Microstructure characterization of heat affected zone after welding in Mod.9Cr–1Mo steel

    Microstructure characterization of heat affected zone after welding in Mod.9Cr-1Mo steel K. Sawada, T. Hara, M. Tabuchi, K. Kimura, K. Kubushiro PII: DOI: Reference:

S1044-5803(15)00015-7 doi: 10.1016/j.matchar.2015.01.013 MTL 7796

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

26 September 2014 26 December 2014 18 January 2015

Please cite this article as: Sawada K, Hara T, Tabuchi M, Kimura K, Kubushiro K, Microstructure characterization of heat affected zone after welding in Mod.9Cr-1Mo steel, Materials Characterization (2015), doi: 10.1016/j.matchar.2015.01.013

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Microstructure characterization of heat affected zone after welding in Mod.9Cr-1Mo steel

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K. Sawada*1, T. Hara**, M. Tabuchi*, K. Kimura*, K. Kubushiro***

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*Materials Reliability Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan

** Surface Physics and Structure Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan

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***IHI Corporation,

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1, Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan Corresponding author: E-mail : [email protected] FAX : +81-29-859-2201

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Phone : +81-29-859-2224

Abstract

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The microstructure of the heat affected zone after welding was investigated in Mod.9Cr-1Mo steel, using TEM and STEM-EDX. The microstructure of thin foil was observed

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at the fusion line, and at the positions of 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm to the base metal side of the fusion line. Martensite structure with very fine lath and high dislocation density was confirmed at all positions. Twins with a twin plane of (112) were locally observed at all positions. Elemental mapping was obtained for all positions by means of STEM-EDX. Inclusions of mainly Si were formed at the fusion line but not at the other positions. No precipitates could be detected at the fusion line or at the position of 0.5 mm. On the other hand, MX particles were observed at the positions of 1.0 mm, 1.5 mm, 2.0 mm, 2.5

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mm, 3.0 mm and 3.5 mm even after welding. M23C6 particles were also confirmed at the

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positions of 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm. Very fine equiaxed grains were locally

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observed at the positions of 2.0 mm and 2.5 mm. The Cr content of the equiaxed grains was

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about 12 mass%, although the martensite area included about 8 mass% Cr.

1. Introduction

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Keywords: Mod.9Cr-1Mo steel, welding, FGHAZ, nonequilibrium microstructure

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Mod.9Cr-1Mo steels (9Cr-1Mo-V-Nb-N) are widely used for components of thermal power plants worldwide. Recently, allowable stresses of Mod.9Cr-1Mo steels were reviewed in

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Japan because the creep strength of Mod.9Cr-1Mo steel decreases in the long term [1, 2]. The creep strength of Mod.9Cr-1Mo steel welds was lower than that of the base metal in the long term since the drop in creep strength was larger in the welds than in the base metal in the long term. The long-term creep data of Mod.9Cr-1Mo steel welds is not enough for the safe design of components. Yaguchi et al reported [3] the evaluation of long-term creep strength of Mod.9Cr-1Mo steel welds, collecting creep data from eight organizations in Japan. They proposed master curve for creep rupture data of welds by using region splitting analysis method.

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In the method, creep rupture data are split into high stress and low stress regions using a

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boundary stress. The creep data for low stress regions are not enough because the longest creep

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data of Mod.9Cr-1Mo steel welds is about 30,000h at 600oC which is steam temperature of ultra-supercritical power plants. Yaguchi et al. compared the creep strength of base metal and

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that of welds. It can be expected from the comparison that the creep strength of welds will be

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larger than that of base metal after 100,000h at 650oC although the creep strength of welds is lower than that of base metal in short term region. However, this may be not nature of creep

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strength of Mod.9Cr-1Mo steel welds because the creep strength of welds basically low due to

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type IV cracking in heat affected zone. The contradictory prediction of long-term creep strength

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may be due to lack of long-term creep data of welds. In order to clarify the reason for the large drop in creep strength of welds, it is very

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important to understand the microstructure of the heat affected zone (HAZ) because creep fracture occurs in the HAZ in the long term [4]. Mod.9Cr-1Mo steels are usually used after post weld heat treatment (PWHT) after welding. Many works have focused on the microstructure of the HAZ after PWHT [5-7]. However, the heating and cooling rates are very high in the case of welding [8, 9], and so a nonequilibrium microstructure tends to be formed after welding. It is difficult to identify the nonequilibrium microstructure in the HAZ after PWHT because it can change during PWHT. Therefore, it is necessary to investigate the microstructure just after

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welding (without PWHT) in order to understand the nonequilibrium microstructure. It has been

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reported [10] that some M23C6 carbides did not dissolve and remaining M23C6 carbides

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coarsened at fine-grained HAZ (FGHAZ) after thermal cycling corresponding to welding in ASME Gr.92 steel. It has also been reported for Mod.9Cr-1Mo steel [11] that fresh nucleation of

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M23C6 carbides with a low Cr/Fe ratio occurred in the HAZ during cooling after welding. Delta

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ferrite was observed at the fusion line after welding in Mod.9Cr-1Mo steel [11]. The Mod.9Cr-1Mo steels have a tempered martensite structure with high dislocation

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density after normalizing and tempering. M23C6 carbides and MX carbonitrides are distributed

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along lath, block, packet and prior austenite grain boundaries, which means that elements such

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as Cr, V, Nb, C and N forming M23C6 carbides and MX carbonitrides are concentrated along these grain boundaries. These elements can move short distances during welding because the

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holding time after heating is very short in the case of welding. Therefore, it is necessary to focus on the local area of the microstructure of the HAZ to clarify the nonequilibrium microstructure after welding. The purpose of this study was to clarify the nonequilibrium microstructure formed in the HAZ after welding (without PWHT), by examining the local area by TEM and STEM-EDX.

2. Experimental procedures

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The material studied was Mod.9Cr-1Mo steel (KA-SCMV28 [12]) plate of 25 mm

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thickness. The chemical composition of the steel and weld metal is listed in Table 1 and Table 2,

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respectively. The chemical composition of the weld metal was measured after welding. The welding was performed by submerged arc welding (380A, 30V, travel speed : 28-40 cm/min),

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using matching filler metal for Mod.9Cr-1Mo steel. The macrostructure of the welded plate is

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shown in Fig. 1. The width of the HAZ was about 2.5 mm at the central part of thickness. A cylindrical specimen was cut from the central part of thickness to make thin foils for

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microstructural observations as shown in Fig. 1. Disks were cut from the cylindrical specimen at

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positions corresponding to the fusion line and at 0.5 mm intervals to the base metal side of the

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fusion line. The disks were mechanically and electrochemically polished to make thin foils. The electrolytic solution was a solution of 3 ml hydrochloric acid and 1 g picric acid in 100 ml of

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ethanol. The microstructure was observed by TEM (Hitachi HF2000) operating at an accelerating voltage of 200 kV. Elemental mapping was obtained using JEOL JEM-2010F with STEM BF/ADF detectors, Schottky thermal type emitter and EDS (Bruker XFlash, Silicon drift detector, detector area : 30mm2, detecting solid angle : 0.13str., energy resolution : 127eV) operating at an accelerating voltage of 200kV.

3. Results and Discussion

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3.1 TEM microstructures

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Figure 2 shows TEM microstructures at the fusion line (0 mm) and at the positions of 0.5

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mm, 1.0 mm, 1.5 mm, 2.0 mm and 2.5 mm. Martensite structure with high dislocation density was observed at all positions. Very fine martensite laths were locally observed. Large particles

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were also confirmed at the fusion line, which may have been inclusions which were introduced

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by the welding process because no large particles were observed at other positions. Dislocation structures inside subgrains are shown in Fig.3. A large number of dislocations were observed at

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all the positions. A very fine lamellar structure like twins was locally confirmed at the fusion

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line and at the positions of 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm and 2.5 mm as shown in Fig. 4.

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The lamellar spacing was about several tens of nanometers which is much smaller than the width of martensite lath (several hundreds of nanometers). Figure 5 shows the lamellar structure

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and its electron diffraction pattern at the position of 2.0 mm. The lamellar structure was clearly twin, with a twin plane of (112). It is well known that twins are formed in steels after martensitic transformation if the carbon concentration is very high [13]. However, the carbon content of the steel in this study was low, as listed in Table 1. M23C6 carbides are distributed in this steel after heat treatment. In the case of welding, the M23C6 carbides will be locally dissolved in the matrix by heating [10], and so the carbon content is locally high immediately after the M23C6 has dissolved. However, most of the carbon from the M23C6 moves long

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distances because carbon diffuses very quickly during welding. The twins can be formed due to

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lower Ms temperature around the local area. If the Cr content is high, the Ms temperature is

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decreased. The Ms temperature is locally low due to the local dissolution of M23C6 because the Cr is included in the M23C6. Therefore, the high Cr content in the local area caused the

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formation of twin.

3.2 Elemental mapping

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Elemental maps were obtained from STEM-EDX at the positions of 0 mm, 0.5 mm, 1.0

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mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm as shown in Fig. 6. Red, green, blue, pink

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and yellow colors denote the elemental maps of Cr, Nb, V, Si and Mo, respectively. The elemental maps were superimposed on bright field images at all positions. The steel studied

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included M23C6 and MX carbonitrides after normalizing and tempering [14]. The particles with red color indicate M23C6 carbides because M23C6 carbides mainly include Cr and C. The mean main metallic compositions of the particles with red color at the position of 2.5mm were Cr : 54.15 , Fe : 37.44, V : 0.62 and Mo : 7.79 in mass percent. These values are very similar to the chemical compositions of M23C6 particles in base metal of ASME T91 steel [14]. The particles with blue and/or green colors are MX carbonitrides because MX carbnoitrides mainly consist of V, Nb and N. At the fusion line (0 mm), large particles with pink color were observed as shown

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in Fig. 6; these particles were already seen in Fig. 2. However, no particles except for the large

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particles were confirmed at the fusion line, indicating that most of the M23C6 carbides and MX

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carbonitrides were dissolved by welding. In the case of welding, the filler metal includes Si as a deoxidizer. Therefore, it is assumed that the large particles with pink color indicate Si oxide. No

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particles that included Si were observed at other positions. No particles were detected at the

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position of 0.5 mm as shown in Fig. 6, indicating that most of the M23C6 carbides and MX carbonitrides were dissolved. It is assumed that the temperature at the position of 0.5 mm during

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welding was high enough to dissolve particles. Large particles with green color were observed

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at the position of 1.0 mm; these mainly consisted of Nb, indicating that the particles were

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Nb-rich MX carbonitrides. Yoshino et al. reported that Nb-rich MX particles remained after normalizing even when the normalizing temperature was high enough (e.g. 1150°C and 1200°C)

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in Mod.9Cr-1Mo steel [15]. In the case of welding, the holding time at high temperature after heating is quite short, so it is difficult for Nb-rich MX particles to dissolve in the matrix during welding. In addition to large Nb-rich MX particles, small particles with blue color were also detected at the position of 1.5 mm; these mainly included V, indicating that the particles were V-rich MX. The V-rich MX can be detected after normalizing if the normalizing temperature is less than 1100°C [15]. In addition to the existence of a small amount of V-rich MX, a small amount of Cr-rich particles (red color) was also observed at the positions of 2.0 mm and 2.5 mm.

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The Cr-rich particles were M23C6 because M23C6 particles mainly include Cr and C. According

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to phase diagram by Thermo-calc., the dissolution temperature of precipitate was estimated to

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be about 880oC and 1310oC for M23C6 and MX, respectively. However, in the case of welding, heating time is very fast in addition to short holding time, indicating that it is difficult for

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precipitates to dissolve. It means dissolution temperature will be increased in the case of

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welding. Therefore, it is difficult to predict temperature during welding in terms of dissolution of precipitates. Very fine grains with red color were observed at the positions of 2.0 mm and 2.5

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mm as indicated by the arrows in Fig. 6. The crystal structure of the fine grains was bcc,

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meaning ferrite grains. A large number of M23C6 (red color) and MX (blue color) particles were

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observed at the positions of 3.0 mm and 3.5 mm as shown in Fig. 6. It was reported that M23C6 and MX particles were distributed along lath, block and packet boundaries in as-received

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specimens of Mod.9Cr-1Mo steel [14]. Therefore, many of the M23C6 and MX particles will not dissolve during welding at the positions of 3.0 mm and 3.5 mm. Recovery of lath structure was locally observed at the position of 3.0 mm as indicated by the arrow. The recovered lath was different from the fine ferrite grains that included high Cr content at the positions of 2.0 mm and 2.5 mm because the Cr content of the recovered lath was the same as that of the matrix. Figure 7 shows the main chemical composition of the matrix at each position by point analysis of EDX. Fifteen points were measured for each position, and the mean values were

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used as the data. The Mo and Si contents do not depend on the position. However, the Cr and V

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contents of the matrix decrease when the position separates from the fusion line. Number

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density of particles is shown in Fig.8. The number densities of particles were measured in area of 70 to 95 m2 for each position using elemental maps. The number of M23C6 and MX particles

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increases with increasing distance from the fusion line. This increase in the number of M23C6

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and MX particles can cause a decrease in the Cr and V content of the matrix because M23C6 and MX mainly include Cr and V, respectively.

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Creep fracture occurs at the FGFHAZ of welded joints for high Cr ferritic steels in the long

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term [16]. The creep strength of the FGHAZ is lower than that of the base metal [17]. According

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to optical micrographs [18], the positions of 2.0 mm and 2.5 mm corresponded to the FGHAZ. The undissolved M23C6 and MX particles at the positions of 2.0 mm and 2.5 mm shown in Fig.

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6 cannot effectively pin the lath and block boundaries because the locations of these boundaries before welding change after welding due to reverse transformation. It is difficult to understand how the fine ferrite grains with higher Cr content at the positions of 2.0 mm and 2.5 mm affect creep strength because it is not clear whether these grains remain after PWHT. In future it is necessary to investigate how fine ferrite grains change after PWHT.

3.3 Mechanism of formation of fine ferrite grains at the positions of 2.0 mm and 2.5 mm

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The main chemical composition was measured for fine ferrite grains with red color shown

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in Fig. 6. The main chemical compositions of each fine ferrite grain and matrix are summarized

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in Fig. 9. The Cr content was higher in the fine ferrite grain than in the matrix at the positions of 2.0 mm and 2.5 mm. The mean Cr content of the fine ferrite grain was 12.4 mass% and 12.1

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mass% at the positions of 2.0 mm and 2.5 mm, respectively, although the mean Cr content of

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the matrix was 8.1 to 8.3 mass%. Figure 10 shows the phase diagram of Mod.9Cr-1Mo steel obtained from Thermo-calc. It was confirmed by optical micrograph that the FGHAZ was

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formed at the positions of 2.0 mm and 2.5 mm, indicating that these positions were heated to the

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temperature from Ac1 to Ac3. According to Fig. 10, the Ac3 temperature was around 850°C for 9

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mass% Cr steel. The equilibrium phase is  phase,  phase, M23C6 and MX at around 900°C when the Cr content is about 12 mass%. If the M23C6 particles locally dissolve during welding,

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the local area of the matrix includes high Cr content immediately after heating. The local area contains no carbon because carbon diffuses very quickly during welding. For example, the diffusion coefficient at 877oC can be estimated to be 1.83×10-10 m2 / s and 3.31×10-15 m2 / s for carbon and chromium in  iron based on literature data [19,20]. In this case, it is possible that the Cr content of the local area becomes about 12 mass%, indicating that equilibrium  phase is locally formed after heating. The  phase can remain because the cooling rate is quite high after welding. The heating rate dependence of transformation temperature was investigated

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in Mod.9Cr-1Mo steel by measuring thermal expansion (sample size : φ3mm, 10mm length).

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The Ac1 andAc3 temperature was 881oC and 969oC when the heating rate was 50oC/s, although

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the Ac1 and Ac3 temperature was 839oC and 893oC for slow heating (1oC/s). Therefore, in the case of welding, all of phase boundaries in Fig.10 will be moved to high temperature region.

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However, it may be possible to discuss about the local presence of  phase depending on the

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local Cr content as mentioned above.

4. Conclusions

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The microstructure of the heat affected zone after welding was investigated in Mod.9Cr-1Mo steel, focusing on dislocation structure and the distribution of precipitates. The

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results are summarized as follows. 1. Martensite structure with very fine lath and high dislocation density was observed in the heat affected zone after welding. 2. Twins with a twin plane of (112) were locally observed in the heat affected zone. The presence of twins did not depend on distance from the fusion line. 3. Si-rich inclusions were observed at the fusion line. MX particles were confirmed at the positions of 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm even after welding. M23C6

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particles were observed at the positions of 2.0 mm, 2.5 mm, 3.0 mm and 3.5 mm.

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4. Fine ferrite grains with high Cr content were formed after welding at the positions of 2.0 mm

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and 2.5 mm. The mean Cr content of the fine ferrite grains was about 12 mass% although the Cr

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content of martensite was about 8 mass%.

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[1] K. Kimura, “ASSESSMENT OF LONG-TERM CREEP STRENGTH AND REVIEW OF

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ALLOWABLE STRESS OF HIGH CR FERRITIC CREEP RESISTANT STEELS”, Proceeding of PVP2005, 2005 ASME Pressure Vessels and Piping Division Conference,

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Denver, CO, USA, July 17–21, 2005. [2] H. Kushima, K. Kimura, F. Abe, “Degradation of Mod.9Cr-1Mo Steel during Long-term Creep Deformation”, Tetsu to Hagane 85 (1999) 841–847. [3] M. Yaguchi, T. Matsumura, K. Hoshino, “EVALUATION OF LONG-TERM CREEP STRENGTH OF WELDED JOINTS OF ASME GRADES 91, 92 AND 122 TYPE STEELS”, Proceeding of PVP2012, ASME 2012 Pressure Vessels and Piping Conference, Toronto, Ontario, CANADA, July 15-19, 2012.

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ed. by D. Gandy, J. Shingledecker, ASM International, (2013) 615–626. [6] K. Charlotte, F. Benjamin, B. Françoise, F. Laurent, D. France, P-F. Giroux, T. Ivan, A-F. “HIGH

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[12] Thermal Power Standard Code, Interpretation of Thermal Power Standard Code, 2007 Revision, Thermal and Nuclear Power Engineering Society, Japan (2008). [13] Z. Nishiyama, “Martensitic Transformation”, Academic Press Inc. (1978) 11. [14] K. Sawada, H. Kushima, M. Tabuchi, K. Kimura, “Microstructural degradation of Gr.91 steel during creep under low stress”, Mater. Sci. Eng. A528 (2011) 5511–5518. [15] M. Yoshino, Y. Mishima, Y. Toda, H. Kushima, K. Sawada. K. Kimura, “Phase Equilibrium between Austenite and MX Carbonitride in a 9Cr-1Mo-V-Nb Steel”, ISIJ Int. (2005)

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107–115.

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[17] H. Hongo, M. Tabuchi, T. Watanabe, “Type IV Creep Damage Behavior in Gr.91 Steel

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Welded Joints”, Metall. Mater. Trans. 43A (2012) 1163–1173. [18] K. Kubushiro, S. Takahashi, K. Morishima, “Microstructure of Welded Joints for High Cr

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Heat Resistant Steels”, J. Soc. Mater. Sci., Japan, 59 (2010) 846–852.

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83 (2011) 054412.

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[19] H. Fujiim S. Tsurekawa, “Diffusion of carbon in iron under magnetic fields”, Phys. Rev. B

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Report, Tech. Rept. Tohoku Univ. 47 (1982) 216.

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Table caption

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Table 1 Chemical composition (mass%) of the steel examined.

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Table 2 Chemical composition (mass%) of the weld metal after welding. Figure captions

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Fig.1 Macrostructure of welded plate. Fig.2 TEM microstructures at fusion line, HAZ and base metal. (a) fusion line, (b) position of 0.5mm, (c) position of 1.0mm, (d) position of 1.5mm, (e) position of 2.0mm, (f) position of 2.5mm Fig.3 Dislocation structure inside subgrain at fusion line, HAZ and base metal. (a) fusion line, (b) position of 0.5mm, (c) position of 1.0mm, (d) position of 1.5mm, (e) position of 2.0mm, (f) position of 2.5mm

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Fig.4

Lamellar

structures

at

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line,

HAZ

and

base

metal.

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(a) fusion line, (b) position of 0.5mm, (c) position of 1.0mm, (d) position of 1.5mm, (e)

Fig.5 Twin observed at position of 2.0mm. Elemental

maps

at

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line,

HAZ

and

base

metal.

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Fig.6

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position of 2.0mm, (f) position of 2.5mm

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(a) fusion line, (b) position of 0.5mm, (c) position of 1.0mm, (d) position of 1.5mm, (e) position of 2.0mm, (f) position of 2.5mm, (g) position of 3.0mm, (h) position of

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3.5mm

Main metallic compositions of matrix at fusion line, HAZ and base metal.

Fig.8

Number density of particles at fusion line, HAZ and base metal.

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Fig.7

Fig.9 Main metallic compositions of matrices and fine ferrite grains at positions of 2.0mm and

Fig.10

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2.5mm.

Phase diagram of the steel studied by Thermo-calc.

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Mn

P

S

Ni

0.099

0.38

0.44

0.019

<0.001

0.078

V

Nb

Al

N

0.19

0.08

0.009

0.059

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Cr

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Si

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C

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Table 1 Chemical composition (mass%) of the steel examined.

8.42

Mo

Cu

0.99

0.026

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Si

Mn

P

S

Ni

0.074

0.16

1.29

0.018

0.001

0.47

V

Nb

Al

N

0.22

0.04

0.003

0.039

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Cr

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Chemical composition (mass%) of the weld metal after welding.

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Table 2

8.53

Mo

Cu

0.94

0.020

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Graphical abstract

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Highlights

Mod.9Cr-1Mo steel.

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Inclusions contain Si were detected at the fusion line.

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Nonequilibrium microstructure of heat affected zone was observed after welding in

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Undissolved M23C6 and MX particles were confirmed in heat affected zone. Twins with a twin plane of (112) were locally observed at all positions.

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Very fine ferrite grains with high Cr content were observed in fine grained heat affected zone.