Overcoming Strength-Ductility Trade-Off at Cryogenic Temperature of Low Carbon Low Alloy Steel via Controlling Retained Austenite Stability
Round 1
Reviewer 1 Report
This manuscript deals with the strength-ductility balance at cryogenic temperatures of 0.07%C-0.36%Si-1.78%Mn-0.51%Ni-0.30%Mo-0.78%Nb-0.02%Ti steel. An interesting result that controlling retained austenite stability enhances the balance. This manuscript is well structured although it is short. The topic is of interest and will bring the attention of the scientific community.
Author Response
We have corrected the errors (highlighted with yellow colour).
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors
My suggestions and questions are marked in attached file.
Thank you.
Best regards
Comments for author File: Comments.pdf
Author Response
Point 1: Why the RA in your steel exhibits thermal/mechanical stability up to -196℃. What is the carbon content in the RA? Please comments and assess the %C in RA?
Response: The main reason why retained austenite can be stable to low temperature (-196℃) is ascribed to the highly enrichment of Mn and C during the two-step intercritical heat treatment. Atom probe tomography analysis revealed that the Mn and C contents were ~6.6wt% and 0.4wt%. In addition, the obtained retained austenite was very fine, the average size was ~300 nm in diameter. Hence, the retained austenite was very stable thermally up to -196°C. These results were reported by us in references [8] and [15].
Point 2: In my opinion, determination of impact toughness is necessary to confirm 'ductility' of the steel in such low temperature. As you know the behaviour of the material in static and dynamic conditions is mostly different!
Response: The author also agrees with the reviewers. But the impact and tensile tests are two different experiments, and the strain rate is also different. This study focuses on the matching of strength and plasticity in low temperature tensile process, and does not involve impact test. However, we will also consider the reviewers' suggestions.
Point 3: Why you did not examine the microstructure of the tensiled/deformed specimens using SEM to confirm TRIP effect? (RA transforms fresh martensite M)
Response: Because the SEM images can only reflect the local micro-area and the field of view is limited, it can not essentially reflect the content of retained austenite in the actual sample. Therefore, in this study, the saturation magnetization method is used to determine the content of retained austenite in deformed and undeformed samples, thus proving the TRIP effect.
Author Response File: Author Response.pdf
Reviewer 3 Report
Review for metals-1045706
Overcoming strength-ductility trade-off at cryogenic temperature of low carbon low alloy steel via controlling retained austenite stability
The authors address an interesting research topic for the journal Metals. In addition, the scientific contribution is relevant and may be appealing to many researchers. Despite this, some recommendations for publication should be considered:
- For the sake of clarity, please include in section 2 a new figure explaining the testing process.
- Since the cryogenic tensile test is not usually done, please include an image in the main text.
- Please, justify the selection of the cryogenic temperatures.
- True stress – true strain curves can only be represented up to UTS point since the necessary equations are derived from the volume constant hypothesis. In this way, Figure 2b should be corrected.
- Please, clarify in the main text how the work hardening curves were obtained in Figure 2b.
- Several images of retained austenite is advisable to justify the different values of 21.1 % (undeformed samples) vs. 4.5-5.2% (-80, -120 ºC) vs. 1.8% (-160 ºC). In this sense, what is the percentage for -196 ºC?
- Fracture analysis is not clear. Please, indicate where the images were obtained from? The fractography could vary depending on the place on the sample section considered.
- Figure 4b seems to be obtained with a lower magnification than the other fractographs. Please, include the magnification for all the fractographs, and confirm that all of them are the same.
- In my opinion, there are too many self-references from authors. Please, include more references from other research groups.
Author Response
Point 1: For the sake of clarity, please include in section 2 a new figure explaining the testing process. Since the cryogenic tensile test is not usually done, please include an image in the main text. Please, justify the selection of the cryogenic temperatures.
Response: The detailed introduction of cryogenic tensile test process has been given in the revised manuscript, please check.
Point 2: True stress – true strain curves can only be represented up to UTS point since the necessary equations are derived from the volume constant hypothesis. In this way, Figure 2b should be corrected. Please, clarify in the main text how the work hardening curves were obtained in Figure 2b.
Response: The true stress-strain curves has been modified. In addition, the calculation of work hardening curves is based on the Hollomon equation, and the related explanation has been given in this paper.
Point 3: Several images of retained austenite is advisable to justify the different values of 21.1 % (undeformed samples) vs. 4.5-5.2% (-80, -120 ºC) vs. 1.8% (-160 ºC). In this sense, what is the percentage for -196 ºC?
Response: The percentage of retained austenite obtained from the sample deformed at -196 ºC is zero according to the calculation results from magnetization curves, which is showed in Figure 3.
Point 4: Fracture analysis is not clear. Please, indicate where the images were obtained from? The fractography could vary depending on the place on the sample section considered. Figure 4b seems to be obtained with a lower magnification than the other fractographs. Please, include the magnification for all the fractographs, and confirm that all of them are the same.
Response: The images were obtained from the fracture surface after tensile test, and the macroscopic fractographs of samples tested at different temperatures were given in Figure 5. We have confirmed that all of the fractographs are the same.
Point 5: In my opinion, there are too many self-references from authors. Please, include more references from other research groups.
Response: The authors have revised it according to the reviewer's suggestion.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Review for metals-1045706
Overcoming strength-ductility trade-off at cryogenic temperature of low carbon low alloy steel via controlling retained austenite stability
The authors address an interesting research topic for the journal Metals. In addition, the scientific contribution is relevant and may be appealing to many researchers. Despite this, some recommendations for publication should be considered:
- Since the cryogenic tensile test is not usually done, please include a real image in the main text. Could authors provide real photography of the testing?
- Please, justify in the main text the selection of the cryogenic temperatures. Why authors choose these temperatures?
- For the sake of clarity, a better explanation should be included in the main text of the values of retained austenite from Figure 4: 21.1 % (undeformed samples) vs. 4.5-5.2% (-80, -120 ºC) vs. 1.8% (-160 ºC).
- Taking into account that the fractography could vary depending on the place on the sample section considered, authors should be reflected where the fractographs of Figure 6 were taken (in the center of the fracture surface, in the middle, etc.).
- The new Figure 5 suggests that the samples exhibit a circumferential notch. Please confirm if it is the case.
- It is curious that the lower the temperature during the test, the more isotropic the fracture surface. To improve the fractographical analysis, could you include a paragraph extending this information in the main text?
Author Response
Response to reviewer's comments
Dear editor and reviewers:
Thank you for reviewing our paper and giving us helpful advices again. We have corrected the errors (highlighted with yellow colour). The detailed responses to the comments are listed as follows:
Reviewer #3:
Point 1: Since the cryogenic tensile test is not usually done, please include a real image in the main text. Could authors provide real photography of the testing?
Response: Because the cryogenic tensile test is carried out in a sealed low temperature control cabin, only the shell of the low temperature chamber can be seen in the actual test photos, as shown in the figure below. Therefore, the paper only gives the schematic diagram of the cryogenic tensile test.
Point 2: Please, justify in the main text the selection of the cryogenic temperatures. Why authors choose these temperatures?
Response: The reason why the tensile test starts at -80°C is that the material has excellent room temperature properties and good low temperature toughness. Please check reference [8]. It has been added in the section of Material and Methods of Experiments.
Point 3: For the sake of clarity, a better explanation should be included in the main text of the values of retained austenite from Figure 4: 21.1 % (undeformed samples) vs. 4.5-5.2% (-80, -120 ºC) vs. 1.8% (-160 ºC).
Response: Because the calculation method of austenite in this paper is magnetic measurement, the specific calculation method has been introduced in detail in reference [14], thus this paper does not give a more detailed description of this method. However, according to the suggestions of the reviewers, the author also makes a proper exposition in the paper.
Point 4: Taking into account that the fractography could vary depending on the place on the sample section considered, authors should be reflected where the fractographs of Figure 6 were taken (in the center of the fracture surface, in the middle, etc.).
Response: In Figure 5, the author marks the observation position of the high magnification fracture morphology in Figure 6.
Point 5: The new Figure 5 suggests that the samples exhibit a circumferential notch. Please confirm if it is the case.
Response: Figure 5 shows not circumferential notch, but fracture section of tensile sample after diameter shrinkage.
Point 6: It is curious that the lower the temperature during the test, the more isotropic the fracture surface. To improve the fractographical analysis, could you include a paragraph extending this information in the main text?
Response: What the author wants to state is that it is not accurate for the reviewers to consider that the isotropic appears with the decrease of temperature. The appearance of petal tearing at -80, -120 and -160℃ is due to the three-dimensional stress in the tensile process, rather than the structure of the material itself.
Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
Review for metals-1045706
Overcoming strength-ductility trade-off at cryogenic temperature of low carbon low alloy steel via controlling retained austenite stability
In my opinion, there are still several aspects that need to be clarified before this article is published:
- The fracture surfaces in Figure 6 reflect anisotropic fracture in the case of Figure 6a, 6b, and 6c, but not in the case of Figure 6d, which is isotropic. The type of fracture surface (isotropic or anisotropic) could occur by the material or by the conditions of testing, or by the synergic influence of both aspects. In my opinion, a discussion of this fact is advisable in the main text.
- Authors affirm that, according to fracture analysis, “This indicated that after large uniform deformation, the plasticity during necking decreased at -196°C”. However, in the true stress-strain curve shown in Figure 3b, the more level of plasticity is reached in the case of -196ºC, just the opposite of the previous sentence.
- I insist that a better explanation should be included in the main text of the values of retained austenite from Figure 4: 21.1 % (undeformed samples) vs. 4.5-5.2% (-80, -120 ºC) vs. 1.8% (-160 ºC). Where is the explanation in the main text? Although the method is explained in detail in reference [14], the authors should explain it briefly here to facilitate understanding of the text by readers of the journal Metals.
Author Response
Please see the attachment. Thank you very much.
Author Response File: Author Response.pdf
