Tempering Behavior of Novel Low-Alloy High-Strength Steel

Round 1

Reviewer 1 Report

p. 2-l.85: The Mn content is low (0.01%). It is better to explain why the Mn content is so low even though the steel concerned is a high strength steel.

 

p.2-l. 86 and p3.-l. 120: The holding time of austenitization should be given.

 

p.2-l.86-88: In the INTRODUCTION, the effects of tempforming were mentioned but this paper does not carry out tempforming. Is it necessary to mention the effects of tempforming in the INTRODUCTION?

 

p.4-l. 152: Give a space before “therefore”!

 

p.4-l. 152-155: What you mean with the sentence that the variation of precipitates changes from M3C + M6C + MX to M3C + M23C6 + M6C + MX? An additional explanation is necessary. According to the Thermo-calc calculation, this change of precipitation kinds occurs with decreasing tempering temperatures. However, the experimental result shown by Equation 2 indicates that this change occurs with increasing tempering temperatures.

 

p. 5-l. 173: Please explain why the yield elongation did not appear after tempering! Are there enough mobile dislocations after tempering?

 

p. 6-l.204: no the flat portions → no flat portions?

 

p. 8-l. 276: If a 0.4% carbon steel is tempered at 200℃, ε-carbide usually precipitate. Why ε-carbide was not observed in your case?

 

p. 10-l. 319: Remove the space before “cementite”!

 

p. 12- l. 366: You don’t mention the existence of retained austenite in the studied steel. Is there any retained austenite?

 

 

p. 12-l. 377: serve→serves Supposedly, there are some spelling errors elsewhere. Please carry out the spelling check!

Author Response

1. p. 2-l.85: The Mn content is low (0.01%). It is better to explain why the Mn content is so low even though the steel concerned is a high strength steel.

The studied steel contains extremely low manganese and it effect on strengthening of the steel is out of our study. Usually manganese has many functions in the steels, including deoxidation, combination with sulfur, stabilization of austenite. However manganese alone is a weak solid-solution-strengthening agent. High strength of the studied steel is provided by alloying with Nb, V, Ti which forms carbide particles and alloying with Si which delays M3C carbide precipitation and prevents embrittlement of the steel after low-temperature tempering.

 

2. p.2-l. 86 and p3.-l. 120: The holding time of austenitization should be given.

The holding time of austenitization was 40 minutes. It has been added to ‘Materials and Methods’ part of revised manuscript.

 

3. p.2-l.86-88: In the INTRODUCTION, the effects of tempforming were mentioned but this paper does not carry out tempforming. Is it necessary to mention the effects of tempforming in the INTRODUCTION?

We agree with reviewer and exclude the consideration of tempforming from introduction part in revised manuscript.

 

4. p.4-l. 152: Give a space before “therefore”!

The space is given in revised manuscript.

 

5. p.4-l. 152-155: What you mean with the sentence that the variation of precipitates changes from M3C + M6C + MX to M3C+ M23C6 + M6C + MX? An additional explanation is necessary. According to the Thermo-calc calculation, this change of precipitation kinds occurs with decreasing tempering temperatures. However, the experimental result shown by Equation 2 indicates that this change occurs with increasing tempering temperatures.

 

We define more exactly precipitation sequences at different temperatures in revised manuscript. This precipitation sequences shows phases which precipitate from martencite at different temperatures in accordance with Thermo-Calc calculations.

6. p. 5-l. 173: Please explain why the yield elongation did not appear after tempering! Are there enough mobile dislocations after tempering?

The yield plateau did not appear after tempering in current work. We are studying conditions at which discontinuous yielding appears in studied steel. However, we plan to discuss this phenomenon in one of our next papers.

 

7. p. 6-l.204: no the flat portions → no flat portions?

The sentence has been corrected in revised manuscript.

 

8. p. 8-l. 276: If a 0.4% carbon steel is tempered at 200℃, ε-carbide usually precipitate. Why ε-carbide was not observed in your case?

The absence of ε-carbide and M3C may be resulted in alloying with Si, which delays precipitation of carbides. The sentence is corrected in revised manuscript.

 

9. p. 10-l. 319: Remove the space before “cementite”!

The space has been removed in revised manuscript.

 

10. p. 12- l. 366: You don’t mention the existence of retained austenite in the studied steel. Is there any retained austenite?

The small quantity of retained austenite was revealed between laths. It is shown in Figure 5c and mention is added in revised manuscript.

 

11. p. 12-l. 377: serve → serves Supposedly, there are some spelling errors elsewhere. Please carry out the spelling check!

The revised manuscript was checked by staff of MDPI editing services.

Reviewer 2 Report

This article studies different tempering temperatures in a novel low-alloy, high-strength steel and correlates the mechanical response with the microstructure.

Several authors have intensively studied this subject with similar alloys, and the novelty of the present work is difficult to elucidate. Nevertheless, the characterization process has been done professionally.

The abstract is difficult to follow.

Through the result, all topics are treated separately, special the mechanical part. Strongly recommend correlating the mechanical result with the different precipitates revealed in the microstructural characterization. There are few mentions of how the microstructure, i.e., carbides, lath width, and subgrain size, affect the mechanical properties.  

Table 3: All measurements should have the error range. How many particles or precipitates were measured?

 

As regards the dislocation density, have the author take into consideration that if no favorable g vectors should be adjusted to a two-beam case in bright field mode (BF) by tilting the sample?, because dislocations are invisible if the g vector adjusted is perpendicular to the Burgers vector of the dislocations (the so-called g x b = 0 or invisibility criterion). This is very important to consider for reliable results.

 

In conclusion number 2, indicates a tempering time of 1 hour, in the materials and methods section state that the tempering time was 2 hours. Please clarify.

 

 

Author Response

1. The abstract is difficult to follow.

The abstract is changed in revised manuscript.

 

2. Through the result, all topics are treated separately, special the mechanical part. Strongly recommend correlating the mechanical result with the different precipitates revealed in the microstructural characterization. There are few mentions of how the microstructure, i.e., carbides, lath width, and subgrain size, affect the mechanical properties.

The discussion about influence of microstructural parameters on mechanical properties is added in revised manuscript.

 

3. Table 3: All measurements should have the error range. How many particles or precipitates were measured?

The error range is added in Table 3. The number of particles for size calculation is added in ‘Materials and Methods’ part.

 

4. As regards the dislocation density, have the author take in to consideration that if no favorable g vectors should be adjusted to a two-beam case in bright field mode (BF) by tilting the sample, because dislocations are invisible if the g vector adjusted is perpendicular to the Burgers vector of the dislocations (the so-called g x b = 0 or invisibility criterion). This is very important to consider for reliable results.

The dislocation densities were evaluated by counting individual dislocations in the grain/subgrain interiors, and each data point represents at least six arbitrarily selected representative TEM images which were obtained using two-beam conditions with {002} or {013} scattering planes and a small positive deviation parameter.

We add this description in ‘Materials and Methods’ part of revised manuscript.

 

5. In conclusion number 2, indicates a tempering time of 1 hour, in the materials and methods section state that the tempering time was 2 hours. Please clarify.

The specimens were tempered for 1 hour. We corrected this in ‘Materials and Methods’ part of revised manuscript.

Reviewer 3 Report

This article investigates the evolutionary behavior of the microstructure and properties of UHSS steels during the tempering process. This article may be helpful to readers of the journal, but I have some comments for the authors of the paper. I hope that my comments will help make the paper better and enable you to get published in Metals.

1.     In section 1. Introduction Line 49-51. The necessary references are missing.

2.     In section 3.1. Thermodynamic analysis. The volume fraction of the precipitated phase is generally used in Figure 1a, and in Figure 1b, the driving force units for the precipitated phase are J/mol, rather than J.

3.     In section 3.1. Thermodynamic analysis Line 155. It is not possible to determine which precipitation is produced in ferrite or austenite, as the austenite and ferrite fractions at different temperatures are not available in the Thermocalc results.

4.     In section 3.2. DSC analysis Line 158. The ε-carbide peak is not fully identified. The authors are requested to give sufficient evidence.

5.     In section 3.2. DSC analysis Line 161. Based on the thermo-calc results, it is possible that M₃C could also precipitate, not just M₂₃C₆.

6.     In section 3.3. Mechanical Properties. In Table 1, the reduction in yield and tensile strength at 400^oC tempering is to be expected, but the reduction in uniform elongation and total elongation is a curious result, with no corresponding microstructural and precipitation explanation given in the paper. Whether this is due to measurement errors or the result of microstructural evolution requires additional corresponding experiments and evidence from the authors.

7.     In section 3.3. Mechanical Properties. In Table 1, the value of the tensile strength for tempering at 500^oC is incorrect.

8.     In section 3.3. Mechanical Properties. In Table 1, P_max and P_GY should be in italics. Are the units kH? The units for load are generally kN.

9.     In section 3.4. Microstructure after quenching. In Figure 5, Nb(c, N) should be Nb(C,N).

10.   In section 3.5. Microstructure after tempering treatment. In Figure 7, The diffraction spots in the M3C dark field should be added.

Author Response

1. In section 1. Introduction Line 49-51. The necessaryreferences are missing.

We have added references in revised manuscript.

 

2. In section 3.1. Thermodynamic analysis. The volume fraction of the precipitated phase is generally used in Figure 1a,and in Figure 1b, the driving force units for the precipitated phase are J/mol, rather than J.

This misprint is corrected in revised manuscript.

 

3. In section 3.1. Thermodynamic analysis Line 155. It is not possible to determine which precipitation is produced in ferrite or austenite, as the austenite and ferrite fractions at different temperatures are not available in the Thermo-calc results.

The ferrite and austenite fractions are added in Figures 1a,b. Precipitation sequences are corrected in the text of manuscript.

 

4. In section 3.2. DSC analysis Line 158. The ε-carbide peakis not fully identified. The authors are requested to give sufficientevidence.

The description of the DSC analysis is corrected in revised manuscript.

 

5. In section 3.2. DSC analysis Line 161. Based on thethermo-calc results, it is possible that M₃C could also precipitate,not just M₂₃C₆

The description of peak at 467^oC in DSC curve is corrected in revised manuscript.

 

6. In section 3.3. Mechanical Properties. In Table 1, the reduction in yield and tensile strength at 400C tempering is to be expected, but the reduction in uniform elongation and total elongation is a curious result, with no corresponding microstructural and precipitation explanation given in the paper. Whether this is due to measurement errors or the result of microstructural evolution requires additional corresponding experiments and evidence from the authors.

We tested another specimen after tempering at 400^oC and revealed that uniform elongation and total elongation is remained the same as after tempering at 280^oC. Probably first specimen presented in previous version of manuscript contained some defect. The result of test after tempering at 400^oC is changed in revised manuscript.

 

7. In section 3.3. Mechanical Properties. In Table 1, the value of the tensile strength for tempering at 500C is incorrect.

The tensile strength for tempering at 500C has been corrected in revised manuscript.

 

8. In section 3.3. Mechanical Properties. In Table 1, P_max and P_GY should be in italics. Are the units kH? The units for load aregenerally kN.

It has been corrected in revised manuscript.

 

9. In section 3.4. Microstructure after quenching. In Figure 5,Nb(c, N) should be Nb(C,N).

It has been corrected in revised manuscript.

 

10. In section 3.5. Microstructure after tempering treatment. In Figure 7, The diffraction spots in the M3C dark field should beadded.

The diffraction spots in the M3C dark field has been added in revised manuscript.

Round 2

Reviewer 3 Report

I am pleased to receive the full and good explanations for my concerns arising from the first version and suggest accepting it for publication as it is.