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Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

Crystals 2023, 13(3), 492; https://doi.org/10.3390/cryst13030492

by Evgeniy Tkachev^1,2,*

, Andrey Belyakov¹

and Rustam Kaibyshev²

Reviewer 1:

Paolo Emilio Di Nunzio

Reviewer 2:

Andrea Di Schino

Crystals 2023, 13(3), 492; https://doi.org/10.3390/cryst13030492

Submission received: 26 February 2023 / Revised: 6 March 2023 / Accepted: 10 March 2023 / Published: 12 March 2023

(This article belongs to the Section Crystalline Metals and Alloys)

Round 1

Reviewer 1 Report

Manuscript: crystals-2277654

Effect of hot-rolling on the microstructure and impact toughness of an advanced 9%Cr steel by E. Tkachev, A. Belyakov, R. Kaibyshev

The paper deals with a modified P92 martensitic steel for power plant applications where Ta and B have been added to improve the impact toughness. Two processing variants have been tested:

1) NT: Austenitization 1050°C x 30 min, air cooling

2) HR+Q: Austenitization 900°C x 90 min + hot rolling in the metastable austenite region, water quenching

The materials have been analyzed in the as quenched state and after different tempering treatments by mechanical testing (tensile, impact) and different microscopic techniques (SEM, TEM, EBSD)

Results indicate that the HR+Q route is able to improve the impact toughness of the steel also exploiting tempering treatments at lower temperatures.

The activity is well explained. The aims are clear. The experimental plan and the quality of the instrumental analysis is appropriate and of good quality. English style is good.

The paper can be accepted with minor revisions.

Below some comments

General remarks:

1) The tempering time is never mentioned. Please add this information somewhere (e.g. in the text or in the caption of fig. 3a).

2) The amount of Boron added to the steel is quite high (130 ppm !) but the effect of B is not explained: can the authors specify whether it forms BN or enters M23C6 particles somehow altering their solubility compared to a B-free steel?

3) It would be interesting to provide a comparison of the mechanical properties of the variants Ta+B studied with a conventional P92 grade steel as reference.

4) In section 3.3 it would be useful to provide some data on the intensity of the overall texture in the NT and HR+Q samples to have a general initial comparison before the detailed discussion of Fig. 13.

5) The presence of coarse M23C6 particles in the as-quenched HR+Q treated steel can be justified by the lower austenitization temperature (900°C) which lies below the solvus temperature of this precipitating phase which is about 1050°C, i.e. close to the austenitizing temperature of NT treatment

6) In line 347, when discussing the role of precipitates on the fracture mode, mention is made not only to the PAGS but also to packets and blocks of the martensitic matrix. Actually these features, that are particularly relevant in determining the fracture behavior, are not considered in the microstructural characterization by SEM-EBSD. See for example: Di Schino et al., Materials Letters, 63 (2009) 1968-1972, Caballero et al. Materials Science and Technology, 28 (2012) 95-102, Javaheri et al., Materials Characterization 142 (2018) 295-308. Could this information be retrieved from the EBSD measurements and discussed in the text?

7) Could the authors provide some qualitative comment on the expected creep behavior of the two analyzed specimens with reference to the precipitation state observed and to its coarsening?

Specific remarks:

Table 1: the amount of Tantalum is not reported in the steel chemical composition.

Fig. 1: please specify the measurement units of Charpy and tensile test specimens.

Fig 3a: please specify the temperature at which the impact tests have been carried out.

Fig 4a and 4c: for obvious reasons the images have different magnifications. Unfortunately, this does not permit an easy comparison between the microstructures. Instead, such a direct comparison could be made exploiting the SEM images in Fig. 8, provided that in place of the current Fig 8b an image with the same format as 8a, 8c and 8d is inserted.

Line 172: a subscript is missing. It should read phi2=45° instead of phi=45°.

Fig. 4f: the (hkl)<uvw> labels of the texture components on the right-hand side of the ODF section at phi2=45° (corresponding to phi1=90°) are not clearly visible. In addition the subscript "1" is missing in the phi1 label on the top left corner of the same plot.

Fig 8: the caption is wrong (it is the same as Fig. 7).

Lines 310-311: the sentence "The subsequent rolling of α-phase with {332}⟨113⟩ texture is accompanied by increasing the intensity of {554}⟨225⟩, {111}⟨112⟩ and {111}⟨110⟩ components" suggests that some ferrite could form during the low temperature rolling of the steel. In the microstructural characterization the presence of deformed or recrystallized ferrite is not mentioned and it seems that the whole microstructure is composed of martensite.

This aspect requires a clarification since the the presence of deformed ferrite would affect the overall texture. As a matter of fact no alpha fiber can be detected in Fig. 4f.

Author Response

Dear Reviewer,

Thank you very much for sending us useful comments regarding our paper. We have revised the paper according to the recommendations as described below. The respective changes have been highlighted in the revised version.

General remarks:

Query1) The tempering time is never mentioned. Please add this information somewhere (e.g. in the text or in the caption of fig. 3a).

Answer1. In the revised manuscript, we added this information to the text and to the caption of Fig. 3a.

Query2) The amount of Boron added to the steel is quite high (130 ppm !) but the effect of B is not explained: can the authors specify whether it forms BN or enters M23C6 particles somehow altering their solubility compared to a B-free steel?

Answer2. We expanded the introduction part of our study to highlight the effect of boron on the microstructure of high-chromium steels as follows:

The enrichment of M₂₃C₆ carbides by B enhances their coarsening resistance under creep and aging conditions [7-10]. This effect was considered as a result of decreased surface energy of the M₂₃(C,B)₆/Ferrite interfaces [7,10,11]. It should be noted that the boron is a strong nitride-forming element and may form coarse BN particles in steels with standard N content of 500 ppm [12]. Thus, the nitrogen content should be controlled to prevent the formation of undesirable brittle BN phase. The balanced content of B=130 ppm and N=70 ppm was shown as promising alloying concept to increase the stability of M₂₃C₆ carbides without the formation of BN particles and to provide the improved creep strength [6,10,13]. (lines 35-44)

Query3) It would be interesting to provide a comparison of the mechanical properties of the variants Ta+B studied with a conventional P92 grade steel as reference.

Answer3. This comparison was incorporated in the revised version of manuscript as follows:

The observed values of YS and UTS for the NT and HR+Q samples after tempering at 750°C are similar to those for a P92 steel (YS=480 MPa, UTS=700 MPa) tempered under the same conditions [34]. (lines 143-145)

Query4) In section 3.3 it would be useful to provide some data on the intensity of the overall texture in the NT and HR+Q samples to have a general initial comparison before the detailed discussion of Fig. 13.

Answer4. In the revised version we added the intensity scale for the ODF in the Fig. 4f and described briefly the reason for the textural changes as follows:

Almost random texture is expected in the NT sample. In contrast, the hot-rolled austenite affected the texture in HR+Q samples. (lines 184-185)

Query5) The presence of coarse M23C6 particles in the as-quenched HR+Q treated steel can be justified by the lower austenitization temperature (900°C) which lies below the solvus temperature of this precipitating phase which is about 1050°C, i.e. close to the austenitizing temperature of NT treatment

Answer5. We used this suggestion in the revised manuscript to clarify the evolution of carbide particles during austenitization and hot rolling as follows:

Therefore, the presence of M₂₃C₆ particles in the as-quenched HR+Q treated steel can be justified by the lower austenitization temperature (900°C) which lies below the solvus temperature of this precipitating phase. (lines 240-242)

Query6) In line 347, when discussing the role of precipitates on the fracture mode, mention is made not only to the PAGS but also to packets and blocks of the martensitic matrix. Actually, these features, that are particularly relevant in determining the fracture behavior, are not considered in the microstructural characterization by SEM-EBSD. See for example: Di Schino et al., Materials Letters, 63 (2009) 1968-1972, Caballero et al. Materials Science and Technology, 28 (2012) 95-102, Javaheri et al., Materials Characterization 142 (2018) 295-308. Could this information be retrieved from the EBSD measurements and discussed in the text?

Answer6. We evaluated the mean size of the martensite packets and added this information to the revised manuscript as follows:

The measured size of the martensite packets was 25 ± 3 µm and 83 ± 15 µm for the NT and HR+Q samples, respectively (Table 2). (lines 179-180)

Then the discussion of the fracture behavior was amended as follows:

The size of the facets in the HR+Q sample is slightly larger than that in the NT sample that correlates with the difference in the sizes of PAGs and packets (Table 2), similar to the observations of Schino et al. [39]. The most common mechanism of the cleavage fracture initiation in steels involves the cracking of the inclusions or second phase particles due to plastic strain in the surrounding matrix [40-42]. Thus, the large packets and the presence of relatively coarse M₂₃C₆ particles in the untempered HR+Q sample facilitates the crack initiation and propagation. (lines 268-274)

Query7) Could the authors provide some qualitative comment on the expected creep behavior of the two analyzed specimens with reference to the precipitation state observed and to its coarsening?

Answer7. The possible effect of the microstructure of the analyzed steels on the creep behavior was commented in the revised manuscript as follows:

The uniform distribution of the M₂₃C₆ precipitates and increased number density of MX nanoprecipitates after HR+Q treatment is also expected to be beneficial for the creep resistance of the studied steel. (lines 373-375)

Specific remarks:

Table 1: the amount of Tantalum is not reported in the steel chemical composition.