Study of the Mechanical Properties and Microstructural Response with Laser Shock Peening on 40CrMo Steel

Round 1

Reviewer 1 Report

The article is devoted to the study of the effect of laser impact treatment on the change in properties, including the mechanical strength and stability of 40CrMo steel. The methods of X-ray diffraction and transmission electron microscopy were used as research methods. Undoubtedly, the results presented by the authors are of high scientific novelty and practical significance, and are also promising for practical research. In general, the presented results of the study can be accepted for publication after the authors provide answers to all the questions raised by the reviewer during the reading of the article.

1. In the abstract, the authors need to more clearly state the purpose and relevance of this work.
   2. The authors should explain the reasons for the increase in the mechanical strength of steel after processing, taking into account the possibility of changing the dislocation density of the near-surface layer or its amorphization.
   3. The authors should explain exactly how the depths at which the properties are modified were determined.
   4. TEM images of steel after processing show the presence of new types of defects, as well as dislocation loops, the authors should explain the mechanism of their formation.
   5. The authors should also clarify whether the proposed methodology can be scaled up for industrial modification of steel.
   6. Conclusion requires significant revision.
2. 7. The abstract also talks about the results of X-ray diffraction, but the article did not find these data in the form of diffraction patterns or their detailed analysis.

Author Response

Reviewer 1#

The article is devoted to the study of the effect of laser impact treatment on the change in properties, including the mechanical strength and stability of 40CrMo steel. The methods of X-ray diffraction and transmission electron microscopy were used as research methods. Undoubtedly, the results presented by the authors are of high scientific novelty and practical significance, and are also promising for practical research. In general, the presented results of the study can be accepted for publication after the authors provide answers to all the questions raised by the reviewer during the reading of the article.

Question 1. In the abstract, the authors need to more clearly state the purpose and relevance of this work.

Answer 1. Thank you for your kind advice. In the abstract, we have stated the purpose and relevance of this work in the revised manuscript.

Question 2. The authors should explain the reasons for the increase in the mechanical strength of steel after processing, taking into account the possibility of changing the dislocation density of the near-surface layer or its amorphization.

Answer 2. Thank you very much for your review. In the manuscript, we have explained the reasons for the increase in the mechanical strength of steel after processing. And the reason was given in the section of 3.4. Tensile strength and elongation.

Question 3. The authors should explain exactly how the depths at which the properties are modified were determined.

Answer 3. Dear reviewer, thank you very much for your comment. At a depth of 600 μm, the test results of residual stress and microhardness of the laser shock peened sample were similar to those of the untreated sample. In sections of 3.1. In-depth residual stress and 3.3. In-depth microhardness, an explanation is given on how to judge the influence depth of laser shock peening.

Question 4. TEM images of steel after processing show the presence of new types of defects, as well as dislocation loops, the authors should explain the mechanism of their formation.

Answer 4. Dear reviewer, thank you very much for your valuable comment. In the revised manuscript, we have explained the formation mechanism of new types of defects, as well as dislocation loops in the section of 3.6.1. Microstructural evolution mechanism.

Question 5. The authors should also clarify whether the proposed methodology can be scaled up for industrial modification of steel.

Answer 5. The proposed methodology can be scaled up for industrial modification of steel. In section 3.6.2 Wear mechanism, we also give the instructions.

Question 6. Conclusion requires significant revision.

Answer 6. Thank you very much for your comment, we have significantly revised our Conclusion in the revised manuscript.

Question 7. The abstract also talks about the results of X-ray diffraction, but the article did not find these data in the form of diffraction patterns or their detailed analysis.

Answer 7. Dear reviewers, we are very sorry for the confusing statement. In the abstract, our claim is to use the XRD technique to measure the residual stress distribution. In the revised manuscript, we have given the correct statement.

Author Response File: Author Response.pdf

Reviewer 2 Report

• dues to the anisotropy of the laser path (according to figure 1), it is extremly important to precise the analysis direction for residual stress study and to indicate clearly the laser path in figure 2; without those precisions, the corresponding results are not meaningful;
• in paragraph 2.5, it is necessary to precise the used X-ray radiation wavelength and the corresponding diffraction plane. In general, for Fe-based alloy and under Cr incident radiation, the {211} family plan diffrects at about 156°, the scanning zone can not be from 129° to 142°; it is important to precise the used elastic moduls for stress calculation and the analysis direciton; the sample preparation for in-depth analysis should also indicated;
• in paragraph 2.6, it is important to precise the exact observation zone with TEM technique;
• in caption of figure 3, it is important to precise the analysis direction dues to the strong anisotropy;
• in figure 4, the indication about the dislocation zone is not correct at this scale. Between cementite slats, there can exsit many dislocations, 
• in figure 10 and 11, it is absolutely necessary to precise the laser path comparing to the wear test direction because of the anisotropy of the surface morphology (surface roughness);

 

Author Response

Reviewer 2#

Question 1. dues to the anisotropy of the laser path (according to figure 1), it is extremly important to precise the analysis direction for residual stress study and to indicate clearly the laser path in figure 2; without those precisions, the corresponding results are not meaningful;

Answer 1. Thank you very much for your comment. In order to test the residual stress at different depths of the sample after laser shock peening, a Struers LectrolPol-5 electrolytic polishing machine was used to peel off the surface of the sample layer by layer, and the residual stress value for each depth position was determined based on the average of the five measured values. For each depth position, the test location for residual stress is shown in Fig. 3. In the revised manuscript, the added statement is given in section 2.5. Residual stress test. Also, the laser path in figure 2 is the same as given in figure 1.

Question 2. in paragraph 2.5, it is necessary to precise the used X-ray radiation wavelength and the corresponding diffraction plane. In general, for Fe-based alloy and under Cr incident radiation, the {211} family plan diffrects at about 156°, the scanning zone can not be from 129° to 142°; it is important to precise the used elastic moduls for stress calculation and the analysis direciton; the sample preparation for in-depth analysis should also indicated;

Answer 2. Thank you very much for your comment. For Fe-based alloy and under Cr incident radiation, the {211} family plan diffrects at about 156°, the scanning zone is from 149° to 169°, we are very sorry for giving the incorrect parameters. Also, the sample preparation for in-depth analysis is also indicated. In section of 2.5. Residual stress test of the revised manuscript, the revised content is also given.

Question 3. in paragraph 2.6, it is important to precise the exact observation zone with TEM technique;

Answer 3. Thank you very much for your advice. The exact observation zone with TEM technique is shown in Fig. 3 in the revised manuscript.

Question 4. in caption of figure 3, it is important to precise the analysis direction dues to the strong anisotropy;

Answer 4.  Thank you very much for your comment. In order to test the residual stress at different depths of the sample after laser shock peening, a Struers LectrolPol-5 electrolytic polishing machine was used to peel off the surface of the sample layer by layer, and the residual stress value for each depth position was determined based on the average of the five measured values. For each depth, the test location for residual stress is shown in Fig. 3. In the revised manuscript, the added statement is given in section 2.5. Residual stress test.

Question 5. in figure 4, the indication about the dislocation zone is not correct at this scale. Between cementite slats, there can exsit many dislocations,

Answer 5. We are very sorry for the inappropriate marking, between cementite slats, there can indeed exsit many dislocations. In the revised manuscript, we have presented the revised picture shown in Fig. 6.

Question 6. in figure 10 and 11, it is absolutely necessary to precise the laser path comparing to the wear test direction because of the anisotropy of the surface morphology (surface roughness);

Answer 6. Dear reviewer, thank you very much for your comment. On the laser shock peening treated surface, the direction of reciprocating wear has also been given in Fig. 12 in the revised manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors answered all questions, the article can be accepted for publication in the form presented.

Author Response

Thank you very much for your approval of our work.

Reviewer 2 Report

The question about the residual stress direction is crucial and extremely important because the residual stress level can be very different when the stress analysis has been done in parallel or in perpendicular direction comparing to laser path. In fact, the residual stress state is at least bi-directional in the laser path plane dues to the very strong anisotropy of ther process. So, it is extremely important to precise the XRD stress analysis direction comparing to the laser path. If not, the presented residual stress in figure 5 is not meaningful. 

So, in the figure 3 and the figure 5, it is necessary to indicate the XRD stress analysis direciton. In the discussison part, it is also important to discuss the stress anisotropy and the their influecne about the studied properities.

 

Author Response

Question 1. The question about the residual stress direction is crucial and extremely important because the residual stress level can be very different when the stress analysis has been done in parallel or in perpendicular direction comparing to laser path. In fact, the residual stress state is at least bi-directional in the laser path plane dues to the very strong anisotropy of ther process. So, it is extremely important to precise the XRD stress analysis direction comparing to the laser path. If not, the presented residual stress in figure 5 is not meaningful. 

        So, in the figure 3 and the figure 5, it is necessary to indicate the XRD stress analysis direciton. In the discussison part, it is also important to discuss the stress anisotropy and the their influecne about the studied properities.

Answer 1. Thank you very much for your comment. For the residual stress analysis on the sample surface, the test direction was parallel to the laser path, while for the residual stress analysis on depth direction, the test direction was perpendicular to the laser path. On the cross-sectional surface, the residual stress values were measured at the depths of every 100 μm. And the residual stress value for each depth position was determined based on the average of five measured values. The test location for residual stress at the surface and depth directions is shown in Fig. 3. In the analysis of the residual stress test results, we also specify the test direction compared to the laser path.

        The added instructions and schematic diagrams of residual stress testing location are also given in the revised manuscript.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

the answer about the residual stress analysis direction is still very very confused:

the sentencs in page n°4 between line 111 and 114 are not understandful. Why the residual stress in surface is determined parellel to the laser path and the residusl stress in depth has been analsyed in the direciton penpendicular to the laser path ? We know the residual stress level is quite different in the two directions in the same analysis zone dues to the anisotropy of the process and the laser path.

According to the description in page n°4 between line 111 and 114, the results presented in figure 5 are not meaningful because one can not superimpose the residual stress value coming from different directions without any precaution or strong hypotheses.

 

Furthermore, in caption of some figures, it is necessary to precise the test conditions or analysis zone:

• in figure 2, it is necessary to precise the laser path direction dues to the strong anisotropy of the process;
• in figure 3, it is also important to indicate the laser path with right direction;
• in figure 5, it is necessary to precise the analysis direction for all XRD stress ananlysis results;
• in caption of figure 6 and 7, it is necessary to precise the observation zone (extreme surface ? near surface ?);
•  

Author Response

Question 1. the answer about the residual stress analysis direction is still very very confused:

the sentencs in page n°4 between line 111 and 114 are not understandful. Why the residual stress in surface is determined parellel to the laser path and the residusl stress in depth has been analsyed in the direciton penpendicular to the laser path? We know the residual stress level is quite different in the two directions in the same analysis zone dues to the anisotropy of the process and the laser path.

According to the description in page n°4 between line 111 and 114, the results presented in figure 5 are not meaningful because one can not superimpose the residual stress value coming from different directions without any precaution or strong hypotheses.

Answer 1. Dear reviewer, thank you very much for your comment, which is important to improve the quality of our manuscript. We have re-measured the residual stress in the depth direction, and the test direction was uniformly chosen to be parallel to the laser path. In order to test the residual stress in different depths of the sample after laser shock peening, a Struers LectrolPol-5 electrolytic polishing machine was used to peel off the surface of the sample layer by layer. And the residual stress value for each depth position was determined based on the average of five measured values.

The revised content has been given in the manuscript.

Question 2. Furthermore, in caption of some figures, it is necessary to precise the test conditions or analysis zone:

in figure 2, it is necessary to precise the laser path direction dues to the strong anisotropy of the process;

in figure 3, it is also important to indicate the laser path with right direction;

in figure 5, it is necessary to precise the analysis direction for all XRD stress ananlsis results;

in caption of figure 6 and 7, it is necessary to precise the observation zone (extreme surface ? near surface ?);

Answer 2. Dear reviewer, thank you very much for your valuable comment.

In figure 2, we have given the precise laser path direction.

In figure 3, we have given the correct laser path direction.

In figure 5, we have revised to precisely the analysis direction for all XRD stress ananlysis results; the measured directions of residual stress were all parallel to the laser path.

In the caption of figure 6 and 7, we have given the precise observation zone, and the test zone on the extreme surface.

Author Response File: Author Response.pdf

Round 4

Reviewer 2 Report

in the revised version of the manuscript, author has made necessary corrections including all remarks from reviewers.