High-Temperature Oxidation Behavior of an Additively Manufactured Alumina-Forming Austenitic Stainless Steel

Comments 1: Figure 11 line 330-332 is unclear as to what is being shown. A-D images are provided do not match the caption.

Response: We have revised the caption for the Figure to make it clearer. The Figure is discussed in the main text:

The EDX area maps of AM-AFA25 alloys (Figure 11) demonstrate β-NiAl denuded zone in both AM alloys and occasionally Mn-rich oxide nodules only in AMAFA25-1, even though the Al2O3 scale remained continuous in these regions. The cross-sectional EDX mapping result for W-AFA25 was discussed in our previous work [3] and the presence of Mn oxide was also observed in oxidized W-AFA25. β-NiAl precipitates are expected to play a critical role in the continuous formation of the oxide scale in the subsequent oxidation; since Aluminum diffuses from the NiAl to the interface, forming a continuous oxide layer as confirmed through microscopy studies.

4. Some areas need work with grammar and word use. 

Response:  Necessary changes have been made following the comments in the attached pdf.

Comment 1: How the wrought alumina forming stainless steel alloy (W-AFA25) was prepared should be introduced in part 2.1.

Response: “Wrought alumina forming stainless steel alloy (W-AFA25) was obtained from the sponsor of this project as a benchmark alloy, and two additively manufactured AFA25 alloys processed by SLM were used as experimental materials in this work. For IP protection purposes we cannot reveal the 16 varying SLM parameters tested for parameter development.

Comment 2: Nb-rich layers were found along the inter-dendritic regions in AM alloys. It is necessary to discuss the possible reasons for this phenomenon, as well as their influence on oxidation behavior.

Response: Nb and C have high affinity for each other thus forming stable and fine size NbC.; Studies by Du et al. [51] and Zhao et al. [52] show that its formation is essential for enhanced high temperature strength and hardness for AFA steels. The morphology of Nb rich phases vary in AM alloys and conventional alloys, where complex heat cycles in AM result in precipitation of Nb rich phases or its segregation along the grain boundaries.

NbC formed in the AM alloys along the grain boundary actively reacts with oxygen, forming NbO. The PBR of NbC and NbO are nearly equal, and twice that of Al2O3.  This results in cracks allowing oxygen to diffuse and react with the underlying aluminum in the steel. The variation in Nb concentrations along the grain boundary results in a higher oxidation rate in AFA AM 25-1 steels than in the AFA AM 25-16 steel type. Further higher defects in AFA AM 25-1 steel result in a higher oxidation rate.

----added in the manuscript.”

Comment 3: In line 241, it is stated that During the early stages of high-temperature oxidation of AFA stainless steel alloys, typically an Fe-oxide scale grows rapidly, which is followed by the formation of a continuous Cr₂O₃ layer below the Fe-oxide scale. Then why was only Al₂O₃ scale, but not Fe-oxide scale, found for the three investigated steels?

Response  “Aluminum has a high affinity towards oxygen. Additionally, the AM alloys have NbC-rich phases along grain boundaries which promotes reaction with oxygen and forming Nb-oxide. Since Nb-oxides have high PBR it cracks and allow oxygen to diffuse and react with underlying Aluminum in the steels.  When the alloy is continuously oxidized at a high temperature (850 °C), the oxygen tends to diffuse inwards through the oxide scale, and eventually Al2O3 forms at the alloy/scale interface. As the oxidation proceeds, the thickness of the β-NiAl precipitate zone increases, and the neighboring β-NiAl precipitates dissolve to supply Al to the oxidation front. β-NiAl precipitates act as an Al reservoir to maintain the Al2O3 sur-face scale [7,47]. β-NiAl denuded zone detected below the oxide scales of Figure 10 supports the conclusion that β-NiAl near the oxidized alloys decompose as Al diffuses outwards (towards the surface) and reacts with oxygen to form the oxide scale. Also, the relative Cr content in the austenite matrix increases as the aluminide forms in the matrix of the alloy due to the depletion of Al and Ni [9,37]. The increase in Cr content leads to the formation of an alumina layer that acts as a protective scale. We have reported the presence of Cr-oxide and Fe-oxide in both the top surface and cross-section EDX evaluation in Figure 5, 6 and 11”. We expected a mixture of oxides in our AFA alloys and we observed that in both AM and wrought alloy.

Comment 4: Ferrite phase was also found in all samples. How does it affect the oxidation behavior?

Response: ”In Alumina Forming Austenitic (AFA) stainless steels, the ferrite phase (α-ferrite) plays a crucial role in influencing both hot cracking susceptibility and the formation of embrittling phases during high-temperature service. While a small amount of ferrite can help prevent solidification cracking during welding, excessive ferrite can lead to the precipitation of brittle phases like sigma (σ) phase, reducing creep resistance and overall mechanical properties [53-54]. All our alloys show lower and nearly equal fer-rite volume fractions. This was not investigated further in this work and can be a scope of future work.---added in manuscript”

Comment 5: The oxidation resistance mechanism needs a further in-depth discussion. Figs. 7, 8, and 14 are all cited from reported works. It is necessary to draw a schematic diagram focusing on the relationship between printed microstructures and oxidation resistance.

Response: “The results and discussion have been modified to include more details.”

Comment 6: Both B2 phase and the Laves phase formed during the oxidation test, as shown in Fig. 11. How does the Laves phase affect the oxidation behavior?

Response: “The presence of Laves phases in AFA steels generally enhances oxidation resistance by stabilizing alumina formation and improving scale adhesion—but only when properly controlled. Excessive precipitation or improper distribution can lead to aluminum depletion and reduced protective scale formation [5]. Note that the Laves phase has not been observed in this study for both the conventional wrought and AM alloys using SEM or XRD.

 Point-by-point response to Comments and Suggestions for Authors

Comment 1: The Abstract section should present quantitative results and not only the most important qualitative results and/or generic considerations. Significant improvements are expected in this section of the manuscript.

Response 1: “This paper shows that a variation of AM builds parameter influences the oxidation properties; where one AM alloy with lower laser power to hatch ratio depicts much better oxidation property compared to conventional wrought AFA alloy-----Changes have been incorporated in the abstract.”

Comment 2: Table 1 – please provide information on what basis the SLM process parameters were selected.

Response: “Wrought alumina forming stainless steel alloy (W-AFA25) was obtained from the sponsor of this project as a benchmark alloy, and two additively manufactured AFA25 alloys processed by SLM were used as experimental materials in this work. For IP protection purposes we cannot reveal the 16 varying SLM parameters tested for parameter development .The AM parameters were developed based on thorough AM parameter development approaches, but this cannot be expressed in detail in the work owing to restrictions due to Intellectual Property (IP) rights.”

Comment 3: Figure 2 - Are these Figures of the microstructure on the surface or in cross-section? What does the microstructure of the printed material look like in cross-section? This should be verified for the longitudinal and cross-sectional sections depending on the direction of the overlapping paths.

Response : “We have reported the top interface in this work (Y-Z; where Y is hatch direction and Z is build diection, as shown in the figure), and at this magnification, we observe dendritic morphology in the AM alloys. This is usually observed along the overlapping interfaces of the AM alloys.

Comment 4: “Several researchers have reported excellent oxidation resistance and mechanical properties of AFA alloys in different high-temperature environments [1,2,14–22,5,7–13]”. Please provide details on excellent oxidation resistance and mechanical properties of AFA alloys. What was this resistance characterized in the various publications? How did it change depending on the test conditions. Referring in one sentence to about 20 publications is not appropriate. This paragraph needs significant revision.

Response : Thank you for the comment. We have tried our best to revise this.

Building on this foundation, numerous studies have systematically examined the oxidation resistance and mechanical performance of AFA alloys under various high-temperature environments. Brady et al. [2,7,9–11] demonstrated that a minimum Al content of ~2.5 at.% enables the formation of a continuous α-Al₂O₃ scale, even in humid air with 10% water vapor at 800 °C. Their long-term oxidation tests—spanning over 1000 h—revealed minimal spallation and parabolic mass gains indicative of slow-growing oxide kinetics. Yamamoto et al. [1,5,20–22] further investigated the influence of microalloying additions such as Nb, Cr, and Ni, which stabilize β-NiAl and Laves phase precipitates, contributing not only to oxidation resistance but also to high-temperature strength. The beneficial role of reactive elements—particularly Hf, Y, and Zr—has been established in enhancing oxide scale adhesion by suppressing void formation at the scale-metal interface and improving grain boundary cohesion [7,13,23]. Hu et al. [16] and Gao et al. [18] linked the precipitation of Laves phases and NiAl to sustained creep resistance at temperatures exceeding 900 °C. Pint and co-workers [14,15] showed that steam exposure and cyclic oxidation conditions further underscore the importance of scale adherence and precipitate stability in real-world scenarios. Taken together, these studies highlight how AFA alloys, through a combination of optimized Al content, reactive element additions, and precipitation hardening, maintain superior oxidation behavior and mechanical reliability across a broad range of harsh service conditions. This unique combination positions AFA steels as viable candidates for advanced energy systems, including ultra-supercritical steam plants and next-generation nuclear reactors [1,12,21,24].

Comment 5: Please add one paragraph in Chapter 4 about the further research.?

Response :” Further research is required to understand how variation of each AM parameter can influence the underlying microstructures and its effect on high temperature oxidation of AFA steels. Additionally, the oxidation studies at each temperature interval needs to be investigated to better understand the oxidation mechanism of AM based AFA alloys---added in the paper.”