Creep Rate, Friction, and Wear of Two Heat-Affected Zone Regions of 9–12 wt.% Cr Steels

Round 1

Reviewer 1 Report

Dear Authors,

I have reviewed your paper titled "Creep rate, friction and wear of two heat-affected zone regions of 9–12wt.% Cr steels". It shows interesting investigations, which fulfil the aims and scope of Metals journal. Article presentes high scientific level. Tests were performed very well, and I suggest minor revision.

General remarks:

• You have presented 26 references. Only two have been pubished in last three years. You should present more of the newest references. The science made big step forward last years, and it will show relevant scientific background for your investigations.
• Please check the style of ref. 14, which is different than others (date). The same with ref. 13.
• Abstract should be supported by values from investigations.

Introduction:

• Please mark the biggest problem during welding materials from investigated group and during their life. It will underline the necessity of your work.

Materials and Methods:

• How many specimens have you performed?
• Fig. 2, and Fig. 3 - please improve the quality. Also, I propos to change pictures to bigger.
• If you used any rules from relevant standards, please mark it in the text. If not, please describe the reasons.

Results:

• I propose to show real views of your specimens after tests. It will be more readable for potential readers. Are there any differences?

Discussion:

• You should compare here your results with other scientiests. Please mark what new have been observed in your tests. Please compare with relevant references.

Conclusions:

• Please support conclusions with values.

Author Response

The authors would like to thank the reviewer for his time to evaluate our manuscript. A point-by-point response to the reviewer`s comments is provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

The following additions/corrections are the mandatory

1. What is the minimum particle size can be distinguieshed using the automathic image analysys of the SEM images in the present work (lines 85-91)?

2.The aging temperature of 750°C is relatively high for these steels and probably can not only induce the coarsening of precipitates but also lead to the significant recrystallization of initial lath/subgrain structure, why did the authors choose this temperature?

3.For a better pesentation, the initial and aged microstructures of the studied steels should be added, even though some of them were published earlier, e. g. in Metall. Trans. A.2016

4. Since the studied steels contains V and Nb, the precipitation of MX-type carbonitrides is expected. Please discuss what effect do these particles have on the friction and wear resistance.

5. As known the major contribution to the decrease in creep strength of martensitic 9-12%Cr steels during creep exposure is subgrain growth/coarsening. Authors should discuss this moment.

Author Response

The authors would like to thank the reviewer for his time to evaluate our manuscript. A point-by-point response to the reviewer`s comments is provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Creep rate, friction and wear of two heat-affected zone regions of 9-12wt.% Cr steels

The paper presents an experimental test program to investigate the friction and wear properties in 9-12Cr steels. Two materials, which represent two heat affected zone materials are investigated, with an emphasis on precipitate evolution. It is recommended that this paper is suitable for publication in Metals if the following major revisions have been made:

1. Abstract: It is stated that 9-12Cr steels are a turbine material. Can the authors clarify that 9-12Cr steels are used in such applications? Thus reviewer is not aware of any such applications of 9-12Cr steels (e.g. for turbine blades etc.).

Introduction

2. Page 1, paragraph 2: This section of the literature on welding can be extended as a lot of work is currently being done on creep and fatigue of welded 9-12Cr steels.
3. Page 1, paragraph 3: The correlation between creep and precipitates is highlighted here. However, the authors do not elude to the full picture, where other microstructural features and evolutions of these features play a pivotal role on the performance of the material. This includes evolution of dislocation density and lath microstructures, as well as the formation of secondary phase particles such as Laves phase or Z-phase in higher Cr materials. These important evolutions of the microstructure should also be discussed. Furthermore, the stress for which the creep tests were performed is also important – different mechanisms may become more important at different applied stresses (e.g. precipitates may not be as important as evolution of dislocation density at higher applied stresses/shorter term creep tests). Thus, the stress range or regime should be described.

Materials and Methods

4. Page 2, paragraph 4: It is stated that the heat treatment simulation is to develop HAZ samples. However, the proposed heat treatment cycle is not representative of a typical welding process (i.e. the temperature is not held constant for 1 hour during welding). This will yield samples with a significantly different (coarser) grain structure to a typical HAZ and this grain structure will, in turn, affect the carbide distributions. This should be made clear.
5. Page 2, paragraph 6: What real-world applications do these very high temperature applications represent?
6. Page 3, paragraph 2: Throughout the text, the authors refer to the number, average spacing and size of precipitates. Which precipitates are these? M23C6 Carbides? The resolution of SEM will not pick up the fine MX carbonitrides which are well known to provide a significant contribution to the creep performance of 9-12Cr steels. Furthermore, how do the authors distinguish between M23C6 carbides, Laves phase particles and Z-phase particles (if any) in the material?
7. Page 3, paragraph 2: How do the automatic measurement processes mentioned in the manuscript account for the shape of precipitates?
8. Page 3, paragraph 4: The additional precipitation that is mentioned in the manuscript is more than likely Laves phase particles. Again, how are these additional secondary phase precipitates distinguished from carbides and accounted for in this analysis?
9. Page 4, paragraph 1: A major drawback in the present work is that the measurements conducted do not distinguish between the different precipitates which evolve (and dissolve) at different rates and temperatures. How do these multiple precipitate types affect the observed results and discussion as presented here? This needs to be discussed in the revised manuscript.
10. Figure 2: Bar charts with the phases side-by-side may be a clearer presentation of the results for the reader.
11. Page 4, paragraph 2: An extension of the literature review to acknowledge the vast wealth of information on precipitation and evolution of precipitates in 9-12Cr steels would answer a lot of the unexplained findings and observations in this article.
12. Page 4. Paragraph 3: The applied stress for the creep tests is extremely high. In fact, at these temperatures, it is probably approaching the yield strength of the material (it would be great to add in some high temperature tensile tests). At these higher stresses, dislocation mechanics and the evolving lath microstructure may dominate. Thus, the authors should discuss the evolution of such microstructural features and comment on their importance to the observations in this manuscript.
13. Page 5, paragraph 1: At what temperature are the friction tests conducted?
14. Page 5, paragraph 4: Have the authors considered the role of thermal loading (e.g. thermomechanical fatigue (TMF) or TMF-wear)?
15. Page 5, paragraph 4: What process do the tests represent?

Results

16. The creep curves from the creep tests need to be presented (see additional comments below).
17. Page 6, paragraph 2: Are the values 0.003 and 0.016 values of strain? If so, what strain?
18. Page 6, paragraph 2: What do the authors mean here when they state grain size in a material with a hierarchical microstructure, including regions with elongated grains? What are the typical values and distributions of these grain sizes?
19. Page 6, paragraph 2: There are a range of other microstructural evolutions is the material and hence, this reviewer thinks that it is difficult to assess the evolution of the creep strain as presented here (linking grain size to what appears to be primary creep strain). Evolution of dislocation density, lath microstructure and formation of secondary phase particles, such as Laves phase particles, and their significant contribution to creep behaviour have been omitted. Once again, the creep curves are also critical here again – a wealth of information is omitted from the manuscript by not presenting these results.
20. Page 7, paragraph 2: Can the authors explain how the frictional force exceeds the normal applied force during the test (i.e. coefficient of friction of 1.4)?
21. Page 8, paragraph 6: M23C6 (mostly Cr23C6) carbides are one of two important initial precipitate types in 9-12Cr steels. The role of MX precipitates, which have been shown to be critical for creep performance with a significant reduction in creep life when these particles are removed (e.g. see work of Sandstrom et al. on Bar 257 and papers associated with this). Furthermore, Laves phase (and Z-phase in higher Cr) particles also form in such materials and play a critical role on creep performance.
22. Page 9. Paragraph 2: How is primary creep strain determined? It is argued by some authors that a secondary creep stage in 9-12Cr steels is not really achieved (possibly due to the constant evolution of the microstructure – Laves phase formation, dislocation density evolution etc.). Thus, how is this point arrived at here? Again, the creep curves are required to clearly illustrate this.
23. Page 9. Paragraph 2: Please provide a unit of stationary creep rate.

Discussion

24. Page 11, paragraph 3: Once again, the authors appear to be only measuring and discussing larger precipitates such as carbides and possibly Laves phase when they form (e.g. the MX precipitates are omitted from this research). This needs to be made clear throughout the manuscript.
25. Figure 12: How does the shape of precipitates affect the performance?

Conclusions

26. Page 15: Can the authors provide any insight into the mechanisms which lead to the observations in points 2 and 3?
27. Page 15: This reviewer believes that the observation in point 7 is due to the omission of other key microstructural strengthening mechanisms and microstructural evolutions. This should be discussed in the discussion section of the manuscript.

Author Response

The authors would like to thank the reviewer for his time to evaluate our manuscript. A point-by-point response to the reviewer`s comments is provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 4 Report

This is a very nice study on the effects of aging and precipitate coarsening etc on the short-term creep and tribological (friction and sliding wear rate) behaviour of two 9-12Cr steels, including two different simulated HAZ conditions.

I have some issues that need to be dealt with however, as follows:

1. the abstract and conclusions refer directly to 'grain size' as a key variable considered for effects. However, no grain size quantification was presented in the paper, so this is somewhat meaningless and vague for the reader. Furthermore, what does 'grain size' mean for such hierarchical steels anyway? Does it mean lath width, sub-grain size, block width, packet size, PAG size? And again, what does 'size' mean in the context of 'grains' or similar features? One needs to be more specific about whether one means diameter, equivalent diameter etc.
2. Additional information should be provided in the Introduction about the context and industrial relevance of this work. Specifically, why would the tribological behaviour of HAZ ever be of interest? Why would a welded connection be used in a sliding component, susceptible to wear? How is this relevant to design of fossil fuel power plant? Please explain and describe in he introduction, for example, with a schematic or similar.
3. p3 -"However, for the P91 steel, when ageing at 650oC, an increase in the number of precipitates was observed ..." - please clarify what this increase is relative to.
4. p5 - please clarify whether or not the contact is plastic - has the plastic normal load, Py, been exceeded? If so, please justify this. 
5. Section 3.1 (and elsewhere) - in the text, the creep rate values given are incorrect, since they do not include the 10^-4, which is very important. PLease correct this in the text - it occurs in numerous places. 
6. Fig 4 (and other figures) - it is not possible to read the label for the y-axis - it is creep rate, but is it 10^-4? please make the graph labels more legible.
7. page 9  - para 2 - line 4 - 'primary creep rate'=> 'primary creep strain'
8. In numerous parts of the paper, the description of results is unwieldy and could be made significantly more succinct. For example, there is often over-detailed description of numerical information already presented in figures. One example of this is on page 12. also paragraph 1 on page 11. ditto page 14. The phrase 'paralysis of analysis'comes to mind - it hinders the efficient digestion of the main contents of the work.   
9. why is there no result presented for alpha in Figure 9? 
10. page 11 - surely "0.5 to 13 precipitates" is essentially the same as "0.5 to 16 precipitates"? does this need special mention? If yes, please briefly explain why in the paper. 
11. For conclusion 2, it should be noted that the difference in creep behaviour between P91 and X20 is more significant than the (marginal) difference in tribological behaviour both prior and post ageing, especially post-ageing.
12. Conclusion 1 - see comment 10 - arguably not worth mentioning this "difference".
13. In order for this conclusion to be evidence-based, Figures 4 to 11 should be annotated to include the 'grain size' (the meaning of which needs to be clarified, see comment 1 above also). 
14. Furthermore, surely the Conclusion 3 is mis-leading, since not only "grain size" changes for the different heat treated simulated HAZ conditions, e.g. phase changes also. Hence it is mis-leading, at best, to conclude a correlation with "grain size" directly.   

 

Author Response

The authors would like to thank the reviewer for his time to evaluate our manuscript. A point-by-point response to the reviewer`s comments is provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 5 Report

Considering obvious flaws of the manuscript regarding the outline of the work topic, false statements on current state of research, serious doubts regarding the applied expermimental methods, and shortcomings in both data presentation and discussion, I can unfortunately only recommend rejection of this manuscript. 

More detailed observations will be provided to the editors of the journal. 

Author Response

The authors would like to thank the reviewer for his time to evaluate our manuscript. We strongly disagree with the statements made by the reviewer, but unfortunately cannot comment on them since they are too general and not grounded.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The symbols on the fig 1 are too small.