Revealing the Dynamic Transformation of Austenite to Bainite during Uniaxial Warm Compression through In-Situ Synchrotron X-ray Diffraction

Round 1

Reviewer 1 Report

Dear colleagues

With no doubt the subject of the paper is of great interest.

I have several concerns regarding the manuscript.

Major:

Line 149-159. It is argued that the decrease in bainite formation rate is related to the introduced dislocations during straining. There is also the possibility that the transformation is reaching its end, it is well reported that last stages of bainitic rection are sluggish, leading to the typical sinusoidal shape  curve when plotting the fraction of bainite/ change in length v.s time.

Line 166: Is there any reference supporting the statement mentioning that austenite strength is lower than that of bainitic ferrite?.

I believe the discussion on the FWHM in relation to dislocation density is a bit more complex than that shown in the manuscript. It seems that the discussion is only focussed in assuming that dislocation are introduced only via deformation, but bainite transformation, being a displacive transformation, implies the generation of dislocation in the parent austenite to accommodate the plastic deformation introduced due to the transformation per se, see Figure 2 of your supplementary material. Also, according to bainite transformation theory, bainite inherits the dislocations from the austenite from where it forms, Figure 2 of your supplementary material, considering the fact that austenite is being continuously plastically deformed, there might be also a contribution to the FWHM of ferrite.

Line 203: It is said that, “…. after the deformation and subsequent recovery, the FWHM 2 in the {211} reflection is significantly higher than that of before deformation”, I don’t thinh is the case, as it also mentioned some paragraphs above, line 174 .

Line 210: The statement made regarding the residual FWHM would only valid if the recovery process is total and dislocation density within bainitic ferrite is brought to cero, being then the crystallite size (scale of the microstructure) the only factor contributing to the  FWHM.

Line 235: The author mention that that “Carbon partition from bainitic  ferrite to remaining austenite is corroborated by fact that the bainite volume fraction (fαb) increases significantly during straining (Figure 2b).”, but such increase in bainite fraction does not corroborate C partitioning from ferrite to austenite, what it probably does is the analysis performed from line 238-244 with the Poisson ratio.

Line 249 is Figure 2 e and not Figure 3 e.

Line 251: I don’t see how the behaviour of bainitic ferrite lattice parameter provides information about the nucleation mechanism (paraequilibrium).

Line 253 to 255. The meaning of that sentences escapes my understanding. Once the conditions for a bainite nuclei (with or without applied deformation) are fulfilled, it forms allowing only for C diffusion. The nuclei then grows by a difussionless and displacive mechanism , bainitic ferrite plate, and, the excess of C in that plate is immediately released to the parent austenite once it stops it growth, regardless if such plate was formed at the beginning of the isothermal step (deformation) or at latter stages (further strains).

Section 3.4.

According to the authors and experimental results, the fraction of bainite formed in the no-deformed case is about 8%, see Figure 5. Image in Figure 7 and the text talk about a fully bainitic microstructure.

Is there any martensite forming on cooling to room T, I would think so for this type of chemical composition. In the case of the deformed sample, does the plastic deformation in the austenite inhibits martensitic transformation (mechanical stabilisation?) ? How martensite presence might affects the interpretation of results?.

In case the authors are using the 430 C microstructure for this part, the whole discussion is futile as it is , there are other microstructural characteristics that affect the HV, as C in austenite and C in ferrite that depends on the transformation T. 

It is mandatory that evidence of refinement against the same microstructure obtained at the same T, is presented in order to talk about “ structural refinement takes place in bainite coming from dynamic transformation”, otherwise is pure speculation.

For all the above reasons, the contradiction found with Ref 4 results can also be explained.

Phase ID in the images is necessary.

Make sure through the text to clarify in all cases the transformation T for the undeformed case, as it seems that sometimes is 430 and other 500.

If, as suggested by the authors, the transformation is no-isotropic (as there is a preferential direction of growth), how does it affect the interpretation of the results?. If measurement were made in a perpendicular direction to that used in this manuscript , you might have found the same or opposite trends?.

Some of the references used in this paper deal with such no-isotropic transformation and the interpretation of results.

Minor:

I believe in line 35 when refereeing to DT , a gamma symbol is missing, also in line 38, line 41…etc

Line 66: Is correct the term Mass % to refer to the fraction of bainite formed, shouldn’t be volume fraction? Was that measured by the HEXRD system?. How long was the time spam to achieve such fraction?

Was the HEXRD system operating and extracting data during the whole heat-thermomechanical treatment, or just in the isothermal step?.

 

Author Response

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Reviewer 2 Report

Dear authors,

Thermomechanical treatment of steels is well known technology widely uses in industry. In the manuscript you sent new approach to bainitic transformation have been presented and sophisticated  examination method. My few questions and comments were marked in the attached file of your manuscript scan.

My questions to the authors are following:

1. Page 1 (abstract): expression - .... microstructure is oriented perpendicular to the ...... is not precise/clear to the readers

What type of phases (ferrite grains, martensite/bainite laths, packets etc.) create dynamically deformed microstructure and which of them are specially oriented to the direction of compression ?

2. Page 1 (abstract): expression - ..... which is ruled by the preferential introduction of dynamic dyslocations into the austenite.  It is mean that at the first stage of plastic deformation in the austenite moveable dislocations are produced ?

3. Paghe 2- Materials and methods. Uniaxial (warm) compression of the cylindrical specimen characterise by non uniform stress and strain distribution in the volume of specimen (this is typical phenomenon of the process due tue friction between sample and tool/anvil). You should examine microstructure in the represantitive area of the deformed samples.

Did you observe and analyse microstructure inhomogeneity resulted from applied way of deformation ?  Which area of the specimens did you examine and why ? 

  4. Page 7 (first line and fifth line). The authors refer to Supplementary Material (see Figure S2 ?). I did not find any Supplement to the manuscript.   5. Page 7 - ...parent austenite.....  Is it PRIOR austenite ?   6. Page 7 ....... isotropic effect of carbon enrichment .... Please explain clearly this phenomenon. 7. Page 8 (line 5 from the top)  Figure 3e - lack of the figure in the manuscript. 8. Page 8 (question to the Figure 3b) - What is the reason of constant temperature during deformation in the true strain range of 0.1-0.3 ? 9. Page 12 (line 5 from the top) - similar to the question no. 1. .....network of structures .... NOT CLEAR (....microstructure of deformed specimens consisted of .......) 10. Page 12 (Conclusions) : ...hot compression.... Deformation at 500 deg. of C  should be called WARM unlike to the typical HOT deformation of the steel in the range of 1200-850 deg. of C.

Best regards

Comments for author File: Comments.pdf

Author Response

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Reviewer 3 Report

The presented paper is devoted to an important issue regarding the promotion of bainitic transformation from plastically deformed austenite. The characteristics of the austenite-bainite reaction and the microstructural evolution during the compressive hot deformation (dynamic transformation) was observed by in-situ high energy synchrotron X-ray diffraction. Authors demonstrate that plastic deformation stimulate continuously bainitic transformation and partitioning of carbon take place during transformation. The paper contains new interesting results, and it can be recommended for publication after taking into account the minor comments below.

1. There are abbreviation “DT( ->alpha b)” in Introduction and Conclusion sections. It looks like the letter “gamma” is missing.

 

1. It would be useful to inform how the used experimental conditions relate to those that are realized in controlled rolling.

 

3. Lines 146-147. “The obtained results in this study can be explained in terms of the dynamic generation of dislocations during deformation.” What mechanism of “dynamic generation” do the authors mean? Should be explain and/or provide a reference.

 

4. Authors conclude about the decisive role of dislocations in the bainite nucleation. At the same time, it usually accepted that grain boundaries are most preferable for bainite nucleation in continuous cooling condition. Could the authors comment on the role of grain boundaries in their experiment.

 

5. Reference 17 should be checked

Author Response

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Round 2

Reviewer 1 Report

Many thanks for the provided answers.
I am very satisfied and thankful for the details provided and changes implemented