Effect of Artificial Defects on the Very High Cycle Fatigue Behavior of 316L Stainless Steel
Round 1
Reviewer 1 Report
Your paper describes very high cycle fatigue properties of stainless steels with an indentation. The results are original and it is of good candidate for publication.
1) About specimen preparation, please clarifiy the timing of indentation. The reviewer thinks that the authors made an indentation pit after surface finishing.
2) Especially for SA, indentation might give severe cold working at the edge of the indentation pit, resulting in enhancement of fatigue strength. This is not mandatory, but it is recommended that nano-indentation testing around the indentation pit might give useful information on cold working due to indentation and deeper insight for discussion.
Author Response
Response to Reviewer 1 Comments
Point 1: About specimen preparation, please clarifiy the timing of indentation. The reviewer thinks that the authors made an indentation pit after surface finishing.
Response 1: Thank you very much for the comments. You are right, the indentations were made after surface finishing. The holding time of the indentation is 15 second. Therefore, the last sentence in Subsection 2.1 was revised to “The indent depths on the SA 316L specimens were 8 μm, 12 μm, and 40 μm (the corresponding loads were 4.9 N, 9.8 N, and 49 N, the hold time is 15 s, as well as that of the following indentation), while for the 20% CW 316L, they were 45 μm and 80 μm (the corresponding loads were 294 N and 490 N).”
Point 2: Especially for SA, indentation might give severe cold working at the edge of the indentation pit, resulting in enhancement of fatigue strength. This is not mandatory, but it is recommended that nano-indentation testing around the indentation pit might give useful information on cold working due to indentation and deeper insight for discussion.
Response 2: Thank you very much for the comments. We quite agree with your comments, i.e. the nano-indentation testing around the indentation pit might give useful information (such as microhardness, load/depth curve and so on) on cold working due to indentation and deeper insight for discussion. I think it is an interesting topic, In the future, I will do some investigation on it.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper is clear and topic is interesting. Discussion of results may be elaborated more in depth, especially about the fatigue crack mechanism. In particular, some images of internal crack surface could help in interpretation of results. More, scatter in fatigue data is not well explained.
Author Response
Response to Reviewer 2 Comments
Point 1: The paper is clear and topic is interesting. Discussion of results may be elaborated more in depth, especially about the fatigue crack mechanism. In particular, some images of internal crack surface could help in interpretation of results. More, scatter in fatigue data is not well explained.
Response 1: Thank you very much for the comments. We agree with your comment. However, the internal crack surface of the specimen without indent and the fatigue failure mechanism has been presented in the previous papers [7,8]. And as said in Subsection 3.2, the fatigue crack behaviour of the specimen with indent is the same as the specimen without indent, therefore, fatigue crack mechanism and the internal crack surface was not presented in this paper. And the explanation of the scatter was revised to “That is, the fatigue life is not affected by the indent with the applied range of depth in this research, the difference in the fatigue life at the same stress amplitude, shown in Figure 5, was not caused by the indent, but by the deformation induced martensite transition. It was reported that the deformation induced martensite transition occurred localized for low stress applied, which allows the formation of microcracks and fatigue failure in the VHCF regime, followed by a pronounced scatter of the fatigue life data [20].”
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors present an interesting study on the influence of artificial defects, i.e. Vickers indents, on the VHCF behavior of austenitic stainless steel in solution annealed (SA) and 20% cold rolled (SA) condition. While the contents of the study in principle fit into the scope of Metals, the following major issues need to be addressed before the manuscript could be considered for publication:
(i) Introduction: It needs to be mentioned that most variants of austenitic stainless steel (including 316L) can, depending on the actual chemical composition and grain morphology, form deformation induced martensite during both, cold deformation, and fatigue loading, see e.g. https://doi.org/10.1179/174328407X179575, https://doi.org/10.1016/j.msea.2013.08.072, https://doi.org/10.1016/j.engfracmech.2017.04.041. The introducing literature review should also reflect the consequences of deformation induced martensite formation on crack initiation and -propagation in the VHCF regime, as well as the precautions to be taken in this context for avoidance of sample overheating and inconstant amplitudes during ultrasonic VHCF testing, see for example https://doi.org/10.1016/j.ijfatigue.2016.05.005 and https://doi.org/10.1051/matecconf/201816504010.
(ii) Materials and Experimental Procedure: (1) A characteristic value of austenite stability (e.g. Md30 temperature) should be derived from the chemical composition (including Nitrogen content which strongly affects austenite stability) of the investigated 316 L batch. The occurrence of VHCF failures beyond 10E7 cycles indicates high austenite stability, which however should be quantitatively specified. (2) In principle, the specimen design according to [7] seems acceptable. However, validation of the correlation between displacement amplitude used as control value in the ultrasonic testing machine and local strain amplitude in the specimen center needs to be validated by strain gauge measurements and/or laser vibrometry. Since the lifetime behavior is plotted in terms of stress amplitude vs. cycles to failure, precise specification of Young's modulus would be essential. (iii) Since the investigated material is rather "soft", forced air cooling combined with pulse-pause loading instead of continuous cycling at 20 kHz seems to be essential for keeping specimen temperature within acceptable limits, see e.g. https://doi.org/10.1016/j.ijfatigue.2016.05.005 and https://doi.org/10.1051/matecconf/201816504010 as well as ref. [7] in the present manuscript. - The countermeasures taken against overheating as well as the limits of specimen temperature need to be specified in detail. Citation [18] is not sufficient in this context because this classic work refers to more general aspects of ultrasonic fatigue testing but not to the specific case considered in the present work.
(iii) Results: (1) "and discussion" should be deleted in the section title. (2) Legibility of temperature scaling in Fig. 7 and 8 needs to be improved! Why is the lowest temperature observed at the specimen center in Fig 7a) / 8a)? Because the material in SA condition is, due to its lower hardness, more prone to self heating, temperature measurement results for this material need to be added. (3) As obvious from the thermographs in Fig 7b) and 8b) as well as from the severe thermally induced oxidation at the crack initiation sites in Fig. 6, crack growth resulted in extreme local overheating. This local change in specimen temperature excludes any evaluation of crack propagation lifetime as used in the subsequent discussion.
(iv) Discussion: As stated above, the crack propagation cycle number may be strongly influenced by significant temperature increase in the crack region. This makes Fig. 9 and the respective discussion extremely questionable. (2) On p.9, the authors correctly state that fatigue life of the material in SA condition stays unaffected by indents below the critical dimensions estimated by Murakami's approach. For the CW material, they also state same fatigue strength in spite of indent sizes above the critical dimension. Considering Fig. 2b), this statement is incorrect at least for an indent depth of 80 microns. In this context, it's unclear why this biggest indent depth is only considered for the CW material but not for the SA variant. (3) The fact that indentation induced work hardening hinders crack initiation and therewith shifts the critical indent size above the values estimated by Murakami's approach is correctly stated in the discussion. However, also local deformation induced martensite formation as well as residual stresses may increase fatigue strength at the indent positions. Both aspects are not addressed in the discussion. In this context, SEM/EBSD analyzes at cross sections through the indents would be essential to clarify whether martensite was formed.
Author Response
Response to Reviewer 3 Comments
The authors present an interesting study on the influence of artificial defects, i.e. Vickers indents, on the VHCF behavior of austenitic stainless steel in solution annealed (SA) and 20% cold rolled (SA) condition. While the contents of the study in principle fit into the scope of Metals, the following major issues need to be addressed before the manuscript could be considered for publication:
Point 1: Introduction: It needs to be mentioned that most variants of austenitic stainless steel (including 316L) can, depending on the actual chemical composition and grain morphology, form deformation induced martensite during both, cold deformation, and fatigue loading, see e.g. https://doi.org/10.1179/174328407X179575, https://doi.org/10.1016/j.msea.2013.08.072, https://doi.org/10.1016/j.engfracmech.2017.04.041. The introducing literature review should also reflect the consequences of deformation induced martensite formation on crack initiation and -propagation in the VHCF regime, as well as the precautions to be taken in this context for avoidance of sample overheating and inconstant amplitudes during ultrasonic VHCF testing, see for example,https://doi.org/10.1016/j.ijfatigue.2016.05.005 and https://doi.org/10.1051/matecconf/201816504010.
Response 1: Thank you very much for your useful comments. These literatures are added in the introduction, the revised parts are marked by red colour as shown in Section 1 in the main text.
Point 2: Materials and Experimental Procedure: (1) A characteristic value of austenite stability (e.g. Md30 temperature) should be derived from the chemical composition (including Nitrogen content which strongly affects austenite stability) of the investigated 316 L batch. The occurrence of VHCF failures beyond 10E7 cycles indicates high austenite stability, which however should be quantitatively specified. (2) In principle, the specimen design according to [7] seems acceptable. However, validation of the correlation between displacement amplitude used as control value in the ultrasonic testing machine and local strain amplitude in the specimen center needs to be validated by strain gauge measurements and/or laser vibrometry. Since the lifetime behavior is plotted in terms of stress amplitude vs. cycles to failure, precise specification of Young's modulus would be essential. (3) Since the investigated material is rather "soft", forced air cooling combined with pulse-pause loading instead of continuous cycling at 20 kHz seems to be essential for keeping specimen temperature within acceptable limits, see e.g. https://doi.org/10.1016/j.ijfatigue.2016.05.005 and https://doi.org/10.1051/matecconf/201816504010 as well as ref. [7] in the present manuscript. -The countermeasures taken against overheating as well as the limits of specimen temperature need to be specified in detail. Citation [18] is not sufficient in this context because this classic work refers to more general aspects of ultrasonic fatigue testing but not to the specific case considered in the present work.
Response 2: Thank you very much for the comments. (1) As reviewer pointed out, concentration of nitrogen is dependent on the material properties. However, in this study, we selecetd standard JIS 316L sitanless steel. In the Japan Industrial Standards, nitrogen concentration is not defined.
(2) As reported in previous papers [T. Naoe et al, J. Nucl. Mater.2017(468) 331-338 and 2018 (506) 12-18], the displacement of the specimen was calibrated in each material using an eddy current gage (Applied Electronics, PU-05). And some typical results have been presented. Furthermore, Although the validation of the correlation between displacement amplitude used as control value in the ultrasonic testing machine and local strain amplitude in the specimen center is not able to measure directly, the relationship between the displacement of the specimen free end and the stress amplitude at the center part has been calculated by numerical simulation (presented in author’s doctor thesis, Z. Xiong, 2016, Ibaraki university, Hitachi, Japan) and the results was coincidence with the theoretical value. However, for precise research, Young's modulus of 316L will be calibrated.
(3) According to the comments, the countermeasures taken against overheating as well as the limits of specimen temperature are specified in detail. The revised parts are tracked with red colour in the last paragraph in Subsection 2.2. Furthermore, the ultrasonic fatigue testing related to present work have already been reported in our previous paper, therefore, the papers are added as citations.
Point 3: Results: (1) "and discussion" should be deleted in the section title. (2) Legibility of temperature scaling in Fig. 7 and 8 needs to be improved! Why is the lowest temperature observed at the specimen center in Fig 7a) / 8a)? Because the material in SA condition is, due to its lower hardness, more prone to self-heating, temperature measurement results for this material need to be added. (3) As obvious from the thermographs in Fig 7b) and 8b) as well as from the severe thermally induced oxidation at the crack initiation sites in Fig. 6, crack growth resulted in extreme local overheating. This local change in specimen temperature excludes any evaluation of crack propagation lifetime as used in the subsequent discussion.
Response 3: Thank you very much for your useful comments.
(1) "and discussion" has been deleted in the section title.
(2) Figs. 7 and 8 have been improved. As shown in Fig 7a) / 8a), the state of the specimen is before fatigue test, in this time, the specimen center is cooled by cooling air, so that the temperature is the lowest. The temperature of specimen center will be the highest during the fatigue testing caused by self-heating. The detailed temperature results of SA316L and 20% CW 316L have been presented in previous paper [7,9].
(3) We agree with your opinion, i.e. crack growth resulted in extreme local overheating. As explained in [7,9,23], the local change in specimen temperature was suddenly changed due to crack propagation. Therefore, the break point of specimen temperature can be utilized to distinguish crack initiation and propagation. In other words, the fatigue life before the break point is contributed to crack initiation and the last part is contributed to crack propagation. Such method was also reported by other researchers [D. Krewerth et al. Ultrasonics 53 (2013) 1441–1449, Z. Huang et al. Acta Materialia 58(2010) 6046-6054].
Point 4: Discussion: (1) As stated above, the crack propagation cycle number may be strongly influenced by significant temperature increase in the crack region. This makes Fig. 9 and the respective discussion extremely questionable. (2) On p.9, the authors correctly state that fatigue life of the material in SA condition stays unaffected by indents below the critical dimensions estimated by Murakami's approach. For the CW material, they also state same fatigue strength in spite of indent sizes above the critical dimension. Considering Fig. 2b), this statement is incorrect at least for an indent depth of 80 microns. In this context, it's unclear why this biggest indent depth is only considered for the CW material but not for the SA variant. (3) The fact that indentation induced work hardening hinders crack initiation and therewith shifts the critical indent size above the values estimated by Murakami's approach is correctly stated in the discussion. However, also local deformation induced martensite formation as well as residual stresses may increase fatigue strength at the indent positions. Both aspects are not addressed in the discussion. In this context, SEM/EBSD analyzes at cross sections through the indents would be essential to clarify whether martensite was formed.
Response 4: (1) We agree with your opinion, i.e. the crack propagation cycle number may be strongly influenced by significant temperature increase in the crack region, the crack propagation rate will be accelerated due to the high temperature in the crack region. However, as shown in Fig.1(in the attached file), it was reported that the fatigue crack growth rate at 600 ℃ is about 10 times of that at room temperature. As shown in Fig.9 in the main text, the proportion of the crack propagation life in the very high cycle regime is still less than 1% even if it was amplified ten times. On the other hand, the maximum temperature is lower than 600℃during crack propagation(see in references [2, 9]. Therefore, the crack propagation life can be ignored comparing to the total fatigue life. The corresponding modification is shown as the red part at the last paragraph in Subsection 4.2.
(2) For the CW material, as discussed in last paragraph of the Subsection 4.2 in the main text, because the plastic deformation around the indent, as well as the residual stress and probable local deformation induced martensite formation (advised by the reviewer), the fatigue crack initiation might be suppressed. Therefore, the fatigue strength was not weakened due to the indent, even its size is 80 microns, which is larger than the value obtained by Murakami’s approach. For the reason of that why this biggest indent depth is only considered for the CW material but not for the SA variant. As said in the introduction section, the research background is related to nuclear plant, and the main purpose is to investigate the effect of the surface defect cause by service conditions. Due to neutron irradiation, 316L will be hardened. Therefore, for the initial state of the service life, the defect depth cause by the service conditions should be smaller. As the service life going on, the material will be hardened and the defect depth will be deeper. Therefore, the indent with the depth of 80 microns is only used for 20% CW 316L. The corresponding modification is shown as the red part at the end of the last paragraph in Subsection 2.2.
(3) We agree with your comments. The effect of residual stress as well as the local deformation induced martensite formation on the fatigue strength at the indent positions should be taken into account. In the future, the SEM/EBSD analyzes at cross sections through the indents will be essential to clarify whether martensite was formed.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The paper can be published in the present form.
Author Response
Thank you very much for your comments. We have rvised some English language and style as shown in the main text tracked by red colour.
Reviewer 3 Report
The authors did serious improvements according to the review comments making the paper now suitable for publication after minor amendments. For details, see the attached pdf document.
Beyond that, the following comment should be considered:
(Fig. 7a) / 8a): It's still unclear why the specimen temperature in the middle of the gauge length (which undergoes highest mechanical loading amplitude) is lower than in the outer sample regions. Moreover, temperature scaling is disturbing: Considering the indicated minimum value of 5°C, i suppose that the distribution of temperature CHANGES with respect to ambient temperature is plotted rather than absolute temperature - please clarify that point.
Comments for author File: Comments.pdf
Author Response
Response 1: Thank you very much for your useful comments. We are sorry that We didn’t explain this question clearly in last reply. Figs.7(a) and 8(a) shows the specimens before fatigue test, i.e. the specimens were not loaded, which means they didn’t undergo mechanical loading amplitude at that time. Meanwhile, the center part of the specimen was cooled due to the cooling air sprayed out from the nozzles of an air cooler. Thus, the specimen temperature in the middle of the gauge length is lower than that of other parts.
Furthermore, according to the comments, Figures 7 and 8 are revised as shown in the main text tracked by red colour. However, the minimum specimen temperature at the initial state is lower than the ambient temperature due to cooled air cooling, thus, we think that the distribution of temperature changes with respect to specimen temperature at initial state is plotted rather than ambient temperature.
Finally, thank you very much for your carefully checking on the main text tracked by red colour. We have revised the English writing according to your comments.
Author Response File: Author Response.pdf
