An Investigation of the Work Hardening Behavior in Interrupted Cutting Inconel 718 under Cryogenic Conditions

Round 1

Reviewer 1 Report

The technical paper entitled: "An investigation of the work hardening behavior in interrupted cutting Inconel 718 under cryogenic conditions" (materials-791307) deals with the influence of dry and cryogenic machining conditions using different cutting speed values (40 m/min, 60 m/min and 80 m/min) on work hardening layer in the case of a nickel-based alloy such as Inconel 718. In my humble opinion and taking its quality into account, the present manuscript is acceptable with minor changes for its possible publication in the Journal of Materials.

Some suggestions that will help to improve the body of the manuscript are as follows:

1) Are the results obtained by these authors reproducible for other Inconel alloys different from 718? Please, clarify this point.

2) In relation to the cutting speed values employed in this study (40 m/min, 60 m/min and 80 m/min), why have the authors selected this interval and not another which is wider? And what about the influence of other machining factors such as feed rate and depth-of-cut, for instance? Maybe, it is also interesting to study the possible influence of both pressure and flow rate of liquid nitrogen coolant. Please, comment about these possible influential machining factors.

3) Figure 7 shows the micro-hardness values as a function of the depth beneath the surface for the different study cases (1 to 9). How many measurements have been taken at each point from Figure 7? In case several measurements were taken to calculate each point, may the authors say something about the variability of these data (for example, through standard deviation values)?

4) Please, try to express units correctly. There must always be a gap between the number and the symbol of the unit. For instance, on page 4 (line 128), it is said: "40m/min, 60m/min, 80m/min" and, in Table 3 from page 8, "20g" is written instead of "20 g". Please, check these errors.

Author Response

Response to the reviewer’s comments

Comments and Suggestions for Authors

The technical paper entitled: "An investigation of the work hardening behavior in interrupted cutting Inconel 718 under cryogenic conditions" (materials-791307) deals with the influence of dry and cryogenic machining conditions using different cutting speed values (40 m/min, 60 m/min and 80 m/min) on work hardening layer in the case of a nickel-based alloy such as Inconel 718. In my humble opinion and taking its quality into account, the present manuscript is acceptable with minor changes for its possible publication in the Journal of Materials.

Some suggestions that will help to improve the body of the manuscript are as follows:

1) Are the results obtained by these authors reproducible for other Inconel alloys different from 718? Please, clarify this point.

Answer:

Investigation on the surface integrity is of key importance in material removal processes. This paper studies the influence of various cutting conditions on the work hardening behavior which should be controlled in cutting operations. The cryogenic cutting condition has a significant influence on the machined affect zone (MAZ) and depth of work hardening (DWH).

In our opinion, the methods to analyze the work hardening behavior in this paper can be used in cutting with other Inconel materials. Ren and Liu [1] used this method in studying the effect of cutting parameters on work hardening behavior.

1. Ren X and Liu Z. Influence of cutting parameters on work hardening behavior of surface layer during turning superalloy Inconel 718. Int J Adv Manuf Technol 2016; 86: 2319-2327.

2) In relation to the cutting speed values employed in this study (40 m/min, 60 m/min and 80 m/min), why have the authors selected this interval and not another which is wider? And what about the influence of other machining factors such as feed rate and depth-of-cut, for instance? Maybe, it is also interesting to study the possible influence of both pressure and flow rate of liquid nitrogen coolant. Please, comment about these possible influential machining factors.

Answer:

In this study, cutting speed with 40 m/min, 60 m/min and 80 m/min was chosen to study its influence on the work hardening behavior. As well know, Inconel 718 is difficult to cut material and the cutting speed is relatively low when compared with other materials such as TC4 and Aluminum-7075. This cutting speed is selected from the recommendation of the tool manufacturer with the carbide tool used in the cutting operation.

We agree with the opinion of the reviewer that cutting factors such as feed rate, depth of cut and cooling factors such as pressure and flow rate also influence the work hardening behavior. The depth of work hardening decreases with the increase of the cutting speed, while the degree of work hardening increases with the increase of the cutting speed. In the new revision, we add some comments about these factors on the work hardening behavior.

3) Figure 7 shows the micro-hardness values as a function of the depth beneath the surface for the different study cases (1 to 9). How many measurements have been taken at each point from Figure 7? In case several measurements were taken to calculate each point, may the authors say something about the variability of these data (for example, through standard deviation values)?

Answer:

In this study, the hardness of each point in every sample has been tested three times and the hardness value was obtained as the mean value. The micro-hardness test Qness Q10A with automatic measurement function is used in the experiment. As shown in Fig.1, the diagonal of the sample is used to estimate the hardness of the sample. Several column test points on the cross-sections of sub-surface are measured in the test program. From the test result analysis, the variability of the data is less than 10% and the statement has added in the manuscript.

4) Please, try to express units correctly. There must always be a gap between the number and the symbol of the unit. For instance, on page 4 (line 128), it is said: "40m/min, 60m/min, 80m/min" and, in Table 3 from page 8, "20g" is written instead of "20 g". Please, check these errors.

Answer:

Thanks to the reviewer’s suggestion, the units in the manuscript has been checked carefully and all the unit has been changed as "20 g".

Author Response File: Author Response.docx

Reviewer 2 Report

REVIEW

on the article

An investigation of the work hardening behavior in interrupted cutting Inconel 718 under cryogenic conditions

Xing Dai, Kejia Zhuang, Donglin Pu, Weiwei Zhang, Han Ding

Summary.

The article considers the important problem of the hardening of Inconel 718 steel parts during intermittent cutting under cryogenic conditions.

Inconel 718 steel belongs to the group of austenitic nickel-chromium heat-resistant alloys. The alloy is difficult to process due to its tendency to harden. Therefore, alloys such as Inconel 718 are machined with deep but slow cutting using carbide tools. However, in the practical application of this high-strength alloy, a non-trivial scientific and technical problem arises of obtaining products of complex shape from it. This problem is caused by its low workability by pressure due to the extremely narrow temperature and speed range of plastic deformation and is also associated with the difficulty of machining due to the high tendency of this material to harden.

Many articles are devoted to the study of the behavior of this steel under conditions of impact, fatigue loading, residual stresses after machining and others.

In general, the work is an interesting study on an urgent problem and is of scientific interest.

Specific comments.

1. In the introduction, it is necessary to formulate the purpose of the study. The title and annotation talk about the development of processing technology by intermittent cutting with cryogenic cooling. That is, new cutting modes are declared. At the same time, in conclusion, the authors do not write about this.
2. It is not clear which problem was solved and what the authors determined. Before describing the results, formulate the statement of the problem.
3. Figure 7 shows the same field of dispersion of results. I do not see a difference in this data, and therefore in the processing modes. It is necessary to describe the difference in detail. I would recommend that the authors give an approximating curve taking into account the error and compare processing modes on them.
4. Section 3.2 yield strength assessment is in doubt. Formulas (3) - (6) give very rough estimates of strength characteristics. Cahoon obtained the results for carbon structural steels and its dependencies operate in the range of ultimate strength from 350 to 800 MPa. This formula has an error of 15% or more. The authors are dealing with special heat-resistant steels and the dependencies are different. The error is even greater. There is a question. Why analyze a very approximate yield strength if the authors measure microhardness accurately? I would recommend the authors focus on the values of microhardness and microstructure of the hardened layer, and not on an indirect assessment of the yield strength.
5.  It is advisable to compare the results obtained by the authors with the data obtained by other researchers.

Author Response

Response to the reviewer’s comments

Summary.

The article considers the important problem of the hardening of Inconel 718 steel parts during intermittent cutting under cryogenic conditions.

Inconel 718 steel belongs to the group of austenitic nickel-chromium heat-resistant alloys. The alloy is difficult to process due to its tendency to harden. Therefore, alloys such as Inconel 718 are machined with deep but slow cutting using carbide tools. However, in the practical application of this high-strength alloy, a non-trivial scientific and technical problem arises of obtaining products of complex shape from it. This problem is caused by its low workability by pressure due to the extremely narrow temperature and speed range of plastic deformation and is also associated with the difficulty of machining due to the high tendency of this material to harden.

Many articles are devoted to the study of the behavior of this steel under conditions of impact, fatigue loading, residual stresses after machining and others.

In general, the work is an interesting study on an urgent problem and is of scientific interest.

Specific comments.

1. In the introduction, it is necessary to formulate the purpose of the study. The title and annotation talk about the development of processing technology by intermittent cutting with cryogenic cooling. That is, new cutting modes are declared. At the same time, in conclusion, the authors do not write about this.

Answer：

In the revision, the introduction and conclusion have been modified as follows:

“It is well accepted that the microhardness generated beneath the machined surface has a significant influence on sequence cut and the service performance. The work hardening behavior has been investigated by numerous literature while few of them are related to the effect of assisted machining methods. The insufficiency of research on work hardening behavior under cryogenic methods limits the use of assisted machining methods. Work hardening generated in cutting operation is affected by cooling methods, which should have an in-depth understanding. This paper focuses on the surface hardening behavior during cryogenic assisted turning of Inconel 718 with uncoated carbide cutting inserts. The effects of different cooling conditions on micro-hardness as well as surface yield strength are studied. The investigation illustrates the effect of cooling conditions on the yield strength base on Meyer’s law.”

“It can be concluded the turning processes with cryogenic cooling method can obtain higher hardness on the surface layer that can improve the wear performance of final production comparing with dry machining.”

2. It is not clear which problem was solved and what the authors determined. Before describing the results, formulate the statement of the problem.

Answer：

In this paper, experiments are employed to study the effect of cryogenic cooling conditions on the generation of surface hardening layer during turning Inconel 718. It can be concluded the turning processes with cryogenic cooling method can obtain higher hardness on the surface layer that can improve the wear performance of final production comparing with dry machining. From the obtained data, the work hardening can be better understood and controlled, which can be used to enhance the machinability and surface integrity characteristics of superalloy Inconel 718.

3. Figure 7 shows the same field of dispersion of results. I do not see a difference in this data, and therefore in the processing modes. It is necessary to describe the difference in detail. I would recommend that the authors give an approximating curve taking into account the error and compare processing modes on them.

Answer：

Fig. 7(a)-(c) show the microhardness profile beneath the machined surface with dry, pre-cooling workpiece and cooling cutting zone, respectively. In the comparison of different cutting condition, the depth of work hardening layer and degree of work hardening has changed. It can be seen from the profiles that the depth of the work hardening layer has a slight decrease with the increase of cutting speed under proposed machining conditions. The depth of work hardening layer maintains about 60 μm without coolant, a bit thicker under pre-cooling method and then, the highest depth obtained in the cooling cutting zone method (85 μm).

An approximating curve of the work hardening profile is given in the revision and the statement in detail has changed.

4. Section 3.2 yield strength assessment is in doubt. Formulas (3) - (6) give very rough estimates of strength characteristics. Cahoon obtained the results for carbon structural steels and its dependencies operate in the range of ultimate strength from 350 to 800 MPa. This formula has an error of 15% or more. The authors are dealing with special heat-resistant steels and the dependencies are different. The error is even greater. There is a question. Why analyze a very approximate yield strength if the authors measure microhardness accurately? I would recommend the authors focus on the values of microhardness and microstructure of the hardened layer, and not on an indirect assessment of the yield strength.

Answer：

We agree with the comments of the reviewer that the strength characteristics estimated from micro hardness are an approximation result. Formulas (3) - (6) give a relationship between the microhardness and yield strength of the material which derived from carbon structural steels. In the previous study [1-2], those formulas can be used in other materials such as superalloy Inconel 718. In this paper, the estimation of yield stress shows the effect of various cutting conditions on the material strength, which may give the engineering advice on the cutting operation.

The microhardness as well as the microstructure of the hardened layer is very important in the investigation on the cutting process and should be controlled. Focus on surface integrity is the research target of our group.

1. Ren X and Liu Z. Influence of cutting parameters on work hardening behavior of surface layer during turning superalloy Inconel 718. Int J Adv Manuf Technol 2016; 86: 2319-2327.
2. Pawade R, Joshi SS and Brahmankar P. Effect of machining parameters and cutting edge geometry on surface integrity of high-speed turned Inconel 718. Int J Mach Tools Manuf 2008; 48: 15-28.

5. It is advisable to compare the results obtained by the authors with the data obtained by other researchers.

Answer：

Thanks to the reviewer’s advice. Some comparison with the data obtained by other researchers will be given in the future.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear colleagues!

        I wish the authors success and recommend the article for publication in the form presented.
        Nevertheless, for future publications, I would recommend that the authors do not use crude indirect estimates of the yield strength, considering that they measure microhardness accurately. Either use your own dependencies for a specific steel grade or build research based on microhardness.