Effect of Gas Nitriding on Interface Adhesion and Surface Damage of CL60 Railway Wheels under Rolling Contact Conditions
Round 1
Reviewer 1 Report
All preious comments were responded to.
Author Response
Thank you very much for the valuable and constructive comments, which helped us to significantly improve our manuscript. We have revised our manuscript according to your comments and those from the other reviewers. In the revised version, the English and grammar have been improved a lot through consulting with colleague of native english speaker.
Reviewer 2 Report
Dear Authors,
The manuscript is improved. I have some additional comments listed below.
Abstract
[12-13] „ The failure mechanisms of wheel/rail surface for the gas nitriding and without gas nitriding...”
Comment: To improve the grammar, I propose: “...surface after the gas nitriding ...”
Introduction
[51-52] „Compared with the other surface treatments, gas nitriding has lower cost...”
Comment: First you should define the “other treatments” because such statement is too general. Secondly, I propose following change: “gas nitriding shows lower cost” or “gas nitriding is characterized by lower cost”. However, the proposed change is just cosmetic. For example, conventional surface hardening would be cheaper than gas nitriding. On the other hand, cladding of hard composite coatings will provide better performance than nitriding. Therefore, please refine the description.
[63] “...the nitrogen diffuses onto the surface...”
Comment: What do you mean “the nitrogen diffuses onto the surface”? Such statement is wrong in general. First the nitrogen from the gaseous atmosphere do not diffuses onto the surface but the atoms or molecules are adsorbed on the surface. Next, the molecules can be decomposed, and atoms (mainly nitrogen ions, N+) are absorbed by the substrate. If the nitriding is conducted at solid state, the absorption will be controlled by diffusion of nitrogen atoms into the bulk. This is the general mechanism of nitriding.
So, please verify the description.
Yours sincerely
Reviewer
Author Response
Q: [12-13] „ The failure mechanisms of wheel/rail surface for the gas nitriding and without gas nitriding...”
Comment: To improve the grammar, I propose: “...surface after the gas nitriding ...”
Reply:
Thank you very much for your comment, this confusing sentence has been changed to “...surface after the gas nitriding ...”.
Introduction
Q: [51-52] „Compared with the other surface treatments, gas nitriding has lower cost...”
Comment: First you should define the “other treatments” because such statement is too general. Secondly, I propose following change: “gas nitriding shows lower cost” or “gas nitriding is characterized by lower cost”. However, the proposed change is just cosmetic. For example, conventional surface hardening would be cheaper than gas nitriding. On the other hand, cladding of hard composite coatings will provide better performance than nitriding. Therefore, please refine the description.
Reply:
We fully agree with your viewpoint.
In the revised manuscript, this confusing sentence has been changed to “Gas nitriding is characterized by lower cost, it is an ideal heat treatment for case hardened machined components with wear surfaces. It is also ideal for parts where distortion is a concern due to tight tolerances since distortion is not typical due to the relatively low temperatures involved in the process.”
Q: [63] “...the nitrogen diffuses onto the surface...”
Comment: What do you mean “the nitrogen diffuses onto the surface”? Such statement is wrong in general. First the nitrogen from the gaseous atmosphere do not diffuses onto the surface but the atoms or molecules are adsorbed on the surface. Next, the molecules can be decomposed, and atoms (mainly nitrogen ions, N+) are absorbed by the substrate. If the nitriding is conducted at solid state, the absorption will be controlled by diffusion of nitrogen atoms into the bulk. This is the general mechanism of nitriding.
So, please verify the description.
Reply:
Thank you for this valuable comment.
Based on the above considerations, the ambiguous sentence has been revised to “...diffusion layer is a compound zone and it existed below the surface layer, which dramatically improves the surface quality”.
In the end, we appreciate very much again for your careful comments.
Author Response File: 
Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Dear Authors,
The subject is interesting and worth of investigation. However, the manuscript must be deeply revised. The English must be also improved. Please find the detailed comments below.
Title :
„Gaseous Nitriding Treatment”
Comment: The proper terminology is „Gas Nitriding”
Abstract:
“...surface gaseous nitriding treatment (GNT)...”
Comment: Is it a new kind of treatment for which the acronym must be defined ?
“unGNT, GNTed”
Comment: Such designations are unusual. Please use comprehensive descriptions.
“The results show that the GNTed samples could form a uniform and dense white layer which was approximately 3-5 μm thick and a diffusion layer which was approximately 20 μm thick on the wheel surface.”
Comment: How the samples could form the white layer? What is relation between samples “which could form the white layer” and the wheels? The wheel is the sample? The sentence is confusing and unclear. Please verify it.
“...GNTed process...”
Comment: Such term is confusing and there is no need for creating any new terminology for the conventional and widely know process of thermochemical treatment – Gas Nitriding.
“Under the wheel surface after the GNTed process, the adhesive coefficient between the wheel/rail was reduced by 11.7% in dry condition, and reduced by 18.4% in water media, but the GNTed samples still could keep the higher adhesive coefficient between the wheel/rail systems, which could prevent the occurrence of the phenomenon such as wheel-slip.”
Comment: The above sentence is incomprehensible. It is not clear what is the difference between wheel surface after the GNTed process and GNTed samples. What do you mean “Under the wheel surface after the GNTed process” ?
“In short, the GNT on wheel materials...’
Comment: How many materials have you examined to formulate such a general statement?
“Which provided a potential application for the actual operating condition, ...”
Comment: Verify the above sentence – it is confusing.
Introduction
“As we all know, surface nitriding treatment has low cost, and has better effect on improving surface hardness and increasing wear resistance [15].”
Comment: Who do you mean “we all” ? Surface nitriding ha better effect on improving hardness and wear resistance in comparison to what? The above sentence is completely mistaken and unauthorized.
“...time and temperatures...”
Comment: “temperatures” is grammatically improper. The English must be revised throughout the text.
“They found that three layers (oxide layer, compound layer and diffusion layer) formed on the surface of the specimen...”
Comment: Are you sure that the “diffusion and compound layers” were formed on the surface? It is unlikely according to the thermodynamics.
“... compared to that of the matrix materials.”
Comment: What do you mean “matrix materials” ? Do you mean “base material” or rather “base metal” ?
Materials and Methods
“The wheel and rail rollers were cut from the heavy-haul railway wheel tread (CL60) in China and rail head (U75V).”
Comment: What condition was the material in? What method of cutting and subsequent machining was applied? What was the influence of cutting and machining on the microstructure of the material? Any heat treatment was applied after machining? This information is necessary.
“The wheel and rail samples diameter are 40 mm, and the arc radius R of...”
Comment: ... diameter is 40 mm... What “arc radius” ?
“The chemical composition in weight percentage of the wheel and rail is listed in Table 1. ”
Comment: The information “in weight percentage” is not needed – it is provided in the caption.
What were the surface conditions of the test wheels? How was the surface machined after Gas Nitriding?
“The normal forces could be detected and adjusted by...”
Comment: “could be” or it was ?
Result and discussion
“Figure 3(a) shows the cross-sectional microstructure of the CL60 wheel sample after the GNT, the nitrided layer of the GNTed sample from the surface to the inside was composed of bright white layer, diffusion layer and the matrix.”
Comment: First, verify the structure of the sentence. The “matrix” is wrong term. Also, the “substrate” in Fig.3(a) is not proper. The substrate term is used in a case of deposition of coatings. In this case the proper term for the unchanged material is “base metal”.
“The bright white layer was the nitride layer with the thickness of approximately 3-5μm, it appears white and bright in the light microscope and it not easily corroded.”
Comment: First, what does the “nitride layer” mean? What kind of nitride did you create? The conventional EDS detector is not reliable for Carbon and Nitrogen analysis. In this case the WDS detector should be applied. The measuring error for EDS is too high. Since the phase analysis was not conducted, the indication of any compound is not reliable.
What kind of corrosion tests did you do to find that the surface layer “not easily corroded” ? The statement is not allowed, because it is not based on your study.
“In fact, the phase composition of the nitriding layer was ε→γ＇→α＋γ＇→α from exterior to the interior [17].”
Comment: Another unauthorized statement. Do not provide results from the Literature in your “Results and discussion” section. The same comment refers to the next sentence in which you provide data from the Literature.
“In addition, it is obvious that the wheel matrix material displays a ferrite-pearlite microstructure which is composed of the mixture of ferrite and pearlite.”
Comment: It is not obvious from the very low quality image of microstructure. The microstructure depends on the heat treatment applied.
“...unGNTed...”
Comment: No consistency in the designations. The “unGNTed” appears first time. Please do not use such strange designations.
“This may be related to the presence of oxide layer and the improvement of surface roughness after the GNT.”
Comment: If the same surface conditions were not provided, the results should not be compared. What was the roughness ?
“According to the detection results of EDX, it can be found that the oxygen content on worn surface in Figure 7(d) was obviously higher than that of the Figure 7(c).”
Comment: These results are missing in the manuscript. There is no way to verify it.
“The reduction of wheel surface damage was mainly due to the improvement of wheel surface hardness after the GNT.”
Comment: First, what is “improvement” of hardness ? Secondly, why do you associate the rate of surface damage just with the hardness ? What about the phase composition in surface layer ?
“In fact, previous studies have confirmed that the use of different lubricants after thousands of dry load cycles would not change the hardening location, but different type and application order of lubricants has a significant influence on the hardening behavior of roller when the lubrication lasts for the whole test.”
Comment: I suggest dividing this long sentence, because its meaning is hard to find. Moreover, what do you mean “would not change the hardening location” ? The strain hardening occurs always in the deformed region.
“However, the plastic deformation layer with higher hardness and different thickness in Figure 9(a-b) was bound to have a greater impact on the nucleation of fatigue crack [6]. ”
Comment: First, “was bound” or “was found” ? Secondly, you refer again to the Literature.
“It was indicated that the water significantly accelerates the growth of RCF cracks, when the medium trapped in the cracks helps to accelerate the propagation of cracks [21]. “
Comment: Please refer your own results.
“Moreover, compared with the unGNTed sample, surface damage was slighter, the wear surface roughness was lower...”
Comment: You refer to roughness even though you have not provided any values.
Conclusions
“1) The surface of the wheel material would form a uniform and dense white bright layer...”
Comment: From scientific point of view the brightness and colour of the surface layer is not important. The phase, chemical composition and microstructure should be determined.
I hope my suggestions and comments will help to improve your manuscript.
Yours sincerely
Reviewer
Reviewer 2 Report
- More information on the nitriding process is needed - particularly the nitriding potential KN of each step in the process.
- A more thorough microstructural characterization is needed:
- XRD of the surface for compound layer phase identification before and after testing
- Higher magnification to identify and characterize any porosity in the compound layer.
- refer to Lehrer diagram for the expected phases in the compound layer.
- A more thorough microstructural characterization of the as-tested samples is needed:
- XRD as described above
- higher magnification of the cross section of the surface to determine if the compound layer has been removed.
