technological laser or electron guns, an internal stress is formed by a hardened section of the material, which is usually observed by thermomechanical processing. During the heat treatment of the material, such internal stresses are not observed. In this regard, the studied steels with electrolyte-plasma hardening have similar results as with thermomechanical processing. But in accordance with the recommendations, we have removed the following sentences. To improve the structure of medium-carbon steels and increase their mechanical properties during electrolytic plasma surface hardening (heating-quenching), it has recently been recommended to use cyclic thermal action, called electrolytic plasma thermocyclic hardening (EPTCH). Unlike other types of chemical-heat treatment, structural and phase transformations during EPTCH occur repeatedly at varying heating–cooling temperatures. The need for repeated processing at specified temperatures, as a rule, is due to the desire to accumulate changes that radically improve the quality of the workpiece and give them properties unattainable with a single chemical-heat treatment. Most often, the resulting changes from cycle to cycle are associated with changes caused by plastic deformation. The nature of the phase interaction of the components in the system largely determines the effectiveness of the effect of electrolytic plasma thermocyclic treatment on changes in the structure and properties of steel. In the case of complete non-miscibility of components in the solid state, thermal cycling is not accompanied by a change in the number of phases in the system, and structural changes in the steel of this system under the influence of electrolytic plasma thermocycling treatment can only be associated with the consequences of microplastic deformation and subsequent recrystallization. Microplastic deformations strengthen steels, and recrystallization increases their plasticity.

2. The authors ignored comment 2 concerning the presence of cementite peaks on the XRD pattern of the EPTCH-treated sample. Cementite should not appear in the structure because of the high cooling rate. Thus, the cementite peak must be explained.

Response: The appearance of cementite (Fe₃C) can be explained by the fact that there are carbon-containing components in the electrolyte. It has been reported that if electrolytic plasma quenching has been carried out properly, the core will still have a low carbon content (0.3% C), while the surface layer should preferably have a carbon content of more than 0.3% C. It is assumed that the carbon content in the hardened layer is presumably greater than 0.3% C, based on the observation of cementite (Fe₃C) in this study. Hardness measurements carried out using a Vickers groove from the surface to the inside of the sample showed that the hardness of the hardened layer was much higher than that of the substrate (Figures 6 and 7). This is a consequence of the presence of solid Fe₃C and martensite, which is determined by X-ray diffraction (Fig. 3).

3. Comments 4, 8, 10, 17, and 18 are not addressed and corrections (explanations) were not performed (added) although the authors claim that they made the corrections. When answering the comments the authors should specify (with line number) where the exact changes were made.

Response:

4. The captions to the Figs. 3 and 7 are not correct: they should start from “Microstructure of the samples processed ……” and “P−h-diagrams curves of…”.

Response: We agree with the reviewer. We have made changes to Figures 4 and 6.

8. Lines 159-160: “Figure 2 a-b shows that the interplate distance of ferrite (light) and perlite (dark) looks different depending on the angle to the surface”. Actually, the interplate distance can not be revealed on these Figures because of their low magnification.

Response: We have removed these suggestions. We studied the microstructure using an optical and scanning electron microscope.

10. Fig. 8 presents three images of the microstructure. What lines do they refer to? What do the inclined dashed lines mean?

Response: The hardness measurements showed three different areas (Fig. 7). These are: the surface layer, designated as (1), consisting of a mixture of Fe₃C, martensite; the zone of thermal influence, designated as (2); and the substrate, designated as (3). The hardness values of the hardened layer, the layer directly under the hardened layer and the substrate are 1000, 500 and 255 HV, respectively. The reason for the higher hardness of region 2 than region 3 is the result of solid solution curing between iron and carbon. It was noticed that the longer the hardening time, the greater the thickness of the hardened layer.

17. The main task of the paper was to study how the number of EPTCH's cycle affects the surface structure and hardening depth. Unfortunately, the authors did not provide any scientific-based explanation of the results obtained. Why the hardening depth is maximal at 3 cycles and minimal at 1 cycle?

Response: Studies have shown that when the electric potential of 320 V is switched on in the electric circuit of the heater, the surface of the sample is heated. The energy power density at the cathode is so high that melting of the surface layer begins in almost 5-10 seconds. The heating rate on the surface of the product reaches 500 ° C/s. When periodically switching on a high voltage of electric potential (320 V), medium (200 V) and low (50 V), a periodic increase and decrease in the heating rate is observed, which makes it possible to increase the time and get a thicker heated layer. The periodic change in the electric field strength between the surfaces of the liquid electrode and the product changes the surface heating power density, which ensures the control of electrolyte-plasma heating and the creation of the necessary thermal regimes for the formation of quenching structures. For example, in sample No. 2, heating was started by turning on the electric potential of 320 V for a time of 1 s. Upon reaching overheating, the electric potential was switched to 50 V for 7 s. After that, the electric potential of 320 V was switched again for a time of 1 s and the heated layer of the sample was cooled by the electrolyte flow. This makes it possible to alternate a high surface heating power density with a low one and, as a result, obtain an average heating rate of the surface layer of the product of 50-200 ° C/s.

18. The paper lacks a deep understanding of the mechanisms and kinetics of the structural evolution in steel under EPTCH treatment.

Response: We agree with the reviewer. A change has been made to the article.