Microstructural Characteristics and Fracture Behavior of the Rotor Magnetic Pole Screw in an Industrial Synchronous Motor
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear Authors,
Your manuscript discusses the microstructure and fracture behavior of a screw. However, there are several issues that need to be clarified:
- Please provide a clear figure of the model, including its dimensions and all relevant properties of the investigated specimen.
- Where is the testing area of the screw? Please clearly indicate the testing location.
- How were the test samples prepared? This procedure should be clearly explained and supported by a figure.
- Please clearly describe and discuss the experimental conditions.
- Please explain the reasons for using optical microscopy (OM) and scanning electron microscopy (SEM) to observe the microstructure.
- Please define and clearly explain the term “fracture behavior” as used in this study.
In general, the available data are insufficient to properly evaluate the research results. Therefore, I recommend that the authors address the issues above and provide the missing information. After these revisions, the results can be more clearly and reliably assessed.
Sincerely yours,
Author Response
Manuscript Number: Coatings-4137327
Title: Microstructural characteristics and fracture behavior of the ro-tor magnetic pole screw in an industrial synchronous motor
Dear Reviewer:
Thank you very much for your comments on our manuscript. These comments are very useful and valuable to polish our manuscript. We have revised the manuscript according to the comments, and the modifications are highlighted in the revised manuscript for easy identification. The following is a point-to-point response to the comments.
Reviewer’s Comments:
1. Please provide a clear figure of the model, including its dimensions and all relevant properties of the investigated specimen.Where is the testing area of the screw? Please clearly indicate the testing location.
RESPONSE: Thank you for your precise comments. The schematic illustration of microstructure characterization and mechanical properties testing for the magnetic pole screw is provided in Fig.1. The dimensions and relevant properties of the investigated specimens are also added.
2. How were the test samples prepared? This procedure should be clearly explained and supported by a figure. Please clearly describe and discuss the experimental conditions.
RESPONSE: Thank you for your valuable suggestions. The specimen preparation and experimental conditions are added in the Experimental procedures.
The tensile properties at the axial center position of the screw were measured by the UTM5105 standard testing machine at a crosshead speed of 5 mm/min at room tempera-ture. The Vickers hardness tests were conducted using an MHV-1000Z microhardness tester with a loading force of 500 gf and a loading time of 15 s. For OM and SEM observation, the samples were mechanically ground and polished fol-lowing standard metallographic preparation procedures, and then etched with a 4% nital solution. The EBSD samples were firstly mechanically ground, and then electropolished with an 8% sodium perchlorate alcohol solution at 0.5 A and 20 s. For fracture characteri-zation, the sample was prepared using a wire cut electrical discharge machining.
3. Please explain the reasons for using optical microscopy (OM) and scanning electron microscopy (SEM) to observe the microstructure.
RESPONSE: OM observation is suitable for the large sample areas, such as the overall thread geometry, crack propagation paths, and hardness indentation distributions (e.g., Fig. 3a macrograph of the thread, Fig. 4a hardness indentation layout). SEM observation is mainly used for fine microstructural and fractographic characterization, such as pearlite lamellar structure (Fig. 2b), fracture micrographs (dimples, cleavage planes, and tear ridges in Fig. 7), crystallographic characteristics (Fig.5). The combined use of OM and SEM in this study provides comprehensive microstructural information across scales from millimeters to micrometers. We appreciate the reviewer’s attention to the analytical methodology.
4. Please define and clearly explain the term “fracture behavior” as used in this study.
RESPONSE: In this study, the term “fracture behavior” refers to the sequence and characteristics of failure under torsional loading, with a focus on macroscopic and microscopic features. The “fracture behavior” was explained through three interrelated aspects of fracture, which were reffered as Crack Initiation Stage (at the thread root, where stress concentration is most severe due to the combined effect of geometric notch and work‑hardened subsurface microstructure), Crack Propagation Stage ( through the ferrite grain, along the phase interface, and through pearlite), Final Rapid Fracture Stage (shear deformation morphology and cleavage fracture).
The related explanations have been added in the revised manuscript.
Other changes in the revised manuscript
(1) Some inaccurate expressions have been modified and highlighted in the manuscript.
Thank you very much for your comments once again. We hope this revised version can meet the requirements of Coatings. If any questions, please do not hesitate to contact us.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis research is very interesting from an industrial perspective. However, 1- The author did not mention in the introduction that this screw had a fracture problem during use, which would justify the interest of this study. 2- The author mentioned several times in the introduction and in the "discussion of results" the terms "stress" and "residual stresses," even the term "concentrated plastic deformation," which are useful for explaining the cause or mechanism of failure. But the use of these terms requires their measurement or quantification by appropriate techniques. For example, to quantify or find the distribution of residual stresses, X-ray diffraction must be used. Through this technique, one can deduce the types of residual stresses in each area of ​​the screw. This technique is absent from this research. This is why the discussion of results is incomplete in this research. For the term "concentrated plastic deformation," it is necessary to use the appropriate characterization technique to counteract the actual concentration of plastic deformation in the different screw zones. Therefore, I propose another formulation of this article.
Author Response
Manuscript Number: Coatings-4137327
Title: Microstructural characteristics and fracture behavior of the ro-tor magnetic pole screw in an industrial synchronous motor
Dear Reviewer:
Thank you very much for your comments on our manuscript. These comments are very useful and valuable to polish our manuscript. We have revised the manuscript according to the comments, and the modifications are highlighted in the revised manuscript for easy identification. The following is a point-to-point response to the comments.
Reviewer’s Comments:
1. This research is very interesting from an industrial perspective. However, the author did not mention in the introduction that this screw had a fracture problem during use, which would justify the interest of this study.
RESPONSE: Thank you for your comments. The wear failure and fracture failure of rotor screws are common forms of failure of screws. With the development of industrial motors towards higher power density, increased oper-ational speeds, and higher sustainability, the reliability (failure prevention) of rotor magnetic pole screws is more concerned. For the high-efficiency forming process (cold thread rolling), the risk of fracture from the gradient microstructure especially at the thread root under complex loading, has become a potential common critical issue in the industry. Therefore, this study aims to systematically analyze the gradient microstructural characteristics of cold-rolled threads and their influence on torsional fracture behavior, which provids theoretical support and process optimization guidance for preventing such fastener failures under demanding operational conditions.
The related description has been added in the Introduction.
2. The author mentioned several times in the introduction and in the "discussion of results" the terms "stress" and "residual stresses," even the term "concentrated plastic deformation," which are useful for explaining the cause or mechanism of failure. But the use of these terms requires their measurement or quantification by appropriate techniques. For example, to quantify or find the distribution of residual stresses, X-ray diffraction must be used. Through this technique, one can deduce the types of residual stresses in each area of ​​the screw. This technique is absent from this research. This is why the discussion of results is incomplete in this research. For the term "concentrated plastic deformation," it is necessary to use the appropriate characterization technique to counteract the actual concentration of plastic deformation in the different screw zones. Therefore, I propose another formulation of this article.
RESPONSE: We sincerely thank the reviewer for this insightful and constructive comment regarding the quantification of stresses and plastic deformation. The reviewer is absolutely correct that direct measurement of residual stress distribution (e.g., by X-ray diffraction, XRD) and detailed quantification of plastic strain concentration would provide more definitive evidence to support our discussion on failure mechanisms.
In this study, the gradient microstructure in the screw thread is obtained by cold thread rolling process. The hardened layer exhibits a sub-millimeter-scale thickness, within which an even thinner region of severe plastic deformation located near the thread surface (the critical site for crack initiation). For traditional X-ray diffraction residual stress measurement, the penetration depth and information collection area pose significant operational difficulties for complex curved components with deep gradients and extremely fine hardened layers. Therefore, we relied on indirect but well-established correlative methods to infer the stress and deformation states:
Microhardness distribution: the macrograph and hardness distribution maps in Fig. 4 provides quantitative data of hardness changes from the very shallow surface to the core with micron-level spatial resolution. The hardness changes was caused by residual stress, dislocation density, and grain refinement.
EBSD analysis and Geometrically necessary dislocation (GND) Density: the results in Fig. 5 provide quantitative data on the plastic deformation distribution at a sub-micron scale. The GND density changes directly corresponded to non-uniform plastic strain, which was caused by residual stress generation.
Microstructural Morphology: The evolution from equiaxed grains to severely elongated fibrous structures (Fig. 3 and Fig. 5) provides qualitative evidence of deformation gradients.
Other changes in the revised manuscript
(1) Some inaccurate expressions have been modified and highlighted in the manuscript.
Thank you very much for your comments once again. We hope this revised version can meet the requirements of Coatings. If any questions, please do not hesitate to contact us.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsGeneral Evaluation
The paper is an interesting case history of a rotor pole screw failure, occurred as a result of interaction of applied torsional load and intensely plastically deformed surface microstructure. Optical and scanning electron microscopy and fractography, EBSD analysis and hardness testing were used as the principal investigation techniques. The paper, although it is marginally adhered to the Journal scope, is well-written and structured and it could be considered for publication provided some further suggested revisions.
Scientific/Technical Comments
- Background information is missing together with some general information regarding the pole screw, e.g. dimensions, specifications, etc.
- Experimental procedure is lacking from details; e.g. EBSD analysis parameters need to be more comprehensively referred (step size, scanned area, etc.).
- Hardness values should be rounded to the closest integer.
- The microstructure characterization is suggested to be enriched with additional results, highlighting also the presence of non-metallic inclusions and their potential impact on fracture behavior.
- In Table 1, please clearly define the Yield ratio (e.g. Yield Strength/Tensile Strength).
- The mechanism of fracture has to be clearly addressed, using well-established terms and definitions, e.g. torsional overload, low-cycle fatigue, etc.
- The estimation of operating load during service and the comparison with the specifications of use will shed light to the root-cause analysis.
- Possibility of coating or surface hardening to hinder crack initiation, will be probably added as a potential failure preventive action together with other suggested measures.
Language/Grammar
The language is sufficient; however, a final proofreading is necessary to improve the language and eliminate potential minor spelling and typographical errors. For instance, total elongation was referred 23.3% (Line 197) while in the Table 1 the elongation was referred equal to 23.2% (rounding of the elongation values to the closest integer is suggested).
Author Response
Manuscript Number: Coatings-4137327
Title: Microstructural characteristics and fracture behavior of the ro-tor magnetic pole screw in an industrial synchronous motor
Reviewer’s Comments:
General Evaluation
The paper is an interesting case history of a rotor pole screw failure, occurred as a result of interaction of applied torsional load and intensely plastically deformed surface microstructure. Optical and scanning electron microscopy and fractography, EBSD analysis and hardness testing were used as the principal investigation techniques. The paper, although it is marginally adhered to the Journal scope, is well-written and structured and it could be considered for publication provided some further suggested revisions.
RESPONSE: Thank you very much for your comments on our manuscript. These comments are very useful and valuable to polish our manuscript. We have revised the manuscript according to the comments, and the modifications are highlighted in the revised manuscript for easy identification. The following is a point-to-point response to the comments.
1. Background information is missing together with some general information regarding the pole screw, e.g. dimensions, specifications, etc. Experimental procedure is lacking from details; e.g. EBSD analysis parameters need to be more comprehensively referred (step size, scanned area, etc.).
RESPONSE: Thank you for your suggestions. The related general informations and measuring parameters have been added in the revised manuscript.
The screw threads were produced by cold thread rolling, with a nominal diameter of 36 mm, length of 360 mm, and thread length of 100 mm. The tensile sample with a diameter of gauge length of 5 mm and gauge length of 25 mm (along the parallel screw axis and perpendicular to the cross-sectional direction). The step size of 0.06 μm and magnification of 4000× are selected for EBSD analysis. For fracture characterization, the sample was prepared using a wire cut electrical discharge machining. The tensile properties at the axial center position of the screw were measured by the UTM5105 standard testing machine at a crosshead speed of 5 mm/min at room temperature.
2. Hardness values should be rounded to the closest integer.
RESPONSE: Thank you for your suggestions. The Hardness values have been rounded to the closest integer.
3. The microstructure characterization is suggested to be enriched with additional results, highlighting also the presence of non-metallic inclusions and their potential impact on fracture behavior.
RESPONSE: Thank you for this valuable comment. The corresponding investigation of the relationship between microstructure and fracture behavior (failure types, crack propagation path, and fracture stage) have been discussed. The primary focus of this study is on the evolution of microstructure and their effect on the fracture behavior. Although our current study did not specifically characterize inclusions, we sincerely acknowledge that non-metallic inclusions are an important factor influencing ductility and fracture. We have enriched the discussion in the revised manuscript to acknowledge the role of inclusions as a potential limitation.
4. In Table 1, please clearly define the Yield ratio (e.g. Yield Strength/Tensile Strength).
RESPONSE: The Yield ratio have been defined in the related parts. The tensile properties with a combination of ultimate tensile strength of 655 MPa, yield ratio (yield strength/tensile strength) of 0.55, and total elongation of 23% are obtained in the screw's central axis zones, as provided in Table 1.
5. The mechanism of fracture has to be clearly addressed, using well-established terms and definitions, e.g. torsional overload, low-cycle fatigue, etc. The estimation of operating load during service and the comparison with the specifications of use will shed light to the root-cause analysis.
RESPONSE: Thank you for your suggestions. In the revised manuscript, we will explicitly use the term “torsional overload fracture” in the “Fracture Behavior”. In addition, we added a fracture classification and load comparison in the Results and Discussion to clearly define the mechanism as torsional overload.
6. Possibility of coating or surface hardening to hinder crack initiation, will be probably added as a potential failure preventive action together with other suggested measures.
RESPONSE: Thank you for this valuable suggestion. The application of coatings or surface hardening treatments represents the effective approaches for improving thread surface performance. We added this potential optimization direction in the Discussion or Conclusion section.
7. The language is sufficient; however, a final proofreading is necessary to improve the language and eliminate potential minor spelling and typographical errors. For instance, total elongation was referred 23.3% (Line 197) while in the Table 1 the elongation was referred equal to 23.2% (rounding of the elongation values to the closest integer is suggested).
RESPONSE: Thank you for your suggestions. The values have been rounded to the closest integer, and some inaccurate expressions have been modified and highlighted in the manuscript.
Thank you very much for your comments once again. We hope this revised version can meet the requirements of Coatings. If any questions, please do not hesitate to contact us.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsDear Authors,
This version could be published.
(*) Please check Fig. 1 : the part should have the center-line
Sincerely yours,
Author Response
Thank you for your precise comment. The center-line has been added in Fig.1.
Reviewer 2 Report
Comments and Suggestions for AuthorsAccepted
