Evaluation of Stationary Creep Rate in Heat-Affected Zone of Martensitic 9–12% Cr Steels

Round 1

Reviewer 1 Report

The manuscript attempted to build a correlation between stationary creep rate and various microstructure features in simulated HAZ of 9-12% Cr steels. The topic should be interesting for the readers of this journal. Unfortunately, I don’t think it can be published due to the following questions.

1. The Abstract needs revisions to represent the main findings of this work.
2. 9-12% Cr steels, especially 9Cr steels, have been extensively studied. In Introduction, more recent works should be cited to discuss major microstructure evolutions in HAZ from real welds and simulated HAZs.
3. X20 and P91 are similar martensitic steels. What’s the point to conduct a parallel work for both steels?
4. The heating rate is critical to phase transformations in HAZ. Why was the heat rate of 225k/s selected?
5. What’s the principle of choosing those peak temperatures? The critical transformation temperatures Ac1 and Ac3 govern phase transformations and microstructure evolution in the HAZ. The Ac1 and Ac3 values of both the X20 and p91 steels should be measured and provided.
6. What’s the dwell time at peak temperatures?
7. Why to choose a tempering temperature of 650C, rather than 760C which is a typical PWHT temperature used for 9Cr steels?
8. For measuring precipitate size, how many SEM images were analyzed for each condition?
9. Could you also overlap an axis of the recorded temperature profiles in Figure 3? This will be better to interpret dilatation changes.
10. There are some significant issues (errors) of the measured PAG size in Figure 5The PAG sizes at after thermal cycles at 900 C and 1000C were reported to be higher than the PAGs size at 1200C and 1350C. The authors explained “because the grain growth is slowed down due to the drag effect by solute atoms (carbon and carbide forming elements)”. This is wrong. It has been widely recognized by enormous prior works that the PAG size increases with the peak temperatures in HAZ. The EBSD maps in Figure 6 and Figure 7 clearly show an opposite trend of the PAG size, especially in the P91 steel. The large PAG in X20 at 900C is probably the prior austenite grains from the parent metals since 900C is not high enough for re-austentinization. For P91 steel, 900-1000C normally promotes formation of fine PAGs, more known as the fine-grained HAZ (FGHAZ). The sub-grain size shown in Figure 8c should be consistent with the PAG size.
11. The PAG size is very important microstructure factor. But it was not used in these three models. Is there a way to integrate the PAG size into the models based on its relation with sub-grain size?
12. Figure 9 needs revision. It is different to distinguish those lines with similar colors and the same thickness. Are the 0-2° sub-grain boundaries included? It is well known that the fraction of lath boundaries in martensitic steels is much higher than fraction of the HAGB (>15°). But the plot doesn’t show this information.
13. Stationary creep rate is a function of creep temperature and stress. However, creep temperature and stress are not mentioned for the results in Figure 17.
14. Although it is expected that different models will give different creep rate values, the general trend should be consistent. Why the creep rates predicated by three models in Figure 17 show an opposite trend between Vodopivec and Sui & Sandström? How to explain this?
15. The model results shows the creep rates of simulated HAZs are lower that the parent metals. It is apparently something wrong with the models. The same creep temperature and stress should be used for comparing creep rates between the parent metal and simulated HAZ, but they were not mentioned.
16. Could the authors make a general comment on which model is the best for these two 9-12% Cr steels?
17. Conclusions 1 and 2 are not new findings. They have been reported in many prior works.
18. Conclusion 3 is not new to me and many others. It is just a well-known statement. It is too descriptive, please be specific.
19. There are some editing and grammar errors. Please review and revise carefully.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The work contains a large number of microstructural measurements of great importance for the physical modeling of the creep behavior of materials. The article is very precise and rigorous on the experimental part. Less precise is the modeling part where the objectives and analysis of the results are unclear. In 9-12%Cr steels the dislocational and diffusion deformation mechanisms are activated at different stress and temperature levels. The microstructure can behave differently and consequently the creep rates can vary greatly. The authors have wrongly omitted the stress and temperature considered for the creep rates evaluation. The authors have wrongly omitted the stress and temperature considered for the creep rates evaluation. Please carefully consider the reviewer comments which are attached in the pdf with notes.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript has been greatly improved, but there are still a few points needed to be addressed before publishing.

1. “Primary Austenite Grain” is wrong, it should be “Prior Austenite Grain”.
2. The prior thermal history of the base metals should be provided.
3. It is still very confusing to me that why the PAG size in 900c-simulated X20 steel is so large (61 um), where the measured Ac1 is 864C. Based on the EBSD map, it looks like those PAGs are not transformed grains, which means no significant austenitization occurred in that sample, they are just PAGs originated from the base metal with a PAG size of 57 um. The number of PAGs in EBSD map Figure 4 is very limited, if calculation is based on that map, it likely gets a slightly higher PAG size (61 um) rather than 57 um. I suspect the specimen was wrong. I strongly suggest the authors repeat that experiment to confirm it.  
4. Conclusion #2 should be removed. This is an incorrect statement based on simplified models, as the authors stated the microstructure evolution is not considered in the models.

Author Response

Dear reviewer,

Thank you for the valuable comments and suggestions. The manuscript was thoroughly read where grammar was corrected as necessary. A point-by-point response to your valuable comments can be found in the attachment.

Sincerely,
Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Well done

Author Response

Dear reviewer,

Thank you for the comment. The manuscript was thoroughly read where grammar was corrected as necessary.

Sincerely,

Authors

Round 3

Reviewer 1 Report

I still hold my opinion that the PAG size data for the 900C-simulated X20 steel is very abnormal, based on the comparison below. And look at the EBSD results in Figure 6, the PAG in 900c-simualted HAZ is even larger than that of the 1350c-simulated HAZ. How could it be in this way?  Please repeat the simulation experiment instead of just measuring that same 900c-x20 specimen.

Materials	Ac1 (C)	Ac3 (C)	900C-HAZ PAG size (um)	1350C-HAZ PAG size (um)	BM PAG size (um)
X20	864	1000	61	54	57
P91	897	1063	11	21	23

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf