Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy

Round 1

Reviewer 1 Report

This is a good research with valuable results and I believe it has a potential to be published in the Journal. It also has a good structure, but before that it is necessary for the authors to provide a major revision of the manuscript.

1- In the literature review, it is not necessary to write the first name of the authors (for example, on page 2 line 75, the phrase "E. vahdat et al." should change to "Vahdat et al."). the same action should be done in the whole introduction. 

2- On page 5 line 180, it seems that 107 cycles should change to 10e7 cycles. 

3- The numbers in Figure 1 are not clear, especially in the right-side image (b). so, please provide a high-quality image. 

4- Related to Figure 6, what is the symbol "?" in front of numbers. The figure should be corrected. 

5- Related to Figure 7, it is better to redraw it. In addition, what is the title of axis?

6- The title 3.2 should change to ''"Hardness and tensile properties".

7- Related to Figure 8, it is stated that three tests are done in each type of treatment, so please report all six tensile test results in this figure. 

8- Related to section 3.4 (Fatigue behaviour), the results of fatigue tests should be reported completely (e.g., stress-life diagram) and explain the data to reach the fatigue strength. In addition, it is necessary to explain more about the method used to find the fatigue strength? 

9- On page 11 line 385, the phrase "106" is true or it should be changed to 10e6?

10- Related to figure 14, the quality of the images is not ok and in addition, what is your main purpose to present this figure? I can not find an useful data on it. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This work mainly addressed the heat treatment effect of tool steels on the microstructure, mechanical properties including tensile, hardness and bending fatigue performance. In general, the author completely presented the research aims, method, and results. It is a complete research paper. After reviewing, I have some comments to the authors for further polishing their paper.

1) some figures are obscure, such as Figure 2b, and Figure 11.

2) when presenting their own results, the authors always cited many references. I do not know what is your purpose. If the results is yours, in my opinion, the citing is not very necessary. 

3) In the page of 6, such as lins 198-204, the authors can reconsidered these sentences. For example, what is the size for large carbides?

4) For bending fatigue tests, why not show the S-N curves for each condition. You gave the standard deviation of tensile and harndess, why not for the fatigue results. In my opinion, the discreteness is more important for the fatigue results than others.

5) The authors stated the substantial absence of segregations, large carbides, inclusions and porosity in the tested steels. but they also found a feature of 'fish eye' on the fracture surface and in your conclusions, the large carbides or inclusions are also responsible for the origin of cracks.  It seems to be some contradictions

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In FIG. 1 add units on the y-axis [° C] and on the x-axis of the time unit [h]

Line 145. For the etchant Nital add that it is a 2% solution

I recommend checking the superscripts and subscripts in units.

In conclusion, define that it is an advantage if the carbides are evenly dispersed in the materials, which is of course true. During the analysis of the current state, you also came across information about possible limit dimensions of carbides of the MC type resp. M6C? Does their size affect the stress concentration in the material and the subsequent initiation of the failure?

Have you also dealt with the economic aspect of changing the heat treatment process? If so, what does the application of a process using cryogenic temperatures look like?

I appreciate the high-quality processing of light and electron microscopy results as well as fractographic analysis. The hill of scientific work is truly visible.

Author Response

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Reviewer 4 Report

The authors did a good work. Congratulations!

In my opinion, they can improve the work if they made a study in the field of residual stresses. This residual stresses must be calculated in the surface but, more important, in depth. To have a profile of the residual stresses for the two types of treatments would certainly help to better understand the results already obtained. And of course the same for the retained austenite!

Anyway, here you have my comments, line by line.

Line 89

“...Molinari and Sola [23, 25, 35] However, to take…”

The . is missing “...Molinari and Sola [23, 25, 35]. However, to take…”

Line 221, 222, 224,

“...size is about 0.40 m with…”

The symbol of micrometer is missing

Line 234, Figure 7

I don’t understand the vertical scale of the two graphics with the two colors. The authors must clarify this. What is the relationship between the two treatments, in the graphics?

Line 371, Figure 12

“...and presence of a flaw (c,d,e) or to the…”

Something is missing at the end of the sentence

In Figures 12 and 13 the authors did not mention the number of cycles until the fracture of the specimens. Maybe it is important to know this.

Line 405

“...This is probably due to the presence of surface compressive residual stresses, induced by the machining processes for sample production…”

In this work we don't know anything about residual stresses. If they are compressive or in tension. If they are compressive, we don't know what is the depth until they reach traction values. I think that the authors can poursuit the work in terms of calculating the residual stresses profile in depth to know exactly what happens in terms of residual stresses.

Line 455

“…Therefore, in conventional quenching and tempering heat treatment, the austenitization temperature is defined in order to avoid an excessive dissolution of primary carbides, and the consequent carbon enrichment of the austenite. As a matter of fact, high C content leads to the formation of harder and thus more brittle martensite and increases the amount of retained austenite (RA) after quenching. In order to enhance toughness and to reduce the amount of retained austenite…”

We don't know what is the reduced retained austenite in type II versus type I?

Line 476

“...This induces precipitation of submicrometric carbides with a reduction of the residual stresses….”

The residual stresses are in compression or in tension? What do the authors want to say about this reduction? 

Line 569 and table 5

“...A comparison of the experimental Kf coefficients (reported in Table 5) highlights…”

I don't understand this sentence and the connection with the results of table 5. In this table all the results are equal, for the two types of treatment.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It seems that the authors did their best to edit the article based on the reviewers' comments. In addition, they carefully responded all comments step by step. Eventually, I believe that the revised manuscript has improved compared to the previous version and Accepted.