Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In my opinion authors must clarify/modify the following issues:

In my opinion, the paper length is too short and authors should include a more extended introduction (it is just 2 paragraphs), and describe in a more detailed way the methods used in the study, for instance the experimental layout of the SSRT tests (including photographs) and the hydrogen pre-charge. In addition, deeper descriptions regarding how results are obtained are needed and additional discussion of the obtained results. Finally, the conclusions section is too general and it should be revised.  

#1) Page 1. Lines 23, 25, 30, 39. Please avoid lumping references. Fort he sake of clarity, please briefly describe each one individually or in groups not higher than 2 or 3 references.

#2) The introduction section should be improved. In my opinion it is too short (just 2 paragraphs) and it should briefly place the study in a broad context and highlight why it is important. It should define the purpose of the work and its significance. The current state of the research field should be reviewed carefully and key publications cited. Finally, the aims of the study based on the research gap obtained in the literature review must be included at the end of the introduction.

#3) Page 2. Line 68 and 73. Please align the equations correctly.

#4) Page 2. Lines 74. Please use italics for variables such as “b”. In addition, please add a blank space after “vector.”.

#5) Page 2. Line 76. Please use capital “T” for 10t.

#6) Page 2 line 75. Please include a detailed description of the SSRT layout including photographs and in addition, a detailed description of the hydrogen pre-charge including photographs. Extensometers were used during the SSRT test?. How load and displacement were measured?.

#7) Page 2. Line 75.  How many SSRT test were carried out for each case of study?. Taking into account the high dispersion in results of this type of tests, this is a key issue and authors should include it in the main text.

#8) Page 2. Line 80. For the sake of clarity, please explain why these crosshead speeds were chosen for smooth and notched specimen. In addition, please include the dimensions of the notch depth (line 79).

#9) Page 2. Line 82. Please include the solution pH.

#10) Page 3. Line 111. In my opinion results shown in Figure 2, 3, 4 and 6 are poorly discussed. Please, include a deeper discussion in the main text of such results. In addition, the size of the figures 3e and 3f are too small, please increase the size.

#11) Page 5. Line 137. Please explain in detail how these results are obtained (Figure 5). In addition, include the SSRT curves. In addition, for analysing trends more points (3 points are too few) are needed.   

#12) Page 5. Line 139. Please include numerical values for this statement based on obtained results “all specimens had similar tensile strengths”.

#13) Page 6. Line 159. The “relative index” is not clearly defined in the main text. It is confusing.

#14) Reference section. For the sake of clarity, please add the title of papers in the reference section. Please revise reference [24]. Such a reference appears twice, see line 243 and line 245.

Comments on the Quality of English Language

Minor editing of English language required.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the role of coalesced bainite in enhancing the hydrogen embrittlement resistance of tempered martensitic steels by analyzing the microstructural characteristics and mechanical properties under varying cooling rates. The paper is well-done and should be published.

1. SSRT curves should be given.

2. Results and Discussion section: compare your results with published works.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript may be interesting, but it is missing some information:

1. Based on the literature data, add in the introduction what strength drops were noted for different steels and hydrogen charge levels.

2. Regardless of Fig. 1b, add the values of bainite and martensite fractions for each cooling rate.

3. lines 125-126, Add the information that the dislocation density was determined from Eq. (2) and add the strain values.

4. "This occurred because a high fraction of tempered bainite and a new generation of coalesced bainite at low cooling rates improved the hydrogen embrittlement resistance by restricting hydrogen transport with the low dislocation density." Please refer to the previously determined dislocation density and the fact that the largest decrease in RRA was for the cooling rate increase from 3.5 to 5.5 C/min.

5. Prove that cracks propagated prior austenite grains.

6. Eq. (2) indicates that samples not charged with hydrogen had to be tested as well. Add SEM images of cracks of samples not charged with H to show the effect of hydrogen charging on the degradation (the difference in destruction).

7. The explanation for the sharp decrease in RRA with increasing cooling rate from 3.5 to 5.5 ^oC/min is not convincing, since the dislocation density differed by only 3%

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The article deals with interesting issues related to microstructural problems of tempered martensitic steels in the context of hydrogen embrittlement. The Authors showed that the introduction of the coalesced bainite significantly increased their resistance to hydrogen embrittlement. The article is well prepared, but it has some shortcomings that require improved.

Detailed comments:

Line 23: Introduction: Please do not use collective references without indicating the achievements of individual Authors. At least several achievements from works [3-11] should be indicated.

Line 55: It is required to indicate the metallographic preparation of samples for microstructural observations.

Line 75: Details of static tensile test are missing. Specifying only a testing machine is not sufficient.

Figure 5: Please indicate the standard deviation in Figure 5a and 5b for RNTS.

Figure 6: How were intergranular and transgranular fractures identified? Microstructural studies should be performed. The fracture path cannot be identified solely based on the shape on the fracture surface. The authors did not present an example of a transgranular crack in Figure 6.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

  

Author Response

 Overall Answer: Thanks to your insightful comments, this paper has been greatly improved.

Reviewer 3 Report

Comments and Suggestions for Authors

I have no comments

Author Response

- Overall comment: I have no comment
- Overall Answer: Thanks to your valuable comments, this paper has been greatly improved.

Reviewer 4 Report

Comments and Suggestions for Authors

The Authors have clearly improved the manuscript. However, I still have some concerns about Figure 7. Why is the linear crack form attributed to grain boundary cracks? A straight crack path is typical for transgranular cracks.

Answer 5 should be introduced in the manuscript. 

I suggest organizing the research methodology - the use of paragraphs or bullet points will promote clarity.

 

Author Response

- Overall comment: The Authors have clearly improved the manuscript. However, I still have some concerns about Figure 7. Why is the linear crack form attributed to grain boundary cracks? A straight crack path is typical for transgranular cracks.

- Overall Answer: Thank you for your insightful comment. We understand your concern regarding the linear crack path in Figure 7. While it is true that transgranular cracks often exhibit a straight way, in our case, the cracks are attributed to grain boundary propagation based on several factors. First, the length of the cracks in Figure 7 closely matches the prior austenite grain size seen in the EBSD image of Figure 2, which strongly suggests that the cracks follow the grain boundaries (Ref. [1]). Additionally, in martensitic microstructures, transgranular cracks typically exhibit deflections due to interactions with the internal structure of blocks and laths. However, the linear cracks paths observed in Figures 7(c) and 7(d) do not show these typical deflections, supporting our interpretation that these are grain boundary cracks rather than transgranular cracks.

[1] S. Bechtle, M. Kumar, B. P. Somerday, M. E. Launey, R. O. Ritchie: Grain-boundary engineering markedly reduces susceptibility to intergranular hydrogen embrittlement in metallic materials, Acta Mater., 2009, vol. 57, pp. 4148-4157. https://doi.org/10.1016/j.actamat.2009.05.012.

 

- Comment 1: Answer 5 should be introduced in the manuscript.

- Answer 1: We have added the manuscript as follows:

[…The length of the cracks in Figure 7, which is similar to the prior austenite grain size seen in the EBSD image of Figure 2, suggests that the cracks propagated along the prior austenite grain boundaries. While transgranular cracks typically propagate with deflections due to interactions with martensitic blocks and laths, the linear propagation observed in Figure 7 indicates a strong correlation with grain boundary cracking rather than transgranular cracking [38]…]

 

- Comment 2: I suggest organizing the research methodology – the use of paragraphs or bullet points will promote clarity.

- Answer 2: We organized the experimental procedure section as the reviewer mentioned.

 

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

The article has been improved according to suggestions.