Laser Surface Hardening of Ni-hard White Cast Iron

Round 1

Reviewer 1 Report

Dear Authors,

Congratulations on your work, which talks about an interesting subject.

However, I have serious concerns about your paper, which is well-written, but presents serious flaws in technical/scientific terms, whose must be revised beore publication. The following comments are divided into two groups: scientific and technical concerns, and grammar/formatting concerns.

Scientific and technical concerns:

1. The introduction needs to introduce the reader to the microstructure of these kind of White Cast Irons. No Ledeburite or Cementite are formed in these alloys and this needs to be properly explained in the Introduction. Please introduce explanations and literature to properly support your development.
2. The Introduction is too poor. The number of references is very low for this kind of work, and just generic information is provided to the reader there. Thus, you must describe developments made by other Authors, as follows: "Regarding the work performed by XXXXX [y], it was found that...". Go directly to the ideas, describing the main objective of the works and the principal findings.
3. The Materials and Methods section is one of the worst sections in your work. No strategy to develop the work is provided. Thus, the reader is lost in your work. Please provide at the beginning a complete strategy for your work, justifying all steps.
4. In the development, you present XRD spectra but, in the Methods, no equipment and test conditions are presented. Please complete.
5. You performed wear tests. However, the surface state was not assessed. Thus, how can we evaluate the wear without that information?
6. You present the rotational speed (785 rpm), but you don't present the linear speed or the diameter of the circle performed on the surface of the ring. How can we evaluate the results?
7. In Methods section you describe the Impact Testing equipment. However, you don't refer the maximum energy available in the equipment. 
8. In Hardness tests, please provide information about the standard used.
9. Regarding the heat-treatment process, please provide information about the cooling rate used in each set od conditions.
10. In 3.1.4, please provide information about the TTT diagrams for this alloy. The information contained in Lines 150 and 151 needs that.
11. Why no measurements about the residual stress in the surface were performed? If you used the XRD technique, this could be made.
12. The rsults are statements about what yu have observed, but without a scientific base behind that. Thus, the comments about the results are less interesting.
13. No DISCUSSION is provided around the RESULTS. Thus, the reader cannot extract the desire information about your paper.
14. Do you think that the gain around the energy in the Impact tests is significant. I don't think so.
15. The Conclusions leave many phenomena in the air...They are statements, but without scientific explanation. The rise in hardness together with a small increase in energy absorbed in the impact tests needs to be properly expained. 

Grammar/formatting concerns:

1. In lines 64 and 65, please be coherent in pointing out the dimensions (spaces between units, values and signals).
2. In Table 1, please use always the same significant number of digits.
3. In line 69, please correct to kW.
4. In Table 2, please correct to kW.
5. In line 93, please insert a space between the unit and the value. There are other similar mistakes throughout the manuscript.
6. In line 114 correct "...samples...".
7. In line 114 correct the spaces in the dimensions.
8. Please correct the verb in the sentence (line 124): "Some secundary carbides (SC) was...".
9. Please put the Figures into the limits of the text.

 

Good luck.

Kind regards.

Reviewer

Author Response

Reviewer #1

 

We would like to thank you for giving us constructive suggestions which would help us in depth to improve the quality of the article.

 

Responses according to the reviewer’s comments:

 

The following is a point-to-point response to the reviewer's comments. All revisions are be clearly highlighted using the "Track Changes" function in Microsoft Word, so that changes are easily visible to the editors and reviewers.

 

Congratulations on your work, which talks about an interesting subject.

1. The introduction needs to introduce the reader to the microstructure of these kind of White Cast Irons. No Ledeburite or Cementite are formed in these alloys and this needs to be properly explained in the Introduction. Please introduce explanations and literature to properly support your development.

Response:

• New part with some new references has been added in the introduction in the revised manuscript.

"Ni-Hard 1 alloys contain about 44 % M3C type carbide embedded in an iron matrix [4]. Their solidification begins with formation of proeutectic austenite followed by eutectic reaction liquid to austenite + M3C [5]. Upon cooling to room temperature, austenite transforms to martensite [6,7]. Ni-Hard 4 is designed to give a microstructure of eutectic M7C3 carbides embedded in a metallic matrix that is predominantly austenite in the as-cast condition. but, after hardening heat treatments, the matrix is predominantly martensitic [8]. Carbon content of Ni-Hard 4 is kept between 2.8 and 3.2 wt% as a compromise between abrasion resistance and fracture toughness. Carbon content determines the volume fraction of eutectic carbides since most of it enters the carbide phase together with iron, chromium and other carbide-forming elements present [9]. A minimum content of about 5 wt. % Ni should be maintained to inhibit pearlite formation during slow cooling of the casting. Silicon addition above 1.5 wt. % hinders the peritectic reaction and thus favors the formation of entirely eutectic M7C3 carbides. Lower silicon contents lead to formation of mixed carbide morphologies of M3C and M7C3, which may impair both abrasion resistance and fracture toughness [10]."

2. The Introduction is too poor. The number of references is very low for this kind of work, and just generic information is provided to the reader there. Thus, you must describe developments made by other Authors, as follows: "Regarding the work performed by XXXXX [y], it was found that...". Go directly to the ideas, describing the main objective of the works and the principal findings.

Response:

• We have added to the literature so that the readers can understand easily. Some new references have been added as well.

"In view of such advantages, many studies are concerned with laser surface hardening e.g. Hwang et al, [18] investigated the effect of laser power and travel speed on the characteristics of the laser-hardened layer of gray cast iron. They found that the hardness of the laser-hardened layer was in a range between 800 and 950 Hv0.1, regardless of the laser power and the wear resistance was doubled. Duffet et al, [19] examined the use of Nd:YAG laser in the treatment of cast iron cylinder bore of automobile engines, the laser beams allowed holes on the initial bore texture to be obtained and significantly yields larger hydrodynamic regime.  Furthermore, Slatter et al, [20] applied a CO₂ laser to treat the valve seat area of a cast iron cylinder head to increase the wear resistance of the components and the results from the impact testing showed a very hard surface that decreased wear. Regarding the work of Kh. M. Adel et al, [21] the use of Nd-Yag pulsed wave laser modified and refined the surface microstructure of acicular bainitic ductile iron, also the wear rate increased with the increasing in the normal loads at constant sliding speed and sliding time. kornȇl Majlinger et al, [22] studied the laser treated layer of cast iron cylinder bores with lamellar graphite, the near surface area of the cylinder bore becomes harder and more wear resistant, furthermore, due to the inhomogeneity of the pearlitic matrix and graphite lamellae, oil reserving holes are formed. Both G.Marest et al, [23] and D. Pantelis et al, [24] used excimer laser to heat treat different types of cast iron alloys, the microstructure and hardness were improved after laser hardening."

 

3. The Materials and Methods section is one of the worst sections in your work. No strategy to develop the work is provided. Thus, the reader is lost in your work. Please provide at the beginning a complete strategy for your work, justifying all steps.

Response:

• Thanks for your valuable comment. We have provided a complete strategy of our work in the Materials and Methods section.

 

"The aim of this study is to achieve a good hardened layer of Ni-hard 1 and 4 alloys by means of laser hardening process, and it was necessary to know the quality and microstructure of the laser hardened zone. The possible strategy was to investigate the hardened layers microstructure by using optical and scanning electronic microscopy, EDX, X-Ray, microhardness and wear tests. These tools could help to analyze whether the layer after laser hardening was suitable for practical use. Through the analysis of the microstructure of the hardened layer, it was possible to know the distributions of the compositions, cracks and porosities, such information and experiment data were very useful for the further process improvement.

Overall Workflow

1. Casting and preparation of two different types of Ni-hard (Ni-Hard 1 & Ni-Hard 4) alloys.
2. Achievement of laser surface hardening on such types of Ni-hard alloys and then, for comparison purpose conventional heat treatment was performed as well.
3. Surface morphology and microstructure investigation by using optical and scanning electronic microscopy followed by X-Ray diffraction (XRD) was performed.
4. Tribological tests (Microhardness and abrasive wear tests) were accomplished.
5. Finally, room temperature impact toughness test was carried out."

 

4. In the development, you present XRD spectra but, in the Methods, no equipment and test conditions are presented. Please complete.

Response:

• Thanks for your constructive comment. We have provided the information about the standard used.

 

"Additionally, the phases present in the surface layer were determined by X-ray diffraction (XRD) with a diffractometer model ( XPert Pro Analytical 45 V, 40 mA with source copper λ = 1.54Å Poland ) using CuKα radiation in a 2θ range of 30–120◦ and to record data at 0.02 degrees steps at a speed of 0.004 degrees/min."

 

5. You performed wear tests. However, the surface state was not assessed. Thus, how can we evaluate the wear without that information?

Response:

• Thanks for your valuable comment. We have added the surface state.

"The wear test was performed on the samples after treatments, without any surface preparation for the samples."

6. You present the rotational speed (785 rpm), but you don't present the linear speed or the diameter of the circle performed on the surface of the ring. How can we evaluate the results?

Response:

• Thanks for your valuable comment. We have added the diameter and the hardness of the ring.

"The pin-on-ring abrasive wear test involved high stress, two-body abrasion, in which one end of a rectangular pin specimen was fixed against rotating abrasive silicon carbide ring having a diameter of 250 mm and hardness of 2600 HV0.05."

7. In Methods section you describe the Impact Testing equipment. However, you don't refer the maximum energy available in the equipment.

Response:

• Thanks for your valuable comment. We have completed all information about the impact testing equipment.

"The impact machine has maximum impact energy of 500 J, maximum impact speed of 5.4 m/s, minimum impact energy resolution of 0.2 J, and rising angel of 150˚. at room temperature."

8. In Hardness tests, please provide information about the standard used.

Response:

• Thanks for your constructive comment. We have provided the information about the standard used and added a reference.

"Vickers surface hardness of the as-cast (AC), CHT and LHT samples were measured under a load of 10 kg, the full load being normally applied for 15 seconds, according to ASM G 132 [27]."

9. Regarding the heat-treatment process, please provide information about the cooling rate used in each set of conditions.

Response:

• Thanks for your valuable comment. A figure has been added to show the heating and cooling cycles.

"To achieve comparison purpose, a conventional heat treatment (CHT) was subjected to the samples; samples were austenitized at 800 °C for 8 hours, and subsequently cooled inside the furnace with slow cooling rate of 30 °C / hour. To improve the impact toughness, subcritical treatments (tempering) were carried out on the CHT samples. The samples were held to 300 °C for four hours followed by air cooling, see Figure 1."

Figure 1. Conventional heat treatment process; (a) hardening heating and cooling cycles and (b) tempering heating and cooling cycles.

10. In 3.1.4, please provide information about the TTT diagrams for this alloy. The information contained in Lines 150 and 151 needs that.

Response:

• Thanks for your valuable comment. We have added this part with a reference that contains all information and figures about the TTT diagram of such alloys.

 

"As a result of its high alloy content, as cast Ni-hard 4 usually contains over 50% retained austenite and more complete austenite transformation to martensite is achieved by destabilization of the austenite at temperature above 750 °C, where austenite stability is lowered by carbide precipitation [30]."

11. Why no measurements about the residual stress in the surface were performed? If you used the XRD technique, this could be made.

Response:

• Unfortunately, we did not measure the residual stresses in this work, and we will consider this in our future work. A previous work discussion with reference has been added in this part.

 

"According to Strum's work [31] the residual stress variation in the hardened layer can be described with the concentration of retained austenite, martensite, and carbides in vol.%. In the hardened layer martensite transformation causes local increase of the microstructure volume and contributes to compressive residual stresses.  Extra investigation of the residual stresses is planned for future work."

12. The results are statements about what you have observed, but without a scientific base behind that. Thus, the comments about the results are less interesting.

Response:

• Both comments of 12 & 13 were done in all Results and Discussion section.
• Thanks for your valuable comment. We have added many new parts within the Results and Discussion section with a new reference in order to provide enough discussion. All new parts are highlighted in red color in the revised manuscript.

Response:

• Both comments of 12 & 13 were done in all Results and Discussion section.

14. Do you think that the gain around the energy in the Impact tests is significant. I don't think so.

Response:

• This part has been added to the revised manuscript.

"Even though the impact toughness data of the hardened samples show some similarity in their values, yet one cannot ignore their levels compared to the as-received impact toughness of the CHT samples. It should be noted that the data reported here regarding impact toughness was rather limited, and mainly directed towards comparing the impact toughness levels between the conventional and the laser treated samples. However, we conclude drawn that the results have some practical significance for impact toughness, as our objective was to find the heat input values that would lead to optimum mechanical properties (hardness, wear resistance and impact toughness)."

15. The Conclusions leave many phenomena in the air...They are statements, but without scientific explanation. The rise in hardness together with a small increase in energy absorbed in the impact tests needs to be properly explained.

Response:

• Many parts have been added to the revised manuscript in order to improve the conclusions.

1. Laser hardening was successfully performed on Ni-hard 1 (S1) and Ni-hard 4 (S2) samples. A homogeneous microstructure is obtained in the laser hardened zone and both hardness and wear resistance were clearly improved. A proper processing window for laser hardening was established as a heat input ranges from 6 to 16.78 J/mm² in order to achieve the optimum condition that achieve acceptable mechanical properties.
2. The hardened depth in both samples was observed to directly proportional to the laser heat input values. The shallowest hardened zone of S1 iron was 25 µm and was associated with the least heat input level of 6 J.mm^-2, while the deepest hardened thickness averages 500 µm and corresponds to the highest heat input level of ~17 J.mm^-2. The corresponding samples of the S2 iron yield values for the hardened depth which range from 12 µm to 400 µm.
3. The microhardness values clearly increased after laser hardening by almost three times for S1 sample (580 HV to 1455 HV). Increment of hardness values of the surface layer is caused by the ultrafine-grained structure and high dislocation densities on the surface after the laser hardening.
4. Analysis of the microstructure and of the measured microhardness of the laser-hardened area leads to the conclusion that the laser hardening induced an increase in the microhardness of the Ni-hard irons and this is due mainly to the appearance, in the matrix, of martensite and to a lesser degree of austenite with an insignificant amount of dispersed eutectic carbides.
5. The wear resistance improved by nearly three times higher due to laser surface hardening. This is due to the martensitic transformation of the austenite dendrites in the microstructure that causes local increase of the microstructure volume and contributes to compressive residual stresses which resulted in the increase of the resistance to subsurface crack formation.
6. The wear resistance depends on the hardened depth, the amount of laser heat input, hardness levels, and compressive stresses obtained during the rapid cooling after laser processing.
7. At lower heat input values (below 10 J.mm^-2), crack formation was observed due to large thermal gradient resulted from the very rapid quenching during laser treatment. With increasing the laser heat input value, no evidence of any subsurface cracking during the wear test was detected. consequently, the laser hardening process should be optimized to avoid any crack formation.
8. The attained impact energy from laser treated samples is approximately equal to that of the as cast alloys although the surface hardness of the LHT is higher by nearly three times as compared to the as-cast.
9. Laser surface treatment imparts resistance to subsurface crack initiation and propagation (at laser heat input above 10 J.mm^-2), the refined microstructure as well as the high stresses induced during rapid cooling of the surface treated surface are expected to retard the crack propagation during the fracture process and accordingly increase the toughness level. This makes laser surface hardening a good choice for improving impact toughness as was recorded by the present findings."

 

 

16. Grammar/formatting concerns:
17. In lines 64 and 65, please be coherent in pointing out the dimensions (spaces between units, values and signals).

 This mistake has been corrected.

2. In Table 1, please use always the same significant number of digits.

This mistake has been corrected.

3. In line 69, please correct to kW.

This mistake has been corrected.

4. In Table 2, please correct to kW.

 

This mistake has been corrected.

 

5. In line 93, please insert a space between the unit and the value. There are other similar mistakes throughout the manuscript.

This mistake has been corrected throughout the manuscript.

6. In line 114 correct "...samples...".

This mistake has been corrected.

7. In line 114 correct the spaces in the dimensions.

This mistake has been corrected.

8. Please correct the verb in the sentence (line 124): "Some secondary carbides (SC) was...".

This mistake has been corrected.

9. Please put the Figures into the limits of the text.

All figures were aligned with the limits of the text. If there is any misalignment, we will take care of it at the final stage.

• Hoping that the changes introduced improved the manuscript in satisfactory way. With my best regards

Author Response File: Author Response.pdf

Reviewer 2 Report

1. To what extent the authors achieved optimum values of laser power, only 3 values were used. Usually to get optimized values far more values should be studied. The same concerns the laser speed, which affect thermal effects on the substrate surface.
2. What are the optimum values of hardened surfaces for practical application and specify the values range and specific applications.
3. In Figs. 12,15,16,18 there are no standard deviations, which does not allow to reveal significance of the obtained differences.
4. Statistics is missing in most of the cases, thus the relevance of the results and difference between results must be checked.

Author Response

Reviewer # 2

 

We would like to thank you for giving us a constructive suggestion which would help us in depth to improve the quality of the article.

 

Responses according to the reviewer’s comments:

 

The following is a point-to-point response to the reviewer's comments. All revisions are be clearly highlighted using the "Track Changes" function in Microsoft Word, so that changes are easily visible to the editors and reviewers.

 

1. To what extent the authors achieved optimum values of laser power, only 3 values were used. Usually to get optimized values far more values should be studied. The same concerns the laser speed, which affect thermal effects on the substrate surface.

Response:

• Thanks for your constructive comment. We executed preliminary experiments from previous works, and we added their references. Then we established a proper processing window for laser hardening as the heat input ranges from 6 to 16.78 J/mm², so around seven different values of laser heat input were studied in this work. These following parts exist in the manuscript.

 

"Preliminary experiments with different laser power values were executed through the previous studies [18–21]. Eventually, a range of laser treatment parameters was chosen to determine the optimum processing conditions for surface treatment."

 

"Heat input values were calculated to combine the effect of laser power with the scanning speed and the laser beam diameter into one value. Heat input was calculated according to the following equation H=P/D.V where: H: heat input (J·mm⁻²), P: laser power (J·s⁻¹), D: beam diameter (mm), and V: laser scanning speed (mm·s⁻¹) [26]."

 

2. What are the optimum values of hardened surfaces for practical application and specify the values range and specific applications?

Response:

• Thanks for your valuable comment. The following part has been added.

" Finally, these cast irons are used extensively in applications requiring superior wear resistance and in practice, hardness is usually the best and only real means of quality control of Ni-hard alloys. These applications include crushing and grinding ores, mixing concrete, pulverizing coal, and rolling metals which require harness values in the range of 500 HV [29]. After laser hardening the hardness values increased by a factor of three higher than that recorded for the untreated alloys which means the increasing in the working lifetime of such component and the expectation of money saving which is an essential factor for successful engineering solutions."

• Also, we mentioned the optimum condition for the mechanical properties in the conclusion as follows:

" The optimum condition for the laser hardening process corresponds to the highest laser heat input value (16.78 J.mm^-2). It resulted in a high quality of hardened zone with crack free layer."

 

3. In Figs. 12,15,16,18 there are no standard deviations, which does not allow to reveal significance of the obtained differences.

Response:

• Thanks for your valuable comment. We have added error bars that represent the standard deviations in all required figures.

 

4. Statistics is missing in most of the cases, thus the relevance of the results and difference between results must be checked.

Response:

Hoping that the changes introduced improved the manuscript in satisfactory way. With my best regards

Author Response File: Author Response.pdf

Reviewer 3 Report

Please:

- change the subtitles: 2.3 and 3.1 only to Microstructural analysis or microstructure characteristics.
- introduce Mechanical and tribological tests into the subtitle.
- correct:
subtitle 3.3: Hardness and microhardness distribution
The signature in Figures 11 and 13 - hardness instead of macrohardness
Please determine the unit of hardness correctly and record the hardness values using it : for Vickers hardness it is HV10 and for microhardness it is HV0.5 (this results from the procedure quoted).
In Figures 6 and 9 correctly mark the carbide phases: Cr3C2 and cementite.

Author Response

Reviewer # 3

 

We would like to thank you for giving us constructive suggestions which would help us in depth to improve the quality of the article.

 

Responses according to the reviewer’s comments:

 

The following is a point-to-point response to the reviewer's comments. All revisions are be clearly highlighted using the "Track Changes" function in Microsoft Word, so that changes are easily visible to the editors and reviewers.

 

1. Extensive editing of English language and style required

 

Response:

• The manuscript has been checked by a native English-speaking colleague

 

2. change the subtitles: 2.3 and 3.1 only to Microstructural analysis or microstructure characteristics.

 

Response:

 

• The subtitles have been changed.

 

3. introduce Mechanical and tribological tests into the subtitle.

 

Response:

 

• The subtitles have been changed and new ones have been added.

 

4. correct:

 

• subtitle 3.3: Hardness and microhardness distribution

Response:

• This subtitle has been corrected

 

• The signature in Figures 11 and 13 - hardness instead of microhardness

 

Response:

 

• Thanks for your valuable comment. The signatures have been corrected.

 

• Please determine the unit of hardness correctly and record the hardness values using it: for Vickers hardness it is HV10 and for microhardness it is HV0.5 (this results from the procedure quoted).

 

Response:

 

• Thanks for your valuable comment. The unit of the hardness has been corrected in each figure.

 

• In Figures 6 and 9 correctly mark the carbide phases: Cr3C2 and cementite.

 

Response:

 

• Thanks for your valuable comment. This part has been corrected in figure 6.

 

• The eutectic carbides in Ni-Hard 4 alloys are Cr7C3 + cementite as shown in the XRD pattern

Hoping that the changes introduced improved the manuscript in satisfactory way.

 

With my best regards

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors,

 

Thank you so much for the huge effort in addressing all the Reviewers' comments.

However, there are more two improvements needed:

1. Table 1 is not OK. I suggest you use the number in the following format: "x.xx" in all of them. Even when you have 2, you need to put 2.00.
2. The references are not OK. There is information missing in some papers, as well as the DOI in another ones. Thus, please complete.

Kind regards,

Reviewer

Author Response

Reviewer #1

 

We would like to thank you for giving us constructive suggestions which would help us in depth to improve the quality of the article.

 

The following is a point-to-point response to the reviewer's comments. All revisions are be clearly highlighted using the "Track Changes" function in Microsoft Word, so that changes are easily visible to the editors and reviewers.

 

Responses according to the reviewer’s comments:

 

Thank you so much for the huge effort in addressing all the Reviewers' comments.

However, there are more two improvements needed:

1. Table 1 is not OK. I suggest you use the number in the following format: "x.xx" in all of them. Even when you have 2, you need to put 2.00.

 

Response:

 

• Thank you for your valuable comment. We have modified it.
• Table 1. Chemical composition of the Ni‐Hard alloy used.

Sample Number	Chemical Composition (%)
	C	Si	Mn	Ni	Cr
Sample 1 (S1) Ni‐hard 1	3.10	0.70	0.77	4.00	2.44
Sample 2 (S2) Ni‐hard 4	2.77	2.00	0.50	5.50	9.40

 

2. The references are not OK. There is information missing in some papers, as well as the DOI in another ones. Thus, please complete.

 

Response:

• Thank you for your valuable comment. All references have been checked and modified as appeared in the revised manuscript.
• Hoping that the changes introduced improved the manuscript in satisfactory way.

 

With my best regards

Author Response File: Author Response.pdf

Reviewer 2 Report

Can be accepted

Author Response

Reviewer # 2

 

We would like to thank you for your opinion on our revised manuscript

 

Responses according to the reviewer’s comments:

 

Reviewer # 2 has no further comments  

 

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	(x)	( )	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )

Comments and Suggestions for Authors

Can be accepted

 

With best regards

 

Author Response File: Author Response.pdf