Optimization of PWHT of Simulated HAZ Subzones in P91 Steel with Respect to Hardness and Impact Toughness

Round 1

Reviewer 1 Report

In the paper, the influence of different  PWHT 's on microstructure, hardness and impact toughness of simulated HAZ subzones grained HAZ was studied systematically. the research work is significant for thermal power plant. and the simulated experiment for grained HAZ and different fine grained HAZ areas are valuable.However, this paper only carry out the studied on simulated material , did not carry out test on actual wleded joint. since there are some differents between the simulated microstructure and welded microstructure. especially, there is inhomogeneity of microstructure in welding joint. suggestion: some supplement study on actural welded joint  should be completed.

Author Response

Dear Sir/Madam,

We have carefully studied your comments. Unfortunately, we were not able to conduct the suggested research on samples cut from a real weld HAZ. It would take months. However, we believe that we could satisfactory explain the reasons why we used simulated microstructures. Corresponding changes were made in the manuscript. Please find a detailed list of changes made in the manuscript below. Changes can also be traced in the revised manuscript. Additionally, all new text in the manuscript is highlighted yellow.

Yours sincerely,

G Lojen and T. Vuherer

Reviewers Comments and Suggestions for Authors

“In the paper, the influence of different PWHT 's on microstructure, hardness and impact toughness of simulated HAZ subzones grained HAZ was studied systematically. the research work is significant for thermal power plant. and the simulated experiment for grained HAZ and different fine grained HAZ areas are valuable. However, this paper only carry out the studied on simulated material, did not carry out test on actual wleded joint. since there are some differents between the simulated microstructure and welded microstructure. especially, there is inhomogeneity of microstructure in welding joint. suggestion: some supplement study on actural welded joint  should be completed.”

Response:

Unfortunately, we were not able to conduct the suggested complementary research on samples cut from a real weld HAZ. It would take several weeks or even months.

However, we recognize that the reasons why we used test pieces with simulated microstructures instead of test pieces cut from real weld HAZ were not sufficiently explained/emphasized. We believe that in the revised manuscript we could provide a satisfactory explanation.

Indeed, there are differences between simulated HAZ material and real weld HAZ material. The most important difference is that a real weld HAZ is a very inhomogeneous transition zone, in which the microstructure (and with it also mechanical properties) continuously changes from the weld metal to the unaffected base metal on a very short distance. Therefore, it is impossible to extract a test piece for Charpy impact test with a sufficiently large volume of homogeneous microstructure from a real weld HAZ. Consequently, the scattering of results is huge and the results cannot be linked to only one certain type of microstructure.  

On the contrary, a sufficiently large volume of material exhibiting a homogeneous microstructure of a certain type can be easily produced by simulation of microstructures [2, 35-35]. According to the literature, simulated material is adequate for the tests if it experiences equal thermal cycles as the real work-piece [2, 53, 56]. Considering all this, simulated microstructures are more suitable for studying the influences of different post weld heat treatments on mechanical properties of individual HAZ subzones, and the results of Charpy impact tests can be regarded as more reliable than those obtained with test pieces cut from a real weld HAZs.

Please find a more comprehensive explanation below, under “Action taken”.

Action taken:

The explanation of the reasons for the decision to use simulated microstructures was substantially extended and four new references were included. The text in the lines 86-91 of the first version was substituted with the following text (lines 87-115 in the revised manuscript):

“A HAZ in a real weld is a narrow and inhomogeneous transition zone, in which the microstructure (and with it also mechanical properties) continuously changes from the weld metal to the unaffected base metal on a very short distance. With respect to mechanical testing, the most inconvenient fact is that the width of individual HAZ subzones is very small. Even the total width of HAZ can be only a few tenths of a millimeter [53], if the welding was done with low heat input. Therefore, any tests (except microscopy and micro hardness measurements) on real weld HAZs are problematic or even impossible [2, 53, 54]. Namely, if in a Charpy test piece the area in which the rupture occurs exhibits inhomogeneous microstructure (like a real weld HAZ does), the energies for initiation and propagation of the crack and the path of the crack are influenced by all different microstructures, through which the crack propagates. Therefore, the results depend on the microstructure at the bottom of the notch, and on number and fractions of different types through which the crack propagates. Furthermore, even microstructures in the vicinity of the fracture surface exhibit influence. It is practically impossible to assure that two test pieces cut from a real weld HAZ would have identical microstructures in the area of rupture. Consequently, scattering of results is enormous, and the results cannot be linked to only one certain type of microstructure. In order to obtain reliable data on mechanical properties in relation to microstructure, a larger volume of homogeneous microstructure is inevitable. The most suitable way to produce sufficiently large volumes of material exhibiting a homogeneous microstructure of a certain type is simulation of microstructures [[2, 35-35]. Several research groups confirmed suitability of simulated material for mechanical tests [2, 53, 56]. They compared microstructures and hardness of real weld HAZs with simulated HAZ microstructures. Good agreement of microstructures and hardness of the simulated HAZ subzones with a real HAZ subzones was established, if the real work-piece and the simulated material experienced equal thermal cycles.

Considering all this, simulated microstructures are more suitable for studying the influences of different PWHTs on mechanical properties of individual HAZ subzones, and the results of Charpy impact tests on simulated test pieces can be regarded as more reliable than those obtained with test pieces cut from real weld HAZs. Nevertheless, simulated material for studies of HAZ subzones was used less frequently than expected.”

New references: [53-56]

Reviewer 2 Report

The manuscript is extremely valuable for the science and industry environment in the field of weld joints in high temperature creep resistant steels. I propose publication in present form.

Author Response

Dear Sir/Madam,

We appreciate your commendatory review.

Yours sincerely,

G. Lojen and T. Vuherer

Reviewer 3 Report

• Fig. 2, b: Please mark Ac1, Ac3, Ms, Mf on the fig.
• Table 3 : t_hold/^oC ==> h 
• Table 4: How many specimens were used? Average of more than three specimens were desired. ICHAZ KV 24==>246.
• Fig. 6, 7: Captions are aqual. Please add before PWHT, and after PWHT.
• Fig .7 : Two methods were used to measure impact energy, but there is no mention of the comparison of the two results.
• Line 325: PWHAT
• Line 327: Please mark fresh martensite and tempered martensite on the photo.
• Line 336: After appropriate PWHT, the microstructures consisted of tempered martensite and carbide precipitates. Before PWHT, were carbide precipitates not observed.
• Line 347: the microstructure was adequate.==> Please explain what microstructure are adaquate, and why.
• Fig 10, 11, 12: Were other microstructures like ferrite,  bainte, perlite not observed?

Author Response

Dear Sir/Madam,

We have carefully studied your comments. We hope that we were able to address your comments and suggestions satisfactory. Please find a detailed list of changes made in the manuscript below. Changes can also be traced in the revised manuscript. Additionally, all new text in the manuscript is highlighted yellow.

Yours sincerely,

G Lojen and T. Vuherer

Reviewers comment/suggestion 1)

“Fig. 2, b: Please mark Ac1, Ac3, Ms, Mf on the fig.”

 Response:

Indeed, the inscriptions A_C1, A_C3, M_s and M_f were missing.

 Action taken:

On the T - DL curve in Fig 2b, the points corresponding to A_C1, A_C3, M_s and M_f were marked.

Reviewers comment/suggestion 2)

Table 3 : t_hold/^oC ==> h 

 Response:

“°C “was wrong. The correct unit is “s”

 Action taken:

The mistake was corrected (lines 198-199 in the revised manuscript).

Reviewers comment/suggestion 3)

Table 4: How many specimens were used? Average of more than three specimens were desired. ICHAZ KV 24==>246.

 Response:

• The values in the Table 3, as all the other values of mechanical properties presented in the manuscript, represent average of three specimens.
• KV = 24 J for ICHAZ is wrong. The correct value in fact is 246 J.

Action taken:

• Following explanation was added (line 249 in the revised manuscript): “All specified values of mechanical properties represent average of three specimens.”
• The value of KV for the ICHAZ in the Table 4 was corrected. In the revised manuscript it is 246 J

Reviewers comment/suggestion 4)

Fig. 6, 7: Captions are aqual. Please add before PWHT, and after PWHT.

 Response:

The captions were equal by mistake. They were corrected.

 Action taken:

Captions of Figs. 6 and 7 were changed. The new captions are (lines 304-305 and 307-309 in the revised manuscript):

Figure 6. The total impact energies KV in individual HAZ subzones: (a) CGHAZ; (b) FGHAZ-1; (c) FGHAZ-2; (d) ICHAZ; Annealing time t = 0 denotes as-welded condition.

Figure 7. The total impact energies KV in individual HAZ subzones: (a) after PWHT at 740 °C; (b) after PWHT at 760 °C; (c) after PWHT at 780 °C; (d) after PWHT at 800 °C; Annealing time t = 0 denotes as-welded condition.

Reviewers comment/suggestion 5)

Fig .7 : Two methods were used to measure impact energy, but there is no mention of the comparison of the two results.

 Response:

We realize that the explanation was insufficient. Measuring of KV in two ways is not intended for comparison. The measurement of total impact energy KV by the encoder is more accurate than by integration of the recorded force vs. time curve (F – t curve). The KV obtained by the encoder is used for calibration of the recorded F – t curve. After that, the calibrated F – t curve is analyzed in order to determine the portion of ductile fracture, and to divide the total impact energy KV into energy for crack initiation E_i and crack propagation E_p.

Action taken:

The sentence in the lines 186-188 of the first version was replaced by the following, more comprehensive explanation (lines 222-230 in the revised manuscript):

“The measurement of total impact energy KV by the encoder is more accurate than by integration of the recorded force vs. time (F – t) curve. Therefore, the KV values presented in this work are values obtained by the encoder. However, determination of the portion of ductile fracture and dividing of the total impact energy KV into energy for crack initiation E_i and crack propagation E_p is only possible by analysis of the F – t curve. Therefore, also the F – t curve is recorded. The KV obtained by the encoder is used for calibration of the recorded F – t curve. By calibration, the error caused by oscillation of the pendulum during impact is eliminated. After that, the calibrated F – t curve is analyzed in order to determine the portion of ductile fracture and to divide the total impact energy KV into E_i and E_p.”

Reviewers comment/suggestion 6)

Line 325: PWHAT

 Response:

It was a mistake. Correct is “PWHT”.

Action taken:

The mistake was corrected and the lines 326-329 were substituted with new text. Please find details about changes made under “Reviewers comment/suggestion 7)”.

Reviewers comment/suggestion 7)

Line 327: Please mark fresh martensite and tempered martensite on the photo.

Response:

Unfortunately, the fresh and the tempered martensite cannot be distinguished in optical micrographs. Even with an electron microscope it can be sometimes difficult. Milović et al. [59] reported that it was not by possible by observation of a carbon replica of ICHAZ in a TEM.

The reasons, why we decided for optical microscopy were explained and a more comprehensive explanation of microstructures was provided.

Action taken:

Additional explanations about the limitations of optical microscopy and about reasons, why in our research in spite of that predominantly optical microscopy was used and three new references were included (Lines 378-391 in revised manuscript:

“As known from the literature [59], microstructures of P91 cannot be investigated in detail by optical microscopy. It is due to presence of numerous carbide particles of the size below the resolution of a conventional optical microscope and due to possible presence of very small fractions of bainite, ferrite or untampered martensite that cannot be distinguished.

However, predominantly optical microscopy was done, considering that the microstructures of the base metal P91 and all HAZ-subzones were already thoroughly investigated [7, 14, 49, 59, 65-67], that this work was focused on optimization of PWHT with respect to mechanical properties, and considering the findings of Li et al. [62] that if the hardness of P91 was above 189 HV, operation was safe, and there was no need to inspect the microstructure. Optical micrographs only enable observation of the grain size of prior austenitic grains and of the coarsest carbide particles. Nevertheless, they provide valuable information. Namely, the grain size influences as well the strength as the impact toughness of the material. Smaller grain size is preferred, as it improves the toughness as well as the strength of the material, and the presence of carbide precipitates ensures appropriate creep resistance [7] and [10] (p. 476).”

 New references: : [65-67].

Reviewers comment/suggestion 8)

Line 336: After appropriate PWHT, the microstructures consisted of tempered martensite and carbide precipitates. Before PWHT, were carbide precipitates not observed.

 Response:

Existence of carbide particles in HAZ depends on the observed subzone. It is known from the literature that in the CGHAZ practically all particles dissolve during the welding thermal cycle. With increasing distance from the weld pool, the peak temperature decreases and towards base metal more and more carbides “survive” the austenitization. However, due to partial dissolution their size decreases and therefore a lot of them can no more be detected by an optical microscope. In addition, those that remained large enough, can hardly be noticed in the fine grained matrix.  

Action taken:

Text in lines 325-328 (of the first version of the manuscript) was extended with additional explanations (Lines 392-413 in the revised manuscript):

“Microstructures of HAZ subzones without PWHT are presented in Figure 10. It can be observed that the grain size increased with higher peak temperatures. It is the coarsest in the CGHAZ and that it decreases towards ICHAZ. Although it can hardly be observed in the micrographs, but is well known from the literature, without PWHAT, the microstructure of CGHAZ consists of untampered martensite, FGHAZ-1 and FGHAZ-2 of untampered martensite and some carbide particles which did not completely dissolve during austenitization, and that ICHAZ only partially transforms, therefore the microstructure contains fresh martensite, tempered martensite and undissolved carbides [36,49]. However, even electron microscopy can be unable to reveal all the details. Marzocca et al. [14] reported that even by observation of carbon replicas in a TEM it was not possible to distinguish the fresh formed martensite from the tempered martensite in the ICHAZ. The problem occurs because in the ICHAZ numerous carbide particles do not dissolve during partial austenitization, and consequently, also in the fresh formed martensitic areas undissolved carbides can be observed. Milović et al. [59] reported that very small amounts of bainite can form prior to martensitic transformation. However, bainite is not problematic. It is tougher than martensite and it already contains carbide precipitates.”

Reviewers comment/suggestion 9)

Line 347: the microstructure was adequate.==> Please explain what microstructure are adaquate, and why.

Response:

Adequate explanations and SEM micrographs of an adequate microstructure were added. Please find details below, under “Action taken”.

Action taken:

• Lines 421-423 in the revised manuscript: “After PWHT, tiny black dots – coarser carbides could be observed on the micrographs (Fig. 11 and Fig. 12). Although finer carbides cannot be observed on the optical micrographs, their presence can be assumed, as it was later confirmed by SEM investigations, Fig. 13.”
• Lines 436-437 in the revised manuscript: “Scanning electron microscopy confirmed the presence of numerous precipitates after PWHT, Figure 13. Herewith the SEM confirmed what was indicated already by optical microscopy:”
• Lines 442-445 in the revised manuscript: A new figure was added Figure 13 in the revised manuscript) – SEM image of an adequate microstructure.
• Explanation why such a microstructure is adequate was added in lines 447-464 of the revised manuscript: “According to the literature [7] and [10] (p. 476), precipitation strengthening is the crucial mechanisms of increasing the creep resistance of steels like P91. The intergranular precipitates (on the prior austenite grain boundaries) provide resistance against grain boundary sliding, whereas finer precipitates inside the former austenitic grains act as barriers for dislocation movement during prolonged exposure to operating temperatures [49]. A micrograph of a typical microstructure of normalized and tempered P91 steel was published by Pandey et al. [67]. It looks identical to microstructure in the Figure 13a. Therefore, the microstructure presented in Figure 13 can be regarded as an example of an appropriate PWHT microstructure. The reports in the literature confirm our observations: The major portion of precipitates are M₂₃C₆ carbides, where M is predominantly Cr, followed by Fe and Mo [14, 49, 65-67]. They are mostly coarser than other carbides and prevail on the former austenite boundaries. Finer particles, observed in the intra-lath regions are predominantly MX type precipitates, where M is V, Cr, Mo, and/or Ta and X can be C and/or N [7, 14, 49]. However, EDS is not suitable to analyze very fine precipitates, because the chemical composition of the matrix can influence the results too strong. Therefore, they are mostly identified by other methods, e.g. electron diffraction in TEM.”

Reviewers comment/suggestion 10)

Fig 10, 11, 12: Were other microstructures like ferrite, bainte, perlite not observed?

Response:

According to the CCT diagram [5], pearlite is very unlikely. Ferrite can be present, if the PWHT temperature was too high [41] and very small amounts of bainite can precipitate from austenite before the martensitic transformation starts [59]. However, none of these constituents can´t be observed neither in the optical micrographs in Figures 10-12 nor in the SEM micrographs in the Fig 13.

Action taken:

Additional explanations regarding possible existence of these constituents were added:

• Lines 404-413 in the revised manuscript: “Milović et al. [59] reported that very small amounts of bainite can form prior to martensitic transformation. However, bainite is not problematic. It is tougher than martensite and it already contains carbide precipitates.”
• Lines 468-471 in the revised manuscript: “Vimalan et al. [41] reported that some fresh martensite and ferrite can form at PWHT even up to 12 °C below A₁. But, as their presence detrimentally lowers the toughness [39], the results of Charpy impact tests indicate that they were not present in our samples.”

Round 2

Reviewer 1 Report

The authors new version did not show the supplement content, which the requirement was asked in last reviewed manuscript. The comparision of HAZ for  actual weld  with simulated HAZ has not curried out.

Author Response

Dear Sir/Madam,

We recognize that a comparison of results obtained with test pieces cut from a real weld HAZ with results obtained with simulated microstructures can reveal important information and is therefore important. However, we avoided the comparison in previous versions in order to keep the text shorter.

Tests of real weld HAZ specimens were made before the present research with simulated microstructures started. In the revised manuscript, the results of those tests are presented in a new chapter and compared with results obtained with simulated microstructures, which were subjected to comparable heat treatments.

Please find the list of changes in the revised manuscript below. The changes can be followed in the word file. In addition, new sections are marked yellow.

Yours sincerely,

G. Lojen and T. Vuherer

Action taken:

• Lines 130-150 and 153-155: additional explanation of the backgrounds of our research (which involves also the real weld HAZ tests), was added.
• Lines 255-257 were modified and the text in lines 265-265 was added, correspondingly to the presentation of new data.
• Lines 384-447: A new chapter “1.1. Comparison of mechanical properties measured on simulated microstructures with values obtained with samples from the real weld HAZ” was added. In order to enable the comparison, also some results obtained with simulated samples are presented more detailed (Table 5). Beside the averages of several measurements, now also the values of individual measurements are presented.
• Chapter 3.1.1., lines 449-446: In the previous version, this text and the Figure were a part of the chapter 3.2. “Microstructures”. Now the number of this figure is 10.
• Lines 565-570 - Conclusions: An additional conclusion was added, drawn from the comparison of results obtained with real weld HAZ and simulated HAZ tests.
• All the new text was spell-checked by an linguistic expert.

Round 3

Reviewer 1 Report

The revised manuscript is much improved. it could be accepted in present form.