An Industrial-Scale Study of the Hardness and Microstructural Effects of Isothermal Heat Treatment Parameters on EN 100CrMo7 Bearing Steel
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this manuscript, the structure and properties of 100CrMo7 steel under different processes are studied, and the reasons for the variations in the structure and properties of this samples under different processes are explained by means of scanning electron microscopy, XRD, hardness tester and other testing methods. The results of the study provide a reference for the process optimization of 100CrMo7 steel. The paper has sufficient experiments, detailed and well-argued arguments, minor revision is recommended and the following comments need to be considered in order to improve the overall scientific quality of the paper:
1. Please supplement the review section with additional literature on the reduction of material strength by residual austenite.
2. Please standardize the scale of the pictures in the paper and make the scale of the figures larger.
3. Please provide the Ac1 and Ac3, bainite and martensite temperature transition ranges for this steel.
4. Please explain that tempering of martensitic structure produces a large amount of coarse carbides, whereas tempering of bainite does not produce a lot of coarse carbides?
5. For experiment No. 2, there is a significant difference in structure morphology and hardness between a 240°C salt bath and a 220°C salt bath, please explain the reason for this phenomenon.
Comments on the Quality of English LanguageModerate editing of English language required
Author Response
ANSWERS TO REVIEWERS
The Authors wish to thank the Reviewers for their valuable revision and for all the insightful suggestions and corrections reported.
Based on the Reviewers' suggestions, the Authors have clarified many parts of the work. Detailed considerations have been incorporated into the manuscript (highlighted in yellow in the text) to fulfill the Reviewers' recommendations and improve the overall quality of the paper. The Authors have reported their considerations and answers to each point raised by every Reviewer.
REVIEWER #1
In this manuscript, the structure and properties of 100CrMo7 steel under different processes are studied, and the reasons for the variations in the structure and properties of this samples under different processes are explained by means of scanning electron microscopy, XRD, hardness tester and other testing methods. The results of the study provide a reference for the process optimization of 100CrMo7 steel. The paper has sufficient experiments, detailed and well-argued arguments, minor revision is recommended and the following comments need to be considered in order to improve the overall scientific quality of the paper:
The Authors would like to thank the Reviewer for his/her favorable comments and his/her positive evaluation. The Authors are glad for his/her appreciation of the study.
- Please supplement the review section with additional literature on the reduction of material strength by residual austenite.
The Authors would like to thank the Reviewer for his/her comment.
As indicated, the following additional references about the reduction of material strength by residual austenite have been added:
A.D. Anoop, A.S. Sekhar, M. Kamaraj, K. Gopinath, Modelling the mechanical behaviour of heat-treated AISI 52100 bearing steel with retained austenite, Proc. Inst. Mech. Eng., Part L 232 (2015) 44–57, https://doi.org/10.1177/1464420715612235.
García-Mateo, Carlos, and Francisca G. Caballero. The role of retained austenite on tensile properties of steels with bainitic microstructures. Materials Transactions 46.8 (2005): 1839-1846.
Pascal Ostermayer, Tarek Allam, Xiao Shen, Wenwen Song, Klaus Burkart, Bastian Blinn, Brigitte Clausen, Wolfgang Bleck, Tilmann Beck, Effect of retained austenite on the fatigue behavior of modified bainitic 100Cr6 steels considering local phase transformation, Materials Science and Engineering: A, Volume 877,
2023, 145204, ISSN 0921-5093, https://doi.org/10.1016/j.msea.2023.145204.
- Please standardize the scale of the pictures in the paper and make the scale of the figures larger.
The Authors would like to thank the Reviewer for pointing this out.
All the OM and SEM micrographs have been amended as suggested.
- Please provide the Ac1 and Ac3, bainite and martensite temperature transition ranges for this steel.
The Authors would like to thank the Reviewer for his/her insightful comment.
Concerning the transformation temperatures for the martensite, from the manufacturer's datasheet, the values are Ms 186 °C, Ac1 730 °C, and Ac3 750 °C. These temperatures are reference values but, as it is known, they vary based on chemical composition, grain size, and austenitization conditions. Various martensite-start formulas have been developed over the years. Since the Authors do not know the actual transformation temperatures curves for the investigated steel, they decided to settle 180 °C for martempering treatment to be around the transformation for an industrial case study. Such information was added in the manuscript.
- Please explain that tempering of martensitic structure produces a large amount of coarse carbides, whereas tempering of bainite does not produce a lot of coarse carbides?
The Authors would like to thank the Reviewer for his/her valuable comment.
From the experimental findings reported in the present study, a qualitative evaluation of the carbides can be achieved. Indeed, no remarkable changes were detected, and an in-depth analysis of the role of carbides resulting from tempering martensitic and bainitic microstructures was beyond the aim of the proposed investigation. Nevertheless, following the Reviewer's suggestion, such analysis could be further investigated by quantifying the microstructure and the carbide particle size, as reported elsewhere (Transformation behavior and acceleration of low-temperature bainite in high carbon chromium steel, 10.1088/2053-1591/aad167).
- For experiment No. 2, there is a significant difference in structure morphology and hardness between a 240°C salt bath and a 220°C salt bath, please explain the reason for this phenomenon.
The Authors would like to thank the Reviewer for his/her valuable comment.
The HRC evolution, depicted in Fig. 7, was related to the observed bainite microstructures, ascribable to upper and lower bainite. These aspects were added to the discussion session.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis study investigates the effects of different temperatures and holding times in austempering on the microstructure and hardness evolution for EN 16 100CrMo7 large-size rings. The manuscript is overall well written, and some minor revisions are needed:
1. The scale bars in all the SEM images should be unified, and the text should be enlarged.
2. The diffractometric method to measure the amount of residual austenite should be clearly stated in Section 2.
3. In Figure 9, the HRC of the ring when using heat treatment time of 180 min is higher than that when using heat treatment time of 300 min at some locations, but lower at other locations. Is there an explanation for the experimental observation？
Comments on the Quality of English LanguageThe English in fine.
Author Response
ANSWERS TO REVIEWERS
The Authors wish to thank the Reviewers for their valuable revision and for all the insightful suggestions and corrections reported.
Based on the Reviewers' suggestions, the Authors have clarified many parts of the work. Detailed considerations have been incorporated into the manuscript (highlighted in yellow in the text) to fulfill the Reviewers' recommendations and improve the overall quality of the paper. The Authors have reported their considerations and answers to each point raised by every Reviewer.
REVIEWER #2
This study investigates the effects of different temperatures and holding times in austempering on the microstructure and hardness evolution for EN 16 100CrMo7 large-size rings. The manuscript is overall well written, and some minor revisions are needed:
The Authors would like to thank the Reviewer for his/her positive evaluation.
- The scale bars in all the SEM images should be unified, and the text should be enlarged.
The Authors would like to thank the Reviewer for pointing this out.
All the SEM micrographs have been amended as suggested.
- The diffractometric method to measure the amount of residual austenite should be clearly stated in Section 2.
The Authors would like to thank the Reviewer for his/her valuable comment.
Accordingly, the adopted diffractometric method has been added to Section 2 that has been changed to read:
“The content of retained austenite was determined by the G.N.R. AreX XRD analyzer, equipped with a Mo X-Ray tube with a 1 mm monocapillary collimator, per the ASTM E975-03 standard. XRD measurements were conducted on the above-described samples for metallographic analyses, i.e. the upper surface and the central region of the ring.”
- In Figure 9, the HRC of the ring when using heat treatment time of 180 min is higher than that when using heat treatment time of 300 min at some locations, but lower at other locations. Is there an explanation for the experimental observation?
The Authors would like to thank the Reviewer for his/her insightful comment.
The observed scattering in hardness evolution (the observed data were without fitting methods) reflects the material homogeneity. The Authors believe that such experimental findings are due to the actual austenitization conditions that do not allow a complete and uniform material transformation. Indeed, as highlighted in the conclusions section, such austenitizing temperature was detrimental to the hardness in the center of the ring, regardless of the holding times and demonstrated that, for real application, the temperatures from the datasheet should be related to the actual condition, as in the case of large size rings.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors,
Your paper is suitable for this journal, some modifications are needed to make it more valuable. Please find enclosed my comments.
Comments for author File: Comments.pdf
Author Response
ANSWERS TO REVIEWERS
The Authors wish to thank the Reviewers for their valuable revision and for all the insightful suggestions and corrections reported.
Based on the Reviewers' suggestions, the Authors have clarified many parts of the work. Detailed considerations have been incorporated into the manuscript (highlighted in yellow in the text) to fulfill the Reviewers' recommendations and improve the overall quality of the paper. The Authors have reported their considerations and answers to each point raised by every Reviewer.
REVIEWER #3
Dear Authors, Here you can find my comments, I mark the rows and figure numbers. Please consider and repair the paper according to the comments.
Abstract
- The Abstract can be more attractive if it contains a small, but interesting part from the results, e.g. mentioning the RA or bainite effect.
The Authors would like to thank the Reviewer for his/her valuable comment.
As suggested, the abstract has been amended to highlight the results obtained.
"The 100CrMo7, commonly employed for bearings in rotating machinery, relies on precise heat treatment parameters to ensure optimal microstructure and, in turn, mechanical properties. Typically, an austenitizing treatment, followed by rapid cooling in a salt bath for martempering or austempering, is crucial in achieving the desired microstructure and hardness. The present industrial-scale study involved a comparative analysis between martempering and austempering routes regarding hardness and microstructure evolution of EN 100CrMo7 large-size rings. The investigation delves into the effects of austempering temperatures and holding times and austenitizing temperature. Besides, the role of tempering in reducing the amount of retained austenite was also considered. Results indicate that martempering yielded the highest hardness values while austempering exhibited a decrease in hardness at the center of the rings, albeit a lower amount of retained austenite (in the range 3.0 - 4.9 vol. %) was detected in comparison with martempering. In addition, the 850 °C austenitizing temperature reduced the hardness by 16 % in the center of the rings and promoted a high content of upper bainite, thus suggesting its inefficacy for the involved large-size rings. In contrast, the 880 °C austenitizing temperature maintained consistently high HRC values across the ring's height. Lastly, the analysis highlighted that the cooling rate played a more crucial role than the austempering holding time. Such industrial-scale investigations performed on real components improve the knowledge and control of heat treatment parameters compared to the nominal guidelines provided by steel suppliers. These outcomes offer insights for optimizing industrial heat treatment parameters, with practical implications for enhancing steel bearings' microstructural and mechanical performance and lifespan."
Introduction
- It is detailed and informative.
The Authors would like to thank the Reviewer for his/her appreciation.
Materials and method
- The chemical composition table contains the standard deviations. So, I suppose, the chemical composition was measured in more points, not just one. In this case, where is it measured?
The Authors would like to thank the Reviewer for pointing this out.
Chemical composition was measured at the ring's core, at half thickness and near the surface: n. 3 samples for each location, on a longitudinal cross-section, were analyzed. Such information was added to the manuscript.
- A drawing of the investigated ring and places of measurement can be very helpful.
The Authors would like to thank the Reviewer for his/her suggestion. Accordingly, a new figure was added.
- The Brinell hardness measurement is described, but in the paper can not be found Brinell hardness results.
The Authors would like to thank the Reviewer, nevertheless, the experimental findings related to the Brinell hardness were reported in the results section (as-received material).
- Why these austenitizing and cooling parameters were chosen? Only the salt bath temperatures were explained.
The Authors would like to thank the Reviewer for his/her helpful comment.
Austenitization temperatures were chosen based on the supplier datasheet: specifically, 880 °C for 300 min is slightly higher than the indicated temperature range combined with a sufficient time for thermal equilibrium of the large-size investigated rings and following practical experience from the industrial perspective. Besides, the 850 °C for 60 min was in accordance with the minimum suggested austenitization temperature based on the supplier datasheet. Finally, the cooling parameters and the martempering and austempering routes were defined based on the supplier datasheet too, since no experimental CCT/TTT curves were known/achievable.
The materials and method section was amended accordingly.
- It can be more informative if the temperature–time diagrams are in the CCT/TTT diagrams. It can show more about the expected phases and hardness.
The Authors would like to thank the Reviewer for his/her insightful comment.
The CCT/TTT diagrams would have been the best solution, also for being more precise about the expected phases and hardness, i.e., with the superimposed cooling curves. The Authors are aware of that. Nevertheless, without the local value of the temperature (e.g. detected by a specific apparatus for continuous recording with k-type exposed thermocouples that was not used in the industrial-scale investigation), the Authors believed that the cooling curves would have been speculative.
- For RA diffractometric measurements were made, but it is not mentioned and described.
The Authors would like to thank the Reviewer for pointing this out.
Accordingly, the adopted diffractometric method has been added to Section 2 that has been changed to read:
"The content of retained austenite was determined by the G.N.R. AreX XRD analyzer, equipped with a Mo X-Ray tube with a 1 mm monocapillary collimator, per the ASTM E975-03 standard. XRD measurements were conducted on the above-described samples for metallographic analyses, i.e. the upper surface and the central region of the ring."
Results
- row 213: it is mentioned that the austempered rings contain bainite, retained austenite, and carbides, but only the upper bainite is marked in fig. 6. The marking of all mentioned phases is better, because not just the bainite has a significant influence on the properties.
The Authors would like to thank the Reviewer for his/her insightful comment.
The Authors are aware that not just the bainite has a significant influence on the properties. Despite that and considering the framework of the present study, the Authors decided to adopt the SEM investigation to be aligned with the aim of an industrial investigation. A detailed microstructural analysis with more accurate techniques to detect and precisely define the phases may be explored. The Authors greatly appreciated this comment and will consider delving deeper into the proposed aspects in a subsequent study.
- Fig. 9: difficult to separate the lines, please use more different markings for different technologies.
The Authors would like to thank the Reviewer for his/her comment.
Fig. 9 was amended accordingly.
- Fig. 10: the scaling is very small.
The Authors would like to thank the Reviewer for pointing this out.
All the OM and SEM micrographs have been amended as suggested, making the scale of the figures larger.
- row 283: grain sizes are not perfectly understandable, please describe it more detailed.
The Authors would like to thank the Reviewer for pointing this out.
The grain size of the investigated steel was detected by the "Mean Line Intercept Method" per the ASTM E112 standard. Accordingly, for the sample quenched from 850 °C the higher G value reflects in a smaller grain size; conversely, for the sample quenched from 880 °C the lower G value reflects in a greater grain size.
- table 2: What does it mean exactly: upper surface, center? Please describe it more precisely.
The Authors would like to thank the Reviewer for his/her valuable comment.
With reference to Fig. 1b, the investigated regions were defined. Table 2 was amended to reflect this update.
Discussion
- It is clear, please don't use almost same sentences as before.
The Authors would like to thank the Reviewer for pointing this out.
Conclusion
- it is ok.
References
- The format of the references is not the same.
- Please write DOI numbers, if possible.
- The reference list contains unreachable publications in this form: e.g. number 30. Please, check it.
The Authors would like to thank the Reviewer for his/her comments.
The references have been checked and amended. All the references (including Journal articles, conference proceedings and book sections) have been listed through Mendely software, according to the Citation Style of the Journal.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThank you for the corrections, now the paper has better quality.
