Influence of the Deformation Degree of Combined Loadings on the Structural and Mechanical Properties of Stainless Steels

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Austenitic stainless steel 316L is widely used in modern industry, especially in such areas as oil and gas and shipbuilding. An important aspect of its application is to understand its behavior under complex mechanical loads combining tension and torsion. This study is devoted to the study of the effect of combined loads on the structure and mechanical properties of this steel. The experiments were carried out on a specially designed setup that allowed tensile and torsion tests at different speeds. It was found that under torsion, the material exhibits greater deformation compared to tension. In addition, the strain rate has a significant effect on the microstructure and mechanical properties of the sample. The importance of the work is that the obtained results allow a more accurate assessment of the material's behavior under real operating conditions and can be used to optimize processing and design of structures where these materials are used, including medical implants.

The article is devoted to an important topic, but there are several comments that need to be clarified:

1. The problem is identified in the introduction (1. Introduction), but the specific goal of the study is not clearly formulated. It would be good to end the introduction section with a clear and well-defined objective of this work.

2. Increase the number of keywords to 6-8, make them more specialized.

3. The introduction refers to some works, but the literature review is not complete. It is necessary to analyze the existing studies on this topic in more depth, identify gaps and justify the novelty of this study. It is necessary to more clearly indicate the basis for the choice of this particular alloy, loading parameters and analysis methods.

4. It is necessary to provide a more detailed analysis of changes in the microstructure, including grain size, dislocation density, and the number of twins. How the microstructure of the sample changes is not described sufficiently. It would be good to supplement the article with at least data on the shape and size of grains.

5. The article notes the relationship between microstructure and mechanical properties, but this relationship requires a more in-depth analysis. What specific structural changes lead to changes in strength and ductility?

6. The torque analysis section provides a qualitative analysis of the curves, but there is no quantitative description of the deformation parameters such as elastic limit, yield strength, etc. Add more numerical values ​​of the obtained data.

7. It is necessary to further explain why the microhardness decreases in the center of the sample and how this data correlates with the microstructure.

8. The conclusions in the “Conclusions” section are somewhat general and are not supported by quantitative analysis. More specific conclusions are needed based on the results presented, i.e. add numerical data both directly in terms of values ​​and percentage change in properties.

Author Response

We thank the reviewer very much for the time he took to analyze the paper and for his pertinent and on-topic recommendations.

Austenitic stainless steel 316L is widely used in modern industry, especially in such areas as oil and gas and shipbuilding. An important aspect of its application is to understand its behavior under complex mechanical loads combining tension and torsion. This study is devoted to the study of the effect of combined loads on the structure and mechanical properties of this steel. The experiments were carried out on a specially designed setup that allowed tensile and torsion tests at different speeds. It was found that under torsion, the material exhibits greater deformation compared to tension. In addition, the strain rate has a significant effect on the microstructure and mechanical properties of the sample. The importance of the work is that the obtained results allow a more accurate assessment of the material's behavior under real operating conditions and can be used to optimize processing and design of structures where these materials are used, including medical implants.

The article is devoted to an important topic, but there are several comments that need to be clarified:

1. The problem is identified in the introduction (1. Introduction), but the specific goal of the study is not clearly formulated. It would be good to end the introduction section with a clear and well-defined objective of this work.

Response: We have inserted the following, as final paragraph of the Introduction section:

In the present work, the authors propose a combined loading protocol of mechanical processing of 316L stainless steel by tensile pre-loading and subsequent torsion using an innovative double-end torsion machine and analyzed the effects of this mechanical processing on the microstructure and several mechanical properties. The aim of the paper is to establish the effects of the combined loading on 316L SS, the maximum limits reached by torsion in these cases and the evolution of the microstructure and mechanical properties (microhardness and indentation modulus).

2. Increase the number of keywords to 6-8, make them more specialized.

Response: we have updated the keywords: microstructure; tensile; torsion; mechanical strength, twist angles, combined loadings, grain refinement.

3. The introduction refers to some works, but the literature review is not complete. It is necessary to analyze the existing studies on this topic in more depth, identify gaps and justify the novelty of this study. It is necessary to more clearly indicate the basis for the choice of this particular alloy, loading parameters and analysis methods.

Response: We have added a paragraph on the medical applications of 316L SS to supplement the first paragraph of the Introduction, which illustrates some of the many areas in which 316L SS is used.

The 316L stainless steel studied is suitable for the medical industry as medical devices and parts, due to its properties such as corrosion resistance [15]. Brackets, the well-known device for straightening teeth, oral instruments, many surgical instruments such as scalpels and tweezers are made from this material. It is important to mention that for this medical purpose a material with high durability, precision and with no adverse reactions such as allergies can be used. 316L SS with an ultra-fine microstructure offers many advantages for biological implants as load-bearing elements [16].

[15] Esmaeili, A., Ghaffari, S.A., Nikkhah, M., Ghaini, F.M., Farzan, F., Mohammadi, S., Biocompatibility assessments of 316L stainless steel substrates coated by Fe-based bulk metallic glass through electro-spark deposition method, Colloids and Surfaces B: Biointerfaces, 198, 2021, 111469.

[16] Muley, S.V., Amey N. Vidvans, A.N., Chaudhari, G.P., Udainiya, S., An assessment of ultra fine grained 316L stainless steel for implant applications, Acta Biomaterialia 30 (2016) 408–419

4. It is necessary to provide a more detailed analysis of changes in the microstructure, including grain size, dislocation density, and the number of twins. How the microstructure of the sample changes is not described sufficiently. It would be good to supplement the article with at least data on the shape and size of grains.

Response: Using VegaTc software we have performed a few determinations of the dimensions suggested (the given values are average values of 5 determinations from different areas).

We have added the following:

For the central part of the sample, the least affected by the torsion, Figure 6 (a), a slight decrease in grain size can be observed, from sizes between 30-70 μm to 20-50 μm, due to the stretching stage in the first part of the deformation and less to the torsion of the material by breaking the boundaries due to the compression of the grains between them. The dislocation density is similar and the number of twins is close to the initial state, confirming that the central area does not deform very much under the influence of the torsion process.

For the part subjected to more intense torsion, i.e. the outer part of the specimens, a reduction in grain size to 10-30 μm was observed over an area of about 65x65 μm (-15300 μm²) due to their crushing under torsional forces. An increase in dislocation density from an initial 25-30% to 60-75% can be seen in Figure 6 (b) and an agglomeration of twins was observed due to a reduction in their width from 1 um to 0.65 μm.

5. The article notes the relationship between microstructure and mechanical properties, but this relationship requires a more in-depth analysis. What specific structural changes lead to changes in strength and ductility?

Response: We have commented the relation between the structure and strength of the material and we have added the following:  

The very good ductility properties of 316L SS are due to the plasticity phenomenon, which induces the formation of twins [25].

The deformation of the material by torsion is accompanied by the formation of twins by mechanical loading. Since the deformation due to the formation of twins depends on the orientation of the grains, this phenomenon is more common in the areas at the edge of the material, Figures 6(a) and 6(b), compared to the central area of the material [26].

The effects of mechanical twinning on material hardening are complex and can be diametrically opposed. In the first case, there is a refinement of the structure, an increase in the microhardness of the material, and in the second case, the concentrations of internal stresses are released and a decrease in the hardness of the material occurs through relaxation of the structure, which is observed in the central area of the deformed material. Depending on which of these two phenomena prevails, the material will suffer an increase or decrease in the modulus of elasticity or hardness, respectively.

We have inserted the references:

[25] Lewandowski, J.J., Seifi, M., Metal additive manufacturing: a review of mechanical properties, Annu. Rev. Mater. Res. 46 (1) (2016) 151–186. 10.1146/annurev-matsci-070115-032024.

[26] Zhai, W., Liu, F., Wang, Q., Nai, S.M.L., Zhou, W., Cryogenic and high temperature tensile properties of 316L steel additively manufactured by laser powder bed fusion, Materials Science & Engineering A 900 (2024) 146461.

6. The torque analysis section provides a qualitative analysis of the curves, but there is no quantitative description of the deformation parameters such as elastic limit, yield strength, etc. Add more numerical values ​​of the obtained data.

Response: The loading protocol used in this study followed the sequential combined loading and evaluation of its effects (influence) on the material response at the macro- and micro-structural levels. For quantitative description, torsional moment and twisting angle values can be used, but not shear stress and strain values (elastic, yield). The specimens were tested (Figure 5(b)) without fracture.

7. It is necessary to further explain why the microhardness decreases in the center of the sample and how this data correlates with the microstructure.

Response: The initial tensile stress, combined with a lower degree of torsion, leads to a relaxation of the material network in the central region and a reduction in the microhardness by accommodating the existing dislocations and twins, which were present in the structure in smaller numbers at the beginning.

8. The conclusions in the “Conclusions” section are somewhat general and are not supported by quantitative analysis. More specific conclusions are needed based on the results presented, i.e. add numerical data both directly in terms of values ​​and percentage change in properties.

Response: We have restructured the main conclusions and revised the text:

The main conclusions drawn from the experimental results are the following:

- Grain refinement occurs during complex mechanical loading and large torsion angles can be applied to 316L stainless steel;

- As the torsion rate increases, the grains undergo plastic deformation and the twins become smaller, affecting the integrity of the material, the grain size decreases by 25-30%; the dislocation density increases with the torsion angle and at the same time the number of twins increases, the width decreases from 1 to 0.65 μm.

- All three torsion curves exhibited a rapid initial increase followed by stabilization (close to the horizontal region). This indicates an initial elastic phase followed by a plastic phase or more stabilized behavior.

- As the torsion rate increases, the grains undergo plastic deformation and the twins become smaller, affecting the integrity of the material;

- The effects of mechanical twinning on the material hardening are complex and can be diametrically opposed, affecting the hardness and elasticity values of the specimens.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. In Fig. 5(a), the tensile strain is inaccurate because no extensometer was applied. This can be seen from the elastic zone of the tensile curve. The Young's modules, E, of 316L is about 190 GPa. However, the E is less than 50 GPa.

2. In Fig. 6, the features of 316L after deformation are deformation-induced twining. I suggest the authors refer to this paper (Materials Science & Engineering A 900 (2024) 146461) to provide a more detailed explanation of the phenomenon. 

3. Perhaps EBSD is more useful in characterizing the deformation-induced twining.

4. A gap is recommended to be added between figures, such as for Fig. 6(a). The gaps in Fig.4 make it easier for readers to visualize. 

5. Why is Young's Modulus measured using indentation so low? Some of the data are even smaller than 50 GPa, which is totally wrong! 

6. The conclusions need to be revised significantly. Some of the conclusions can be found in a textbook without doing the experiment. 

7. "for medical implants because the obtained val-420 ues are very close to those of bone". Please remove this statement since Young's Modulus was measured wrongly.

Comments on the Quality of English Language

NA

Author Response

We thank the reviewer very much for the time he took to analyze the paper and for his pertinent and on-topic recommendations.

1. In Fig. 5(a), the tensile strain is inaccurate because no extensometer was applied. This can be seen from the elastic zone of the tensile curve. The Young's modules, E, of 316L is about 190 GPa. However, the E is less than 50 GPa.

Response: The loading protocol used in this study followed the sequential combined loading and evaluation of its effects (influence) on the material response at macro and micro structural levels. The first test, Figure 5 (a), was a tensile test in the elastic range without the use of a gauge (extensometer). We have mentioned in the manuscript the estimated value for Young's modulus (page 11): “The value of Young's modulus of the initial material determined from different areas of the tensile failure curve (Figure 2) was found to be approximately 190~200 GPa, a value close to those reported in the literature.” The value of 50 GPa or around was obtained/determined by indentation [reference 9] and not by tensile testing (if that is the uncertainty).

2. In Fig. 6, the features of 316L after deformation are deformation-induced twining. I suggest the authors refer to this paper (Materials Science & Engineering A 900 (2024) 146461) to provide a more detailed explanation of the phenomenon.

Response: We have added the following:

The very good ductility properties of 316L SS are due to the plasticity phenomenon, which induces the formation of twins [25].

The deformation of the material by torsion is accompanied by the formation of twins by mechanical loading. Since the deformation due to the formation of twins depends on the orientation of the grains, this phenomenon is more common in the areas at the edge of the material, Figures 6(a) and 6(b), compared to the central area of the material [26].

The effects of mechanical twinning on material hardening are complex and can be diametrically opposed. In the first case, there is a refinement of the structure, an increase in the microhardness of the material, and in the second case, the concentrations of internal stresses are released and a decrease in the hardness of the material occurs through relaxation of the structure, which is observed in the central area of the deformed material. Depending on which of these two phenomena prevails, the material will suffer an increase or decrease in the modulus of elasticity or hardness, respectively.

We have inserted the references:

[25] Lewandowski, J.J., Seifi, M., Metal additive manufacturing: a review of mechanical properties, Annu. Rev. Mater. Res. 46 (1) (2016) 151–186. 10.1146/annurev-matsci-070115-032024.

[26] Zhai, W., Liu, F., Wang, Q., Nai, S.M.L., Zhou, W., Cryogenic and high temperature tensile properties of 316L steel additively manufactured by laser powder bed fusion, Materials Science & Engineering A 900 (2024) 146461. https://doi.org/10.1016/j.msea.2024.146461.

3. Perhaps EBSD is more useful in characterizing the deformation-induced twining.

Response: Yes, it is true that an EBSD analysis has been used in many studies of 316L SS and has been helpful in many discussions of the results (References: 3,6,7,8,10,12,14,21,22,33), however the lack of equipment at this time makes it very difficult for us to perform such tests. We thank you for your suggestion and will try to collaborate with a laboratory that has this detector and can perform such tests.

4. A gap is recommended to be added between figures, such as for Fig. 6(a). The gaps in Fig.4 make it easier for readers to visualize.

Response: Ok, thank you, we've added the space.

5. Why is Young's Modulus measured using indentation so low? Some of the data are even smaller than 50 GPa, which is totally wrong!

Response: To evaluate the influence of combined loadings on the behavior of the material, of some parameters, the indentation method was used (ref. 9). We especially monitored the change in these parameters between the initial sample and the stressed (deformed) ones, but also to be able to compare the values obtained with those identified in the specialized literature by indentation on bone materials (biological, ref 29 (now 33)).

6. The conclusions need to be revised significantly. Some of the conclusions can be found in a textbook without doing the experiment.

Response: We have restructured the main conclusions and revised the text:

The main conclusions drawn from the experimental results are the following:

- Grain refinement occurs during complex mechanical loading and large torsion angles can be applied to 316L stainless steel;

- As the torsion rate increases, the grains undergo plastic deformation and the twins become smaller, affecting the integrity of the material, the grain size decreases by 25-30%; the dislocation density increases with the torsion angle and at the same time the number of twins increases, the width decreases from 1 to 0.65 μm.

- All three torsion curves exhibited a rapid initial increase followed by stabilization (close to the horizontal region). This indicates an initial elastic phase followed by a plastic phase or more stabilized behavior.

- As the torsion rate increases, the grains undergo plastic deformation and the twins become smaller, affecting the integrity of the material;

- The effects of mechanical twinning on the material hardening are complex and can be diametrically opposed, affecting the hardness and elasticity values of the specimens.

7. "for medical implants because the obtained values are very close to those of bone". Please remove this statement since Young's Modulus was measured wrongly.

Response: The Young's modulus was determined by the indentation method, reference 9 for materials subjected to combined loading. There are many references that address 316L SS as an implant material for biological environments.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The article has been revised by the authors. All my questions have been answered.

Reviewer 2 Report

Comments and Suggestions for Authors

The revised version can be published in my opinion.