Study on the Effect of Microstructure Gradients Caused by Heat Gradients on Hydrogen Embrittlement Sensitivity in Heavy Forgings
Round 1
Reviewer 1 Report
Review for metals-1617977
Study on Effect of Microstructure Gradient Caused by Heat gradient on Hydrogen Embrittlement Sensitivity of Heavy Forgings
The authors address an interesting research topic for the journal Metals. Overall, I think it could be an innovative and useful study but the article needs to be clarified in some sections. Therefore, some changes should be considered:
Comments and Suggestions for Authors
- Line 28: “The mechanical properties of the steel will decrease due to the interaction of hydrogen with steel,…” In my opinion, it is necessary to reference this statement.
- In the introduction section, there are several references 3-13 indicating the relationship between HE and microstructure. To complete the state of the art, the role of microstructural anisotropy in the hydrogen-assisted fracture should also be included.
- Since the strain rate directly influences on the hydrogen concentration into a material (e.g. 1051/epjconf/201818303015 ; https://doi.org/10.3390/met6010011), please justify the selection of 1x10-5 s-1.
- Figure 8, Tables 3-5: Please, specify what are A, B, C, D, and E samples. Previously, these letters were used to identify points into samples. This aspect should be clarified in the manuscript in order to understand the Discussion Section.
- Tables 3 and 4. What is “H (%)”? Please define correctly.
- Although the number and the selection of references are adequate, it would be advisable to include some papers from the journals of MDPI editorial (Applied Sciences, Materials, Metals, etc.) related to the topic of the manuscript.
- In the next revision, please submit a version without strikethroughs.
Author Response
Manuscript ID: metals-1617977
Article Title: “Study on Effect of Microstructure Gradient Caused by Heat gradient on Hydrogen Embrittlement Sensitivity of Heavy Forgings”
Dear reviewers:
Thank you very much for your helpful comments and suggestions on our manuscript to make it a better work. The manuscript has been carefully revised according to the opinions and suggestions. We have made a detailed description of the problems raised by the reviewers and corrected them in the corresponding position in the manuscript. The revised part of the manuscript was yellow highlighted. The specific replies and modifications are as follows.
Reviewer # 1
Concern # 1: Line 28: “The mechanical properties of the steel will decrease due to the interaction of hydrogen with steel,…” In my opinion, it is necessary to reference this statement.
Author response:
Thank you very much for your comments, we very much agree with you. We will add some references here.
Concern # 2: In the introduction section, there are several references 3-13 indicating the relationship between HE and microstructure. To complete the state of the art, the role of microstructural anisotropy in the hydrogen-assisted fracture should also be included.
Author response:
Thanks for your comments, adding a description of the role of microstructural anisotropy in the hydrogen-assisted fracture can complete the state of the art. Enhance the logic and rationality of the article. We will add a description of this part to the article.
Concern # 3: Since the strain rate directly influences on the hydrogen concentration into a material (e.g. 1051/epjconf/201818303015 ; https://doi.org/10.3390/met6010011), please justify the selection of 1x10-5 s-1.
Author response:
Thank you for your comments, the selection of the stretching rate is very important. We took this into account when designing our experiments and searched for relevant literature and materials, but they were not given in the article, and we will add the content of this section in the revised version.
Concern # 4: Figure 8, Tables 3-5: Please, specify what are A, B, C, D, and E samples. Previously, these letters were used to identify points into samples. This aspect should be clarified in the manuscript in order to understand the Discussion Section
Author response:
We are very sorry that we have shown the meaning of these dots (R174-175) in an inconspicuous place in the article. Ambiguous samples can cause serious trouble for readers. We will add an introduction to this part (R306-309) at the appropriate place in the text to make the text clearer and more reasonable.
Concern # 5: Tables 3 and 4. What is “H (%)”? Please define correctly.
Author response:
We are very sorry for the ambiguity in understanding caused by our abbreviated writing. The meaning we want to express here is Elongation and Section shrinkage of hydrogen-charged and non-hydrogen-charged samples. We will change the writing here to make the article clearer
Concern # 6: Although the number and the selection of references are adequate, it would be advisable to include some papers from the journals of MDPI editorial (Applied Sciences, Materials, Metals, etc.) related to the topic of the manuscript.
Author response:
Thank you very much for your suggestion, we understand and support your suggestion very much. We will appropriately increase the citations of the papers in MDPI journals in the article.
Concern # 7: In the next revision, please submit a version without strikethroughs.
Author response:
We are very sorry that due to our negligence, there was a problem with the uploaded version. We checked the manuscript in both PDF and WORD versions, and found that the phenomenon you described appeared in the PDF. Once again, we are very sorry for affecting your reading experience. We will upload the reworked manuscript with care and guarantee that this problem will never occur.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors,
I read the authors' papers interestingly and thank you for your many responses. I can feel that you have put a lot of effort for this paper. Unfortunately, this paper has some problems.
The expression "two drops of Na2S " in R207 is not appropriate. Quantitative addition amount should be presented in Si units.
Figure 7 shows the microstructure of 5 samples. However, the quality is very low, and the magnification is too low to distinguish between martensite, bainite, and pearlite. In addition, it is difficult to clearly identify the characteristics of each microstructure. In order to see the simulation results and the suitability of the microstructure of the sample, the phase fraction for each sample seems to be necessary. Therefore, please present a figure of the microstructure characteristics of each phase and microstructure in the figure and the microstructure with a higher magnification than the current one.
In general, the cross-sectional area is considered due to brittle fracture in the case of a tensile specimen of hydrogen embrittlement. Generally, the term is written as reduction of area (RA). However, in this paper, it is marked as section linkage, so I am worried that it will confuse readers. Please correct it to "Reduction of area".
The question has not been resolved in the hydrogen emblem sensitivity index mentioned earlier. Samples of A and C were found to have high hydrogen intensity index and brittle fracture sections were observed. However, the test specimens of B, D, and E are also confirmed to be very high, with , and exceeding 50% and 30%. Despite the high index, they need to explain why brittle fracture did not occur.
In this paper, it was presented based on carbide, grain size, and dislocation density due to the influence of hydrogen embrittlement. However, the only data that can clearly see their impact is grain size. Evidence is needed related to carbide and dislocation density. There are also several papers that say that if hydrogen becomes trap by dislocation, strength hardening occurs. However, in this paper, there is a phenomenon that the strength is lowered rather than non-precharged samples. These papers should be need to clarified as mentioned point.
In addition, the effects of fracture and trap for each microstructure were explained based on hydrogen diffusion. However, hydrogen diffusion for each microstructure is expected to be a different level, but their evidence seems to be insufficient. For example, it seems that evidence is need such as the depth of the brittle fraction layer of the sample of A-E and the amount of hydrogen trapped.
Author Response
Manuscript ID: metals-1617977
Article Title: “Study on Effect of Microstructure Gradient Caused by Heat gradient on Hydrogen Embrittlement Sensitivity of Heavy Forgings”
Dear reviewers:
Thank you very much for your helpful comments and suggestions on our manuscript to make it a better work. The manuscript has been carefully revised according to the opinions and suggestions. We have made a detailed description of the problems raised by the reviewers and corrected them in the corresponding position in the manuscript. The revised part of the manuscript was yellow highlighted. The specific replies and modifications are as follows.
Reviewer # 2
Concern # 1: The expression "two drops of Na2S " in R207 is not appropriate. Quantitative addition amount should be presented in Si units.
Author response:
Thank you very much for your suggestion. The "two drops" we have expressed is indeed not rigorous. To publish an article in a good journal does need to ensure that the article is rigorous enough. We reviewed the original data records and converted the sodium sulfide content to SI in the article.
Concern # 2: Figure 7 shows the microstructure of 5 samples. However, the quality is very low, and the magnification is too low to distinguish between martensite, bainite, and pearlite. In addition, it is difficult to clearly identify the characteristics of each microstructure. In order to see the simulation results and the suitability of the microstructure of the sample, the phase fraction for each sample seems to be necessary. Therefore, please present a figure of the microstructure characteristics of each phase and microstructure in the figure and the microstructure with a higher magnification than the current one.
Author response:
We are very sorry that the picture here is not clear. We used standard metallographic etching techniques and magnifications for easy viewing. It is only for the preliminary determination of the microstructure, and the specific phase score cannot be obtained. At the same time, this purpose is also to verify the accuracy of the A-E point microstructure obtained by the simulation. We have replaced clear pictures in the article, and the replaced pictures can clearly distinguish various microstructure.
Concern # 3: In general, the cross-sectional area is considered due to brittle fracture in the case of a tensile specimen of hydrogen embrittlement. Generally, the term is written as reduction of area (RA). However, in this paper, it is marked as section linkage, so I am worried that it will confuse readers. Please correct it to "Reduction of area".
Author response:
Thank you very much for your suggestion, the content of this part is really not rigorous enough, we will correct the inappropriate vocabulary in this part of the paper.
Concern # 4: The question has not been resolved in the hydrogen emblem sensitivity index mentioned earlier. Samples of A and C were found to have high hydrogen intensity index and brittle fracture sections were observed. However, the test specimens of B, D, and E are also confirmed to be very high, with , and exceeding 50% and 30%. Despite the high index, they need to explain why brittle fracture did not occur.
Author response:
Thank you very much for your comments, the test specimens of B, D, and E do have high sensitivity, but some traces of brittle fracture can still be observed in the fracture images. We will add details to the original fracture images, annotate some features, and also add fracture descriptions to the test specimens of B, D, and E.
Concern # 5: In this paper, it was presented based on carbide, grain size, and dislocation density due to the influence of hydrogen embrittlement. However, the only data that can clearly see their impact is grain size. Evidence is needed related to carbide and dislocation density. There are also several papers that say that if hydrogen becomes trap by dislocation, strength hardening occurs. However, in this paper, there is a phenomenon that the strength is lowered rather than non-precharged samples. These papers should be need to clarified as mentioned point.
Author response:
We are very sorry for the lack of evidence for carbide and dislocation densities due to our lack of accurate descriptions in the text. Dislocations can be divided into geometrically necessary dislocations(GND) and statistical storage dislocations(SSD). The GND can be calculated by KAM. The formula is ρGND = 2θ/μb (μ is the EBSD laboratory Select the step length, θ is the average value of the local misorientation in the selected area, calculated by KAM). In this study, using EBSD's OIM software, the GND map were proposed to characterize the dislocation density information. The brighter the color in the GND diagram, the greater the dislocation density.
Regarding the microstructure, different microstructures must have different distributions of carbides. Due to the fact that there are many kinds of carbides and different morphologies, the effects are not the same. In this study, no in-depth discussion is made, but it is proposed that the difference in carbides may be one of the reasons for the difference in hydrogen embrittlement susceptibility.
For the strength hardening you mentioned, it's the effect of hydrogen on the steel that doesn't necessarily make it strength harden. In the study of other types of steel by many scholars [1,2,3], the material does not have strength hardening under the influence of hydrogen, and even many of them show a reduction in strength.
[1]Shibata, A.; Takeda, Y.; Kimura, Y.; Tsuji, N. Hydrogen-Related Fracture Behavior under Constant Loading Tensile Test in As-Quenched Low-Carbon Martensitic Steel. Metals 2022, 12, 440. https://doi.org/10.3390/met12030440
[2]Zhang, C.; Yu, Hang.; Zhi, H.H.; Antonov, S.; Su, YJ. Twinning behavior and hydrogen embrittlement of a pre-strained twinning-induced plasticity (TWIP) steel. Corros. Sci. 2021, 192, 109791. https://doi.org/10.1016/j.corsci.2021.109791
[3] Momotani, Y.J.; Shibata, A.; Tsujia, N.; Hydrogen embrittlement behaviors at different deformation temperatures in as-quenched low-carbon martensitic steel. Int. J. of Hydrog. Energy 2022, 47,3131-3140. https://doi.org/10.1016/j.ijhydene.2021.10.169
Concern # 6: In addition, the effects of fracture and trap for each microstructure were explained based on hydrogen diffusion. However, hydrogen diffusion for each microstructure is expected to be a different level, but their evidence seems to be insufficient. For example, it seems that evidence is need such as the depth of the brittle fraction layer of the sample of A-E and the amount of hydrogen trapped.
Author response:
Thank you very much for your comments, the description of the sample brittle layer depth is indeed missing from the manuscript and we are sorry for that. We have calculated and consulted the literature before the experiment. The maximum solid solubility of hydrogen atoms in iron is 2.68ppm. In the experiment, because the electrochemical reaction is microscopically inhomogeneous, it is difficult to strictly quantitatively control the hydrogen concentration in the metal. Therefore, we adopted a long hydrogen charging time and a large current density to nearly saturate the hydrogen in the sample. The diffusion coefficient of hydrogen in various tissues is 0.15-3x10-6cm2/s [1-4], and the thickness of the sample is 1mm. According to Fick's theorem, after 60min of hydrogen charging at this current density, the hydrogen concentration in the metal will reach approximately saturation. Therefore, the hydrogen concentration of the samples of different tissues is in a saturated state.
[1]Pereira, V.; Hincapie-Ladino, D,; Nishikawa, L.P.; Goldenstein, H. Effect of Microstructure on Hydrogen Diffusivity, Trapping and HIC Resistance in Two API X65 Steels. OTC Brasil. 2017,24-26. https://doi.org/10.4043/28154-MS
[2]Yang, J.; Huang, F.; Guo, Z.; Rong, Y.; Chen, N. Effect of retained austenite on the hydrogen embrittlement of amedium carbon quenching and partitioning steel with refined microstructure. Mater. Sci. Eng. A 2016, 665, 76-85. https://doi.org/10.1016/j.msea.2016.04.025
[3]López-Martínez, E.; Vergara-Hernández, H.J.; Flore,s O.; Campillo, B. Hydrogen diffusivity in the welding zone of two high strength experimental microalloyed steels. ISIJ Int. 2015, 55, 2435-2442. https://doi.org/10.2355/isijinternational.ISIJINT-2015-259
[4]Araújo, D.F.; Vilar, E.O.; Palma, C.J. A critical review of mathematical models used to determine thedensity of hydrogen trapping sites in steels and alloys. Int. J. Hydrogen Energy 2014, 39, 12194-12200. https://doi.org/10.1016/j.ijhydene.2014.06.036
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The paper was improved, and only one question should be still clarified:
- Tables 3 and 4. Please, clarify the units used in the manuscript for elongation and RA (%, mm, ....).
Author Response
Manuscript ID: metals-1617977
Article Title: “Study on Effect of Microstructure Gradient Caused by Heat gradient on Hydrogen Embrittlement Sensitivity of Heavy Forgings”
Dear reviewers:
Thank you very much for your helpful comments and suggestions on our manuscript to make it a better work. The manuscript has been carefully revised according to the opinions and suggestions. We have made a detailed description of the problems raised by the reviewers and corrected them in the corresponding position in the manuscript. The revised part of the manuscript was yellow highlighted. The specific replies and modifications are as follows.
Reviewer # 1
Concern # 1: Tables 3 and 4. Please, clarify the units used in the manuscript for elongation and RA (%, mm, ....).
Author response:
Thank you very much for your comments. Data is very important as valid evidence to support the paper. We will carefully check the data of the article and indicate the unit. Once again, I apologize for the impreciseness of the paper.
Author Response File: Author Response.pdf
