Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel

Round 1

Reviewer 1 Report

REVIEW COMMENTS

Manuscript ID: metals-metals-1208245

Color light metallography versus electron microscopy for detecting and estimating various phases in a high-strength multiphase steel

 

The authors present an analysis of the use of different metallographic preparation strategies to identify the phases present in multiphase steels. For this, they use a medium carbon steel to which they apply a heat treatment obtaining a multiphase microstructure formed by bainite, martensite and retained austenite. The observation and estimation of the fraction of these phases by means of a metallographic preparation is a complex task because there is no single reagent that allows obtaining an adequate contrast between the different phases. For this reason, the authors decide to analyze the performance of several attack steps in order to adequately reveal the different phases present in the microstructure of the steel. Likewise, in order to validate the results obtained with the different metallographic techniques, they estimate the fraction of the phases obtained by image analysis of the microstructures obtained with the fraction of the phases obtained by other techniques (SEM, EBSD, XRD). This allows them to conclude that the four-step metallographic technique that they propose allows a good estimate of the residual austenite fraction in the steel.

The work is interesting and very useful for researchers working with multiphase steels. In addition, it is presented neatly and clearly, which makes it very pleasant to read. The experimental methodology used is described in detail and the results are presented and discussed clearly. However, some minor errors have been observed and some aspects could perhaps be better explained. For this reason, I would appreciate you consider the following suggestions:

 

Introduction

1. The summary of the state of the art of the subject studied is very complete. The interest of a correct identification of the different phases in, for example, 3^rd generation multiphase steels is justified. Likewise, the description of works by other authors who have analyzed the use of different types of etching techniques to identify the different phases in multiphase steels is complete and clear.

 

Materials and Methods

2. Line 133: What is the Bs temperature of the analyzed steel? The authors calculate it in their work doi:10.3390/met9050492, based on an empirical formula, but I think it would be convenient to indicate it in the present work as well.
3. Line 154: In some cases, it is indicated that the Marble's reagent was used at 30 ° C or 40 ° C. In the rest of the etching techniques used, the temperature of the reagent is not indicated. What was the temperature in these cases?

 

Results and discussion

4. Line 191: Why with the nital etch the white-colored constituents are identified as RA and with the Marble´s reagent they are identified as M/RA? Please, correct it or explain it.
5. Lines 247-249: I think it would be interesting to include an image at higher magnification to see more clearly the RA particles, speckled with fine, orange-coloured, martensite laths.
6. Line 363: According to what is indicated in Table 4, the calculated fraction of bainite in the double-step metallography techniques is approximately 50%. However, in the three-step and four-step techniques the bainite fraction is approximately 70%. This seems to indicate that the two-step techniques do not allow to fully reveal all the areas that present bainitic structure, even though the Marble´s reagent is used, which is indicated to reveal the bainite. It would be interesting for the authors to discuss this fact and explain the possible causes of the difference in bainite fraction observed as a function of the different metallographic techniques analyzed.

Author Response

The authors present an analysis of the use of different metallographic preparation strategies to identify the phases present in multiphase steels. For this, they use a medium carbon steel to which they apply a heat treatment obtaining a multiphase microstructure formed by bainite, martensite and retained austenite. The observation and estimation of the fraction of these phases by means of a metallographic preparation is a complex task because there is no single reagent that allows obtaining an adequate contrast between the different phases. For this reason, the authors decide to analyze the performance of several attack steps in order to adequately reveal the different phases present in the microstructure of the steel. Likewise, in order to validate the results obtained with the different metallographic techniques, they estimate the fraction of the phases obtained by image analysis of the microstructures obtained with the fraction of the phases obtained by other techniques (SEM, EBSD, XRD). This allows them to conclude that the four-step metallographic technique that they propose allows a good estimate of the residual austenite fraction in the steel.

The work is interesting and very useful for researchers working with multiphase steels. In addition, it is presented neatly and clearly, which makes it very pleasant to read. The experimental methodology used is described in detail and the results are presented and discussed clearly. However, some minor errors have been observed and some aspects could perhaps be better explained. For this reason, I would appreciate you consider the following suggestions:

Introduction

1. The summary of the state of the art of the subject studied is very complete. The interest of a correct identification of the different phases in, for example, 3^rdgeneration multiphase steels is justified. Likewise, the description of works by other authors who have analyzed the use of different types of etching techniques to identify the different phases in multiphase steels is complete and clear.

We are thankful to the reviewer for his positive comments.

 

Materials and Methods

1. Line 133: What is the Bs temperature of the analyzed steel? The authors calculate it in their work doi:10.3390/met9050492, based on an empirical formula, but I think it would be convenient to indicate it in the present work as well.

We have now included the bainite start (B_s) temperature in the Experimental section in a manner similar to our previous work (Line 137).

 

1. Line 154: In some cases, it is indicated that the Marble's reagent was used at 30 ° C or 40 ° C. In the rest of the etching techniques used, the temperature of the reagent is not indicated. What was the temperature in these cases?

Etching in these cases were carried out at room temperature (about 20 °C). Accordingly, Table 3 has been revised (Line 163).

 

Results and discussion

1. Line 191: Why with the nital etch the white-colored constituents are identified as RA and with the Marble´s reagent they are identified as M/RA? Please, correct it or explain it.

In Figure 2, all of the white-colored constituents are identified as M/RA phase by using both nital and Marble’s reagent. We made a mistake by writing M/RA in Figure 2 (a). So we have corrected it.  

 

1. Lines 247-249: I think it would be interesting to include an image at higher magnification to see more clearly the RA particles, speckled with fine, orange-coloured, martensite laths.

We had used the possible highest magnification of 800x for light optical micrographs. So we have now tried to put enlarged images to show the details more clearly. However, the following phrase has been added to the text “Due to the development of very fine mixture of bainite, martensite, and retained austenite in multiphase microstructure, high magnification light optical microscopy was used.” (lines 185-187).

1. Line 363: According to what is indicated in Table 4, the calculated fraction of bainite in the double-step metallography techniques is approximately 50%. However, in the three-step and four-step techniques the bainite fraction is approximately 70%. This seems to indicate that the two-step techniques do not allow to fully reveal all the areas that present bainitic structure, even though the Marble´s reagent is used, which is indicated to reveal the bainite. It would be interesting for the authors to discuss this fact and explain the possible causes of the difference in bainite fraction observed as a function of the different metallographic techniques analyzed.

We can perhaps relate these results to the similarity between bainite and martensite microstructure and so it is impossible to make difference between them completely by using double-step etching techniques in comparison to the triple-step and quadruple-step etching methods. Also, we used warm Marble’s reagent 30 ^oC in triple- and quadruple-step etching techniques which are more sensitive for etching bainite. Please read Line 244 “Marble's reagent heated at 30 ^oC color-tinted the bainite plates with better contrast [28,29].”

Reviewer 2 Report

The article discusses an interesting problem - the identification of various phases (martensite, bainite, retained austenite) in steels by the color light metallography using multistage etching. There are difficulties in identifying the phases by the SEM EBSD methods in multiphase steels. Only fcc and bcc phases differ well. It is not possible to distinguish between bainite and martensite. The most complete information on the structure and phase composition can be obtained by the TEM method on thin foils, where all phases are distinguishable. However, this information is obtained from local areas and does not include the inhomogeneity of the phase composition of the steel. Color light metallography helps expanding the knowledge about the phase composition of steels after heat treatment, in particular after quenching and bainitic holding.

There are significant comments on the illustrative material of the article that have to be reconsidered after major revision.

Figures 2 a and b do not allow the reader to unambiguously determine any phase components of the steel structure. These elements are distinguishable only in Figure 2c.

The magnification used in Figures 2-6 fails to provide a qualitative distinction between the phases. Have any statistical evaluations been made by Image J software in these figures? If so, how accurate are they? What is the accuracy of determining the boundaries between the phases and how accurately the phases are separated by color? What are the errors of these estimates? It is necessary to show Figures 2-6 at a higher magnification in order to provide a visual difference between the phases.

Line 341-344. “…..the achieved volume fractions (in vol. %) of RA phase using light color 341 metallography methods (single-step nital 2%, triple-step, and quadruple-step) estimated by Image J software is shown in Figure 10, and corresponding fractions are 8.2 % (single step 2% nital), 12.82 % (triple-step) and 13.34 % (quadruple-step)…”. Why the hundredth fractions? Are these really meaningful and reasonable?

A comparison of the color light metallography data with the SEM EBSD and XRD results is appropriate. It would also be useful to compare your data with the TEM results.

The conclusions of the article are very extensive and require significant reduction.

What was new in this work? Quenching and bainitic holding treatment and DIN1.5025 steel are quite well studied. The novelty of the results obtained is contained only in the methods of using various etchants to identify the multiphase structure of steels. As presented, the results raise many questions. Therefore, a major revision is required.

 

Author Response

The article discusses an interesting problem - the identification of various phases (martensite, bainite, retained austenite) in steels by the color light metallography using multistage etching. There are difficulties in identifying the phases by the SEM EBSD methods in multiphase steels. Only fcc and bcc phases differ well. It is not possible to distinguish between bainite and martensite. The most complete information on the structure and phase composition can be obtained by the TEM method on thin foils, where all phases are distinguishable. However, this information is obtained from local areas and does not include the inhomogeneity of the phase composition of the steel. Color light metallography helps expanding the knowledge about the phase composition of steels after heat treatment, in particular after quenching and bainitic holding.

There are significant comments on the illustrative material of the article that have to be reconsidered after major revision.

 

Figures 2 a and b do not allow the reader to unambiguously determine any phase components of the steel structure. These elements are distinguishable only in Figure 2c.

As we discussed in section 3.1.1 General/ Single-Step Metallography starting at Line 182,

it is difficult to make a good contrasting resolution between bainite, martensite, and retained austenite after using just nital or Vilella’s or marble’s reagent according to the light micrograph in Figure 2. We have chosen these micrographs to emphasise the purpose of this paper. These techniques can only partially reveal the microconstituents in this multiphase microstructure.   

 

The magnification used in Figures 2-6 fails to provide a qualitative distinction between the phases. Have any statistical evaluations been made by Image J software in these figures? If so, how accurate are they? What is the accuracy of determining the boundaries between the phases and how accurately the phases are separated by color? What are the errors of these estimates? It is necessary to show Figures 2-6 at a higher magnification in order to provide a visual difference between the phases.

We had used the maximum magnification of 800x for light optical micrographs shown in Figures 2-6. However, we tried to include enlarged images by keeping the resolution to show details in Figures 2-6. Moreover, the following phrase has been added to the text “Due to the development of very fine mixture of bainite, martensite, and retained austenite in multiphase microstructure, high magnification light optical microscopy was used.” (lines 185-187).    

 

Line 341-344. “…..the achieved volume fractions (in vol. %) of RA phase using light color 341 metallography methods (single-step nital 2%, triple-step, and quadruple-step) estimated by Image J software is shown in Figure 10, and corresponding fractions are 8.2 % (single step 2% nital), 12.82 % (triple-step) and 13.34 % (quadruple-step)…”. Why the hundredth fractions? Are these really meaningful and reasonable?

We obtained these results according to ASTM E562 standard conditions using ImageJ software. The standard deviation (S.D) and error bars are added in Table 4 and Figure 11, respectively. The experimental procedure was revised in “Lines 156-160”.

 

A comparison of the color light metallography data with the SEM EBSD and XRD results is appropriate. It would also be useful to compare your data with the TEM results.

Thankful for your comment. We also added typical TEM images for the heat treated sample “section 3.2. Microstructral characterization with FE-SEM, EBSD, TEM, and XRD techniques, Line 325-340”.

 

 

The conclusions of the article are very extensive and require significant reduction.

 

The conclusion part was reduced according to your comment. 

 

What was new in this work? Quenching and bainitic holding treatment and DIN1.5025 steel are quite well studied. The novelty of the results obtained is contained only in the methods of using various etchants to identify the multiphase structure of steels. As presented, the results raise many questions. Therefore, a major revision is required.

We tried to use different color etching techniques to identify and make a good contrasting resolution between various microcostituents by selectively coloring them. As color metallography has advantages such as simple sample preparation, low cost, commonly used worldwide, and … that they make it a suitable candidate method for microstructural investigation in these multiphase microstructures. We try to revise whole of the paper as much as possible as indicated by yellow background text. 

Reviewer 3 Report

As a person focused on the development of imaging techniques, I may not be completely objective, but I appreciate the idea of ​​a precise discussion of the tools used for our research. Color light metallography remains an unpopular but highly useful method for rapidly assessing the microstructure of complex alloys. It was somewhat forgotten when the SEM methods, especially SEM-EBSD, were popularized, but it remains useful for the workshop assessment of the microstructure. Of course, it cannot be compared to TEM methods, which, despite being much more time-consuming, give the opportunity to image structures and phases with the smallest dimensions.
In the introduction, the authors include extensive and well-described literature knowledge as well as the methodology described in the same way. I positively assess the accuracy of the description of the color etching methods, as temperatures and times will be the key to repeat these analyzes by readers. However, I believe that temperatures should be specified for each process step, even those at normal temperatures (line 154). The entire text is well-edited, but I have a few comments listed below:

- The quality of the description of the experimental results is satisfactory, but the micrographs themselves do not match the level of the rest of the work. They look like they were made with a 10x objective and a total magnification of 100x, when fine structures should be more visible for readers at 10x higher magnifications (especially with the use of immersion methods). Such a low magnification, although it can rightly be used in quantitative analysis, does not allow the reader to orientate in the actual colors of the microstructures. I highly recommend supplementing the work with high-resolution light microscope images.
- 142 - Does describing the values ​​835, 765, 471 and 281 in the graph have any specific purpose?
- on what surfaces and data sets the data presented in table 4 and figure 10 were prepared? I have the impression that the work lacks a description of quantitative analysis methods, they quite fundamentally affect the final result. Maybe it is worth comparing sample images with their analysis for each method in Supplementary Material?
- what statistical methods were used to verify the measurement certainty?
- Could the authors in the discussion or summary attempt to numerically evaluate the time-consumption and cost of individual methods?

Author Response

As a person focused on the development of imaging techniques, I may not be completely objective, but I appreciate the idea of ​​a precise discussion of the tools used for our research. Color light metallography remains an unpopular but highly useful method for rapidly assessing the microstructure of complex alloys. It was somewhat forgotten when the SEM methods, especially SEM-EBSD, were popularized, but it remains useful for the workshop assessment of the microstructure. Of course, it cannot be compared to TEM methods, which, despite being much more time-consuming, give the opportunity to image structures and phases with the smallest dimensions.
In the introduction, the authors include extensive and well-described literature knowledge as well as the methodology described in the same way. I positively assess the accuracy of the description of the color etching methods, as temperatures and times will be the key to repeat these analyzes by readers. However, I believe that temperatures should be specified for each process step, even those at normal temperatures (line 154). The entire text is well-edited, but I have a few comments listed below:

- The quality of the description of the experimental results is satisfactory, but the micrographs themselves do not match the level of the rest of the work. They look like they were made with a 10x objective and a total magnification of 100x, when fine structures should be more visible for readers at 10x higher magnifications (especially with the use of immersion methods). Such a low magnification, although it can rightly be used in quantitative analysis, does not allow the reader to orientate in the actual colors of the microstructures. I highly recommend supplementing the work with high-resolution light microscope images.

We had used the maximum magnification of 800x for light micrographs shown in Figures 2-6. However, we tried to put enlarged images while maintaining high resolution to show details in Figures 2-6. Moreover, the following phrase has been added to the text “Due to the development of very fine mixture of bainite, martensite, and retained austenite in multiphase microstructure, high magnification light optical microscopy was used.” lines 185-187.

- 142 - Does describing the values ​​835, 765, 471 and 281 in the graph have any specific purpose?

We presented the specific heat treatment cycle with actual data, and hence no approximations were made.

- on what surfaces and data sets the data presented in table 4 and figure 10 were prepared? I have the impression that the work lacks a description of quantitative analysis methods, they quite fundamentally affect the final result. Maybe it is worth comparing sample images with their analysis for each method in Supplementary Material?

We have described more in the experimental part about the methods we used to determine the data presented in Table 4 and Figure 11. Please see Lines: 156-160.  

- what statistical methods were used to verify the measurement certainty?

All the volume fraction data have been measured according to ASTM E562 standard conditions using ImageJ software. The standard deviation (S.D) and error bars have been added in Table 4 and Figure 11, respectively. The associated experimental procedure has been revised. Please see Lines: 156-160 and 169-173.

- Could the authors in the discussion or summary attempt to numerically evaluate the time-consumption and cost of individual methods?

We have now briefly described in the Conclusions section about the cost of operation and advantages and disadvantages of different microstructural characterization techniques. 

Round 2

Reviewer 2 Report

The authors made significant changes to the text of the article. The figures added with higher magnification. In these figures, the discussed phases are better distinguishable. TEM images confirm the complex microstructure of the steel. Changes made to the discussion and the conclusions were reduced. Indeed, multistage color metallography techniques can be interesting for quantifying the phase constituents of multiphase steels.

In the presented form, the article can be published.