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Issue
Metall. Res. Technol.
Volume 117, Number 4, 2020
Article Number 402
Number of page(s) 5
DOI https://doi.org/10.1051/metal/2020033
Published online 25 June 2020

© EDP Sciences, 2020

1 Introduction

The welding of dissimilar materials has been widely employed in various industrial fields, such as the power plant, automobile, and petrochemical industry etc [14]. Much attention has been paid to Co-based alloys owing to their good high-temperature strength, excellent ductility, hot corrosion resistance and weldability [5]. Meanwhile, the AISI 410 stainless steel was extensively utilized in pump gates, impellers, and pressure vessels attributing to its high strength, ductility, and corrosion resistance [6]. Furthermore, the Co-based alloys/stainless steels dissimilar welded joints can reveal unique advantages by taking the superior comprehensive mechanical properties and economic benefits into account. Up to now, this dissimilar welded joint has been successfully utilized in the brush seals of industrial steam turbines to increase the power plant efficiency by reducing the leakage [7], in which Co-based alloys were considered as the ideal bristle material [8] while the AISI 410 stainless steel was used as the plate material [9].

Generally, the service life of welded joints at high temperature was remarkably affected by their microstructure, especially precipitation evolution [10,11]. Thus, the formation site, size, and volume fraction for precipitates should be taken into consideration [1214]. The microstructure changes could take place under the thermal effect [15,16]. The grain boundaries in metallic materials could accelerate the element diffusion, generate the element segregation, and also promote the phase precipitation [17,18]. The carbides distributed along the grain boundary could cause stress concentration and micro-cavity formation, and therefore deteriorated the impact toughness and creep resistance of the welded joints [1921]. On the contrary, the dispersed fine carbides in grain boundaries could improve the mechanical properties such as yield strength [22] and fatigue resistance etc [23]. The grain boundary carbides could serve as dislocation sources and facilitate the crack blunting [24,25]. In addition, the grain boundary migration under long-term high-temperature condition could be effectively inhibited attributing to the resisting pinning effect by the grain boundary carbides [26]. As an effective method to evaluate the changes in microstructure and mechanical property of weldments, the microhardness was investigated in some relative studies [2729]. To ensure the reliability of welded components serving at elevated temperature, the phase precipitation at high temperature should be appropriately controlled.

The present study focused on the microstructure evolution of the WM before and after the high-temperature aging treatment in Co-based alloy/AISI 410 stainless steel dissimilar welded joints. Moreover, the formation process of the nano-sized clustered carbides distributing along the grain boundaries in the WM during high-temperature aging treatment was correspondingly discussed.

2 Materials and methods

The base metals (BMs) for the dissimilar welded joint were Co-based alloy and AISI 410 stainless steel. The chemical composition of the two BMs was listed in Table 1. Electron beam welding without filler technique was utilized to fabricate the dissimilar welded joint. Considering the technical feasibility and cost reduction, 566 °C was one of the typical working temperatures for the steam turbines in the coal-fired power plants [30,31], and thus the temperature 566 °C was chosen to simulate the practical working temperature. To study the microstructure evolution during aging treatment, the dissimilar welded joints were aged at 566 °C for 200, 400, 600, and 800 h, respectively. In particular, the specimen aging treated for 800 h was emphasized in the present study owing to its nano-sized clustered characteristic microstructure. The solution HCl + HNO3 + H2O with the volume proportion of 3:1:4 was applied to reveal the microstructure of the weldments. The microstructure details of the WM before and after aging treatment were examined by optical microscopy (OM, Zeiss Imager A2m) and scanning electron microscopy (SEM, JSM-6700F). The element distribution of the WM with and without the aging treatment was obtained by electron probe micro-analyzer (EPMA, Shimadzu 8050G). In addition, the microhardness along the axial direction of the welds was measured by a Vickers hardness tester (MH-5L) with the constant load of 200 gf and dwell time for 10 s.

Table 1

Chemical composition of Co-based alloy and AISI 410 stainless steel (wt. %).

3 Results and discussion

Figure 1 displayed the microstructure and element mapping results of the WM in Co-based alloy/AISI 410 dissimilar welded joint without aging treatment. Figures 1a1c showed the γ-Co matrix with the typical dendritic microstructure. The dendrite growth was affected by the direction of heat transfer of the molten pool during the solidification process [32]. The element distribution was exhibited in Figures 1d1g. Clearly the elements Cr and W were slightly segregated in the interdendritic regions. The element segregation was mainly caused by the rapid cooling and controlled by the solute redistribution behavior [33,34].

Figure 2 illustrated the microstructure and element distribution of the WM for the Co-based alloy/AISI 410 stainless steel dissimilar welded joint after aging treatment. A dendritic microstructure with clear grain boundaries was displayed in Figure 2a. Figure 2b exhibited the SEM image of the aged WM. It was found that a large number of nano-sized clustered carbides existed along the grain boundaries. The details of the clustered carbides were displayed in Figure 2c. Most of these carbides were ellipsoidal with the size of 100–300 nm. The element mapping results in Figures 2d2g suggested that these clustered carbides were rich in elements Cr, W, and C. The element segregation occurred during the aging treatment from the grain interior to its boundary played a crucial role in the formation of nano-sized clustered carbides. Considering the element distribution results and morphology of the second phase particles, the nano-sized clustered carbides rich in elements Cr, W, and C could potentially be M23C6 or M7C3 (M = Cr, W) carbides for high Co alloys [3537]. Further analysis on the composition and type of these carbides is already in progress.

The precipitation of the nano-sized clustered carbides along the grain boundaries during high-temperature thermal aging was closely related to the element diffusion. Figure 3 schematically depicted the formation process of the nano-sized clustered carbides in the WM during the aging treatment. During the solidification of the molten welding pool, the slight segregation of the elements Cr and W occurred at the grain boundaries, as displayed in Figures 3a and 3b. Figures 3c and 3d reflected the carbide formation process after the aging treatment. At high temperature, Cr and W atoms tended to diffuse from the grain interior to its boundary. The main causes of this process were rooted in the element diffusion resulted from the imperfect structure of grain boundaries and high diffusion coefficient for element diffusion under high-temperature condition [3840]. The carbide precipitation was promoted by the grain boundary segregation of the carbide forming elements and their changed driving force in system free energy. To decrease the free energy of the whole system, the carbides would precipitate when the atom concentrations of various elements were suitable [41]. Finally, the nano-sized clustered carbides formed at grain boundaries during the high-temperature aging treatment.

To shed light on the microstructure and mechanical properties of the weldment, the microhardness distribution of the WM before and after aging treatment was exhibited in Figure 4. To reflect the overall hardness trend of the WM and avoid the possible effect of heterogeneous microstructure in the WM near the fusion line on the hardness test, the indentation in the WM with about 1 mm away from the fusion line was determined as the starting point to measure hardness. The microhardness of the WM before aging was approximately 275 HV. As for the specimen aging at 566 °C for 800 h, its microhardness was not varied obviously with respect to the one without aging treatment. In addition, no significant microhardness fluctuation was detected between the grain interior and their boundaries, as shown in the insets of Figure 4. Zieliński et al. [42] indicated that the high-temperature aging treatment could weaken the steel and result in microhardness reduction. In the present study, the nano-sized clustered carbides could strengthen the grain boundaries by their effectiveness as dislocation sources and posing the obstacles for grain boundary migration. As a result, the weakening effect on the weldment during high-temperature aging treatment could be compensated and a stable performance was anticipated for this dissimilar welded joint even worked under high-temperature condition.

thumbnail Fig. 1

Microstructure and element distribution of the WM without aging treatment: (a) OM image; (b) and (c) SEM images showing the dendritic microstructure; and (d)–(g) the element distribution of the dendrites.

thumbnail Fig. 2

Microstructure and element mapping of the WM aged at 566 °C for 800 h: (a) OM image; (b) SEM image; (c) details of the nano-sized clustered carbides along the grain boundary; and (d)–(g) element maps of the region indicated by the rectangle in (c).

thumbnail Fig. 3

Schematic of the formation process of the nano-sized clustered carbides: (a) and (b) before aging treatment; and (c) and (d) after aging treatment.

thumbnail Fig. 4

Microhardness distribution of the WM before and after aging treatment. The inset OM images displayed the hardness indentation positions in the WM after aging treatment at 566 °C for 800 h.

4 Conclusions

In this study, the formation of the nano-sized clustered carbides in the WM of Co-based alloy/AISI 410 stainless steel dissimilar welded joint during high-temperature aging treatment was systematically studied. The results showed that the WM without aging treatment was mainly γ-Co matrix with dendritic microstructure. The elements Cr and W segregated slightly in the interdendritic regions. Comparatively a large number of nano-sized clustered carbides precipitated along the grain boundaries in the WM after being aged at 566 °C for 800 h. These carbides, with a size of 100–300 nm, were rich in Cr and W elements. The carbide precipitation at grain boundaries was attributed to the element segregation during solidification and the element diffusion induced by high-temperature aging treatment. Similar hardness distribution (∼ 275 HV) between the WM before and after aging treatment was observed. Though the precipitated nano-sized clustered carbides revealed a negligible influence on the microhardness of the weld, yet a stable performance was anticipated for this dissimilar welded joint even worked at high temperature attributing to the strengthening effect of grain boundaries as far as the nano-sized carbides were concerned.

Acknowledgments

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant no. U1760102) and Shanghai Science and Technology Committee (Grant no. 13DZ1101502).

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Cite this article as: Yuanheng Zhang, Kai Ding, Guanzhi Wu, Bingge Zhao, Yuanfang Wang, Tao Wei, Yulai Gao, Aging-induced formation of the nano-sized clustered carbides in the weld metal of Co-based alloy/AISI 410 stainless steel dissimilar welded joint, Metall. Res. Technol. 117, 402 (2020)

All Tables

Table 1

Chemical composition of Co-based alloy and AISI 410 stainless steel (wt. %).

All Figures

thumbnail Fig. 1

Microstructure and element distribution of the WM without aging treatment: (a) OM image; (b) and (c) SEM images showing the dendritic microstructure; and (d)–(g) the element distribution of the dendrites.

In the text
thumbnail Fig. 2

Microstructure and element mapping of the WM aged at 566 °C for 800 h: (a) OM image; (b) SEM image; (c) details of the nano-sized clustered carbides along the grain boundary; and (d)–(g) element maps of the region indicated by the rectangle in (c).

In the text
thumbnail Fig. 3

Schematic of the formation process of the nano-sized clustered carbides: (a) and (b) before aging treatment; and (c) and (d) after aging treatment.

In the text
thumbnail Fig. 4

Microhardness distribution of the WM before and after aging treatment. The inset OM images displayed the hardness indentation positions in the WM after aging treatment at 566 °C for 800 h.

In the text

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