High-Temperature Oxidation Behaviors of 321 Steel with Y or Nb Micro-Alloying

Round 1

Reviewer 1 Report

 

Th paper deals with the effects of Y or Nb addition on the oxidation behavior of 321 steel at high temperatures. The paper is interesting and the effects of adding elements are well explained. however, the English is completely to be revised and this makes reading unclear. In addition please change the following points

-line 85. Please add °C at all temperature

-the pictures in figure 5 are not clear and you cannot see any difference between the samples. please edit them or insert other pictures

Author Response

1. line 85. Please add °C at all temperature

Re: Thank you for suggestion and we have corrected it.

 

2. the pictures in figure 5 are not clear and you cannot see any difference between the samples. please edit them or insert other pictures

Re: Thank you for your suggestion. We have removed Figure 5 and observed and analyzed the oxidation performance of the alloy through XRD and SEM detection.

Author Response File: Author Response.docx

Reviewer 2 Report

This is an interesting work; however, before proceeding to the next step, the authors should address the following comments.

1. The language of the manuscript has to be improved.

2. Provide a more in-depth discussion of related previous works.

3. Authors should also provide more meaningful discussions regarding the repeatability and reproducibility of the conducted tests/analysis.

4. In the “Conclusion” section, I recommend presenting more quantitative data as the main results of the research study.

Author Response

1. The language of the manuscript has to be improved.

Re: Authors are thankful to the reviewer for identifying this issue. In the updated version of this manuscript, we have double checked and tried to nullify all the typos and grammatical mistakes with the help of native speaker.

 

2. Provide a more in-depth discussion of related previous works.

Re: Thank you for your suggestion and we have revised it.

Nb is considered an important microalloying element. The addition of Nb has obvious effects on grain refinement and moderate precipitation reinforcement of steel. The nailing effect of Nb precipitates on grain boundaries can inhibit grain growth, thus improving the oxidation resistance and spalling resistance of the steel, and thus improving the adhesion between the oxide layer and the substrate. From Figure 5 (d), it can be seen that the surface oxide layer is significantly denser, and the surface of the oxide layer is distributed in a granular manner. Through scanning electron microscopy analysis, the addition of Nb element can also hinder the inward diffusion of O ions, thereby slowing down the formation of the oxide layer. At the same time, the addition of Nb can also prevent the inward diffusion of thus slowing down the formation of the oxide layer.

 

3. Authors should also provide more meaningful discussions regarding the repeatability and reproducibility of the conducted tests/analysis.

Re: Thank you for your suggestion and we have revised it. We have added a discussion on the impact of Nb microalloying and summarized the experiment.

 

When fitting the data of each alloy separately, they all showed a good degree of fitting. Y₀: R²=0.987, Y_0.015: R²=0.937, Y_0.045: R²=0.954, N_0.5: R²=0.848, respectively. It can be seen from the figure that at the same temperature, the oxidation rate of Y0.045 alloy and Nb0.5 alloy is lower than that of Y0 alloy, which again indicates that adding 0.045%Y or 0.5%Nb helps to enhance the alloy's high-temperature oxidation resistance. And alloy Nb0.5 has the best high-temperature oxidation resistance, which is consistent with the previous testing results.

 

 

Nb is considered an important microalloying element. Adding Nb has obvious effects on grain refinement and moderate precipitation reinforcement of steel. The nailing effect of Nb precipitates on grain boundaries can inhibit grain growth, thus improving steel's oxidation and spalling resistance, thus improving the adhesion between the oxide layer and the substrate. Figure 5 (d) shows that the surface oxide layer is significantly denser, and the surface of the oxide layer is distributed in a granular manner. Using scanning electron microscopy analysis, the addition of Nb element can also hinder the inward diffusion of O ions, thus slowing down the oxide layer formation. At the same time, the addition of Nb can also prevent the inward diffusion, thus slowing down the formation of the oxide layer.

 

4. In the “Conclusion” section, I recommend presenting more quantitative data as the main results of the research study.

Re: Thank you for your suggestion and we have revised it.

The effect of Y and Nb microalloying on the oxidation behavior of 321 heat-resistant steel at 600 ℃, 650 ℃ and 700 ℃ in air was studied. The main conclusions are as follows:

(1) Nb_0.5 alloy heat-resistant steel has the most excellent oxidation resistance at high temperatures among the five alloy groups. The oxide gradient of the alloy will not detach throughout the entire oxidation process. After oxidation at 700 ℃ for 200 hours, the gain of the alloy is only 0.156mg/cm². There has been a significant improvement in the alloy's resistance to oxidation at high temperatures.

(2) Adding an appropriate amount of active elements can effectively improve the alloy's microstructure. Improve high-temperature antioxidant performance. When rare earth elements are added separately, if the amount is small, it will not work and even accelerate the growth of the oxide film, thereby deteriorating the oxidation resistance of the alloy.

(3) In the high-temperature oxidation process of the alloy, rare earth Y promotes the diffusion of Cr in the matrix. It increases the content of Cr on the oxide scale, which can quickly form a relatively dense and complete Cr₂O₃ oxide film on the surface. As a result of the addition of Nb, the inward diffusion of O ions can be prevented. Oxidation rate is reduced and oxide film adhesion rate is increased. The oxidation resistance of 321 stainless steel has significantly improved.

Author Response File: Author Response.docx

Reviewer 3 Report

1. What phases are present in the microstructure in Fig. 1?

2. Why are these temperatures chosen for oxidation analysis? In Fig. 1 e-h are not signed.

3. Does the addition of niobium and yttrium affect the phase composition?

4. In fig. 3 e-h not signed.

5. What is the reason for the release of chromium oxide at 650C?

6. In fig. 5 the structure is not visible. Is it possible to show surface oxidation in some other way? Or select areas in this figure.

7. In fig. 7 b,d,f not signed.

8. Fig. 6 e-h no signatures.

9. What is the reason for the formation of Fe3O4 oxide?

Author Response

1. What phases are present in the microstructure in Fig. 1?

Re: Thank you for your suggestion. After oxidation at 600 ℃ for 200 hours, due to the excellent oxidation resistance of the alloy, only slight oxidation occurs, so the main phase is still the matrix phase. A thin layer of oxide skin appears on the surface, with the main phase being Fe₂O₃, which is consistent with the XRD detection results in Figure 2.

2. Why are these temperatures chosen for oxidation analysis? In Fig. 1 e-h are not signed.

Re: Thank you for your suggestion. This alloy is mainly used in boilers at 600 ℃, 650 ℃, and 700 ℃ in practical applications, so this temperature is chosen.

3. Does the addition of niobium and yttrium affect the phase composition?

Re: Thank you for your suggestion. The addition of Y and Nb will affect the phase composition. As a microalloying element, Nb can replace Cr and C to form stable NbC, which can refine the grains. Moreover, according to Hyung et al. ^[1], a small amount of Nb element was found in ferritic stainless steel. It was found that Nb is prone to segregation at the interface between the oxide film and the metal matrix, forming Nb₂O₅ and Laves phases.

The addition of Y can refine the grains and accelerate the formation of Fe2O3 and Cr2O3 phases.

[1] Seo, Hyung Suk;Yun, Dae Won;Kim, Kyoo Young.Oxidation behavior of ferritic stainless steel containing Nb, Nb–Si and Nb–Ti for SOFC interconnect[J].International Journal of Hydrogen Energy,2013,Vol.38(5): 2432-2442

4. In fig. 3 e-h not signed.

Re: Thank you for suggestion and we have corrected it.

5. What is the reason for the release of chromium oxide at 650C?

Re: Thank you for your suggestion. According to the Richardson Ellingham diagram in the principle of oxidation thermodynamics, the relationship between the Gibbs free energy of oxide formation and temperature can be seen, so the possible order of S321 steel oxidation products at 650 ℃ is Cr₂O₃>FeO>Fe₃O₄>NiO>Fe₂O₃. And the Cr content in the alloy is very high, so the O on the surface of the matrix immediately reacts with Cr, and the surface rapidly oxidizes to form Cr₂O₃.

 

 

 

 

 

6. In fig. 5 the structure is not visible. Is it possible to show surface oxidation in some other way? Or select areas in this figure.

Re: Thank you for your suggestion. We have removed Figure 5 and observed and analyzed the oxidation performance of the alloy through XRD and SEM detection.

7. In fig. 7 b,d,f not signed.

Re: Thank you for suggestion and we have corrected it.

8. Fig. 6 e-h no signatures.

Re: Thank you for suggestion and we have corrected it.

9. What is the reason for the formation of Fe₃O₄oxide?

Re: Thank you for your suggestion. According to the Richardson Ellingham diagram in the principle of oxidation thermodynamics, the relationship between the Gibbs free energy of oxide formation and temperature and the oxidation mechanism of steel can be seen that the formation order of Fe oxides in the oxidation process is FeO → Fe₃O₄ → Fe₂O₃. So Fe₃O₄ will appear during the oxidation process.

 

Author Response File: Author Response.docx