Review Reports

Round 1

Reviewer 1 Report

The manuscript entitled: The Affect of Aging on Precipitates, Mechanical and Magnetic Properties of Fe-21Cr-15Ni-6Mn-Nb Low Magnetic Stainless Steel deals with the low magnetic stainless steel and tests their mechanical and magnetic properties.

Comments for improvement of the manuscript:

• The correlation between the thermal treatment (precipitates) and the magnetic properties is still not convincing
• Ingot with the dimension of 100×50×L - what is L?
• Figure 2(a) - Scale bar is barely visible
• Figure 2(b) - X and Y axis are barely visible
• Figure 3(b) should in indexed properly. Scale bar is also missing
• What is MX - which is indexed in the XRD pattern in Fig. 4. It has to be mentioned directly in the XRD pattern in the caption
• Error bars for data points in Figures 10, 11 should be included
• Legends in Fig. 9(b,d) are barely readable
• Typos in the manuscript need to be rectified. For instance, space should be introduced between numbers and units. For ex. 100nm should be written as 100 nm.

Author Response

Reviewer #1

Comment (1): The correlation between the thermal treatment (precipitates) and the magnetic properties is still not convincing

Response: This suggestion is appreciated. We have made a supplement in the paper as follows.

Chapter 3.5 When aging treatment is carried out under different conditions, the number and size of precipitates in the experimental steel are different. However, through the magnetic properties test, it is found that the hysteresis loop of the experimental steel after various aging treatments is still a straight line, showing paramagnetic properties. Therefore, the precipitates produced in the aging process of the experimental steel will not affect the paramagnetic properties of the experimental steel. Aging treatment will not destroy the paramagnetic stability of the experimental steel.

Comment (2): Ingot with the dimension of 100×50×L - what is L?

Response: We are very sorry for our incorrect writing. We have made correction according to the reviewer’s comments as follows.

Chapter 2.1 (2) the ingot is forged into a billet of 100×100×50 mm.

Comment (3): Figure 2(a) - Scale bar is barely visible

Response: We have redrawn the scale bar in Figure 2(a).

Comment (4): Figure 2(b) - X and Y axis are barely visible

Response: This suggestion is appreciated. We have redrawn the X and Y axis in Figure 2(b).

Comment (5): Figure 3(b) should in indexed properly. Scale bar is also missing

Response: This suggestion is appreciated. We have redrawn Figure 3.

Figure 3. TEM morphology of the steel after solution treatment.

(a) bright field image of retained particles, (b) diffraction pattern calibration of precipitates.

Comment (6): What is MX - which is indexed in the XRD pattern in Fig. 4. It has to be mentioned directly in the XRD pattern in the caption

Response: This suggestion is appreciated. MX are (Nb, V)(C, N) particles. We have redrawn Figure 4(a).

Comment (7): Error bars for data points in Figures 10, 11 should be included

Response: This suggestion is appreciated. We have redrawn Figure 10, 11.

Figure 10. Calculation of coarsening rate of precipitates (a) at different aging temperatures with 2 h, (b) comparsion of the calculated and measured precipitates inside the grains at 650 °C with different aging times.

Figure 11. Influence of aging parameters on YS, UTS and elongation for the experimental steel.

(a) aging at different temperatures with 2h, (b) aging at 650 °C with different time.

Comment (8): Legends in Fig. 9(b,d) are barely readable

Response: This suggestion is appreciated. We have redrawn Figure 9(b,d).

Figure 9. TEM and EDS analysis of elements in the steel after aging treatment.

(a) (b) aging at 650 °C for 10 h, (c) (d) aging at 750 °C for 2 h.

Comment (9): Typos in the manuscript need to be rectified. For instance, space should be introduced between numbers and units. For ex. 100nm should be written as 100 nm.

Response: We are very sorry for our incorrect writing. The phrase "100nm " in the introduction has been corrected as "100 nm". And the similar problems in the paper have been modified.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

It is not quite correct to consider that sigma phase in stainless steels generally converts from M23C6 during aging treatment. In the initial state of austenitic stainless steel which contains 0.18% carbon will be present a ferrite a-phase. As a result at the temperature of 450 -550 C stratification of the high-chromium a-solid solution will occur and a brittle s-phase is formed. Therefore, aging is best carried out in the temperature range 600-650 C, then the microstructure of the steel will be precisely balanced.

 

The second remark is related to the chemical affinity to carbon of alloying elements of steel. It is well known that the affinity of niobium and vanadium to carbon is higher than that of chromium and iron. But the authors do not take into account the rather high amount of nitrogen (0.368 %) in the austenitic stainless steels. That is, nitrogen is partially consumed for the stabilization of austenite and for the formation, first of all, of nitride phases with niobium and vanadium. Its affinity for these elements is higher than that of carbon.

Author Response

Reviewer #2:

Comment (1): It is not quite correct to consider that sigma phase in stainless steels generally converts from M₂₃C₆ during aging treatment. In the initial state of austenitic stainless steel which contains 0.18% carbon will be present a ferrite a-phase. As a result at the temperature of 450 -550 °C stratification of the high-chromium a-solid solution will occur and a brittle s-phase is formed. Therefore, aging is best carried out in the temperature range 600-650 °C, then the microstructure of the steel will be precisely balanced.

Response: Considering the Reviewer’s suggestion, we have rewritten the sentence and made a supplement in the paper as following:

Chapter 4.1 Formation of an intermetallic phase known as σ-phase is a severe problem when using standard austenitic stainless steels at elevated temperatures [32,33]. In the initial state of austenitic stainless steel which contains 0.18% carbon will be present a ferrite α-phase. As a result at the temperature of 450 -550 °C stratification of the high-chromium α-solid solution will occur and a brittle σ-phase is formed. Therefore, aging is best carried out in the temperature range 600-650 °C, then the microstructure of the steel will be precisely balanced.

Comment (2): The second remark is related to the chemical affinity to carbon of alloying elements of steel. It is well known that the affinity of niobium and vanadium to carbon is higher than that of chromium and iron. But the authors do not take into account the rather high amount of nitrogen (0.368 %) in the austenitic stainless steels. That is, nitrogen is partially consumed for the stabilization of austenite and for the formation, first of all, of nitride phases with niobium and vanadium. Its affinity for these elements is higher than that of carbon.

Response: This suggestion is appreciated. We have rewritten the sentence and made a supplement in the paper as following:

Chapter 4.1. In this paper, due to the high amount of N (0.368 %) and the higher affinity for Nb, V than that of carbon, the nitrogen in experimental steel is consumed for the stabilization of austenite partially and for the formation of nitride phases with niobium and vanadium firstly. At the same time, the affinity of Nb and V for C is stronger than that of Cr and Mo, which is not conducive to the formation of intergranular precipitation phase M₂₃C₆. By reducing the Cr-depleted region and the diffusion rate of Cr, the probability of σ phase formation decrease.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

In presented manuscript the authors the effect of aging on the precipitates, mechanical and magnetic properties of Fe-21Cr-15Ni-6Mn-Nb low magnetic stainless steel were investigated. The authors noticed that the during the aging treatment, the (Nb, V)(C, N) particles gradually precipitated in the grain, which were coherent or semi-coherent with the matrix. Additionaly when the aging temperature was beyond 650 °C, the coarsening rate of (Nb, V)(C, N) particles increase rapidly and the coherent orientation between (Nb, V)(C, N) particles and the matrix was lost gradually. They analysed the coarsening behavior of (Nb, V)(C, N) precipitates in the grain was analyzed, and the size of the particles precipitated after aging treatment at 650°C during different time were studied.

The presented manuscript of the authors are interesting. The some issue are not clearly described in manuscript. Therefore before publishing should be consider the following comments:

1. In introduction the authors wrote that the their studies have laid the foundation for the research and development of high strength low magnetic stainless steel. Please provide examples of industrial use. For what purposes will such steels with such properties be used?
2. Figure 7 present the stress-strain curve of the experimental steel after different aging treatment. Are there any changes observed in the curve before breaking? The compilation of all curves on one graph does not allow you to look closely at the curve separately. Please consider the information about this in the text of the manuscript. 
3. Figure 12 is clipped.Please correct it.
4. There are some editorial errors in the manuscript.Please read the manuscript carefully and correct it. For example: page 2 line 7.

Author Response

Reviewer #3:

Comment (1):In introduction the authors wrote that the their studies have laid the foundation for the research and development of high strength low magnetic stainless steel. Please provide examples of industrial use. For what purposes will such steels with such properties be used?

Response: This suggestion is appreciated. The high strength low magnetic stainless steel is mainly used for the shell of naval submarines, but it has not yet reached the stage of industrial production. Considering the Reviewer’s suggestion, we have rewritten the sentence as following:

Chapter 1 These studies can promote the research and application of high strength low magnetic stainless steel.

Comment (2): Figure 7 present the stress-strain curve of the experimental steel after different aging treatment. Are there any changes observed in the curve before breaking? The compilation of all curves on one graph does not allow you to look closely at the curve separately. Please consider the information about this in the text of the manuscript.

Response: This suggestion is appreciated. We want to compare the yield strength, tensile strength and elongation of different aging treatments intuitively through the stress-strain curve in Figure 7. In order to extract the information in the graph more clearly, we made a table as following:

Table 2 Tensile properties of the experimental steel after different aging treatment

Aging treatment	Yield strength /MPa	Tensile strength /MPa	Elongation /%
550 °C−2 h	660	901	48
600 °C−2 h	680	973	43
650 °C−2 h	705	1002	38
700 °C−2 h	693	963	37
750 °C−2 h	676	931	32
650 °C−0.5 h	661	898	48
650 °C−1 h	693	971	42
650 °C−4 h	685	979	38
650 °C−10 h	672	965	34

Comment (3): Figure 12 is clipped. Please correct it.

Response: This suggestion is appreciated. We have adjusted the position of Figure 12.

Comment (3): There are some editorial errors in the manuscript. Please read the manuscript carefully and correct it. For example: page 2 line 7.

Response: This suggestion is appreciated. Considering the Reviewer’s suggestion, we have rewritten the sentence as following. And the similar problems in the paper have been modified.

Chapter 1. The main reason for the decrease at high temperature strength of 310S austenitic stainless steel is the coarsening of Laves phase during long time aging treatment.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

The submitted paper discusses about effect of aging on the precipitates, mechanical and magnetic properties of Fe-21Cr-15Ni-6Mn-Nb low magnetic stainless steel. The authors concluded that the carbonitride particles were gradually precipitated in the grain. The size of precipitates was increased by the increase of aging temperature. In my opinion, the paper has been well organized and can be accepted for publication by Metals. However, the following comments are recommended before its publication:

1) There are some minor grammar and spelling errors in the paper. For example, in the title of paper “affect” should be replaced by “effect”. Please take a careful look at the paper and edit the English of paper.

2) The innovation of the paper should be obviously explained in the introduction section.

3) Please explain how the Image Pro Plus image processing software calculate the size of precipitates.

4) Based on Fig. 7, the mechanical properties of steel are increased with the aging treatment for different times. Is there any optimum condition, based on temperature and time, to get the highest mechanical properties?

5) Please increase the resolution of EDS analysis images as shown in Fig. 9b and 9d.

6) According the Fig. 12b, after aging at 650 °C for 2 h, “a large number of particles at the bottom of the dimple, which indicates that dislocations pile up around the second phase particles during aging”. How did the authors understand that the particles are sign of dislocations pile up?

7) The authors should explain why the ductile fracture transforms to the brittle one with the increase of aging temperature and its time.

8) An important question is that what is the main mechanism for the improvement of mechanical properties with aging treatment by precipitates formation?

 

Author Response

Reviewer #4:

Comment (1): There are some minor grammar and spelling errors in the paper. For example, in the title of paper “affect” should be replaced by “effect”. Please take a careful look at the paper and edit the English of paper.

Response: This suggestion is appreciated. The “affect” in the title of paper have been replaced by “effect”. And the similar problems in the paper have been modified.

Comment (2): The innovation of the paper should be obviously explained in the introduction section.

Response: Considering the Reviewer’s suggestion, we have made a supplement in the introduction as following:

Chapter 1 The precipitates evolution through aging treatment for low magnetic stainless steel was investigated. The coherent or semi-coherent relationship of (Nb, V) (C, N) precipitates with the matrix in the aging treatment was researched. The coarsening behavior of intragranular (Nb, V) (C, N) particles during the aging process was analyzed by LSW theory. The relative magnetic permeability of experimental steel with different aging process were studied.

Comment (3): Please explain how the Image Pro Plus image processing software calculate the size of precipitates.

Response: Firstly, the precipitates were made black by adjusting the contrast of the image, and then "select color" was checked on the count/size interface. The total area and number of precipitates can be obtained by selecting black. The area and diameter of average precipitates can be obtained by equal area method.

Comment (4): Based on Fig. 7, the mechanical properties of steel are increased with the aging treatment for different times. Is there any optimum condition, based on temperature and time, to get the highest mechanical properties?

Response: Considering the Reviewer’s suggestion, we have made a supplement in the paper as following:

Chapter 3.4. The optimal aging treatment process parameters is at 650 °C for 2 h, while the yield strength, the tensile strength, and the elongation is 705.6 MPa, 1002.3 MPa, and 37.8%.

Comment (5): Please increase the resolution of EDS analysis images as shown in Fig. 9b and 9d.

Response: This suggestion is appreciated. We have redrawn Figure 9b and Figure 9d.

Figure 9. TEM and EDS analysis of elements in the steel after aging treatment.

(a) (b) aging at 650 °C for 10 h, (c) (d) aging at 750 °C for 2 h.

Comment (6): According the Fig. 12b, after aging at 650 °C for 2 h, “a large number of particles at the bottom of the dimple, which indicates that dislocations pile up around the second phase particles during aging”. How did the authors understand that the particles are sign of dislocations pile up?

Response: This suggestion is appreciated. We are sorry for the incorrect expression. The “durnning aging” in the title of paper have been replaced by “during the tensile process”. In plastic deformation, it is easy to produce deformation disharmony and stress set at the interface between intermetallic compound and matrix, which lead the form of crack source. When the size of precipitates is small, the critical shear stress required for dislocation cutting is large, and the stress concentration caused by stacking dislocations lead to the interface separation between precipitates and matrix, resulting in the formation of microcracks and dimples.

Comment (7): The authors should explain why the ductile fracture transforms to the brittle one with the increase of aging temperature and its time.

Response: Considering the Reviewer’s suggestion, we have made a supplement in the paper as following:

Chapter 4.3. With the increase of aging temperature and aging time, the precipitates on grain boundary begin to gather, the size of precipitates in grain becomes larger, and the coherent relationship with matrix is lost. On the one hand, the crystal structure of the coarse intermetallic compound is different from that of the matrix, so it is easy to produce the uncoordinated deformation in the plastic deformation, resulting in the stress concentration at the interface between the intermetallic compound and the matrix, forming the crack source. On the other hand, the mechanism of dislocation slip changes from shear to bypass. When intragranular dislocation slip encounters precipitated phase, the critical shear stress required to bypass precipitated phase is lower than that required by shear. Dislocations tend to bypass precipitates and gather at grain boundaries, leading to stress concentration, intergranular fracture and brittle fracture.

Comment (8): An important question is that what is the main mechanism for the improvement of mechanical properties with aging treatment by precipitates formation?

Response: The main mechanism for the improvement of mechanical properties with aging treatment by precipitates formation is that fine intragranular precipitates are obtained by aging treatment, and they are coherent with the matrix. Only in this way can the conflict of strength and plasticity be solved.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have satisfactorily addressed the comments and the manuscript may now be accepted for publication in the present form.