Dynamic Coercivity of Tempered Ferritic Steel Subjected to Creep-Fatigue for Nondestructive Evaluation by Reversible Permeability
Round 1
Reviewer 1 Report
1. Results and discussion section need relevant literature review to support results.
2. Why the author only tests two time cycles? How they come up with this time frame?
3. Fig. 2 is really hard to understand and identify what’s happening? The Author needs to clarify with arrow what’s going on.
4. Fig.2 notation (e.g. d) is not appropriate. Should use d1 & d2 or d & e. Some miller indices are not visible.
5. it is hard to the scale bar at fig. 3.
6. Line 120, Fig. 1 notation a, b misplaced.
7. Which ASTM method used for testing the specimen.
Author Response
- Results and discussion section need relevant literature review to support results.
As the reviewer’s comments, the results and discussion section were revised and added using some references. The reference about the correlations of magnetic coercivity with Vickers hardness was added and supported in this study. In the text, the reference [20] was added as follows; Byeon et al. also reported that the good correlations of magnetic coercivity with Vicker's hardness. They studied magnetic coercivity of 2.25Cr steel subjected to long-term thermal aging and showed the precipitate dependence on the magnetic coercivity. The reference [21] was also added. For the dislocation recovery and lath width growth, in the text, the reference [22] was added as follows; Abe reported that the martensite lath increased with recombination and migration of laths during high-temperature creep of 9Cr steel.
- Why the author only tests two time cycles? How they come up with this time frame?
The creep-fatigue test was achieved with two hold time, 60s and 600s to investigate the effects of hold time on the creep-fatigue damage of tempered steel. This is for the creep effect under cyclic fatigue at high temperature. Of course, the creep effect of 600s on the cyclic fatigue was more dominant and clear in relation to microstructural variation such as dislocation density and precipitates. The typical example is the result of Figure 1(a) showing variation in dynamic coercivity with fatigue cycles. It clearly shows the hold time dependence between 60s and 600s.
- 2 is really hard to understand and identify what’s happening? The Author needs to clarify with arrow what’s going on.
Figure 2 shows the transmission electron micrographs at each fatigue cycles of 600s hold time and selected area diffraction pattern of typical precipitates. The martensitic lath structures are the general characteristics of tempered martensite steel. This martensite steel consists of dislocation substructures and fine precipitates along the lath boundaries. The lath boundary and precipitate were clarified with arrows in the TEM micrographs. The as-tempered specimen exhibited a high dislocation density in the lath interior and fine precipitates on the martensite lath boundaries. This is also clarified with arrows.
The tempered specimen showed elongated martensite lath structures, as shown in Fig. 2(a). The dislocations within the martensite laths were tangled with a high density. The dislocation density decreased with increasing creep-fatigue for both stress hold times, as shown in Fig. 2(b). Figure 2(c) shows the microstructures just after fatigue failure of the hold time 600s. The martensite lath continuously increased with creep-fatigue, and the dislocation density within the lath interior was lower than in Fig. 2(a).
- 2 notation (e.g. d) is not appropriate. Should use d1 & d2 or d & e. Some miller indices are not visible.
Figure 2 was revised and expressed with some arrows to explain more detail for microstructural changes. Also, the selected area diffraction (SAD) pattern of Fig. 2(d1) and Fig. 2(d2) was revised. The notation of miller indices also revised to make clear. The notation of SAD patterns was expressed with Fig. 2(d1) and Fig. 2(d2).
- it is hard to the scale bar in fig. 3.
As the reviewer’s comments, the scale bar was revised to make clear.
- Line 120, Fig. 1 notation a, b misplaced.
As the reviewer’s comments, the notation of Fig. (a) and Fig. (b) was revised and positioned.
- Which ASTM method used for testing the specimen.
The hardness test was applied and followed the ASTM E92-17. The micro Vicker’s hardness test was carried with the same samples of optical micrograph observation after vibration polishing with 0.01mm Al2O3 suspension.
Reviewer 2 Report
Table 1: explain better the meaning of Nt.
The details of experiment should be described more detailed. The most important evaluated parameter is the dynamic coercivity, but is mentioned only that “The more detailed experimental system was previously reported [19].” For the proper understanding of the present paper, author is requested to describe what parameter and how were measured.
Also the description of experimental arrangement is missing. A scetch of the experiment is suggested to give. Shape of sample, how magnetizing yoke is attached, etc. Why a ferrite yoke was applied instead of iron yoke? Usually it is believed in similar types of measurement, that the material of sample and yoke should be close to each other.
Author mentions that 12 kA/m magnetic field wad applied. Does he know the exact value of magnetic field? Is it calculated or measured? In similar experiments the value of magnetization current (in Amps) is usually known.
Figs. 1, 4 and 5: two values of holding time were applied in experiments (60s and 600s). It is clearly seen in Fig. 1a that the dynamic coercivity depends very much on this value. However, in Figs. 1b, 4 and 5 where correlation between dynamic coercivity and Vickers hardness / precipitate number / dislocation densitiy are given, there is no difference between 60s and 600s if the linear correlation is considered, It would be nice if the author interpreted this phenomenon.
Author Response
- Table 1: explain better the meaning of Nt.
As the reviewer’s comments, the fatigue life fraction was explained in the text. The Nf means the fracture cycle of creep-fatigue test.
- The details of experiment should be described more detailed. The most important evaluated parameter is the dynamic coercivity, but is mentioned only that “The more detailed experimental system was previously reported [19].” For the proper understanding of the present paper, author is requested to describe what parameter and how were measured.
A ferrite yoke type probe was applied to eliminate the demagnetization effect for an open magnetic circuit and wound by pick-up coils, an AC perturbing coils, and a DC magnetizing coils. The test specimen was prepared in a dimension of 15 mm ´ 7 mm ´ 1 mm by the wire cutting process. The slow varying mantic field of 12 kA/m at 0.05 Hz was measured by the shunt resistor of 1 ohm. Also, a perturbation field was applied with 80 A/m at 40 Hz. The induced voltage in the pick-up coil was obtained by a lock-in amplifier (EG&G PAR 5210). The more detailed experimental system was previously reported [19]. The reversible permeability (RP) profile can be successfully obtained during a slow varying cycle with current using an I/O board.
- Also the description of experimental arrangement is missing. A scetch of the experiment is suggested to give. Shape of sample, how magnetizing yoke is attached, etc. Why a ferrite yoke was applied instead of iron yoke? Usually it is believed in similar types of measurement, that the material of sample and yoke should be close to each other.
The schematic diagram is introduced in the reference. In this study, the test specimen was prepared in a dimension of 15 mm ´ 7 mm ´ 1 mm by the wire cutting process. A ferrite yoke type probe was applied to eliminate the demagnetization effect for an open magnetic circuit. This ferrite has superior magnetic properties than the electric sheet metal and iron. Also, this ferrite has more efficient for magnetization and demagnetization.
- Author mentions that 12 kA/m magnetic field wad applied. Does he know the exact value of magnetic field? Is it calculated or measured? In similar experiments the value of magnetization current (in Amps) is usually known.
As the reviewer’s comments, the unit of a magnetic field is Amps. In this study, the slow varying mantic field was measured by a current using the voltage across a shunt resistor of 1 ohm.
- 1, 4 and 5: two values of holding time were applied in experiments (60s and 600s). It is clearly seen in Fig. 1a that the dynamic coercivity depends very much on this value. However, in Figs. 1b, 4 and 5 where correlation between dynamic coercivity and Vickers hardness / precipitate number / dislocation densitiy are given, there is no difference between 60s and 600s if the linear correlation is considered, It would be nice if the author interpreted this phenomenon.
In this study, the dynamic coercivity depends very much on the microstructural variation of dislocation and precipitate. Most of all, it has a good relationship with dislocation density and precipitates number. But, it has a little discrepancy with theoretical models. In this study, the dynamic coercivity is depending on the Cr23C6 precipitate concentration with more than. This discrepancy may cause by several microstructural characteristics of materials but, in this study, there are difficulties in measuring the nano-size precipitates with an image analyzer. Therefore, it is not considered the dependence of nano-size precipitates and less than several tens of nanometer particles on the dynamic coercivity. In this study, the dynamic coercivity is depending on the dislocation density with more than. This discrepancy may cause by the complexity of material microstructures such as precipitates, grain boundaries, lath boundaries, etc.
Round 2
Reviewer 1 Report
Great work
Author Response
As the reviewer's comments, English is edited.
Reviewer 2 Report
I repeat my question No. 4, because author's response is not acceptable. The unit of magnetic field is A/m. The unit of current is Amper (A, Amps).
Author writes that "the slow varying magnetic field was measured by a current using the voltage across a shunt resistor of 1 ohm". Magnetic field can be calculated from the current only in case of a closed magnetic circuit. Using an open magnetic curcuit, as done in the presented experiments the magnetic field inside the sample is evidently different from the value, which is calculated from the current. This fact has no influence on the results, discussed in the paper, but it is not correct to say that "the unit of a magnetic field is Amps".
I think that the easiest way to solve the problem is to use expression "magnetizing current (in Amper)" instead if magnetizing field. Proper value of magnetizing current can be really easily determined from the voltage across a shunt resistor of 1 ohm.
Author Response
As the reviewer’s comments, the unit of a magnetic field is A/m. The unit of current is Amper (A, Amps). There are some mistakes for the 1st author’s response about the unit of a current and magnetic circuit.
The detail response to the comments is uploaded with a file.
Author Response File: 
Author Response.docx
