Microstructural characterization of dissimilar welds

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Microstructural characterization of dissimilar welds between alloy 800 and HP heat-resistant steel R. Dehmolaei, M. Shamanian⁎, A. Kermanpur Department of Materials Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran

AR TIC LE D ATA

ABSTR ACT

Article history:

In this study, dissimilar welds between HP heat-resistant steel and Incoloy 800 were made

Received 22 February 2007

with four different filler materials including: 309 stainless steel and nickel-based Inconel 82,

Received in revised form

182 and 617. The microstructure of the base metals, weld metals and their interfaces were

20 November 2007

characterized by utilizing optical and scanning electron microscopy. Grain boundaries

Accepted 9 January 2008

migration in the weld metals was studied. It was found that the migration of grain boundaries in the Inconel 82 weld metal was very extensive. Precipitates of TiC and M23C6

Keywords:

(M = Cr and Mo) in the Inconel 617 weld metal are identified. The necessary conditions for the

Alloy 800

formation of cracks close to the fusion line of the 309-HP joints are described. Furthermore

HP heat-resistant steel

unmixed zone near the fusion line between HP steel base metal and Inconel 82 weld metal is

Dissimilar welds

discussed. An epitaxial growth is characterized at the fusion line of the 309-Alloy 800 and

Weldability

Inconel 617-Alloy 800 joints.

Unmixed zone

1.

Introduction

Superalloys and heat-resistant steels are widely used in high temperature environments, such as steam generators, reformer and pyrolysis tubes in oil refineries, petrochemical factories and nuclear power plants, where a combination of strength and resistance to corrosion is required [1–6]. Dissimilar metal welds between superalloy and heat-resistant steel tubes are commonly employed in the above-mentioned applications. Incoloy 800 (Alloy 800) and HP heat-resistant steel (25%Cr–35%Ni) are frequently used in refinery and petrochemical industries. The microstructure of Alloy 800 consists of an austenitic matrix with precipitations of titanium carbide and titanium nitride (or carbonitride) along the grain boundaries and in the matrix [7,8]. On the other hand, the microstructure of HP steel consists of an austenitic matrix and a network of primary carbides along the grain boundaries [9–11]. Aging of this alloy at elevated temperatures may cause secondary phases, such as chromium carbide and G phase to precipitate [9,10,12].

© 2008 Elsevier Inc. All rights reserved.

Dissimilar welding of Alloy 800-HP steel in aged condition is usually associated with several problems: thermally induced cyclic stresses resulting from the difference in thermal expansion coefficients (TEC) of HP steel and Alloy 800 as well as crack initiation in the heat affected zone (HAZ) [7]. In order to overcome these problems and to prolong the life of such transition joints, using a trimetallic configuration has been recommended; it is to use a material having a TEC between those of HP steel and Alloy 800 as the filler material. In addition, for optimizing the choice of filler material, apart from TEC factors such as weldability, solidification cracking, metallurgical compatibility and longterm stability in elevated temperatures of service must also be considered. Among the materials which can be employed as the filler material, nickel-base alloys are the most attractive, due to their excellent resistance to creep and oxidation as well as their appropriate TEC [7,13]. Other suggested methods for eliminating the above-mentioned problems are: selection of proportion heat input, controlled interpass temperature and solution annealing heat treatment of base metal before welding [8,13].

⁎ Corresponding author. Tel.: +98 311 3915737; fax: +98 311 3912752. E-mail address: [email protected] (M. Shamanian). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.01.013

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Table 1 – Chemical composition of the base and the filler metals (wt.%) Sample HP steel Alloy 800 Inconel 82 Inconel 182 Inconel 617 309 stainless steel

Fe

Ni

Cr

C

Nb

Ti

Al

Mo

Mn

Si

S

Co

35.4 39.5 3 Rest 1.5 Rest

35.8 31.1 67 72.6 Rest 13.5

24.4 17.9 18.2 19.6 22 22.5

0.4 0.09 0.1 0.015 0.07 0.025

1.3 – 2–3 2.68 – –

– 0.36 0.75 0.37 0.3 –

– 0.25 0.3 – 1.2 –

0.04 – – – 9 2.6

1.3 1 2.5 2.8 0.5 0.7

1.3 0.7 0.5 0.1 0.5 0.9

– – – – 0.008 –

– – – – 12.5 –

In the current work, a trimetallic joint has been studied for dissimilar welding of as-cast HP steel and solution-annealed Alloy 800 using several types of alloys as the filler materials. Microstructures of two joints were investigated: one between HP steel and filler metal and the other between filler metal and Alloy 800.

2.

Materials and Experimental Procedures

HP heat-resistant steel and Alloy 800 were used as the base materials in this study. The HP steel was manufactured by centrifugal casting in tube shape with external diameter 250 mm and thickness13.5 mm. The Alloy 800 was also manufactured by forging in tube shape with external diameter 320 mm and thickness 13.5 mm. The four consumables were 309 stainless steel, Inconel 82, Inconel 617 (gas tungsten arc filler wires) and Inconel 182 (manual metal arc electrode). The chemical compositions of the base and the filler materials are given in Table 1. Small coupons with 100 mm long were cut from each tube for the welding experiments. The welding was carried out on three different types of joint, including the as-cast HP steelsolution-annealed Alloy 800, the as-cast HP steel-aged Alloy 800 and the aged HP steel/solution-annealed Alloy 800, which the study of two latter types of the joints will be reported in the other paper. All butt joints were machined into 37.5° V-grooves. Welding routes were then performed by gas tungsten arc welding with direct current electrode negative (GTAW-DCEN) for three types of filler metals (309 stainless steel, Inconel 82 and Inconel 617) and by shielded metal arc welding (SMAW) for Inconel 182 electrode. The welding parameters are listed in Table 2. Shielding was done with 99% pure argon using 25CHF and 40CHF flows for back and shielding. Specimens of 10× 20× 100 mm in size were cut from the welded samples. All specimens were ground on silicon carbide paper of 80–2000 grit and then were polished on a nylon cloth with 0.3 μm alumina. The etching reagent used for revealing the microstructure was Marbel (10 g CuSO4 + 50 cc HCl+ 50 cc H2O). The microstructures were characterized using optical micro-

scopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) point analysis.

3.

Results and Discussion

3.1.

Base Metal Microstructures

Fig. 1 shows the microstructure of the as-cast HP steel. The optical image (Fig. 1a) shows the austenitic matrix containing chains of eutectic carbides in a lamellar or skeleton structure. The SEM image (Fig. 1b) shows two different primary carbides, which are distinguished by their tonality, the dark and the white carbides. Different colors of these carbides can be attributed to the difference in atomic weight of the concentrated elements in the particles. Fig. 1c and d show the EDS spectrum of the eutectic carbides. It is observed that the darker carbides are chromium-rich, which has been reported to be Cr23C6, and the clearer carbides are niobium-rich, which are NbC. These results verify the previous results reported by other [9,12,14]. The microstructure of Alloy 800 is shown in Fig. 2. The optical image (Fig. 2a) shows the fully austenitic equiaxed matrix containing several types of precipitates. These precipitates are found in the austenitic matrix and along the grain boundaries. Alloy 800 contains 0.36% titanium and 0.09% carbon and hence tends to form titanium carbide and titanium nitride or carbonitride during high-temperatures exposure. The SEM image (Fig. 2b) shows two different precipitates; first is the large precipitates with cuboidal morphology in the austenitic matrix and along the grain boundaries that might be titanium nitride formed during solidification process; second is the finer precipitate with nearly spherical morphology and white color, probably titanium carbides, mainly in the austenitic matrix. These results confirmed by EDS analysis (Fig. 2c and d). Similar results have been also reported by others [7,12]. It has been reported that titanium nitrides or carbonitrides can not be dissolved easily during solution annealing even if high soaking temperatures are employed [7].

3.2. Table 2 – The welding parameters Parameter Current (A) Voltage (V) Travel speed (mm/s) Heat input (kJ/mm)

Root pass

Fill and cap passes

105 14 1.4 0.791

120 14 1.1 1.15

Weld Metal Microstructures

The microstructure of the weld metals are shown in Fig. 3. Fig. 3a shows the microstructure of the 309 weld metal. This reveals an almost fully austenitic structure with a dendritic morphology showing well-developed side branches. There is also a region in the weld metal exhibiting more ferritic– austenitic (FA) and less austenitic–ferritic (AF) modes of solidifications. The regions solidified in the (FA) mode and

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Fig. 1 – Metallography of the as-cast HP steel: (a) light optical micrograph; (b) SEM image using backscattered electrons. EDS spectra of the eutectic carbides: (c) Dark carbide and (d) clear carbide.

consisting of small amount of delta ferrite, segregated in interdendritic (or intercellular) boundaries. However, in the regions that solidified in the (FA) mode, there is a greater amount of vermicular ferrite. The solidification mode in welds of austenitic stainless steels are sensitive to chemical composition (Creq/Nieq ratio, Creq = Cr%+ Mo% + 1.5 Si%+ 0.5Nb% and Nieq = Ni% + 30 C% + 0.5 Mn%) and kinetic factors (welding velocity) [7,8]. In higher Creq/Nieq ratio and lower welding velocities, the mode of solidification is shifted to the FA mode. Since the Creq/Nieq ratio in the 309 stainless steel is relatively high (1.695) and its welding velocity is low (1.1 mm/s), the solidification mode is predominantly FA [15,16]. The microstructures of the Inconel 82 and 182 weld metals (Fig. 3b and c) are fully austenitic. This is due to the fact that because they do not undergo allotropic transformation during welding. The microstructures of weld metals are similar with extensively migrated grain boundary (MGB) [7,15]. The MGBs carry a high angle misorientation of the parent solidification

grain boundary (SGB). The driving force of their migration is the same as simple grain growth in base metals (that is to lower the boundaries energy) [15]. It is also possible that some segregation might occur along the MGBs, possibly due to a “sweeping” mechanism [15]. The Inconel 182 weld metal deposited by manual metal arc welding shows, however, a large number of fine inclusions. Fig. 3d shows that the weld metal microstructure of the Inconel 617 is austenitic with a dendritic morphology. A small amount of precipitates in the different forms are also present. A comparison between Fig. 3b and d shows a lower number of MGB for Inconel 617 weld metal than Inconel 82 weld metal. MGBs are mostly prevalent in fully austenitic weld metals such as Inconel 82, while in the Inconel 617 weld metal, precipitates form along the solidification subgrain boundaries (SSGB) and SGBs. These precipitates are rather effective in “pinning” the crystallographic component of the SGBs. Therefore, they prevent it from migrating away from the parent SGB [15]. The

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Fig. 2 – Metallography of Alloy 800: (a) light optical micrograph; (b) SEM image using backscattered electrons. EDS spectra of the precipitates: (c) cuboidal and (d) spherical.

microstructures of the different weld metals have a significant effect on deciding their cracking susceptibility and mechanical properties. For instance, a quantitative evaluation of weld solidification cracking showed that the 309 weld metal exhibited a greater tendency to cracking than the others weld metals. It is believed that the greater sensitivity of 309 weld metal to cracking is attributed to its pronounced dendritic morphology. It is found that dendritic structures are associated with a greater degree of segregation and are more prone to cracking [17].

3.3.

Interfacial Microstructures

The interfaces of Inconel 82 weld metal with the base metals are observed in Fig. 4. A continuous weld line with an unmixed zone in both interfaces can be seen. However, the unmixed zone in Inconel 82/HP steel base metal interface (Fig. 4a) is much wider than the one in Alloy 800 side. Such an unmixed zone is observed when the melting range of filler materials is similar to or higher than the melting ranges of base metal [18].

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Fig. 3 – Microstructure of the weld metals: (a) 309 stainless steel; (b) Inconel 82; (c) Inconel 182 and (d) Inconel 617.

The existence of unmixed zones affects weld corrosion resistance. Previous investigations indicated that in environments that base and weld metals are resistant to corrosion, the unmixed zone can preferentially be sensitive to corrosion [19]. Fig. 4b shows grain growth in the HAZ and near the interface of Alloy 800. It can be attributed to an increase in temperature during different passes of welding. Fig. 4b also shows a partially melted zone that is characterized by grain boundary melting and thickening. The tendency of grain boundaries to melt in HAZ of Alloy 800 is well known and is attributed to the precipitation of titanium at these boundaries. Titanium at these boundaries not only lowers the melting point constitutionally, but also forms low-melting carbide– austenite eutectics during solidification [20]. The interfaces of Inconel 182 weld metal with base metals are almost similar to the interfaces of Inconel 82 weld metal with base metals. Fig. 5a shows a fully continuous interface between 309 weld metal and Alloy 800 base metal without any crack. No considerable variation is observed in the HAZ of Alloy 800, but grain growth is occurred near the fusion boundary. In addition, an epitaxial growth is clearly evident at the fusion line in Fig. 5a,. Epitaxial growth is attributed to the alloys having the same crystal structures (here FCC) [16]. Far from the fusion line, competitive growth is dominated. This type of growth is attributed to the different growth directions of grains far from fusion line (easy growth direction is b100N in FCC materials) [15,16].

The interfaces between 309 weld metal and HP steel base metal are shown in Fig. 5b and c. Fig. 5b shows important variations in the HAZ of HP steel base metal. A melted zone of base metal (MZBM) exists between fusion boundary and HP base metal. This melted zone has finer grains than the base metal. The melting point of 309 stainless steel (weld metal) is higher than HP heat-resistant steel (base metal), which results in formation of finer grains close to the fusion boundary, due to rapid cooling. Chromium and molybdenum of the weld metal (309 stainless steel is rich of Cr and Mo elements) can diffuse in the MZBM and form chromium and molybdenum carbides. On the other hand, carbon of the base metal from MZBM can diffuse to the weld metal and forms carbides with chromium and molybdenum. Fig. 5c shows a crack in the MZBM adjacent to the fusion boundary running parallel to it. The weld metal has higher melting point than the base metal and can solidify sooner. Therefore, MZBM is still liquid while, weld and base metals are solid. Solidification of this liquid causes tensile stress on the solidified MZBM that can form the cracks. These cracks usually initiate from the carbide precipitates in the MZBM. The interfaces between Inconel 617 weld metal and the base metals are shown in Fig. 6. Fig. 6a and b illustrates that the interfaces between weld metal and base metals are fully continuous without any crack. Fig. 6a reveals the formation of secondary precipitations in the grain boundaries in the HAZ of HP steel near the fusion line. The weld metal is rich in chromium and molybdenum (Inconel 617) and the base metal is rich in

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carbon (HP steel). Therefore, chromium and molybdenum can diffuse from the weld metal liquid (even solid) to the base metal and then form secondary precipitations. Carbon can diffuse from the base metal to the weld metal and form precipitates near the fusion line of the weld metal. The presence of Cr and Mo in the precipitates was confirmed by EDS analysis (Fig. 6b). Fig. 6a also shows some unmelted grains near the fusion line, due to the difference between melting temperature of the base and weld metals [7]. Grain growth has occurred in the HAZ of Alloy 800 (Fig. 6c) due to the heating cycles during different passes of welding. From the welds shown in Fig. 6c, epitaxial growth at the fusion line is evident. Similarity of crystal structure and the chemical composition of the base and weld metals have caused epitaxial growth to occur. Fig. 6d shows SEM image of the epitaxial growth. Two types of grain boundaries may occur near the fusion boundary in the weld metal of dissimilar welds: type I (in a direction roughly perpendicular to the fusion boundary caused by epitaxial growth) and type II (in a direction roughly parallel to the fusion boundary). Type I boundary is usually observed in the similar welds (similar base and filler metals), while type II boundary is a result of allotropic transformation in the base metal that occurs on cooling of weld in dissimilar welds (BCC/FCC) [7,15]. However, in the present study all the micrographs obtained from the interface regions revealed only type I boundaries. It should be noted that, although dissimilar

Fig. 5 – Interface between 309 stainless steel weld metal and base metals: (a) epitaxial growth of Alloy 800; (b) as-cast HP steel; (c) crack in the HAZ of as-cast HP steel base metal.

welds are produced, there is no allotropic transformation during the cooling of two base metals.

4.

Fig. 4 – Interface between Inconel 82 weld metal and base metals. (a) As-cast HP steel; (b) Alloy 800.

Conclusion

Dissimilar welds between HP heat-resistant steel and Alloy 800 can be successfully produced with nickel-base alloy filler metals, such as Inconel 82, 182 and 617. Inconel 82, 182 and 617 weld metals exhibited a dendritic structure. The weld fusion lines of base metals and Inconel 82, 182 and 617 weld metals showed continuous weld lines, without any cracks; while in the

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Fig. 6 – Interface between Inconel 617 weld metal and base metals: (a) HP steel; (b) EDS spectrum of precipitates in the HAZ of HP steel; (c) Alloy 800; (d) SEM image using backscattered electrons of Alloy 800.

HAZ of HP steel with 309 weld metal, some cracks were formed. Therefore, 309 stainless steel can not be recommended for such dissimilar joints. Some precipitations occurred in the Inconel 617 weld metal that might promote creep resistance of joint at the elevated temperature. The Inconel weld metal, when deposited from flux coated electrodes (such as Inconel 182), revealed a higher inclusion content than welds produced by gasshielded welding (such as Inconel 82 and 617).

Acknowledgements The authors would like to acknowledge Isfahan Oil Refinery Company in Iran for performing welding experiments, their

partial financial support and permission for publishing this paper.

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