Scholarly article on topic 'Influence of Second Thermal Cycle on Reheat Cracking Susceptibility of Welding CGHAZ in Vanadium-Modified 2.25Cr1Mo Steel'

Influence of Second Thermal Cycle on Reheat Cracking Susceptibility of Welding CGHAZ in Vanadium-Modified 2.25Cr1Mo Steel Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Y.C. Han, X.D. Chen, Z.C. Fan, H.Q. Bu

Abstract The welding coarse grained heat affected zones (CGHAZ) of Vanadium-modified (V-mod) 2.25Cr1Mo steel are prone to reheat cracking because of its poor ductility in the temperature range of intermediate stress relief treatment (650∼680 ̊C). Considering the multiple-pass welding procedure of V-mod 2.25Cr1Mo steel, the microstructure, high temperature ductility and reheat cracking susceptibility of the CGHAZ subjected to second welding thermal cycle with different peak temperatures (Tp2) were studied by the thermal simulation technique. The results show that the high temperature ductility of double-cycle CGHAZs with all second peak temperature (730 ̊C, 890 ̊C, 1020 ̊C, 1250 ̊C) exhibits recovery in comparison to the single-cycle CGHAZ. Under the condition of Tp2=890 ̊C and Tp2=1020 ̊C, the recrystallization of prior austenite grains in CGHAZ results in a remarkable elevation of the high temperature ductility and a significant reduction of the reheat cracking susceptibility.

Academic research paper on topic "Influence of Second Thermal Cycle on Reheat Cracking Susceptibility of Welding CGHAZ in Vanadium-Modified 2.25Cr1Mo Steel"

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Procedía Engineering 130 (2015) 487 - 496

Procedía Engineering

www.elsevier.com/locate/proeedia

14th International Conference on Pressure Vessel Technology

Influence of Second Thermal Cycle on Reheat Cracking Susceptibility of Welding CGHAZ in Vanadium-Modified

2.25CrlMo Steel

Y.C. Han3 *, X.D. Chena, Z.C. Fana, H.Q. Bua

aHefei General Machinery Research Institute (National Safety Engineering Technology Research Center for Pressure Vessels and Pipelines, Anhui Province Safety Technology Laboratory for Pressure Vessels and Pipelines), Hefei, 230031, China

Abstract

The welding coarse grained heat affected zones (CGHAZ) of Vanadium-modified (V-mod) 2.25CrlMo steel are prone to reheat cracking because of its poor ductility in the temperature range of intermediate stress relief treatment (650-680°C). Considering the multiple-pass welding procedure of V-mod 2.25CrlMo steel, the microstructure, high temperature ductility and reheat cracking susceptibility of the CGHAZ subjected to second welding thermal cycle with different peak temperatures (TP2) were studied by the thermal simulation technique. The results show that the high temperature ductility of double-cycle CGHAZs with all second peak temperature (730°C, 890°C, 1020°C, 1250°C) exhibits recovery in comparison to the single-cycle CGHAZ. Under the condition of TP2=890°C and TP2=1020°C, the recrystallization of prior austenite grains in CGHAZ results in a remarkable elevation of the high temperature ductility and a significant reduction of the reheat cracking susceptibility.

©2015 The Authors.Published byElsevierLtd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

Keywords: reheat cracking susceptibility; welding coarse grained heat affected zones; second thermal cycle; welding thermal simulation

1. Introduction

V-mod 2.25CrlMo, compared with traditional 2.25CrlMo steel, exhibits a higher strength and creep resistance at elevated temperature, increased resistance to hydrogen attack, a lower susceptibility to temper

* Corresponding author. E-mail address: shinehic@163.com

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

doi:10.1016/j.proeng.2015.12.245

embrittlement, and increased resistance to weld overlay disbanding, which can better satisfy the development demand of modern hydrogénation reactor towards higher-parameters, larger-dimensions and lighter-weight [1-5]. Unfortunately, the addition of strong carbide forming elements (such as Vanadium and Titanium) usually results in increasing reheat cracking susceptibility of welded joints during Intermediate Stress Relief treatment (ISR) [6,7]. During the first half of 2008, multiple reactor fabricators in Europe experienced reheat cracking in the Submerged Arc Welding (SAW) weld metal. The cracking affected numerous welds on more than 30 reactors, causing many repairs, delays and cost overruns on the projects [8]. In china, reheat cracks have been detected many times in the welding coarse grained heat affected zones (CGHAZ) of V-mod 2.25CrlMo reactor during the course of fabrication and inspection [5,9]. These problems have been becoming one of the main bottlenecks in the reliability design and fabrication of hydrogénation reactors. To solve these problems, a lot of investigations on the reheat cracking in weld metal were conducted by many laboratories in Europe [10]. In February 2010, a Joint Industrial Program (JIP) was proposed to develop a weld metal/flux screening test procedure for reheat cracking susceptibility [11]. In March 2012, the standard constant strain rate tensile test procedure was established and incorporated in American Petroleum Institute (API) recommended practice "API RP 934-A" as anew appendix [12].

In the past, most researches on reheat cracking dealt with the behavior of the weld metal of V-mod 2.25CrlMo welds, and only a few researchers have investigated the susceptibility to reheat cracking of welding CGHAZ[13-16], In china, Y-groove cracking test were normally used by reactor fabricators to assess reheat cracking susceptibility of CGHAZ, but the test results is seldom consistent with the actual situation. Shinya et al. studied the effect of calcium treatments and strain rate on reheat cracking susceptibility of CGHAZ by isothermal constant strain rate tensile test [13]. Lundin et al. conducted some comparative researches to reheat cracking behavior between the conventional and V-mod 2.25CrlMo steel using notched C-Ring reheat cracking test [14]. Recently, Gleeble tensile test on the simulated CGHAZ was also used to investigate the effect of stress relief temperature and welding heat input on reheat cracking susceptibility [15,16].

All of the research objects in above were single-cycle CGHAZ. However, welding Cr-Mo steels for thick-wall component typically required a multiple-pass procedure. The second thermal cycles of subsequent passes can dramatically change the microstructure and properties of single-cycle CGHAZ. Double-cycle CGHAZ can be divided into the four following typical zones according to the peak temperatures of second thermal cycle (TP2).

(1) Sub-critically CGHAZ (SC-CGHAZ): A tempering zone with coarse grains is tempered by the subsequent passes, because it is heated to a temperature lower than the lower critical transformation temperature (Acl).

(2) Inter-critically reheated CGHAZ (IC-CGHAZ): A partial transformation zone with coarse grains heated in the inter-critical field, between Acl and the upper critical transformation temperature (Ac3).

(3) Super-critically reheated CGHAZ (SCR-CGHAZ): A recrystallized zone with coarse grains is normalized by the subsequent passes. It is heated to a temperature slightly higher than Ac3.

(4) Unaltered CGHAZ (UA-CGHAZ): A zone with coarse grains is unaffected by the subsequent passes, with Tp2 higherthan 1200 °C.

The actual welding CGHAZ always consists of these four zones. Understanding the differences in microstructure and properties between them will be conducive to determine which zone is most vulnerable to reheat cracking, and to find optimal welding procedure to mitigate reheat cracking susceptibility of CGHAZ.

This paper is concerned with the influence of second thermal cycle with different peak temperatures on reheat cracking susceptibility of CGHAZ in V-mod 2.25CrlMo steel, with emphasis on the high temperature ductility, microstructure and hardness of simulated double-cycle CGHAZ. The relationship between welding procedure, material microstructure, and reheat cracking susceptibility were also discussed.

2. Material and experimental procedure

2.1. Material

The experimental materials were prepared from the 280mm thick V-mod 2.25CrlMo forged material which was subjected to quenching and tempering. The actual chemical composition of the steel is listed in Tab. 1.

Table 1. Chemical composition ofV-mod 2.25CrlMo forged material.

c Si Mn P S Cr Mo V Cu Ti Nb Ni Fe

0.133 0.07 0.56 0.0094 0.0032 2.45 0.99 0.268 0.022 0.0069 0.036 0.161 Bal.

2.2. CGHAZs simulation

Gleeble tensile specimens shown in Fig. 1 were initially subjected to a welding thermal cycle on Gleeble 3180 Thermal-Mechanical Simulator. The typical thermal cycle curve for IC-CGHAZ simulation is shown in Fig. 2.

The peak temperature for the first thermal cycle (Tpi) was selected at 1320 °C, and cooling times between 800 and 500 °C (A t8/5) was 26s for the heat input of 35kJ/cm. In addition, the heating rate during austenitization (^hi) was 1000°C/s, the holding time at peak temperature (tpi) was Is, and the preheat temperature was 200°C. Following the first thermal cycle, the second thermal cycle with various peak temperatures was superimposed. In order to obtain different types of double-cycle CGHAZs, the critical transformation temperatures (Acl and Ac3) under welding condition for V-mod 2.25CrlMo steel were determined, as shown in Tab. 2. The second peak temperatures (TP2) for the various CGHAZs simulation were present in Tab. 3. 730°C, 890°C, 1020°C, and 1250r were selected for SC-CGHAZ, IC-CGHAZ, SCR-CGHAZ, and UA-CGHAZ simulation respectively. Besides, the interpass temperature, heating rate and heat input during second welding thermal cycle

simulation were also listed in Tab. 3.

Fig. 1. Sample geometry for the Gleeble tensile test.

p 1000 0)

a> 600

u 400 200 0

0 400 800 1200 1600 2000 Time (s)

T pi T,: 1320 °C pi

T : 890 °C p2

t ,, t ,: Is pi' p2

T wH1: 1000°C/s

- I wH2: 300°C/s

- ^0=35kJ/cm ^g=35kJ/cm

- interpass temp.:220°C i.i.i.i.

Fig. 2. The thermal cycle curve for IC-CGHAZ simulation.

Table 2. The critical transformation temperatures under welding condition forV-mod 2.25CrlMo steel [17].

Thermal cycle type CGHAZ CGHAZ+CGHAZ CGHAZ+FGHAZ

Critical trans, temp. Acl Ac3 Acl Ac3 Acl Ac3

(°C) 870 950 825 930 840 935

Note: FGHAZ represents fine grain heat affected zone. The peak temperature of CGHAZ thermal cycle is 1315°C, and the peak temperature for FGHAZ simulation is 955°C. Acl and Ac3 are the lower and upper critical transformation temperature respectively.

Table 3. The thermal simulation parameters for double-cycle CGHAZs.

Interpass temp. (°C) Heating rate (°C/s) Second peak temp. (°C) Heat input (kJ/cm)

800 1250

500 1020

220 35

300 890

150 730

2.3. Reheat cracking susceptibility test (RHC test)

Subsequent to welding thermal simulation, constant strain rate tensile tests at stress relief temperature on the simulated CGHAZ specimens were carried out utilizing the Gleeble simulator. The test protocol is summarized below:

(1) Heating of the specimen in 3 minutes to reach the test temperature, and soaking 10 minutes at test temperature.

(2) The specimens were pulled at strain rate of 5 X 10"4/s and 5 X 10"5/s at the most susceptible temperature to reheat cracking ofV-mod 2.25CrlMo steel (675°C) [15].

(3) The reduction of area (RoA) was measured after breaking of the specimens using a vernier caliper to assess the reheat cracking susceptibility of the various CGHAZs.

2.4. Microstructural observation, hardness test and fractographic analysis

In order to investigate the effect of second thermal cycle on the microstructure and hardness of CGHAZ, characterize the micro-morphology of reheat cracks, tensile specimens both before and after RHC test were sectioned in half along the longitudinal direction and inlayed in resin. The metallographic specimen were mechanically polished and then etched in 4% nital solution. The hardness ofbase metal, simulated CGHAZs and fractured specimens (test positions away from fracture surface of 0.5mm) was tested on the metallographic sections by an OLYMPUS-DVK-1S Vickers hardness tester using the load of lOkgf. Every sample was tested at 4 points and the average hardness value was calculated.

Moreover, in order to compare the failure modes of various CGHAZs, the fracture morphology of tensile specimens was observed and analyzed by a ZEISS-SUPRA40 Field Emission Scanning Electron Microscope (FESEM).

3. Results and analysis

3.1. Microstructural characterization

The optical micrographs of the various CGHAZs are shown in Fig. 3. TP2 equaling to room temperature (R. T.) denotes the simulated single-cycle CGHAZ. The microstructure ofV-mod 2.25CrlMo base metal is tempered granular bainite, with the prior-austenite grain size of about 25p.m. The single-cycle CGHAZ exhibits the lath-

like bainite microstructure, with dramatically coarsened prior austenite grains (size of about 75 pm) and straight prior-austenite grain boundaries (PAGB), as shown in Fig. 3a.

Under the conditions of TP2=730 °C, the second peak temperature is lower than Acl. This will result in tempering of the single-cycle CGHAZ. But owing to the rapid heating and cooling during welding thermal simulation, the microstructure of SC-CGHAZ still keeps lath-like bainite morphology of the single-cycle CGHAZ, as shown in Fig. 3b.

Under the conditions of TP2=890°C, the single-cycle CGHAZ is heated to a temperature between Acl and Ac3. This will cause partial normalizing of coarse grains. The high-angle grain boundaries in CGHAZ (such as PAGB) have higher energy than grain interiors, which become preferential nucleation site during austenization. It can be seen from Fig. 3d that the regions adjacent to PAGB (about 7-8^m wide) have transformed to austenite upon heating and retransformed to very fine bainitic microstructure upon cooling. However, the center of prior austenite grains remains bainite due to inadequate driving forces for phase transformation, which experiences high temperature tempering at the intercritical region, resulting in the disappearance of lath-like morphology characteristic to some extent, as shown in Fig. 3c.

Under the conditions of TP2=1020 °C , the second peak temperature is slightly higher than Ac3. The recrystallization will occur both within grain interiors and on grain boundaries. It can be seen from Fig. 3e that both the prior austenite grains size and the width of bainitic lath are refined by second thermal cycle, coupled with the formation of zigzag PAGB.

Under the conditions of TP2=1250°C, the single-cycle CGHAZ is heated to the overheated temperature zone again. So, the grain size and microstructure of UA-CGHAZ is similar to the single-cycle CGHAZ, as shown in Fig. 3f.

Fig. 3. Optical metallographs ofthe various CGHAZs (a) Single-cycle CGHAZ, Tp2=R.T.; (b) SC-CGHAZ, Tp2=730°C; (c) IC-CGHAZ, Tp2=890°C; (d) IC-CGHAZ, Tp2=890°C, shallow etching; (e) SCR-CGHAZ, Tp2=1020°C; (e) UA-CGHAZ, Tp2=1250°C.

3.2. Ductility and reheat cracking susceptibility

Fig. 4 summarizes the reheat cracking susceptibility test results of the various CGHAZs of V-mod 2.25CrlMo steel. It can be seen that the ductility of double-cycle CGHAZs at 675°C with all second peak temperature (730°C, 890°C, 1020 °C, 1250°C) exhibits recovery in comparison to the single-cycle CGHAZ.

Under the condition of TP2=890°C and TP2=1020°C, the second welding thermal cycle results in a remarkable elevation ofthe RoA value associated with the IC-CGHAZ and SCR-CGHAZ.

Second peak temp. (°C )

Fig. 4. Relation between RoA ofthe double-cycle CGHAZs and second peak temperature.

According to the previous researches [17-21], the criteria applied to assess the degree of reheat cracking susceptibility are generally given in terms of the RoA associated with a simulated CGHAZ microstructure, as follows: RoA < 5%, "extremely susceptible"; 5% < RoA < 10%, "highly susceptible"; 10% < RoA < 15%, "slightly susceptible"; RoA > 20%, "not susceptible".

The reheat cracking susceptibility of various CGHAZs was assessed base on their RoA value, as shown in Tab. 4. The single-cycle CGHAZ and UA-CGHAZ can be considered "highly" to "extremely susceptible" to reheat cracking at 675 °C. The reheat cracking susceptibility of SC-CGHAZ is slightly lower than the single-cycle CGHAZ. It can be considered "slightly" to "highly susceptible" to reheat cracking. IC-CGHAZ gives an average RoA value of 20.6% at 5xl0"4/s and 14.6% at 5xl0"5/s, which can be considered "not" to "slightly susceptible" to reheat cracking. The average RoA value of SCR-CGHAZ also is more than twice compared to single-cycle CGHAZ both at two strain rate. It can be considered "slightly susceptible" to reheat cracking.

Table 4. Reheat cracking susceptibility ofthe various CGHAZs.

CGHAZ type Stain rate

5xl0-4/s 5xl0-5/s

Single-cycle CGHAZ highly susceptible extremely susceptible

SC-CGHAZ slightly susceptible highly susceptible

IC-CGHAZ not susceptible slightly susceptible

SCR-CGHAZ slightly susceptible slightly susceptible

UA-CGHAZ highly susceptible extremely susceptible

3.3. Hardness

The Vickers hardness value of V-mod 2.25CrlMo base metal is 206 HV10. Due to the formation of lath-like bainite microstructure with oversaturated carbon and alloy element and high dislocation density, the hardness of single-cycle CGHAZ remarkably increases to 383 HV10.

The hardness ofthe various CGHAZs subjected to double thermal cycle is listed in Tab. 5. The SCR-CGHAZ reveals an increase in hardness of 23 HV10 compared to the single-cycle CGHAZ. Because the transformation product in the single-cycle CGHAZ and SCR-CGHAZ is the same (lath-like bainite), the increase in hardness is due to a decrease in grain size caused by recrystallization. The hardness of IC-CGHAZ is 369 HV10, lower than single-cycle CGHAZ. This could be due to the temper softening of the center of prior austenite grains. The

hardness of SC-CGHAZ and UA-CGHAZ is 389 and 378 HV10 respectively, little difference compared to single-cycle CGHAZ.

The hardness of all CGHAZs decreases after RHC test, as shown in Tab. 5. It can be noted that the degree of temper softening can be ranked as follow: SCR-CGHAZ> IC-CGHAZ> SC-CGHAZ> UA-CGHAZ> single-cycle CGHAZ. Under the stain rate of 5xl0"4/s, the percent decrease in hardness of SCR-CGHAZ is 12.9%, but only 0.5% and 2.3% associated with single-cycle CGHAZ and UA-CGHAZ. From the ductility and hardness response of various CGHAZs, it can be deduced that the more prone to softening the material is, the lower susceptibility to reheat cracking it usually exhibits.

Table 5. Hardness ofthe various CGHAZs before and after RHC test.

As simulated After RHC test (5 xlO-4/s) After RHC test (5 xlO-5/s)

CGHAZ Type

Hardness Hardness Percent decrease Hardness Percent

(HV10) (HV10) (%) (HV10) decrease (%)

Single-cycle CGHAZ 383 381 0.5 356 7.0

SC-CGHAZ 389 362 6.8 348 10.6

IC-CGHAZ 369 344 6.9 320 13.3

SCR-CGHAZ 406 353 12.9 318 21.6

UA-CGHAZ 378 369 2.3 343 9.31

3.4. Crack morphology and fractographic analysis

Fig. 5. The micro-morphology ofreheat cracks in various CGHAZs (a) SC-CGHAZ, Tp2 = 730°C; (b) IC-CGHAZ, Tp2 = 890°C; (c,d) SCR-CGHAZ, Tp2 = 1020°C; (e) UA-CGHAZ, Tp2 = 1250°C; (f) single-cycle CGHAZ, Tp2 = R.T..

In order to understand the intrinsic relationship between the microstructure, hardness and high temperature ductility of the various CGHAZ, the crack morphology in the fractured tensile specimens was observed and analyzed, as shown in Fig. 5. Through a comparative analysis, some phenomena can be noted as follows: (1) the crack morphology in all types of CGHAZ exhibits a characteristic of intergranular fracture along PAGB. (2) In single-cycle CGHAZ (Fig. 5f), SC-CGHAZ (Fig. 5a) and UA-CGHAZ (Fig. 5e), the crack tips are very sharp and the PAGBs are straight. (3) In SCR-CGHAZ (Fig. 5c, d), the crack tips are blunted and the PAGBs are

zigzag. (4) In IC-CGHAZ (Fig. 5b), the crack width is corresponding to the width of the recrystallization zone adjacent to PAGB, and the microcrack propagation is seemingly blocked by triplejunction.

The fractographs of various CGHAZs are shown in Fig. 6. The single-cycle CGHAZ exhibits a fully intergranular microvoid coalescence fracture modes (Fig. 6a) with dense and fine dimples on the PAGB (Fig. 6d) that is free of micro-ductility.

Fig. 6. Fracture morphology ofthe various CGHAZs (a,d) single-cycle CGHAZ, Tp2=R.T.; (b,e) IC-CGHAZ, Tp2=890°C; (c,f) SCR-CGHAZ, Tp2=1020°C

By contrast, the fracture morphology of SCR-CGHAZ presents a mixed fracture modes of intergranular and transgranular (Fig. 6c), associated with obviously increasing dimple size and a significant extent of micro-ductility (Fig. 6f). The fracture mode of IC-CGHAZ is also mainly intergranular, but with increasing dimple size in comparison to single-cycle CGHAZ, as shown in Fig. 6b. A micro-ductility zone can be observed in the triple junction of IC-CGHAZ (Fig. 6e), indicating that the recrystallization near the PAGBs could significantly improve the ductility of CGHAZ at stress relief temperature.

4. Discussion

From the test result in this paper, it can be seen that the single-cycle CGHAZ, UA-CGHAZ (TP2=1250°C) and SC-CGHAZ (TP2=730 °C) exhibits lower ductility and higher reheat cracking susceptibility at stress relief temperature, these types of microstructures with the straight prior-austenite grain boundaries and the lower degree of temper softening. Crack morphology and fractographic analysis indicates that these types of CGHAZs can only undergo very little micro-plastic deformation before rupture.

However, for the IC-CGHAZ (Tp2=890°C) and SCR-CGHAZ (Tp2=1020°C), the crack tips are blunted and the grain boundaries dimples are bigger, associated with a significant increasing ductility and much less reheat cracking susceptibility. Previous studies have shown that both the strengthening grain interiors induced by intragranular carbides precipitation and the weakening PAGBs are the leading causes of high temperature ductility deterioration in single-cycle CGHAZ of V-mod 2.25CrlMo steel [22]. In IC-CGHAZ, both the initiation and propagation of micro-cracks could be blocked by the recrystallization zone adjacent to PAGBs (Fig 5b and 5e), resulting in the decrease of the extent of grain boundary weakness and the increase of high

temperature ductility. In SCR-CGHAZ, the high extent of temper softening, the zigzag PAGBs, combined with the refined prior-austenite grains make a noticeable improvement of ductility.

Therefore, taking advantage of recrystallization in prior-austenite grain interiors and boundaries result from the second welding cycle, obtaining a maximum range of IC-CGHAZ and SCR-CGHAZ, could efficiently reduce the reheat cracking susceptibility of the single-cycle CGHAZ. Base on this point, two recommendations for optimization of welding parameters can be proposed.

(1) Increase the coefficient of weld moulding (^=width of weld bead / weld bead height), because of relatively thin and flat weld bead could promote recrystallization in CGHAZ during subsequent weld pass.

(2) Re-normalising of cap bead: for the cap beads which are not undergo second thermal cycle, recrystallization in CGHAZ can be achieved by surface induction heating, Tungsten Inert Gas Welding (TIG) remelting or plasma reheating.

5. Conclusion

In this study, the influence of second welding thermal cycle with various peak temperatures on the microstructure, high temperature ductility and reheat cracking susceptibility of the CGHAZ in V-mod 2.25CrlMo steel were studied by the thermal simulation technique. The main conclusions were drawn as follows:

(1) The high temperature ductility of double-cycle CGHAZ with all second peak temperature (730 °C, 890 °C, 1020°C, 1250°C) exhibited recovery in comparison to the single-cycle CGHAZ.

(2) The second thermal cycle with peak temperatures of 890 °C and 1020 °C promote the recrystallization of prior austenite grains in CGHAZ, which resulted in a remarkable elevation of the ductility and a significantly reduction of the reheat cracking susceptibility.

(3) Taking advantage of recrystallization in prior-austenite grain interiors and boundaries result from the second welding cycle, obtaining a maximum range of IC-CGHAZ and SCR-CGHAZ, could efficiently reduce the reheat cracking susceptibility of CGHAZ in V-mod 2.25CrlMo steel.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Program, No. 2015CB057603).

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