On the Q&P Potential of a Commercial Spring Steel

Round 1

Reviewer 1 Report

Q&P treatment are popular for high-strength steels, especially for automotive steels. The manuscript investigated the microstructure and properties of a commercial spring steel treated by Q&P process. However, several key points remain unclear (please see comments to authors). I would like to see how authors to reply these questions.

1. The motivation of the study is not clear.

Q&P treatment was generally summarized in Section Introduction. The background and motivation of the study should be also given.

2. Section 3.1. Microstructure

The volume fraction of RA in sample partitioned at 400 °C is much larger than that of sample partitioned at 300 °C (Figs.3c and 3d), which is attributed to austenite reversion at 400 °C. This is maybe correct. But, you don’t have any proof to prove that more RA is caused by austenite reversion. In this case, the more partitioning of carbon at 400 °C cannot be ignored. At higher temperature, more carbon content in austenite caused by more partitioning stabilize untransformed austenite, resulting in more RA at room temperature.

3. Section 3.2. Mechanical properties

(1) The sample dimension for tensile and compression tests should given.

(2) Although RA fractions are similar (6.4%, 5.8%), the total elongation of sample partitioned at 300 °C is much larger than that of sample partitioned at 250 °C. Detailed explanation should be given for this result.

4. Sample in condition 2 (partitioned at 300 °C) exhibits the best mechanical properties than sample in condition 3 (partitioned at 400 °C) although the later has much more RA. Authors attributed this to the fine distribution of retained austenite for both samples in abstract, whole text and conclusions. My question is: how to explain the lower mechanical properties of sample 3.

5. The normal elastic module of steels is about 200 GPa, but from Fig.4a, we can see that elastic module of the tested steel is only about 100 GPa, much smaller than normal value.

Author Response

Dear Reviewer,

thank you for taking time to give detailed feedback to our manuscript. With the help of your kind comments we were improving our paper. We really appreciate this and we hope that all changes in manuscript as well as our answer to your comments and question are acceptable for you.

 

1. The motivation of the study is not clear.

Q&P treatment was generally summarized in Section Introduction. The background and motivation of the study should be also given.

 

Thanks for this comment. We tried to improve with some more details and results from other groups working on QP of spring steels to figure out why we were conducting this study.

 

2. Section 3.1. Microstructure

The volume fraction of RA in sample partitioned at 400 °C is much larger than that of sample partitioned at 300 °C (Figs.3c and 3d), which is attributed to austenite reversion at 400 °C. This is maybe correct. But, you don’t have any proof to prove that more RA is caused by austenite reversion. In this case, the more partitioning of carbon at 400 °C cannot be ignored. At higher temperature, more carbon content in austenite caused by more partitioning stabilize untransformed austenite, resulting in more RA at room temperature.

Thank you for this comment. We change the manuscript slightly in lin. We are not sure about the reasons and will investigate this more in details in our next big project with the help of more detailed XRD investigations and hopefully also some SEM in situ methods on high temperature SEM at Otto-von-Guericke University in Magdeburg, Germany.

3. Section 3.2. Mechanical properties

(1) The sample dimension for tensile and compression tests should given.

Can be found in line 135-37. Those values were already in the first draft and you probably overread them while they are without a picture. We avoided a figure due to the easy design of the specimen.

 

(2) Although RA fractions are similar (6.4%, 5.8%), the total elongation of sample partitioned at 300 °C is much larger than that of sample partitioned at 250 °C. Detailed explanation should be given for this result.

The differences in the mechanical properties, despite similar resulting retained austenite contents, can be seen in the different heat treatment process routes. For example, Q&P 2 with PT = 300 °C has a higher quenching temperature, so that in comparison to Q&P 1 with PT = 200 °C and QT = 175 °C there may be increased tempering effects which increase the toughness of the martensite present. Furthermore, it was observed that, compared to Q&P 1, there is an increase and a finer distribution of retained martensite. Further investigations using SEM and XRD are planned in future work to better understand the mechanisms involved.

4. Sample in condition 2 (partitioned at 300 °C) exhibits the best mechanical properties than sample in condition 3 (partitioned at 400 °C) although the later has much more RA. Authors attributed this to the fine distribution of retained austenite for both samples in abstract, whole text and conclusions. My question is: how to explain the lower mechanical properties of sample 3.

Due to the higher retained austenite content, the strengths decrease. Furthermore, it can be assumed that due to the higher retained austenite content more carbon is diffused from the martensite, i.e. that the martensite has a low C content locally. Carbide precipitations may also be present. In addition, a higher partitioning temperature is present, so that the existing martensite is additionally tempered to a higher degree and thus relaxed more. This also leads to a decrease in strength. In total, the effects mentioned lead to a reduction in strength. We added this in the manuscript. A more detailed breakdown of the respective effects is planned in further investigations.

5. The normal elastic module of steels is about 200 GPa, but from Fig.4a, we can see that elastic module of the tested steel is only about 100 GPa, much smaller than normal value.

Thank you for your comment. We made an analysis of the Young’s moduli of our testing data. The measured Young’s moduli were between 173 and 200 GPa during tensile testing determined between 50 and 500 MPa. The differences arise from the small deviation in the alignment of the specimen and the DIC measuring system. If this is not 100 % perpendicular such small deviations of 10-15 % in Young’s modulus may happen. For the determination of the mechanical properties we were subtracting the elastic part of value with the Young’s modulus of each measurement. The much lower Young’s moduli from compression testing are arising from the strain measurement with the internal displacement. The stiffness of the test rig is influencing and falsifying that value. Anyway, the determined yield strengths of those compression tests are not affected from this. That is why we hope that this answer is okay for you. If we would have had main focus on determine Young’s moduli we would have used strain gauges of course, especially for the compression tests.

Reviewer 2 Report

The article provides relevant interesting information on the possibility of improving the mechanical properties of commercial spring steel by means of the QnP process. The optimal regime of QnP processing is proposed to match the industrial application. The paper has the potential to be cited by the researchers dealing with QnP treatment. However, some points should be addressed before the paper's acceptance.

1. The introduction seems to be too brief. There is a lack of specific data related to the topic of the article. Specific attention in the introduction should be addressed to the application of the QnP-process for commercial spring steels. Corresponding references, for example, are:

- Materials Science and Engineering: A. 2019. vol. 745, 307-318. doi.org/10.1016/j.msea.2018.12.106

- Journal of Alloys and Compounds? 2017. vol. 697, 43–54, https://doi.org/10.1016/j.jallcom.2016.12.134.

- Journal of Iron and Steel Research, International, 2011, vol. 18 (2), 70-74,

https://doi.org/10.1016/S1006-706X(11)60026-5.

2. What was the medium for the quenching to 170 oC and 200 oC? What was the medium for partitioning holding? Was the specimen’s surface protected from decarburizing?
3. Lines 212-214: “We need to conclude that this Q&P treatment failed in order to compare those results with data from the actual applied quenching and tempering (QT)”. Please specify the regime of quenching and tempering corresponded to the properties presented.
4. Lines 239-240: “Furthermore, we assume that the partitioning at 400 °C also lead to the formation of brittle carbides (e.g. M23C6)…”. Why M23C6? Is there any proof for this type of carbide? More likely, cementite or transitional carbide may occur in studied steel.
5. Lines 267-272: “The remarkable conclusion is that not only volume fraction and fine distribution of retained austenite determines the potential of improving mechanical properties by Q&P in commercial alloys: also the process step when the retained austenite is developing (and stabilizing or not), as well as occurring parallel formation of carbides, may strongly influence this potential.” Explaining the reasons for improving properties, the authors ignored the possible TRIP effect due to the transformation of retained austenite under the loading.
6. The beginning of the abstract (four sentences) gives no new information. They may be omitted.

Author Response

Dear Reviewer,

thank you for taking time to give detailed feedback to our manuscript. With the help of your kind comments we were improving our paper. We really appreciate this and we hope that all changes in manuscript as well as our answer to your comments and question are acceptable for you.

 

1. The introduction seems to be too brief. There is a lack of specific data related to the topic of the article. Specific attention in the introduction should be addressed to the application of the QnP-process for commercial spring steels. Corresponding references, for example, are:

- Materials Science and Engineering: A. 2019. vol. 745, 307-318. doi.org/10.1016/j.msea.2018.12.106

- Journal of Alloys and Compounds? 2017. vol. 697, 43–54, https://doi.org/10.1016/j.jallcom.2016.12.134 .

- Journal of Iron and Steel Research, International, 2011, vol. 18 (2), 70-74,

https://doi.org/10.1016/S1006-706X(11)60026-5 .

Thank you for your kind input. We strongly tried to improve the introduction/motivation to show why this study was performed. Find the changings in the revised manuscript marked with yellow.

Thank you for this comment and those articles. You are absolutely right and we tried to improve the introduction regarding figuring out why this study has been conducted recently. Find the changed parts in yellow in the revised manuscript.

 

2. What was the medium for the quenching to 170 oC and 200 oC? What was the medium for partitioning holding? Was the specimen’s surface protected from decarburizing?

The quenching occured in the induction-based heat treatment tool of DeltaSigma GmbH which is combine with a contact cooling system (cooled with water tubes in the tool). Several thermocouples are measuring in real time. So the control unit can adjust the temperature in the tool. So no specific medium like liquid salts are required. That is another remarkable benefit for potential industrial application. We were added this information to the revised version. 

The specimen in the tool itself is not capsuled completely from the outside. So we cannot exclude decarburizing, but the open surfaces are reduced and the is no recirculation like in standard furnaces which results overall in reduced decarburisation.

 

1. Lines 212-214: “We need to conclude that this Q&P treatment failed in order to compare those results with data from the actual applied quenching and tempering (QT)”. Please specify the regime of quenching and tempering corresponded to the properties presented.

We did not perform mechanical testing of Q&T condition. We just took the data from a commercial library which we are using in thyssenkrupp company for obtaining mechanical properties for FE simulation. So we are not able to specify the corresponding heat treatment, but we assume that those values are determined from a commercial optimised and common heat treatment. Comparable heat treatments in the literature exhibits comparable values.

3. Lines 239-240: “Furthermore, we assume that the partitioning at 400 °C also lead to the formation of brittle carbides (e.g. M23C6)…”. Why M23C6? Is there any proof for this type of carbide? More likely, cementite or transitional carbide may occur in studied steel.

Thanks for this comment. You are right. We were reading this in the literature, but mixed this with a steel with lower carbon content. Some quick JMatPro calculation were showing that cementite is dominating and few M7C3 may occur. We change this in the revised version.

 

4. Lines 267-272: “The remarkable conclusion is that not only volume fraction and fine distribution of retained austenite determines the potential of improving mechanical properties by Q&P in commercial alloys: also the process step when the retained austenite is developing (and stabilizing or not), as well as occurring parallel formation of carbides, may strongly influence this potential.” Explaining the reasons for improving properties, the authors ignored the possible TRIP effect due to the transformation of retained austenite under the loading.

Thank you for this comment. We add this in that conclusion even though the TRIP effect is not that pronounced such as in typical TRIP steel sheets where the work hardening almost leads to a factor of 2 between yield strength and ultimate tensile strength. Furthermore the content of austenite after the mechanical testing was not decreased so much that we would publish this as typical TRIP effect. That is why we were not focussing on TRIP explanations.

5. The beginning of the abstract (four sentences) gives no new information. They may be omitted.

You are absolutely right. Those four sentences without new information were addressed to potential applications in the industry which should help the engineers to introduce the topic to their managers (that is why we choose open access journal!). In order to keep this idea at least a little alive we killed two from those four sentences. I hope you can live with this compromise of small introducing to the field of QP before getting serious with information about our study.

Reviewer 3 Report

Effects of reversed austenite on mechanical properties is not yet well investigated. The authors undertook research on this issue. “Quenching and partitioning” (Q&P) proces is applied on tailor-made materials with high purities or prototype alloys. Authors performed investigations on the commercial 0.54C-1.45Si-0.71Mn spring steel. They found that with increased quenching temperature and increased partitioning temperature, fraction of austenite increases.  The authors carried out well-designed and properly performed studies of the structure and properties of various variants of steel heat treatment. Stretch curves are shown, however there is no strengthening curve which would allow a clearer comparison of the effects of the heat treatment. The conclusions drawn result from the research carried out and are correct. The research results have a large practical aspect and are very valuable. In my opinion, with a slight correction regarding the graphic illustration of the steel property, the article can be published. 

Author Response

Dear Reviewer,

thank you for taking time to give positive feedback to our manuscript.  We really appreciate this and we hope that  our answer to your comment/question is acceptable for you.

 

"Effects of reversed austenite on mechanical properties is not yet well investigated. The authors undertook research on this issue. “Quenching and partitioning” (Q&P) proces is applied on tailor-made materials with high purities or prototype alloys. Authors performed investigations on the commercial 0.54C-1.45Si-0.71Mn spring steel. They found that with increased quenching temperature and increased partitioning temperature, fraction of austenite increases.  The authors carried out well-designed and properly performed studies of the structure and properties of various variants of steel heat treatment. Stretch curves are shown, however there is no strengthening curve which would allow a clearer comparison of the effects of the heat treatment. The conclusions drawn result from the research carried out and are correct. The research results have a large practical aspect and are very valuable. In my opinion, with a slight correction regarding the graphic illustration of the steel property, the article can be published."

Interestingly, you are mentioning hardening/strengthening curves. Such hardening curves and differences in flow behaviour under tension and compression should be content of our next research paper in near future. Of course, those properties and its implementation in models for FE simulation are always in the focus from a big steel producer like thyssenkrupp. We would kindly ask you to accept that we will not publish them within this study.

Round 2

Reviewer 1 Report

It can be published in its current form.