Microstructure and Fracture Toughness of Nitrided D2 Steels Using Potential-Controlled Nitriding

Round 1

Reviewer 1 Report

Dear authors, your paper entitled "Microstructure and Fracture Toughness of Nitrided D2 Steels using potential-Controlled Nitriding" is very interesting, hence, it should be published. Before publishing, however, please correct the following issues:

1) Lines 132-133, and 198-199: The statements "Coarse and fine carbides were precipitated in tempered martensite owing to quenching and tempering (Figure 3a)." and "As shown in Figure 8b, micro-cracks were formed in the precipitates when the Palmqvist..." are false. These carbides belong to eutectic- and/or secondary particles. The precipitates are not visible on these micrographs because of low magnification. For revealing the precipitates transmission electron microscopy is needed - compare the size of carbides marked in Figure 3 with real precipitates reported in literature: V.G. Gavriljuk et al., Metall. Mater. Trans. A 45A (2014) 2453 – 2465, T. Nykiel, T. Hryniewicz, J. Mater. Eng. Perform. 23 (2014) 2050 – 2054 or Ďurica J et al. Vacuum. 2019;170:108977.
Please correct to the proper way.

 

2) Figures 7, 8, and micrographs a, c, and e in Fig. 9 are plan-view micrographs. For independent reader it may not be clear. Please extend the figure captions accordingly.

 

3) The principal finding that precisely controlled nitriding can lead to better fracture toughness of nitrided layers may bring certain benefits into the industrial practice. However, it is known that nitriding (and other thermo-chemical processes) commonly lead to embrittlement of materials or tools, eg. Kwietniewski, C. et al., Surf. Coat. Techn. 2004, 179, 27 – 32, or Jurci, P., Materials 2019, 12(10), 1660; https://doi.org/10.3390/ma12101660. Please try to (at least) delineate the effect of controlled nitriding on resulting toughness of the steel, in Discussion for instance.

 

Your reviewer.

Author Response

Dear reviewer, this paper entitled "Microstructure and Fracture Toughness of Nitrided D2 Steels using potential-Controlled Nitriding" was revised. We would like to send you the revised file. We would appreciate it if you could check the corrections.

Point 1: Lines 132-133, and 198-199: The statements "Coarse and fine carbides were precipitated in tempered martensite owing to quenching and tempering (Figure 3a)." and "As shown in Figure 8b, micro-cracks were formed in the precipitates when the Palmqvist..." are false. These carbides belong to eutectic- and/or secondary particles. The precipitates are not visible on these micrographs because of low magnification. For revealing the precipitates transmission electron microscopy is needed - compare the size of carbides marked in Figure 3 with real precipitates reported in literature: V.G. Gavriljuk et al., Metall. Mater. Trans. A 45A (2014) 2453 – 2465, T. Nykiel, T. Hryniewicz, J. Mater. Eng. Perform. 23 (2014) 2050 – 2054 or Ďurica J et al. Vacuum. 2019;170:108977.

Response 1: Lines 132-134, The statements were revised.

Response 1: Lines 202-203, The statements were revised. (revised “the precipitates” to “along the primary carbides”)

Response 1: Lines 206-207, The statements in the Figure 8 were revised. (revised “Figure 8. SEM images of the nitrided surface (compound-free) showing (a) semi-circular cracks, and (b) crack formation along the coarse carbides.”)

(Explanation)

We absolutely agree with the reviewer’s comment. We’d like to explain Primary carbides (micro scale) of the QT base material by using OM(Optical Microscope) and SEM(Scanning Electron Microscope) as shown in Figure 3 and Figure 8 (, not Secondary carbides which can be seen with nano scale measurement). Also, the word ‘precipitate’ were deleted and added “Primary carbides”.

Point 2: Figures 7, 8, and micrographs a, c, and e in Fig. 9 are plan-view micrographs. For independent reader it may not be clear. Please extend the figure captions accordingly.

Response 2: Line 195, The statements in the figure 7 were revised. (revised “SEM images” to “SEM images of the nitrided surface”)

Response 2: Lines 206-207, The statements in the figure 8 were revised. (revised “Figure 8. SEM images of the nitrided surface (compound-free) showing (a) semi-circular cracks, and (b) crack formation along the coarse carbides.”)

Response 2: Line 227, The statements were revised in the figure 9. (revised “beneath indentation corner” to “on the nitrided surface”)

Response 2: Line 228, The statements were revised in the figure 9. (revised “the microstructural evolution of compound layers” to “the cross-sectional microstructures of compound layers”)

Point 3: The principal finding that precisely controlled nitriding can lead to better fracture toughness of nitrided layers may bring certain benefits into the industrial practice. However, it is known that nitriding (and other thermo-chemical processes) commonly lead to embrittlement of materials or tools, eg. Kwietniewski, C. et al., Surf. Coat. Techn. 2004, 179, 27 – 32, or Jurci, P., Materials 2019, 12(10), 1660; https://doi.org/10.3390/ma12101660. Please try to (at least) delineate the effect of controlled nitriding on resulting toughness of the steel, in Discussion for instance.

Response 3: Lines 275-277, The statements were added. (added “Thus, the control nitriding for D2 steel can be effectively used to form the non-porous compound layer which lead to the increase in the fracture toughness by controlling the K_n.”)

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper investigates the effect of surface microstructure on fracture toughness by the adjustment of nitriding potential. The mechanism how the fracture toughness is affected by the surface morphology is analyzed in detail. This paper is deserved publication, whereas, some revisions should be made.

 

1. Line 36, the nitriding treatment and physical vapor deposition should be simply compared.
2. Line 60-61, the effect of thickness and phase evolution with the potential should be discussed based on the literature review.
3. Line 79, why gas nitriding should be indicated.
4. Line 87 “as per AMS 2759” and Line 94 “as per ISO18203”, what does “as per” mean ?
5. Line 99 for XRD and Line 102 for EBSD, what is the step-size?
6. Line 110, why use Equation (4) should be given.
7. For Figure 3, the microstructure of base material should be described.
8. For Figure 8, how to prepare the sample should be simply described.
9. Line 248, “the crack length increased again”, which cannot see from the Figure 9. Or the number should be given for the crack length.
10. The figure 11 should be merged into figure 9, which will help the readers to understand the discussion easier.

Author Response

Dear reviewer, this paper entitled "Microstructure and Fracture Toughness of Nitrided D2 Steels using potential-Controlled Nitriding" was revised. We would like to send you the revised file. We would appreciate it if you could check the corrections.

Point 1: Line 36, the nitriding treatment and physical vapor deposition should be simply compared.

Response 1: Lines 36-39, The statements were revised by adding comparison of two technology. (“Well-known techniques for surface modification, such as nitriding treatment(e.g. compound-free, ion-nitriding) and physical vapor deposition(e.g. TiN, AlTiN coatings), have been studied and implemented to improve the working performance of molds and dies [6,7,11].”)

Response 1: Line 39, The reference [11] was added again.

Point 2: Line 60-61, the effect of thickness and phase evolution with the potential should be discussed based on the literature review.

Response 2: Line 63, The reference was added. (added “[22]”)

Response 2: Lines 365-366, The statements for the reference [22] was added. (added “Kovacı, H.; Yetim, A.F.; Baran, Ö.; Çelik, A. Fatigue crack growth analysis of plasma nitrided AISI 4140 low-alloy steel: Part1-constant amplitude loading. Mater. Sci. Eng. A. 2016, 672, 257-264. https://doi.org/10.1016/j.msea.2016.07.002.”)

Response 2: All references after the reference [22] have been corrected.

Point 3: Line 79, why gas nitriding should be indicated.

Response 3: Controlled nitriding is a technology that control the gas atmosphere during the gas nitriding process.

Point 4: Line 87 “as per AMS 2759” and Line 94 “as per ISO18203”, what does “as per” mean ?

Response 4: AMS 2759 (controlled nitriding method), ISO18203 (hardening depth measurement standard), and “as per” means that the method was carried out according to these standards.

 Point 5: Line 99 for XRD and Line 102 for EBSD, what is the step-size?

Response 5: XRD step size (0.02°/s), EBSD step size (0.08㎛/s)

Point 6: Line 110, why use Equation (4) should be given.

Response 6: Lines 108-109, The statements were added . (added “To determine the fracture toughness of the nitrided layer, the Palmqvist-method was used by estimating the total crack length.”)

Point 7: For Figure 3, the microstructure of base material should be described.

Response 7: Lines 133-134, The statements were added. (added “In the QT base material, primary carbides were observed in tempered martensite owing to quenching and tempering.”)

Point 8: For Figure 8, how to prepare the sample should be simply described.

 Response 8: Lines 198-199, The statements were added. (added “, obtained by polishing the compound layer on the nitrided surface (K_n = 0.2 atm^-1/2).”)

 Point 9: Line 248, “the crack length increased again”, which cannot see from the Figure 9. Or the number should be given for the crack length.

Response 9: Line 252, The statements were added. (added “to approximately 70.4 μm”)

 Point 10: The figure 11 should be merged into figure 9, which will help the readers to understand the discussion easier.

Response 10: Line 226, the figure 11 were merged into figure 9.

Response 10: Lines 228-230, the figure title was added (added “, and schematic diagrams of crack patterns near the nitrided surface showing the different crack growth behaviors at K_n = (g) 0.2, and (h) 5.0 atm^-1/2.”)

Response 10: Line 266, the statements were revised. (revised “Figure 11” to “Figure 9g,h”.)

Response 10: Line 275, the statements were revised. (revised “Figure 11b” to “Figure 9h”.)

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Accepted