Effect of Hot Rolling on Microstructural Evolution and Wear Behaviors of G20CrNi₂MoA Bearing Steel

Round 1

Reviewer 1 Report

The authors of the paper “Effect of hot rolling on microstructural evolution and wear behaviors of G20CrNi2MoA bearing steel” show the positive effect of hot rolling on the microstructure and properties of the bearing steel. The increases of the carbon diffusion and carbides volume fraction after hot rolling was shown. The paper is well written. However, some parts of the paper are questionable and are needed to be modified accordingly following comments:

1. In the Abstract and conclusion #2, the authors wrote that “refined austenite grains promoted the diffusion of carbon atoms during carburization”. It seems to be incorrect. It is hardly that such an insignificant change in the grain size (from 13 to 10 μm) may influence the diffusion. The increases in the diffusion may be due to the increase of the defects (dislocations, vacancies) inside the grains during the hot deformation.
2. In conclusion #3 the authors wrote that “the fine grains provided more nucleation sites for the carbides”. It is again incorrect. The carbides nucleate mainly inside the grains, not on the grain boundary. The slight refinement of the grains cannot significantly increase the distribution density of the carbides. The reason again is in the increase in defects number inside the grains.
3. What phase (ferrite or martensite) is shown in Figure 2? How did the authors measure the austenite grain size using the EBSD maps obtained at room temperature?
4. The difference in the grain size after hot rolling is insufficient according to grain size distribution in Figure 3a. The authors should add the confidence intervals to the values of average grain size through all text and in Figure 4.

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Reviewer 2 Report

This paper presented the effect of hot rolling on the microstructure and wear properties of G20CrNi2MoA bearing steel. A variety of literature has been reviewed to justify the basis of this research. The experimental procedure is reproducible. It was concluded that the fine grains are produced due to hot rolling by the series of heat treatments known as carburizing, quenching, and tempering (CQT) and reheating, quenching, and tempering (RQT). The conclusion was drawn based on experimental observation and the paper has sufficient merit for publish. However, the following needs to be addressed before publication:

1. A massive English editing is required. Please check with the journal's editing service.
2. The abbreviation should be presented properly in the abstract and as well throughout the manuscript. Just as an example,  carburizing, quenching, and tempering (CQT) ; reheating, quenching, and tempering (RQT); and so on.
3. Please avoid grouped citation such as [1-3], [4-6], [10-13], [14-16], [21-23], [24-27] etc.
4. There is a mixture of fig. and figure throughout the manuscript. Please use only one form.
5. Page 3 line 103: Please rewrite the sentence as below: The raw material used in this work was a typical G20CrNi2MoA carburized bearing steel (same as SAE 4320) which was received as an annealed bar with a size of ϕ 120×120 mm with varying thicknesses.
6. Please write the conditions for HR 1 -4, CQT 1-4, and RQT 1-4 in a table form in the experimental section for ease of reading.
7. Figure 4: Are the different values of d represent the average values of grain size? The values are not obvious from the figure. 
8.  Figure 5 shows a linear relationship with the depth reduction. Is there any relationship between CQT and RQT?
9. Figure 6: Why CQT-2, 3 were not presented?
10. Figures 6 and 7 can be merged.
11. Please include CQT information in figures 9 and 11.

Author Response

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Reviewer 3 Report

1. the current study investigates the influence of hot rolling process on the evolution of microstructure and wear behaviour of bearing steel. For this, the authors apply carburizing, quenching and tempering and reheating, quenching and tempering. The authors found that grain size was influenced due to recrystallization. The authors found that hardness increased  while friction and wear were reduced.
2. The abstract can be improved, it requires some restructuring, please start with a swift 1-2 lines about the nature of this work (something generic) then Please consider reviewing the abstract and highlight the novelty, major findings and conclusions.
3. Line 51 why the authors use bold font here?
4. After line 91 the authors should consider answer the following question: What is the research gap did you find from the previous researchers in your field? Mention it properly. It will improve the strength of the article.
5. Please add the info in line 104-105 in a table and reference it if it was not measured by the authors
6. Line 156-157 please discuss why the grain size showed a decreasing trend with the increase of hot rolling reduction and support with references. Also what did past studies found out about this. Does their results agree or disagree with yours?
7. “size was significantly reduced after the reheating and quenching, which was consistent with the research [24, 25]” I am not sure why the authors cite this sentence. Please try to expand your discussion and give more details about your results and comparing them to previous studies
8. Please expand figure 5 it is difficult to read the legends and the small bar chars in them
9. Line 230-231 “rather than in the form of coarse undissolved carbides.” Was there an expectation that carbides wont dissolve? Was this something observed in earlier studies?  Please discuss and elaborate further
10. Line 251 “specimens increased slightly” slightly is not a scientific way to express a measured data please either say how much in value or % but not just say slightly. Check this elsewhere in the manuscript as well

Author Response

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Round 2

Reviewer 1 Report

The authors have answered comments and made necessary changes in the paper. The manuscript may be accepted for publication.

Author Response

Thank you very much for your recognition on the research of this paper. The pdf file of the Revised Manuscript has been upload. Please check.

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Reviewer 2 Report

The authors have addressed all the issues raised by the reviewers. It is ready to publish.

Author Response

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Reviewer 3 Report

The manuscript have improved significanlty. please answer the following remaining queries:

Line 224-228: this is good conclusion but the authors should attempt to compare this result with past studies similar to their work, did they find similar effect of multiple austenitizing on grain refinement? Also what does the funamental literature say about multiple austenitizing. 

Line 260-261 "differences in the microstructures of RQT specimens, especially carbides." how serious are these differences, please elaborate further, for example if the temperature difference is not large, would there be huge microstructural differences or the other way around, please discuss further. 

Author Response

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