Effect of Deep Cryogenic Time on the Microstructure and Mechanical Property of Cr-Mn-Si High-Strength Alloy Steel

Round 1

Reviewer 1 Report

This paper deals with the effect of cryogenic heat treatment after quenching and tempering with various heat treatment times on the mechanical properties and microstructures of steels. I cannot recommend this paper because of the following issues:

1. The authors have used the cryogenic heat treatment 'AFTER' the quenching and tempering. Is there any specific reason for this? The alloy used for this study is known to have almost no residual austenite after the quenching and tempering and the reviewer wonder why the cryogenic heat treatment is applied after making the semi-stable microstructure by the tempering treatment.

2. In figures 3 and 4, the stress-strain behaviors of the samples with different cryogenic heat treatment times are presented. They look more or less identical with just little variation at stresses near the fracture. It is recommended to include the behavior of the sample just after the quenching and tempering, without the cryogenic heat treatment if the cryogenic heat treatment has some effect. The reviewer seriously wonders if there is a pronounced effect of the cryogenic heat treatment.

3. The discussions for the microstructural changes due to the different cryogenic heat treatment time is not convincing. The authors provided XRD patterns that do not show any peak of the austenite phase but the authors stated that the reason for the different microstructures is from the retained austenite. The SEM microstructures of the samples cryogenically heat treated for 1 and 3 hours, shown in Fig.7, look nearly identical. In the same SEM image, the authors indicated the retained austenite phase. Was that phase also detected in the EBSD analysis?

4. Can the microstructure of the studied steel change pronouncedly at such a low temperature for a few hours during the cryogenic treatments? Are there any reference paper that showed similar behavior for the steels cryogenic treated after tempering?

4. The objective of the paper is not clearly described in the introduction section. Is the main objective of the paper to exam the effect of cryogenic heat treatment time?  

Extensive English editing is needed. The reviewer is pretty sure that at least some parts of the paper are directly pasted from the translation software/website. There are many strange expressions such as 'tissues', 'home and abroad', 'domestic' and so on.  Sentences are in general unnecessarily long. As a non-native English speaker, the reviewer can infer the original meanings but many expressions are wrong and need to be revised.  

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Submitted manuscript is devoted to interesting subject – the possibility to improve physical properties of Cr-Mn-Si steel by the cryogenic treatment. The title of the work is confusing: “Effect of deep cryogenic treatment on the microstructure and property of Cr-Mn-Si high-strength alloy steel” – what property is most important, if they are “properties” it is better to mention them.

Work has serious problem with metrological part of description – neither Tables, no graphs have error bars or corresponding discussion of the experimental errors. For example, it is quite difficult to evaluate the average size of the grains for such non-uniform structures with indicated accuracy (Fig. 10).

Figure 9 describes schematics of the interaction mechanism between the second phase particles and dislocation: looping mechanism. However, without careful analysis of the data of transmission electron microscopy proposed mechanism is simply a guess work. Please, show the lamellar martensite data and their analysis. Authors must propose the phase diagram indicating phase transitions in order to clarify possible microscopic mechanism of the processes at low temperature treatments.

Manuscript contains a lot of misprints and lost intervals. The size of the symbols is very different for different figures and some of them are not readable.

Moderate editing of English language required

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The article is interesting.

But there are some questions about it.

1. Why were such temperatures and times chosen for heat treatment and cryogenic treatment?

2. What are the test statistics for mechanical properties? Many values are very close to each other, for example 404 MPa and 392 MPa.

3. Figure 3 does not show the curve for the 3H test.

4. In Figure 4, the histogram does not show the measurement error and, accordingly, it is not clear how significant the changes are or not.

5. What is the reason for the significant increase in the proportion of low-angle boundaries during 2 and 3-hour cryogenic treatment? While the proportion of high-angle boundaries does not change so significantly.

6. Figure 13 The units of change in the density of dislocations shown in the figure are not very clear.

7. What is the reason for the peak in dislocation density at 2H treatment and then the observed decrease? While broadening and grain size analysis tell a different story.

8. What changes will be observed in the parameters of the structure and properties if the cryogenic treatment time is increased?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript discusses the effects of deep cryogenic treatment on Cr-Mn-Si alloyed high-strength steel. It states that after quenching and tempering (Q-T), the steel was subjected to deep cryogenic treatment, and the resulting microstructure and properties were studied. After proper revisions, the manuscript may deserve to publish in this journal.

While the manuscript provides a general overview of the effects of deep cryogenic treatment on the microstructure and mechanical properties, it needs more specific quantitative data, statistical analysis, and detailed explanations. It would be beneficial to offer insights into the underlying mechanisms through which deep cryogenic treatment influences the microstructure, and mechanical properties of the alloyed steel would provide a more comprehensive understanding of the research findings.

Discussing the potential implications of these observations on the mechanical properties, such as impact toughness and fracture resistance, would enhance the manuscript. Furthermore, providing an interpretation of the observed changes based on underlying mechanisms and existing theories in the field would add depth to the analysis.

Explaining the underlying mechanisms, such as carbon diffusion, precipitation strengthening, and fine crystal strengthening, would add depth to the analysis.

Please provide specific numerical values, correlations between grain boundary changes and mechanical properties, or an interpretation of the observed phenomena based on underlying mechanisms.

Providing statistical analysis and discussing the significance of the observed changes in dislocation density on the material's mechanical properties would strengthen the claims. Additionally, a more thorough explanation of the underlying mechanisms, such as the interplay between dislocation strengthening and matrix stabilization, would add depth to the analysis.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The paper is improved. However, I still cannot recommend this paper to be published due to the following reason. It seems that the authors did conduct only one experiment for one condition for the tensile test. When looking at the very tiny deviation between the test samples, I am not sure if the deviation observed is from the experimental error or from the effect that the authors stated. I would like to recommend the authors to do at least 5 tensile tests for each heat treatment condition and provide statistical results, e.g. with error bars in the yield strength, tensile strength and etc.. 

Personally I do respect the hard work done by the authors but I cannot find significant change in the microstructure, fractography and tensile behavior in terms of the heat treatments. I think it is necessary to do some statistical analysis at least on the tensile behavior to support the conclusions which were drawn by the authors.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Figure 9 describes schematics of the interaction mechanism between the second phase particles and dislocation: looping mechanism. However, without careful analysis of the data of transmission electron microscopy proposed mechanism is simply a guess work. Please, show the lamellar martensite data and their analysis. Authors must propose the phase diagram indicating phase transitions in order to clarify possible microscopic mechanism of the processes at low temperature treatments. 

Moderat corrections are necessary

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

All my previous concern has been addressed by the authors. 

Reviewer 2 Report

Authors did not respondmy questions. "Speculations" should not be used in scientific publication. Proposed mechanism is not justified - this part must be either deleted or supported by TEM.

English is limitd by it is almost understandable.

Author Response

The authors have taken your suggestion seriously, removed the schematic diagrams and related content that could not be represented by TEM. Thank you very much for your suggestion to eliminate speculation without experimental verification in future work!