Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment
Round 1
Reviewer 1 Report
This paper studied the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. The work is somehow good, while some improvements need to be conducted.
1) For the specimen in Figure 1, I think the authors designed the geometry according to ASTM or other standards, please provide the standard number.
2) Please provide the preparation procedure of SEM and other microstructural specimens.
3) In Line 124 "Chemical composition analysis using EDX showed that for both TMT and HT specimens the critical crack initiating inclusions were always oxides of type AlCaO", how could the authors give this conclusion without any EDX data?
4) The author mentioned the EDX technique was applied in Line 111, while no such data at all in the manuscript.
5) For the data in Figure 5, how many data points have you used? Data deviation should be provided if these are averaged data.
6) In the conclusion part, there are six points, which is not good. It should be condensed to 3-4 points.
Author Response
Dear Reviewer 1,
First, thank you very much for your effort. We really appreciate your constructive feedback. Therefore, we have considered your comments. Please find our changes of the paper and some more feedback below. Thank you once again for investing your valuable time to review the paper!
Best regards
(Please see the attachment for detail)
Response to Reviewer 1 Comments
Comment 1: For the specimen in Figure 1, I think the authors designed the geometry according to ASTM or other standards, please provide the standard number.
Response 1: The technical drawing of specimen geometry is according to DIN 406-10 and 406-11. The general tolerances of specimen is based on the DIN ISO 2768-m. A new technical drawing of the specimen geometry has been added to the manuscript and the respective standard numbers are mentioned in the description now.
Comment 2: Please provide the preparation procedure of SEM and other microstructural specimens
Response 2: Actually, there is no specific procedure for the fracture surfaces before SEM observations. The fractured specimens went directly to the SEM without further treatment. In the reviewed version we added now microstructural pictures of sections. These were polished by standard methods and etched with Nital.
Comment 3: In Line 124 "Chemical composition analysis using EDX showed that for both TMT and HT specimens the critical crack initiating inclusions were always oxides of type AlCaO", how could the authors give this conclusion without any EDX data?
Response 3: The EDX result of an inclusion as an example was added to the manuscript to support the statement.
Comment 4: The author mentioned the EDX technique was applied in Line 111, while no such data at all in the manuscript.
Response 4: Please see response above.
Comment 5: For the data in Figure 5, how many data points have you used? Data deviation should be provided if these are averaged data.
Response 5: Figure 5 (now Fig. 7) shows the area of critical inclusions of each tested specimens (8 HT specimens and 8 TMT specimens). Therefore no average data is presented in this diagram.
Comment 6: In the conclusion part, there are six points, which is not good. It should be condensed to 3-4 points.
Response 6: We agree that the conclusion part was rather long. We shortened and reduced it to 4 points.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Since thermomechanical treatment (TMT) in the temperature range of dynamic strain ageing (DSA) can increase the fatigue limit of quenched and tempered steels, this study considered the influence of TMT on the shapes, sizes and positions of the non-metallic inclusions and the internal crack propagation behavior after TMT and HT. The results might be useful for the heat treatment methods, however, some of the problems in the contents need to be modified:
1. Authors should describe the method a little detail in the abstract.
2. Section 2 materials and experimental procedures was insufficient, A. authors had better added the mechanical and physical properties of the specimen in the content. B. authors should state how to make the specimen before the experiments? And are there any production requirements for preparing the specimen?
3. In section 3, authors had better described the mechanisms of the results between TMT and HT in Fig. 2.
4. The content in the line 187 to the line 188 stated that “… TMT strengthening mechanism of a more stable dislocation structure around inclusions.”, authors had better to explain “the stable dislocation structure” by graph or other illustration in the content.
5. Some English writing style in the content should be modified or the meaning of the contents are not easily to understand for the reader, authors should check the content again, for example, A. line 22 stated that “… When the fisheye size reaches a certain threshold, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types. What is the certain threshold in the research? B. from line 236 to 237, “A longer soaking time at TMT temperature could enable this. Further investigations are necessary to proof this.”. what is this? The statements were not clearly, please check and modify the contents in this research like the above statements.
Author Response
Dear Reviewer 2,
First, thank you very much for your effort. We really appreciate your constructive feedback. Therefore, we have considered your comments. Please find our changes of the paper and some more feedback below. Thank you once again for investing your valuable time to review the paper!
Best regards
(Please see the attachment for detail)
Response to Reviewer 2 Comments
Comment 1: Authors should describe the method a little detail in the abstract.
Response 1:
The TMT is now described with more detail in the abstract. Additionally, a more detailed description of the TMT was added to section 2.
Comment 2: Section 2 materials and experimental procedures was insufficient, A. authors had better added the mechanical and physical properties of the specimen in the content. B. authors should state how to make the specimen before the experiments? And are there any production requirements for preparing the specimen?
Response 2: Thank you for the remark. The material was delivered in a soft-annealed state, in the form of round bars, from which a near-net-shape geometry of the specimens was machined by turning. After the quenching and tempering, the specimen geometries were again machined to avoid dimensional changes after heat treatment. The quenched and tempered specimen has the 0.2% yield strength of 1500 MPa and the ultimate tensile strength of 1900 MPa. Both mechanical properties and the machining procedure of the specimen before the experiments were added to the manuscript. There is no specific production requirements for preparing the specimens.
Comment 3: In section 3, authors had better described the mechanisms of the results between TMT and HT in Fig. 2.
Response 3: Done
Comment 4: The content in the line 187 to the line 188 stated that “… TMT strengthening mechanism of a more stable dislocation structure around inclusions.”, authors had better to explain “the stable dislocation structure” by graph or other illustration in the content.
Response 4: The exact mechanism of the stabilisation is still not clear. It is part of an ongoing study to extract TEM lamellae near critical inclusions of TMT and HT specimens in order to find more information about these dislocation structures. However, at the moment we cannot give a reasonable illustration.
Comment 5: Some English writing style in the content should be modified or the meaning of the contents are not easily to understand for the reader, authors should check the content again, for example, A. line 22 stated that “… When the fisheye size reaches a certain threshold, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types. What is the certain threshold in the research? B. from line 236 to 237, “A longer soaking time at TMT temperature could enable this. Further investigations are necessary to proof this.”. what is this? The statements were not clearly, please check and modify the contents in this research like the above statements.
Response 5: Thank you for your remarks. We carefully checked the manuscript for unclear statements and corrected them accordingly.
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear authors,
The reviewed work: Comparison of the internal fatigue crack initiation and propagation behavior of quenched and tempered steel with and without thermomechanical treatment by A. Khayatzadeh et al. (metals-1745891) concerns a very important and difficult area of science, namely the fatigue strength of the material. The importance of the subject is all the greater as it concerns the steel currently used for very important components of drive units in motor vehicles, ships and airplanes.
The research were carried out and presented very interesting, however in an article I noticed some shortcomings, which does not affect my positive opinion. The main drawback of the reviewed work is deficiencies in the description of the initial state of the material (where it came from, chemical composition, properties), thermomechanical treatments, and their effects (1). No descriptions or photos of the microstructures of the material as delivered and after TMT and HT were presented (2). It is not known what TMT consisted of, what its parameters and effects were (3). A mere reference to work [22] is not sufficient here. There was also no information on the basis of which standards / recommendations or own research were used to determine the conditions of heat treatment treatments. There is no information about what the samples 1-16 are and what were the differences in their processing, since this resulted in differences in fatigue properties (Fig. 2).
The presented article concerns almost 50% of the issue of the presence of non-metallic inclusions in the tested material and their influence on its strength properties. However, in the work there is not even a hint of an attempt to refer to the purity of the material in terms of the amount and distribution of these impurities (4).
In my opinion, the presented results are very important, but their publication in the form of an article should be possible only after removing the above-mentioned shortcomings, which result from the incomplete description.
I also recommend making the following corrections:
Fig. 1 – The drawing was made contrary to the rules of imaging and dimensioning of objects in the technical drawing. Visible lack of thought regarding the recording of the accuracy of the dimensions listed.
106 – The stress amplitude of 775 MPa … - no literature indication for such a statement.
110 - … were used to 110 measure the size of inclusions … - by what method or program the research was carried out.
Fig. 2 – The figure should be presented vertically; usually the individual factors are presented on the horizontal axis and the answer to them vertically. No second axis or value is defined for each sample. What is the physical sense of specifying the number of cycles with an accuracy of 4 decimal places, since it is only to an accuracy of 1 decimal place?
123 - … failed… - this word is of a general nature, please write specifically, e.g. "cracked".
Fig. 3,4,5,7 – Please remove the frames under the photos and transfer the information contained therein to the photo, remembering about their visibility.
Fig. 5 – Invalid symbol describing the cycle amplitude (sa). In descriptions and formulas, root expressions are written in a power form with an exponent of an appropriate value. Please improve the readability of the unit indicators on both axes. Also in Fig. 6 and 7.
Fig. 6 – The authors' intention regarding the composition of this drawing is incomprehensible. If "o" correspond to the measurement points, why the attached photos are not properly adjusted to them in size and exceed the area limited by the frame. Is it just round zones or "fisheye" zones? Why is there so much empty space above the photos? Correct the description of the vertical axis and its unit.
Fig. 7 – Improve the clarity of the inscriptions and other markings. Why is there a different way of marking the fisheye region and its descriptions in figure "c"? Arrange the arrangement of all the captions.
216 - … the inclusion depth… - what does it mean and how was it measured?
Fig. 8 – Remove the parentheses next to the descriptions "smooth fisheye...”. Analyse the validity of such descriptions and their distribution. Standardize the stress amplitude symbol.
Author Response
Dear Reviewer 3,
First, thank you very much for your effort. We really appreciate your constructive feedback. Therefore, we have considered your comments. Please find our changes of the paper and some more feedback below. Thank you once again for investing your valuable time to review the paper!
Best regards
(Please see attachment for detail)
Response to Reviewer 3 Comments
Comment 1:The research were carried out and presented very interesting, however in an article I noticed some shortcomings, which does not affect my positive opinion. The main drawback of the reviewed work is deficiencies in the description of the initial state of the material (where it came from, chemical composition, properties), thermomechanical treatments, and their effects
Response 1: Thank you very much for your comment. We added a more detailed description of the material in section 2. The material was delivered in a soft-annealed state, in the form of round bars, from which a near-net-shape geometry of the specimens was machined by turning. After the quenching and tempering, the specimen geometries were again machined to avoid dimensional changes after heat treatment. After the heat treatment, a fully martensitic microstructure was obtained with the 0.2% yield strength of 1500 MPa and the ultimate tensile strength of 1900 MPa, which exhibits a hardness of 594 HV 0.5.
Comment 2: No descriptions or photos of the microstructures of the material as delivered and after TMT and HT were presented (2).
Response 2: Very good hint, as it is mentioned above the material was delivered in a soft-annealed state and after a classical heat treatment a fully martensitic microstructure was obtained (HT). As it was expected no microstructural changes after TMT were observable and thus the microstructure before and after TMT is similar. Based on your hint microstructural photos of the samples before and after TMT are added to the manuscript (Figure 2).
Figure 2. Microstructural images magnified 500X in optical microscope. (a) HT specimen; (b) TMT specimen.
Comment 3: It is not known what TMT consisted of, what its parameters and effects were (3). A mere reference to work [22] is not sufficient here. There was also no information on the basis of which standards / recommendations or own research were used to determine the conditions of heat treatment treatments.
Response 3: The TMT process and parameters were added to the manuscript as well as referencing it to work [22]. The quenching and tempering was according to the standard for the steel SAE4140, which can be literally found everywhere. We think it is not necessary to rationalize why we used standard a standard heat treatment.
Comment 4: There is no information about what the samples 1-16 are and what were the differences in their processing, since this resulted in differences in fatigue properties (Fig. 2).
Response 4: The processing of all the 16 samples are completely the same, the only difference is that 8 specimens after heat treatment are thermomechanical treated and 8 specimens are not thermomechanical treated.
Comment 5: The presented article concerns almost 50% of the issue of the presence of non-metallic inclusions in the tested material and their influence on its strength properties. However, in the work there is not even a hint of an attempt to refer to the purity of the material in terms of the amount and distribution of these impurities (4).
Response 5: The raw material of SAE4140 was procured according to standard EN ISO 683-2, which specifies the purity. In this form, the steel is available worldwide. It was not our objective to investigate the influence of purity on the materials fatigue properties. Instead we wanted to study for a material, as it is used in technical applications, whether and how a thermomechanical treatments affects parameters of critical inclusions after fatigue tests e.g. size, depth and composition. Hence, absolute values of purity play a minor role for our study. Nevertheless, we added the standard number specifying the purity in the materials section.
Comment 6: Fig. 1 – The drawing was made contrary to the rules of imaging and dimensioning of objects in the technical drawing. Visible lack of thought regarding the recording of the accuracy of the dimensions listed.
Response 6: Actually, a reviewer of a former Metals manuscript of us (Ref. 22) asked us to remove the accuracy of the dimensions….
In this case, we included them again in Fig. 1 along with a more detailed description.
Comment 7: The stress amplitude of 775 MPa … - no literature indication for such a statement.
Response 7: The according literature was added.
Comment 8: were used to 110 measure the size of inclusions … - by what method or program the research was carried out.
Response 8: We applied the software KLONK Image Measurement on SEM images. This was clarified in the text.
Comment 9: The figure should be presented vertically; usually the individual factors are presented on the horizontal axis and the answer to them vertically. No second axis or value is defined for each sample. What is the physical sense of specifying the number of cycles with an accuracy of 4 decimal places, since it is only to an accuracy of 1 decimal place?
Response 9:
In standard S-N diagrams, the number of cycles to failure corresponds to x axis, therefore, we decided to prepare the figure horizontally. But as you mentioned and it is true, the vertical axis for results is more common in scientific articles and we changed it accordingly. The number of cycles was given in millions, therefore we used decimal places. To avoid confusion, we changed the number of cycles now to absolute values.
Comment 10: … failed… - this word is of a general nature, please write specifically, e.g. "cracked".
Response 10: The word “failed” was replaced by “fractured”.
Comment 11: Fig. 3,4,5,7 – Please remove the frames under the photos and transfer the information contained therein to the photo, remembering about their visibility.
Response 11: Done
Comment 12: Invalid symbol describing the cycle amplitude (sa). In descriptions and formulas, root expressions are written in a power form with an exponent of an appropriate value. Please improve the readability of the unit indicators on both axes. Also in Fig. 6 and 7.
Response 12: The changes have been applied to the diagrams.
Comment 13: Fig. 6 – The authors' intention regarding the composition of this drawing is incomprehensible. If "o" correspond to the measurement points, why the attached photos are not properly adjusted to them in size and exceed the area limited by the frame. Is it just round zones or "fisheye" zones? Why is there so much empty space above the photos? Correct the description of the vertical axis and its unit.
Response 13: The diagram was adjusted and it was clarified that the photos show inclusions (thanks).
Comment 14: Fig. 7 – Improve the clarity of the inscriptions and other markings. Why is there a different way of marking the fisheye region and its descriptions in figure "c"? Arrange the arrangement of all the captions.
Response 14: The clarity of the inscriptions has been improved and unified. The captions have been rearranged.
Comment 15: … the inclusion depth… - what does it mean and how was it measured?
Response 15: It is the minimum distance between the center of the inclusion and the surface of the specimen measured on SEM images with the software KLONK image measurement.
The text was changed for more clarity.
Comment 16: Fig. 8 – Remove the parentheses next to the descriptions "smooth fisheye...”. Analyse the validity of such descriptions and their distribution. Standardize the stress amplitude symbol.
Response 16: The description has been removed. Stress amplitude symbol was adjusted.
Author Response File: Author Response.pdf
Reviewer 4 Report
Manuscript ID: metals-1745891
Type of manuscript: Article
Title: Comparison of the internal fatigue crack initiation and propagation behavior of a quenched and tempered steel with and without a thermomechanical treatment
This manuscript performs a study on the influence of a thermomechanical treatment on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. They showed that the thermomechanical treatment can increase the average lifetime by about 40 %, while there was no effect of the thermomechanical treatment on the form or size of critical inclusions. Also, they claimed that there is also no correlation between inclusion depth and lifetime, and mentioned that the crack propagation stage covers only a small portion of the overall lifetime. Although the results of this manuscript can be attractive, it needs a minor revision before acceptance.
Comments:
· The characteristics thermomechanical treatment should be mentioned in “materials and methods”.
· The novelty of this article compared to reference 12 should be carefully clarified
· The EDS results should be added to SEM images.
Author Response
Dear Reviewer 4,
First, thank you very much for your effort. We really appreciate your constructive feedback. Therefore, we have considered your comments. Please find our changes of the paper and some more feedback below. Thank you once again for investing your valuable time to review the paper!
Best regards
(Please see attachment for detail)
Response to Reviewer 4 Comments
Comment 1: The characteristics thermomechanical treatment should be mentioned in “materials and methods”
Response 1: The thermomechanical treatment is now described in more detail in the respective section.
Comment 2: The novelty of this article compared to reference 12 should be carefully clarified
Response 2: The central points of our manuscript are the investigation of inclusion size and depth as well as fisheye formation and the influence of a thermomechanical treatment on them. Reference 12 is mainly about the formation of fine granular area (FGA) on the fracture surface for high strength steels. It does neither treat TMT, nor fisheye formation. Hence, we think there is enough delimitation between our study and reference 12. In the results of reference 12 it has been shown that not necessarily all internal cracks initiating at non-metallic inclusions exhibit FGA formation, which is in good agreement with our experimental results. Therefore, reference 12 has been used to support our results for the inclusions without FGA formation.
Comment 3: The EDS results should be added to SEM images.
Response 3: EDX results were added for a representative inclusion (Fig. 4)
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The revision is acceptable for publication
Author Response
Dear Reviewer 1,
Thank you very much for your effort. We really appreciate your constructive feedback.
Reviewer 2 Report
Authors were already modified all the comments from the reviewer, I think the paper can be published to this journal.
Author Response
Dear Reviewer 2,
Thank you very much for your effort. We really appreciate your constructive feedback.
Best regards
Amin Khayatzadeh, Stefan Guth, and Martin Heilmaier
Reviewer 3 Report
Dear authors,
The comments I have sent are only a form of discussion on a topic in the article. They mainly result from a misunderstanding of the presented content or the lack of information known to you, but not provided in it. That is why the assessment of the person standing on the sidelines is so important for the quality of the description of the presented phenomena. I am glad that my comments found your approval. And the wide range of corrections introduced by you positively affects the quality of the article.
I propose two small fixes:
Fig. 1 - My remark regarding the non-compliance of the drawing with the standards concerned the lack of symmetry of symmetrical dimensions, namely 11.49 and 16.49 - do not do it this way (delete). Only the dimension of the length of the central section 5 should be given (add), similarly to 27.98 (rounded up to 28), and the value of the rounding radius R ... (add). The editor's remark was correct and in this type of drawing the deviations of the accuracy of the dimensions and the position tolerances are unnecessary. Correct the original drawing as suggested. Shorten the caption, leaving only a reference to the standard specifying the shape and dimensions of the sample or the course of the tensile test.
Fig. 2 - I believe that in the figure, specifying the average lifetime is sufficient to 1 decimal place, so Nf = 5.0 and 3.5 x 106 cycles.
Author Response
Dear Reviewer 3,
Thank you very much for your effort. We really appreciate your constructive feedback. Therefore, we have considered your comments. Please find our changes of the paper and some more feedback below. Thank you once again for investing your valuable time to review the paper!
Best regards
Amin Khayatzadeh, Stefan Guth, and Martin Heilmaier
Response to Reviewer 3 Comments
Comment 1: Fig. 1 - My remark regarding the non-compliance of the drawing with the standards concerned the lack of symmetry of symmetrical dimensions, namely 11.49 and 16.49 - do not do it this way (delete). Only the dimension of the length of the central section 5 should be given (add), similarly to 27.98 (rounded up to 28), and the value of the rounding radius R ... (add). The editor's remark was correct and in this type of drawing the deviations of the accuracy of the dimensions and the position tolerances are unnecessary. Correct the original drawing as suggested. Shorten the caption, leaving only a reference to the standard specifying the shape and dimensions of the sample or the course of the tensile test.
Response 1: Thanks a lot for your comment, the technical drawing is changed based on your comments
Comment 2: Fig. 2 - I believe that in the figure, specifying the average lifetime is sufficient to 1 decimal place, so Nf = 5.0 and 3.5 x 106 cycles.
Response 2: Probably you mean the Fig. 3, according to your comment the average lifetimes are changed to Nf = 5.0 and 3.5 x 106 cycles.
