Effects of PWHT on Microstructure and Mechanical Properties of Weld Metals of Ni-Based Superalloy 617 and 263 for Hyper-Supercritical Power Plants

doi:10.1007/s40195-016-0494-y

Abstract

Abstract:

It is well known that solid solution-strengthened alloy 617 and γ′ precipitation-strengthened alloy 263 have excellent mechanical properties and corrosion resistance at high temperatures. Hyper-supercritical power plants work at temperatures above 700 °C, and these superalloys are considered candidate materials for steam turbines components of these power plants. In this study, gas tungsten arc weldability of these superalloys was evaluated, and the effects of post-weld heat treatment (PWHT) on the microstructural characteristics and the mechanical properties of their weld metals were investigated. Scanning transmission electron microscopy, energy-dispersive spectroscopy and electron probe microanalysis were utilized for the investigation. The experimental results confirmed that these weld metals had different characteristics in microstructure and mechanical properties. PWHT resulted in the precipitation of intergranular carbides, γ′ particles and an increase in tensile strength of these superalloy weld metals. Furthermore, fine γ′ particles, which were not detected in the as-welded metal of alloy 263, were precipitated after PWHT and those particles were the reason for the drastic increase in tensile strength.

Key words: Superalloy, Welding, Precipitation, Segregation

Jeong Kil Kim,Hae Ji Park,Deog Nam Shim,Jeong Kil Kim. Effects of PWHT on Microstructure and Mechanical Properties of Weld Metals of Ni-Based Superalloy 617 and 263 for Hyper-Supercritical Power Plants[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(12): 1107-1118.

Figures/Tables 21

Table 1 Chemical compositions of alloy 617 and alloy 263 (wt%)

Alloys	C	Si	Mn	Cr	Mo	Al	Co	Ti	Ni
Alloy 617	0.06	0.1	0.02	22.3	8.8	1.0	11.5	0.4	Bal.
Alloy 263	0.05	0.2	0.04	19.9	5.8	0.5	20.0	2.1	Bal.

Fig. 1 OM and SEM micrographs of solution-treated alloy 617 and alloy 263

Fig. 2 Schematics of groove a, welding position b

Table 2 Welding conditions

Process	Position	Interpass temp.	Welding current	Welding voltage	Traveling speed	Heat input	Wire feeding speed	Shielding gas/flow rate
GTAW	Horizontal	30 °C	140 A	12.5 V	10 cm/min	10.5 kJ/cm	80 cm/min	99.99% Ar/20 L/min

Fig. 3 Heating cycle used in this study

Table 3 Chemical compositions of alloy 617 and alloy 263 weld metals (wt%)

Weld metals	C	Si	Mn	Cr	Mo	Al	Co	Ti	Ni
Alloy 617 weld metal	0.07	0.11	0.12	21.67	8.82	1.3	11.42	0.36	Bal.
Alloy 263 weld metal	0.05	0.13	0.08	19.9	5.80	0.45	19.72	2.06	Bal.

Fig. 4 Penetration inspection results on the surfaces of the as-welded specimens: a alloy 617 weld metal, b alloy 263 weld metal

Fig. 5 Macrostructures of as-welded alloy 617 and alloy 263 welds In the side bending test for evaluating the formability of the weld metals, the results show that cracking did not occur in any of the welded specimens before and after PWHT as shown in Fig. 6. It can be recognized that weld metals and HAZ of Ni-based superalloys were susceptible to hot cracking and therefore to degrade their formability [15-17, 23, 31, 32]. However, the results stated earlier clarified that these superalloys and the welding consumables in this study have good weldability and that the formability of the welds is excellent.

Fig. 6 Specimens after side bending tests

Fig. 7 OM micrographs of alloy 617 weld metals before a, after b PWHT

Fig. 8 EPMA of the as-welded alloy 617 weld metal

Fig. 9 EPMA of the heat-treated alloy 617 weld metal

Fig. 10 STEM analysis result on intergranular precipitation in heat-treated alloy 617 weld metal

Fig. 11 OM micrographs of alloy 263 weld metals before a, after b PWHT

Fig. 12 EPMA analysis result on as-welded alloy 263 weld metal

Fig. 13 EPMA analysis result on heat-treated alloy 263 weld metal

Fig. 14 STEM analysis result on intergranular precipitation in heat-treated alloy 263 weld metal

Fig. 15 High-magnification SEM image around the grain boundary of heat-treated alloy 263 weld metal

Fig. 16 STEM analysis result on heat-treated alloy 263 weld metal

Fig. 17 Tensile strength of alloy 617 and alloy 263 welds before and after PWHT

Fig. 18 Calculated γ′ precipitation temperatures of alloy 617 a, alloy 263 b by JmatPro

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