Review Reports - Temperature-Dependent Effectiveness of Ti, Nb, Zr, and Y in Controlling Grain Growth of AISI 304 Austenitic Stainless Steel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the effects of microalloying elements (Ti, Nb, Zr, and Y) on austenite grain growth in AISI 304 stainless steel at elevated temperatures. The manuscript presents publishable findings and offers valuable insights for selecting optimal microalloying strategies based on the maximum operating temperature of this steel. However, several issues should be addressed to improve the manuscript. The following points require clarification or revision before the paper can be considered for publication. The queries are as follows:

Q1: The title, "Temperature-Dependent Effectiveness of Ti, Nb, Zr, and Y in Controlling Grain Growth," does not explicitly mention the specific material under investigation. Given that the research focuses on AISI 304 steel, it is recommended to revise the title to "Temperature-Dependent Effectiveness of Ti, Nb, Zr, and Y in Controlling Grain Growth of AISI 304 Stainless Steel" to enhance clarity and precision.

Q2: Metallographic Sample Preparation. The paper provides optical and scanning electron microscope images, yet the etchant employed for microstructure examination is not specified.

Q3: Delta Ferrite Measurement (line 164). The sentence states: “The Feritscope MP30 detects between 0.1 and 80 vol.% of delta ferrite in austenitic steel and has a maximum permissible measurement deviation of ± 0.4 vol.%. (46)” What does “(46) refer to? Clarification is required regarding the reference "(46)"; please specify whether it denotes a citation, a standard, or another source.

Q4: The following revisions are recommended for Tables 14 and 15:

Please ensure the headers and line structures (i.e., the row and column layout) are consistent between the two tables.

The unit of measurement for the average crystal grain sizes should be clearly included in the respective table title(s).

Q5: The following revisions are recommended for Figure 23:

Please provide the unit for the annealing time on the horizontal axis in Fig. 23. The current label is incomplete without it.

Author Response

Firstly, we would like to express our gratitude to the reviewer for the insightful and constructive comments. We have corrected our manuscript accordingly.

A1: Thank you for the recommendation, we have expanded the title to: ”Temperature-Dependent Effectiveness of Ti, Nb, Zr, and Y in Controlling Grain Growth of AISI 304 Austenitic Stainless Steel”.

Q2: Metallographic Sample Preparation. The paper provides optical and scanning electron microscope images, yet the etchant employed for microstructure examination is not specified.

A2: Thank you for noticing the missing info. The samples were etched with aqua regia 10 ml glycerol, 15 ml HCl, and 5 ml HNO₃.

A3: We have added the appropriate reference.

American Welding Society (AWS). Standard Procedures for Calibrating Magnetic Instruments to Measure the Delta Ferrite Content of Austenitic and Duplex Ferritic-Austenitic Stainless Steel Weld Metal; AWS A4.2M:2020 (ISO 8249:2018, MOD); American Welding Society: Miami, FL, USA, 2020.

Q4: The following revisions are recommended for Tables 14 and 15:

Please ensure the headers and line structures (i.e., the row and column layout) are consistent between the two tables.

The unit of measurement for the average crystal grain sizes should be clearly included in the respective table title(s).

A4: We have modified tables 14 and 15 accordingly. As the grain size according to ASTM is dimensionless, there is no unit, that is why we later convert it to µm in graphs.

Q5: The following revisions are recommended for Figure 23:

Please provide the unit for the annealing time on the horizontal axis in Fig. 23. The current label is incomplete without it.

A4: The graph has been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper describes the influence of annealing temperature on austenite grain growth in dependance of different microalloying elements. It is a good paper that clearly summarizes the results of many experiments.

Just a few minor comments:

1.) The inclusions visualized in chapter 3.3 are very large and are expected to appear in a low density in the steel matrix. Therefore the impact of these particles to Zener pinning seems to be very low. It would be interesting to know how high the particle density is, especially of those in the lower size range (like the ones in figure 18).

2.) The micrographs in figures 18, 19, 20, and 21 are not well selected. The differences in the microstructures described are not apparent. Either images with the same annealing time and different grain sizes or the same grain size and different annealing times should be used here.

3.) The grain sizes for the Y-alloyed steel at 1200°C are all significantly lower than at 1150°C (compare figures 16 and 17), this is not the expected behavior and should be dicussed.

4.) In line 155 it is refered to a diagram explaning the ASTM grain sizes, but this seems not to be the mentioned figure 21.

Author Response

We are grateful for the reviewer's input. We have improved the paper according to the review.

A1: You are correct, the non-metallic inclusions that are larger than a few micrometers are not efficient at pinning. We did not measure the number density of NMI in the samples. The precipitates and inclusions that are a part of Zener pinning are too small for automatic SEM analysis, and, therefore are very difficult to count. Figure 18 is etched, so they seem much larger than they are in reality due to pitting corrosion effect of the etchant.

A2: We have made a new selection of the images to better present the described process. We have also taken your suggestion into account.

3.) The grain sizes for the Y-alloyed steel at 1200°C are all significantly lower than at 1150°C (compare figures 16 and 17), this is not the expected behavior and should be dicussed.

A3: We have added discussion. The Y-alloyed samples only showed pinning at 1200 °C, where the grain sizes were the largest. This means that the Y-rich particles were sparsely located and did not show any pinning effect due to the relatively large distances between them. Yttrium additions are known to inhibit grain growth [45,46].

4.) In line 155 it is refered to a diagram explaning the ASTM grain sizes, but this seems not to be the mentioned figure 21.

We have removed the references, this is a part of the text that remained from an old version of the paper, that had figures from the ASTM standard included. We have removed them in the process of modifying the paper.